Temperature dependence of the tensile and thermal fatigue cracking properties of laser-deposited cobalt-based coatings for brake disc application

A new cobalt-based coating, CoNiTi, was developed for brake disc applications. The effect of temperature on the tensile and thermal fatigue cracking properties of CoNiTi was evaluated between room temperature (RT) and 700 °C, and compared with those of the commercial CoCrMoW and Stellite 6 coatings. The strength of all coatings decreased with increasing test temperature, and the elongation to fracture of Stellite 6 increased with increasing test temperature. However, CoNiTi and CoCrMoW presented deterioration in elongation to fracture at 400–500 °C. In addition, the crack propagation rate of CoCrMoW and CoNiTi also presented a trend of accelerated increase under RT ∼700 °C thermal fatigue condition. By combining the finite element simulation of thermal fatigue and in-situ tensile test, the grain boundary weakness at middle temperature was revealed to clarify the reason for the deterioration of plasticity and crack growth resistance in CoCrMoW and CoNiTi. The microstructure evolution of γ -Co → ε -Co martensitic transformation during tests depended on the temperature and chemical composition associated with stacking fault energy (SFE). Increasing temperature and nickel content could improve SFE, stabilize γ -Co, and make dislocation glide in cross-slip mode, on the contrary, γ -Co tended to undergo martensitic transformation, and the dislocation was in planar-slip mode, when SFE was low. • The temperature effect on the tensile and thermal fatigue cracking properties of cobalt-based coatings was revealed. • PLC band and grain boundary weakening at medium temperature deteriorated plasticity and crack resistance of coatings. • γ -Co → ε -Co martensitic transformation depended on SFE, and presented preferred selection obeying Schmid's law.