Strengthening mechanisms and thermal stability of arc deposited nanostructured TiAlN-based thin films
Titanium aluminium nitride (TiAlN)-based coatings are designed specially for applications where oxidation and wear resistance are primary demands, as in the case of cutting tools. Moreover the coatings are required to be thermally stable as well as have good "hot" hardness and fracture toughness for temperatures around 1000°C. Once these requirements are fulfilled, improved wear resistance is expected for standing extremely high shear forces developed on the tool rake face during high speed machining (HSM) in dry conditions. HSM also requires good shock resistance of coatings particularly demanding at the first second of the cutting process when the tool temperature has not reached "high temperature enhanced" plasticity. In this work, nanocrystalline films have been found to better respond to the complex paradox of conciliating high wear resistance with high fracture toughness. The synthesis of nanostructures with elevated mechanical properties became possible by using ultra high vacuum (UHV) deposition technologies such as cathode arc deposition. Compared to sputtering, for the same deposition conditions, arc permits to increase the average energy of particles extracted from the cathode. Moreover physical vapour deposition (PVD) techniques (Arc and Sputtering) permit to have good adherent coatings deposited at low temperature (<500°C). In this manner atoms lose the mobility necessary to find their preferential sites before new atoms arrive and grain growth is relented, leading to the formation of small crystallites. Alternatively the formation of nanostructures can be achieved either by layering or limited miscibility of phases as it is the case of TiN and AlN; in the form of thin films, large extent of miscibility can occur in a metastable state where phase decomposition is probable at high temperatures. Some addition of Si, B or Cr atoms can be done for obtaining refined or nanocomposite structures. The structure analysis was done in the as deposited state and after annealing performed under vacuum. Extensive transmission electron microscopy (TEM) studies combined with x-ray diffraction (XRD) spectras could give us reliable information concerning phase formation, structure morphology, preferential growth, grain sizes, etc. The mechanical characterization was done with nanoindentation by extracting hardness and elastic modulus. Relationships between residual stress (compressive) and depth sensing hardness could be found. The observation of the indenter impression in topographic and in cross section allowed to estimate values of fracture toughness. For analysing cross section of indents, TEM foils were prepared by tripod method for nanocomposite TiAlN coating with an average grain size lower than 10 nm. For structures with larger grain size and multilayers focused ion beam (FIB) was done for preparing cross section prior to observation of crack propagation in scanning electron microscope (SEM) and atomic force microscopy (AFM). Milling tests contributed for understanding processes of degradation in nanocrystalline TiAlN. For that TEM lamellas of the tool rake face were performed (when film was not too much cracked) and oxygen diffusivity to the film bulk as well as formation of oxide layers could be studied.