ABSTRACT

A considerable amount of research results on the manufacture, characterization, modeling and design of ceramic-matrix composites (CMCs) have been obtained at both the University of Tennessee, Knoxville (UTK) and the Oak Ridge National Laboratory (ORNL). It is crucial to incorporate the completed research advances into both undergraduate and graduate curricula at UTK. Students who enroll in the proposed courses have been exposed to the complete and state-of-the-art facilities at ORNL and UTK. Moreover, students interact with scientists and engineers who are knowledgeable and have extensive research and hands-on experience on CMCs. The courses are interdisciplinary so that students are exposed to not only materials science but also mechanics and design of CMCs. Instructional modules, such as textbooks, video tapes, slides or CD-ROMs, are being developed and available for future UTK students as well as students and professionals in other institutions.

INTRODUCTION

The research in ceramic-matrix composites is of industrial and national importance. For instance, continuous fiber reinforced ceramic composites (CFCCs) have been successfully fabricated by chemical vapor infiltration techniques at ORNL and industrial companies, such as Amercom, DuPont, B. P. Chemicals Ltd., 3M/Delta G, B. F. Goodrich and Refractory Composites. The CFCCs are being considered for elevated-temperature structural usage. The pertinent applications include boiler components, combustors, heat exchangers and hot gas filters in power generation systems, and high heat flux surfaces and first walls in fusion reactors. The science and technology for the manufacture, characterization, modeling, design and application of CMCs are of paramount importance for improving U.S. industrial competitiveness in the worldwide market.

A three-year project on "Ceramic Matrix Composites - A Combined Research-Curriculum Development (CRCD) Program" has been funded by the National Science Foundation to incorporate the long-standing research advances, achieved by UTK and ORNL, on ceramic-matrix composites (CMCs) into the interdisciplinary undergraduate and graduate level curricula at UTK, which integrates the disciplines of Materials and Mechanics.

A significant amount of research advances, covering a broad class of technologically important areas of CMCs, which include the manufacture, characterization, modeling and design, have been accomplished at both ORNL and UTK.[1-45] The research progresses of CMCs are ready for being integrated into curriculum development. The continued research efforts have been funded by the Department of Energy [Fossil and Fusion Energy Materials Programs and the Continuous Fiber Ceramic Composites (CFCC) Program] as well as the Air Force Office of Scientific Research. The UTK/ORNL research accomplishments [l-45] concentrate on the following three interrelated areas of CMCs:

(1) Manufacture and Processing

Using the traditional CMC manufacture technique, such as hot-pressing, fiber damage can occur because of the presence of elevated temperatures and pressures during processing. A great amount of progresses have been achieved in the fabrication of CMCs using the forced chemical vapor infiltration (FCVI) method developed at ORNL, which has overcome the difficulties of slow diffusion, fiber-damage, and restricted permeability.[1-3] The FCVI methods have been modeled using finite-volume analyses for achieving optimal fabrication conditions.[4-9] Moreover, the time to manufacture ceramic composites with acceptable density has been significantly shortened, which greatly decreases the fabrication cost for structural applications.[5-9]

(2) Material Characterization

A considerable amount of research advances have been made related to the understanding of the thermomechanical behavior of CMCs, covering shear, fracture, fatigue, creep, tensile, compressive properties at UTK and ORNL.[1-45] Furthermore, nondestructive evaluation has been conducted to determine the composites' elastic, thermal, and physical properties.[18,l9] Mechanistic understanding of the damage and failure mechanisms of CMCs has been provided. Furthermore, interface problems have been investigated to quantify the effect of coating thickness on fracture behavior[20,21,40,44] and to develop oxidation-resistant coatings.[20] A great amount of progresses have been made concerning the understanding of the influence of fiber fabric orientation on the fatigue and fracture behavior of CFCCs.[1l,l3,4l,43] Microstructural characterization of CMCs using the latest technological advances including atomic force microscopy (AFM)[33], high resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM),[32] and an interfacial fiber microprobe,[29,31] have been conducted before and after the application of thermomechanical loading to provide a mechanistic understanding of the microstructural evolution in the composites.

