The Mechanics of Creep Deformation in Polymer Derived Continuous Fiber-Reinforced Ceramic Matrix Composites [report]

E Lara-Curzio
2001 unpublished
The objective of this Cooperative Research and Development Agreement between Lockheed Martin Energy Research Corporation and Dow Corning Corporation was to study the effects of temperature, stress, fiber type and fiber architecture on the timedependent deformation and stress-rupture behavior of polymer-derived ceramic matrix composites developed by the Dow Corning Corporation. Materials reinforced with CG-Nicalon™, Hi-Nicalon™ and Sylramic® fibers were evaluated under fast fracture,
more » ... ation, and stress-rupture conditions at temperatures between 700°C and 1400°C in ambient air and for stresses between 50 and 200 MPa. Some of the stress-rupture tests conducted as part of this program are among the longest-duration experiments ever conducted with these materials. The possibility of using accelerated test techniques to evaluate the very-long term stress-rupture/creep behavior of these materials was investigated by means of stressrelaxation experiments. However it was found that because these materials exhibit nonlinear stress-strain behavior at stresses larger than the matrix cracking stress and because of environmentally-induced changes in the micro and mesostructure of the material, particularly at elevated temperatures, this approach is impractical. However, the results of stress-relaxation experiments will be useful to predict the behavior of these materials in applications where stresses are thermally-induced and therefore driven by strains (e.g., when components are subjected to thermal gradients). The evolution of the microstructure of the fibers, matrix and fiber-matrix interface was studied as a function of stress and temperature, using analytical electron microscopy. The results from these analyses were essential to understand the relationships between environment, stress, temperature and processing on t he microstructure and properties of these materials. Objectives The original technical objectives of the CRADA were: • to determine the effect of fiber type and fiber architecture on the time-dependent deformation of ceramic matrix composites densified by polymer-infiltration and pyrolysis by the Dow Corning Corporation. • To study the evolution of the microstructure of the composite constituents when the composite is subjected to stress in air and simulated combustion environments at elevated temperatures. On January 1997, an extension was requested and granted to redirect the focus of research in order to address two new objectives: • The effect of exposure to air at intermediate temperatures on the thermal and mechanical stability of these composites. • The effect of various compositions on the stability of polymer-derived SiC-based matrices. Meeting Objectives The original structure of the program had three phases, with each phase having a different number of tasks involving both ORNL and Dow Corning. Phases IV and V were added after the amendment to the scope of work on January 1997. Phase I 3 tasks Phase II 6 tasks Phase III 6 tasks Phase IV 2 tasks Phase V 2 tasks All tasks were completed except task 3 of Phase I due to technical difficulties. CRADA Benefit to DOE The results from this study have helped develop an understanding of the mechanisms responsible for the mode of deformation of continuous fiber-reinforced ceramic matrix composites. This understanding will facilitate the selection of materials, the design of components using these materials, and the development of a new generation of these materials, to address the demanding needs of the energy industries in the U.S. particularly in applications at elevated temperatures.
doi:10.2172/777651 fatcat:bb37ewg2efayrihufofutbzc6q