Quenching stress and fracture of paraffin droplet during solidification and adhesion on metallic substrate

Abstract

A model experiment was carried out to evaluate strain/stress development and fracture behaviors during solidification and adhesion process of molten droplet, which partly simulated thermal spraying of thermal barrier coatings. In the experiment, a molten paraffin droplet was dropped from a definite height onto a pre-cooled 430 stainless steel substrate, and the strain at the substrate back surface was measured with a bi-axial strain gauge. During the solidification and cooling process of the paraffin splat, tensile quenching strain was developed at the substrate back surface. Effects of substrate pre-set temperature, drop height, and paraffin material properties on quenching strain were investigated. Experimental results suggested that the quenching strain decreased with the increase of drop height and the increase of the substrate pre-set temperature. Cracking, debonding, and delamination occurred depending on the experimental conditions. A finite element analysis was conducted to calculate splat stress and interfacial stresses as driving forces for cracking and debonding. The calculated splat stress provided a reasonable explanation for the cracking behaviors observed in the experiments. Based on the numerical investigation, the development of quenching strain and resultant fracture behavior were discussed and associated with the effects of experimental conditions and paraffin material properties, including melting points, Young's moduli, and thermal expansion coefficients. Findings from a series of experiments and analyses showed some similarities to actual phenomena during thermal spray, and these can provide important guidance for optimizing thermal spraying processes.