Thermal spraying of functionally graded coatings for biomedical applications

Biomedical requirements in a prosthesis are often complex and diverse in nature. These multifaceted requirements can be met by functionally graded materials (FGMs). Biomaterials for implants have to display high adaptability in order to suit the various stages of the biointegration of foreign material into the human body. The present paper analyses the preparation of functionally graded bioceramic coatings composed of calcium phosphate compounds. Two FGM systems were investigated. System 1 utilised the thermal response of different size ranges of hydroxyapatite (HA) powders in the plasma flame to form gradients in biological and physical properties while system 2 combined two types of calcium phosphates, namely α tricalcium phosphate and HA, to form the FGM. The coating was graded according to adhesive strength and bioresorbability. The bond coat on the metal stub was deposited with a range of HA particles that provides high adhesive strength but has poor biocompatibility. The top coat of system 1 is composed of HA particles of varying size that elicit a highly biocompatible layer while in system 2, the top coat consists mainly of highly bioresorbable α tricalcium phosphate. This arrangement allows the accelerated biointegration of the coating by the body tissues. Processing steps and problems are highlighted in the present paper as are the results of post-spray treatment with conventional heat treatment and hot isostatic pressing (hipping).