Particle modification for additive manufacturing

Abstract

To provide a precise and conformal coating which is used for changing the properties of powder for additive manufacturing (AM) by atomic layer deposition (ALD) and molecular layer deposition (MLD).SOLUTION: It turned out that the improvement of fluidity can be obtained by using restricted numbers of ALD cycles. In various embodiments, a coating can bring out one or more advantages such as new material properties, the increase of fluidity, the improvement of sintering, the improvement of stability during storage and the prevention of early sintering.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] Related applications This application claims priority to U.S. Provisional Patent Application No. 62 / 695,665, filed on July 9, 2019. [Background technology]

[0002] Additive manufacturing (AM), also known as solid freeform fabrication or 3D printing, refers to any manufacturing process in which a three-dimensional object is constructed from raw materials (filaments, powders, liquids, suspensions, sheets, or molten solids) in a series of two-dimensional layers or cross-sections. In contrast, conventional manufacturing techniques typically involve casting and molding or subtractive processes. This invention applies, among other things, to 3D printing of metals, ceramics, and cermets.

[0003] Various processes can be used in additive manufacturing, including vat photopolymerization, material extrusion, material jetting, binder jetting, powder bed fusion bonding, directed energy deposition, and sheet lamination. These processes differ depending on the energy used to create the layers, whether the fabricated object requires subsequent densification or other improvements in a method of layering to create a three-dimensional object, and the types of raw materials used, such as the materials compatible with each process. Within the scope of these processes, some methods (e.g., selective laser melting (SLM) or direct metal laser sintering (DMLS), selective laser sintering (SLS), fused deposition modeling (FDM), etc.) melt or soften the material to create layers, while others use various techniques such as stereolithography (SLA) to cure the liquid material.

[0004] Other approaches to AM include binder jetting (or powder bed and inkjet 3D printing) and cold spraying. In binder jetting, a liquid binder is sprayed in a pattern on the particles to create a "green body" (molded body). After the binder has cured, the green body can be directly sintered, or, in some cases, with additional materials or penetrants to create a matrix composite in the next step. Cold spraying is included in directed energy deposition and uses the conversion of kinetic energy instead of a fusion heat source. Particles are bonded by spraying high-speed particles onto / on a substrate that can be incorporated into the final product.

[0005] Sintering is a densification process achieved by using thermal energy below the material's melting point for a predetermined period of time. When powder material is heated to a sufficient temperature during the sintering process, atoms in the powder particles diffuse across the boundaries of adjacent particles, fusing the particles together to form a solid piece. In contrast to melting, the powder used in sintering does not need to reach the liquid phase. Because the sintering temperature does not need to reach the material's melting point, sintering is used in commercial processes such as metal injection molding (MIM) to create complex, dense parts or for high-melting-point materials. Both sintering and melting can be used in additive manufacturing. SLM is used for additive manufacturing of metals or metal alloys (e.g., titanium, gold, steel, Inconel, cobalt, chromium, etc.), which have individual melting points and typically melt during the SLM process.

[0006] An ideal powder for 3D printing is one that flows or spreads well and has a given particle size distribution (PSD). Flowability can be achieved or improved by spherical morphology, coarse powder, or a specific size distribution. The main goal is to obtain a high-density powder bed. The price of metal powders can range up to several hundred dollars per kilogram (e.g., titanium). Powders can be manufactured more cheaply but tend to lack good flowability. Improving flowability through coatings can enable the production of higher-quality parts using lower-cost processes, thus offering a significant advantage to additive manufacturing processes.

[0007] Post-processing of 3D printed parts is also crucial for their final strength and usability. This includes processes such as heat treatment to relieve internal stress and strain that occurs during the printing process, as well as removing any surface defects, such as partially sintered metal or ceramic particles. Post-processing may also include removing the part from the temporary 3D printed support. [Overview of the project]

[0008] Numerous embodiments of the invention are disclosed herein, including the deposition of films onto particles to modify core metals, ceramics, and other powders to create superior properties for additive manufacturing. In various embodiments, the coating may provide one or more advantages, such as novel material properties, increased fluidity, improved sintering, enhanced storage stability, and prevention of premature sintering.

[0009] In a first aspect, the present invention provides a method for manufacturing a molded article, the method comprising: providing a powder containing core particles having a metal or ceramic core; reacting the powder with a first reactant (reactant A) and then sequentially reacting it with a second reactant (reactant B) in an atomic layer deposition (ALD) or molecular layer deposition (MLD) process to produce a powder containing an ALD / MLD coating having alternatingly bonded reactant A portions and reactant B portions; and subjecting the coating powder to an additive manufacturing process and suspending it in a matrix containing the ALD / MLD coating, or sintering it together in a shaped green product suspended in a powder bed containing a powder containing core particles having a metal or ceramic core and an ALD / MLD coating having alternatingly bonded reactant A portions and reactant B portions, thereby producing a shaped green product containing bonded core particles. The method may further be characterized by a limited number of ALD cycles and / or increased fluidity. Typically, ALD / MLD coating imparts at least 10% higher fluidity to uncoated powders with metal or ceramic cores. Surprisingly, we have found that when ALD is performed for 2 to about 50 cycles, about 25 cycles, or 5 to 25 cycles, the resulting powder exhibits improved fluidity compared to uncoated powders and also compared to powders subjected to a greater number of cycles. Instead of cycles, the particles may be characterized by an ALD / MLD coating thickness preferably in the range of 0.2 nm to 5 nm or 10 nm. Fluidity can be measured by the methods described or referenced below.

[0010] Any aspect of the present invention (as referred to above or below) may, in some embodiments, further feature one or any combination of the following features: a powder containing an ALD / MLD coating, wherein the powder has a fluidity at least 10%, at least 20%, or at least 30% higher than that of an uncoated powder having a metal or ceramic core, prior to the step of subjecting the coating powder to an additive manufacturing process; The molded green product is heated to sinter the core particles together; here, the core particles are metal, and the particles or coating, or both, contain at least 0.5 mass% of one or more rare earth elements; the coating has a higher melting temperature than the core; the core particles include at least two compositionally different core particles, and furthermore, the compositionally different core particles include a coating that is compositionally identical; this method includes ALD or MLD of 2-25 cycles or 5-25 cycles (or the coating has a thickness of about 0.2 nm to about 5 nm); the core particles are metal, and the coating contains at least 0.5 mass% of one or more rare earth elements; the coating includes an imide or fluorinated organic moiety; the powder includes a spontaneously combustible metal core; the additive manufacturing process includes a cold spray process; a laser is used to bond the powder to produce a green intermediate product. The ALD / MLD coating enhances the laser absorption efficiency by forming a product; the ALD / MLD coating includes an ABC structure deposited by the ALD / MLD process; the additive manufacturing process includes a binder jet process in which powder is propelled onto the surface; the ALD / MLD coating includes elements or compounds that reduce surface tension during laser melting; further comprising coating the ALD coating with a hydrophobic organic coating (this arrangement can be achieved, for example, by bonding hydrophobic end groups to the ALD-coated metal oxide); and / or the ALD / MLD coating includes carbon-containing materials such as oxides, metal fluorides, lanthanides, silanes, silide and other silicon-containing materials, as well as polymers (e.g., polyamides, polyethylene, polyamides, polyureas, polyurethanes), hydrocarbons, polymer or amino acid fragments or other bio-related molecules and polymers, and other materials), fluorinated polymers (e.g., fluoropolyamides or perfluoropolyamides, -polyethylene, -polyamides, -polyureas, -urethanes, -hydrocarbons). The term "polymer" includes oligomers, or in other words, organic compounds that have repeating units rather than monomers.

[0011] In another embodiment, the present invention provides an additive manufacturing system comprising a container comprising core particles bonded together via a coating on the particles, wherein the particles comprise an ALD / MLD coating having alternating layers of reactant A portion and reactant B portion.

[0012] In a further embodiment, the present invention provides an additive manufacturing system comprising a container including a powder bed, the powder bed including coating particles comprising metal or ceramic core particles comprising an ALD / MLD coating having alternating layers of reactant A portion and reactant B portion; and an energy source and patterning system adapted to apply energy to bond the coating particles into a three-dimensional shape. In a preferred embodiment, the coating having alternating layers of reactant A portion and reactant B portion comprises a metal oxide and further comprises an organic polymer binder.

[0013] In another embodiment, the present invention provides a cermet comprising a metal phase and a crushed ceramic sphere having a thickness in the range of 0.2 to 200 nm, preferably 0.5 to 100 nm, where the crushed ceramic sphere is dispersed in the metal phase. The metal phase may contain sintered particles surrounded by the crushed ceramic sphere, resulting in porosity including gaps between the sintered particles. Preferably, at least 90% by mass of the crushed ceramic sphere has a thickness of 30% or less, preferably 20% or less, of the mass-average thickness of the crushed ceramic sphere.

