Hydroxyapatite coating on stainless steel 316L using investment casting technique

Abstract

Decades of research had found that the use of hydroxyapatite (HA) could promote osseointegration and increase the mechanical stability and strength of the metallic implant. Currently, medical implants are produced by machining, metal forming and casting particularly investment casting technique. Comparatively, the investment casting technique is more practical to mass produced implant due to its simplicity, superior surface finish, relatively cheap process, capability to produce complex and near-net shape implants. Investment casting technique provides a single stage processing technique in producing coated implant i.e. HA coated on the cast implant. In this respect, the possibility of applying HA layer during casting for implant substrate using investment casting was explored. Previous studies used paint brush in applying HA layer onto the internal cavity of the ceramic mould. However, the layer thickness was inconsistence which affects the quality of the HA layer coated on the casting and impractical for small and complex shapes components. In this study, medical grade 316L stainless steel (316L-SS) was coated with HA using investment casting technique by pouring molten 316L-SS into a HA coated ceramic mould at temperature of 1650?C in argon gas. The coated samples were sintered in a furnace at four different temperatures (600, 800, 1000 and 1200?C) for 1 hour. The as-cast and sintered HA coated samples were characterised using scanning electron microscopy, energy dispersive x-ray spectroscopy and x-ray diffraction analysis. Results showed that the as-cast samples produced good HA coating bond, formed amorphous phases and complex calcium-chromium oxide (CaCrO) layer at the interface between HA and 316L-SS. Sintering process was conducted to recrystallise and improve the properties of the as-cast HA coating. The results confirmed that crystallinity and purity of the coating increased with increasing sintering temperature whilst Ca/P ratio and porosity decreased. Increasing sintering temperature from 600 to 800ºC did not significantly alter the crystallinity and purity. The crystallinity and purity recorded at that temperature range were 57.35% - 58.55% and 61.80% - 63.21% respectively. This temperature range was considered insufficient to recrystallise and purify the coating to an acceptable value for implant applications. Increased of sintering temperature from 1000 to 1200ºC increased the crystallinity from 73.52% - 74.47% and purity from 61.80% - 81.8%. Simultaneously, the Ca/P ratio and porosity were reduced to 1.51 and 14.14% respectively which is acceptable to human body. Sintered as-cast specimen at 1000ºC immersed into Simulated Body Fluid (SBF) solution showed increased in Ca/P ratio with increasing immersion time indicating that the coating was bioactive.