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8%Y 2 O 3 -stabilized zirconia (8YPSZ) and mullite (3Al 2 O 3 Á2SiO 2 ) powders, which were made plasma sprayable by using an organic binder (polyvinyl alcohol), have been plasma spray coated on to the piston head, valves and cylinder head of a 3.8 kW single-cylinder diesel engine, previously coated with Ni±Cr±Al±Y bond coat. The engine with components coated with 250 mm thick 8YPSZ and 1 mm thick mullite thermal barrier coatings has been evaluated for fuel eciency and for endurance during 500 h long rigorous tests. Improved fuel eciency was shown by the engine with coated components and the results are discussed. The coatings and the coated components have also been examined for phases, microstructure and chemical composition by X-ray diractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX). Mullite coatings were found to exhibit increased resistance to microcracking compared with 8YPSZ during the 500 h endurance test. Keywords: thermal barrier coating, partially stabilized zirconia, mullite, diesel engine, thermomechanical fatigue, fuel eciency NOTATION b.s.f.c. brake speci®c fuel consumption BHP brake horse power EBPVD electron beam physical vapour deposition ECC engine with coated components EDAX energy dispersive X-ray analysis EMULC engine components coated with mullite EUC engine with uncoated components E8YZC engine components coated with 8YPSZ FHP frictional horse power s.f.c. speci®c fuel consumption SEM scanning electron microscope TBCs thermal barrier coatings XRD X-ray diractometry 8YPSZ 8%Y 2 O 3 -stabilized zirconia BACKGROUNDThermal barrier coatings (TBCs) have been a topic of much scienti®c interest worldwide for several decades [1±4]. They have been increasingly used as thermal insulation for engine components in turbine and aircraft engines in order to achieve improved engine performance and fuel eciency [5±7] by increasing the actual temperature of engine operation. Generally, TBCs are thermally insulating materials, and a ®lm of TBC of about 250 mm applied to metals produces a temperature drop of about 150±200 C between ceramic and metal sides of the coating at temperatures of about 1000 C. The coating also increases the life of the metal component by protecting it against high temperature degradation and eliminates the need for complex cooling systems. In the 1980s, eorts were focused upon extending the advantages of using TBCs in diesel engines, with the aim of:(a) reducing speci®c fuel consumption, emissions and noise,The MS was

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