[0014] In further embodiments, the present invention provides a method for producing a molded article, comprising: providing a powder comprising core particles having a metal or ceramic core and a surface coating; etching the particles; and using the etched particles in an additive manufacturing process. In some preferred embodiments, the core particles having a metal or ceramic core and a surface coating are particles that are recycled in the additive manufacturing process. Preferably, the surface coating is a natural oxide; the method further comprises: reacting the powder with a first reactant (reactant A) and then a second reactant (reactant B) in an atomic layer deposition (ALD) or molecular layer deposition (MLD) process after etching the particles to produce a coating powder containing an ALD / MLD coating having alternatingly bonded reactant A portions and reactant B portions; and subjecting the coating powder to an additive manufacturing process to produce a molded green product containing bonded core particles, either suspended in a matrix containing the ALD / MLD coating or sintered together in a molded green product suspended in a powder bed containing the powder having the ALD / MLD coating having alternatingly bonded reactant A portions and reactant B portions, which contains core particles having metal or ceramic cores.

[0015] In another embodiment, the present invention relates to a method for producing a molded article, the method comprising: providing a first core particle having a first composition and including a metal or ceramic core; coating the first core particle with an ALD / MLD coating; providing a second core particle having a second composition and including a metal or ceramic core; and coating the second core particle with an ALD / MLD coating, wherein the ALD / MLD coating applied to the second core particle is the same as the ALD / MLD coating applied to the first core particle; and using a mixture of the coated first and second core particles in an additive manufacturing process.

[0016] In yet another aspect, the present invention provides a method of manufacturing a shaped article, the present invention comprising: providing a powder comprising core particles comprising a metal or ceramic core; subjecting the coating powder to a laminated fabrication process to produce a shaped green product comprising core particles adhered together; reacting the shaped green product with a first reactant (reactant A) in a process of atomic layer deposition (ALD) or molecular layer deposition (MLD), and then sequentially reacting with a second reactant (reactant B) to produce a shaped green product comprising an ALD / MLD coating having alternately bonded reactant A portions and reactant B portions.

[0017] In an alternative embodiment of the present invention, any of the methods of the present invention can be modified and the core comprises a polymer or other material.

[0018] The present invention includes shaped articles made by any of the methods described herein. The present invention includes laminated fabrication using any of the powders described herein.

[0019] The present invention may be described using the term "comprising" which means including the following. In any of the concepts of the present invention, in a narrower embodiment, the term "comprising" can be replaced with "consisting essentially of" to exclude elements that substantially reduce the properties of the layer, or in the narrowest embodiment, "consisting of".

Brief Description of the Drawings

[0020] [Figure 1] It is a figure which shows the result of the Brookfield powder flow tester of the aluminum alloy powder coated with the ALD alumina coating. The graph shows the flow coefficient corresponding to the major principal consolidation stress of the ALD coating applied using 0.5, 1, 25, 50, and 100 ALD cycles. [Figure 2] It is a figure which shows the normalized plot of the data of FIG. 1. [Figure 3] It is a diagram showing the results of a TGA test of titania powder coated with an alumina or titania ALD coating.

Mode for Carrying Out the Invention

[0021] The powder of the present invention can be any powder useful for additive manufacturing. Some preferred starting powders are as follows.

[0022]

Table 1

[0023] The materials listed are well-known. For example, some preferred starting material powders are as follows. EOS AlSi10Mg: Al (balance), Si (9 - 11 wt%), Mg (0.2 - 0.45 wt%); PSD: 34 μm (average) EOS Ti64: Ti (balance), Al (5.5 - 6.75 wt%), V (3.5 - 4.5 wt%) PSD: 45 μm (average) H13 tool steel: Fe (balance), Cr (5 wt%), Mo (1.5 wt%), V (1 wt%), Si (1 wt%); PSD: 16 μm (average)

[0024] General ALD / MLD chemistries that can be used in the present invention for 3D printing, either alone or in combination, include, but are not limited to, the following.

[0025]

Table 2

[0026] "Polymer" can be an oligomer containing two or more repeating units. The chemical properties of ALD will be described in more detail below.

[0027] 1. General Description of Atomic Layer Deposition or Molecular Layer Deposition Atomic layer controlled growth technology makes it possible to deposit coatings with a thickness of approximately 0.1 to 5 angstroms per reaction cycle, thus providing a means to control the coating thickness very precisely. Thicker coatings can be prepared by repeating the reaction sequence and sequentially depositing additional layers of coating material until the desired coating thickness is achieved.

[0028] The coating is deposited using atomic layer deposition (ALD) or molecular layer deposition (MLD) processes. In the ALD / MLD process, the coating formation reaction proceeds as a series of (typically) two half-reactions. In each of these half-reactions, a single reagent (precursor) is introduced into contact with the substrate surface. The state is such that the reagent is in gaseous form. In most cases, the reagent reacts with functional groups on the surface of the particles and becomes bound to the particles. Because the reagent is a gas, it penetrates into the pores in the substrate and deposits on the inner surface of the pores and the outer surface of the substrate. This precursor is designed to react with the surface at all available surface sites, but does not react with itself. In this way, the first reaction takes place to form a single monolayer or sub-monolayer, creating new surface functionality. Next, any excess reagent is removed. This helps prevent the growth of undesirable, larger inclusions in the coating material. Then, each of the remaining half-reactions proceeds in sequence, with the first reagent being introduced each time, reacting with the surface of the particles, and any excess reagent being removed before introducing the next reagent. Typically, reagents are introduced using an inert carrier gas, and the reaction chamber is usually swept with the carrier gas during the continuous introduction of reagents to help remove excess reagents and gaseous reaction products. Vacuum sweeping can be performed during and between the continuous administration of reagents to further remove excess reagents and gaseous reaction products.

[0029] After being exposed to a first precursor, the surface is exposed to a second precursor, which is typically dispersed in an inert carrier gas. This precursor is designed to react with the functional groups positioned in the first reaction step. This reaction occurs until all available surface sites react. The second precursor also does not react with itself. Any excess second precursor is removed in an optional inert gas purging step. If the gas is properly metered, the purging step may not be necessary. This can be a process of at least four steps (precursor 1, purge, precursor 2, purge) to deposit one monolayer of the growing film. This process is repeated the required number of times to form the desired film thickness. The ALD / MLD process may be initiated with a “linker” agent that promotes covalent bonding to the surface, or terminated with a termination agent that may be hydrophobic or otherwise designed in a different way for a particular purpose.

[0030] For the purposes of the present invention, the ALD / MLD process may consist of only half-reactions rather than a complete cycle. However, at least one complete cycle, more preferably at least five cycles, is preferred.

[0031] A simple method for coating particulate substrates is to form a fluidized bed of particles or other agitated bed, and then pass various reagents through the fluidized bed under reaction conditions. Methods for fluidizing particulate matter are well known and generally involve supporting the particles on a porous plate or screen. A fluidizing gas passes upward through the plate or screen, slightly lifting the particles and expanding the volume of the bed. When properly expanded, the particles behave like a fluid. Reagents (gas phase, liquid phase, or solid phase) can be introduced into the bed to react with the surface of the particles. Liquid or solid reagents are converted to gaseous form in the bed before reacting with the particles. In this invention, the fluidizing gas can also act as an inert purging gas to remove unreacted reagents and volatile or gaseous reaction products. Furthermore, the reaction can occur on the particle surface in a rotating cylindrical vessel, rotating tube, or vibrating bed. This vibrating bed method is particularly suitable for continuous processes.

[0032] The reaction conditions are selected to meet three main criteria. The first criterion is that the reagents are gaseous under the reaction conditions. Therefore, the temperature and pressure conditions are selected so that the reactants volatilize before the reaction. The second criterion is reactivity. The conditions, in particular the temperature, are selected so that the desired reaction between the film-forming reagent (or, at the start of the reaction, the reagent initially introduced and the particle surface) occurs at a commercially reasonable rate. The third criterion is that the substrate is thermally stable from a chemical and physical standpoint. The substrate must not decompose or react at the process temperature, except in cases where it may react with one of the ALD precursors on its surface functional groups in the early stages of the process. Similarly, the substrate must not melt or soften at the process temperature, thereby maintaining the physical shape of the substrate, particularly its pore structure. The reaction is generally carried out at a temperature of about 270–1000K, preferably 290–450K, where the specific temperature in each case is lower than the temperature at which the substrate melts, softens, or decomposes.

[0033] During the continuous administration of the reagents, the particles are subjected to conditions sufficient to remove the reaction products and unreacted reagents. This is done after each reaction step, for example, about 10 -5 This can be achieved by subjecting the particles to a high vacuum, such as Torr or higher. Another method to achieve this, which is more readily applicable to industrial applications, is to sweep the particles with an inert purge gas between reaction steps. This purge gas can also act as a fluidizing medium for the particles and a carrier for the reagents.

