Rolling Contact Fatigue Life Of Silicon Nitride Ceramic Balls

Ceramic bearings are a kind of new style bearings, which are suitable for super high-speed, corrosion, high temperature, and formidable working condition. The ceramic balls are one of the important parts of ceramic bearings. The rolling contact fatigue (RCF) life of ceramic balls is a reliable technique to asses whether or not they are suitable to be used in rolling bearings. The fatigue properties of Silicon Nitride ceramic balls were analyzed in theory and experiment in this paper. The research work was supported by the Science and Technology Development Fund of Shanghai (No.0152nm031). The main works are as follows:1,A new pure rolling fatigue test rig with three contact points was developed. All three non-uniformly spaced contact points of the tested bearing ball are in a same maximal circle cross-section and the contact stress are equal. The test rig overcoms the disadvantages that existed in the using test rig such as in rolling/slide working conditions or requiring some appendage, and reduces influencing factors on experimental results resulted from conventional test rigs.2,The measure-control system for bearing balls RCF test rig was designed, making full use of sensing device technology, data acquisition, signal analysis process technology and microcomputer controlling technology. The running state of the test rig can be automatically monitored. The working state of the specimen balls can be identified. The faults can be checked out and measures will be taken by the system while something being wrong with test rig, to ensure that the test rig is safe in operation and lighten the working strength of tester and increase test efficiency.3,The fatigue properties of GCr15 steel balls and two kinds of Si₃N₄ ceramic balls(GSN-200 and NBD-200) produced with Gas Pressure Sintering (GPS) and Hot Isostatic Pressing (HIP) technologies respectively were compared using the new fatigue test rig. Ball surfaces were microexamined after failure. It was identified by tests that the failure mode of ceramic balls was the same surface spall as steel balls. Life tests data, summarized in accordance with the Weibull theory, showed that the life of GSN-200 balls was close to that of GCr15 balls, whereas the life of NBD-200 balls was much longer than those of GSN-200 and GCr15. Under the same working condition, the temperature rise of all ceramic balls was lower than that of steel balls, and their crack propagation rates were slower than that of steel balls.4,Ball surfaces were examined after failure with optical microscopy and scanning electron microscopy. The failure cause, fatigue phenomenon and mechanics were analyzed. The research shows that subsurface cracks play a dominant role in the formation of spalling failure. The crack growth path was from subsurface to surface. These cracks originated from volume defects of the material, and propagate, to from elliptical-subuliform fatigue spalls under the action of principal tensile stresses.5,The effect of microstructure on rolling contact fatigue performance of ceramic balls is studied making use of scanning electron microscopy and accelerated test technology. The research indicates that the microstructure affects not only toughness, strength, etc. mechanical properties but also RCF performance.6,The contact stress field in subsurface was analyzed using elastic contact mechanics and Hertz theory under pure rolling condition. The maximum principal tensile is obtained. The pure rolling contact fatigue test was performed correspondent to theoretical calculation. A comparison between the theoretical calculation and experiment result indicates that the crack directions are generally consistent with the contours of the principal tensile stress. Consequently it is verified that critical stress of silicon nitride ceramic balls is the maximum principal tensile stress.7,The relation expression between failure probability and rolling contact life was deduced with Weibull fracture statistic method for the silicon nitride ceramic ball in ball-cylinder geometric model. The tensile stress life model about silicon nitride ceramic balls was set up between rolling contact fatigue life and contact stress on the basis of the correlative numerical solution between the rating life and the maximum contact stress. The tensile stress life model is conceived basing on maximum principal tensile stress. It is indicated that the tensile stress life model is feasible through rolling contact fatigue test with different stress levels. Comparing the tensile stress life model with the L-P shear stress life model, the former is more suitable for rolling contact fatigue life prediction of silicon nitride ceramic balls than the latter. So the tensile stress life model is verified again.The research provides a new test method and theoretical analysis method for ceramic balls. The study is useful for designing together with technology improving and using of ceramic balls in reason. It provides a good foundation for the analysis of the rolling contact fatigue life of ceramic beatings.