Field of the invention

[0001] The present invention relates to antifriction coating formulation compositions, antifriction coatings formed from the compositions, and sliding members having the antifriction coatings.

Background of the invention

[0002] Antifriction coatings are known in the art to improve sliding properties of components used for industrial machines, construction machines and automobiles. Typical antifriction coating compositions comprise resin binders, solid lubricants and solvents. Solid lubricants work to reduce friction and wear of contacting surfaces in relative motion and provide protection from damage. Well-known solid lubricants include molybdenum disulfide (M0S2), graphite and polytetrafluoroethylene (PTFE).

[0003] Although antifriction coatings comprising molybdenum disulfide show excellent sliding properties, it is always desirable to continue to improve the wear properties. WO2016/073341 A discloses a connecting rod comprising a wear resistant coating. The wear resistant coating comprises a polymer matrix, solid lubricant and hard particles, wherein the solid lubricant is selected from molybdenum disulfide, graphite, tungsten sulfide, hexagonal boron nitride, polytetrafluoroethylene and metal sulfides. It can contain one or more solid lubricant. US7,368, 182B discloses a multiple coating layers to improve wear resistance.

[0004] Mixed-metal sulfides are known in the area of catalysis, such as WO2011/008513 A and US4,752,623B. These prior art references disclose cobalt-molybdenum disulfide, in which small amount of cobalt metal is incorporated in the parent M0S2 structure. In the use of catalyst, a second metal (i.e. cobalt) incorporated into M0S2 structure acts as a catalyst promoter. However, these prior art references do not mention about the use of the mixed metal sulfides as solid lubricants of antifriction coatings.

Summary of the invention

[0005] Disclosed herein are an antifriction coating formulation composition comprising: (a) a resin and (b) a metal sulfide comprising molybdenum and cobalt, and optionally (c) solid lubricant other than the metal sulfide and (d) a solvent. Such antifriction coating formulation composition can provide an antifriction coating which exhibit higher wear resistance. [0006] Also disclosed herein is a coated film formed from the antifriction coating formulation composition.

[0007] Further disclosed herein is a sliding member having a lubricating film formed from the antifriction coating formulation composition.

Brief description of the figures

Fig. 1 shows a geometry of the test setup of ball-on-plate wear test.

Fig. 2 shows a geometry of LFW-1 (block on ring) test.

Detailed description of the invention

[0008] The antifriction coating (AFC) formulation composition disclosed herein comprises at least two ingredients: (a) a resin and (b) a metal sulfide comprising molybdenum and cobalt, in which the molar ratio of molybdenum and cobalt in the metal sulfide is from 99 to 1 to 1 to 99

Resin (a)

[0009] Resin (a) used in the antifriction coating formulation composition works as a matrix polymer of a coating film described later. Examples of resin include

polyamideimide, polyimide, polyamide, epoxy resin, phenol resin, polybenzimidazole, polyphenyl sulfonate, polyether ether ketone, polyurethane, poly-butyltitanate, polyacryl- alkyd resin, polyetherketoneketone (PEKK), polyoxymethylene (POM), polybutylene terephthalate (PBT), fluoropolymers, and mixtures thereof. Preferred resin includes polyamideimide, (polyimide) and (polyamide), with polyamideimide most preferred.

[0010] Preferably, the resin present in the antifriction coating formulation composition ranges from 10 to 90 parts by weight with respect to 100 parts by weight of the solid contents of the antifriction coating formulation composition. More preferably, the resin content is from 20 to 80 parts by weight, and even more preferably from 30 to 70 parts by weight, with respect to 100 parts by weight of the solid content of the antifriction coating formulation composition. In this specification, the weight of the solid contents of the antifriction coating formulation composition means the total weight of the solid contents of the AFC formulation composition (i.e., resin, metal sulfide, solid lubricant and additional ingredients with solid form).

Metal sulfide (b) [0011] The metal sulfide used in the antifriction coating formulation composition comprises molybdenum and cobalt. Since the metal sulfide comprises at least two metals, it is also called as mixed metal sulfide. When the metal elements of the metal sulfide are cobalt and molybdenum, the metal sulfide can also be called cobalt-molybdenum disulfide, and can be described by the formula (Co,Mo)S2 or CoxMo _(i -x ₎ S2. In the formula, x is a number less than 1.

[0012] The molar ratio of molybdenum and cobalt in the metal sulfide ranges from 99 to 1 to 1 to 99. The molar ratio can be selected based on the required properties of the antifriction coating formulation composition comprising the metal sulfide. When the antifriction coating formulation composition comprising the metal sulfide is used for antifriction coatings with higher wear resistance, preferably the molar ratio of

molybdenum and cobalt ranges from Mo:Co 85: 15 to Mo: Co 98:2. In such molar ratio range, it is considered that a small amount of cobalt metal replaces the molybdenum metal of the parent M0S2 structure, so the wear resistance of a film comprising the metal sulfide is improved while the basic antifriction property is maintained. More preferably, the ratio of molybdenum and cobalt in the metal sulfide is from Mo:Co 85: 15 to 95:5, further more preferably, the ratio is from Mo:Co 90: 10 to 95:5.

