Study On Silicon Nitride And Fatigue Life Of Its Sphere

Engineering ceramics have excellent physical and mechanical properties, and their application in bearings which belong to a type of important basic component has become ripe, especially the silicon nitride (Si3N4) material, which is most widely used. However, silicon nitride material used in bearings often show unstable performance, short rolling contact fatigue life and poor reliability due to the influences of preparation progresses, raw material and sintering aids. Therefore, Si3N4material was selected as the research object, and ways to optimize the structure and performance of the material were put forward to provide technical support for the quality improvement of ceramic rollers.The rolling contact fatigue tests were conducted on Si3N4ceramic balls, the existing problems were analysed and scanning electron microscope was used for microscopic observation. The differences of rolling contact fatigue peformances of Si3N4, alumina (Al2O3), zirconia (ZrO2) and bearing steel were studied. It was found that the fatigue life of the Si3N4balls used in the test was generally short, and the dispersion was big. The defects resulted from grain boundary phase segregation were main causes of fatigue failure of the Si3N4balls. Impurities in raw material and inhomogeneous mixing can result in formation of grain boundary segregation. In the comparative test of different materials, the fatigue properties of Al2O3, ZrO2and bearing steel were not as good as Si3N4.Sialon was introduced into ceramic material for bearings. The influences of the raw material purity and introducing Sialon on the properties and microstructure of Si3N4were studied. And the sphere prepared by Si3N4which contained Sialon was tested in the rolling contact fatigue test. Through absorption of grain boundary phase during the sintering of Sialon, with the decrease of Al2O3and increase of A1N in sintering aids, Sialon increased and the content of grain boundary phase decreased gradually in the microstructure, the influences of grain boundary phase segregation were reduced, which was helpful to improve the rolling contact fatigue performance of Si3N4balls. With the increase of Sialon in S13N4, for S13N4prepared by high purity raw material, the number of elongated grains reduced, the size of the grians increased gradually, the ratio of length to diameter decreased gradually, the crush load decreased, the bending strength decreased gradually, the hardness (HV10) and elastic modulus increased gradually; for Si3N4prepared by low purity raw material, the bending strength decreased gradually, the hardness and elastic modulus increased firstly and then decreased. The strength of Si3N4prepared by low purity raw material was lower than Si3N4prepared by high purity raw material, and abnormally growed grains appeared in its microstructure. The Weibull modulus of bending strength and crush load made a great difference in results. In the comparative test with foreign ceramic balls, fatigue life did not show big difference, for wear resistance, Norton ceramic balls were the best of the three, ceramic balls prepared in this study took the second place, Toshiba ceramic balls were the worst.