(3) Theoretical Modeling

It is well known that the microstructure plays an important role in controlling the thermomechanical and damage behavior of CMCs. The modeling and prediction capabilities of the microstructure-property relationships provide guidelines for the design and manufacture of CMCs tailored for specific structural applications. A great amount of progresses have been made concerning the prediction of the effects of microcracks and/or microvoids on the elastic properties and stiffness degradation of CMCs. These micromechanics models consider the constituents' distribution, size, shape and properties.[10-l3,l8-l9,34-40] Moreover, the extent of progressive damage, including transverse matrix crack propagation and creation, interfacial and interlaminar slipping, and fiber breakage, in CMCs has been modeled and predicted.[29-31,34-40] Interface mechanics has also been used to model the fiber push-in and push-out characteristics of CMCs in light of the application of the newly developed nanoindentation techniques.[29-31]

The vision and rationale for the proposed program could be affected by the following questions:

(a) Could the students in most of the educational institutions have access to the state-of-the-art facilities of materials fabrication, characterization, and theoretical modeling? Would they have opportunities to witness how materials are fabricated, or even to machine specimens themselves, followed by careful microstructural evaluations, including optical, scanning and transmission electron microscopy, or even AFM, and thermomechanical testing, such as creep, tensile, or flexural experiments, further followed by theoretical analyses, and therefore, having an overall understanding of CMCs? and

(b) Could they design structural components using CMCs?

The answers to the above questions are probably not positive at most educational institutions.

In the CRCD project, the students at UTK have the unique opportunity of receiving the combined materials and mechanics disciplines on CMCs. In addition, they will be exposed to the state-of-the-art facilities at ORNL and UTK, and the research progresses of the four important areas of the fabrication, characterization, modeling, and design of CMCs. The undergraduate and graduate courses on CMCs are cross-listed under both Materials Science and Engineering (MSE), and Mechanical and Aerospace Engineering and Engineering Science (MAES) Departments in the College of Engineering (CoE) at UTK, and will have three (3) credit hours with one-and-a-half (1.5) design credits in the undergraduate course. The undergraduate course is provided every Fall and will be a pre-requisite of the graduate course, which is offered every Spring semester. Drs. Liaw and/or Yu have conducted and attended all of the class lectures and labs.

(1) Undergraduate Curriculum:

At the undergraduate level, basic and overall knowledge about CMCs along with hands-on training is the focus. Students who enroll in this class have been exposed to an introduction of the features of ceramics, fibers, interfaces and CMCs. The following five interrelated topic areas have been extensively studied:

(a) Macromechanics: Stress and strength analyses of laminae and laminates have been taught. Existing interactive computer codes are available to students;

(b) Fabrication: Demos on FCVI, traditional powder, sol-gel and microwave processing methods have been introduced;

(c) Microstructural Characterization: Projects on how to determine important physical properties including the fiber volume fraction, density and porosity of the manufactured CMCs have been given to students. Students have employed optical and scanning electron microscopy to characterize the microstructures of the fabricated specimens;

(d) Mechanical Performance: The basic knowledge of the mechanical behavior of CMCs has been taught; and

(e) Design and Applications: The design of CMCs with good mechanical properties has been provided based on macromechanical models. A survey of the advantages and limitations of CMCs has been conducted.

(2) Graduate Curriculum:

The five topics of mechanics, manufacture, microstructural characterization, mechanical behavior, and design and applications have been the foundation of the graduate course. However, at the graduate level, an in-depth understanding of recent research advances along with hands-on training is the focus:

(a) Micromechanics: Elastic properties and damage evolution, such as interfacial debonding, of CMCs have been discussed;

(b) Manufacture: Students have understood the FCVI processes and visited the facilities at ORNL:

(c) Microstructural Characterization: Projects on SEM analyses of fabricated CMCs have been given to students;

(d) Thermomechanical Behavior: Recent advances in the mechanistic understanding and theoretical modeling of CMCs have been covered. Demos on fiber push-in and push-out tests using nanoindentation techniques have been conducted; and

(e) Design and Applications: Projects on the design of CMCs, based on micromechanics models, have been conducted by the students.

IMPLEMENTATION OF COURSES

The two courses on CMCs have been proposed by the authors, and approved by MSE and MAES Departments, CoE, Undergraduate and Graduate Councils at UTK. The newly developed undergraduate course - MSE 429/ES 429: Introduction to Ceramic Matrix Composites - and graduate course - MSE 528/ES 528: Ceramic Matrix Composites: Materials and Mechanics - are cross-listed under both MSE and MAES Departments in CoE at UTK, and have three (3) credit hours with one (1) design credit hour for the undergraduate course. The undergraduate course (MSE 429/ES 429) is provided every Spring semester and is a pre-requisite of the graduate course (MSE 528/ES 528), which is provided every Fall semester. Both courses serve as technical electives for all engineering majors at UTK.

(1) Personnel:

The Co-principal investigators (Co-PIs) of the project are Prof. P. K. Liaw, MSE Department, and Prof. N. Yu of MAES Department at UTK. Dr. Liaw's research and teaching interest concentrate on the fatigue, fracture, fiber coating, and thermomechanical behavior of CMCs. Dr. Yu's research and teaching experience focus on the micromechanics of the elastic properties, nonlinear characteristics, and damage of CMCs. Both Drs. Liaw and Yu are responsible for the curriculum development and liaison with ORNL. Moreover, they have been effectively working together to conduct a synergistic investigation of materials and mechanics of CMCs,[l0-l2,l8,l9] which has been integrated into the proposed materials/mechanics courses.

