Induced formation of solid lubricants

Abstract

The method and device for induced formation of solid lubricants includes providing (S10) an article to be treated. The article is exposed (S20) to a process fluid (34) that includes a solvent, an impact medium (20), and a solid lubricant precursor additive. The solvent is a low volatility high flash solvent. The impact medium is a non-abrasive hard particle. The solid lubricant precursor additive is a surface reactive compound that serves as a carrier for at least one of S, P, B, and at least one refractory metal. A velocity difference is created (S30) between the surface (12) of the article and the impact medium. This causes an impact between the impact medium and the article. A solid lubricant material is formed on the surface by a chemical reaction (S40). The chemical reaction includes the solid lubricant precursor and is induced by the energy of the impact. The chemical reaction occurs at the surface of the article.

Description

[Technical field]

[0001] The present technology relates generally to the lubrication of articles, and more particularly to methods and devices for the induced formation of solid lubricants on the surfaces of articles. [Background technology]

[0002] Wear and friction are always a problem in all devices that include moving parts. In some cases, mechanical contact can be avoided, thus reducing friction and wear. However, in most cases, moving parts are moved in mechanical contact with other parts of the device. In such cases, measures to reduce friction and wear by providing lubricant substances to the interaction areas are common. The most common lubricant substances, even today, are of the liquid type, such as different types of lubricating oils. An alternative is to provide a solid lubricant on the surfaces that are in mechanical contact. The lubricant keeps the surfaces apart and is itself easily shearable, which reduces the force required to achieve relative motion.

[0003] The use of certain solid film lubricants has been known for some time. Either graphite itself or graphite embedded in a binder are common examples. Vaporized reactive substances containing phosphorus, sulfur, selenium or halogen atoms have also been used for a long time. Different approaches are also known, for example using molybdenum disulfide held in a matrix of resin, which can be deposited, for example, by mechanical interaction between the surface and the tool in the presence of the resin. A group of prior art can also be mentioned that describes low-friction films produced by PVD, CVD and / or plasma sputtering.

[0004] Low friction surfaces can also be produced using dynamic methods such as fine particle peening, fine particle shot peening, and ultrafine shot peening, either by adding solid lubricants to the shot media or by using the above solid lubricant powders as the shot media. Molybdenum disulfide and tungsten disulfide are the most commonly used solid lubricants for that purpose (see e.g. Y. Yoshimi, et al., Surface Treatment Technology for Sliding Parts of Compressors, International Compressor Engineering Conference at Purdue, July 17-20, 2006, C063 or Y. Ishida et al., Frictional Properties of Textured Surfaces by Fine Particle Peening in Lubricated Condition. The Proceedings of the Machine Design and Tribology Division meeting in JSME. 2008. 8. 165-166.).

[0005] There are also numerous other coating methods used to reduce friction wear (see e.g. Ali Eldemir, Low-Friction Materials and Coatings, in Multifunctional Materials for Tribological Applications (Robert JK Wood), Jenny Stanford Publishing, New York, 2015, Chapter 8.). For example, laser cladding techniques can also be used to produce wear-resistant composite coating layers incorporating solid lubricants (see CN112575324). Various thermal spray methods, such as HVOF spray deposition as described in US9,162,424, are another option. However, such techniques cannot meet stringent precision without post-processing of the coated object. US2011 / 0165331 describes a method for producing polymer-bonded low-friction coatings that include a resin binder and a ceramic particulate filler. Another invention disclosed in US10,266,783 describes a low-friction surface featuring protruding nanotubes. Unfortunately, the practical utility of the methods disclosed therein is significantly limited due to the thermal expansion coefficient mismatch between the glass binder and the metal base, which leads to rapid coating loss under combined load and thermal cycling, and to the adverse health safety profile of the nanotubes.

[0006] Common to most solid lubricant systems is that the lubricant is deposited onto a surface, either as a pure lubricant material or as a lubricant in a carrier material. Deposition can be followed by different types of post-treatment, typically thermal or mechanical treatment. The lubricant is thus provided as a layer on the surface to be lubricated. It is difficult to obtain good adhesion to a surface while at the same time exhibiting low friction relative to neighboring surfaces.

[0007] Mechanochemical surfacing is another attractive alternative for producing low-friction surfaces. An example of an industrial process that relies on mechanochemistry is Triboconditioning® (see, for example, WO 2012 / 008890). This process combines elements of mechanical burnishing of the component surface with the tribochemical deposition of a low-friction tribofilm. In doing so, the running-in behavior of the treated components is improved and their service life is extended (see e.g. B. Zhmud, In-manufacture Running-in of Engine Components by Using the Triboconditioning Process, M. Abdel Wahab (Ed.): FFW 2018, LNME, pp. 671-681, 2019.).

[0008] The Triboconditioning® treatment relies on the in-situ chemical generation of solid lubricants that are in tribological contact during the process. This ensures that an optimal amount of solid lubricant is retained by the surfaces, minimizing raw material waste and potential cleanliness issues in the end application.

[0009] In conventional Triboconditioning® methods, significant technical challenges can be found in tool alignment, tool load balancing, and work surface wear compensation. In particular, special tool designs are required to minimize part twisting under applied load, to adjust tool load to maintain constant contact pressure, and to maintain tool / workpiece contact at constant conditions. This precludes the use of the method for the processing of geometrically complex parts that present surfaces with non-uniform curvature and material thickness.

[0010] Therefore, there is a need for improved methods for the processing of geometrically complex parts exhibiting surfaces with non-uniform curvature and material thickness. Summary of the Invention

[0011] The overall object of the present technology is to provide improved methods and devices for the formation of solid lubricants by mechanochemical surface processing.

[0012] The above object is achieved by a method and a device according to the independent claims. Preferred embodiments are defined in the dependent claims.

[0013] Generally speaking, in a first aspect, a method for the induced formation of a solid lubricant on an article includes providing an article to be treated. The article is exposed to a chemically reactive process fluid and an impact medium. The chemically reactive process fluid includes a solvent and a solid lubricant precursor additive. The solvent is a low volatility high flash solvent. The solid lubricant precursor additive includes an oil soluble metal carboxylate in combination with a surface reactive compound that serves as a carrier for at least one of S, P, B, and a surface reactive compound that serves as a carrier for at least one refractory metal, and / or a sulfurized additive. The impact medium is a non-abrasive hard particle. A velocity difference is created between the surface of the article and the impact medium. This causes an impact between the impact medium and the surface of the article, resulting in a vanishing action. A solid lubricant material is formed on the surface of the article by a chemical reaction. The chemical reaction includes a solid lubricant precursor and is induced by the energy of the impact in the presence of said chemically reactive process fluid. The chemical reaction occurs at the surface of the article.

