Humidity Dependence On The Tribological Properties And Mechanism Of Annealed Polycrystalline Diamond

The polycrystalline diamond?PCD?is widely used in cutting tools and geological drilling tools for its high hardness,good impact toughness and excellent wear resistance.In the processes of drilling and cutting,the PCD drill bits are used in the complex boundary lubrication environment composed of water,drilling fluid and drilling mud,and the terrestrial heat and local flash temperature of the asperity will accelerate thermal damage.The structural changes and interface physical and chemical reactions of the PCD caused by the humid and heat working conditions affect its tribological properties and wear mechanism.In this paper,the PCD was annealed at 700 ? in ambient air,and the surface of the annealed sample was graphitized and oxidized,and a large number of micro-spalling pits appeared.The tribological properties of the 700 ? annealed PCD/Si₃N₄ ball were investigated by the CSM tribometer in 5%-99.9% relative humidity?RH?,and the wear morphology of samples was analyzed.Based on SEM,EDS,Raman and XPS analysis,the wear mechanism of the 700 ? annealed PCD/Si₃N₄ in different humid environment was studied.The results showed that,?1?The average friction coefficient of the 700 ? annealed PCD/Si₃N₄ increases with reletive humidity increasing.In the 5%-99.9% RH,the wear rate of the 700 ? annealed PCD decreases with the increased relative humidity,and almost no wear occurs in 70%-99.9% RH.The wear rate of Si₃N₄ ball decreases firstly and then increases with the increasing of relative humidity,and achieves the lowest wear rate in 30% RH.?2?Seriouse graphitization of the 700 ? annealed PCD occurs in the low relative humid environment.The micro-diamond grains spalled from the 700 ? annealed PCD surface deposite on the wear scars of Si₃N₄,forming a carbonaceous transfer film.The anti-friction transfer film leads to the low friction of 700 ? annealed PCD/Si₃N₄ and severe wear of the 700 ? annealed PCD.?3?The 700 ? annealed PCD is more likely to form a carbonaceous transfer film on the Si₃N₄ wear surface than the original sample,and the formation of the transfer film starts in the run phase,and finishes during the whole tribological process.?4?The water molecules result in oxidative hydrolysis of Si₃N₄ balls in high relative humid environment,and the silicic acid produced by the action fills in the micro-spalling pits on the 700 ? annealed PCD surface,resulting in the reduced surface roughness of sample,which protects the 700 ? annealed PCD from seriously wearing.The micro-diamond grains spalled from the 700 ? annealed PCD surface embed in wear surface of Si₃N₄ balls,which hinders the formation of completely covered silicic acid layer on the wear scar of mating balls.The tribochemistry reaction dominates the wear behaviors of Si₃N₄ in high relative humid environment.