Mechanical and chemical consequences of the residual stresses in plasma sprayed hydroxyapatite coatings

The residual stresses in thick hydroxyapatite coatings, deposited by plasma spraying, have been determined experimentally using Raman piezo-spectroscopy. The stress dependence of the centre position of the 980cm−1 Raman band, owing to the symmetric stretching of the phosphate ion, PO43−, has been established and found to be 2.47cm−1 GPa−1. Using this calibration, the residual stresses in hydroxyapatite coatings deposited onto Ti-6AI-4V substrates in air have been found to be 100 MPa (tensile), whereas those deposited in a vacuum have been found to be 60 MPa (compressive). Although desirable from a mechanical point of view, it is shown that coatings under residual compression are thermodynamically more stable and, hence, the dissolution of the ionic species, necessary in the exchange between bone and hydroxyapatite coating, can be impeded. It is calculated that for the coatings under examination the stresses have an effect comparable with almost an order of magnitude change of the [OH−] concentration. The analysis explains the dissolution behaviour of hydroxyapatite coatings subject to cyclic stress reported previously.