Laser cladding of ZrO₂-(Ni alloy) composite coating

The microstructure of laser-clad 60 vol.% ZrO₂ (partially stabilized with 2 mol% Y₂O₃) plus 40 vol.% Ni alloy composite coating on steel 1045 was investigated by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, energy-dispersive X-ray analysis and microhardness tests. The composite coating consists of a pure ZrO₂ clad layer in the outer region and a bonding zone of Ni alloy adjacent to the substrate. The pure ceramic layer exhibits fine equiaxed ZrO₂ grains in the outer zone and columnar ZrO₂ dendrites in the inner zone, growing from the ceramic layer-bonding zone interface. This ceramic layer is composed of metastable t′-ZrO₂ phase and a very small amount of m-ZrO₂ phase and displays a microhardness of 1700 HV_0.2. The high heating and cooling rate caused by laser cladding restrains the t → m phase transformation in the ZrO₂ ceramic layer. Interdiffusion of alloy elements takes place in the bonding zone, in which the coexistence of ZrO₂ particles, Ni-based solid solution and (Fe,Cr)₂₃C₆ particles in the interdendritic regions was found.