Nanocomposite TiSiN coatings were deposited on tool steels. Detailed mechanisms that govern the corrosion of these coated steels were revealed, following immersion tests in a 70% nitric acid solution. Pitting originated preferentially from coating defect sites and expanded with increasing immersion time. Both Young's modulus and hardness measured by nanoindentation decreased as the corrosion damage intensified. A thin oxide layer formed from the thermal annealing of the as-deposited samples at 900 C was found to be effective against corrosive attack. In addition, compressive residual stress was noted to suppress the 2 propagation of corrosion-induced cracks. The role of residual stress in controlling the corrosion resistance of these ceramic-coated steels is clarified by finite element analysis.

Nanostructured TiSiN coatings were deposited onto a tool steel substrate. The coated samples were then annealed under vacuum at temperatures ranging from 4001 to 9001C. Both mechanical properties and residual stresses in the coatings were determined using nanoindentation methods, assisted by finite element analysis. Intrinsic residual stress was found to be dominant in the as-deposited coatings, but decreased with the increased annealing temperature. In contrast, thermal annealing has little impact on either the Young’s modulus or hardness of the coatings at temperatures up to 8001C. Grazing incidence X-ray diffraction analysis indicated that stress relaxation occurred in nanocrystalline TiN grains during thermal annealing. Direct subsurface observation, enabled by focused ion beam microscopy, revealed that microstructural characteristics, responsible for both the Young’s modulus and hardness of the coatings remained unaffected during thermal annealing. The degradation of mechanical properties for the coatings annealed at 9001C resulted primarily from the formation of a thin, soft titanium oxide layer at the outer surface

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Ceramic coatings of distinct microstructures, namely TiSiN and CrN, were deposited on tool steels. The corrosion resistance of the coated steels was measured. Following nanoindentation, immersion tests were conducted in 70% HNO3 solution for 96 h. Deformed CrN, where deformation occurred through shear sliding of columnar grains, was insensitive to corrosion attack. In contrast, open cracks formed in the indented TiSiN coating, serving as pathway for reactive agents to penetrate through the coatings. Finite element analysis was applied to evaluate the damage tolerance of the coatings and its impact on the corrosion resistance of the coated steels.

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