Mechanical testing and quantification of crack healing in self-healing yttria-stabilized zirconia thermal barrier coating

Abstract

Thermal barrier coatings (TBC) are often applied to hot components of modern turbojet engines. These coatings increase the performance of the engine and enhance the lifetime of its structural components. Thus, improving the TBC lifetime is related with a reduction of maintenance costs and increased reliability. As an alternative to conventional design approaches, a self-healing mechanism is implemented in the TBC material. To this end, alumina encapsulated MoSi2B-based YSZ composites were prepared via spark plasma sintering. First, a qualitative study of the crack healing behaviour was conducted using an indentation technique. In most of the cases, indentation cracks fracture the particles in line with the crack path. Then, a sufficient particle-matrix interface strength exists. Upon oxidation at 1100 ºC for 1, 4 and 16 h in laboratory air, the fractured particles oxidize and form glassy SiO2, that filled the crack. It has been observed that the SiO2 further reacts with the YSZ matrix and forms a well-bonded healing product, ZrSiO4. As a second step, a quantitative characterization of the recovery of load bearing capability for the self-healing TBC was performed. For this purpose, two novel mechanical test methods were developed to create a controlled crack that can be stopped, healed and re-tested. This enables to measure in the same specimen the virginal, residual and healed strength of the material. These two set-ups are the Brazilian Disc (BD) test and the Wedge-Loaded Double Cantilever Beam (WL-DCB) test. The application of both the WL-DCB and BD test to the self-healing TBC material was successful because the growth of a crack could be controlled in the brittle material. The WL-DCB test is potentially more suitable for quantification of recovery of load bearing capability. The BD test is more straightforward for determining strength of the material. Application of this BD test to benchmark YSZ material gave a strength value of around 550 MPa, which is accordance with previously reported values for this material. Following the successful achievement of controlled cracking with both methods, the quantification of load bearing capability in the healed material remains an issue of future work.