Tribological Properties Of Plasma Electrolytic Oxidation Composite Lubricating Coating Deposited On Titanium Alloy Surface

Due to the low hardness and poor sliding friction properties,the application of titanium alloys in friction and wear-related components is restricted in the aviation and aerospace fields.Especially,titanium alloys will become thermally unstable at elevated temperatures?>500 ??and their mechanical and frictional properties get severely aggravated.In present work,the antifriction and wear resistant coating for titanium alloys using over a wide temperature range were designed on the basis of hard ceramic framework,surface texture morphology and multicomponent synergistic lubrication.The hard and wear resistant framework was prepared by plasma electrolytic oxidation?PEO?process.The electrolyte compositions and electrical parameters were optimized in present work.The PEO/solid lubricating composite coatings were deposited using one-step and multi-step process,wherein the graphite/MoS₂ nanoparticles and the hard PEO coating acted as lubricants and wear resistant framework,respectively.Regular dimple arrays were fabricated on the Ti6A14V substrate by laser surface texturing?LST?method.The storage and load bearing for lubricating films,which was provided by the combination of surface textures and hard ceramic framework,were investigated.The Ag/MoS₂ multilayered lubricating coating was prepared on the porous surface of PEO coating and the synergistic lubrication of multicomponent lubricants was used.On one hand,the MoS₂ and soft metal Ag could provide lubrication at room and medium temperatures.On the other hand,the diffusion of stored Ag and the formation of authigenic lubrication oxide were responsible for the lubrication at high temperature.The tribological properties of PEO coatings and their related composite lubricating coatings were evaluated at RT to high temperature?600??.The combined and crossed influences of hard ceramic framework,surface texture and solid lubricants on tribological properties were discussed.The formation,failure and evolution of solid lubricants on the friction surface were elaborated.The surface and sectional morphologies of coatings as well as the worn surfaces of frictional pairs were observed by the optical microscope and fieldemission scanning electron microscope.The 2D and 3D morphologies of textured dimples were observed by a contact-type profilometer and a confocal microscope.The phase constitutions of coatings were detected by X-ray diffraction.The element distribution and their chemical states of coatings and related worn surfaces were analyzed by EDS,Raman and XPS.The results showed that the constitutions,quality and properties of PEO coatings were influenced by the electrolyte composition and the discharged energy of single pulse during each cycle.The optimal quality and growth rate of the PEO coatings were obtained using the optimized electrolyte?NaAlO₂ 12g/L,Na₃PO₄ 1.6g/L,NaOH 0.3g/L?and electrical parameters?voltage 450-500V,pulse frequency 400Hz,duty ratio 20%-30%?.The coating having high thickness and low porosity mainly consisted of Al₂TiO₅ phase and the contents of?-Al₂O₃ and ?-Al₂O₃ with high hardness were also increased.The hardness of optimal coating was 2.5 times higher than that of Ti6Al4V substrate,which led to the higher load bearing capacity and wear resistance.The wear rate was in the order of magnitude?10-5mm3/?Nm?,which was one order of magnitude lower than that of titanium substrate.The lubricating films deposited on LST/PEO duplex-treated surface by multi-step process showed better tribological properties than the PEO composite coatings containing nano-solid lubricants prepared by one-step process.For the one-step process,the addition of graphite and MoS₂ nanoparticles provided benefit for the increase of compactness and wear resistance of PEO coatings.The composite coating got a lower friction coefficient?0.45-0.5?as the additive content was increased tolOg/L.For the multi-step process,the MoS₂ films could remarkably decrease the friction coefficients of PEO coatings?0.35-0.38 for dipping films and 0.1-0.25 for burnished films?.A new surface dimple texture consisting of large textured dimples?diameter 200?m?and small discharged dimples?diameter 5-9?pm?was fabricated on Ti6A14V substrate after the LST/PEO duplex treatment.On one hand,the new dimple texture could enhance the storage of solid lubricants and provide traps for wear debris.On the other hand,the PEO coating could provide a hard layer for load bearing and a porous surface for good adhesion.The two aspects were helpful for prolonging the wear life of lubricating films.The increase of textured dimple density from 8%to 55%and the decrease of surface roughness from Ra=2.9?m to Ra=1.0?m could enhance the storage of solid lubricants and were helpful for transferring stored MoS₂ lubricants to the sliding surface and forming new lubricating films,whilst the abrasive wear of lubricating film inducing by the hard wear debris was also decreased.The above-mentioned aspects led to a fact that the wear life of MoS₂ films was increased more than 6 times.The Ag/MoS₂ multilayered lubricating coating deposited on porous PEO coating showed adaptive-lubricating behavior and high wear resistance over a wide temperature range?RT-600??.The electroplating Ag started to nucleate at the discharged dimple regions and filled the dimple defects of the PEO coating,giving rise to the improvement of wear resistance at high temperature.The Ag and MoS₂ with low shear strength could provide effective lubrication at RT and medium temperature?350??,which led to a low friction coefficient?0.1-0.2?.The friction coefficient sharply decreased to a low level?0.2 at high temperature?600??,which was attributed to a combined effect of the diffused Ag from discharged reservoirs,the presence of high temperature lubricants?Ag2MoO4?and the formation of lubricants/transfer lubricating layers contact.