FRETTING FATIGUE FAILURE OF ENGINEERING CERAMICS

Rollers made of engineering ceramic are very well suitable for hot rolling of high-strength steels. During the shaping process the ceramic rollers are highly loaded. The appearing complex loading is caused by a cyclic loading from the rotation of the rollers and high Hertzian stresses connected with friction, caused by the contact between the rollers and the rolling stock during the shaping process. To investigate the lifetime behavior of potential roller materials, fretting fatigue tests are carried out in a modified four-point-bending unit, where a fretting pad is pressed onto the tensile loaded side of the bending bar and oscillated. In the test series, carried out on silicon-nitride and alumina, different types of the fretting loading were realized. In all test series the ceramics are damaged by the fretting loading by the initiation and/or activation of cracks. In static tests a fretting fatigue loading leads to lower residual strengths. The strong tensile field in the specimen caused by the oscillating fretting pad leads to an activation of the surface defects, but doesn't influence the R-curves of the materials. In cyclic tests the additional cyclic four-point-bending-loading causes a strong degradation of the reinforcement elements, which leads to a more lower R-curve than expected from the pure four point bending and the lifetime gets significant lower. The variation of the fretting loading helps to separate and understand the damage mechanisms.