Mechanical characterization of mullite environmental barrier coatings (EBCs)

Abstract

Current gas turbines have reached their maximum operating temperature due to the limitations imposed by the turbine blades materials. In order to improve the gas turbine efficiency it is mandatory to be able to achieve higher temperatures in the combustion chamber. Therefore, ceramic materials are excellent candidates for turbine blades materials. Silicon based ceramics like silicon carbide have shown great properties for this application, but they suffer severe hot corrosion in combustion environments. Thus, several environmental barrier coatings (EBCs) solutions based on mullite has been tested for years, as it presents great thermal expansion match with silicon based ceramics. One of the researches in this field involves the deposition of mullite by chemical vapour deposition (CVD), which allows the gradation of the composition to alumina-rich mullite at the outer surface that gives great results at inhibiting the environmental degradation of these materials. The evaluation of the mechanical properties of these coatings is, therefore, relevant in order to grant their adherence and avoid their pull out after thermal cycles or little impacts. Hence, different CVD mullite coated silicon carbide samples have been studied in order to extract their mechanical properties, estimate their fracture toughness and study their behaviour under scratch conditions. Furthermore, the possible evolution of these properties after long term expositions to high temperatures was also assed in order to grant the mechanical stability of these coatings. The results show a relation between the mechanical properties and the characteristics of the coating, especially with the Al/Si ratio. Good adherence to the substrate and a high tendency of the coating to absorb damage without delamination were also found, with higher ductility in the case of the near stoichiometric mullite samples. In addition, thanks to the heat treatment applied to samples, it was found that samples with low Al/Si ratio tend to improve their hardness and elastic modulus, but behave slightly more brittlely; while the samples with higher ratios apparently maintain their properties but may suffer severe cracking as a result of possible residual stresses introduced during the deposition process.