Arc-evaporation allows for alloying TiN hard coatings with Al to form a homogenous but metastable cubic Ti(1-x)Al(x)N coating with signficant performance improvements in metal-cutting applications. However, above the deposition temperature the coating decomposes to the equilibrium phases of TiN and hexagonal-AlN, which is accompanied by a deterioration in mechanical properties. A unique feature of this phase transformation is that it occurs via spinodal decomposition, which first produces nano-domains of coherent, metastable cubic-AlN, resulting in an increase in hardness via a precipitation hardening effect. Control over this process is thus highly desirable to produce optimised or self-adaptive coatings that harden in response to the tool’s working conditions.
Preliminary work using high-energy x-ray diffraction has shown that the initially high compressive residual stresses induced by arc-evaporation may have a large influence on the decomposition rate and initiation temperature. Furthermore, significant anisotropy was seen in the strain evolution during heating of these films, affected by the apparent preferential phase separation into TiN +AlN domains along the in-plane direction.
In this work the results of a detailed investigation into the effect of residual stress on the decomposition behaviour of Ti(1-x)Al(x)N films is reported. The 10µm thick Ti(1-x)Al(x)N coatings with x = 0.5 or 0.66 were produced in a commercial chamber onto WC-Co substrates, with 3 different biases to produce initial residual stresses ranging from -0.7 to -3.8 GPa. Simultaneous wide-angle and small-angle x-ray scattering data (WAXS and SAXS) were collected using 80.72 keV high-energy synchrotron radiation at beamline 1-ID the Advanced Photon Source. The films were placed in a specially designed furnace to allow in-situ analysis of the phase transformations during heating, with a sampling interval of 3°. The transmission geometry allowed simultaneous collection of diffraction data around 360°, providing 4 quadrants of sin2Ψ data for residual stress determination. DSC and nano-indentation was used to supplement the x-ray diffraction data.
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