Method for manufacturing a part made of technical ceramic

Abstract

The present invention relates to a method for manufacturing a part made of technical ceramic (10), the method comprising the following steps: i) a step (S1) of depositing in a mold (M) a composition based on at least one ceramic powder (P1; P2..., Pn) and at least one metal oxide powder; ii) a step (S2) of carrying out a first SPS sintering cycle which provides a preform (PF) with a density lower than a final density of the part made of technical ceramic (10) in order to facilitate subsequent machining of the preform (PF); iii) a step (S3) of carrying out at least one machining operation on the preform (PF) in order to obtain a blank (E); iv) a step (S4) of placing the blank (E) in the mold (M) and surrounding it with a sacrificial powder (PS) until any empty space within the mold is filled; v) a step (S5) of carrying out a second SPS sintering cycle which provides the blank (E) with the final density; and vi) a step (S7) of grinding the blank (E) having the final density in order to obtain the part made of technical ceramic (10) with final dimensions. The composition of the sacrificial powder (PS) is selected so as not to adhere to the blank (E) during the second sintering cycle.

Description

Process for manufacturing a technical ceramic part technical field

[0001] The present invention relates to a method for manufacturing a technical ceramic part, in particular a watch case component in the field of watchmaking, especially a watch case. State of the art

[0002] Processes employing, in particular, a sintering operation on a ceramic powder-based composition for the manufacture of ceramic watch components are well known. For example, obtaining a watch case requiring a sintering step is described in publication CH718069. Processes using a SPS-type sintering operation for the manufacture of watch components are also known; an example is described in publication EP4080293.

[0003] Ceramic parts sintered using SPS are generally machined from densified material, the hardness of which complicates machining operations to achieve the final dimensions of the part. It is known to create a rough draft of a ceramic part by 3D printing, with dimensions close to the final shape of the part. 3D printing is typically performed by stereolithography (SLA) or binder jetting. The part obtained by SLA or binder jetting is then subjected to an SPS sintering process to densify the ceramic part and achieve the desired mechanical properties.

[0004] The above-mentioned process involves a particular and additional step and technology which has the disadvantage of complicating the implementation of the process and increasing production costs.

[0005] It is also known to use a composition based on a ceramic powder mixed with a binder and then perform a process of AUDEMA-35-PCT Pre-sintering, using a conventional sintering process, produces a raw part that is easily machinable. However, a debinding operation followed by a final sintering operation is necessary after machining. Consequently, the part's volume is reduced due to the densification of the composition and the loss of binder. This volume reduction is typically between 10% and 25% of the raw part's volume, i.e., the volume of the part before debinding and sintering. Therefore, it is essential to accurately determine this volume reduction in all directions to obtain a final part with the desired dimensions after sintering.

[0006] Furthermore, conventional sintering processes, characterized by uniform and often very high temperatures combined with prolonged exposure to these conditions, promote the diffusion of chemical elements from one area to another. This can cause undesirable chemical reactions between the pigments in the different colored areas, thus compromising the clear separation between them.

[0007] This process also has the disadvantage of being complex to implement, as it involves numerous steps, including the preparation of a composition based on ceramic powder, a binder, and a stabilizing agent, as well as a debinding step. Consequently, the complexity of this process has a significant impact on the cost of the parts produced.

[0008] US2019248709 discloses a process for preparing ceramics for dental applications comprising two distinct steps. The first is a pre-densification by SPS sintering, which yields a machinable porous body. The second is a final densification by conventional air sintering, which overcomes problems associated with final-stage SPS sintering, such as color browning. Brief summary of the invention

[0009] One aim of the present invention is therefore to propose a method for manufacturing a technical ceramic part, AUDEMA-35-PCT in particular a watch component such as a watch case, the implementation of which is simplified compared to the aforementioned prior art.

[0010] Another objective of the present invention is to propose a method for manufacturing a technical ceramic part, in particular a watch component such as a watch case, exhibiting a clear demarcation between areas of different colors.

