Paul Heinz Mayrhofer

Structure of sputtered nanocomposite CrCx∕a-C:H thin films

This work presents the structural evolution of nanocomposite CrCx∕a-C:H coatings prepared by unbalanced magnetron sputtering of a metallic Cr target in Ar+CH4 glow discharges using low negative dc bias voltages. Raman spectroscopy and x-ray photoelectron spectroscopy were used to characterize the phase composition and the chemical bonding in the films deposited at different experimental conditions. The results were correlated to the chemical composition obtained by elastic recoil detection analysis. The coating microstructure was investigated on selected samples by high-resolution transmission electron microscopy combined with electron energy-loss spectroscopy analysis. The nanocomposite coatings can be divided into hard CrCx dominated films, when prepared at low CH4 partial pressure to total pressure (pt) ratios (pCH4∕pt 0.4. The structure of the low-friction a-C:H dominated coatings consists of 2–10nm sized fcc CrC crystallites ...

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.

Structure Models of Massively Transformed High Niobium Containing TiAl Alloys

Ab-initio calculations using the Vienna ab-initio simulation package (VASP) were performed for a high Nb bearing γ TiAl based alloy with a composition of Ti-46at.%Al-9at.%Nb in order to evaluate the effect of Nb on the crystal structure. The calculations revealed that upon doping with Nb the resulting structure can have Ti and Nb atoms on Al-sites, which leads to a reduction of the c/a ratio of the tetragonal γ TiAl cell to ~1.In contrast, the c/a ratio is increased, compared to the binary phase, if the Nb atoms occupy solely Ti sites and if Ti antisite defects (i.e. Ti on the Al sublattice) are formed. The relaxed structure models were used to perform high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image simulations. The results showed that the positions of the Nb atoms should be detectable by these high spatial resolution methods, although it might be easier by HAADF-STEM investigations due to the stronger dependence of the signal on the atomic number Z.

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.