Effect Of Liquid PH On Tribological Properties Of Polycrystalline Diamond/Silicon Nitride Friction Pairs

Because of its excellent mechanical properties and chemical properties,such as high strength,good toughness,good thermal conductivity and good corrosion resistance,polycrystalline diamond(PCD)is widely used in geological drilling or cutting processes.Super hard tools and geological drills,and polycrystalline diamond drills or cutting tools will be completely immersed in the drilling fluid or cutting fluid during service,due to the complexity of the liquid composition,especially the pH of the liquid,for polycrystalline diamond,drilling rock or cutting material occured physical and chemical reactions on the surface,which affects its friction and wear properties.This paper mainly explores the friction and wear properties of PCD on silicon nitride milled at different pH values of 0 to 13.5.The friction and wear tester was used to perform friction and wear tests under different pH conditions.The friction coefficient,surface morphology of silicon nitride,and Raman and XPS testing methods were combined to investigate the friction and wear mechanism between polycrystalline diamond and silicon nitride,research results show:(1)Under different pH(0-7)HCl solution conditions,when the pH value of the solution is in the 0-5,the friction coefficient of each pH value is lower than 0.07;with the increase of pH value,firstly the friction coefficient decreases.When the pH value is 0.5,the friction coefficient is reduced to a minimum of 0.009;when the pH value of the solution is greater than 0.5,the friction coefficient gradually increases,but when the pH value is in the range of 2.5-5,the friction coefficient changes little in each state.In addition,silicon nitride wear increases with the pH,the size of the silicon nitride ball wear spot will increase firstly and then decrease.When the pH value is 2.5,the wear spot is the largest,and there are some furrows and wear debris on the surface of the wear spot.(2)Under the condition of different pH(7-13.5)NaOH solution,the friction coefficient decreases gradually with the increase of pH value,when the pH value is 13,the friction coefficient decreases to the minimum value of 0.035;when the solution pH value is greater than 13,the coefficient of friction increases quickly,increasing to 0.086,probably due to the precipitation of sodium hydroxide in the solution;but when the pH value is in the range of 7-11.5,the friction coefficient in each state changes slightly,and they are all at 0.06.To about 0.07.The size of the silicon nitride ball wear spots increases firstly and then decreases with increasing pH.When the pH value is 10,the wear spot is the largest;there are some furrows and wear debris on the surface of the silicon nitride ball wear spots at different pH conditions.(3)Under acidic conditions,the hydrogen ions in the solution can undergo surface protonation reaction with the hydrolyzed products of silicon nitride and the polycrystalline diamond oxidation products,resulting in the formation of a double electron layer structure on the surface,lowering the friction coefficient,and hydrogen in the solution.When the concentration of ions decreases,C-OH and Si-OH bonds on the surface of the grinding subsidiary material are exposed in the partial region,forming a C-O-Si bridge at the interface and increasing silicon nitride wear;in alkaline conditions,due to high concentration Sodium hydroxide affects the viscosity of the solution,thereby reducing the friction coefficient and the wear of the silicon nitride.In addition,the bridging between the hydroxyl groups at the interface increases the wear of the silicon nitride.