The projects, including hand-on experimentation, demos, and design, have been assisted by the following ORNL scientists: Drs. P. F. Becher, T. M. Besmann, D. Braski, A. Choudhury, M. K. Ferber, R. A. Lowden, O. O. Omatete and T. N. Tiegs. Furthermore, the projects involve four graduate teaching assistants and undergraduate students for equipment setup, specimen preparation, demo, grading, etc.

Other invited lecturers are Prof. C. R. Brooks (microstructural characterization, UTK), Dr. C. H. Hsueh (interface mechanics, ORNL), Dr. T. T. Meek (manufacture, UTK), and Mr. D. P. Stinton (applications, ORNL).

(2) Instructional Modules:

Instead of traditional instructional equipment, such as chalk, blackboards, transparencies, slides, etc., a multimedia projector has been employed in the classroom. Thus, every item that can be presented on the screen of a computer monitor, including the text, 3-D graphics, color pictures, and even video clips, can be projected onto a large screen with full motion and rich sound.

Experimentation of materials fabrication, specifically, chemical vapor infiltration and gelcasting methods, and mechanical characterization including flexure and fatigue tests has been videotaped for instructional purposes. The videotapes help the students to be well prepared before going to the laboratory and save costs for repeated demonstrations.

On-line class notes have been implemented using hypertext techniques incorporated with multimedia resources, i.e., hypermedia. At this moment, only the notes for the undergraduate course, MSE 429/ES 429: Introduction to Ceramic Matrix Composites, which was taught in Spring '96, are available at http://www.engr.utk.edu/~cmc. The notes for the graduate course, MSE 528/ES 528: Ceramic Matrix Composites: Materials and Mechanics, which is to be offered in Fall '97, are still in preparation.

(3) Evaluation and Assessment:

Students' evaluation of the present curriculum is conducted by UTK twice every semester. The questions on the mid-semester evaluation, performed by the UTK Learning Research Center, cover a broad aspect of teaching activities. The results of the mid-semester evaluation have been forwarded to the Co-PIs promptly for their use in the modification and improvement of the current course offering. Note that in Spring '96, the students gave the undergraduate course a quite positive evaluation on almost all aspects of the curriculum.

The Campus Teaching Evaluation Program (CTEP) is done by the UTK Office of Academic Affairs at the end of each semester. The results of CTEP are used to improve future offerings of the proposed course and are available to the Co-PIs after the students have received their grades. The results of the evaluation are also compared with the average performance of the Department(s), CoE, and the University.

A significant amount of research progresses, including the manufacture, characterization, modeling and design of CMCs, have been made at both UTK and ORNL. These advances have been integrated into both undergraduate and graduate curricula at UTK. Students who enroll in the proposed courses have been exposed to the state-of-the-art facilities at ORNL and UTK.

The proposed undergraduate curriculum emphasizes the basic concepts, overall knowledge, and hands-on skills of the fabrication, and microstructural and mechanical characterizations of CMCs, as well as fundamental theories of mechanics of CMCs. On the other hand, the proposed graduate course provides in-depth knowledge of the processing, thermomechanical behavior, and micromechanics models of CMCs. The developed courses are interdisciplinary so that students are exposed to not only materials science but also mechanics and design of CMCs. Instructional modules, such as video tapes or slides, will be developed and available for future UTK students, and students and professionals in other institutions.

ACKNOWLEDGMENTS

The present program is supported by the National Science Foundation (NSF) Combined Research-Curriculum Development (CRCD) Program under contract number EEC9527527, and the Office of Research Administration (ORA), the Center for Materials Processing (CMP) and the Office of Dean of Engineering (ODE) at UTK. We are very grateful to Ms. M. Poats, the CRCD Program Manager, Dr. K. Walker of ORA, Dr. C. McHargue, the Director of CMP, and Dean J. E. Stoneking of ODE for their financial support.

The authors would also like to thank Drs. P. F. Becher, T. M. Besmann, D. Braski, C. R. Brooks, A. Choudhury, D. F. Craig, M. K. Ferber, C. H. Hsueh, R. A. Lowden, T. T. Meek, O. O. Omatete, D. P. Stinton, and T. N. Tiegs for their involvement in teaching the class, Drs. M. Devine, C. McHargue, J. E. Stoneking, and K. Walker for providing cost sharing, and Mr. J. W. Baldwin, Mr. R. Lichtwardt, Ms. G. Worley, Mr. J. Kim, Mr. T. Somphone, Mr. M. Webb, Ms. Y. Zhang, Mr. W. Zhao and Ms. L. Ziegler for their assistance.

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