Microstructure evolution in yttria stabilized zirconia during laser hybrid induction modification

To minimize crack susceptibility, a laser hybrid induction modification (LHIM) technique, involving the lowest laser energy required for continuously remelting at a preset inducting heating temperature, was used to partially remelt atmospheric plasma spraying (APS) yttria stabilized zirconia (YSZ) coating. Computational fluid dynamics (CFD) simulations indicate that a combination of elevated preheating temperature and low laser energy is essential to lower the cooling rate and temperature gradient and increase the solidification rate of melt pool. This is consistent with the experimental results obtained by eliminating the undesirable monoclinic phase and achieving the desired columnar and equiaxed grains. More importantly, the intersplat pores are healed to a higher degree, and the APS coatings become denser. These obvious structural changes decrease the differential shrinkage rate, mainly responsible for the initiation of cracks existing between the remelted layer and residual APS coatings during laser thermal shock. The healing of intersplat pores is also helpful for achieving less interfacial imperfections in the expansion of molten pool, thus developing a well-bonded remelted layer. Thus, a significantly decreased mismatch strain resulting from the lower thermal gradient and reduced differential shrinkage rate produces an acceptably low stress level for the crack susceptibility of remelted coating. Instead, the stress relief related to splat sliding is beneficial to achieve enlarged intercolumn gaps with high strain tolerance.