Thermal fatigue cracks in WC-10Ni + AgCuTi composite coatings

In this study, a WC-10Ni + AgCuTi/Cu composite coating was applied to a copper substrate using vacuum brazing. The initiation and propagation of thermal fatigue cracks in the composite coating were investigated under various upper limit temperatures of thermal fatigue, and the law governing crack initiation was explored. Additionally, the mechanism of thermal fatigue behaviour was also analysed. The findings reveal that no cracks are observed in the composite coating when the upper limit temperature of the thermal fatigue test is set at 400 °C and subjected to 50 cycles. However, as the upper limit temperature and the number of cycles increase, the rate of crack initiation and propagation significantly accelerates, eventually leading to macroscopic cracking and coating failure. The behaviour of thermal fatigue cracks in composite coatings is characterized by the coating undergoing alternating tensile and compressive stresses during the thermal fatigue process. This stress cycle causes the hard layer to develop longitudinal cracks that propagate inward. Moreover, transverse cracks initiate near the interface layer of the hard layer and extend along the interface direction. The study identifies four primary modes of crack propagation.