[0034] Several techniques are useful for monitoring the progress of the reaction. For example, vibrational spectroscopy studies can be performed using transmission Fourier transform infrared techniques. Deposited coatings can be investigated using in situ spectroscopic ellipsometry. Atomic force microscopy studies can be used to characterize the roughness of the coating relative to the roughness of the substrate surface. Depth profiling and confirmation of the crystalline structure of the coating can be performed using X-ray photoelectron spectroscopy and X-ray diffraction.

[0035] The aluminum oxide coating is easily deposited using trimethylaluminum and water as precursors, as illustrated in reaction sequence A1 / B1. The reactions illustrated are unbalanced and are intended only to show the reaction at the surface of the substrate (i.e., not an interlayer or intralayer reaction). Substrate-XH * +Al(CH3)3 = Substrate-X-Al * -CH3 + CH4 (precursor reaction) Substrate-X-Al * -CH3 + H2O = Substrate-X--Al-OH * +CH4 (A1) Substrate-X-Al-OH * +Al(CH3)3 = Substrate-X-Al-O-A-l * -CH3 + CH4 (B1)

[0036] In reaction A1 / B1, X is typically oxygen, nitrogen, or sulfur, and the asterisk ( * ) represents surface species where the following half-reactions can occur. The aluminum oxide film is formed by alternately repeating reactions A1 and B1 until the desired coating thickness is achieved. The aluminum oxide film tends to grow at a rate of about 0.3 nm / cycle using this reaction sequence.

[0037] The titanium oxide coating is easily deposited using titanium tetrachloride and water and / or hydrogen peroxide as precursors, as illustrated in reaction sequence A2 / B2. As before, the reactions illustrated are unbalanced and are intended only to show the reaction at the surface of the particle (i.e., not an interlayer or intralayer reaction). Substrate-XH * +TiCl4 = Substrate-X--Ti * -Cl3 + HCl (precursor reaction) Substrate-X--Ti * -Cl3 + H2O2 = Substrate-X-T-i * -OH + HCl + Cl2 (A2) Substrate-X-Ti * -OH + TiCl4 Substrate-X-Ti-O-Ti * -Cl3+-HCl(B2)

[0038] In reaction A2 / B2, X is typically oxygen, nitrogen, or sulfur, and is represented by an asterisk ( * ) represents a surface species in which the following half-reactions may occur. The titanium oxide film is formed by alternately repeating reactions A2 and B2 until the desired coating thickness is achieved. Using this reaction sequence, the titanium oxide film tends to grow at a rate of approximately 0.05–0.1 nm / cycle.

[0039] As is known in ALD / MLD processes, the order may be AB, ABC, ABCD, ABCDABABCD, or any desired order if the chemical entities react with each other in the desired order. Each reactant has at least two reactive moieties (this includes the possibility that the reactant can be modified to have two reactive moieties, such as having a first reactive moiety and a second reactive moiety that are temporarily blocked by protecting groups or require subsequent activation of the reaction, such as UV activation). In some preferred embodiments, the reactants have exactly two reactive moieties, as a greater number of reactive groups can result in a lower packing density. In some preferred embodiments, the film has at least three repeating units (e.g., ABABAB), or at least five, or at least ten, or at least 50, sometimes in the range of 2 to 1000, or 5 to 100. “Reactive” means under normal MLD conditions and on a commercially appropriate timescale (e.g., at least 50% reacted within 10 hours under suitable reaction conditions). To control the quality of the film, the reactants must be single-reactive during each step of the MLD process, avoiding two reactions with the surface, and the reactants must not self-react and condense on the surface.

[0040] In some preferred embodiments, the reactive moiety of reactant A may include isocyanates (R-NCO), acrylates, carboxylic acids, esters, epoxides, amides, and amines, as well as combinations thereof. In some preferred embodiments, reactant A includes diisocyanates, diacrylates, dicarboxylic acids, diesters, diamides, or diamines. In some preferred embodiments, the reactive moiety on reactant B includes alcohols or amines, and combinations thereof. In some preferred embodiments, reactant B includes diols, amine alcohols, or diamines.

[0041] In some cases, particularly in the case of MLD, a gas-phase reactant is selected to react monofunctionally with the substrate or growing polymer chain only. That is, only one group or part of the gas-phase reactant can react with the substrate or growing polymer chain under the conditions of this reaction. This prevents unwanted crosslinking or chain termination that may occur if the gas-phase reactant can react multifunctionally. A reactant is considered to react "monofunctionally" if, during the reaction, it forms a bond with only one polymer chain and does not self-polymerize under the reaction conditions used. In certain embodiments of the present invention, as fully described below, it is possible to use a gas-phase reactant that can react bifunctionally with the substrate or growing polymer chain, provided that the gas-phase reactant contains at least one additional functional group. In this aspect of the present invention, reactants having exactly two functional groups with substantially equal reactivity are preferably avoided.

[0042] A preferred gas-phase reactant of the first class is a compound having two different reactants, one of which is reactive with a functional group on the substrate or polymer chain, and the other which does not readily react with a functional group on the polymer chain but reacts with a functional group introduced by the other gas-phase reactant. Examples of reactants of this class are as follows: a) Hydroxyl compounds having vinyl or allyl unsaturation. These can react with carboxylic acids, carboxylic acid halides, or siloxane groups to form esters or silicon-oxygen bonds, introducing vinyl or allyl unsaturation into polymer chains. Alternatively, the unsaturated group can react with a primary amino group in a Michael reaction to extend the polymer chain and introduce a hydroxyl group into the chain. b) Amino alcohol compounds. The amino group can react with a carboxyl group, carboxylic acid chloride, vinyl or allyl group, or isocyanate group to, for example, extend polymer chains and introduce hydroxyl groups into the chains. Alternatively, the hydroxyl group can react with a siloxane species to form a silicon-oxygen bond and introduce free primary or secondary amino groups.

[0043] The second class of suitable gas-phase reactants includes a variety of cyclic compounds that can participate in ring-opening reactions. Ring-opening reactions generate new functional groups that do not readily react with the cyclic compound. Examples of such cyclic compounds include, for example, the following: a) Cyclic azasilanes. These can react with hydroxyl groups to form silicon-oxygen bonds, generating free primary or secondary amino groups. b) Cyclic carbonates, lactones, and lactams. Carbonates can react with primary or secondary amino groups to form urethane links and generate free hydroxyl groups. Lactones and lactams can react with primary or secondary amino groups to form amide links and generate free hydroxyl groups or amino groups, respectively.

[0044] The third class of gas-phase reactants includes compounds containing two different reactive groups. Both reactive groups react with the functional groups of the polymer chain, but one is far more reactive than the other. This allows the more reactive group to react with the functional groups of the polymer chain, while the less reactive group remains unreacted and can be used to react with other gas-phase reactants.

[0045] A fourth class of gas-phase reactants includes compounds containing two reactive groups, one of which is blocked, or otherwise masked or protected, so that it cannot be used in the reaction until a blocking, masking, or protecting group is removed. The blocking or protecting group may be removed chemically in some cases, or otherwise by thermal decomposition of the blocking group to produce the underlying reactive group, by irradiating the group with visible or ultraviolet light, or by a photochemical reaction. Unprotected groups may include, for example, amino groups, anhydride groups, hydroxyl groups, carboxylic acid groups, carboxylic acid anhydride groups, carboxylic acid ester groups, and isocyanate groups. Protecting groups may, after removal of the protecting group, result in one of the aforementioned functional groups.

[0046] The reactants of this fourth class may have hydroxyl groups protected by leaving groups such as benzyl, nitrobenzyl, tetrahydropyranyl, -CH2OCH3, or similar groups. In these cases, the hydroxyl groups can be deprotected by various methods, such as treatment with HCl, ethanol, or possibly irradiation. Carboxyl groups can be protected by leaving groups such as -CH2SCH3, t-butyl, benzyl, dimethylamino, and similar groups. These groups can be deprotected by treatment with species such as trifluoroacetic acid, formic acid, methanol, or water to produce carboxylic acid groups. Amino groups can be protected by groups such as R-OOC-, which can be removed by reaction with trifluoroacetic acid, hydrazine, or ammonia. Isocyanate groups can be protected by carboxyl compounds such as formic acid or acetic acid.

[0047] The fifth class of gas-phase reactants contains a first functional group and a precursor group that can undergo further reactions to produce a second functional group. In such cases, the first functional group reacts to bond to the polymer chain, and then a chemical reaction occurs at the precursor group to produce the second functional group. The first functional group can be any of the aforementioned types, including siloxane groups, amino groups, anhydride groups, hydroxyl groups, carboxylic acid groups, carboxylic acid anhydride groups, carboxylic acid ester groups, isocyanate groups, etc. A wide variety of precursor groups can exist on this type of reactant.