[0014] In a second embodiment, the device for inducing the formation of a solid lubricant on an article includes an exposure vessel. The exposure vessel has an inlet for a chemically reactive process fluid and an impact medium. The chemically active process fluid includes a solvent and a solid lubricant precursor additive. The solvent is a low volatility high flash solvent. The solid lubricant precursor additive includes an oil-soluble metal carboxylate in combination with a surface reactive compound that serves as a carrier for at least one of S, P, B, and a surface reactive compound that serves as a carrier for at least one refractory metal, and / or a sulfurized additive. The impact medium is a non-abrasive hard particle. The device for inducing the formation of a solid lubricant on an article further includes an article holder device for the article to be placed in the exposure vessel, and a device for creating a velocity difference between the surface of the article and the impact medium.

[0015] One advantage of the proposed technology is that it allows for the processing of dissimilar components using the same finishing equipment and media composition. Also, different components can be processed simultaneously in the same batch. Other advantages shall be understood upon reading the detailed description.

[0016] The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken together with the accompanying drawings, in which: [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 illustrates the runnability window for a conventional Triboconditioning® process. [Figure 2A-C] FIG. 1 illustrates tool control challenges associated with a conventional Triboconditioning® process for a built part. [Diagram 3] FIG. 1 illustrates the mechanics of oblique mechanical impact. [Figure 4] FIG. 2 is a flow diagram of steps of an embodiment of a method for induced formation of a solid lubricant on an article. [Figure 5A-B]1 is a schematic diagram of an embodiment of a device for inducing the formation of a solid lubricant on an article based on vibrational motion. FIG. [Figure 6A-B] 1 is a schematic diagram of an embodiment of a device for inducing the formation of a solid lubricant on an article based on rotational motion. FIG. [Figure 7] 1 is a schematic diagram of an embodiment of a device for inducing the formation of a solid lubricant on an article based on directing an impact medium flow. FIG. [Figure 8A] FIG. 1 illustrates a test pin surface profile before processing using the present method. [Figure 8B] FIG. 13 illustrates a test pin surface profile after processing using the present method. [Figure 9] FIG. 1 presents the results of compressive stress measurements. [Figure 10A] FIG. 1 illustrates the micropitting areas and wear for an untreated article in a twin disc test. [Figure 10B] FIG. 1 illustrates micropitting areas and wear for treated articles in a twin disc test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0019] For a better understanding of the proposed technology, it may be useful to start with a brief overview of the conditions of a conventional Triboconditioning® process. In a conventional Triboconditioning® process, the process runnability window will typically be found empirically for each individual application. The basic variables are the tool slide speed, the contact pressure, and the processing time. The product of speed x contact pressure can be expressed, for example, in J / m 2s defines the friction energy flux. The contact pressure must be high enough to trigger plastic deformation of the surface asperities, but not too high to prevent any workpiece deformation. For a given contact pressure, the sliding speed must be high enough to provide sufficient energy for tribochemical reaction activation, but not too high to prevent overheating and workpiece damage. This is illustrated diagrammatically in FIG. 1. The minimum energy to initiate the tribo-reaction is illustrated by the dotted line 100. Contact pressures above this limit result in the necessary conditions for forming solid lubricant material from precursors being available at the contact point. The dotted line 101 indicates the practical limit where the applied contact pressure approaches the material yield stress sufficiently close to the material yield stress where damage to the article cannot be eliminated. The Y-axis of the diagram indicates the number of passes between a single point on the article and the tool. This can be expressed, for example, as the tool sliding speed for a fixed cycle time or the number of tool passes per unit time times the processing time. A minimum limit 102 can be assigned, which corresponds to the minimum processing time for a sufficient film thickness. A maximum limit 103 can be assigned where wear and reagent waste become significant, resulting in low power output. Together, these limits define a runnability window 104 where high power production of a useful solid lubricant coating is achieved. For illustrative purposes, curve 105 shows the average post-treatment coefficient of friction (CoF) between a 60 HRC 100Cr6 steel surface with an initial Ra=0.3um and a friction probe, in this particular case a 10 mm 100Cr6 bearing steel ball at 10N load in lubricated contact using pure PAO2 base oil as the lubricant. The CoF decreases as the surface roughness decreases and the low friction tribofilm increases.

[0020] Moreover, as noted in the Background, tool control can be a challenge in conventional Triboconditioning® processes. 2A-C illustrate different situations that arise when a camshaft is to be provided with a conventional Triboconditioning® solid lubricant layer. The Triboconditioning® tool 106 is pressed against an article 10, in this case a camshaft, having a non-circular cross section. If the Triboconditioning® tool 106 is pressed with the same force while the camshaft is rotated. The contact pressure in the different Figs. 2A-2C changes and the contact points on the tool 106 change as well. To ensure constant operating conditions for all surface segments of the article 10, the contact force must therefore be synchronized with the movement of the article. Preferably, the orientation of the tool 106 relative to the article should also be changed synchronously. Such an arrangement is complex and often needs to be adapted for each individual article shape.

[0021] A requirement for the chemical reaction of the conventional Triboconditioning® process to occur is that high pressure and temperature are available within the volume in which the chemical reaction is to take place. In the conventional Triboconditioning® process, this is provided by mechanical interaction between the tool and the surface to be treated in a sliding or rubbing relative motion of the tool along the surface to be treated. Thus, the energy for the chemical reaction is provided by friction and not by hitting the surface.

[0022] However, energy can also be provided to a localized volume at the surface of the article in other ways. Replacing the fixed tool with a group of non-abrasive, dispersed impact members that impact the surface to be treated can provide high energy transfer. The impact members can be, for example, hardened metal carbide balls, ceramic beads, aluminum oxide beads, zirconium oxide beads, or similar. In contrast to continuous tool action, the impact of small objects can transfer high localized forces due to the short impact time. The impact energy can be controlled by the kinetic energy of the impact members. The kinetic energy is proportional to the impact member mass times the impact member velocity squared. The energy flux per unit area is then proportional to the impact member density times the velocity squared times the radius.

