MECHANICAL AND CHEMICAL CONSEQUENCES OF THE RESIDUAL-STRESSES IN PLASMA-SPRAYED HYDROXYAPATITE COATINGS

The residual stresses in thick hydroxyapatite coatings, deposited by p lasma spraying, have been determined experimentally using Raman piezo- spectroscopy. The stress dependence of the centre position of the 980 cm(-1) Raman band, owing to the symmetric stretching of the phosphate ion, PO43-, has been established and found to be 2.47 cm(-1) GPa(-1). Using this calibration, the residual stresses in hydroxyapatite coatin gs deposited onto Ti-6Al-4V substrates in air have been found to be 10 0 MPa (tensile), whereas those deposited in a vacuum have been found t o be 60 MPa (compressive). Although desirable from a mechanical point of view, it is shown that coatings under residual compression are ther modynamically more stable and, hence, the dissolution of the ionic spe cies, necessary in the exchange between bone and hydroxyapatite coatin g, can be impeded. It is calculated that for the coatings under examin ation the stresses have an effect comparable with almost an order of m agnitude change of the [OH-] concentration. The analysis explains the dissolution behaviour of hydroxyapatite coatings subject to cyclic str ess reported previously. (C) 1997 Elsevier Science Limited.