[0048] Precursor groups may not react with polymer chains themselves, but can be converted into functional groups that can grow chains by reacting with other gas-phase reactants. Two notable types of precursor groups are vinyl and / or allyl unsaturated, and halogen-substituted, particularly chlorine or bromine. The vinyl and allyl unsaturated can be converted into functional groups using a variety of chemical reactions. These can react with ozone or peroxides to form carboxylic acids or aldehydes. They can also react with ammonia or primary amino acids to produce amines or imines. Halogens can be substituted with a variety of functional groups. They can react with ammonia or primary amines to introduce amino groups, and, if desired, can react with phosgene to produce isocyanate groups.

[0049] Reactants used to convert precursor groups to functional groups, or to demask or deprotect functional groups, are introduced into the gas phase. Excess reactants of this type are typically removed before introducing the next reactant, usually by sweeping a high vacuum into the reaction area, purging the chamber with a purge gas, or both. Reaction byproducts are removed in the same manner before introducing the next reactant into the reaction area.

[0050] In some preferred embodiments, at least one or all of the reactants within the MLD repeating unit have chain lengths between reactive parts of 2 to 20 atoms, or 2 to 10 atoms, or 2 to 5 atoms (heterogroups such as oxygen may be present, but typically carbon atoms). In some preferred embodiments, the reactants have straight chains between reactive parts (i.e., no branching) to increase packing density. In some preferred embodiments, the chains between reactive parts are non-reactive. However, in some embodiments, these may be parts within a chain that can bridge to adjacent chains. In some embodiments, the capping layer and / or MLD layer on or very close to the surface (e.g., the capping layer or within 5 cycles or 2 cycles of the surface) are branched to increase hydrophobicity.

[0051] The inorganic layer applied to the particles in the first step preferably becomes covalently bonded to the substrate. Covalent bonding can occur when the precursor compound initially applied reacts with functional groups on the substrate surface under the conditions of the atomic layer deposition process. Examples of such functional groups include, for example, hydroxyl, carbonyl, carboxylic acids, carboxylic acid anhydrides, carboxylic acid halides, and primary or secondary amino acids.

[0052] Some ALD coatings are aluminum oxide and / or titanium oxide coatings. “Aluminum oxide” is used herein to refer to a coating composed substantially entirely of aluminum and oxygen atoms, regardless of specific stoichiometry. In many cases, aluminum oxide coatings are expected to correspond somewhat closely to the empirical structure of alumina, i.e., Al2O3, but deviations from this structure are common and can occur. “Titanium oxide” is used herein to refer to a coating composed substantially entirely of titanium and oxygen atoms, regardless of specific stoichiometry. In most cases, titanium oxide coatings are expected to correspond closely to the empirical structure of titania, i.e., TiO2, but deviations from this structure are common and can occur. Similarly, considerations apply to understanding other formulations described herein, but in some embodiments, the present invention can be more specifically defined by using terms such as “consisting.”

[0053] Except for the half-reactions included in a broader embodiment of the present invention, the atomic layer deposition process is characterized by requiring at least two different reactants to form a coating layer. The reactants are introduced into the reaction region individually, sequentially, and in the gas phase. Excess reactants are removed from the reaction region before introducing the next reactant. Reaction byproducts are also removed between the sequential introduction of reagents. This procedure ensures that the reaction occurs on the surface of the substrate rather than in the gas phase.

[0054] To further aid in the removal of excess reactants, a purge gas is typically introduced between alternating feeds of reactants. A carrier gas, which does not necessarily have to be the same as the purge gas, is generally introduced (though not always required) between each reactant feed. The carrier gas can perform several functions, including (1) facilitating the removal of excess reactants and reaction byproducts, and (2) distributing the reactants through the reaction zone, thereby helping to expose all surfaces to the reactants. The purge gas will not undergo undesirable reactions with the ALD reactants or deposited coatings, nor will it interfere with their reactions on the substrate surface.

[0055] Since gaseous reactants are still required, the temperature and pressure conditions vary depending on the specific reaction system. As is well known in ALD / MLD processes, the temperature needs to be high enough for the reaction to occur in the gas phase, but not so high that the product degrades.

[0056] 2. Particle and coating properties In some embodiments, the core particles are metal, preferably comprising one or more transition metal elements, alkali metal elements, alkaline earth metal elements, post-transition metal elements, or metalloid elements. Preferred elements include copper, aluminum, silicon, titanium, chromium, iron, cobalt, nickel, molybdenum, tungsten, rhenium, silver, platinum, palladium, gold, zirconium, hafnium, zinc, indium, tin, gallium, and germanium, as well as alloys of any of these elements. Other elements such as magnesium or lithium may also be present. In some preferred embodiments, the metal particles include superalloys, which can be used, for example, in 3D printing of jet engine parts. In some preferred embodiments, either the particles, individual particles in the mixture, or the sum of all particles in the mixture constitute at least 80% by mass or at least 90% by mass of a single element. In alternative embodiments, instead of metal, the core particles may be ceramics, e.g., oxides, nitrides, and carbides, or polymers, e.g., ABS, PLA, PVA, nylon, HDPE, PETT, or other materials, e.g., carbon fiber, or biological materials. For the purposes of the present invention, the core particles have a surface that reacts under ALD or MLD conditions to form a surface layer (or are coated with a layer having a surface). In some preferred embodiments, the surface that reacts under ALD or MLD conditions is a metal or metal oxide surface. For the purposes of the present invention, the metal may include a metalloid.

[0057] Particles containing any coating typically have a diameter in the range of 0.01 to 500 μm, preferably 10 to 300 μm; more preferably 10 to 100 μm. In some embodiments, at least 90% by mass of the particles have a diameter of 100 μm or less. For asymmetric particles, the particle diameter is typically determined by optical microscopy based on the minimum dimension passing through the center of the particle.

[0058] The core particles preferably have one or more of the following characteristics: spherical, spherical with satellites, identical chemical composition, little or no internal porosity, and / or low surface contamination. Satellites are small nodules on the outside of larger (usually spherical) particles. Satellites are typically formed during the production of core metal particles, particularly during the production of these materials from gas atomization processes. When gas atomization is used, particles are produced in various sizes and coalesce together. Satellites are relatively small particles that adhere to larger particles during production.

[0059] The coating may include any coating that can be applied by depositing molecular or atomic layers. Some well-known coatings that can be applied to core particles of metal or other materials include oxides or mixed oxides (e.g., Al2O3, TiO2, ZnO, ZrO2, SiO2, HfO2, Ta2O5, LiNbxOy), nitrides (e.g., TiN, TaN, W2N, TiY2N), sulfides (e.g., ZnS, CdS, SnS, WS2, MoS2, ZnIn2S4), and phosphides (e.g., GaP, InP, Fe 0.5 Co 0.5P) may be included. Some lesser-known materials that can be coated onto core particles include metal fluorides (e.g., AlF3, MgF2, ZnF2), oxyfluorides and oxynitrides of transition metals (e.g., Al, Cu, Co, W, Cr, Fe, Zn, Zr, Pt, Pd), lanthanides in any form of elements, oxides, fluorides, nitrides, borides, or sulfides (e.g., Y, YN, La2O3, LaF3, Nb, Dy2O3, Nd, LaB6, La2S3, etc.), borides (e.g., TiB2), carbides (e.g., For example, B4C, WC), silanes, silide and other silicon-containing materials, and, not limited to, polymers (e.g., polyamides, polyethylene, polyamides, polyureas, polyurethanes), hydrocarbons, polymer or amino acid fragments or other biologically related molecules and polymers, and other materials), fluorinated polymers (e.g., fluoropolyamides or perfluoropolyamides, -polyethylene, -polyamides, -polyurethanes, -urethanes, -hydrocarbons) and other carbon-containing materials. The coating exhibits high uniformity throughout the particles, preferably with a variation in coating thickness across the entire surface of the particles of 20% or less, more preferably 10% or less, or 5% or less. This high level of uniformity is characteristic of the ALD / MLD process. Particles coated by ALD / MLD are distinguishable from particles coated by other methods by 1) uniformity of film thickness and 2) no change in the particle size distribution of individual core particles, which cannot be distinguished by other methods.

[0060] The coating on the core powder typically has a thickness in the range of 0.1 to 100 nm, preferably 0.2 to 50 nm, and more preferably 0.5 to 10 nm. The coating thickness can be measured by transmission electron microscopy (TEM).

[0061] The ALD / MLD coating preferably covers at least 60%, more preferably at least 80%, more preferably at least 95%, and even more preferably at least 99% of the surface area of ​​the particles on the surface.

[0062] The ALD / MLD coating is preferably conformal. "Conformal" means that the thickness of the coating layer is relatively uniform across the surface of the particles (i.e., for example, the thickest region of the coating is no more than 3 times, preferably no more than 2 times, and in some embodiments, no more than 20% thicker than the thinnest region).