[0023] The conditions for the dynamics of the particles of the impact medium are illustrated in FIG. 3. A particle of the impact medium 20, typically a bead, impacts the surface 12 of the article 10 at an angle α0 with a velocity V0. After the impact, the particle of the impact medium 20 leaves the surface 12 at an angle α1 with a velocity V1 and with a rotation of ω1. Thus, the impact action involves a velocity component towards the surface to be treated before the impact occurs, and a velocity component away from the surface to be treated after the impact occurs. These changes in the dynamics of the particles are caused by the interaction with the surface 12. Elastic and inelastic deformations are caused at the surface 12 as a result of the local pressure at the impact point, which at least in part causes a temperature increase at the surface 12. Friction between the particle 20 and the surface 12 may also impose changed rotation conditions for the particle 10.

[0024] 1MJ / m 2 s and preferably 5MJ / m 2 It is estimated that an energy flux delivered at the impact region of surface 12 greater than s may generally be sufficient to induce the formation of solid lubricant material, however, even lower energy fluxes may be useful for particular choices of process parameters and / or for particular types of articles.

[0025] At the same time, the energy flux transmitted in the impact area of ​​the surface 12 must not be too high, since this may favor different undesirable wear processes. Here, it is generally considered that the energy flux is less than 250 MJ / m 2 s and preferably 50MJ / m 2 It is believed that energy fluxes of as much as s should be avoided. However, for the lower energy flux limits, even higher energy fluxes may be operable for a particular choice of process parameters and / or for a particular article.

[0026] It has been found that a greater transfer of the initial kinetic energy to heat occurs when the velocity of the impacting medium 20 has a non-zero component parallel to the surface 12. In other words, more energy of the impacting medium 20 can be used to cause a chemical reaction when the angle α0 is different from 90 degrees. This is probably due to the increased importance of the friction contribution at smaller angles α0. If there is controllability of the velocity direction of the impacting medium 20 relative to the surface 12, it may be possible to select the energy transfer ratio.

[0027] If a solid lubricant precursor suitable for forming a solid lubricant material is present on the surface of the article and the energy flux provided by the impacting medium is sufficient to induce a chemical reaction involving the solid lubricant precursor, a solid lubricant material similar to that produced by the conventional Triboconditioning® process may be formed on the surface of the article.

[0028] FIG. 4 is a flow diagram of steps of an embodiment of a method for induced formation of a solid lubricant on an article. In step S10, an article to be treated is provided. The article can be of any material used for mechanical contact purposes, typically a metal or metal alloy. Preferably, the material in the article includes iron, since S, P, B based solid lubricants often also react with iron. In step S20, the article is exposed to a chemically reactive process fluid and an impact medium. The chemically reactive process fluid includes a solvent and a solid lubricant precursor additive. The solvent is a low volatility high flash solvent. The solid lubricant precursor additive includes a surface reactive compound that serves as a carrier for at least one of S, P, B, and a surface reactive compound that serves as a carrier for at least one refractory metal, and / or an oil soluble refractory metal carboxylate. The impact medium is a non-abrasive hard particle. These hard particles are typically beads, balls, pins, etc. made of ceramic or hardened metal carbide that can be provided in a wide range of shapes and sizes. Preferred embodiments are discussed further below. In step S30, a velocity difference is created between the surface of the article and the impact medium. This velocity difference is used to induce an impact between the impact medium and the surface of the article. Different approaches to achieve the velocity difference are discussed in more detail below. In step S40, a solid lubricant material is formed on the surface of the article by chemical reactions involving solid lubricant precursors. These chemical reactions are induced by the energy of the impact. The chemical reactions occur at the surface of the article. Preferably, the step of forming the solid lubricant material is performed by chemical reactions further involving a surface material of the surface of the article.

[0029] Methods and devices for creating a velocity difference between the surface of an article and an impact medium are known per se in the prior art. Manufacturing processes such as mass finishing exploit the velocity difference between the impact medium and the surface. The term "mass finishing" refers to a group of manufacturing processes that allow large quantities of parts to be subjected to a finishing process at the same time. One main type of mass finishing is tumble finishing, also known as barrel finishing, and vibratory finishing. The goal of this type of finishing is to polish, deburr, clean, radius, deburr, descale, descaling, polish, brighten, surface harden, prepare the part for further finishing, or interrupt the die cast runner. In other words, mass finishing uses abrasive contact between the workpiece and the finishing media surface to achieve the desired surface finish quality for the workpiece. Various finishing media types may be used. Mass finishing may be performed dry or wet. Wet processes use liquid lubricants, coolants, or detergents together with abrasives. Cycle times can vary from minutes to hours depending on the process conditions, workpiece material, and finishing media used. Mass finishing processes can be run as either batch or continuous processes and can also be sequenced, which involves passing the workpiece through multiple different mass finishing stages.

[0030] Mass finishing methods generally involve surface preparation, where some surface material is typically removed from the surface. This polishing action may be combined with treatment of the remaining portion of the surface, for example by surface hardening. However, mass finishing methods generally do not involve surface coating processes.

[0031] However, similar techniques in which the article surface is coated, such as fine particle peening, fine particle shot peening, and ultrafine shot peening, are also available in the prior art, as briefly discussed in the background. This is typically done by adding a solid lubricant to the shot media, or by using a solid lubricant powder as the shot media itself. In these methods, the material intended to coat the article is provided as or in the shot media itself. No additive of any solid lubricant precursor in the form of a surface-reactive compound to any process liquid is used.

[0032] In contrast to well-known chemically accelerated vibratory finishing, such as the chemical mechanical polishing (CMP) process used in the semiconductor industry or the isotropic superfinishing (ISF) used in mechanical engineering, the function and composition of the reactive fluid used by the process proposed here are completely different. Unlike CMP and ISF fluids, the process fluids of the present technology are not expected to chemically etch the surface to accelerate the process. Instead, their main function is to provide a chemical structure for tribofilm generation, as well as optional complimentary corrosion prevention and cleaning functions that favor process stability. It should therefore be noted that the conventional Triboconditioning® process and the present technology conceptually differ from chemically accelerated vibratory finishing in the composition and function of the process fluid and the characteristics of the finished surface.

