Friction plays an intricate role in our everyday lives, it is therefore critical to understand the underlying features of friction to better help control and manipulate the response anywhere two surfaces in contact move past each other by a sliding motion. Here we present results targeting a thorough understanding of soft material friction and how it can be manipulated with patterns. We found that the naturally occurring length scale or periodicity (λ) of frictionally induced patterns, Schallamach waves, could be described using two materials properties (critical energy release rate Gc and complex modulus (E*), i.e. λ∞ Gc /E*). Following this, we evaluated the effect of a single defect at a sliding interface. Sliding over a defect can be used to model the sliding from one feature to another in a patterned surface. Defects decreased the sliding frictional force by as much as 80% sliding and this decrease was attributed to changes in tangential stiffness of the sliding interface. The frictional response of surface wrinkles, where multiple edges or defects are acting in concert, was also evaluated. Wrinkles were shown to decrease friction (F) and changes in contact area (A) could not describe this decrease. A tangential stiffness correction factor (fx) and changes in the critical energy release rate were used to describe this deviation (F ∞ Gc *A*fx/ℓ, where ℓ is a materials defined length scale of dissipation). This scaling can be used to describe the friction of any topographically patterned surface including the Gecko's foot, where the feature size is smaller than ℓ and thus replaces ℓ, increasing the friction compared to a flat surface. Also, mechanically-induced surface defects were used to align osmotically driven surface wrinkles by creating stress discontinuities that convert the global biaxial stress state to local uniaxial stresses. Defect spacing was used to control the alignment process at the surface of the wrinkled rigid film/soft elastomer interface. These aligned wrinkled surfaces can be used to tune the adhesion and friction of an interface. The work presented here gives insight into tuning the friction of a soft polymeric surface as well as understanding the friction of complex hierarchical structures.^

Abstract not available

Abstract not available

The post-yielding behavior of some common thermoplastics was examined in uniaxial tension to determine if these materials were ideally plastic from a thermodynamic viewpoint. Various polyethylenes, poly(methyl methacrylate) and polycarbonate, polyarylate and polysulfone based on bisphenol A were studied. Thermodynamic measurements were made during deformation using a novel isothermal deformation calorimeter capable of measuring the work and heat of deformation. Thermodynamically ideal plasticity was not observed for any of the polymers examined. The polyethylenes stored approximately 30% of the input work as a latent internal energy change while this value was 40-50% for the amorphous glasses. Differential scanning calorimetry results for the deformed polyethylenes indicated that the heats of transition were less for the drawn samples than for the isotropic samples. This result was primarily due to the stored deformation energy and was not necessarily indicative of a change in crystallinity. The energy stored during drawing was explained using some commonly accepted models for the deformation of polyethylene. Additional experiments were performed to determine the mechanism of deformation energy storage and to ascertain the implications of this stored energy in engineering applications. Relaxations at temperatures much less than the glass transition temperature (T$\sb{\rm g}$) were observed for the drawn amorphous glasses using dynamic mechanical and differential scanning calorimetry measurements. Substantial thermal shrinkage was found in unconstrained drawn glassy samples exposed to thermal cycles never exceeding T$\sb{\rm g}$. Considerable stress buildup was also observed for uniaxially constrained glassy samples cycled at temperatures much less than T$\sb{\rm g}$. The values of these stresses were typically greater than 50% of the yield stress. These thermally induced events were attributed to a partial release of the energy stored during deformation. These experimental observations were explained in terms of a proposed deformation model which involves chain deformation with breakage and re-formation of intermolecular secondary bonds.

