K. T. Faber

Crack deflection processes—I. Theory

A fracture mechanics approach has been used to predict fracture toughness increases due to crack deflection around second phase particles. The analysis is based on a determination of the initial tilt and the maximum twist of the crack front between particles, which provides the basis for evaluating the deflection-induced reduction in crack driving force. Features found to be important in determining the toughness increase include the volume fraction of second phase, the particle morphology and aspect ratio, and the distribution of interparticle spacing. Predictions are compared with expected surface area increases.

Crack deflection processes—II. Experiment

Two microstructural characteristics, particle morphology and size, have been examined with respect to toughening by crack deflection. Particle morphology effects were evaluated in a series of hot-pressed silicon nitrides comprised of rod-shaped grains of various aspect ratios and a bariumsilicate glass ceramic containing spherulites. Lithium-alumino-silicate glass ceramics containing Li_2Si_2O_5 lath-shaped crystals were studied for particle size effects. Independent measures of the fracture toughness and the crack deflection process were performed and results were correlated with a crack deflection model.

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y2O3-ZrO2 (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y2O3 stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y2O3 in the t-zirconia began to decrease as predicted by the Y2O3-ZrO2 phase diagram. The c-ZrO2 phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO2, which has a higher thermal conductivity than tetragonal or cubic zirconia.

Fracture of ceramics and glasses

The rich variety of fracture behavior exhibited by glasses and ceramic materials is reviewed with particular emphasis on the understanding gained through the use of deliberately introduced, controlled cracks. After a brief summary of the mechanics of indentation cracks four major topics are discussed, the structure of crack tips, environment assisted crack growth, high temperature fracture and the toughening of ceramics. Resolution of the sharp vs blunt crack dilemma is presented together with recent microscopy observations of crack tips in brittle solids. In describing fracture in polycrystalline ceramics we explore some of the complexities beyond the simple Griffith behavior relating strength to flaw size, and show how the scale of the microstructure with respect to the crack length affects the observed toughness. It is shown that the interaction of a crack with the microstructure provides a unifying theme for interpreting much of the current work in the literature and leads to important concepts discussed here, such as the discrete-continuum transition, R-curve behavior, toughening due to crack deflection and crack bridging, transformation toughening and stress-induced microcrack toughening.

Characterization of AIN ceramics containing long-period polytypes

Two AIN-SiO2 materials have been investigated by means of electron microscopy; fracture toughness has been used to characterize both materials. Three new long-period polytypes, close to the 2H hexagonal AIN structure, have been identified by high-resolution electron microscopy, namely 33R, 24H and 39R. These polytypes are built on the same stacking principle as those in the previously observed shorter polytypes in the AIN system. Indentation measurements indicate similar hardness and toughness for the different polytypes in these ceramics.

Interfacial shear stresses in fiber-reinforced glasses

The single fiber pull-out test has been used to measure interfacial properties of SiC fibers in soda-borosilicate glass matrices. Two parameters are examined: the SiC to free carbon ratio at the surface of the SiC fiber is varied to test the effect of interfacial chemistry and the glass expansion is varied to investigate the effect of residual stresses. A carbon coating is shown to be effective in preventing strong fiber-matrix bonding and oxidation of the SiC fibers by the glass. However, coatings with higher carbon content result in stronger bonding to the soda-borosilicate glass. Both the interfacial shear strength and the frictional shear stress increase linearly with residual stress and reach a maximum of nearly double the strain-free value above which the interfacial strength decreases as a result of radial crack formation in the glass matrix. The measured interfacial shear properties are also found to be stress rate dependent.

Thermal properties of pitch-derived graphite foam

Graphite foams make potentially desirable engineering materials because of their high thermal conductivity coupled with their low density. Compared to conventional thermal management materials, such as copper and aluminum, graphite foams have specific thermal conductivity values up to five times higher. A high specific thermal conductivity combined with relatively high specific strength make graphite foam an attractive material for use in thermal management applications. Other properties of graphite foam, such as a relatively low coefficient of thermal expansion (CTE), make the material dimensionally stable, and thus, well suited to thermal applications.

Optimization of small-particle plasma-sprayed alumina coatings using designed experiments

Statistically designed experiments and multiple regression analysis have been used to determine the effects of processing parameters on three properties of small particle plasma sprayed alumina coatings – permeability, hardness, and thickness. First, an initial screening study was conducted with six variables at two levels each. Based on the results from this preliminary set, a second set of experiments was then conducted to probe the effects of four variables at three levels each. Multiple regression analysis was used to create models describing the effects of the variables on the three coating properties. The models were then used to identify the optimum conditions for producing coatings with the desired properties. It was found that the optimum conditions for low permeability did not correlate with those for high hardness due to defect structures that formed in the hardest coatings. The morphology of the defect structures was such that they did not affect indentation measurements, but did affect permeability. These defect structures formed as a result of copious splashing during splat formation and correlated to coatings that had high surface roughness.

Microcracking and R-curve behavior in SiC-TiB2 composites

The fracture toughness of SiC-TiB_2 composites is studied as a function of TiB_2 size and fraction with emphasis on the effect of stress-induced microcracking. R-curve behavior in these materials is observed from double cantilever beam tests and is found to be a major strength-controlling factor. The microcrack process zones are confirmed by both small angle X-ray scattering and transmission electron microscopy. The process zone size scales with the fraction of TiB_2 present. For composites containing fine TiB_2 particles, quasi-catastrophic crack growth and arrest are observed during R-curve measurements. The various contributions to the R-curve are discussed.

Interfacial debonding and sliding in brittle-matrix composites measured using an improved fiber pullout technique

A modified fiber pullout technique has been developed which allows direct experimental evaluation of the force-displacement relation for a crack-bridging fiber. The technique allows a continuous, accurate measurement of stable, progressive interfacial debonding and frictional sliding. Coupled with an appropriate analysis, the test provides a quantitative determination of interfacial properties relevant to the toughening of brittle materials through fiber-reinforcement. The technique has been used to measure interfacial debonding and sliding in three SiC fiber/glass composite systems. The fibers differ primarily in diameter and surface roughness characteristics allowing a unique evaluation of the effects of these parameters. The results indicate that fiber surface roughness plays a significant role in the interfacial debonding and sliding behavior. Load fluctuations observed during both monotonic and cyclic loading are explained by invoking the concept of asperity-asperity interaction during sliding at a rough interface.