[0033] In the present technology, high density impact media tend to produce higher impact pressures than low density impact media. Oblique impact angles can be used to induce sliding action, while straight impacts can be used to maximize impact pressure. Different impact media shapes can also be used as needed.

[0034] Centrifugal and vibratory equipment has proven suitable for carrying out the process presented herein using small balls or beads made of hardened metal carbide, nitride, zirconium oxide, or ceramic. 3 Hardened tungsten carbide balls with a density of 3.2-3.4g / cm 3 Sintered silicon nitride beads with a density of 5.6-5.8g / cm 3 and zirconium oxide beads having a density of g / cm 3 Sintered bauxite balls having a density of 0.1 to 1.0 have all been advantageously used as impact media.

[0035] The preferred ball size in these cases was 1-5 mm. Smaller sizes are needed to process recessed surfaces, e.g., to access the flank and root surfaces in the case of large diameter pitch gears.

[0036] Maximum 18g / cm 3 Balls made of tungsten heavy alloys having a density of 1000 .mu.m have also been used as impact media.

[0037] Additionally, silicon nitride or tungsten carbide powder slurries (particle size 0.1-250um) may be used in place of or in addition to other impact media, such as the impact media examples listed above, to provide a light abrasive action. The use of hardened tungsten carbide media is particularly preferred, as it exhibits unanticipated effects due to carbide nanoparticles being released from the media and lapping onto the workpiece surface. The possibility of nanoparticle encrustation has been previously described for nanodiamonds (see Jiang X et al, in Mechanistic features of nanodiamonds in the lapping of magnetic heads. Scientific World Journal. 2014 (2014) 326427.). However, it was quite surprising that it occurred during the solid lubricant formation process using sintered ceramic media as the impact media. The presence of embedded tungsten carbide nanoparticles at the surface of the treated parts has been confirmed by SEM-EDX analysis. Such particles have beneficial effects on the break-in performance and wear resistance of the treated parts.

[0038] In other words, in one embodiment, the impact medium comprises hardened metal carbide, metal nitride, zirconium oxide, ceramic, and / or tungsten heavy alloy. Preferably, the size of the impact medium is 1-5 mm.

[0039] In one embodiment, the impact medium comprises silicon nitride or tungsten carbide powder. In a further embodiment, the impact medium comprises hardened tungsten carbide powder. Preferably, the particles of the silicon nitride or tungsten carbide powder have an average size between 0.1 and 250 μm.

[0040] The process is a wet process involving a chemically reactive process fluid to which an impact medium is provided. The process fluid includes a solvent and a solid lubricant precursor additive. The solvent is a low volatility high flash solvent. This allows the impact medium to be used in the presence of a reactive fluid that provides the reagents for the tribochemical reaction.

[0041] The preferred fluid selection depends on the individual application requirements and the device being used. For example, when dealing with low density impact members such as bauxite or silicon nitride balls, low viscosity neat oil type fluids are preferred because higher viscosity conventional fluids provide excessive impact damping. Furthermore, when some level of corrosion protection is essential along with improved tribology, aqueous zirconium phosphate based formulations may be selected. Many particulate systems such as graphene, colloidal titanium oxide, and inorganic fullerene-like structures may also be deployed in both aqueous and oil-based formulations to achieve specific performance goals.

[0042] In one embodiment, the solvent includes mineral oils and spirits, synthetic polyalphaolefins, isoparaffins, alkylated naphthalenes, esters, ethers, alcohols, carboxylated or alkoxylated polyols, water, and / or ionic liquids.

[0043] In one embodiment, the solvent is selected to have a kinematic viscosity at 40° C. of less than 50 cSt, and preferably less than 10 cSt.

[0044] Some non-exclusive examples of suitable fluid formulations are provided below. Neat Oil Type 1 Polyalphaolefin 2 (Durasyn 162, INEOS) ………43 Alkylated naphthalene 8 (Na-lube KR-008, King Industries)………33 Oil-soluble organotungstate (Vanlube W324, Vanderbilt)…20 Sulfurized olefins (Anglamol 33, Lubrizol)………2 Zinc dialkyldithiophosphate (Lubrizol 1371, Lubrizol)………2 Antifoaming agent (Viscoplex14-520, Evonik)……200ppm Characteristics: Kinematic viscosity @40C=14.1cSt; Specific gravity=0.88gcm-3 Neat oil type 2 (low viscosity) Naphtha (Exxol D100, ExxonMobil)………………86 Oil-soluble organotungstate (Vanlube W324, Vanderbilt)…10 Sulfurized olefin (Additin RC2540, Rhein Chemie)…2 Zinc dialkyldithiophosphate (Lubrizol 1371, Lubrizol)………2 Antifoaming agent (Viscoplex14-520, Evonik)……200ppm Characteristics: Kinematic viscosity @40C=3.6cSt; Specific gravity=0.83gcm-3 aqueous fluid Phosphoric acid, 85% aqueous solution……………………………………15g Zirconium phosphate, powder………………………………30g Zinc nitrate hexahydrate, powder………………………………15g Water…………………………………………………………940g

[0045] Typical solvent densities are between 0.5 and 1.5 g / cm 3 is within the range.

[0046] The solid lubricant precursor additives are, as mentioned above, surface-reactive compounds that act as carriers for at least one of S, P, B, and at least one refractory metal. These are the types of precursors that are operable for the conventional Triboconditioning® process.

[0047] In one embodiment, the surface-reactive compound that serves as a support for at least one refractory metal is a salt and / or an organic complex.

[0048] In one embodiment, the refractory metal is Mo and / or W.

[0049] In further embodiments, when the surface-reactive compound comprises W, the surface-reactive compound serving as a support for at least one refractory metal is preferably a simple tungstate, a thiotungstate, a tungsten dithiocarbamate, a tungsten dithiophosphate, a tungsten carboxylate and dithiocarboxylate, a tungsten xanthate and a thioxanthate, a polynuclear tungsten complex containing carbonyl, cyclopentadienyl, and sulfur as ligands, a halogen-containing complex of tungsten with pyridine, bipyridine, nitrile, and phosphine as ligands, and / or an adduct of tungstic acid with fatty acid glycerides, amides, and amines.