The thermodynamics of uniaxial solid deformation was studied experimentally for a number of polymeric solids, including two polyurethane-urea elastomers, natural rubber, a thermoplastic elastomer and low density polyethylene. A deformation calorimeter was developed to measure the heat and work of uniaxial solid deformation and measurements were made on the above materials. A differential scanning calorimetry method was developed to characterize the melting behavior of stretched elastomers which were found to undergo stress-induced crystallization during stretching as deduced from the large but recoverable internal energy changes measured by deformation calorimetry during uniaxial extension and contraction. Wide angle x-ray diffraction and thermostatic measurements were also performed on the elastomers held in the extended state in order to characterize the amorphous-crystalline phase transition which occurs during deformation. The motivation for this work was to evaluate the performance of the two polyurethane-urea elastomers which were found to function effectively as working substances in rubber heat engines. These elastomers could generate 1 Joule of work per gram of elastomer at about 3% of Carnot efficiency in experimental Sterling cycles.

The major theme of this dissertation is structure/property relationships in fiber-reinforced thermoplastic-matrix composites. Effort has been focused on the interface: interfacial crystallization and fiber/matrix adhesion. Included are investigations on interfacial nucleation and morphology, measurement of fiber/matrix adhesion, effects of interfacial adsorption and crystallization on fiber/matrix adhesion, and composites reinforced with thermotropic liquid crystal copolyester fibers. Crystallization of a copolyester and poly(butylene terephthalate) with glass, carbon, or aramid fibers has been studied with regard to interfacial mophology. Techniques employed included hot-stage optical microscopy and differential scanning calorimetry. Nucleation by the fibers was found to be a general phenomenon. Morphology could be varied by changing the cooling rate. In order to better monitor fiber/matrix adhesion, a buckled plate test has been developed. The test measures transverse toughness as the parameter characterizing interfacial adhesion in unidirectional, continuous-fiber composites. The test is simple to perform yet has advantages over other interfacial evaluation techniques. The buckled plate test was found to be a sensitive measure of fiber/matrix adhesion. The buckled plate test has been used along with the transverse tensile test to examine how interfacial adsorption and crystallization affect fiber/matrix adhesion in polycarbonate/carbon fiber composites. Adsorption was found to be of primary importance in developing adhesion, while crystallization is a secondary effect. The toughness data have been fit successfully for annealing time and temperature dependence. The dependence of adsorption and transverse toughness on matrix molecular weight was found to be large, with higher molecular weights adsorbing more effectively. Studies of the fiber/matrix interface have been extended to composites reinforced with thermotropic liquid crystal copolyester fibers. Composites made with these fibers had poor transverse properties, regardless of matrix. Surface treatment such as ozonation increased transverse properties, but values were still low. Scanning electron micrographs of fracture surfaces indicated that fiber splitting occurs, especially for surface treated fibers. Poor fiber transverse properties rather than fiber/matrix adhesion thus appear to limit composite transverse properties.

Defects in semiconductors can play a vital role and even dominate the performance of optoelectronic devices. Thus, understanding the relationship between structural defects and optoelectronic properties is central to the design of new high-performance materials. In this dissertation, we apply state-of-the-art first-principles approaches based on density functional theory (DFT) and many-body perturbation theory (MBPT) to quantitatively describe trap state energies and optical excitation spectra of defective bulk gallium nitride (GaN) and monolayer germanium selenide (GeSe). GaN is a technologically important wide bandgap semiconductor used as a power electronics and blue light emitting material, and naturally contains performance-degrading defects. For GaN containing a charged nitrogen vacancy, we systematically study the trap-state energies and excitonic properties. We benchmark the accuracy of hybrid DFT by comparison to MBPT studies of defective bulk GaN and determine that the HSE functional (Heyd–Scuseria–Ernzerhof) predicts trap-state energies in excellent agreement with MBPT, and that a recently developed solid-state screened range-separated hybrid (SRSH) functional can quantitatively reproduce MBPT-predicted defect energetics, including optical excitations. Additionally, we utilize MBPT to quantify the localization of the Wannier-Mott exciton in the presence of a point defect, introducing an analysis technique of the exciton envelope and center-of-mass functions to extract the Wannier exciton Bohr radius and quantify the perturbation of the exciton wavefunction due to the defect. We then utilize (TD)SRSH to study the excited-state properties of three other important defects in GaN and predict that the carbon impurity may result in the well-known yellow luminescence in bulk GaN. Finally, we apply MBPT with the same analysis techniques developed for GaN to study the optoelectronic properties of defects in monolayer semiconducting GeSe, a material that has promising applications in next-generation optoelectronic devices; we determine that a selenium vacancy strongly modifies the optoelectronic properties of the material. Overall, this dissertation provides a recipe for performing quantitatively accurate MBPT and TDDFT calculations on defective semiconductors, with a systematic study of calculation convergence and defect-defect interactions. Additionally, by an analysis technique of the BSE-computed exciton wavefunction, we introduce a framework for describing defect-induced exciton localization that can be broadly applied to many classes of materials.

