AlTiN and AlTiON-coated hot work tool steels for tooling in steel thixoforming

X32CrMoV33 hot work tool steel samples coated with AlTiN and AlTiON were submitted to thermal cycling under conditions that approximate thixoforming of steels and to sliding wear tests at 750 degrees C, measured to be the cavity surface temperature shortly after the steel slurry is forced into the thixoforming die. AlTiN and AlTiON coatings provide adequate protection against oxidation of the tool steel substrate, but fail to avoid thermal fatigue cracking. This is attributed to the extensive softening of the substrate, the thermal expansion mismatch between the hot work tool steel and the coatings and residual compressive stresses inherited from the deposition process. The impact of AlTiN and AlTiON coatings on the high temperature wear resistance, on the other hand, is favourable. The improved wear resistance is attributed to the stable, protective surface oxide films.