[0050] In a further embodiment, when the surface-reactive compound comprises Mo, the surface-reactive compound serving as a support for at least one refractory metal is preferably a simple molybdate, a thiomolybdate, a molybdenum dithiocarbamate, a molybdenum dithiophosphate, a molybdenum carboxylate and dithiocarboxylate, a molybdenum xanthate and thioxanthate, a polynuclear molybdenum complex containing carbonyl, cyclopentadienyl, and sulfur as ligands, a halogen-containing complex of molybdenum with pyridine, bipyridine, nitrile, and phosphine, and / or an adduct of molybdic acid with fatty acid glycerides, amides, and amines.

[0051] In one embodiment, the surface-reactive compound serving as a carrier for at least one of S, P, and B is a surface-reactive compound serving as a carrier for S. The surface-reactive compound serving as a carrier for S can be elemental sulfur in one embodiment. Alternatively, or in combination, the surface-reactive compound serving as a carrier for S is an organic sulfide and / or an organic polysulfide. Preferably, the surface-reactive compound serving as a carrier for S is dibenzyl disulfide, sulfurized isobutene, sulfurized fatty acid, and / or dialkyl polysulfide.

[0052] The refractory metal and S, P, and / or B form different compounds, possibly including iron from the article surface. Such compounds are typically easily shearable and therefore present advantageous solid lubricant properties. These compounds are typically relatively similar to, for example, molybdenum disulfide or tungsten disulfide, or similar compounds based on P or B. However, due to the heterogeneous conditions at the article surface and the possibility of also including elements originally present at the article surface in the chemical reaction, the compounds formed may be of various uncharacterizable compounds that are bound to the article surface with various strengths.

[0053] It is also possible to use materials that are supports for both the refractory metal and S, P, and / or B. In other words, in one embodiment, the surface-reactive compound that serves as a support for at least one refractory metal and the surface-reactive compound that serves as a support for at least one of S, P, B are both thiocarbamates, thiophosphates, and / or thioxanthates.

[0054] Another group of solid lubricant precursors includes the oil-soluble metal carboxylates. Overbased carboxylates of copper and zinc have been shown to produce solid lubricant compounds such as metal sulfides, particularly when used in combination with sulfurized additives such as sulfurized fats, sulfurized acids, dialkyl and diallyl polysulfides, sulfurized olefins, and the like.

[0055] The basic idea of ​​this technique to provide high energy levels on a limited surface area of ​​an article is to utilize the collision of a large amount of impacting medium particles with the article to be treated. The collision is caused by creating a velocity difference between the impacting medium and the article, crossing their respective paths. This can be obtained in different ways:

[0056] In one embodiment, a device for inducing the formation of a solid lubricant on an article includes an exposure vessel, an article holder apparatus for an article to be placed in the exposure vessel, and an apparatus for creating a velocity differential between a surface of the article and an impact medium. The exposure vessel has an inlet for a chemically reactive process fluid and an impact medium. The chemically reactive process fluid includes a solvent and a solid lubricant precursor additive. The solvent is a low volatility, high flash solvent. The impact medium includes non-abrasive hard particles. The additives of the solid lubricant precursor are surface-reactive compounds that act as carriers for at least one of S, P, B, and at least one refractory metal.

[0057] FIG. 5A illustrates one possible realization of an embodiment of the device 1 for inducing the formation of a solid lubricant on an article. This embodiment is based on obtaining a velocity difference by vibration. The device 1 for inducing the formation of a solid lubricant on an article includes an exposure chamber 30 having an inlet 32 ​​for a process fluid 34 and an impact medium 20. The process fluid 34 includes a solvent and an additive. Only a small part of the impact medium 20 is illustrated. An article holder device 36 is arranged in the exposure chamber 30 to support the article 10 to be treated. The article holder device 36 is arranged to allow the flowing process fluid to reach the surface of the article 10. The device 40 for creating a velocity difference between the surface of the article 10 and the impact medium 20 in the exposure chamber 30 includes, in this embodiment, a vibration device 38. The vibration device 38, when activated, vibrates the article holder device 36 and its contents, the article 10, relative to the process fluid 34. In other words, the device 40 for creating a velocity difference includes a means for vibrating the article holder device 36.

[0058] FIG. 5B illustrates another embodiment of the device 1 for inducing the formation of a solid lubricant on an article. This embodiment is also based on obtaining a velocity difference by vibration. Most of the parts are similar to the previous embodiment. However, in this embodiment, the device 40 for creating a velocity difference between the surface of the article 10 and the impact medium 20 in the exposure chamber 30 includes two vibration device plates 39. The vibration device plate 93 is arranged to apply a vibration motion to the process fluid 34 in the exposure chamber 30 and, in particular, to the impact medium 20 provided therein. In other words, the device 40 for creating a velocity difference includes a means for vibrating the process fluid 34.

[0059] If the energy of the process is required to be further increased, the article holder device 36 may be movable within the exposure chamber 30 to force the article 10 through a vibrating impact medium.

[0060] In a further embodiment, the principles of FIGS. 5A and 5B may be combined to vibrate both the article holder device 36 as well as the process fluid 34.

[0061] FIG. 6A illustrates another embodiment of the device 1 for inducing the formation of a solid lubricant on an article. This embodiment is based on obtaining a speed difference by rotation. Most of the parts are similar to the previous embodiment. However, the exposure chamber 30 is now a cylinder that is made rotatable about a horizontal axis. In this embodiment, the device 40 for creating a speed difference between the surface of the article 10 and the impact medium 20 in the exposure chamber 30 includes a motor 41 arranged to rotate the exposure chamber and thus impart a rotational motion to the process fluid 34 and also to the impact medium 20 therein. In other words, the device 40 for creating a speed difference includes a means for rotating the process fluid 34.

[0062] FIG. 6B illustrates another embodiment of the device 1 for inducing the formation of a solid lubricant on an article. This embodiment is also based on obtaining a speed difference by rotation. Most of the parts are similar to the previous embodiment. However, the article holder device 36 is now connected to a rotatable shaft 45. A motor 41 now drives the shaft 45 by a drive means 43, resulting in the article holder device 36 being rotated in the exposure bath. In other words, the device 40 for creating a speed difference comprises a means for rotating the article holder device 36.

