Improved sinterability of hydroxyapatite functionally graded materials strengthened with SS316L and CNTs fabricated by pressureless sintering

Recently, functionally graded materials (FGMs) have attracted particular research attention for their potential use in biomedical implants and joint prosthesis. In the present investigation, carbon nanotubes (CNTs) and stainless steel 316L (SS316L) reinforced hydroxyapatite (HA) hybrid FGMs were fabricated by pressureless sintering. Three types of FGMs were successfully fabricated with varying amounts of SS316L (0–40 wt%) in the discrete layers, in which SS316L was then combined with micro-sized HA, mixture of micro-sized HA with CNTs and a mixture of (1:1) mass ratio micro and nano-sized HA with CNTs. In the ceramic reinforced composites, it is difficult to surmount the thermal cracks generated during conventional pressureless sintering due to differences in thermal expansion coefficients of metal and ceramic. In this study, the emphasis was on elimination of the cracks generated in FGMs during the pressureless sintering by controlling process parameters such as temperature and time at different sintering stages and binder concentration. The microstructural analysis revealed uniform structure with optimized sintering cycle. No cracks were found around the inter-layer boundaries and within the composite layers. Moreover, improvement in the densification and sintering shrinkage was observed with the addition of CNTs and nano HA.