Application of Sinboron Fibrous Monoliths for Air Breathing Engine Applications

Poor thermal shock and resistance to crack propagation have had a negative influence on the acceptance of ceramics for components in air breathing engine applications. In comparison with their metallic counterparts, they are less predictable and there is limited historical in-use data available. They have therefore been considered primarily for non-load bearing applications such as flaps and seals, flame holders and covers. A fibrous monolith, ceramic hybrid between a Si3N4 core with a weak BN interface has been evaluated for non-load bearing applications in air breathing engines where a high degree of thermal shock resistance is required. The fibrous monolith architecture confines damage to local regions and although the overall strength is typically reduced, the macroscopic damage tolerance and thermal shock resistance is improved over monolithic S3N4 of similar compositions. Two different architectural scales with a cell size of 150μm and 30μm were investigated. Both architectures showed a similar response with no loss in strength when quenched from below 800°C and no loss in modulus when quenched from 1000°C into water. Cyclic thermal shock from 600°C showed no change in modulus but at 1000°C a gradual drop in the modulus was observed. Microstructural assessment of the Si3N4 core and BN phases showed microcracking of the BN phase following thermal shock but no discernable changes in the Si3N4 phase. The thermal shock resistance of the FM Si3N4 / BN structure is improved over monolithic Si3N4 due to the ability of the weak BN interface to preferentially microcrack and accommodate strain. Improved thermal shock resistance has been demonstrated in water quench tests as well as in a gas burner rig test where heating rates and temperatures more representative of those observed in engine applications have been investigated.