[0063] In the process of producing coatings (and resulting coating particles) containing hydrocarbon reactants, it may be desirable that the hydrocarbon reactants be partially or completely fluorinated, with fluorine replacing hydrogen within the hydrocarbon chains connecting the reactive parts. For example, the reactants (or film) may have an F / H atomic ratio of at least 0.1, or at least 0.5, or at least 1, or at least 5, or at least 10. In many preferred embodiments, the hydrophobic film should not be readily hydrolyzable, so in some embodiments, the film does not contain anhydride linkages. Similarly, in some embodiments, metal atoms may be undesirable, and in some embodiments, the film contains less than 10% by mass, or less than 5% by mass, or less than 1% by mass of a transition metal on the outside (surface) of a film thickness of 2 nm, or 5 nm, or 10 nm, and in some embodiments, the same limitations apply to metalloids.

[0064] 3. Rheology 3D printing uses powders that do not flow completely. When adding layers of powder to the bed to fabricate a part, if the powder does not flow sufficiently, defects in the part can occur in the voids of the powder bed. A high-density powder bed with a thickness of 30-100 μm is optimal. This invention provides a method for improving the fluidity of metal powders. In some embodiments, this invention improves the fluidity of the powder, reduces voids in the bed, and minimizes defects in the final part.

[0065] Powder size significantly impacts the rheology of the particles. Smaller powders tend to be more cohesive (sticky), resulting in reduced reliability when spreading, shaking, or moving the powder to form layers. Furthermore, smaller powders, particularly metals and non-oxide ceramics, typically have a higher oxygen content, which tends to be dominant on the surface. When oxygen is present in the finished part, it typically alters the composition of the finished part in an undesirable way. However, smaller powders result in finer feature sizes and a smoother surface finish of the finished part. In some embodiments, the present invention allows for the use of smaller metal or non-oxide ceramic particles for AM without increasing the oxygen content of the finished part.

[0066] The particle coating can be applied in alternating cycles according to the ALD process. Surprisingly, it has been found that at least 5 cycles of the ALD process significantly improve fluidity. Preferably, at least 10 or at least 25 cycles of ALD are performed to enhance fluidity and oxidation resistance. In some embodiments, the ALD process is performed for 50 or fewer or 100 or fewer cycles, and it has been found that beyond that, there is little additional benefit.

[0067] In some preferred embodiments, the ALD coating includes alumina.

[0068] In some embodiments, the ALD process increases the flow coefficient by at least 10%, at least 20%, or in the range of 5 to about 40%. The flow coefficient can be measured at a Major Principal Consolidation Stress of 5 or 10. Preferably, testing is performed using a Brookfield powder flow test apparatus (PFT3115) equipped with a PFT-405 5'' sample trough and a PFT-515F'' 304SS vane lid, or the flow coefficient can be measured according to ASTM B213-17 using a Hall flowmeter funnel, or, if the powder does not flow through a Hall flowmeter funnel, it can be measured according to test method B964 using a Kearney flowmeter funnel. In some preferred embodiments of the present invention, the coating powder can be described as having any of the values ​​shown in the Examples section, ±50%, ±30%, or ±10%. These values ​​may be combined with one or any combination of the other descriptions provided herein.

[0069] In the ALD and MLD processes, unique physical properties arise. Powders coated by the ALD or MLD process can be identified or characterized by measurable properties. Thus, the particles are distinguishable from powders produced by other methods, for example, by general spectroscopic methods. In some embodiments, the powder can be characterized by a flow coefficient of at least 10, or a flow coefficient in the range of 10–15, as measured according to the apparatus and conditions described above (see also Examples).

[0070] 4. Chemical reaction resistance Some powders used for 3D printing are prone to water oxidation or absorption, which can lead to undesirable elemental composition or poor rheological properties. The shelf life of 3D printing powders can be extended by coating them with an H2O / O2 barrier coating (approximately 10 nm or less) and optionally vacuum drying the powder.

[0071] Some preferred coating methods utilize one or more cycles in which the powder reacts with an oxidizing source such as water (H2O2 or O3) followed by a reaction with trimethylaluminum (TMA) or diethylzinc (DEZ). More generally, the strength of particle-particle interactions can be reduced by exposing the powder to one or more chemicals (e.g., TMA) to modify the surface groups, for example, by changing an Al-OH surface to an Al-X surface, where X can be equal to CH3, N, F, S, etc.

[0072] The coating may be a hydrophobic organic coating, such as an imide or fluorinated organic compound. This can be deposited after one or more cycles of applying an inorganic coating.

[0073] Some preferred powders have oxidation resistance such that, as measured by thermogravimetric analysis, heating to 450°C at 10°C / min in oxygen results in no substantial increase in mass (in some embodiments, an increase of 5% or less, 3% or less, or 1% or less in mass). Also, as with any of the properties discussed herein, in some embodiments, the particles may be characterized by oxidation resistance by any selected combination of properties, e.g., particle size and / or composition. The present invention also includes a method of additive manufacturing using the powders described herein.

[0074] Most metal powders have a surface layer of natural oxides. This surface layer often inhibits the diffusion of metal atoms, resulting in insufficient sintering or requiring a higher temperature for sintering. Another embodiment of the present invention involves coating a metal powder with an arbitrary material, the coating material being less stable than the natural oxide on the powder. For example, a ZnO coating on aluminum powder. In this embodiment, the less stable material (ZnO deposited by ALD or a similar technique) improves sintering by allowing or increasing the diffusion of metal atoms from the powder. In this embodiment of the present invention, "less stable" means that the powder sintersects together with less energy than the powder with the natural oxide. With this less stable material, the powder can be coated immediately after an etching process (e.g., atomic layer etching or spontaneous etching) so that the original natural oxide (e.g., Al2O3) is substantially "replaced" with the new material (e.g., ZnO), thereby enabling good sintering. For example, in the table below, the natural oxide can be replaced with any of the oxides listed in the table that are more stable than the natural oxide.

[0075] [Table 3]

[0076] More specifically, a less stable coating is used to replace the natural oxide. "Less stable" is defined as being able to sinter at a lower heat input. For example, if a powder containing the natural oxide requires a heat value of X to sinter, a powder containing a less stable coating replacing the natural oxide will require less than X (e.g., 0.9X or 0.8X or less) to sinter at the same rate. Therefore, whether or not a coating is less stable can be confirmed by routine experiments.

[0077] The particles or particle beds may be characterized by having a carbide coating, a nitride coating, or an organic coating. Additionally or alternatively, the particles or particle beds may be characterized by having oxides with an average size of less than 1 nm, or oxides with an average size of less than 0.5 nm, or substantially no oxides on the surface of the particles.

[0078] Another distinctive advantage of ALD / MLD coated particles is that their rheology can be improved by making them less susceptible to condensation, allowing for greater reuse of the powder before it is purified or otherwise modified to return it to specifications. By modifying the surface chemical groups to be less hydrophilic, the amount of water absorbed by the powder over time can be reduced, which can lead to less oxygen being incorporated into the powder.

[0079] More generally, surface chemical modification to prevent unwanted chemical species from reacting with powders can be achieved by ALD or MLD coatings. For example, coating an Ag or Li powder or part with Al2O3ALD can prevent the powder or part from reacting with atmospheric or environmental species such as sulfur, oxygen, water, nitrogen, or others. The ALD coating acts as a barrier to prevent these species from interacting with the underlying powder or part. In other embodiments, other ALD coatings such as fluorides, nitrides, and carbides are used.

[0080] 5. Cermet formation A ceramic coating can be applied to metal powder to form a cermet. ALD can be used to coat metal particles with a ceramic coating. The resulting powder can be printed into a desired shape in the form of a cermet, which is a composite material of a metal phase and a ceramic phase. For example, 3D printing conditions can be controlled so that particles having the ALD coating (e.g., metal oxide, metal nitride, or metal carbide) are either dispersed within a metal matrix or as a thin layer separating the metal regions, resulting in a printed composite having each phase (e.g., metal oxide, metal nitride, or metal carbide phase).

[0081] 6. Reduction of oxygen content Rare earth elements (REEs) are added to the particle coating to bond oxygen. In Al, oxygen can exist in islands or deposits of Al2O3. Furthermore, it can advantageously form a dense surface layer to protect the underlying metal. In Ti, O is highly soluble and tends to migrate towards the interior of the particles. Placing REEs on the outside of the particles slows down the oxidation of the bulk metal.