[0011] The object of the present invention is to achieve at least one of these goals by means of a process for manufacturing a technical ceramic part, comprising the following steps: i) a step of depositing in a mold a composition based on at least one ceramic powder and at least one powdered metal oxide; ii) a step of carrying out a first SPS sintering cycle to obtain a preform with a density lower than the final density of the technical ceramic part to facilitate subsequent machining of said preform; iii) a step of carrying out at least one machining operation of the preform in order to obtain a blank; iv) a step of placing the blank in the mold and surrounding it with a sacrificial powder until all empty spaces within the mold are filled;(v) a step consisting of carrying out a second SPS sintering cycle to obtain the blank with said final density, and (vi) a step consisting of rectifying the blank of final density in order to obtain said technical ceramic part with the final dimensions. The composition of the sacrificial powder is chosen so as not to adhere to the machined part during the second sintering cycle.

[0012] According to one embodiment, the rough draft before the second SPS sintering cycle is at most one fifth, preferably one tenth, or even one twentieth larger than the technical ceramic part with final dimensions.

[0013] According to one embodiment, the preform obtained after the first sintering cycle has a porosity in the range of 25 to 50%, preferably 45 to 50%. AUDEMA-35-PCT

[0014] According to one embodiment, the composition comprises one or more oxides selected from iron oxide, aluminum oxide, bismuth oxide, copper oxide, cerium oxide, zinc oxide, titanium oxide, manganese oxide, cobalt oxide, nickel oxide, chromium oxide, europium oxide, tin oxide, silicon oxide, vanadium oxide, neodymium oxide, praseodymium oxide, hafnium oxide, calcium oxide and yttrium oxide, or at least any combination of these oxides.

[0015] According to one embodiment, the preform obtained after the first sintering cycle has a density in the range of 3 to 6 g / cm³ 3 preferably 4 to 6 g / cm³ 3 or 3 to 3.3 g / cm² 3 .

[0016] According to one embodiment, the sacrificial powder has a sintering temperature at least 100°C higher than the average sintering temperature of said composition.

[0017] According to one embodiment, the blank is placed in the mold on a first layer of sacrificial powder and is completely covered with a second layer of sacrificial powder.

[0018] According to one embodiment, each of the first and second layers has a thickness between 0.5 and 5mm, preferably between 1 and 2mm.

[0019] According to one embodiment, the sacrificial powder is chosen from alumina oxide, amorphous silica or a combination of alumina oxide and amorphous silica, or from a refractory powder whose sintering temperature is above 2000°C.

[0020] According to one embodiment, the composition deposited in the mold forms an inhomogeneous arrangement of powders. AUDEMA-35-PCT

[0021] According to one embodiment, the final density roughing further undergoes a step aimed at carrying out one or more additional marginal machining operations.

[0022] According to one embodiment, the step of rectifying the roughing is carried out by grinding, by diamond tools, by laser and / or by water jet.

[0023] According to one embodiment, the process further comprises a reoxidation step of the blank carried out between the step of carrying out the second sintering cycle and the grinding step or after said grinding step.

[0024] Another aspect of the invention relates to a watch component, in particular a casing such as a case, a bezel, a bracelet or a case back, obtained by the process according to one of the aforementioned embodiments, as well as to a timepiece comprising the watch component. Brief description of the figures

[0025] Examples of implementation of the invention are given in the description illustrated by the accompanying figures, in which: - Figures 1a and 1b illustrate the main steps in the manufacturing process of a watch case, according to one embodiment, - Figure 2 schematically illustrates an SPS press with the sintering chamber, and - Figures 3 to 5 illustrate arrangements of several ceramic-based powders of different composition in the mold according to different arrangements. AUDEMA-35-PCT Examples of embodiments of the invention

[0026] The following description focuses on a method for manufacturing a watch case in various forms. Naturally, a person skilled in the art can apply the same method to the manufacture of other ceramic watch parts without departing from the scope of this invention.

