Comparison of Bioceramic Coatings on Metals from Sol-Gel, PACVD and Precipitation Processes

Orthopaedic implants and the associated bone graft material industry have an estimated world-wide market of #~ US$2.5 billion. Much of this is based on the use of titanium, titanium alloys, cobalt chromium and surgical stainless steel substrates. Based on body tissue responses, these substrates are classified as bioinert producing fibrous soft tissue of variable thickness around the implant but not strong adhesive bonding to hard tissue (bone) by inducing its growth. The application of bioactive ceramic thin films to these substrate surfaces, inducing growth of bone at the interface, is therefore an international research target. This bioactivity has been shown to be produced with bioceramic layers including hydrolysed silica, hydrolysed titania and hydroxyapatite (HA). The structure and bonding of the thin film material to the metal or alloy substrate is, however, critical to the lifetime of the implant in the body. The half-life of hip implants, before replacement is required for patients under the age of 50, is of the order of 5-7 years. Failure generally occurs at the metal/ceramic boundary due to mismatch in physical and chemical properties between the substrate and the ceramic. The design of improved interfacial boundaries promoting strong adhesive bonding and graded physical and chemical properties from the metal to the bioceramic requires understanding of the reaction mechanisms and resulting structures across this boundary.