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Multi-ion doping in synthetic HA was carried out using high energy planetary ball milling followed by calcination at 1250 °C for 2 h. The influence of Sr +2 , Zn +2 , Ag + , and F - ion doping on crystallinity and crystallite size was analyzed using Taguchi design of experiments (DOE) and optimal concentration of different dopants has been identified to achieve desired crystallinity and crystallite size. The doped HA samples have been characterized using X-ray diffraction and Fourier transform infrared spectroscopy to determine their phase purity, degree of crystallinity, crystallite size and functional groups. Standard Analysis of variance (ANOVA) showed relatively high contribution of Sr +2 and Zn +2 doping in changing the crystallinity and crystal size of HA compared to the effect of Ag + and F - doping. Our analysis demonstrated strong interaction between dopants at binary level doping, while ternary and quaternary doping of elements did not exhibit any interaction in influencing the crystallinity and crystallite size of HA. In general, multi-ion doping in HA found to decrease its crystallinity from 92% to 72% (max.), but enhance the hardness, depending on the type and concentration of doping element. Similarly, a minimum crystallite size of 31 nm was achieved with some binary compositions and other combinations resulted in crystallite sizes up to 59 nm. The compositions that ensure desired crystallinity and crystallite size can also provide high hardness. Our results can be used to tailor the composition of HA in achieving desired functional properties, dependent on crystallinity and crystallite size, such as strength, bioactivity and degradation to suit variety of implant applications.

Zirconium (Zr) and its alloys are considered as promising materials for implant applications due to their excellent biocompatibility. However, the poor tribological properties of Zr are limiting its widespread applications. Therefore, in this investigation thermal nitriding of laser processed Zr was carried out with an aim to improve its wear and corrosion properties for potential wear resistant implant applications. The influences of nitriding temperature and time on the nitride layer thickness, phase constituents, wear and corrosion properties of nitrided Zr layer were studied. The Zr samples were nitrided at 600 °C and 800 °C for 2 and 6 h in flowing nitrogen. The results demonstrated that the nitriding temperature had relatively stronger influence on the nitride layer thickness, wettability, hardness and wear resistance than nitriding time. In general, the nitride layers found to contain ZrN and ZrO2 as major phases and Zr2N and Zr7O8N4 as minor phases. The nitride layer thickness increased from 5 ± 0.7 μm to 26 ± 5 μm with increase in the nitriding temperature and time. Nitriding also enhanced the wettability and passivity of Zr. The samples nitrided at 600 °C for 2 h resulted in noblest Ecorr of − 130 mV vs SCE with Icorr of 0.001 μA/cm2 in Hank's balanced salt solution. Lowest in vitro wear rate of 4.4 × 10− 7mm3/N·m was recorded, against Al2O3,when the samples were nitrided at 800 °C for 6 h, which is 93% lower than that of pure Zr. These results demonstrate that thermal nitriding of Zr can be used to increase the corrosion and wear resistance of Zr in physiological environment.