Florian Rovere

Experimental and computational study on the phase stability of Al-containing cubic transition metal nitrides

The phase stability of Al-containing cubic transition metal (TM) nitrides, where Al substitutes for TM (i.e. TM1−xAlxN), is studied as a function of the TM valence electron concentration (VEC). X-ray diffraction and thermal analyses data of magnetron sputtered Ti1−xAlxN, V1−xAlxN and Cr1−xAlxN films indicate increasing phase stability of cubic TM1−xAlxN at larger Al contents and higher temperatures with increasing TM VEC. These experimental findings can be understood based on first principle investigations of ternary cubic TM1−xAlxN with TM = Sc, Ti, V, Cr, Y, Zr and Nb where the TM VEC and the lattice strain are systematically varied.However, our experimental data indicate that, in addition to the decomposition energetics (cubic TM1−xAlxN → cubic TMN + hexagonal AlN), future stability models have to include nitrogen release as one of the mechanisms that critically determine the overall phase stability of TM1−xAlxN.

Impact of yttrium on structure and mechanical properties of Cr–Al–N thin films

Cr1−xAlxN coatings are known for superior mechanical and thermal properties, which make them promising candidates for advanced machining and high temperature applications. Their oxidation resistance can further be improved by alloying oxygen active elements such as yttrium, which promote growths of dense and adherent oxide scales. Based on x-ray diffraction and transmission electron microscopy studies the authors can conclude that our films with an Al∕Cr ratio of ∼1.2 exhibit a single phase NaCl-type crystal structure for the investigated Y-content variation. With increasing YN mole fraction from 0% to 8% the lattice parameter increases from ∼4.11to4.18A, the hardness increases from ∼31to38GPa, the indentation modulus decreases from ∼497to488GPa, and the residual biaxial compressive stresses decrease from ∼−1.6to−0.6GPa, respectively. The mean crystallite feature size is found to be in the range of 28–43nm for the coatings investigated.

Thermal stability and thermo-mechanical properties of magnetron sputtered Cr-Al-Y-N coatings

Cr1−xAlxN coatings are promising candidates for advanced machining and high temperature applications due to their good mechanical and thermal properties. Recently the authors have shown that reactive magnetron sputtering using Cr-Al targets with Al/Cr ratios of 1.5 and Y contents of 0, 2, 4, and 8 at % results in the formation of stoichiometric (Cr1−xAlx)1−yYyN films with Al/Cr ratios of ∼1.2 and YN mole fractions of 0%, 2%, 4%, and 8%, respectively. Here, the impact of Y on thermal stability, structural evolution, and thermo-mechanical properties is investigated in detail. Based on in situ stress measurements, thermal analyzing, x-ray diffraction, and transmission electron microscopy studies the authors conclude that Y effectively retards diffusional processes such as recovery, precipitation of hcp-AlN and fcc-YN, grain growth, and decomposition induced N2 release. Hence, the onset temperature of the latter shifts from ∼1010 to 1125 °C and the hardness after annealing at Ta=1100 °C increases from ∼32 to ...

Protective transition metal nitride coatings

Hard coatings based on ternary Ti–Al–N and Cr–Al–N are commercial products currently employed in many industrial applications due to their outstanding chemical and physical properties, including high hardness and toughness and thermal as well as chemical stability. In this chapter the current understanding of mechanisms relevant for the thermal and chemical stability of these coating systems will be summarized based on state-of-the-art experimental and computational data. Synthesized by low-temperature (substrate temperatures below 500 °C) plasma-assisted vapor deposition (PVD) techniques, ternary Ti1−xAlxN, Cr1−xAlxN, and related coatings form metastable solid solutions. Depending on the chemical composition (and the deposition parameters used, like substrate temperature, gas pressure, and ion bombardment), the coatings crystallize in a face centered cubic (fcc) NaCl-type (c) or a hexagonal close packed (hcp) wurtzite-type (w) phase. For the main engineering applications, the cubic modification is preferred due to the superior mechanical, tribological, and oxidation properties. For example, the hardness of as-deposited c-Ti1−xAlxN and c-Cr1−xAlxN coatings, with AlN content (x) close to its metastable cubic solubility limit of x ∼ 0.7, can be as high as 37 and 30 GPa, respectively. During thermal treatments above the deposition temperature (e.g., during cutting application), the coatings undergo various processes to reach equilibrium. While for single-phase cubic Ti1−xAlxN the decomposition into the stable phases c-TiN and w-AlN occurs across the formation of cubic Al-rich and Ti-rich domains, the decomposition of Cr1−xAlxN is driven by nucleation and growth of w-AlN as well as by the release of N2 starting at temperatures around 1000 °C. The combination of experimental (e.g., x-ray diffractometry, calorimetry, nanoindentation, scanning and transmission electron microscopy, and atom probe tomography) with computational (e.g., density functional theory and continuum mechanics) studies allows for identifying, describing, and understanding the mechanisms and processes that govern the thermally induced decomposition. Thermal stability is discussed for Ti1−xAlxN-based coating systems, while chemical stability is analyzed for Cr1−xAlxN-based coating systems. Furthermore, the influence of alloying elements such as Y, Nb, Ta, Zr, and Hf on the phase formation, structure, mechanical, and thermal properties of these ternary Ti1−xAlxN and Cr1−xAlxN coatings is discussed.

Comprehensive Materials Proceedings

Hard coatings based on ternary Ti–Al–N and Cr–Al–N are commercial products currently employed in many industrial applications due to their outstanding chemical and physical properties, including high hardness and toughness and thermal as well as chemical stability. In this chapter the current understanding of mechanisms relevant for the thermal and chemical stability of these coating systems will be summarized based on state-of-the-art experimental and computational data. Synthesized by low-temperature (substrate temperatures below 500 °C) plasma-assisted vapor deposition (PVD) techniques, ternary Ti1−xAlxN, Cr1−xAlxN, and related coatings form metastable solid solutions. Depending on the chemical composition (and the deposition parameters used, like substrate temperature, gas pressure, and ion bombardment), the coatings crystallize in a face centered cubic (fcc) NaCl-type (c) or a hexagonal close packed (hcp) wurtzite-type (w) phase. For the main engineering applications, the cubic modification is preferred due to the superior mechanical, tribological, and oxidation properties. For example, the hardness of as-deposited c-Ti1−xAlxN and c-Cr1−xAlxN coatings, with AlN content (x) close to its metastable cubic solubility limit of x ∼ 0.7, can be as high as 37 and 30 GPa, respectively. During thermal treatments above the deposition temperature (e.g., during cutting application), the coatings undergo various processes to reach equilibrium. While for single-phase cubic Ti1−xAlxN the decomposition into the stable phases c-TiN and w-AlN occurs across the formation of cubic Al-rich and Ti-rich domains, the decomposition of Cr1−xAlxN is driven by nucleation and growth of w-AlN as well as by the release of N2 starting at temperatures around 1000 °C. The combination of experimental (e.g., x-ray diffractometry, calorimetry, nanoindentation, scanning and transmission electron microscopy, and atom probe tomography) with computational (e.g., density functional theory and continuum mechanics) studies allows for identifying, describing, and understanding the mechanisms and processes that govern the thermally induced decomposition. Thermal stability is discussed for Ti1−xAlxN-based coating systems, while chemical stability is analyzed for Cr1−xAlxN-based coating systems. Furthermore, the influence of alloying elements such as Y, Nb, Ta, Zr, and Hf on the phase formation, structure, mechanical, and thermal properties of these ternary Ti1−xAlxN and Cr1−xAlxN coatings is discussed.