ICMCTF2011 Session B4-1: Properties and Characterization of Hard Coatings and Surfaces

Hard coatings based on transition metal aluminum nitrides are well established and routinely used for various industrial applications due to their outstanding properties like high hardness, wear and corrosion resistance. However, the favourable phenomena are often strictly connected to their crystalline structure. While Ti-Al-N coatings are known to crystallize either in cubic or wurtzite structure (or a mixture of both) depending on the Al content, the crystallization behavior is more complicated for Zr-Al-N and Hf-Al-N, which can also form Zr₃N₄ and Hf₃N₄ based structures.

Here we present experimental and ab initio studies with emphasis on phase stability ranges and lattice structures of ternary Ti_1-xAl_xN, Zr_1-xAl_xN_y and Hf_1-xAl_xN_y alloys. The concept of special quasi-random structures to simulate more random-like alloys is compared with studies of ad hoc structures which can mimic also clustering of ions. Several crystallographic configurations (like cubic, wurtzite, hexagonal) are considered, as these are the stable and metastable phases of the binary constituents.

A strong dependence of the various phase-stability-ranges on the alloyed elements is obtained. In particular, the maximum metastable solubility limit for Al in the cubic structure decreases from ~0.7 for Ti_1-xAl_xN to ~0.6 for Zr_1-xAl_xN and to ~0.55 for Hf_1-xAl_xN. This result is rationalised by an analysis of the structural and electronic configurations and is confronted with experimental data showing a strong dependence of the evolving crystalline structure on the N₂ partial pressure during growth. We furthermore show that the Al-content of the films also strongly depends on the N₂/Ar partial pressure ratio when sputtering a powder metallurgically prepared alloy target.

TiAlN alloys are of interest for hard wear protective coatings, for example on metal machining cutting inserts. Arc evaporated TiAlN coatings can be grown as metastable solid solutions in the cubic (c) B1 structure over a wide range of Ti/Al ratios. This B1-structure possesses a broad miscibility gap and may decompose spinodally into c-TiN and c-AlN rich domains when heated. The evolving differences in lattice parameter and elastic constants during heat treatment affect the decomposition behavior and the final microstructure. Here the phase field approach is used to solve the Cahn-Hilliard equations including the elastic energy term to describe the microstructure evolution during decomposition. In order to appropriately address TiAlN alloys enthalpy of mixing and the elastic stiffness tensor, determined by first-principles density-functional theory (DFT) calculations, are also taken into account as input parameters.

The microstructure and local strain evolution is studied during spinodal decomposition. At compositions above 40 at.% AlN the microstructure reveals nanosized domains with preferred growth directions in the elastically compliant directions <100>. At AlN contents below 40 at.% where the elastic anisotropy is less pronounced spherical domains are instead formed. Hence, there is a strong correlation between the evolved microstructure and the elastic properties. The initiation of the spinodal decomposition is notably slower when the composition is displaced from the center of the miscibility gap towards lower amounts of AlN. In addition, the evolved modulation in elastic properties itself slows down the decomposition since ­it results in an increased amount of stored elastic energy. The origins of the observed kinetic differences are discussed and the simulated microstructure evolution is compared to experimental observations by small angle x-ray scattering, scanning transmission electron microscopy, and atom probe tomography.

This study was intended to investigate the effect of internal stress on cutting performance of coated carbide tools.

In order to control the internal stress, PVD films were deposited under a lot of bias conditions. The depth profiles of internal stress measurement were carried out by using synchrotron radiation ‘SPring-8’ in Japan. In case of the PVD-TiAlN film, the bias voltage was changed linearly gradient from -50V up to -150V during the deposition, the compressive residual stress was increased to a maximum value of 5.5 GPa gradually from a substrate to a surface of film. Such a gradient stress film enhanced a balance between fracture property of cutting edge and adhesion of film. The film microstructure was evaluated by using Electron Backscatter Diffraction Pattern (EBSP) and Scanning Ion Microscopy (SIM). Furthermore, the cutting test was carried out with different stress condition films of TiAlN, TiSiN, AlCrN etc.

On the other hand, in order to control the internal stress of CVD-TiCN/Al₂O₃ film, shot-peening was treated from film surface. The internal stress of CVD-TiCN/Al₂O₃ film was changed from tensile stress (0.24GPa) to compressive stress (-0.17GPa) by shot-peening treatment. Such CVD film shows high chipping resistance in steel machining.

In this investigation, plasma nitriding treatment parameters have been optimized to achieve a modified nitrided microstructure to deal with wear issues while maintaining other mechanical properties. For this purpose, the nitriding treatment was performed at low temperatures to minimize grain growth of the bulk material. Also, the active nitriding gas composition was diluted for minimal formation of hard, brittle nitride phases on the surface and enhanced diffusion of nitrogen atoms. Prior to the treatment, samples’ surfaces were polished and possible sources of surface contamination like oxide scales were removed. The SEM, FIB and TEM examinations of the nitrided samples showed a thin compound layer (< 2μm) composed of TiN and Ti₂N formed on top of a relatively deep diffusion zone (~ 40μm). The surface-and depth-profiling chemical analysis was established by incorporating XRD, GDOES, and XPS results. Conventional metallographic preparation technique was used to prepare samples for nano-hardness versus depth measurements. Micro-scratch tests under progressive loading were performed to determine the resistance to surface deformation and the critical failure load of the compound layer to the diffusion zone as well as its friction behaviour. The results showed good interfacial bonding as well as load bearing capacity. The scratch paths were investigated using FIB, SEM and optical surface profilometry. The results showed that ductile β particles embedded at α-grain boundaries in the diffusion zone served as crack deflection sites. The hardened diffusion zone suppressed extensive plastic deformation of the substrate and provided good mechanical support for the compound layer, thus early spallation was inhibited and surface fracture toughness was increased.