Phase evolution in calcium phosphate coatings obtained by in situ laser cladding

Calcium phosphate coating was fabricated by in situ laser cladding using mixed powders of CaCO ₃ and CaHPO ₄, which presented a complex phase constitution since the reactions between CaCO ₃ and CaHPO ₄ would produce not only hydroxyapatite (HA) in the coating, but also other phases, such as Ca ₄(PO ₄) ₂O (TTCP) and α-Ca ₃(PO ₄) ₂ (α-TCP). In order to realize the control of the phase constitution, the effects of the Ca/P molar ratio of mixed powders, laser power, scanning velocity and heat treatment on the phase constitution of the coatings were investigated through X-ray diffraction analysis. It is found that the variation of the Ca/P molar ratio of the mixed powders, laser power and scanning velocity can adjust, to a certain extent, the proportion of HA, α-TCP, and TTCP in the coating. However, the α-TCP and TTCP cannot be eliminated from the coating due to the intrinsic high cooling rate of the laser melt pool during laser cladding. By suitable post heat treatment, the TTCP and α-TCP in the coating can be partially or completely transformed into HA. Therefore, HA coating or coatings with desirable proportion of HA, α-TCP and TTCP can be obtained by in situ laser cladding plus post heat treatment.