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Processing and Properties of Nanophase, Transparent and Layered Composites
Materials with nanophase microstructures can have unique electrical, magnetic, tribological, and mechanical properties. Similarly, layered composites are expected to display unique tribological and mechanical properties. However, some of these unique property attributes are either lost or not attainable by conventional processing techniques, which require higher temperatures. This project is exploring the use of electrochemical deposition for synthesizing nanocomposites, layered composites, and fiber-reinforced composites near room temperature. The influence of processing parameters on the microstructure and tribological/mechanical properties is under study.
Reaction-Based Processing of Fiber-Reinforced Ceramic Composites
Processing approaches that lead to dense and net-shape composites are needed for fabricating fiber-reinforced composites in a cost-effective manner because conventional sintering and hot-pressing methods are either not desirable or do not work for fabricating dense composites. Reaction-based processing approaches offer net-shape processing at lower temperatures and without the application of pressures. This project is exploring Si-C and Si-C-Metal reaction-based processing approaches for fabricating SiC composites. In addition, fundamental aspects of reaction kinetics, preform processing, and their influence on the microstructure are being studied. The influence of processing variables on the microstructure and thermal/mechanical properties is also investigated.
Electromechanical Response of Ferroelectric/Piezoelectric-"Smart" Materials
Ferroelectric/piezoelectric ceramics have the capability of sensing (stresses) and actuating (strains) in response to an electrical or mechanical stimulus. In many of their applications these materials are often subjected to extremes of stresses and electrical fields under which cracks can form and grow leading to failure of components. In this research project, a fundamental study of the influence of mechanical and electrical stresses on the crack formation and propagation is underway which has already demonstrated that the crack propagation can be influenced (enhanced or stopped) by certain type of the applied electric field. This knowledge is being used in designing "Smart" materials that respond in a certain way because of the applied electric field and may display superior reliability in real applications. These studies are being performed on a variety of PZT and electrostrictive PMN-PT ceramics processed in our group.
Study of Field-Induced Phase Transformation in Ferroelectric Ceramics
Ferroelectric ceramic compositions near the morphotropic phase boundary in PLZT system show field-induced antiferroelectric(AFE) to ferroelectric (F) phase change with concomitant enhancement in electrical (dielectric constant) and electromechanical (strain) properties. Interesting properties have been reported in Sn-modified PLZT compositions. Present research is studying modifications to PZT compositions with a variety of cation substitutions and their roles on the phase stability, induced-phase transformations, and electrical properties. Synthesis of dense PZT compositions is being done by powder processing and sintering routes. Studies of induced-phase transformation are accomplished by characterizing structural changes using x-ray diffraction and TEM, and by measuring electrical/electromechanical response.
High-Temperature Electronics Based on Diamond and C-BN Thin Films
High temperature electronics has emerged as a very important area because the dominant silicon electronics provides low reliability or fails to function altogether at elevated temperatures. Applications, which depend on high temperature electronics, are not only limited to military systems, but include commercial utilization in air/space travel, automobiles, smog control, geothermal energy, well logging, and nuclear reactor monitoring and control.
The primary objectives of this project are to synthesize and characterize thin films of high resistivity (undoped), and p- and n -doped polycrystalline diamond (PCD) and cubic-boron nitride (c-BN) suitable for the fabrication of high temperature electronic devices, and application/demonstration of these films for the fabrication of high temperature microelectronic devices. The thin films of undoped diamond have been prepared by ECR (Electron Cyclotron Resonance) plasma enhanced chemical vapor deposition (ECR-PECVD) on (100) Si substrates. The influence of different process parameters (e.g., plasma gas composition, substrate temperature, pressure, and plasma density) on the composition, crystal quality, and properties of the PCD and films are being studied. In addition, electrical properties of the films are being determined by resistivity measurements. A plasma etching technique has been applied to open windows through the Si substrate and form capacitors using Mo electrodes for probing C-V and resistivity of the diamond films. This will provide information on the resistivity of the diamond films for fabricating electrical devices. In particular, the following two basic circuits are proposed for investigation: (1) a voltage multiplier stack (VMS) which requires the integration of MSM Schottky diodes with MSM interdigitated capacitors, and (2) an optical bridge of four MSM detectors for DC-to-AC conversion and for driving the voltage multiplier stacks.
