Resistivity changes of magnetron sputtered, amorphous Cr₂AlC thin films were measured during heating in vacuum. Based on correlative X-ray diffraction, in-situ and ex-situ selected area electron diffraction measurements and differential scanning calorimetry data from literature it is evident that the resistivity changes at 552 ± 4 and 585 ± 13 °C indicate the phase transitions from amorphous to a hexagonal disordered solid solution structure and from the latter to MAX phase, respectively. We have shown that phase changes in Cr₂AlC thin films can be revealed by in-situ measurements of thermally induced resistivity changes.

The oxidation behavior of Cr₂AlC MAX phase coatings with columnar and equiaxed morphologies is compared. After oxidation at 1100°C for 240 min the Cr2AlC MAX phase coatings with equiaxed grains showed a 20% lower scale thickness and a 52% larger scale grain size as the coatings with columnar grains. Transmission electron microscopy data suggests that this is primarily caused by nanopores (2–25 nm) present in the columnar grain boundaries. As their presence has previously been overlooked, a 20% improvement of the high temperature oxidation behavior of Cr₂AlC coatings is obtained if the formation of nanopores is avoided. GRAPHICAL ABSTRACT IMPACT STATEMENT This is the first report on the formation of nanopores at grain boundaries of Cr₂AlC coatings and their implications for the oxidation behavior. Avoiding the previously overlooked presence of nanopores at the grain boundaries of the as grown coatings enables a significant improvement in high temperature oxidation behavior of Cr₂AlC coatings.

We investigate the effect of continuous vs. periodically interrupted plasma exposure during cathodic arc evaporation on the elastic modulus as well as the residual stress state of metastable cubic TiAlN coatings. Nanoindentation reveals that the elastic modulus of TiAlN grown at floating potential with continuous plasma exposure is 7%–11% larger than for coatings grown with periodically interrupted plasma exposure due to substrate rotation. In combination with X-ray stress analysis, it is evident that the elastic modulus is governed by the residual stress state. The experimental dependence of the elastic modulus on the stress state is in excellent agreement with ab initio predictions. The macroparticle surface coverage exhibits a strong angular dependence as both density and size of incorporated macroparticles are significantly lower during continuous plasma exposure. Scanning transmission electron microscopy in combination with energy dispersive X-ray spectroscopy reveals the formation of underdense boundary regions between the matrix and TiN-rich macroparticles. The estimated porosity is on the order of 1% and a porosity-induced elastic modulus reduction of 5%–9% may be expected based on effective medium theory. It appears reasonable to assume that these underdense boundary regions enable stress relaxation causing the experimentally determined reduction in elastic modulus as the population of macroparticles is increased.

Monitoring the structural health and integrity of coated components is of vital importance to increase their lifetime and the overall sustainability of the targeted applications. Here, the temporal oxidation behavior of TiN thin films is tracked using in situ sheet resistance measurements. Based on correlative film morphology, structure, and local composition data, it is evident that observed resistance changes are caused by oxidation of TiN. Thickness measurements of the remaining TiN under the oxide layer are in very good agreement with thicknesses deduced from in situ sheet resistance measurements. Hence, the in situ measured sheet resistance is an autonomous self‐reporting property useful for tracking the temporal oxidation behavior of TiN coatings.