[0082] 7. Surface properties Using ALD / MLD, a uniform surface coating can be applied that reduces the surface tension of the molten pool. In some embodiments, these coatings are used to have a beneficial effect on the final microstructure of the part by influencing particle growth, precipitation, and other microstructure-related properties. Referring to page 107 of 2017-Chen et al., in J. Mater. Processing Tech. “Improving additive manufacturing processability of hard-to-process overhanging structure by selective laser melting (SLM),” it is stated that “small amounts of surfactant elements such as boron and niobium are added to the original powder. The surfactant elements have the function of reducing the surface tension of the molten pool during SLM. This is very useful in increasing the stability of the molten pool and improving the processing quality of overhanging structures by SLM.” In this invention, the coating is applied to the particles to reduce surface tension, improve the stability of the molten pool, and reduce capillary action, thereby reducing the tendency of the molten material to take particles from the printing bed and generate dross. Coating is particularly advantageous because it allows for the concentration of surfactant elements or compounds on the surface of the particles, resulting in better control of the composition and superior properties of the resulting printed articles. Furthermore, the combination with improved fluidity provides a synergistic advantage over uncoated particles. Examples of such coatings include sulfur-containing materials (e.g., ZnS, CaS, BaS, SrS, CdS, PbS, In2S3, Cu) xExamples include S, WS2, TiS2, Sb2S3, SnS, GaSx, GeS, MoS2, Li2S), boron-containing materials (for example, but not limited to these, TiB2, B2O3, BN, TaB, TaB2, B-doped ZnO, W2B5, AlB2, B4C, Mo2B5, Re2B5, Os2B5), tantalum-containing materials (TaN, TaB, tantalum oxide, etc.), and Nb-containing materials (for example, niobium oxide, NbN). Alternatively, an active "coating" can be applied simply by surface-treating the metal powder by exposing it to a chemical substance containing the element of the desired element (for example, in the case of B, these could be BBr3, tetrakis(dimethylamino)diboron, B(C2H5)3, B(OCH3)3[(CH3)2CHO]3B, B(CD3)3, (C6H5)3B; in the case of S, this could be H2S; and in the case of Nb, there are several options).

[0083] In other embodiments, ALD coatings, or layers within multi-material ALD coatings, modify grain growth, typically inhibiting it, although in some embodiments, grain growth in high-temperature alloys increases. Here, fewer particles are desirable because the particle slip becomes brittle, and the single-crystal portion often performs best at extremely high temperatures. Examples of such grain growth-controlling coatings include AlPO4, TiPO4, TiP, AlP, GaP, Ni2P, and the aforementioned coatings for reducing the surface tension of the molten pool.

[0084] In some embodiments, the coating can function as a sintering aid. Since the ALD forms a uniform, typically thin layer, the intermediates and products after sintering or other heat treatment have controlled properties at the interparticle interfaces.

[0085] It is possible to reduce oxides during ALD deposition using hydrogen or a foaming gas to create a metal coating, or to reduce the oxide ALD coating immediately before use. This works best, especially when the particles may be carried out within a 3D printing system after being deposited on a powder bed. In the case of metal particles in particular, once the surface oxide is removed, the particles tend to aggregate (or loosely sinter). Therefore, they will not flow or disperse optimally on the printing test bed.

[0086] 8. Etching In some embodiments, the coating layer or contaminating film (e.g., undesirable surface oxides) can be removed before use, preferably immediately before use, such as by atomic layer etching (ALE). ALE is performed on recycled particles and can regenerate or "activate" them for reuse by removing unwanted materials or elements (e.g., oxygen or metals, metal nitrides, or other elements or compounds). This process can be either true ALE (self-limiting by exposure) or spontaneous etching of the surface material (e.g., spontaneous etching of oxides from the surface of metal powder upon exposure to HF gas). In some embodiments, this ALE or spontaneous etching process has the additional benefit of modifying the original surface chemical, for example, from an oxide to another surface chemical, such as a fluoride or oxyfluoride. This new surface (in the case of a fluoride) is less hydrophilic than the original surface (e.g., originally an oxide / -OH surface). A low-hydrophilic surface is beneficial for reducing oxygen / H2O uptake by the film during storage or operation. An additional advantage is that fluorides are often more thermodynamically stable than oxides, so they do not revert back to oxides.

[0087] In any of the methods of the present invention, the particles can be subjected to an etching step or a reduction step before ALD / MLD.

[0088] 9. Resistance to spontaneous combustion Another application of coatings is to protect flammable powders from ignition or to reduce the spontaneous combustion of such powders. These powders may include fine metal powders that are spontaneously combustible below a certain particle size, such as titanium, aluminum, or zinc powders; or spontaneously combustible powders, particularly hafnium, zirconium, and tin; or alkali metals such as calcium and magnesium. Coatings can also be used to prevent smoke (where particles are blown away in the printer by hot airflow). Films that optimize particle cohesiveness can be fabricated using ALD films. These films strike a balance between improving powder fluidity, which is desirable for reliable casting of each powder layer, and providing a sufficiently cohesive film to reduce the particle's sensitivity to smoke. The smooth and highly uniform nature of ALD / MLD coatings allows for excellent fluidity, and intermolecular forces between particles in the coating prevent smoke. Furthermore, ALD / MLD allows for complete surface coverage with less material than other methods, resulting in safer operation and superior results. Spontaneously combustible powders are generally recognized and, in some embodiments, can be defined as having the ability to spontaneously combust at 10% humidity and standard pressure.

[0089] 10.Anti-reflection The precise thickness and composition of the coating on particles for additive manufacturing can be selected and controlled to reduce the reflection of the applied laser beam. The coating can be any material deposited by ALD or MLD, e.g., (MgF2, ZnO, Zn, polyamide, fluorocarbon, etc.). Films (e.g., metal films) can provide high crystallinity. Coating the surface of the main particles with highly crystalline nanonodules can increase laser diffraction and scattering, reducing reflectivity. ALD can be used to form nanonodules on the particle surface. Coatings of very low levels of highly crystalline materials, or films that do not readily nucleate on the support, grow in the form of film islands. Furthermore, selecting an ALD film that preferentially grows on its own results in larger island growth, in contrast to an increase in islands. Alternatively, alternating layer nanolayers can reduce reflectivity. Nanolayers are designed to maximize absorption by alternating high and low refractive indices. The thickness and composition of the coating can be selected to optimize absorption at different laser wavelengths. The coating increases energy absorption, reducing the input energy required to sinter or melt the powder to form parts. This reduced input energy can result in faster scanning speeds, correspondingly faster part production, less laser energy required, or lower energy costs for running the system. The powder can be coated with a coating containing one or more chromophores, such as polymers containing non-conjugated double bonds, to absorb laser light, accelerate sintering, and increase efficiency.

[0090] Multilayer coatings can be formed using ALD or MLD to increase the absorption of laser radiation. These multilayer coatings have two, three, or more layers of different compositions, and suitable compositions can be selected by known optical science. Preferably, anti-reflective coatings increase absorption by at least 1%, 2%, or at least 5% (or decrease reflectivity).

[0091] 11. Enhancement of the mixture One problem in the powder industry is producing a uniform powder blend. Powders often separate due to incompatible surface chemistry and particle size. By modifying the surface chemistry, mixing between two or more powders can be facilitated, resulting in better processing and part formation. Another method provided by the present invention is to coat different particles with the same coating to improve mixing. In one embodiment, the same ALD coating is applied to two or more powders. In another embodiment, the coating is selected to mimic the surface of one powder and is applied only to the second powder. By applying a coating of a given material to one or more powders, a better blended powder mixture can be produced. For example, when trying to produce a blend of powders X and Y, powder X can be coated with the surface chemistry present in Y. Thus, both powders have the same surface chemistry and mix better. A specific example is coating metal W powder with a carbon coating. This ensures that when the metal W powder is mixed with carbon powder (for the final WC part printing), the surface chemistry of all particles becomes C-based. Alternatively, the surface functional groups of carbon can be easily modified to add WOx (tungsten oxide) functional groups to the carbon surface. This second approach results in WOx surface groups on all particles, which also leads to better mixing and less segregation. Thus, particles with different core compositions have substantially similar or identical surface chemicals, allowing for better mixing and less segregation before or during the formation of printed parts. This concept can be extended to multiple material powders, where the number of different powder materials is preferably 2 to 10, more preferably 2 to 5, or more preferably 2 to 3.

[0092] In another example, a carbon-containing coating is applied to metal (e.g., tungsten) particles. When the W metal powder is mixed with the carbon powder (for the final printing of WC parts), the surface chemicals of all particles become C-based. Alternatively, the surface functional groups of carbon can be easily modified to add WOx (tungsten oxide) functional groups to the carbon surface. This second approach results in WOx surface groups on all particles, which also results in better mixing and less segregation. This concept is extended to multiple material powders, where the number of different powder materials is preferably 2-10, more preferably 2-5, or more preferably 2-3. Tungsten can be coated onto the particle surface by known ALD processes. See, for example, Herrmann et al., "Multilayer and functional coatings on carbon nanotubes using atomic layer deposition," Appl. Phys. Lett. 87, 123110 (2005). Tungsten oxide can bond to the surface of carbon particles via carbon-containing defects, such as edge defects, or defects that are present in the substrate or can be generated in the substrate by plasma or O3 pretreatment.

