Environmentally induced damage propagation with localised stresses in CMCs

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Key Objectives and AimsCurrent variants of SiCf/SiC ceramic matrix composites (CMC) produced by Rolls-Royce High Temperature Composites (RRHTC) contain matrix additions that control oxidation degradation via the formation of a low viscosity high oxygen permeability borosilicate glass. These CMCs undergo internal oxidation at elevated temperatures, however this internal oxidation is split into two regimes - a lower temperature 'pesting' regime and a high temperature 'self-healing' regime. Similarly, high temperature oxidation in steam results in surface recession of the CMC by volatilisation of silicon hydroxide species. This programme will investigate degradation mechanisms of CMCs and examine how the variables of temperature, humidity, pressure (total and partial pressure of water vapour) and areas of localised mechanical stress interact to affect internal embrittlement and/or surface oxidation/recession, and hence CMC lifing. Specific objectives include:1. Quantify/qualify the influence of cooling hole stress raising features in a water vapour environment. 2. Detailed fractography characterisation of steam fatigue specimens. 3. Exploration of advanced characterisation techniques such as X-CT and damaging monitoring methods in steam environments, whether in-situ or pre-exposed.4. Determine the influence of an environmental barrier coating (EBC) on environmental damage at stress concentrations.Novel MethodologiesRolls-Royce and Swansea University will collaborate together to design an implement a test configuration to investigate CMC material degradation with representative geometrical cooling holes. It is proposed that a furnace will be used to place hollow CMC tubes at temperatures equivalent to engine service along with a flow of cooling air (containing a partial pressure of water vapour) through the tube. The aim is to vary the conditions such as temperature and humidity levels, the cooling air pressure and the manufacture of the tubes to see which variables cause the greatest changes in recession rates and/or embrittlement over specific time durations. Previous test data and specimens will be provided by Rolls-Royce to the student when it is applicable to supplement the new specimens evaluated by the student.Steam-fatigue (in-plane and interlaminar) testing shall be carried out to investigate environmentally induced damage propagation with localised stresses. Tests will be analysed over a range of conditions such as pre-exposure, specimen geometries, kt factors, load vs. strain control; as well as the influence of environmental barrier coatings. A coordinated effort will also be pursued to implement damage monitoring techniques, such as electric resistance, acoustic emission and image correlation to further the understanding of the steam-fatigue results.ExploitationCMCs are focussed towards future applications in the high temperature stages of aero and land-based gas turbines. The information and methods developed in this research will inform future CMC material developments and lifing techniques. Understanding the resulting damage mechanisms and effects from various humidity's, pressures and temperatures at highly localised stresses in a range of testing environments (time-dependent, fatigue) can help reach the potential of these materials.