| This thesis is a part of National Natural Science Foundation (51405278). For sliding couples of Si3N4-hBN/metal under the air condition, a significant influence of external factors (sliding speed and load) and internal causes (metal struature and the content of hBN) on the tribology performance of the tribocouples have been obtained. In order to systematiclly explore the effect of external and internal factors on the tribological properties of Si3N4-hBN/metal tribocouples under the air condition, the friction and wear tests of silicon nitride which added the solide lubricant hexagonal boron nitride (hBN) sliding against austenitic stainless steel 0Cr18Ni9Ti (ASS) have been carried on. And another conventional metal 45# steel was chosen for the contrast test. By means of MMW-1 vertical universal tester in which an upper pin mated with a bottom disc, friction and wear tests were carried out. By using Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Laser Scanning Microscope (LSM), the morphologies of worn surfaces and debris were observed and the phase compositions of the tribo-film and debris were analyzed. Combined with the measurement of the friction temperature of the tribocouples by using the infrared thermography, the influence and interactive effect of the internal and external factors on the tribological performance of Si3N4-hBN/metal tribocouples were revealed. The conclusions through systematically analysis and research are as follows:For Si3N4-hBN/ASS sliding pairs under the air condition and a normal load ION, the friction coefficient of the pairs was decreased with the increased speed. Only the value of the SN10/ASS pair was decreased further while that of other pairs were all increased when the speed was continuous increased. When the content of hBN was 10%, the friction coefficient of the SN10/ASS tribocouple was decreased from 0.69 at a speed of 0.52m/s to 0.51 at a speed of 0.86m/s, then continuously decreased to 0.12 at a speed of 1.73m/s. There was a surface film whose main components were Fe2O3, SiO2 and B2O3 formed on the worn surface of the ASS disc mated with the SN10 pin at a speed of 1.73m/s. It protected and lubricated the worn surface so that the SN10/ASS tribocouple had obtained a better tribological performance. Moreover, the negative wear which closed to zero appeared on the SN10 pin, the value was 6.5x10-6 mm3·N-1m-1. And the wear rate of the the ASS disc sliding against the SNIO pin obtained a minimum value,1.26×10-5 mm3·N-1m-1. The wear mechanism of Si3N4-hBN/ASS tribocouples was dominated by adhesion wear. As for Si3N4-hBN/45# sliding pairs under the same test conditions, the friction coefficients were higher than that of Si3N4-hBN/ASS sliding pairs, especially reaching up to 1.38 at a speed of 1.73m/s. Under this condition, the worn surface of the 45# disc mated with the SN10 pin was rough and there were obvious ploughing and spalling wear happened, so that there was no formation of the continuous surface film which have the function of lubrication on the were surface. The wear rate of the SNO pin at a speed of 0.52m/s and the SN30 pin at a speed of 1.73m/s were at 10-5 mm3·N-m-1 magnitude while the wear rates of the rest pins were all at 10-6 mm3·N-m-1 magnitude. Besides, the wear rates of the 45# discs were all higher than 1×10mm3·N-m-1. The main wear mechanism of the Si3N4-hBN/45# tribocouple was adhesion wear and abrasive wear.The test load also had a significant effect on the tribology performance of the Si3N4-hBN/ASS tribocouples. The friction coefficients decreased with the increased load, and it rose again when the load continuously increased. Under the condition of a certain speed of 0.52m/s and a normal load of 20N, the wear mechanism of the SNO/ASS and SN20/ASS sliding pairs was dominated by abrasive wear and adhesion wear while that of the SN30/ASS sliding pair was mainly abrasive wear. With the increased load, the friction coefficient of the SN10/ASS tribocouple was obviously decreased, from 0.69 under the load of ION to 0.27 under the load of 20N, then increased to 0.48 and 0.74 under the loads of 30N and 50N, respectively. And, the wear rates of pins and discs were all increased with the increase of the load. There was a surface film whose main components were Fe2O3 and SiO2 formed on the worn surface of the ASS disc mated with the SN10 pin under the load of 20N. It protected and lubricated the worn surface. At the same time, a layer of debris was formed on the worn surface of the SN10 pin, it could effectively prevent the direct contact of the pin and disc and played a certain protective effect to the worn surface so that the SN10/ASS pair showed a relative good tribological behaviour. For Si3N4-hBN/45 tribocouples under the same conditions, the friction coefficient was higher than that of Si3N4-hBN/ASS tribocouples, especially the friction coefficient of the SN30/45# pair was reach up to 1.49 under the load of 50N. The wear rates of the pin mated with the 45# discs under different loads were all at 10-5 mm3·N-m-1 and 10-6 mm3·N-m-1 magnitude. However, in addition to the wear rates of some individual 45# discs were at 10-5 mm3·N-m-1 magnitude, the wear rates of the rest 45# discs were all above 1×10-4 mm3·N-1m-1.The addition of the hBN could effectively improve the self-lubricating property of the Si3N4-hBN composites. A better tribological performance of the tribocouples was obtained when the content of the hBN was 10%; the tribological performance of the Si3N4-hBN composites was bad due to the poor physical and mechanical property when the content of the hBN was higher. During the process of the friction, the friction temperature of the tribocouples rose with the increase of the speed and load. It made the friction chemical reaction of the debris piled up in the spalling pits on the worn surface more easily to be happened, which provided favorable conditions for the formation of the surface film on the worn surface. However, when the friction temperature was too high, the formation mechanism of the tribo-chemical film was destroyed, there was no surface film formed on the worn surface of the tribocouple so that the friction coefficients were obviously increased.The alloy elements contained in the austenitic stainless steel (Ni, Cr, Mn, Ti etc) make the hardness, toughness, wear resistance are all improved. Besides, the surface hardness of the ASS disc has been improved because of the martensitic transformation induced by the friction. And the formation of the surface hardend layer open the way to the formation of the surface film. The instantaneous abrasive wear resistance ability was also improved, so as to improved the wear resistance of the material. However, during the friction process, the main wear form of the 45# disc was dominated by adhesion, peeling off and spalling pits due to the poor toughness of 45# disc, and the phase transition could not happeded. Therefore, it is difficult for the formation of the surface film on the worn surface to which attributed the accumulation of the friction chemical reaction products, which could not play a role of lubrication.To sum up, the internal and external factors could significantly influence the tribological properties of the Si3N4-hBN/ASS tribocouples. In this thesis, under the variable speed, the SN10/ASS pair obtained the best tribological performance at a speed of 1.73m/s; under the variable load, the SN10/ASS pair obtained the best tribological performance under a load of 20N. |
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