[0093] 12. Binder Jet Coating ALD or MLD can be used to coat metal or ceramic particles with organic or other removable coatings. After shaping the particles, the coating can be removed (e.g. by burning) and further heating can form porous parts that can be completely densified. ALD / MLD increases the density of green parts (manufactured using a binder jet process). ALD / MLD coatings on particles are advantageous because they allow for stronger binder-particle interactions, or result in harder or denser green parts. In one embodiment, the ALD / MLD coating is applied to particles designed to chemically react with a binder to become harder, denser green parts. In another embodiment, the ALD / MLD coating is designed to interact with the binder in a beneficial way without covalently reacting (for example, a benzene ring-based coating on the powder results in π-π stacking interactions with the binder (in the case of an aromatic-containing binder)). In yet another embodiment, the ALD / MLD coating is applied to the particles to be added to the binder before the binder is sprayed onto the powder bed. The powder in the powder bed is also coated with the ALD / MLD layer(s). The coatings are designed to interact with each other to form denser green parts. In yet another embodiment, the ALD coating is applied only to the powder, and the ALD / MLD coating acts as an initiator, curing or polymerizing the binder upon contact with the powder to create harder, denser green parts.

[0094] Currently, binders for powder jet processing consist of furfuryl alcohol, bisphenol A, or resorcinol. Phenol binders bind more readily to methylated surfaces than to hydroxylated surfaces. As an example, particles are coated with aluminum oxide using a TMA / H2O chemical, but instead of ending the chemical with a water dose, the process ends with a TMA cycle. This leaves methyl groups on the surface that can be used to react with the phenol material. Alternatively, carbon-containing ALD / MLD chemicals (e.g., polyamides) can be terminated with non-hydrolyzable groups (e.g., methyl groups on the carbon chain) to facilitate better and more stable interactions with the binder.

[0095] In some embodiments, reactive groups can be coated onto the exterior of particles to be activated during the additive manufacturing process using ALD or MLD processes. For example, the reactive groups can be ring-opened by the application of heat, light (photoactivation), electron beam, or catalyst (e.g., a catalyst applied in the liquid or gas phase), or blocked by otherwise activated blocking groups. Activation may occur simultaneously with or after the formation of the aggregated layer or green body. One example involves coating particles with MLD chemicals that have hidden functions that are only revealed during the reaction. Many ring-opening reactions produce hydroxyl, amine, or carboxylic acid groups during the reaction. For example, cyclic azasilanes (AZs), such as 2,2-dimethoxy-1,6-diaza-2-silacyclooctane (CAS 182008-07-7), can react with surface hydroxyls to form silicon-oxygen bonds. The cyclic azasilane can also unfold, leaving behind amine species that are not expected to react with surface hydroxyl groups. Next, cyclic carbonates such as ethylene carbonate (EC) (CAS-96-49-1) can react with surface amines to form urethane bonds. The ethylene carbonate also unfolds, generating hydroxyl species that are not expected to react with surface amine groups. This chemical property can be terminated at any step, leaving block groups that open optimally during green formation, which can increase particle adhesion.

[0096] 13. Melt resistance / sintering resistance of core particles In some optional embodiments, the coating has a melting point higher than the melting point of the core. In these cases, the core may soften before coating during the additive manufacturing process. One embodiment would be the development of a film that prevents particles from undesirably sintering together in a heated bed or under pressure during the printing process. While this feature is not present in all embodiments of the invention, the invention may include this feature in any of the embodiments described herein in alternative embodiments.

[0097] In another embodiment of the present invention, a coating of a given material can be applied to one or more powders to slow down the sintering of one of the materials in a powder containing two or more materials. One problem in fabricating materials such as cermets or other composite metal / inorganic materials (such as metal matrix composites, MMCs, or multilayer metal / ceramic structures such as multilayer ceramic capacitors (MLCCs)) is that the metal components typically sinter at a much lower temperature than the inorganic (ceramic) components. The method described herein is to coat a metal or more generally lower sintering temperature powder or material with a coating that slows down, reduces, delays, or otherwise affects the sintering of the metal or lower sintering temperature material. In some preferred embodiments, this coating increases the sintering temperature of the lower sintering temperature material. By increasing the sintering temperature of the lower sintering temperature material to be closer to that of the higher sintering temperature material, less segregation occurs during sintering, thus making it possible to produce a more uniform composite material. A specific example is coating metal powder (e.g., Ni) with an oxide coating (e.g., lithium titanate) when metal (Ni) is sintered together with a ceramic material (e.g., titanium carbide, BaTiO3, etc.), thereby enhancing the sintering process to reduce material segregation compared to when one of the two components is not coated. Examples of cermet and MMC materials include carbides or nitrides of titanium, tantalum, tungsten, niobium, or molybdenum, and other materials combined with binders of nickel, cobalt, molybdenum, and other materials.

[0098] 14. Coatings that enable a better cold spray process. The coating can be applied by ALD / MLD (cold spray or direct energy technology) which reduces the drag on particles when sprayed toward the build plate. These films may include fluorocarbons, aluminum fluoride (any transition metal fluoride), polyamide or polyimide, polyurethane, polyurea, and base hydrocarbons.

[0099] Coatings can be applied by ALD / MLD, which increases the coefficient of adhesion between cold-sprayed particles. This may include a soft, deformable material or a sintering aid for the core particles of interest. The impact energy is sufficient for localized sintering. For example, polymer coatings with lower thermal conductivity can be applied to metal powder by ALD / MLD so that the impact energy is contained more locally at the point of impact. This can result in finer control over feature size.

[0100] 15. Coating powders to improve extruded 3D printing filaments (inks). One element of the present invention is to increase the dispersion of powder filling a resin or liquid matrix (e.g., solvent or dispersant). This makes it possible to improve uniformity and add a larger amount of solids without adversely affecting the processing of the filled resin or ink.

[0101] 16. Use of an ALD-like process to remove H2O or O2 from the printing environment during printing. During the printing process, TMA (or other preferably reactive gas) is used on the printing bed to remove H2O from the atmosphere in the printing environment. This reduces oxidation of both the powder and the manufactured parts materials. The reactive gas acts as a "getter" in situ, reacting unwanted species and precipitating the reaction products from the gas phase. In most cases, no part is formed. This removes unwanted H2O, O, or other unwanted chemical species from the printing environment in most cases.

[0102] 17. Coating and etching for adding and subtracting elements to produce tailored alloys. Using ALD / MLD, a small amount of an element or compound can be added to the powder to produce a modified alloy. Alternatively, atomic layer etching or spontaneous etching can be used to remove the target element or compound from the powder to produce a modified alloy. Current AM powders are typically sold to conform to elemental composition specifications defined by a range, or by upper or lower bounds. In practice, the AM process may require certain elements or compounds to some extent. Therefore, this embodiment of the present invention results in the creation of custom alloys before or during part manufacturing.

[0103] In another embodiment of the present invention, an ALD-like process is added during printing to return elements or compounds to a low concentration in the powder / part / build that are volatilized by a laser and therefore not present in the final part. The ALD / MLD-like process in this embodiment would allow for the replacement of necessary elements / compounds that would otherwise be lacking in the final part. Such defects may degrade the performance of the part, for example, due to microstructural issues.

[0104] 18. ALD / MLD coating on intermediate processed parts Densification—sintering aids are added to the formed green material. This eliminates the need to coat the contact points between particles, thus reducing the amount of material added to the finished part.

[0105] Mechanical stabilization involves adding a higher structural layer before the material is moved from the BJP (Binder Jet Printer) or extrusion tool. This layer improves the structure of the part before the material is finally densified. The coating is not necessarily done with the same tool, but can be done outside the 3D printer before final firing.

[0106] To improve the hygroscopicity of bronze materials, a thin film of another metal on the part provides a good wettable surface for the bronze material to flow sufficiently into the pores. Several films exist that enhance the fluidity of the material. [Examples]

[0107] Materials and methods To evaluate the rheology and several other advantages of the present invention and the various coatings described above, tests were developed and carried out on core metal powders with coatings. The methods and results of such tests are described below.

[0108] procedure The steps used for each execution are listed below. 1. For each sample number shown in Table 2, approximately 100 g of AlSi10Mg powder was added to a 75 ml atomic layer deposition (ALD) reactor. The reactor was evacuated, and the powder was allowed to dehydrate overnight at room temperature. This was done to limit any reaction between water and the high-temperature substrate powder. A residual gas analyzer (RGA) detected only the desorption of water from the floor. 2. The following morning, the reactor was heated to 180°C and the powder was dried for 4 hours. 3. The powder was coated with an appropriate number of cycles according to Table 2. RGA was used to detect the gas phase product and ensure that the deposition always coincided with the alumina deposition. 4. The reactor was cooled to room temperature and then pressurized. 5. The powder was collected and packaged.

[0109] [Table 4]

[0110] Analysis method Residual Gas Analyzers - Residual Gas Analyzers (RGAs) are mass spectrometers used to sample exhaust gases from a process. They provide real-time information about the progress of the ALD process or any side reactions and are typically used for feedback control of the ALD process. RGAs also provide leak detection and substrate degassing information.

[0111] Thermogravimetric Analysis of Oxidation - The thermogravimetric analyzer (TGA) is used as an ex-situ analytical tool to investigate the mass change behavior of a sample when it is heated to high temperatures. The substrate is tilted using an N2 / O2 atmosphere, and the oxidative protection provided by the ALD film is observed to estimate the film's effectiveness or thickness.

