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Comparative investigation into effects of ZrO2 and Al2O3 addition in fluorapatite laser-clad composite coatings on Ti6Al4V alloy

Composite coatings consisting of fluorapatite mixed with 20 wt% yttria (3 mol%) stabilized cubic phase zirconia (c-ZrO2, 3Y-TZP) or 20 wt% alumina (α-Al2O3) were deposited on Ti6Al4V substrates using a Nd:YAG laser cladding system. The interface morphology, phase composition, micro-hardness and biological properties of the two coatings were examined and compared. The results showed that the fluorapatite/Al2O3 specimen underwent a greater inter-diffusion at the interface between the coating layer and the transition layer than the fluorapatite/ZrO2 specimen. During the cladding process, the ZrO2 and Al2O3 components of the coating were completely decomposed or underwent phase transformation. In addition, the fluorapatite was partially decomposed. For both specimens, the coating layers contained fluorapatite, CaF2 and CaTiO3 phases. The coating layer of the fluorapatite/ZrO2 specimen additionally contained TTCP, CaO, CaZrO3 and m-ZrO2 (monoclinic phase), while that of the fluorapatite/Al2O3 specimen contained β-TCP, CaAl2O4 and θ-Al2O3. The average micro-hardness of the fluorapatite/ZrO2 coating layer (1300 HV) was approximately 200 HV higher than that of the fluorapatite/Al2O3 coating layer (1100 HV). Both specimens generated dense bone-like apatite following immersion in simulated body fluid for 3 days. In other words, both specimens had a good in vitro bioactivity. However, the fluorapatite/ZrO2 specimen showed a better initial attachment and spread of osteoblast-like osteosarcoma MG63 cells than the fluorapatite/Al2O3 specimen in in vitro biocompatibility tests performed for 24 h.