In-vitro investigation of air-plasma sprayed hydroxyapatite coatings deposited on two geometrically different substrates

Abstract

In this study, coatings of hydroxyapatite (HAp) prepared by thermal spraying technique are investigated for biological response. Spraying was done on two geometrically different Ti-6Al-4V alloy substrates under atmospheric conditions. Subsequent immersion experiments, mimicking physiological environment, were carried out using simulated body fluids (SBF). Non-destructive techniques utilizing conventional and high-energy synchrotron diffractometry were employed for depth–resolved investigations of phase composition, crystallinity and residual stresses within the coating for both substrate geometries. Microscopy techniques were used to examine surface morphology and microstructure. In both substrate geometries and for all immersion periods, HAp is the predominant phase with tetra-calcium phosphate (TTCP) and tri-calcium phosphate (TCP) the two main thermal products. The coating deposited on cylindrical rod substrate show a higher volume fraction of HAp at the near-surface region for the as-sprayed condition and samples immersed for 7 and 28 days. Further immersion shows the former decreasing gradually while the latter saturates after 28 days. Thermal products TTCP and TCP, for the coating deposited on the flat geometry substrate decreased with immersion, while those deposited on the cylindrical rod remains roughly the same before increasing slightly. Through-thickness behavior shows the as-sprayed HAp increasing almost linearly with depth, reaching a maximum around the coating midpoint before decreasing with further depth. Immersion in SBF does not alter the general trend across the coating however it increased the volume fraction of HAp within the first half of the coating with the biggest change occurring between 7 and 28-days of immersion. The variation of HAp with depth and immersion at the three lateral positions shows agreement within error bars indicating the coating to be homogenous. The trend is observed for coating deposited on cylindrical substrate geometry. Both substrate geometries show high near-surface region crystallinity with an index of ~90%. The interface region is less crystalline with a degree of crystallinity index of 67% and 56% for the flat and cylindrical substrates, respectively. Immersion in SBF does not alter the general through-thickness trend however it increased the degree of crystallinity at both ends. Residual stresses for the coating deposited on both substrate geometries are tensile and small, not exceeding 42 MPa and 65 MPa, respectively. The stresses are mainly confined in the near-surface region with the interior region showing neglible stresses, <10 MPa. Immersion in SBF relaxes the stress with the interior to almost zero. In the as-sprayed condition, the magnitudes of the average normal stresses 11 and 22 are 36.1 ± 2.9 MPa and 36.2  3.0 MPa, respectively; the stresses increase by ~10% and ~13% to 39.6 ± 2.6 MPa and 41.0 ± 2.6 MPa respectively upon immersion. With further immersion i.e. after 28 days, they relax and stabilize around 25.6 ± 2.8 MPa and 28.4 ± 2.8 MPa. Coatings deposited on cylindrical rod show similar trend with 11 and 22 increasing from 57.7 ± 3.2 MPa to 37.2 ± 3.1 MPa to 63.4  2.6 MPa and 41.9  3.5 MPa, respectively and subsequently decreasing 51.0 3.5 MPa and 39.3  3.3 MPa, respectively. Microscopy analysis of the coating show typical plasma sprayed coating morphology with glassy smooth regions, pancake splats, cracks as well as partial molten particles across coating surface. Immersion in SBF resulted in dissolution of ions from the coating hence increasing the surface roughness. Further immersion led to the formation of a precipitate layer which grew in thickness with immersion period. The precipitate extended deeper into the coating through a 3-D network of channels. Overall, the first 7 days of immersion are crucial with SBF-induced changes of the above-mentioned occurring during this period and the near-surface region being the most affected. Substrate geometry seems to have an effect on the crystallinity, phase composition, residual stress as well as their dissolution.