Influence of defects on the phase stability of arc evaporated cubic titanium aluminum nitride and oxynitride coatings

Holzapfel, Damian Mauritius; Schneider, Jochen M. (Thesis advisor); Mitterer, Christian (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Book, Dissertation / PhD Thesis

In: Materials chemistry dissertation 39 (2022)
Page(s)/Article-Nr.: 1 Online-Ressource : Illustrationen

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

In the first part of this thesis, the influence of macroparticles in (Ti,Al)N coatings on the thermal stability is characterized at the nanometer scale. The presence of macroparticles in cathodic arc evaporated coatings is considered to be one major drawback of this synthesis method. We demonstrate with transmission electron microscopy (TEM) and atom probe tomography (APT) that the thermal stability of a macroparticle in the industrial benchmark coating (Ti,Al)N does not limit the overall coating thermal stability up to 1000 °C because metal-rich macroparticles exhibit a higher thermal stability than the c-(Ti,Al)N matrix. It is shown that the superior stability of the macroparticle is enabled by the self-organized formation of a c-TiN based diffusion barrier shell around the macroparticle. In the second part, the influence of oxygen on the thermal stability of (Ti,Al)(O,N) coatings is investigated. Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800 °C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000 °C to 1300 °C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase. In the third and last part, the influence of ion irradiation induced defects on the thermal stability of (Ti,Al)N is explored theoretically and experimentally. The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Materials Chemistry [521110]