Dissolution response of hydroxyapatite coatings to residual stresses

The effect of residual stress on the dissolution of hydroxyapatite (HA) coa tings was investigated. The examined coatings of 80-, 110-, and 200-mum thi ckness were prepared by a plasma spraying technique under identical conditi ons. Residual stresses in the coatings were measured with a hole-drilling m ethod. Dissolution of the coatings was monitored along with an examination of the phase composition. The results showed that both tensile residual str ess and amorphous HA existed throughout the entire depth of the coatings an d tended to increase from the surface to the interface of the coating and s ubstrate. The thicker the coatings were, the higher the maximum residual st ress was. Correspondingly, the pH value and calcium concentration of the so lutions tended to increase with the coating thickness. On the basis of thes e phenomena and a thermodynamic analysis of the dissolution of the HA subje cted to stresses, we concluded that besides structural effects, residual st ress was also an important intrinsic factor influencing dissolution of HA c oatings, and the dissolution can be delayed or even restrained by compressi ve residual stress. (C) 2001 John Wiley & Sons, Inc. J Biomed Mater Res 55: 596-602, 2001.