Influence of plasma and cold spray deposited Ti Layers on high-cycle fatigue properties of Ti6Al4V substrates

Abstract

This paper presents a summary of the research on the influence of plasma and cold spray deposition of bio-grade Ti powder on the high-cycle fatigue properties of Ti6Al4V substrates. Four sets of flat specimens (as-received, grit-blasted, plasma and cold sprayed) were prepared and subjected to cantilever–beam cyclic bend loading with a constant deflection of the free end. It was found that the grit-blasting procedure significantly increased the fatigue lives of the specimens compared to the as-received set (1.81x increase). The deposition of the coatings onto grit-blasted specimens led to deterioration of fatigue properties. The average fatigue lives of the plasma sprayed and cold sprayed samples reached 1.16x and 0.91x of the as-received specimens, respectively. In order to understand the positive effect of the grit-blasting procedure and the adverse effect of the coatings deposition on fatigue lives, chemical analyses, fractographic analysis, microstructural investigations as well as layers elastic moduli estimation were carried out. It was found that the cold spray deposition retained the composition of the powder feedstock while a complete transformation of Ti into nitrides and oxides was detected in the plasma deposited layers. Owing to the different coating build-up principles and their respective different porosity levels, the moduli of the coatings were found to vary (7.2 GPa for plasma sprayed and 36.7 GPa for cold sprayed layers). The fatigue crack initiation sites and propagation directions in the plasma and cold sprayed specimens were found to differ substantially. Based on the obtained results, two explanations of the fatigue loading results are suggested in the paper.This paper presents a summary of the research on the influence of plasma and cold spray deposition of bio-grade Ti powder on the high-cycle fatigue properties of Ti6Al4V substrates. Four sets of flat specimens (as-received, grit-blasted, plasma and cold sprayed) were prepared and subjected to cantilever–beam cyclic bend loading with a constant deflection of the free end. It was found that the grit-blasting procedure significantly increased the fatigue lives of the specimens compared to the as-received set (1.81x increase). The deposition of the coatings onto grit-blasted specimens led to deterioration of fatigue properties. The average fatigue lives of the plasma sprayed and cold sprayed samples reached 1.16x and 0.91x of the as-received specimens, respectively. In order to understand the positive effect of the grit-blasting procedure and the adverse effect of the coatings deposition on fatigue lives, chemical analyses, fractographic analysis, microstructural investigations as well as layers elastic moduli estimation were carried out. It was found that the cold spray deposition retained the composition of the powder feedstock while a complete transformation of Ti into nitrides and oxides was detected in the plasma deposited layers. Owing to the different coating build-up principles and their respective different porosity levels, the moduli of the coatings were found to vary (7.2 GPa for plasma sprayed and 36.7 GPa for cold sprayed layers). The fatigue crack initiation sites and propagation directions in the plasma and cold sprayed specimens were found to differ substantially. Based on the obtained results, two explanations of the fatigue loading results are suggested in the paper.