Research Program: MECHANICAL PROPERTIES OF BRITTLE MATERIALS

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-weight 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, ceramic materials offer the potential for major improvements in component design for a wide range of applications. On the debit side, however, ceramics 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 for detecting and quantifying 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 inter-laboratory 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 both at room temperature and at elevated temperatures. At room temperature, mechanical properties and failure processes are investigated in polycrystalline ceramics, glasses, and ceramic matrix composites as a function of microstructure, environment, and processing conditions. Material systems include glasses for spacecraft windows, thermal barrier coatings, and aluminum nitride substrates. Microstructural stresses related to enhanced fracture toughness and damage mechanisms 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 stresses and microcrack damage 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, strength, 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 are designed which permit higher stresses with reduced non-gage section failures. Intra- and inter-laboratory studies demonstrated the robustness of these geometries. International inter-laboratory studies are underway to elucidate their relationship to alternate testing geometries.

Finally, techniques to predict lifetimes of ceramic materials and glasses 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 fused silica and other glasses for spacecraft window applications. A new experimental procedure is being explored for characterizing time-dependent failure under static loads.

Contact:  G. White, (301) 975-5752

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Date created October 1999
Latest revision made July 2000