Nitride multilayer hardcoating materials

Over the past decades, transition metal nitrides have found wide-spread application as hardcoating materials for improving wear, oxidation, and corrosion resistance of cutting tools and machine parts. Initial research on such nitrides focused on single and multilayers of several micrometers in thickness (TiN,
CrN, NbN, VN, etc.). As the properties required for hard coatings are nowadays far more demanding than those achieved by such thick layers, there is a need for the development of more advanced coating systems. An efficient way of improving the mechanical properties of hardcoatings lies in the preparation of
compositionally modulated nanoscale multilayers using physical vapor deposition techniques. Maximum hardness values ranging from 3500 to 5000 kg/mm^2 could be achieved in several nitride multilayer systems (e.g. TiN/AlN, TiN/NbN, TiN/VN, TiN/CrN, AlN/CrN). Such a high hardness is comparable to cubic-BN and is only exceeded by diamond. However, nitride multilayer coatings have the advantage that often they not only provide excellent mechanical properties but also high corrosion and oxidation resistance. On the other hand, one of the main drawbacks of multilayers is their limited thermal stability. Due to the large amount of internal interfaces, multilayers are in a metastable state. In dry-cutting applications, the operation temperatures can exceed 1000°C resulting in the dissolution of the layered structure, e.g., due to interdiffusion. Such a change in microstructure usually leads to a substantial deterioration of the mechanical properties.

A quantum mechanically based theory-guided materials design of nano-scaled superlattices containing metastable phases is critically important for future development of advanced lamellar composites with application-dictated stiffness and hardness.Our study combining theoretical and experimental methods exemplifies the strength of this approach for the case of the elastic properties of AlN/CrNsuperlattices that were deposited by reactive radio-frequency magnetron sputtering with a bilayer period of 4 nm. Importantly, CrN stabilizes AlNin ametastable B1 (rock salt) cubic phase only in the formof a layer that is very thin, up to a few nanometers.Due to the fact that B1-AlN crystals do not exist as bulk materials, experimental data for this phase are not available. Therefore, quantum-mechanical calculations have been applied to simulate an AlN/CrN superlattice with a similar bilayer period.The ab initio predicted Youngʼs modulus (428GPa) along the [001] direction is in excellent agreementwithmeasured nano-indentation values (408±32 GPa).Aiming at a future rapid high-throughputmaterials design of superlattices,we have also tested predictions obtained within linear-elasticity continuummodeling using elastic properties of B1-CrNand B1-AlN phases as input. Using single-crystal elastic constants fromab initio calculations for both phases, we demonstrate the reliability of this approach to design nano-patterned coherent superlatticeswith unprecedented and potentially superior properties.

This study is about the microstructural evolution of TiAlN/CrN multilayers (with a Ti:Al ratio of 0.75:0.25 and average bilayer period of 9 nm) upon thermal treatment. Pulsed laser atom probe analyses were performed in conjunction with transmission electron microscopy and X-ray diffraction. The layers are found to be thermally stable up to 600 1C. At 700 1C TiAlN layers begin to decompose into Ti- and Al-rich nitride layers in the out-of-plane direction. Further increase in temperature to 1000°C leads to a strong decomposition of the multilayer structure as well as grain coarsening. Layer dissolution and grain coarsening appear to begin at the surface. Domains of AlN and TiCrN larger than 100 nm are found, together with smaller nano-sized AlN precipitates within the TiCrN matrix. Fe and V impurities are detected in the multilayers as well, which diffuse from the steel substrate into the coating along columnar grain boundaries.

Microstructural and compositional changes in TiAlN/CrN multilayered films occurring at temperatures up to 1000 °C were studied at different length scales by a combination of atom probe tomography, transmission electron microscopy and X-ray diffraction. We observe the onset of decomposition of the multilayer structure at 700 °C via the mechanism of interface-directed spinodal decomposition of TiAlN layers, where Al atoms preferentially move toward the nearest interface and segregate there. The interface-directed mechanism
later transforms into isotropic spinodal decomposition and is accompanied by intense interdiffusion between the constituting layers. Distinct compositional gradients across columnar grain boundaries (extending perpendicular to the multilayers) are detected at this stage of decomposition. Drastic differences in decomposition behavior across the film depth were observed at elevated temperatures (800–1000 °C): the layered structure completely dissolves in the near-surface part but persists in the regions distant from the surface. The influence of residual stresses caused by the sputter deposition process on the thermally induced evolution of the multilayer thin films is discussed.

AlN/CrN superlattices with a B1 cubic crystal structure and a bilayer period of 4 nm were deposited by reactive radiofrequency magnetron sputtering. The coatings were investigated with respect to their thermal stability and changes in microstructure and chemical composition at 900 °C. The AlN layers show high chemical stability but undergo dissolution by pinching off at grain boundaries. A transformation from cubic to hexagonal AlN with subsequent coarsening at grain boundary triple junctions is observed. In contrast to AlN, the CrN layers show poor chemical stability and their compositions are shifted towards Cr2N upon annealing in a protective argon atmosphere due to nitrogen loss. However, even after establishing Cr2N stoichiometry the crystal structure of the layers remains cubic.