[0027] One of the distinctive features of this process is the performance of two SPS sintering cycles, with a machining operation carried out between them. This machining operation is performed to obtain a roughing whose dimensions and shape closely resemble the final technical ceramic component, as described later.

[0028] The parameters of the first SPS sintering cycle are determined to obtain a preform with a density lower than the final density of the technical ceramic part, thus facilitating subsequent machining. However, the preform's density must exceed a predetermined threshold to ensure sufficient mechanical strength to withstand the significant stresses encountered during machining and prevent crumbling or cracking. The parameters of the second SPS sintering cycle are then determined to impart the desired mechanical properties to the final ceramic part.

[0029] In general, and with reference to Figure 1 which illustrates a non-limiting embodiment, the process for manufacturing a watch case includes a first step S1 which aims to place in a mold or matrix M, typically made of graphite, a composition based on at least one ceramic powder P1... Pn and further comprising at least one metal oxide in powder form, preferably at least two metal oxides of different types, each oxide having a certain content, for example between 10% and 20% of the composition. AUDEMA-35-PCT

[0030] A composition based on various ceramic powders and metal oxides can be deposited in the mold according to different arrangements, which will be described below without limitation. The composition will be defined as a set of powders in which this set consists, on the one hand, of at least one ceramic powder and at least one powdered metal oxide, and, on the other hand, is arranged in a mold according to a particular arrangement of these powders relative to each other.

[0031] Each ceramic powder P1, P2, P3 can for example be chosen from a powder based on alumina, zirconia, silicon carbide, silicon nitride, boron nitride and aluminium nitride.

[0032] According to an alternative, at least one of the ceramic powders is a composite chosen from among the following three composites: Alumina matrix zirconia (ZTA), Zirconia matrix alumina (ATZ) and Yttrium-stabilized zirconia at least partially (Y-TZP).

[0033] According to another alternative, at least one of the ceramic powders is composed of a mixture of alumina matrix zirconia (ZTA), zirconia matrix alumina (ATZ), and zirconia stabilized at least partially with yttrium (Y-TZP).

[0034] The metal oxide can, for example, be selected from among iron oxide, aluminum oxide, bismuth oxide, copper oxide, cerium oxide, zinc oxide, titanium oxide, manganese oxide, cobalt oxide, nickel oxide, chromium oxide, europium oxide, tin oxide, silicon oxide, vanadium oxide, neodymium oxide, praseodymium oxide, hafnium oxide, calcium oxide, and yttrium oxide, or at least any combination of these oxides. At least any combination made from at least two of these oxides is also possible.

[0035] As an example, several ceramic-based powders P1, P2, P3 of different types can be arranged sequentially in AUDEMA-35-PCT the mold M layer by layer so as not to be mixed according to the example illustrated by figure 3.

[0036] Alternatively, a first main powder based on ceramic P1 can be placed in the mold M, while other powders P2, P3 based on ceramics of different natures in the form of clusters are embedded in the main powder as schematically illustrated by figure 4.

[0037] According to the embodiment illustrated in Figure 5, at least two powders P1, P2, based on ceramics of different types, are distributed in the mold M to obtain an assembly of unmixed powders. The two powders can, for example, be distributed so as to have an inclination or shapes, arbitrary or predefined, complementary in order to form an interface with no (or almost no) concentration gradient between the two powders P1, P2.

[0038] Depending on the requirements, other arrangements of different ceramic-based powders can be implemented, for example random arrangements or to form geometric figures.

[0039] According to Figure 1, the first step S1, consisting of filling the mold M with different ceramic powders according to different arrangements, is followed by a step S2 consisting of carrying out a first cycle of SPS (Spark Plasma Sintering) in order to obtain a preform PF of the case.

