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 CaCO3 and CaHPO4, which presented a complex phase constitution since the reactions between CaCO3 and CaHPO4 would produce not only hydroxyapatite (HA) in the coating, but also other phases, such as Ca4(PO4)2O (TTCP) and α-Ca3(PO4)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.

► The phase constitution varies with the Ca/P molar ratio of the mixed powders of CaCO3 and CaHPO4.
► High laser powder and scanning velocity are disadvantages for the synthesis of HA.
► Heat treatment with furnace cooling can make α-TCP and TTCP transform into HA.