Diffusion barrier coatings on ductile particles for protection in bioactive glass–ceramic matrix

Bioactive Apoceram glass-ceramics reinforced with ductile Ti particles are promising surgical implant materials. Unfortunately, there is a detrimental reaction between Ti particles and the Apoceram matrix leading to the formation of a brittle reaction layer of Ti5Si3 which prevents the attainment of the full toughening potential of the ductile particles. This paper describes the use of protective coatings on titanium particles to prevent interdiffusion, and thus to minimise the formation of the brittle reaction layer. The coating systems investigated were (i) a titanium diboride layer combined with an outer carbon layer (TiB2/C), and (ii) a titanium boride nitride coating Ti(B,N). These coatings were deposited onto Ti particles using a magnetron sputter method prior to the incorporation into the Apoceram matrix. The structure and mechanical properties of the composites reinforced with Ti particles with and without diffusion barrier coatings were determined. It was found that the Ti(B,N) coating reduced the extent of the Ti,Si reaction layer by a factor of about 2. The coating process did not affect the particle size distribution of the titanium powder nor change the main characteristics of the crystallisation of the matrix. The coatings improved the fracture toughness and flexural strength of the composites, especially the composites reinforced with TiB2/C coated titanium.