[0063] In a further embodiment, the principles of Figures 6A and 6B may be combined to rotate both the article holder device 36 and the process fluid 34 in opposite directions.

[0064] Rotation-based principles for achieving velocity differentials, i.e., centrifugal techniques, tend to deliver higher impact energy compared to vibration techniques. Efficient formation of solid lubricant material is typically relatively easy to obtain, especially when counter-rotating process fluid and article holder devices are used.

[0065] FIG. 7 illustrates another embodiment of the device 1 for inducing the formation of a solid lubricant on an article. This embodiment is based on obtaining a velocity difference by a directed particle flow. As a device 40 for creating a velocity difference, a means 44 for directing a medium flow against the surface of the article is used. Typically, a pressurizer combined with a nozzle can be used. The article holder device 36 can be stationary, for example, if a specific angle of incidence for the impact medium is required. Alternatively, the article holder device 36 can be displaced and / or rotated to expose different surfaces to the medium flow. In other words, the device 40 for creating a velocity difference includes a means 44 for directing a medium flow against the surface of the article.

[0066] In the rotation and vibration approach, a stochastic motion of the impacting media and article passing each other is achieved. This further results in a solid lubricant formation that is more or less stochastic with respect to direction. By using a directed impacting media flow around a fixed article, the flow conditions such as the impact angle and the selectively exposed surfaces can be controlled. The article can be mounted or rotated at a specific angle, for example, to preferentially expose different surfaces to the impacting media impact. This allows for better control over the entire process.

[0067] Different combinations of impact media, process fluids, and treatment environments can be used to achieve different outputs, depending on the desired characteristics for a particular application.

[0068] In terms of the method or process, in one embodiment the velocity difference has a non-zero component parallel to the surface of the article.

[0069] In one embodiment, the impact between the surface of the article and the impact medium is 1 MJ / m 2 s and preferably 5MJ / m 2 s.

[0070] In one embodiment, the impact between the surface of the article and the impact medium is 250 MJ / m 2 s and preferably less than 50MJ / m 2 s.

[0071] In one embodiment, the step of creating a velocity differential includes relative rotation between the process fluid and the article.

[0072] In one embodiment, the step of creating a velocity differential includes relative vibration between the process fluid and the article.

[0073] In one embodiment, the step of creating a velocity differential includes directing a stream of media against a surface of the article.

[0074] As a more detailed embodiment, a suitable process setup for carrying out the present treatment using a centrifuge barrel installation is now described below. The workpiece, i.e. the item to be treated, is placed inside the barrel together with the impact medium and reactive process fluids, including solvents and necessary additives. The combined filling rate of the impact medium and fluid inside the barrel is 50-90%, of which the impact medium filling rate is 20-80%, but preferably 40-60%.

[0075] The barrel rests on a cradle that is mounted on the turret. The turret rotates about a horizontal axis, resulting in a Ferris wheel-like motion with a one-to-one ratio of barrel rotation to turret rotation. Inside the barrel, the rotational motion induces collisions between the impact medium and the workpiece, changing the surface condition of the workpiece. Heat from the impact medium-workpiece collisions also stimulates chemical reactions between the workpiece and the process fluid. The amount of heat generated depends on the turret rotation speed, the impact member size and density, and the workpiece dimensions.

[0076] The process dynamics depend on the barrel and turret dimensions. For a barrel with a diameter of 20 cm, and a turret with a diameter of 60 cm, the turret must typically rotate at 150-220 rpm. 160-180 rpm has been found to be the optimum window for heavy density media (e.g., hardened metal carbides), while 180-200 rpm is preferred for media with lower density. The speed increases with the dimensions to keep the factor "diameter x rpm squared" constant. Processing times can vary from a few minutes up to an hour, but in most cases 10-30 minute intervals are targeted.

[0077] When evaluating the results of the method described herein above, different aspects such as surface roughness, compressive stress, and noise excitation were considered.

[0078] Surface roughness parameters are specified according to established standards for geometric dimensioning and tolerancing (GD&T) as defined by ISO / TC213. The presence of a tribofilm is established by applicable surface chemical analysis methods such as scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) or scanning electron microscopy with X-ray fluorescence (XRF). Residual stress measurements are performed using conventional X-ray diffraction methods.

[0079] The methods presented above not only provide a solid lubricant film, but also slightly modify the surface of the article. An example of a typical surface roughness profile modification resulting from the surface treatment methods described herein is shown in Figures 8A and 8B. Figure 8A illustrates a test pin surface profile before treatment, and Figure 8B illustrates a similar test pin surface profile after treatment.

[0080] As a result of the treatment, a significant decrease in the amplitude roughness parameters (Ra, Rz, Rpk, Rk) and the development of a plateau-like surface roughness profile characterized by an increasingly negative skewness (Rsk) should be highlighted. Apart from that, the gradient roughness is reduced, as evidenced by the drop in the Aq value measured by angle-resolved light scattering. In the boundary lubrication regime, which occurs at high loads, the surface asperities come into direct contact with each other and are deformed during the contact. The resulting friction coefficient is proportional to the energy required to deform the asperities. This energy is then proportional to the square of the deformation amplitude times the number of asperities per unit length. The deformation amplitude is related to the amplitude roughness (expressed by Rk) and the number of asperities per unit length is related to the gradient roughness (expressed by Rdq or Aq). Therefore, friction losses can be reduced by reducing both the slope and amplitude roughness.

[0081] Similar to the conventional Triboconditioning® process according to WO 2012 / 008890, which is carried out as a fixed tool machining operation, the novel method presented here also imparts surface compression due to plastic deformation in a subsurface material layer extending up to 10-20 μm in depth. This can be linked to the deformation hardening phenomenon obtained by shot peening, which is considered beneficial for component tribology (see e.g. K. Tosha, Influence of residual stresses on the hardness number in the affected layer produced by shot peening, Proc. 2 nd(Asia-Pacific Forum on Precision Surface Finishing and Deburring Technology, Seoul, Korea, July 2002, pp. 48-54.). In FIG. 9, compressive stress profiles of two examples of surface treatment according to the present technology based on rotational relative motion 201 and directed impact medium flow 202, respectively, are shown. Surface compression up to a depth of 10-20 μm can be seen. As a reference, the surface compression caused by shot peening 203 is also illustrated. The example using directed impact medium flow 202 had even higher compression than shot peening in the outermost part.

