Splat deposition stress formation mechanism of droplets impacting onto texture

Abstract

This study focuses on understanding the crucial phenomenon of deposition stress during the splat formation of plasma-sprayed droplets, which is essential for comprehending coating stress in applications such as plasma spraying. A novel three-dimensional coupled fluid–solid and thermo-mechanical model was developed, which employed the volume of fluid method and finite element method. This model was applied to simulate the formation of deposition stress in molybdenum droplets during plasma spraying onto a stainless steel substrate with laser-textured groove patterns. It successfully replicated various stages of the splat formation process, including impacting, spreading, cooling, and solidification, which captured the evolution of temperature, strain, and stress in the splat and groove walls. The accuracy of the model was validated through X-ray diffraction measurements of splat deposition stress on the planar substrate surface. The simulation results revealed that deposition stress was induced during the simultaneous spreading and solidification of the droplet. This stress was attributed to quenching and solidification contraction, which resulted in tensile stress in the splat, with higher stress at the bonding interface edges. By contrast, the groove walls experienced compressive stress, which peaked in the central groove region. The mean isotropic stress and von Mises stress of the splat were quantified as 176 and 141 MPa, respectively. In-situ curvature tests demonstrated that stress relaxation, which was triggered by splat deposition stress, resulted in a 24.6 % reduction in cracking, yielding, and slipping stresses. In addition, a refined theoretical equation for splat deposition stress was proposed, with correction coefficients of 0.119 determined from simulations and experiments. The developed simulation model offers reliable insights into the state of splat deposition stress, which provides valuable information for optimizing plasma-spray processes and enhances our understanding of coating stress in such applications.