[0013] The metal sulfide can be obtained by the method described in the following publications: 1) Cobalt molybdenum sulfide catalysts prepared by in situ activation of bimetallic (Co Mo) alkylthiomolybdates. Nava, et al, Catalysis Letters 2003, Vol. 86, No. 4, p. 257, and 2) The Role of Structural Carbon in Transition Metal Sulfides Hydrotreating Catalysts. Berhault, et al. Journal of Catalysis 2001, Vol. 198 (1), pp. 9-19.

[0014] The metal sulfide synthesized by this method produces a very dark solid with a platelet structure, similar in appearance to M0S2. The primary particle size of the metal sulfide tends to agglomerate in clusters preferably from 0.1 to 10 micrometers, more preferably from 1 to 6 micrometers. The size can be measured by a particle analyzer such as laser diffraction scattering, or it can be estimated from Scanning Electron Microscope (SEM) images.

[0015] The amount of the metal sulfide in the resin composition ranges from 10 to 60 parts by weight, preferably from 20 to 40 parts by weight, with respect to 100 parts by weight of the solid content of the antifriction coating formulation.

Solid lubricant (c) [0016] The antifriction coating formulation composition can optionally comprise solid lubricant (c). The solid lubricant is different from the metal sulfide (b) disclosed above. Non- limiting examples of solid lubricants include graphite, polytetrafluoroethylene (PTFE), polyethylene (PE) and mixtures thereof. Graphite is preferable.

[0017] The solid lubricants in the antifriction coating formulation composition described herein are typically platelet-like in structure, with these“sheets” sliding relatively easily against each other. The materials naturally cluster into larger agglomerates that are easily broken down into smaller particles during the preparation and mixing of the antifriction coatings. The average primary particle size of the solid lubricants is preferably from 0.1 to 10 micrometers, more preferably from 1 to 6 micrometers.

[0018] When the antifriction coating formulation composition comprises a solid lubricant, the amount of the solid lubricant ranges from 1 to 100 parts by weight, preferably from 5 to 50 parts by weight and more preferably from 10 to 30 parts by weight, with respect to 100 parts by weight of the solid content of the antifriction coating formulation composition.

Solvent (d)

[0019] The antifriction coating formulation composition can optionally comprise a solvent (d) for the purpose of improving coating properties. The solvent can be selected depending on the type of binder resin. Usable solvents include, for example, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; esters such as methyl acetate and ethyl acetate; aromatic hydrocarbons such as toluene and xylene;

alcohols such as ethanol, 2-propanol, diacetone alcohol (DAA); organic halogen compounds such as methyl chloroform, trichloroethylene and trichlorotrifluoroethane; N- methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), l,3-dimethyl-2- imidazolidinone (DMI), 3-methoxy-N,N-dimethylpropanemide, methylisopyrrolidone (MIP), dimethylformaldehyde (DMF), dimethylacetaldehyde (DMAC), and mixtures thereof. Preferred solvents are DMI, NEP and xylene.

Additional ingredients (e)

[0020] The antifriction coating formulation composition described herein may optionally include additional ingredients such as a UV absorber, a stabilizer, an antioxidant, a leveling agent, a deformer, a thickener, a pigment, a dye and a dispersant as long as the object of the present invention is not impaired. When present, the amount of additional ingredients will preferably range from 0.1 to 5 parts by weight, with respect to 100 parts by weight of the solid content of the antifriction coating formulation composition. [0021] Although the metal sulfide (b) of the composition comprises cobalt and molybdenum (CoxMo _(i -x ₎ S2), other metal sulfide (M _x Mo _(i -x ₎ S2, M is tungsten, tantalum or nickel) can be used.

[0022] The antifriction coating formulation compositions described herein may be prepared using methods known to those skilled in the art, for example, mixing the described ingredients using conventional apparatus in any suitable order. For example, dissolving resins and introducing the metal sulfide and other ingredients if present.

Coated film

[0023] The second aspect of the present invention relates to a coated film formed from the above-mentioned antifriction coating formulation composition. The film is formed by applying the composition described above onto the surface of a substrate and then heating it to cure the applied composition. The substrate can be metal, plastics, wood, elastomers, composites, etc. The coating can be applied to the surface by any conventional method, for example brushing, dipping and spraying. The coating thickness is determined from the required properties and the life of the film, but it is typically from 5 to 20 micrometers. Once the antifriction coating formulation composition is applied on the surface of a substrate, it is dried to evaporate the solvent (if applicable) and is cured to form a coated film. The curing process depends on the nature of the substrate and the kind of resin. For example, the cure can be conducted in an oven for 30 to 90 minutes at a temperature of between 100 to 280 degrees C.

