Preventing contact loading induced surface failure of ceramic components by a shot peening treatment

The failure of ceramic components in, e.g., roller bearing and roller tool applications generally is initiated at small cracks at or near the surface and is controlled by the tensile contact stresses. The common way to improve static, cyclic and rolling load capacity is therefore to decrease the size of natural and machining induced flaws and to increase the strength by optimizing the material and the fabrication procedures. Unfortunately, the gain of material quality is limited by the capabilities of mass-production methods and economical restrictions. The aim of the presented investigation was to use our novel shot peening technique in order to introduce high compressive residual stresses and by this to increase the static, cyclic and rolling load capacities. The material we studied was a commercially available silicon nitride. This type of silicon nitride has recently been used as a material for full ceramic bearings. The process of shot peening used in our investigations involves the impact of cemented carbide beads on the surface under controlled velocity. The impact velocity and the hardness of the shot are high enough to cause limited plastic flow near the surface. Up on completion of the process, the workpiece exhibits a layer with compressive residual stresses. For the assessment of the surface strengthening the X ray diffraction analysis (XRD), the static and cyclic ball-on-plate strength test and a rolling contact test (Amsler test) were used. The XRD investigations allowed evaluating the peening-induced residual stress depth profiles. In the ball-on-plate test, the samples were loaded with a spherical indenter up to Hertzian failure (surface cracks) of the sample. In the cyclic ball on plate test the samples were tested 106 cycles at different loads. The rolling contact tests were preformed with and without water lubrication. The results show that high compressive residual stresses in the GPa-range can be introduced in silicon nitride which may boost the static load capacity of the near surface layer by a factor of up to 4. Only little effect on the surface integrity could be obtained. Also the cyclic load capacity increased by a factor of 4. The rolling contact tests showed that it was possible to increase the Hertzian pressure by a factor of 3 without initiation of any cracks or pitting in the raceway.