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from the MSEL Annual Report 1997:  

ADVANCED MATERIALS PROGRAMS:  Mechanical Properties of Brittle Materials

Contact:  Edwin Fuller (301) 975-5795

Mechanical properties are the source of the greatest benefits as well as the most severe limitations of ceramic materials. Owing to their high strength-to-mass ratio, their relatively inert behavior in aggressive environments, their high hardness and wear resistance, and their ability to withstand significantly higher temperatures than metals or polymers, ceramics offer the potential for major improvements in component design for a wide range of applications. On the debit side, however, ceramic materials typically exhibit statistically variable brittle fracture, environmentally enhanced subcritical crack growth, sensitivity to machining damage, and creep/deformation behavior at elevated temperatures. Additionally, a lack of techniques, which can detect and quantify critical flaws before failure ensues, severely curtails current uses of ceramics. Unpredictable failure behavior of ceramics stems from three sources: (1) limited data and a deficiency of basic understanding of failure processes in ceramics; (2) limited standard test techniques to permit interlaboratory comparisons of materials behavior and collection of engineering data; and (3) inadequate models and statistical techniques for life prediction and reliability analyses. The Mechanical Properties of Brittle Materials Program has components specifically addressing each of these issues.

Basic understanding of mechanical behavior of ceramics is investigated at both room temperature and elevated temperatures. At room temperature, mechanical properties and failure processes are investigated in fiber-reinforced ceramic matrix composites as a function of microstructural scale and in aluminum nitride substrates as a function of processing conditions, phase content, and microstructure. Microstructural stresses related to enhanced fracture toughness are measured via micro-Raman techniques in heterogeneous microstructures and correlated with micro-mechanical modeling. Micro-mechanical computer simulations are used to elucidate distributions of residual stress distributions in highly anisotropic ceramics as a function of crystallographic texture. At elevated temperatures, the basic mechanisms responsible for crack growth, creep and creep rupture are investigated for various silicon nitride compositions, and for membrane and fuel cell materials.

To improve interlaboratory comparisons and to increase confidence in generated data, new standard test techniques for hardness and toughness are being developed and tested in round-robin experiments. Research and interlaboratory studies in instrumented indentation address the use of this technique for measuring elasticity and hardness of thin films and coatings. Micro-Raman techniques are being developed and calibrated so that quantitative assessments of microstructural residual stresses can be mapped for heterogeneous microstructures. At elevated temperatures, new creep specimens were designed which permit higher stresses with reduced non-gage section failures. Intra- and inter-laboratory studies demonstrated the robustness of these geometries. International interlaboratory studies are underway to elucidate their relationship to alternate testing geometries.

Finally, techniques to predict lifetimes of ceramics under constant and variable loading conditions are being developed. A nonparametric bootstrap approach for assessing the confidence of lifetime predictions is investigated and compared with analytical techniques. Work includes applying these techniques to aluminum nitride materials for thermal management systems and to fused silica and other glasses for spacecraft window applications. A new experimental procedure was developed for characterizing time-dependent failure under static loads.