Study on the High Temperature Strength of Ceramics and Ceramics Coating Materials Using Laser Speckle Strain/Displacement Gage

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In this study, fatigue crack growth tests for alumina ceramics and silicon nitride ceramics were performed at both room and elevated temperature. The crack opening displacement was also measured during the tests. Then the crack growth characteristics of ceramics were discussed using these experimental data. The results concerned with the crack growth tests are as follows.1.The crack growth rate was found to decrease gradually with a further crack growth, and after some crack extension a crack stopped growing temporary for both ceramics materials.2.It was found from the relationship between COD and stress intensity factor, K,that a crack opening level, Kop, increased with increasing the number of cycles. A little bit wide hysteresis was also observed in these curves. These hysteresis were thought to correspond with the effect of particle bridging on the crack opening behavior. Furthermore, the compliance of sample varied discontinuously with increasing the number of cycles.3.Both the cr … More ack arrest mentioned the above and the variation on the compliance related to the existence of the particle bridging effect near the crack tip. The stress intensity factor at crack tip was reduced by subtracting the friction stress due to this bridging effect.Next, fatigue tests for ceramics coating materials were conducted at both room and elevated temperature. The results for coating materials are as follows.1.While the blasted sample at room temperature has a higher fatigue limit than the annealed sample at the same temperature, the alumina coated sample has a lower fatigue limit that the blasted sample.2.Though the ceramics coated sample shows the elastic deformation at an early stage of fatigue life under a higher applied stress level, the plastic deformation increased with increasing the number of cycles at a middle stage of fatigue life. This behavior corresponded with the cyclic softening behavior of substrate. At a final stage of fatigue life the surface strain of the sample varied discontinuously. This means that the region with no strain due to the delamination among a coat material and a substrate appeared.3.Under a lower applied stress level, the sample was under an elastic state at an early stage, and gradually showed the plastic deformation behavior. At a final stage of fatigue life, however, the plastic strain range of sample decreased with increasing the number of cycles. This means that a single main crack can only propagate under this condition. Less