Produção e caracterização de revestimentos nanoestruturados em multicamadas de TiAlN/Mo

Abstract

Multilayers have been an object of growing interest over the past years. As suggested by Koehler in 1970, a strengthening effect can be obtained by using alternate layers of materials with high and low elastic constants. Several examples of this effect have been reported where multilayers revealed hardness values higher than those calculated by the rule-of-mixtures. This behaviour requires a nanometer-scale multilayer periodicity below a certain value in order to reduce dislocation motion across layer interface. Below this critical period, in most cases the hardness decreases as the period decreases. The multiple interfaces have an important role on this behaviour, working as stress relaxation areas and preventing crack propagation, influencing the mechanical properties of the system. The interfaces depend strongly on the nucleation and growth mechanisms of each layer on the previous one, and on their physical and chemical interaction. Understanding the origin of these effects requires knowledge of the interface structure, where the interfacial roughness is of prime importance. For specific engineering applications, there is an important need of modelling in order to find the optimum conditions for the individual components of the bilayer, which is not straightforward. Deposition parameters such as working and reactive gas partial pressure, ion current density and relative thickness of the multilayer material constituents, to name just a few, are crucial for the overall performance. Hence the design aspects of the coating architecture in the initial stage of development require systematic experimental investigations and are extremely time consuming, which is only surpassed by the relative ease of the subsequent reproducibility. Nanocomposite Ti0.4Al0.6N/Mo multilayers were produced by reactive magnetron sputtering with modulation periods between 1.4 and 20 nm. By growing these coatings with bilayer thickness in the nanometer range the mechanical properties are ameliorated and therefore become attractive for tribological applications. The non-isostructural system that is studied in this work comprises nitride/metal alternate layers, where the nitride (Ti0.4Al0.6N - fcc) accounts for the high hardness while the metal (Mo - bcc) provides a soft and ductile layer. Generally, when combining two different materials in a multilayer the net hardness is substantially greater than both constituents and the corresponding rule of mixtures value. Hall-Petch strengthening and dislocation blocking at interfaces are models that have been adapted to a great extent in order to explain this feature, depending on the structure and type of materials that combine the multilayer. In order to investigate the effect of the deposition parameters and layer thickness on the interface structure and roughness parameters of the films, different characterisation techniques were used. Hence, combined X-ray diffraction (XRD) and Rutherford Backscattering Spectroscopy (RBS) measurements were performed on these multilayer thin films in order to study their structural parameters with the aid of modelling. Low-angle and high-angle XRD provided us information regarding the modulation periodicity, layer thickness ratio and interfacial structural disorder, while RBS was used to study the composition profile and average interlayer roughness as a function of the number of bilayers and of the multilayer period. Furthermore, the misorientation of the textured grains was probed through XRD asymmetric experiments, while the Extended X-Rays Aborption and Fine Structure (EXAFS) analysis yielded the atomic coordination and composition of the nitride layer. Atomic Force Microscopy (AFM) analysis provided the surface roughness as a function of the lateral length scale and an estimation of the columnar grain size. Cross-sectional High Resolution Transmission Electron Microscopy (HRTEM) was also used to investigate the interfacial roughness of the multilayers and to derive the layer-bylayer crystalline texture growth. Atomic scale information on the structure of both the layers and the interfaces can be obtained as well as general features like columnar growth, grain size and orientation and grain boundaries. The diffusion between adjacent layers and waviness of the interfaces are also readily monitored and compared with the structural data available from the X-ray diffraction experiments.