Evaluation of fatigue delamination strength of WC-Co thermal sprayed coating by torsion-tension pin-test method

Abstract

In this report, the fatigue delamination strength of WC-Co thermal sprayed coatings under combined torsion and tension is evaluated using the torsion-tension pin-test method. First, the effects of both the pin-diameter and the coating thickness on the apparent delamination strength were investigated experimentally. Second, the stress distributions around the interface edge between the pin and the coating were numerically obtained by using the finite element analysis program "MARC". Third, delamination fatigue strength was investigated under cyclic torsion with or without tensile stress, experimentally. It was confirmed that the fractured plane of the torsion pin coincides with the interfacial plane between the coating and the pin. Similar stress distributions were observed at the interface when delaminations occurred for rather thick coatings, independent of the pin diameter. The critical combination of strength of shear stress field (Ks) with that of tensile stress fields (Ka), i.e., the delamination criteria of the coating under combined shear and tensile loadings, was obtained for a WC-12Co thermal sprayed coating. These combinations were found to be independent of pin diameter and coating thickness. The apparent shear fatigue delamination strength obtained experimentally decreased with increasing pin diameter and tensile stress. The normalized shear fatigue delamination strength curves (S/N curves) can be obtained using the strength of shear stress fields (Ks) observed at the interface when delaminations occurred, independent of pin-diameter.

Keywords

• thermal sprayed coating
• fem analysis
• shear delamination
• fatigue strength
• stress singularity
• combined stresses
• strength of stress fields
• delamination criteria