Damage induced bending strength variation and crack initiation mechanism during thermal cycling shock of laser deposited Stellite alloy coating for engine valve

Stellite alloy coatings can significantly enhance the wear resistance of valves; however, cyclic thermal loading can induce thermal cycling shock, thereby reducing their service life and overall performance. This study systematically investigates the mechanisms of damage evolution, variations in bending properties, and crack initiation under thermal shocks in laser-deposited Stellite alloy coatings through quenching experiments coupled with multiscale characterization and numerical simulations. The results indicate that the critical temperature of macroscopic cracking is in the range from 350 °C to 400 °C. The damage, such as slip bands, micropores, and microcracks, is primarily induced by thermal stress concentrations driven by rapid cooling. The bending strength of the coatings deteriorates with an increasing damage fraction, which is attributed to the increase of the stress concentration factor in the damage area. Higher peak temperatures produce higher thermal stress, accelerating crack initiation and rapid propagation along the interfaces between the eutectic carbides and the matrix above the critical temperature. Conversely, at lower peak thermal shocking temperatures, the high density of geometrically necessary dislocations (from 74.786 × 10 12 to 178.269 × 10 12 /m 2 ) leads to significant localized stress concentrations, which synergistically with the interfacial damage degradation promote crack initiation.