ROLLING-CONTACT FATIGUE AND WEAR OF CVD-SIC WITH RESIDUAL SURFACE COMPRESSION

The rolling-contact fatigue life and wear of CVD-SiC coatings with sur face compression were studied using a three-ball-on-rod rolling-contac t fatigue (RCF) tester. Two levels of surface compression in the coati ngs, approximately 190 mu m thick, were introduced by using substrates of a liquid-phase sintered SiC and a SiC-30 vol% TiC to obtain desire d thermal-expansion mismatches with the CVD-SiC. Residual surface comp ressions of 250 and 680 MPa were measured in the CVD-SiC coatings on S iC and SiC-30 vol% TiC substrates, respectively, by a strain-gage tech nique, In the RCF test, the fatigue life of the CVD-SiC coating with t he moderate surface compression was limited by severe wear by a brittl e fracture mechanism at a Hertzian contact stress of 5.5 GPa. The CVD- SiC coating on the SiC-30 vol% TiC composite, on the other hand, did n ot show measurable wear. A majority of the tests (11 out of 16) were s uspended at 100 h or stopped due to failure of the steel balls, Five t ests stopped due to spalling of the CVD-SiC coating at weak interfaces 10 to 15 mu m below the contact surface, Examination of the microstru cture of the CVD-SiC coating in cross sections revealed that the weak interfaces that led to the spalling were related to discontinuous grow th of the CVD-SiC in the form of nodules or growth regions. Eliminatio n of these defective structures is likely to enhance the tribological performance of surface-toughened CVD-SiC.