[0112] Powder Flow Test - The powder flow function of the material was tested using Brookfield PFT.

[0113] Results and Discussion Table 2 summarizes the coatings that were performed.

[0114] [Table 5]

[0115] [Table 6]

[0116] T326a The material dried without issue and became sufficiently fluid. The RGA data was consistent with the alumina deposit.

[0117] TGA analysis Samples were tested in TGA by heating to 450°C at 10°C / min in oxygen. Samples with 5 cycles or less were oxidized under these conditions, but no oxidation was observed in samples with 25 cycles or more. Uncoated materials showed considerable mass loss when heated at lower temperatures, which may be an indicator of adsorbed water.

[0118] Powder flow rate test Powder samples were tested using a Brookfield powder flow analyzer, and the flow coefficient was determined in correspondence with the main consolidation stress. The results are shown in Figure 1. Compared to uncoated powders, coatings of 0.5 to 50 cycles increased the flow coefficient across the entire range of samples tested, and coatings of 100 cycles initially increased the flow coefficient, but this advantage diminished as the consolidation stress increased.

[0119] By normalizing the flow coefficient of the coated powder relative to the uncoated powder, the benefits of coating can be seen more directly. See Figure 2. The greatest increase in the flow coefficient was observed at 5 cycles of running, which was also the thickest film that failed during the TGA test. While this data shows a surprising result of observing a significant increase in flowability with just one ALD cycle, at 100 cycles, a decrease in flowability is shown under higher consolidation stresses. If you want to increase oxidation resistance, the desirable range is 2 to less than 100 cycles, preferably 2 to 50 cycles or 2 to 25 cycles or 5 to 25 or 50 cycles. Note that the 50-cycle run curve shows a different shape from the other curves. This may indicate a systematic error either during coating running or during powder flow analysis.

[0120] Another test performed to evaluate the fluidity of these powders is the Hall flow test, which is recommended to be performed before the Carney flow test. However, some fine powders may not flow through the small opening of the Hall flowmeter funnel. If a larger orifice is required, the Carney flow test can be used to determine a significant flow rate. The timing of the metal powder is adjusted when it flows through the calibrated orifice (diameter 5.08 ± 0.13 mm) of the Carney flowmeter funnel.

[0121] AlSi10Mg powder (at the time of receipt, drying, or coating) does not flow through a Hall flowmeter funnel. Therefore, a Carney flow test was used. The results from the Carney flowmeter are as follows:

[0122] [Table 7]

[0123] A faster Carney flow time indicates higher powder fluidity. This test supports Brookfield measurements, which suggest there is an optimal film thickness for improving fluidity.

[0124] Example 2: ALD coating of Al2O3 or TiO2 on Ti64 alloy to improve oxidation resistance

[0125] Ti6Al4V (Ti64) powder was obtained from EOS. 300.0 g of Ti64 powder was loaded into a 150 mL reactor and placed in a fluidized bed reactor. The substrate was heated to 185°C for 15 hours to remove excess moisture. Once dried, the fluidization properties of the powder were evaluated. The minimum fluidization point was approximately 10 sccm, so 30 sccm was used as the operating flow rate during ALD coating. Using standard processing conditions, either Al2O3 ALD for 2, 5, or 10 cycles, or TiO2 for 15 cycles, was deposited. An onboard residual gas analyzer (RGA) was used to detect when unreacted TMA, H2O, or TiCl4 penetrated the bed during each cycle, thus ensuring complete surface saturation during each ALD cycle. After the cycles, the reactor was cooled to room temperature, pressurized to atmospheric pressure, and the substrate was removed.

[0126] The oxidation resistance of each sample, as well as the uncoated sample, was evaluated by TGA analysis. Each sample was heated in air while its mass was measured. The results are shown in Figure 3.

[0127] The oxidation of the powder is represented as an increase in mass during heating. It is easy to see that the uncoated powder rapidly begins to oxidize at approximately 220°C. In two cycles of Al2O3, the oxidation onset temperature does not change significantly. By the time five cycles of Al2O3 are performed, the oxidation onset temperature has shifted to approximately 270°C. This represents a significant improvement in the oxidation resistance of the powder. The improvement in oxidation resistance is not limited to Al2O3 coating and can also be achieved via TiO2 coating, as demonstrated by 15 cycles of TiO2 coating which increases the oxidation resistance of Ti64 powder up to approximately 270°C.

[0128] Example 3: Use of Al2O3ALD and SAM capping layers to improve the flowability of stainless steel powder

[0129] 10 kg of stainless steel powder was loaded into a 3 L fluidized bed reactor vessel. Al2O3ALD was performed on the powder for either 6 or 16 cycles using the method of Example 1. Immediately afterward, the powder was exposed to molecules that form a self-assembled monolayer (SAM). To provide optimal conditions for the molecules selected in this example, the powder was cooled to 140°C. Next, the powder was exposed to FOMB(DMA)S (tridecafluoro-1,1,2,2-tetrahydrooctylmethylbis(dimethylamino)silane). The administration of FOMB(DMA)S was carried out while monitoring byproducts with RGA. Administration was continued until the byproduct signal disappeared. Under these conditions, this took approximately 16 hours. The system was cooled to room temperature.

[0130] These powders were tested using the Hall and Carney flow test systems. Neither the coated nor uncoated powders flowed through the funnel, but the silane-coated powder began to flow and then stopped before the entire test was completed. This indicates a significant improvement in this highly cohesive powder. Visually, these powders exhibit very different flow characteristics compared to the uncoated material.

Claims

1. A method for manufacturing a molded article, To provide a powder containing core particles including a metal or ceramic core, In an atomic layer deposition (ALD) or molecular layer deposition (MLD) process, the powder is reacted with a first reactant (reactant A), and then sequentially reacted with a second reactant (reactant B) to produce a powder containing an ALD / MLD coating having alternately bonded reactant A portions and reactant B portions. The further step includes applying a hydrophobic coating on the ALD / MLD coating, In the first additive manufacturing process, the powder is used to form a three-dimensional object constructed from the powder in a series of two-dimensional layers, The method, including the method described above.

2. A method for producing a molded article, To provide a powder containing core particles including a metal or ceramic core, In an atomic layer deposition (ALD) or molecular layer deposition (MLD) process, the powder is reacted with a first reactant (reactant A), and then sequentially reacted with a second reactant (reactant B) to produce a coating powder containing an ALD / MLD coating having alternately bonded reactant A portions and reactant B portions. The process includes subjecting the coating powder to an additive manufacturing process to produce a molded green product containing bonded core particles, The method wherein the coating is a nanolayer containing alternating high refractive index and low refractive index layers.

3. A method for manufacturing a molded article, To provide a powder containing core particles including a metal or ceramic core, The process includes reacting the powder with a first reactant (reactant A) and then sequentially reacting it with a second reactant (reactant B) to produce a powder containing an ALD / MLD coating having alternatingly bonded reactant A and reactant B portions, ALD is the method described above, performed over 2 to 25 cycles.

4. The method according to claim 3, wherein the ALD coating comprises alumina.

5. The method according to claim 3, wherein the powder is subjected to atomic layer etching before being recycled and used in a second additive manufacturing process.

6. An additive manufacturing process comprising a binder jet or cold spray, To provide a powder containing core particles including a metal or ceramic core, The process includes reacting the powder with a first reactant (reactant A) and then sequentially reacting it with a second reactant (reactant B) to produce a powder containing an ALD / MLD coating having alternatingly bonded reactant A and reactant B portions, The method wherein the ALD / MLD coating is designed to react with a binder, and the powder containing the ALD / MLD coating binds to the binder.

7. A method for manufacturing a molded article, To provide a first core particle having a first composition and including a metal or ceramic core, The first core particle is coated with an ALD / MLD coating, To provide a second core particle having a second composition and a metal or ceramic core, The second core particle is coated with an ALD / MLD coating, Includes, The ALD / MLD coating applied to the second core particle is the same as the ALD / MLD coating applied to the first core particle. In the additive manufacturing process, the molded article is produced using a mixture of the coated first core particles and the second core particles. The method, including the method described above.

8. A method for manufacturing a molded article, To provide a powder containing core particles including a metal or ceramic core, The coating powder is subjected to an additive manufacturing process to produce a molded green product containing bonded core particles, In an atomic layer deposition (ALD) or molecular layer deposition (MLD) process, the molded green product is reacted with a first reactant (reactant A), and then sequentially reacted with a second reactant (reactant B) to produce a molded article containing an ALD / MLD coating having alternately bonded reactant A portions and reactant B portions. The method, including the method described above.

9. The method according to claim 1, wherein the coating comprises an imide or a fluorinated organic portion.

10. The method according to claim 6, wherein the binder comprises furfuryl alcohol, bisphenol A, or resorcinol.

11. The method according to claim 6, wherein the ALD / MLD coating is bonded to the binder via a π-π stacking interaction.

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