| Nowadays, the researches on the tribological characteristics of Si3N4-hBN composite ceramics become more and more. But, the research results are not systematic and uniform. At present, the investigations were mainly concerned in the influence of load or speed on the tribological characteristics of Si3N4-hBN composite ceramics. However, there are few investigations on the influence of temperature and relative humidity on the tribological characteristic. This thesis, which is a part of the National Natural Science fund project (51405278), aimed at study the influence of external environment factors (such as temperature and relative humidity) on the tribological properties of Si3N4-hBN composite ceramics.In this paper, the friction and wear tests were carried out by means of MMW-1 vertical universal tester, in which an upper pin mated with a bottom disc. The test is under the different temperatures (25℃、50 ℃ and 150 ℃) and the different relative humidity (30%RH、60%RH and 90%RH). Other test parameters are at a speed of 0.87 m/s, under the distance of 1000m and at the load of 10 N. Meanwhile, hBN volume contents increased from 0% to 30%. The worn surface morphology and composition of tribo-couples were conducted by using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometer (EDS). And the surface profile and roughness were analyzed by Laser Scanning Microscope (LSM). The element chemical structure of tribology surface was analyzed by X-ray Photoelectron Spectrometer (XPS). The results can be described as followings.(1) The relative humidity had a significant effect on the tribology performance of the Si3N4-hBN/austenitic stainless steel tribo-couples. The results showed that at the relative humidity of 60%RH, the tribological performance of Si3N4-hBN/austenitic stainless steel (ASS) was the best. The friction coefficient of Si3N4-10%hBN/ASS tribo-couple at 60%RH was as low as 0.03. At the relative humidity of 30%RH, the friction coefficients of Si3N4-hBN/ASS composite ceramics tribo-couples were around 0.27～0.4, and the wear coefficients of pins were all higher than 10-5mm3·N-1m-1 magnitude. At the relative humidity of 60%RH, Si3N4-10%hBN/ASS composite ceramics tribo-couples showed the lowest friction coefficient and wear rate. At relative humidity of 90%RH, the friction coefficients of Si3N4-hBN/ASS composite ceramics tribo-couples were around 0.26～0.14, which was higher than the friction coefficients under 60%RH, and the wear coefficients of pins also were higher than 10-5mm3·N-1m-1 magnitude.(2) The best tribological properties of Si3N4-10%hBN/ASS were obtained under the humidity of 60%RH, and the wear mechanism was dominated by the chemical wear. The existence of water molecule greatly increases the chance of wear debris hydrolyzed and oxidized, which could avoid happiness of the abrasive wear. Fe element, B element and Si element of friction surface were hydrolyzed and oxidized. The surface film containing B2O3 and SiO2 could effectively reduce the direct contact of ceramic and metal to decrease the friction coefficients and wear coefficients. As a result, the addition of hBN into Si3N4 has a good effect on the tribological performance.(3) The temperature also had a significant influence on the tribology performance of the Si3N4-hBN/austenitic stainless steel tribocouples. With the temperature increasing, the friction coefficients of the Si3N4-hBN/ASS tribo-couples showed an obviously changeable trend. At the room temperature, the friction coefficients of Si3N4-hBN/ASS tribo-couples were around 0.27～0.4, which shows that the influence of solid lubricant hBN on the tribological characteristics for silicon nitride based ceramics at room temperature was not obvious, and the wear coefficients of pins were higher than 10-5mm3·N-1m-1 magnitude. At the temperature of 50 ℃, the tribological properties were all not satisfying. And the maximum friction coefficients obtained for Si3N4/ASS tribo-couple was 0.87, and the wear coefficients of pins were 7.1 X 10-4mm3·N-1m-1. Also, at temperature 150 ℃, the tribological properties were improved with the solid lubricant hBN content in the Si3N4-hBN composite ceramics tribo-couple increasing. The maximum friction coefficients for Si3N4/ASS group was 0.72, the minimum friction coefficients obtained for Si3N4-20%hBN/ASS was 0.1. The result showed that the anti-friction effect of the solid lubricant hBN under high temperature was obvious.(4) Under temperature 150℃ condition, the surface of the Si3N4-20%hBN pin in Si3N4-20%hBN/ASS tribo-couples was smooth without apparent grinding grain production. At the same time, there is a uniform smooth surface film forming on this surface. The wear mechanism was dominated by the friction chemical wear. In addition, the tribo-chemical product from the wear debris on the friction surface effectively improves the tribological performance of the Si3N4-hBN ceramics composite. The tribology reaction rate was accelerated because of high temperature. The surface film on the surface under temperature 50 ℃ was more than the oxidation product on the surface under normal temperature. Simultaneously, the tribology reaction product SiO2 had larger plastic deformation ability at high temperature because it had lower hardness than S13N4. Plastic deformation may reduce the brittle fracture of the wear particles and strengthen the adhesion effect of the wear debris. In this case, the abrasive wear degree was weakened.(5) The friction chemical reaction, which occured in friction surface of Si3N4-hBN/ASS, was analyzed by thermodynamic analysis. The Gibbs free energy of friction chemical reacting equations were calculated, and the results showed the Gibbs free energy were both negative at 25 ℃ and 150 ℃. And, the friction chemical reaction is furthermore confirmed to be spontaneous. And, the reaction products SiO2, B2O3 and H3BO3 were all the good lubricants. |
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