Study On Preparation And Tribological Behavior Of Ni₃Al Matrix Self-lubricating Composite Coating

The Ni₃Al intermetallic has higher melting point and better high-temperature mechanical properties than Nickel-alloy matrix,and has better toughness than ceramic materials,so it has been considered as a new material with important application value in the field of high-temperature structural materials.In this paper,a feed of micron spherical Ni₃Al-Cr₂O₃-Mo-Ag-BaF₂/CaF₂ self-lubricating composite powder was fabricated by high-energy ball milling combined with spray granulation methods.Then it was deposited on the steel surface by plasma spraying methods to form a self-lubricating coating.The friction and wear properties of the Ni₃Al matrix self-lubricating composite coating at different temperatures and loads and counterpart balls in atmosphere were tested on a HT-1000pin-on-disk type high-temperature tribometer.The main findings are as follows:The tribological performance were carried out by sliding the Ni₃Al matrix self-lubricating composite coating disk against Inconel 718 pin,the testing temperature range was from 25℃to 800℃.The results showed that the coating had excellent antifriction and wear resistance in a wide temperature range,with the friction coefficient between 0.14 and0.42 and the wear rate between 2.41 and 5.76×10^-4 mm³/（N·m）.The anti-temperature yield effect of Ni₃Al intermetallic compound at high temperature ensured the wear resistance of the coating.And its good self-lubrication performance in a wide temperature range is attributed to the"precipitation effect"of the lubricants Ag and BaF₂/CaF₂,as well as the"co-lubrication effect"of NiO,Ag₂MoO₄,NiCr₂O₄ and other high-temperature lubricating phases generated by high-temperature friction reaction.The tribological behaviors of the coating at different temperatures（25℃to 800℃）and different loads（5 N,10 N and 20 N）were evaluated by a pin-on-disk HT-1000 friction-meter.SEM,EDS and Raman techniques were adopted to study the lubrication and wear mechanisms of the coating tested at different loads,by analyzing the microstructure and phase content of the wear debris,wear scars of the coating and the counterpart pins.The results indicated that within temperature range of 25℃-200℃,the increase of load from 5 N to 10 N accelerated the precipitation of the lubricant resulting from the synergism between the friction heat and applied load,providing a friction and wear reduction.However,with further increasing of the load to 20 N in this temperature range,the coating was deformed plastically,leading to“blocking effect”and thus increasing of the friction and wear.From400℃to 600℃,the oxidation process was accelerated by the friction heat caused by the increased loads,which decreased the shear strength of the worn surfaces,then reduced the friction and wear.At 800℃,the worn surfaces tested at all the loads were covered by a continuous and oxidized glaze layer,which enriched with NiCr₂O₄,Ag₂MoO₄ and NiO.The distinction was that when the load of 20 N is applied,the local high stress will destroy the integrity of the lubricating film,thereby decrease the tribological properties.The tribological performance of the coating against different counterpart balls（316L,Al₂O₃,and WC）within temperature range of 25℃to 800℃are significantly impacted by the mechanical properties and wear chemistry of counterpart.From 25℃to 400℃,the friction coefficient and wear rate decreased with an increasing of temperature.Due to the high hardness of the WC ball at 25℃,the coating undergoed severe plastic deformation,which inhibited the precipitation of the Ag lubrication phase;But with the increase of temperature,the red hardness of WC and the temperature work together to promote the formation of a part of NiO lubrication film on the worn surfaces,which resulted in the friction coefficient of the WC system in the temperature range decreased the most.From400℃to 600℃,the soft metal Ag was oxidized and lossed lubricity,BaF₂/CaF₂ eutectic brittle-plastic transition was incomplete,and the content of NiO,NiCr₂O₄,Ag₂MoO₄ and BaMoO₄ lubrication phases generated by high temperature oxidation and friction-induced reaction were relatively low.As a result,the friction coefficient in this temperature range all showed an upward trend.From 600℃to 800℃,BaF₂/CaF₂ eutectic brittle-plastic transition was more complete with the increasing temperature,and the glazed layer contained a large number of high temperature lubrication phases,such as NiO,NiCr₂O₄,Ag₂MoO₄ and BaMoO₄ formed on the worn surfaces.At the same time,316L ball had a higher affinity with the coating and formed a discontinuous transfer film;WC ball undergoed high temperature oxidiation and formed a partial oxide film.The transfer film and the oxide film effectively blocked the damage of the coating,and the wear rate of the coating were significantly reduced to 0.03×10^-4 mm³/（N·m）and 0.09×10^-4 mm³/（N·m）.