By means of the inclusion of a dispersed phase with low Young's modulus, the generally low thermal stress resistance of brittle materials for high‐temperature structures can be improved significantly. An analysis of this improvement on the basis of continuum and micromechanical theory is presented in this paper.
The low‐E phase is shown to cause a significant decrease in Young's modulus, with a negligible effect in the coefficient of thermal expansion. The relative change in thermal conductivity is a function of the thermal conductivity of the dispersed phase.
The tensile fracture stress is reduced significantly, primarily due to the mismatch in elastic properties, with smaller effects due to the mismatches in the coefficient of thermal expansion and thermal conductivity.
The relative changes in Young's modulus and tensile fracture stress are such as to result in an increase in the strain‐at‐fracture and a simultaneous decrease in the elastic energy at fracture, the driving force for catastrophic crack propagation. The accompanying increase in fracture energy also contributes to the improvement of thermal shock resistance. By changing the size of the low‐E inclusion, tradeoffs can be made between strain‐at‐fracture and elastic energy at fracture.
The fracture of CaF2, SrF2, and BaF2 crystals by {111} cleavage requires critical fracture energies (γIC) which increase with decrease in cation size, as would be predicted. The γIC of single crystals, however, can be affected by the crystallographic direction of crack propagation and by annealing treatment. The introduction of grain boundaries can also result in some increase in γIC, particularly for finer grain sizes. Slow crack growth, which is sensitive to the environment, is observed but differs from that noted in other brittle materials in that it is often accompanied by crack arrest. This observation, as well as the stair-step crack motion during slow crack growth in concentrated HF, indicates that dislocation motion at the crack tip is associated with the slow-crack-growth phenomenon. Because of the extremely strong influence of the stress intensity on crack growth velocity, the effects of delayed failure should be minimal in CaF2, SrF2, and BaF2.
The adherence of a glass bonded Pt-Au thick film conductor to various alumina substrates is degraded by changes in the surface composition of and by the presence of (0 0 0 1) crystallographic texture in the substrate. Using the critical fracture energy (7ic) required to separate the thick film from the substrate, it was found that 3'm was reduced from a maximum of 3.7 J m -2 using an as-received 96+ wt % alumina substrate to ~ 2 J m -2 using an as-received 99+ wt % alumina. In addition, the thick film adherence 3'ic using (0 0 0 1) sapphire substrates was less than that using (1 1 53) sapphire. The 96 wt % substate exhibited essentially a random crystallographic surface texture and a considerable amount of surface silicates. The as-received 99+ wt % Al2 03 substrate was charactarized by a high (0 0 0 1) surface texture and, while exhibiting a similar composition silicate layer as that of the as-received 96 wt % alumina, the total amount of the glass layer is greatly diminished. Fractographic analysis of the separated thick films and substrates showed that changes in the substrate crystallographic texture and the glass layer diminish the interpenetrating nature of the glass-metal interface and weaken the glass-substrate interface. Such changes in thick film microstructure lead to poorer thick film adherence.
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