Study On Tangential Fretting And Impact Wear Properties Of HVOF Sprayed CoMoCrSi Superalloy Coatins

Co-based superalloy coatings are extensively used in the related components of steam turbines to improve their service life due to their excellent corrosion resistance,oxidation resistance and wear resistance.During the service of steam turbines blades,their working conditions are very complicated and harsh,and they are affected by high temperature,high pressure,fluid,stress,and impurities.Corrosions(oxidation corrosion,cavitation,etc.)and mechanical damages(fretting wear,fretting fatigue,sliding wear,impact wear,etc.)may occur in key parts such as turbine blades and valves,which may cause breakage,even lead to failure in serious cases.Especially for fretting wear,this type of wear is difficult to discover.Once it is discovered,the damage can be very serious,even catastrophic,resulting in immeasurable economic losses.In this paper,in view of the fretting wear and impact wear of the turbine blade coating during service,a Co-based coating was prepared on the surface of the Ni-based substrate by high velocity oxygen fuel(HVOF).The structure and properties of the Co-based coatings were systematically analyzed by means of scanning electron microscopy(SEM),X-ray diffraction(XRD)and Vickers microhardness tester,etc.The tangential fretting wear and impact wear properties of the coating were investigated by a new high-temperature tangential micro-motion device and energy-controlled mode impact wear equipment using ball-to-plane contact mode.The microscopic morphology of wear scars were analyzed by and optical microscopy(OM),SEM and EDS.A white light interferometer was used to obtain the profiles and volumes of the wear scars.The mechanisms of fretting wear and impact wear of Co-based coatings are systematically revealed and the following conclusions are obtained:1.The spraying process parameters have certain influence on the microstructure of the coating.Under different spraying parameters,the deposition rate of the powder,the porosity and cracks of the coating are obviously different.However,within the range of the designed spray process parameters,the porosity of each coating is very low,which is less than 8%.The spraying process parameters have no significant effect on the hardness distribution of the coatings and their tangential fretting wear performance.According to the evaluation of microstructure and wear volume,the optimum parameters of spraying process can be obtained,which include a powder feed rate of 6.5 RPM,a fuel flow rate of 6.0 GPH,an oxygen flow rate of 1950 SCFH,and a spraying distance of 380 mm.2.The tangential fretting wear properties of the coatings are significantly affect by temperature.When the temperature increases from room temperature to 400℃,the friction coefficients decreases obviously.Due to the significant adhesive wear and a large quantity of material transfer,the wear morphology of the coating changes from pit to bulge.At room temperature,the wear mechanism is abrasive wear,delamination and spalling.At elevated temperatures,adhesive wear become the primary wear behavior.Under different temperatures and different fretting cycles,the wear volume of the coating always far below that of the substrate,indicating that the coating has better fretting wear resistance relative to the substrate.3.During the single impact process,the impact contact force,the velocity absorption and the energy absorption increase with the increase of the initial kinetic energy of impact,but the change of the energy absorption rate has no obvious regularity.In addition,as the initial kinetic energy increases,the wear volume increases;but the wear rate increases when the impact velocity is lower than 130mm/s,while decreases higher than 130mm/s.When the initial kinetic energy is constant,as the number of impact cycles increases,the impact contact force and the return velocity decrease first and then increase,while the energy absorption rate increases first and then decreases.The wear volume continues to increase as the number of impact cycles increases,but at the initial stage of impact,the wear rate of the substrate is much larger than the value of the coating,and the wear rate continues to decrease.The wear rate of the coating keeps low relative to the substrate and relatively stable.Under different initial kinetic energy and impact cycles,the wear mechanism is mainly plastic deformation and spalling,and the wear volume of the coating is always lower than that of the substrate.