Nanoindentation and nanoscratch properties of mullite-based environmental barrier coatings: Influence of chemical composition - Al/Si ratio

The mechanical properties and structural integrity of mullite (3Al(2)O(3)center dot 2SiO(2)) environmental barrier coatings (EBCs) are key factors towards the implementation of these systems in real applications. By using chemical vapor deposition, the chemical composition of mullite in these coatings may be tailored to obtain high Al/Si ratios in the outer surface, for an optimum corrosion protection in gas turbine environments. In this work, the influence of such Al/Si ratio on the mechanical properties and structural integrity of different Al-rich mullite coatings is evaluated by means of nanoindentation and nanoscratch tests. It is found that hardness and elastic modulus of the coating increases as the Al/Si ratio raises. On the other hand, an inverse trend is discerned for the apparent fracture toughness, although this is stated to be an indirect consequence of the resulting residual stress state, without any relevant effect on the intrinsic toughness of the mullite coatings. Regarding structural integrity, an increasingly brittle response to the sliding contact is evidenced as chemical composition moves towards higher Al/Si ratios. However, within the load range studied, brittle-like damage at the coating surface did not translate into critical decohesion at the coating/substrate interface. It sustains the effective protective role of the studied layers as EBCs. Therefore, all the studied coatings are found to exhibit a suitable structural integrity for their potential use as EBCs. (C) 2013 Elsevier B.V. All rights reserved.