[0082] In one test, a bevel gear was treated with the goal of achieving noise reduction. This treatment resulted in a reduced surface roughness with a progressively negative skewness. Ra decreased from 0.65 to 0.49, Rz decreased from 3.67 to 2.85, Rpk decreased from 0.71 to 0.53, Rk decreased from 1.7 to 1.08, and Rvk decreased from 1.99 to 1.85. Furthermore, an increase in compressive stress from 900 to 1400 MPa was determined. These changes contributed to a significantly reduced noise excitation. At 60°C, the maximum noise level decreased from 68 dB to 63 dB.

[0083] In another test, micropitting and wear were investigated by using a twin-disc test on untreated articles and articles treated by the method described above. The results are illustrated in Figures 10A and 10B. In Figure 10A, a twin-disc test was performed on an untreated article, presenting an early and extensive production of micropitting, which corresponds to curve 204. At the same time, a high degree of wear was found, curve 205. A corresponding test was performed after treatment by the method described above, and the results are illustrated in Figure 10B. Compared to the untreated article, the time to onset of micropitting was increased (curve 206) and wear was reduced (curve 207).

[0084] High speed gear transmissions are articles that would benefit from treatment according to the present technology. For that purpose, aural tests were carried out. Standard A-profile gears with different surface finishes were used to carry out the FZG (Forschungsstelle fur Zahnrader und Getriebebau) scuffing load test A / 8.3 / 90 according to ASTM D 5182. Standard FZG gears (Ra 0.3-0.7um) were used as reference. Isotropically finished (ISF) gears (Ra 0.03um) and gears mechanochemically finished according to the principles presented here (Ra 0.1m) were investigated. Two different gear oil types were compared: a commercial SAE 75W-90 API GL-5 hypoid oil and a specially formulated viscosity-matched additive-free oil. With the GL-5 oil, no differences in the scuffing load were observed and all gears showed no scuffing up to the final load stage. However, the isotropically finished and mechanochemically finished gears demonstrated the lowest wear during the test, 10 and 8 mg compared to 43 mg for the reference. In the pure oil, both the reference and the isotropically finished gears failed at load stage 5, while the mechanochemically finished gears survived until stage 7. This shows that mechanochemical finishing makes it possible to match the wear behavior of isotropically finished gears while increasing the scuffing resistance in low-additive oils, such as dual-clutch transmission (DCT) fluids and e-fluids for electric vehicles. Mechanochemically finished gears according to the principles presented above are therefore particularly suitable for use in high-speed gear units.

[0085] The above examples provide clear evidence that the treatment according to the present technology provides clear advantages compared to what can be achieved by the prior art, e.g., classical abrasive mass finishing processes. First of all, the tribofilm created provides a low friction surface with improved wear resistance. Thus, the present method provides the full range of benefits provided by the conventional Triboconditioning® process, since the present process produces a solid lubricant tribofilm containing the same chemical structure as that disclosed, e.g., in WO 2012 / 008890. The compressive stress contributes to the beneficial tribological properties. Secondly, there is also a significant difference in the surface profile. A negatively skewed surface roughness profile with reduced gradient and amplitude roughness values ​​is obtained. In comparison, the abrasive process tends to reduce the amplitude roughness (Rt, Rz, Ra), but has little effect on the gradient roughness (Rdq or Aq). A multi-stage abrasive process using progressively finer abrasives can provide a negative skewness, but this comes at an additional cost.

[0086] The most prominent advantages of this method are: A low friction solid lubricant tribofilm is generated during the process by tribochemical reaction with the process fluid. The workpiece macro-geometry is adhered to within the available specifications. The surface roughness profile is modified by obtaining a negative skewness (Rsk=-0.5 to -3), reduced Rpk and Rk, and reduced gradient roughness (Rdq or Aq). Compressive stresses (negative residual stresses) are generated below the surface.

[0087] For optimal process stability and consistent quality, before the first use the impact medium should preferably be "activated" using a dedicated break-in sequence. A typical break-in sequence duration is 10-20 minutes under identical conditions to the actual treatment process. The only difference is that no components are loaded into the device, only the impact medium and process fluid are loaded, and the impact medium elements are activated by rubbing against each other. The process fluid does not necessarily contain additives. Because the break-in sequence generates a large amount of particulate matter, the impact medium should preferably be washed afterwards and new process fluid is filled into the device for the subsequent production run.

[0088] In other words, in one embodiment, the method for induced formation of a solid lubricant on an article includes a further step of running-in the impact medium prior to the step of immersing the article in the process fluid, the running-in step including creating a velocity differential between the impact medium in the process fluid in the absence of the article. This causes an impact between different objects of the impact medium.

[0089] Since the above described processes are prone to generating particulate contamination, mainly iron and metal ceramic fines originating from the impact medium and the parts being processed, a suitable fluid management system is preferred to achieve good process stability and part quality.

[0090] The simplest method of fluid regeneration has been filtration. Unfortunately, the use of conventional band and precoat filters may not produce the desired results, since such filters are quickly blocked by oleogel composed of fine particulate matter and additive degradation products. Instead, it has been found that the use of cascaded systems including settling tanks, or multiweir systems complemented by cyclones, does not have this drawback and allows the removal of heavy particles while leaving most of the gel in the fluid, thus minimizing additive depletion. After reaching the end of its life, the burnishing media must be regenerated and cleaned, and new fluid is filled into the unit. The latter operation is scheduled as planned maintenance. Under normal use, such a maintenance step is preferably carried out after approximately 100-200 processing cycles, or as soon as a deterioration in the surface finish quality is noticed.

[0091] In one embodiment of the device for inducing the formation of a solid lubricant on an article, the device further includes a settler and / or decanter for the process fluid being used in inducing the formation of the solid lubricant, and a cyclone fluidly connected to an outlet from the settler and / or decanter.

[0092] In a method aspect, in one embodiment, the method for induced formation of a solid lubricant on an article comprises a further step of post-treatment of the process fluid after the steps of creating a velocity differential and forming the solid lubricant material, The step of post-treatment of the process fluid comprises settling and / or decanting of the process fluid followed by treatment of the process fluid in a cyclone.

