ICMCTF2016 Session F4-1: Functional Oxide and Oxynitride Coatings

The low temperature synthesis of single phase α-Al₂O₃ coatings by physical vapour deposition has been subject of intense research activities in recent years. Among various technological advancements and coating concepts aiming a controlled oxide phase formation, the alloying with Cr is of particular interest as it facilitates the formation of an (Al,Cr)₂O₃ solid solution in the desired corundum structure. This material demonstrates comparable mechanical properties as, e.g., chemical vapour deposited Alumina. However, while for Al/Cr-ratios smaller than 2 the single phase corundum structure can be realised, the oxide coatings with smaller Cr-contents still exhibit additional metastable phases, which bears the risk of thermally-induced phase transformations– representing a distinctive loss of the coating’s efficiency for protective applications. Recently, the effect of Fe-doping on the phase formation of cathodic arc evaporated (Al,Cr)₂O₃ films was investigated and it was demonstrated that the addition of up to 5 at.% Fe distinctively increases the hexagonal phase fraction [1]. Here, we address the question whether these microstructural modifications also result in an enhancement of mechanical properties (indentation modulus and hardness), and how the latter scale with the Fe concentration within the coatings and deposition parameters applied. In order to examine the thermal stability of the (Al,Cr,Fe)₂O₃ solid solution vacuum and ambient air annealing up to 1100 °C is carried out. Results are discussed and compared with (Al,Cr)₂O₃ films.

[1] Koller et al. Scripta Materialia 2015 (97) 49–52

The introduction of oxygen (O) into thin films of aluminum nitride (AlN) allows for a wide range of variability in physical parameters of this transparent, hard and noncorrosive material. The already broad application field of AlN can even be expanded by aluminum oxynitride (AlON), e.g. in terms of oxidation resistant protection layers due to the inherent O content in the films.

AlON was deposited as approximately 1 μm thin films by magnetically unbalanced reactive closed field direct current magnetron sputtering (R-CFDCMS). Different O contents were established by changing the O/N ratio in the reactive gas administered to the process. O was found to incorporate into the AlN wurtzite crystallites in the form of a solid solution up to 8 at%. The resulting films were under tensile stress and experienced lattice shrinkage caused by Al vacancies and grain refinement related to enhanced renucleation rate. Increasing O insertion lead the ternary material system gradually from O-doped AlN towards N-doped aluminum oxide (Al₂O₃), hereby inducing considerable changes in crystallinity, optical refraction and residual stress state. In the regime of 8-16 at% O an intergranular amorphous Al₂O₃ matrix enveloped the AlON crystallites, reducing the tensile stress and allowing for hydrogen (H) incorporation. The threshold of 16 at% O represented the onset of compressively stressed and thus H free films consisting mainly of an X-ray amorphous AlON network in which O-N interactions were discovered. The elucidation of the different film regimes helps to reproducibly deposit films with desired physical qualities tied to the intrinsic O content and opens the way to the practical application of AlON thin films.

The aim of this study is to identify the formation of chromium oxide-nitride films by reactive sputtering of chromium target using oxygen and nitrogen as reactive gases along with helium that was used as an inert gas. The objective was to explore the consequence of temperature and nitrogen flow rate variation on formation of chromium oxide-nitride films. The identification of respective oxide/nitride phases of chromium was done by X-ray diffraction. The contact angle and surface energy of the deposited films were studied using contact angle measuring system. Initially at lower deposition temperature of 200ºC, the deposited films are amorphous and hydrophilic. When temperature is increased from 200ºC to 600ºC, the formation of crystalline films is observed and its behavior gets transformed from hydrophilic to hydrophobic. Tribological properties of chromium oxide-nitride films are examined in different testing conditions. Tribological properties of these films are dependent on variation of temperature and nitrogen flow rate.

Cathodic arc evaporation, being an extremely successful and widely used physical vapour deposition (PVD) technology, features high flexibility for the preparation of protective coating systems. Aluminium-chromium-based oxides and nitrides, for instance, are typically applied to cutting and forming tools, or milling devices, owing to their outstanding thermo-mechanical properties, wear and oxidation resistance, representing crucial capabilities for such forming and machining tools.

Another major benefit of cathodic arc evaporation is the possibility to significantly influence the coating microstructure by the adaption of deposition parameters, which allows for enhanced and specifically tailored properties.

In this regard, we have studied the architectural design and resulting mechanical properties of Al-Cr-based oxide, nitride, and oxide/nitride coatings. Therefore, Al_xCr_1-xN and (Al_xCr_1-x)₂O₃ multilayers as well as mixtures thereof were synthesised by reactive arc evaporation using powder metallurgically prepared Al_0.7Cr_0.3 targets. By careful adjustment of deposition time and reactive gas configuration, the individual (Al_xCr_1-x)₂O_3, Al_xCr_1-xN, and multilayers could be designed with different bilayer periods and interfaces. Scanning electron microscopy (SEM) analyses revealed the influence of the deposition time per layer to the architectural structure and surface topology. By reducing the time per layer during the deposition (thus increasing the total number of layers), the (Al_xCr_1-x)₂O₃ and Al_xCr_1-xN layers have different thicknesses, resulting in a decreased bilayer period, decreasing roughness and an increasing hardness. According to TEM and XRD results the Al-Cr-based multi-layered coatings indicate a single phase face centred cubic (fcc) structure with a preferred (200) and (220) orientation. With decreasing layer thickness, we observe a slight shift of the diffraction peaks to higher 2ϴ angles, which suggests reduced compressive stress formation. Evaluation of the full width at half maximum, based on the B1-like cubic (200) XRD peak, reveals a lower grain size for coatings with lower deposition time per layer.

Based on our results we can conclude that arc evaporation and the knowledge-based reactive gas flow control and arrangement leads to Al_xCr_1-xN/(Al_xCr_1-x)₂O₃ multilayers with excellent hardnesses and moderate surface rouhgnesses, when the bilayer period is optimized.

We seek to gain new insights into the fundamental deformation mechanisms of self-healing oxides in response to thermal and/or mechanical stimuli. Model systems that will be investigated are binary oxides that are at least partially restored through the extrinsic or intrinsic addition of silver or silver oxide nanoscale elemental inclusions that form ternary oxides (e.g., Nb₂O₅ + Ag à AgNbO₃).The selected oxides were produced in bulk (sintering) and in thin film (unbalanced magnetron sputtering) form. A surface crack was created and a healing agent was added intrinsically or extrinsically. The sample was annealed to stimulate the self-healing process by forming a ternary oxide in the crack region. X-ray diffraction was used to explore phase evolution, chemical compositions, and structural properties of the sintered and sputtered samples before and after annealing. SEM equipped with EDS were used to investigate the chemical and morphological properties of the fracture surface. Mechanical testing techniques were also applied to estimate the self-healing properties and performance.