Sliding member

[0024] The third aspect of the present invention relates to a sliding member having a lubricating film formed from the above-mentioned antifriction coating formulation composition. The sliding member can be selected from a swash plate of a compressor, an engine tappet, a camshaft, a crankshaft, an engine metal, an engine piston, an engine fastener, a slide bearing, a piston ring, a gear, a door lock, a brake shim or a brake clip.

EXAMPLES

Examples Series T Wear resistance tests

[0025] The raw materials shown in Table 1 were used to prepare compositions in the Examples. Table 1

Preparation of CoxMo _(i -x ₎ S2

[0026] Stoichiometric amounts of ammonium sulfide [(NELfkS] and ammonium heptamolybdate [(NH4)2Mq7q24-4H2q] were combined in water solution and stirred at 60°C for 1 hour (during which solids will completely dissolve). The resulting water solution was co-dripped with the stoichiometric amount of a water solution of cobalt acetate [Co(C2H302)2] from an addition funnel into an acetic acid solution at 60°C and allowed to stir for one hour. The resulting solid material |(NH4)4[Co(MoS4)3]} was filtered and dried at 80°C. The dry material was then placed into a purged nitrogen furnace, ramped up to 500°C, and held for about one hour to reduce the solid to the final sulfide product. After heating the furnace was allowed to cool down naturally while remaining under nitrogen atmosphere.

Characterization of CoxMo _(i-X) S2

[0027] As synthesized, CoxMo _(i-X) S2 show the same phases by powder X-ray diffraction as those present in the parent M0S2 structure; however, the peaks are weaker and broader because of a nanocrystalline structure. By scanning electron microscopy / electron dispersive spectroscopy (SEM-EDS), the cobalt is relatively evenly distributed over the grains, and particle sizes are estimated at roughly 2 microns or less. These appear to be clusters of a smaller primary particle size on the order of 100s of nm, and some large agglomerates are also present. It is likely that the large agglomerates are broken up in the antifriction coating formulation during the milling process.

[0028] Mixed metal sulfide were analyzed by X-ray fluorescence to get true stoichiometric ratios of Co:Mo. Data in examples are shown as rounded ratios for simplicity.

Test Methods

TEST 1 : Ball-on -plate wear test

[0029] Ball-on-plate wear test was conducted following ASTM G-133. A ½” diameter steel ball (11) was brought into contact with the anti-friction coating (21), which has been applied to a steel (or other material) coupon, with a force of 10N. The load was maintained throughout the test as the test sample was reciprocated back and forth with a stroke length of 4mm for a total of 10,000 passes (or 5000 cycles). The geometry of the test setup (1) from ASTM G-133 is shown in Fig. 1 for reference. TEST 2: LFW-1 test

[0030] LFW-1 test is another wear test frequently conducted on antifriction coatings, which follows ASTM-D 2714. This dry test is done at relatively high load (2860N), at 72 rpm for the coated test ring (Rockwell hardness 60); geometry is an upper block applying the load on the ring spinning on a shaft underneath. See Fig. 2 for the wear test geometry schematics.

Examples

[0031] Antifriction coating formulation compositions disclosed in Tables 2 and 3 were prepared and tested. Ingredients (resin, M0S2 or CoxMo _(i -x ₎ S2, solid lubricant, solvent and additive) were mixed by milling and subsequent filtration, then sprayed onto a substrate to make test films. The test films were heated at 80 degrees C for 10 minutes, followed by 230 degrees C for 1 hour, in order to cure the resin. Table 2

Table 3

Examples Series II: Wear life tests

[0032] Using the formulation of Samples 8 and 11, long term ball-on-plate tests were conducted. The film thickness of Samples 8 and 11 were 13.7 and 11.0 micrometers respectively. The formulation with the Sample 8 lasts much longer before failure than Sample 11. Control samples of Sample 11 were tested in winter and in summer in order to ensure that there was not a significant influence from large changes in relative humidity. Table 4

[0033] S:Mo ratios are also shown in Table 4. Using X-ray photoelectron spectroscopy, the ratios of sulfur to molybdenum were calculated from the peaks corresponding to the relevant bonding states and are corrected for relative sensitivity. These estimates are reasonable, as indicated by the values on the film surfaces of 1.94 (Sample 11) and 1.93 (Sample 8). The Sample 11 containing standard M0S2 indicates with all of the samples with high wear times or failure that the sulfur is present as sulfate, not sulfide. This is a key difference from the Sample 8 with mixed metal sulfide, as the sulfide :Mo ratios remain quite similar to the original film surface values. While it is still not completely understood, this difference does support the idea that the presence of the cobalt in the M0S2 structure could possibly delay or inhibit the oxidation that correlates with wear.