Investigation of residual stress generation in plasma-sprayed hydroxyapatite coatings with various spraying programs

Abstract

In the past studies, the various results of the residual stresses in the hydroxyapatite coatings (HACs) prepared vis the plasma spraying were investigated; some arguments discussed in these studies especially focused on the stress values and the stress states ( i.e. tension or compression). In this study, in order to obtain the complete and accurate value and state of the residual stress, the nonstressed free HAC slice was separated from the substrate. According to the curvature theory, the liner residual strain and curvature strain of HAC were obtained. For the purpose of inferring the generation mechanism of residual stress, the particular types of the substrate and different directions of cooling jets were employed. The results showed that the difference of temperature between the HAC and substrate is the main factor that dominates the states of residual stress. HAC on the hollow rotational holder with back cooling from substrate side displayed the higher temperature than titanium substrate (Ti-substrate) during the plasma spraying and resulted in tensile residual stress in the HAC. On the contrary, HAC on the non-hollow holder with front cooling from coating side displayed much lower temperature than Ti-substrate and therefore showed the compressive residual stress. These could be attributed to the differences of volume shrinkage between the HAC and Ti-substrate when the specimen cooled from different initial temperature to room temperature. In other words, the residual stress is not always tensile or compressive; the stress state might be dominated by the cooling directions and the substrate types.