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.
