Effect of target power density on the chemical composition, phase formation, and mechanical properties of MAX-phase Cr$_{2}$AlC coatings and prediction of mechanical properties of VAlN

Abstract

Huge progress has been made in the areas of material design and synthesis process development for protective coatings in the last decades. The common strategies for the industrial-scale deposition of multi-element containing coatings are based on reactive sputtering of elemental or multielement targets or based on non-reactive sputtering utilizing compound or composite targets. In both cases, it is desired to conserve either the elemental ratio of the target (reactive) or the whole target composition (non-reactive) in the coating. In case of non-reactive depositions, it is well-known, that the coating composition may drastically deviate from the target composition. To overcome this limitation, a methodology based on a combinatorial synthesis approach is derived within the first and second part of this thesis to enable the development of a processing parameter-dependent target composition, which furthermore allows to address application-related requirements (e.g. single-phase and dense coatings) for the synthesis of Cr$_{2}$AlC MAX-phase coatings by ion-assisted growth. First, the effect of target power density, substrate bias potential and substrate temperature on the coating composition was studied. A Cr-Al-C composite target was sputtered utilizing direct current (DCMS: 2.3 W/cm²) and high power pulsed magnetron sputtering (HPPMS: 373 W/cm²) generators. At floating potential, all Cr-Al-C coatings showed similar compositions, independently of the applied target power density. However, as substrate bias potential was increased to -400 V, aluminum deficiencies by a factor of up to 1.6 for DCMS and 4.1 for HPPMS were obtained. Based on the measured ion currents at the substrate, preferential re-sputtering of Al is suggested to cause the dramatic Al depletion. As the substrate temperature was increased to 560 °C, the Al concentration was reduced by a factor of up to 1.9 compared to the room temperature deposition. This additional reduction may be rationalized by thermally induced desorption being active in addition to re-sputtering. Hence, it is evident that the composition of these films sputtered from the here employed composite target is strongly affected by the deposition conditions and that for a certain intended film composition - for example Cr$_{2}$AlC - a processing parameter specific composite target composition is required. Secondly, to identify such a processing parameter specific composite target composition for the synthesis of the MAX-phase Cr$_{2}$AlC, which is indispensable for industrial application, a split Cr$_{0.46}$-Al$_{0.25}$-C$_{0.29}$ / Cr$_{0.15}$-Al$_{0.75}$-C$_{0.10}$ composite target was designed, which, due to the combinatorial approach, enables the compensation of the target power density and substrate bias potential induced Al-deficiencies. Furthermore, to utilize Cr$_{2}$AlC as protective coatings in nuclear, energy conversion or aerospace applications, several processing-related requirements need to be fulfilled. For example, the deposition process has to yield a dense and single-phase coating at substrate temperatures compatible with metallic substrates. Therefore, the effect of target power density and substrate bias potential on phase formation, microstructure evolution, and mechanical properties of sputtered Cr$_{2}$AlC coatings was studied. At a deposition temperature of 560 °C, DCMS and HPPMS were utilized. Generally, HPPMS resulted in coatings with superior density and hence larger elastic moduli compared to DCMS, indicating that ion bombardment by ionized film-forming species is beneficial. However, as the substrate bias potential was decreased to -200 V for DCMS and -100 V for HPPMS, the ion bombardment induced formation of the disordered (Cr,Al)$_{2}$C$_{x}$ solid solution was observed. It is evident that there is an optimum moderate ion energy for the formation of dense, single-phase Cr$_{2}$AlC MAX-phase coatings. Too small energy results in the formation of under-dense coatings. Too large energy results in the formation of the disordered (Cr,Al)$_{2}$C$_{x}$ solid solution in addition to the MAX-phase. Besides identifying an optimum processing window for the formation of Cr$_{2}$AlC coatings utilizing a split Cr$_{0.46}$-Al$_{0.25}$-C$_{0.29}$ / Cr$_{0.15}$-Al$_{0.75}$-C$_{0.10}$ composite target, the combinatorial synthesis approach furthermore enables the identification of the required processing parameter-dependent composite target composition for the deposition of stoichiometric Cr$_{2}$AlC coatings from a single-composition composite target. In the third part of this thesis, the extensive and theoretically unexplained spread in experimentally obtained elastic moduli ranging from 254 to 599 GPa for (V$_{1-x}$Al$_{x}$)$_{1-y}$N$_{y}$, as reported in literature, was explored. To identify its origin, the effect of chemical composition (0 ≤ x ≤ 0.75), non-metal to metal ratio (N/M-ratio: 0.48 ≤ y ≤ 0.52), and stress state (-6 ≤ σ ≤ 2 GPa) on the elastic modulus at room temperature was studied systematically by density functional theory employing the Debye-Grüneisen model. As the Al concentration is increased from x = 0 to x = 0.75, strong Al-N sp³d² hybridization causes an increase in elastic modulus of 26 %. The effect of the N/M-ratio on the elastic properties is also Al content dependent. As y is increased from y = 0.50 to y = 0.52, decreasing bond distance upon vacancy formation causes an anomalous increase in the elastic modulus of 6 % for V$_{1-y}$N$_{y}$, while a decrease in elastic modulus of up to 5 % occurs for (V$_{1-x}$Al$_{x}$)$_{1-y}$N$_{y}$. A stress state variation from +2 to -6 GPa increases the elastic modulus e.g. for (V$_{0.5}$Al$_{0.5}$)$_{0.5}$N$_{0.5}$ by 70 GPa and hence 13 % due to shifts in density of states towards lower energies implying bond strengthening. Thus, it is suggested that the extensive spread of 58 % in reported elastic moduli for (V$_{1-x}$Al$_{x}$)$_{1-y}$N$_{y}$ can at least in part be rationalized based on variations in chemical composition, off-stoichiometry induced point defects, and stress state