Thermal fatigue performance of yttria-stabilized zirconia-coated hot work tool steel was investigated under conditions encountered by thixoforming dies in semi-solid processing of steels. The thermal barrier effect of the YSZ coating was evident from the relatively smaller and more uniform temperature gradients set up across the section of the sample, which in turn, had a direct impact on the magnitude of thermal stresses generated at the surface of the tool steel. In spite of some thermal expansion mismatch with the substrate tool steel and occasional discontinuous thermally grown oxides along the splat boundaries and between the BC and TC, the YSZ coating survived thermal cycling under steel thixoforming conditions for over 12,000 cycles with no evidence of debonding and spallation. This is a nearly ten-fold increase in thermal fatigue life with respect to the uncoated counterpart. An increasingly dense YSZ layer and the toughening linked with the tetragonal to monoclinic ZrO2 phase transformation are credited for the remarkable performance of the YSZ-coating. It is fair to conclude that the YSZ coating offers adequate protection for the underlying tool steel, which without YSZ coating, suffers severe oxidation after only several hundred cycles and extensive cracking after 1000 cycles.
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