An evaluation method for interface toughness of environmental barrier coatings (EBCs) on ceramic matrix composites (CMCs)

1. Introduction Application of SiC fiber-reinforced SiC matrix composites (SiC/SiCs) in civil jet engines has been started and is expected to be expand. To use SiC/SiCs in combustion gas at high temperature of 1300-1400 ˚C, an environmental barrier coatings (EBCs) are needed to protect SiC/SiC from corrosion by water vapor. EBC and SiC/SiC substrate is subject to severe heat cycle from service temperature to ambient temperature and thus the thermomechanical durability is required as well as chemical environmental barrier properties. It is necessary to measure interface fracture toughness for the prediction of EBC life time and safe use of SiC/SiC. A variety of test methods evaluating interface adhesion of coating has been proposed. However only a few tests can provide a quantitative interface toughness as a function of the phase angle. Specifically a test for mode I rich loading which is likely to occur at the interface between the EBC and the SiC/SiC substrate with larger coefficient of thermal expansion in the EBC is lacking. In this study, a variation on the asymmetric double cantilever beam (ADCB) test is proposed so that the interface toughness can be measured in a small specimen of simple shape. 2. Experimental procedure A multilayer coating system of mullite topcoat and Si bond coat on monolithic SiC substrate was used. The coating layers of Si and mullite were deposited by atmospheric plasma spray (APS). The thickness of the layer was 100 μm in the Si bond coat and 250 μm in the mullite topcoat The coated SiC plate was cut into the specimen of 4 × 3 mm. A notch was introduced using a diamond wire saw to the depth of 0.5-1 mm along the interface. The notched specimen was mounted on an x-z stage located below a wedge. The insertion of the wedge was done by raising the motorized z stage with a constant speed. The load and notch opening displacement was measured during the test. The test was done until the load decrease almost zero and the coating was removed completely. Please click Additional Files below to see the full abstract