| The power and efficiency of industrial gas turbines is directly related to the maximum cycle temperature, that is, the temperature of the air exiting the combustor. This temperature is limited by the maximum operating temperature of the metallic materials used in the combustion zone. Previously, thermal barrier coatings (TBC) composed of plasma sprayed partially-stabilised zirconia (PSZ) have, not only dramatically improved the service history of metallic components, but have served to increase component operating temperatures by as much as 170°C. However, the current state-of-the-art thermal barrier coatings are restricted in use to operation below 1300°C and to a thickness of 0.35mm. The promise of increased operating temperatures and longer service life has prompted significant interest in the development of thicker thermal barrier coatings (>0.50mm in thickness). A study has been conducted to determine the microstructure, thermal shock resistance and oxidation resistance of thick thermal barrier coatings (TTBC) produced using standard processing parameters. By careful inspection of the coating both before and after environmental testing it has been possible to deduce the cause of premature failure in TTBCs and determine how the life can be elongated. A performance study to assess the heat transport properties across TTBCs has been undertaken on prototype combustors in a high pressure combustor facilities. The scientific investigations, currently being conducted by the European COMBCOAT consortium, have been reported. The COMBCOAT consortium have extended the understanding of the thermal spraying process through the use of advanced measuring techniques. Marked progress has been made in improving the durability of TTBCs through tighter control of processing parameters. Despite, advances made in this field, the use of TTBCs is limited by the maximum operating temperature of PSZ. The main area of interest has been in an alternative thermally-insulating technique which has been conceived by European Gas Turbines; this is the concept of porous ceramic liner materials. These materials, which can be either fibrous or foam in structure, boast excellent insulating properties, chemical stability up to 1700°C and are not constrained by geometrical factors. The friable nature of porous ceramics, however, makes them vulnerable to erosion attack and a protective coating is required to prevent deterioration. In addition, an intimate joint must be formed between the ceramic and the nickel-based superalloy component that will simultaneously restrain the movement of the insulating liner and hinder the transference of mechanical vibrations from the substrate. The combustion chamber liner development programme has been addressed by a multidisciplinary European consortium. For the duration of the programme, the author acted as project coordinator as well as having a significant input in to the experimental programme. The progress achieved by the consortium has been described in this thesis. For convenience, the material developments have been split into the four major elements of the system; bulk porous ceramic insulation, protective coating, joining media and substrate. For each element, the proposed materials, environmental testing procedures and the results of ranking trials are given. Particular attention has been paid to the scientific study on ceramic fibreboard production. This study was an ongoing investigation conducted by the author working in close liaison with the material technologist. Preliminary investigations into the materials to be used as bulk insulation and protective surface coatings have been very encouraging. However, the major difficulty stems from the development of a satisfactory joint between the porous ceramic insulation and the substrate. After extensive studies of a comprehensive range of joining methods, including both mechanical and chemical adhesion, it was not possible to achieve a firm join between the ceramic and metal. Efforts were then switched to the use of a CMC substrate which proved to be successful. However, current deficiencies in CMC materials which are currently available on the market, means that the successful production of a thermal liner is reliant on the fabrication of a cheap, good quality CMC. |
