Preparation And Tribological Properties Of Nano Boron Nitride/Silicon Nitride/PTFE Composite Coating

Polytetrafluoroethylene(PTFE)has the characteristics of stable chemical properties,high and low temperature resistance,good self-lubricating performance and extremely low friction coefficient.It is the latest engineering plastic at present,but its poor wear resistance limits its application in many fields.In order to apply polytetrafluoroethylene to more occasions,it is necessary to study and improve the tribological properties of polytetrafluoroethylene coating.In response to the above problems,this thesis prepared PTFE composite coatings by optimizing the ratio of each component,and filled nanoparticles to further improve the tribological properties of PTFE composite coatings and systematically studied the effects of nanoparticles mechanism of action in the coatingFirstly,the composition optimization design of PTFE composite coating was carried out.Based on PTFE,this paper used PAI as the binder,Al₂O₃ as the filler,and then developed a three-factor three-level orthogonal test table with the evaluation index including Vickers hardness,contact angle,adhesion,impact resistance.The test results were put into the orthogonal test table,and the range analysis method was used to analyze the influence of each factor on the index,so as to select the best combination scheme:PTFE(45 wt.%),Al₂O₃(15wt.%),PAI(15 wt.%).Secondly,the effect of nano-h-BN on the PTFE composite coating was studied.The test results show that with the continuous increase of h-BN content,the microhardness of the composite coating continues to decrease.In terms of friction coefficient,the friction coefficient of PTFE composite coating decreases first and then increases with the increase of h-BN content,and the optimal friction coefficient decreases by 23.7%compared with the case without adding h-BN.In terms of wear rate,when the mass fraction of h-BN reaches the optimal value,the wear rate is reduced by 69.0%.When too much h-BN is added,the wear rate of the PTFE composite coating increases instead.The main reason is that h-BN has good lubrication.When h-BN falls off and enters the friction pairs under the action of external force,it plays a role of lubrication.However,if too much nano-h-BN is added,the phenomenon of clusters will appear,the density of the coating will be affected,and the wear rate will increase.Then,the influence of nano Si₃N₄ on the PTFE composite coating was studied.In order to uniformly disperse silicon nitride into the composite coating,the surface of Si₃N₄ was modified.The surface morphology(SEM),Fourier transform infrared spectroscopy(FT-IR),and thermogravimetric analysis(TGA)were used to characterize the nano-silicon nitride before and after the modification,which proved that KH-570 has been successfully connected to the surface of nano-Si₃N₄.The hardness of the composite coating increases first and then decreases with the increase of the nano-Si₃N₄ content,showing an inverted"V"shape.The optimal value of microhardness increased by 13.3%compared with the case without Si₃N₄.The friction coefficient and wear rate of the composite coating first decrease and then increase with the increase of the nano-Si₃N₄ content,showing a"V"shape.At the optimal value,the wear rate is reduced by 70.0%compared with no Si₃N₄.The main reason is that the addition of nano Si₃N₄can fill the gap of the composite coating,the coating will be more dense,and at the same time improve the hardness of the coating.As a rigid support point,it gives priority to carrying load,and has the role of helping to dispersion and strengthening.However,when excessive Si₃N₄ is added,the surface performance of the composite coating becomes worse,mainly because it is difficult for Si₃N₄ to be uniformly dispersed in the composite coating,resulting in stress concentration and decreased tribological properties of the coating.Finally,this paper studied the effect of nano-h-BN/nano-Si₃N₄ on the PTFE composite coating.It is found that h-BN acts as a solid lubricant and Si₃N₄ as a rigid support point has a good synergistic effect.When h-BN:Si₃N₄=1:1.5,the friction coefficient reaches the lowest value,which is reduced by 21.9%.When h-BN:Si₃N₄=1.5:0.5,the hardness reaches the maximum value,and the wear rate also reaches the lowest,which is 83.3%lower than that of the composite coating without nanoparticles and 44.5%lower than that of the silicon nitride composite coating alone.The tribological behavior of h-BN:Si₃N₄=1.5:0.5 composite coating at high temperature was studied.It was found that the friction coefficients of the composite coating at 120?,200?and 300?were only 4.5%,17.3%and 2.1%different from those of the mature non-sticky coating on the market.The wear rate at 120?,200?,300?is 11.1%,8.7%,15.6%higher than that of mature non-sticky coating on the market,which provides scientific guidance for the application of PTFE composite coating at high temperature.