[0093] In one embodiment, the method for induced formation of a solid lubricant on an article includes the further step of separating the impact medium from the process fluid at least partially depleted of the solid lubricant precursor additive. The impact medium is regenerated. The impact medium is then preferably washed to remove remaining solvent. Preferably, the impact medium is washed in a solvent-based cleaner containing the same solvent and dispersant as used in the process fluid. This regenerated impact medium is reused with additional solid lubricant precursor additive in new solvent to form a new process fluid useful in a subsequent solid lubricant formation process.

[0094] The above described embodiments shall be understood as some illustrative examples of the present invention. It shall be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in different embodiments may be combined in other configurations, if technically possible. However, the scope of the present invention is defined by the appended claims.

Claims

1. A method for the induced formation of a solid lubricant on an article (10), comprising: - providing the article (10) to be treated (step S10); - exposing the article (10) to a chemically reactive process fluid (34) (step S20); wherein the chemically reactive process fluid (34) comprises a solvent and an additive of a solid lubricant precursor, the solvent is a low volatility, high flash point solvent, and the additive of the solid lubricant precursor comprises at least one surface reactive compound serving as a carrier for at least one of S, P, B, and at least one surface reactive compound serving as a carrier for at least one refractory metal, and an oil-soluble metal carboxylate combined with a sulfurizing additive and is at least one of them, the exposing step (S20) further comprises exposing the article (10) to an impact medium (20), the impact medium (20) being non-abrasive hard particles, the method further comprising - creating a velocity difference between the surface (12) of the article (10) and the impact medium (20) (step S30), thereby causing an impact between the impact medium (20) and the surface (12) of the article (10) to produce a vanishing effect; - forming a solid lubricant substance on the surface (12) of the article (10) by a chemical reaction involving the solid lubricant precursor induced by the energy of the impact in the presence of the chemically reactive process fluid (step S40); and the chemical reaction occurs at the surface (12) of the article (10).

2. The method according to claim 1, wherein the impact medium (20) is particles of a hardened metal carbide, a metal nitride, zirconium oxide, a ceramic, and / or a tungsten heavy alloy.

3. The method according to claim 1, wherein the impact medium (20) is particles of silicon nitride or tungsten carbide powder.

4. The method according to claim 3, wherein the impact medium (20) is particles of hardened tungsten carbide powder.

5. The method according to claim 3, wherein the particles of silicon nitride or tungsten carbide powder have an average size between 0.1 and 250 μm.

6. The method according to claim 1, wherein the speed difference has a non-zero component parallel to the surface (12) of the article (10).

7. The impact between the surface (12) of the article (10) and the impact medium (20) is greater than 1 MJ / m ２ s, and preferably greater than 5 MJ / m ２ s, creating an energy beam in the impact area of the surface (12) of the article (10), The method according to claim 1, characterized in that.

8. The impact between the surface (12) of the article (10) and the impact medium (20) is less than 250 MJ / m ２ 2, and preferably less than 50 MJ / m ２ 2 to create an energy beam in the impact area of the surface (12) of the article (10), The method according to claim 1, characterized in that.

9. The method according to claim 1, further comprising a step of conditioning the impact medium (20) before the step (S10) of exposing the article (10) to the process fluid (34), the conditioning step including creating a speed difference between the impact media (20) in the process fluid (34) without the article (10).

10. After the step (S30) of creating a speed difference and the step (S40) of forming a solid lubricant substance, the method further includes a step of post-treating the process fluid, the post-treatment step of the process fluid comprising: - sedimentation and / or decantation of the process fluid (34), followed by treatment of the process fluid (34) in a cyclone, the method according to claim 1.

11. - separating the impact medium (20) from the process fluid (34) in which the additive of the solid lubricant precursor is at least partially depleted; - regenerating the impact medium (20) by washing the impact medium (20) to remove the remaining solvent; - reusing the regenerated impact medium in a fresh solvent and with an additive of additional solid lubricant precursor to form a fresh process fluid (34) useful for subsequent solid lubricant formation processes The method according to claim 1, further comprising.

12. The method according to claim 11, wherein the regenerating comprises washing the impact medium (20) in a solvent-based detergent comprising the same solvent and dispersant used in the process fluid (34).

13. The method according to claim 1, wherein the surface reactive compound serving as a carrier for at least one refractory metal is a salt and / or an organic complex.

14. The method according to any one of claims 1 to 13, wherein the step (S40) of forming the solid lubricant substance is carried out by a chemical reaction further comprising a surface substance of the surface (12) of the article (10).

15. A device (1) for inducing the formation of a solid lubricant on an article (10), - An exposure tank (30) having an inlet (32) for a chemically reactive process fluid (34) and an impact medium (20), wherein the chemically reactive process fluid (34) comprises a solvent and an additive of a solid lubricant precursor, the solvent is a low volatility, high flash point solvent, the additive of the solid lubricant precursor is at least one surface reactive compound serving as a carrier for at least one of S, P, B, and at least one surface reactive compound serving as a carrier for at least one refractory metal, and an oil-soluble metal carboxylate combined with a sulfurizing additive and is at least one of them, the impact medium (20) is non-abrasive hard particles, the exposure tank (30), - an article holder device (36) for the article (10) disposed within the exposure tank (30), and - a device for creating a velocity difference (40) between the surface (12) of the article (10) and the impact medium (20), characterized by a device (1).

16. The apparatus for creating the speed difference (40) is 1 MJ / m ２ exceeding, and preferably exceeding 5 MJ / m ２ s, and is configured to create an energy beam in the impact area of the surface (12) of the article (10), characterized in that, the device according to claim 15.

17. The device for creating a velocity difference (40) includes means (41) for rotating at least one of the process fluid (34) and the article holder device (36), and is characterized by the device according to claim 15.

18. The device for creating a velocity difference (40) includes means (38, 39) for vibrating at least one of the process fluid (34) and the article holder device (36), and is characterized by the device according to claim 15.

19. The device for creating a velocity difference (40) includes means (44) for directing an impact medium flow against the surface (12) of the article (10), and is characterized by the device according to claim 15.

20. - A sedimentation device and / or a decantation device for a process fluid used in the induced formation of a solid lubricant, and - Further including a cyclone fluidly connected to an outlet from the sedimentation device and / or the decantation device, and is characterized by the device according to any one of claims 15 to 19.

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