Study On Tribological Properties Of Polytetrafluoroethylene Filled With Boron Nitride/Polyimide

Polytetrafluoroethylene(PTFE)is widely used for sealing in the machinery manufacturing industry,petrochemical industry,aerospace industry,etc.due to its excellent self-lubricating properties,corrosion resistance and resistance to high and low temperatures.However,deformation of PTFE caused by excessive wear in high friction environments such as heavy loads at high speeds can easily lead to the seal failure.To improve the global performance of PTFE,filling modification is an effective method.This paper examines the mechanical properties,the thermal conductivity,the tribological properties and the thermal properties of modified PTFE filled with boron nitride(BN),graphene nanosheets(GN),molybdenum disulfide(MoS2),fluorinated graphite(FGr)and mixed with polyimide(PI).Then the modified PTFE parameters are applied to the combined seal structure for a seal analysis.The principal work and conclusions are as follows:(1)The PTFE grade M-18F was determined,and the effects of molding pressure,sintering temperature and cooling rate on the tensile properties of PTFE were investigated.It was found that when the sintering temperature and cooling rate are unchanged,the tensile strength of the specimen formed under 50MPa could reach 65MPa,the elongation at break could reach 380%,and the tensile mechanics was better.When the molding pressure and cooling rate are the same,the tensile strength of PTFE sintered at 360? is 74MPa,the elongation at break is 380%,and the tensile mechanical properties are the best.At unchanged molding pressure and sintering temperature,as well as the cooling rate of 50?/h,the tensile mechanical properties of the sintered sheet are optimal,while warping hardly occurs.Hence,the parameters of the subsequent forming process are as follows:the molding pressure is 50 MPa,the sintering temperature is 360? and the cooling rate is 50?.(2)Different composites such as MoS2/PTFE,BN/PTFE,GNs/PTFE have been prepared and the influence of the lubricating charge on the properties of PTFE has been investigated.The results of the tensile mechanics show that when the GN content is less than 1%wt,the tensile mechanical properties of the material increase.This is due to the fact that the graphene itself has a small particle size and a large specific surface area,which can fully exert the surface effect and effectively increase the interface strength of the composite material.In return,the tensile properties of the composite material increase.When the GN content is 1%wt,the tensile strength and elongation at break of the composite reaches 73MPa and 368%respectively.With BN and MoS2,the mechanical tensile properties of the composite further decrease.The thermal conductivity of the three composites has increased significantly.The wear resistance of the composites added with three kinds of lubricating fillers has been improved.The coefficient of friction of a small amount of NG(1%wt)added to PTFE is practically unchanged,and the wear resistance is multiplied by 20.The addition of BN makes the wear resistance of the composite increase while the friction coefficient decreases significantly.When the BN content is 10%wt,the friction coefficient of the composites is 0.19.The addition of MoS2 obviously increases the friction coefficient of composites.When the MoS2 content is 10%by weight,the coefficient of friction of the composites is 0.25,which is not suitable for reducing friction.The addition of three lubricating fillers to PTFE causes the expansion of the composites to show an upward trend.(3)The performance of PI/BN and FGr/BN respectively filled with PTFE was compared,and the mixing and wear mechanisms were analyzed.Tribological properties,mechanical properties,hardness,thermal conductivity and thermal properties of the composites were examined using the ring block friction testing machine,universal material testing machine,shore D hardness tester,thermal conductivity tester,DSC,etc..The results show that both PI/BN/PTFE and FGr/BN/PTFE composites have better mechanical properties and better thermal conductivity under the same reinforcing filler content(taking 15%wt as an example),which can meet the technical requirements.The hardness of PI/BN/PTFE composites is 63HD,which is obviously greater than 56HD of FGr/BN/PTFE composites.The DSC results indicate that the change in PI content has a minimal effect on the crystallization capacity of PTFE composites.The addition of FGr accelerates the crystallization rate and increases the crystallinity of the composites.The tribological properties of PI/BN/PTFE composites are considerably better than those of FGr/BN/PTFE composites.Under the optimal ratio of PI/BN/PTFE composite(15%wt PI),the friction coefficient of the composite is 0.187,the wear rate is 2.9×10-6 mm3/(N·m),which is nearly 300 times higher than PTFE and nearly 27 times higher than PTFE-BN composite.And it can be used at either low speed and high friction load(300N)or low load and high friction speed(300r/min).At the optimum ratio of FGr/BN/PTFE composite(5%wt of FGr),the coefficient of friction of the composites is 0.17,the wear rate is 181×10-6 mm3/(N·m),which is only 7 times higher than that of PTFE.(4)The structural design and finite element model analysis of the combination seal were performed using the parameters of the PI/BN/PTFE and FGr/BN/PTFE composites at the optimal ratio.It was observed that the maximum contact stress of the combination seal occurred at the lower part of the modified PTFE bushing the seal groove,and the size was 48.46MPa.The equivalent maximum mises stress occurring where the modified PTFE bushing contacts the O-ring,and the size is 22.61 MPa.The location of maximum mises changes with increasing pressure,while the location of maximum contact stress remains essentially unchanged.The magnitude of the stress value has a positive correlation with the sealing pressure.Material performance parameters have a great influence on the sealing performance of the combination seal.Combination seal using the parameters of PI/BN/PTFE composites cannot only meet the actual technical sealing specifications,but also have better sealing performance than FGr/BN/PTFE composites.