[0040] SPS sintering is a process similar to hot isostatic pressing, but it uses the Joule effect to heat a pre-compacted powder in a mold between two graphite electrodes under an inert atmosphere or vacuum. The SPS sintering chamber contains the mold holding the pre-compacted powder, which is subjected to a pressure of several megapascals by pistons. A direct or alternating current of several kiloamperes, pulsed or continuous, is applied between the electrodes at a voltage of a few volts, thus creating a AUDEMA-35-PCT much faster densification of the pre-compacted powder than conventional sintering processes.

[0041] The parameters of the first SPS sintering cycle are determined so that the resulting preform PF exhibits sufficient mechanical strength to allow one or more machining operations to be performed according to step S3, described below, in order to obtain a blank E of predetermined shape. Sufficient mechanical strength in the context of the present invention is obtained for a stabilized zirconia-based ceramic having one or more of the following properties: density: 3 to 6 g / cm³ 3 preferably 4 to 6 g / cm³ 3 or 3 to 3.3 g / cm² 3 , porosity: 45 to 50%, tensile strength: 50 to 90 MPa.

[0042] According to the illustrated example, the case is made from a stabilized zirconia-based composition so that the PF preform is cylindrical in shape with a diameter that can vary between 40mm and 60mm, for example approximately equal to 50mm and whose thickness is between 15 and 20mm, for example 18mm.

[0043] The first S2 sintering cycle may include, in particular, the following parameters: - temperature rise profile located between 100°C / min and 150°C / min up to a target TIFRI temperature typically between 1200°C and 1600°C; - Maintaining the target TIFRI temperature between 1 and 5 minutes - temperature descent profile located between 25°C to 100 °C / min after the sintering phase; - PIFRI sintering pressure is between 40 MPa and 80 MPa; - currents between 3000 and 4000 Amperes; - voltage between 5 and 10 Volts.

[0044] The machining step S3 consists of machining the preform PF, notably by grinding, laser cutting, milling, turning and / or waterjet cutting, to obtain a rough E resembling the shape of the part at the final dimensions. The dimensions of the rough E must nevertheless be determined to take into account the volume loss occurring during the AUDEMA-35-PCT Second sintering cycle. This volume loss results from the densification of the material and therefore not from a loss of binder due to its mere absence. This volume loss is generally between 2 and 20%, typically between 5 and 10% of the volume of the machined part. Thus, the blank E must be machined so as to have a volume no more than one-fifth, preferably one-tenth, or even one-twentieth greater than that of the finished part.

[0045] The machining step S3 is followed by step S4, which consists of placing the blank E, surrounded by a sacrificial powder PS, into the mold M. The blank is thus completely immersed in the PS sacrificial powder within the mold before a second SPS sintering cycle is undertaken. The objective of this second cycle is to give the part the desired mechanical properties, particularly in terms of hardness and toughness. This PS sacrificial powder creates a homogeneous volume, ensuring that pressure and heat are transmitted uniformly across the entire blank E during the second sintering cycle and that the blank is supported without causing deformation that could result from thermal stresses during this second cycle.

[0046] In a preferred embodiment, a first layer of sacrificial powder Cpsi, preferably between 0.5 and 5 mm thick, and preferably between 1 mm and 2 mm, is placed at the bottom of the mold. The blank E is then placed in the mold on this first layer, and the empty spaces are then filled with sacrificial powder until the blank is completely covered, forming a second layer of sacrificial powder Cps2 above it, with a thickness similar to the layer located below the blank.

[0047] Preferably, the sacrificial powder PS will possess certain specific properties, notably the ability to withstand high temperatures without sintering itself. In this way, the sacrificial powder will not stick or fuse with the blank E and will avoid any adhesion to the part or mold M, thus facilitating its removal after sintering. The sacrificial powder PS can, for example, be chosen from refractory powders. AUDEMA-35-PCT amorphous silica, silicon carbide or a combination of at least some of these components.

