Influence  of micro(nano) structures on the mechanical properties of functional coatings 

This PhD work deals with the synthesis and performance in use of ceramic coatings of titanium aluminum nitrides. Nanostructured coatings of metallic nitrides are commonly used in the industry as protecting coatings for cutting tools due to an excellent combination of properties including a high hardness, improved wear properties and oxidation resistance. To improve their functionality towards various applications it is necessary to test new deposition techniques beyond the conventional Physical Vapor Deposition (PVD) techniques such as magnetron sputtering. The coatings here studied are synthesized in collaboration with researchers form the team MINAMAS (Institute FEMTO.ST, Montbéliard, France) by using a “Reactive Gas Pulsing Process” (RGPP) which is a promising new technique allowing one to obtain multilayer compositionally modulated coatings with potential excellent mechanical properties.

Ceramic/ceramic multilayers are currently synthesized, cyclical variations of the nitrogen flux being imposed during the deposition process to obtain a stacking of Ti_0.5Al_0.5N/Ti_0.5Al_0.5N_x bilayers with 0≤x≤0.8 and different bilayer thickness for a total thickness of the coating of 2 microns. To better understand the relationships between the microstructure and the properties, the coatings are systematically characterized by using mechanical testing (friction test, scratch test, nanoindentation) and structural characterization techniques (X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Electron Energy Loss Spectroscopy (EELS), …).

Among the different problems identified since the beginning of this work, of particular importance is the control of the nitrogen content in the Ti_0.5Al_0.5N_x layers due to interdiffusion phenomena and/or nitrogen enrichment in the residual atmosphere and/or poisoning effect of the target during the deposition process. The concentration profile for nitrogen as a function of the experimental set of deposition parameters will be deduced from EELS experiment performed in a TEM, allowing an optimization of the deposition process for the desired nitrogen content and function in use. Moreover, preliminary results reveal that the formation of the TiAl intermetallic phase is detrimental as far as tribological properties of the coating are concerned. This can simply be attributed to the very brittle behavior of such a phase at room temperature. On the contrary, the more ductile behavior Ti_0.5Al_0.5N_0.4 with a rock salt structure should be beneficial for the desired tribological properties of our coatings.