Fiber-Matrix Interfacial Properties in Ceramic and Metallic Composites
Fiber-matrix interfaces play a dominant role on the mechanical properties of fiber-reinforced Ceramic- and Metallic-composites. In particular, optimum interfaces are essential for toughening composites via fiber-bridging and pullout mechanisms. This research is aimed at measuring mechanical and morphological characteristics of fiber-matrix interfaces using fiber pushout tests and high-resolution TEM. In addition, the influence of fiber coatings, surface roughness, and expansion mismatch in modifying interfacial properties are studied. Another objective of this research is to relate interfacial characteristics to mechanical response of composites.
Role of Interfacial Properties on Mechanical Behavior of Ceramic Composites
Changes in interfacial properties can influence matrix cracking strength, ultimate strength, and toughness of ceramic composites. In addition, interfaces play a dominant role on the crack-fiber interaction and consequently toughening potentials of ceramic composites. In one aspect of this research, the influence of changes in interfacial properties on the first-matrix cracking stress is being studied, and in another aspect the influence of crack size in the matrix on the matrix cracking strength is under study (steady state vs. non-steady state). Also, under study are the micro-scale observations of interactions of cracks in the matrix with reinforcing fibers or fiber coatings and relating these observations to the overall mechanical response and toughening mechanisms of composites.
Mechanical Behavior of Fiber-Reinforced Ceramic Composites at Elevated Temperatures
Ceramic composites offer unique advantages over polymer- and metal-matrix composites in mechanical properties at elevated temperatures. However, the mechanical properties of ceramic composites at elevated temperatures are not well understood. This research program is aimed at studying plasticity and creep behaviors of fiber-reinforced ceramic composites at elevated temperatures. Of particular interest are the influence of the fiber-matrix interfacial and constituent properties on the creep/plasticity of composites, and mechanisms of plastic deformation/creep. These objectives are being achieved by fabricating composites with controlled interfaces, microstructures, and fiber architectures, and then performing mechanical tests at elevated temperatures in controlled atmospheres. TEM studies of the deformed microstructures are also planned.
Thermal Shock Behavior of Continuous Fiber-Reinforced Ceramic Composites
Ceramic composites with toughening potentials far superior to the monolithic ceramic have been developed. These composites are often exposed to thermal transients in real applications, which can induce damage because of the thermal shock. Such thermal shock damaged composites containing matrix cracks are further susceptible to additional damage by fatigue and oxidation at elevated temperatures. A fundamental study is underway to characterize damage to composites by thermal shock using the water quenches technique, and the damage to composites is being characterized by destructive (strength) and non-destructive (ultrasonic/thermal diffusivity) techniques. In addition, the influence of fiber, fiber architecture, interfacial properties, and method of composite fabrication on the thermal shock behavior of composites is being studied. These results are then analyzed to identify mechanisms of the thermal shock damage in ceramic composites.
Fracture and Crack-Tip Behavior in Fiber-Reinforced Ceramic Composites
Crack propagation and crack-fiber interactions play dominant roles in fracture and toughening of ceramic composites. In particular, processes in the vicinity of a crack tip such as crack-fiber interaction, fiber-bridging stresses, interfacial sliding, and fiber failure are important in understanding crack propagation and fracture in ceramic composites. In-situ studies are underway to characterize the influence of interfacial properties on the crack tip behavior and crack fiber interactions in ceramic composites. Of particular interests are measurements of crack shape, crack opening displacements, and crack bridging stresses as a function of applied stress and crack length. This knowledge is then used to model fracture and crack growth resistance of ceramic composites using a cohesive crack growth model.
Finite Element Modeling of Materials Behavior
A broad-based modeling activity is underway in support of some of the above mentioned experimental studies to simulate behavior of polycrystalline materials, fiber-reinforced composites, and electromechanical response of ferroelectric/piezoelectric ceramics. Finite element modeling (FEM) of polycrystalline material properties using single crystal values has been done using a novel Voronoi-Tesselation approach. This approach can simulate real polycrystalline microstructures, which are otherwise difficult to model using other approaches. Thermal properties of polycrystalline materials have been calculated from single crystal values, and the results are in excellent agreement with the experimental values. 3-D FEM models have also been developed to determine properties of fiber-reinforced composites, such as expansion behavior, elastic properties, and thermomechanical residual stress distributions as a result of expansion mismatch. Recently, modeling of electromechanical response of piezoelectric ceramics has also been successful in explaining the influence of electric field and mechanical load on the crack propagation in PZT. These FEM models are also being used in planning critical experiments to illustrate unique response of materials to mechanical and electrical stimulus, and in demonstrating/simulating behavior of materials under extremes of operating conditions.