Next generation ceramic matrix composites for turbine engine efficiency

Ceramic matrix composites (CMCs) are gaining significant attention in aerospace gas turbines due to their superior performance, surpassing traditional Ni-based superalloys. However, they face environmental challenges such as water vapour and calcium magnesium aluminosilicate (CMAS), which necessitate protective coatings to improve their thermal efficiency and corrosion resistance. oxide-oxide CMCs, though less studied, show promise due to their cost-effectiveness. This study focuses on understanding and enhancing the thermo-mechanical and corrosion properties of oxide-oxide CMCs, specifically AS-N610, by investigating its thermos-mechanical properties, fatigue life, corrosion and response to notches, alongside the effectiveness of environmental barrier coatings (EBCs) for corrosion resistance.

AS-N610, made from NextelTM610 based oxide fibres with mullite matrices, demonstrates impressive tensile strength (~420 MPa) but is prone to fibre cracking and debonding, particularly in shoulder regions at temperatures up to 1200 °C. Finite Element Method analysis validates these observations. Thermal shock tests indicate a ~10% tensile strength reduction at 1100 °C and a ~34% decrease at 1200 °C due to fibre pull-out. Notched samples reveal high stress concentration, especially with circular notches. Fatigue tests indicate a service life of 1 million cycles under low cyclic loads, but this decreases with higher loads, especially in pre-manufactured notches. Notch evaluation, considering factors such as position and shape, is essential. Overall, AS-N610 exhibits excellent thermo-mechanical resistance, making it suitable for high-temperature applications.

CMAS (Calcium magnesium aluminosilicate) coated oxide-oxide CMC substrates experience a maximum weight gain of ~8% due to CMAS adhesion and penetration. Experimental results align with an analytical model, and XRD analysis confirms substrate stability up to 1400 °C. Scanning Electron Microscopy reveals calcium aluminosilicate formation at 1000 °C, which leads to substrate degradation, with delamination at higher temperatures suggesting CMAS infiltration. These findings highlight the need for protective coatings like EBCs to protect the CMC in corrosive environments. In response to this, two protective coating strategies are explored. The first involves a single-layer approach using MAX phase materials (Ti2AlC or Ti3SiC2), known for their corrosion resistance to CMAS. Ti2AlC shows superior resistance due to inward oxygen diffusion, forming protective coatings of rutile TiO2 and Al2O3. Mathematical models predict post-corrosion oxidation thickness, and both MAX phases ii demonstrate good oxidation resistance in water vapour environments, though challenges in synthesis limit their widespread adoption.

The second strategy utilises a multilayer approach with lanthanum silicate and zirconate, synthesized via solid-state method and spark plasma sintering. The prepared lanthanum silicate and zirconate shown incomplete densification and CMAS corrosion test demonstrated the superiority of lanthanum zirconate providing better resistance against CMAS due to formation of protective apatite layer. Characterisation reveals defects like void formation and crack propagation, highlighting compatibility concerns within the multilayer system. A Taguchi model identifies temperature as a key factor influencing deformation and stress distribution, with an optimal coating thickness of 300 µm for rectangular samples.

This research highlights the promising role of oxide-oxide CMCs in high-temperature aerospace applications and underscores the importance of protective coatings to enhance their performance and longevity. Further exploration into corrosion patterns and the development of cost-effective coatings is essential for the effective utilisation of oxide-oxide CMCs in high-temperature environments.