A range of carbon-graphites with differing properties has been evaluated for wear resistance. These include carbons with a high degree of graphitic order (natural and synthetic graphite), those with little or no such order (pitch bonded cokes, glassy carbons) and impregnated grades. Testing has been carried out using abrasive wear, dry sliding wear, particle erosion, slurry erosion, cavitation erosion and the corresponding wear rates have been related to the bulk properties of the different materials. In all tests; hardness, elastic modulus, porosity and the presence of fillers were found to influence the wear rates of the various grades. Maximum wear rates were consistently observed with the softer, more porous unfilled carbons.

Bibliography: pages 129-139. / Silicon nitride, Si₃N₄, ceramics were produced using either silicon or silicon nitride powder. The silicon was reaction bonded in nitrogen atmosphere to form reaction bonded Si₃N₄,which was then sintered between 1700°C and 1800°C to form a dense Si₃N₄ ceramic. The silicon nitride powder compacts were also sintered between 1700°C and 1800°C. In order to achieve densification Y₂O₃-A1₂O₃ additive combination was used in both processing routes. The physical and mechanical properties of the Si₃N₄ materials was found to be dependent on the processing conditions. The post sintered reaction bonded Si₃N₄ materials had the highest densities and hardness values, while the sintered Si3N4 materials had the highest strength and toughness values. The microstructure was also influenced to a great extent by the processing conditions, and this in tum influenced the mechanical properties of the ceramics.

A study has been made of the effects of combinations of pressure and temperature ori six polycrystalline, synthetic graphites, in the high pressure domain (> 1 GPa).The graphites were investigated in three different conditions: (1) the "as received" condition (AR condition),(2) after exposure to pressures of about 3 GPa at room temperature (in a piston-cylinder device – PC condition) and (3) after exposure to temperatures of about 1500°C at pressures of about 5.5 GP a (high temperature- high pressure, or HTHP, condition). Their microstructures have been compared on the basis of X-ray diffraction measurements to determine their crystallite sizes (L˳ and L˳), interplanar spacings (c and a) and textures. Optical and scanning electron microscopy were used to examine their fracture surfaces and macro porosity. Mercury porosimetry provided a means of establishing the pore size distribution of pores of less than 20 1-1m diameter. Bulk and skeletal densities were determined from mercury porosimetry and helium pycnometry respectively. The effects of PC and HTHP conditioning on their mechanical properties, were measured by both uniaxial compression fracture tests, and by electrical resistivity measurements. In addition, the triaxial behavioursof the six graphites in the AR condition were evaluated from piston-cylinder compression tests. All the isopressed graphites were found to have very similar crystallite sizes, interplanar spacings and textures in the AR condition. The extruded graphite had larger crystallite dimensions, and was slightly less isotropic, than the other grades. Fracture occurred due to cleavage of the basal planes of crystallites in the filler particles or binder. The size, shape and orientation of filler particles and porosity with respect to the applied stress field determined whether fracture was intergranular, or trans granular, in nature. Limited basal plane slip and sub-critical microcracking caused uniaxial compressive stress-strain curves typical of those of polycrystalline graphites,i.e. convex with respect to the strain axis. Fracture strengths and strains were related to the proportion of amorphous, intercrystallite bonding and, to a lesser extent, to porosity.