Thermal shock characterization and lifetime prediction of ceramics by means of laser irradiation

The application of ceramics in gas turbines requires high resistance of the material with respect to thermal shock and thermal cycling. To study this behaviour a special test arrangement has been developed. Thin disks are centrally heated by means of a high power laser. By measuring the time-dependent temperature profile across the diameter the stress distribution can be computed. As an advantage of this method, thermal stresses can become so high that even unnotched samples of high strength ceramics are damaged. A batch of unnotched samples of hot-pressed silicon nitride was tested. For every sample the fracture stress was calculated from the measured temperature field at the instant of fracture. From this data the Weibull parameters of the ceramic were determined. To study the cycling behaviour of this silicon nitride a second batch of samples were subjected to repeated thermal shocks but with slightly reduced load. From the resulting distribution of the numbers of cycles to failure the parameter for subcritical crack growth can be determined. This parameter can be used to calculate strength-probability-time or SPT diagrams. To demonstrate thermal stress relaxation by creep, tentative experiments were made on disks of a special IR absorbing glass. At high temperature the compressive stress relaxed by creep and therefore tensile stress built up during cooling down. This tensile stress gave rise to radial cracks which soon turned into spalling cracks.