Advances on refractory ceramic coatings - improving oxidation and fracture resistance

Abstract

Refractory ceramic coatings comprising transition metal carbides and borides are well known for their protective properties such as super hardness or highest thermal stability. However, their unique strength, based on their highly specific bonding nature – i.e. mixed covalent, metallic, as well as ionic contributions – is accompanied by two great weaknesses, namely brittle character and limited oxidation resistance. In order to overcome these deficiencies, two different strategies are elaborated within this thesis. To develop compound specific strategies (e.g. boride vs. carbide) for improving the oxidation resistance was the main motivation to comparatively describe the oxide scale formation of a metal (Hf), carbide (HfC0.96), nitride (HfN1.5), and boride (HfB2.3) coating synthesized by physical vapor deposition. Interestingly, the non-metal species decisively affect the onset temperature of oxidation, ranging between 550 °C for HfC0.96 to 840 °C for HfN1.5. HfB2.3 and HfN1.5 obtain the slowest oxide growth kinetics following a parabolic law with kp values of 4.97∙10-10 and 5.66∙10-11 kg2m-4s-1 at 840°C, indicating HfB2.3 as a quite good oxidation resistant coating material. For the HfB2.3 coating a layered oxide scale is formed with an amorphous boron oxide on top and a crystalline HfO2 region beneath. To further enhance the oxidation resistance of transition metal diboride coatings, the addition of Si was found to be highly effective, by modifying the amorphous oxide layer. Studying five different TM-Si-B2±z (TM=Ti, Cr, Hf, Ta, W) based coating systems clearly indicated Si as a predominant part in the oxide scale formation, leading to highly stable Si rich scales even up to 1400 °C. Best results are obtained for Hf0.21Si0.18B0.61 and Cr0.26Si0.16B0.58 (both around 2.4 μm thick) revealing drastically retarded oxidation kinetics (kp < 10-11 kg2m-4s-1) forming only 400 nm thin oxide scales after 3 h at 1200 °C in ambient air. A detailed study on the high temperature oxidation of Hf-Si-B2±z coatings further revealed a strong influence of the Si diffusion within the coating material. In long term oxidation treatments at 1200 °C (up to 60 h), the formation of Si precipitates within the remaining coating (related to a non-homogenous Si in the deposition), but also pure Si layers on top and bottom of the Hf-Si-B2±z coatings, next to the well adherend SiO2 based scales, highlights the different Si diffusion pathways. Nevertheless, the extremely low oxide growth rates (1.5 μm after 60 h at 1200 °C) demonstrate the excellent protectability of Hf-Si-B2±z coatings against oxidation at high temperatures.In the second part of the thesis, the enhancement of the limited fracture resistance of face centered cubic transition metal carbide coatings by non-metal alloying – substituting C with N atoms – was proven to be an effective approach. Through the addition of nitrogen, the valence electron concentration (VEC) is increased leading to a change in the bonding nature towards more metallic character, also described in detail by density functional theory calculations for Hf-C-N and Ta-C-N. These theoretical predictions are experimentally verified by sputter depositing these period VI transition metal carbides and carbonitrides using HfC, TaC, and WC targets operated in either pure or mixed Ar/N2 atmospheres. Indeed, an increased fracture toughness was observed from HfC1.3 (VEC=8) to WC0.67 (VEC=10) of 1.89 to 3.3 MPa·m1/2, respectively. The enhanced ductile character of these superhard coatings was investigated by different micro mechanical testing setups – i.e. cantilever bending tests or pillar compression – proving Ta0.47C0.34N0.19 as the best compromise obtaining 43 GPa hardness at a yield strength of 17 GPa accompanied by a fracture toughness of 2.9 MPa·m1/2.In summary, this thesis emphasizes the large potential of adapting materials properties on the atomic scale range i.e. by adapting the VEC, but also highlights the great influence of oxide scale constitution, alloying elements and diffusion driven processes during oxidation.