[0048] Prior to the second S5 sintering cycle, the filled mold is placed in the SPS sintering chamber according to the schematic representation shown in Figure 2. It must be positioned on the SPS machine's platform or support to ensure optimal contact with the electrodes. The mold can be mounted on a support to maintain its stability during the sintering process.

[0049] Step S4 is followed by step S5, which consists of performing a second SPS sintering cycle with parameters determined to obtain the desired mechanical properties of the case. These mechanical properties are obtained in the context of the present invention for a stabilized zirconia-based ceramic preferably having one or more of the following properties: density: greater than 6 g / cm³ 3 , porosity: less than 0.5%.

[0050] The parameters of the second S5 sintering cycle may include, in particular, the following parameters: - temperature rise profile located between 100°C / min and 150°C / min up to a target temperature Δ2FRI typically between 1200°C and 1600°C; - maintaining the target temperature Δ2FRI between 5 and 20 minutes - temperature descent profile located between 25°C to 100 °C / min after the sintering phase; - P2FR1 sintering pressure is between 40 MPa and 80 MPa; - currents between 3000 and 4000 Amperes; - voltage between 5 and 10 Volts.

[0051] The sintering pressure P2FR1 is achieved by actuating the two pistons Pis1, Pis2 against respectively the upper and lower layers of sacrificial powder. AUDEMA-35-PCT

[0052] Once the second S5 sintering cycle has been completed, the blank is removed from the M mold and cleaned of any powder residue to ensure the cleanliness of the blank.

[0053] Depending on atmospheric conditions, the materials used, and the sintering parameters, the SPS sintering process may induce chemical reduction. In particular, in a vacuum or inert atmosphere, there may be partial reduction of certain oxides because oxygen is removed.

[0054] Although complete reduction of stabilized zirconia is unlikely under normal SPS sintering conditions, due to its thermodynamic stability, it contains one or more metal oxides which may be in a reduced or partially reduced state, giving the part dull colors.

[0055] In this case, the second sintering cycle can advantageously be followed by an S6 reoxidation operation allowing reoxidation of the metal oxides, thus giving the case areas bright colors depending on the type and content of the metal oxides present.

[0056] The reoxidation process typically takes place in an oxygen-rich atmosphere, often using air or pure oxygen. If more controlled reoxidation is required, a partially oxidizing atmosphere (such as a mixture of oxygen and argon) can be used to prevent excessive oxidation or the formation of secondary oxides. The case is heated in a furnace or heat treatment system to a temperature high enough to facilitate oxygen diffusion throughout the case. Reoxidation temperatures vary depending on the type of metal oxides present in the case but are generally between 800°C and 500°C.

[0057] Thus, after the S6 re-oxidation operation, the visual appearance of the case displays one or more colors in different areas depending on the type, content, and distribution of the metal oxides within AUDEMA-35-PCT ceramics. The case may, for example, have an area with a reddish tint for cerium oxide, an area with bright green or blue for copper oxide and blues, or an area with a purple tint for manganese oxide.

[0058] A grinding operation S7 is then performed on the case to achieve the desired final dimensions. Since the dimensions of the blank E, densified to its final value after the second sintering cycle S5, are close to the final dimensions, the grinding operation is significantly shortened and can typically be performed primarily by milling or grinding. In the context of this process, the grinding step S7 may also include additional marginal machining operations on the final density piece, particularly to create, in the case of a case, the opening for the crown stem and the lug holes to receive the ends of a bracelet bar.

[0059] Depending on the embodiment, a decorative operation S8 can be carried out after the grinding step to obtain the final part 10. This optional operation consists of applying a decoration to one or more areas of the case, particularly on the side of the case, using a mechanical process. The mechanical process may include polishing, sandblasting, satin finishing (straight, circular, or spiral), microblasting, guilloché work, perlage, Côtes de Genève finishing, or hand engraving.

[0060] According to one embodiment variant, the reoxidation operation can be carried out after the S7 rectification operation and before or after the S8 decoration operation.

[0061] Although the process was described primarily for the manufacture of a watch case in technical ceramic, the process can be applied to other ceramic parts, particularly to any watch case or movement component, without deviating from AUDEMA-35-PCT the invention as defined by the claims. This method can, for example, be applied to any type of watch component, in particular to components of a watch movement intended to be visible on the back of a watch case through a sapphire crystal, or to casing components. AUDEMA-35-PCT

Claims

Demands 1. A method for manufacturing a technical ceramic part (10), comprising the following steps: i) a step (S1) of depositing into a mold (M) a composition based on at least one ceramic powder (P1; P2....Pn) and at least one powdered metal oxide; ii) a step (S2) of carrying out a first SPS sintering cycle to obtain a preform (PF) with a density lower than the final density of the technical ceramic part (10) to facilitate subsequent machining of said preform (PF); iii) a step (S3) of carrying out at least one machining operation of the preform (PF) in order to obtain a blank (E); characterized in that it further comprises iv) a step (S4) of placing the blank (E) in said mold (M) and surrounding it with a sacrificial powder (PS) until all empty spaces within the mold are filled;(v) a step (S5) consisting of carrying out a second SPS sintering cycle to obtain the blank (E) with said final density, and (vi) a step (S7) consisting of rectifying the blank (E) of final density in order to obtain said technical ceramic part (10) with the final dimensions, and in that the composition of the sacrificial powder (PS) is chosen so as not to adhere to the blank (E) during the second sintering cycle.

2. A method according to claim 1, wherein the blank before the second SPS sintering cycle is at most one fifth, preferably one tenth, or even one twentieth, larger than the technical ceramic part (10).

3. A process according to any one of the preceding claims, wherein the preform (PF) obtained after the first sintering cycle has a porosity in the range of 25 to 50%. AUDEMA-35-PCT 4. A process according to any one of the preceding claims, wherein said composition comprises one or more oxides selected from iron oxide, aluminum oxide, bismuth oxide, copper oxide, cerium oxide, zinc oxide, titanium oxide, manganese oxide, cobalt oxide, nickel oxide, chromium oxide, europium oxide, tin oxide, silicon oxide, vanadium oxide, neodymium oxide, praseodymium oxide, hafnium oxide, calcium oxide and yttrium oxide, or at least any combination thereof.

5. A method according to any one of the preceding claims, wherein the preform obtained after the first sintering cycle has a density in the range of 4 to 6 g / cm³ 3 .

6. A method according to any one of the preceding claims, wherein the sacrificial powder has a sintering temperature at least 100°C higher than the average sintering temperature of said composition.

7. A method according to any one of the preceding claims, wherein the blank is placed in the mold on a first layer of sacrificial powder and is totally covered with a second layer of said sacrificial powder, each of said first and second layers having a thickness of between 0.5 and 5 mm, preferably between 1 and 2 mm.

8. A method according to any one of the preceding claims, wherein the sacrificial powder is selected from alumina oxide, amorphous silica or a combination of alumina oxide and amorphous silica, or from a refractory powder whose sintering temperature is above 2000°C.

9. A method according to any one of the preceding claims, wherein said composition deposited in the mold (M) forms an inhomogeneous arrangement of powders. AUDEMA-35-PCT 10. A method according to any one of the preceding claims, wherein the final density blank (E) further undergoes a step aimed at carrying out one or more additional marginal machining operations.

11. A method according to any one of the preceding claims, wherein the step (S7) of rectifying the rough (E) is carried out by milling, grinding, diamond tools, laser and / or water jet.

12. A process according to any one of the preceding claims, further comprising a step (S6) of reoxidation of the blank (E) carried out between the step (S5) of carrying out the second sintering cycle and the step (S7) of grinding or after said step (S7) of grinding. AUDEMA-35-PCT

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