Terry N. Tiegs

Toughening of Ceramics by Whisker Reinforcement

These studies focus on improvements in the mechanical performance of ceramics by consideration of composite toughening approaches. Analysis of toughening processes indicated that both crack deflection and whisker pull out mechanisms can contribute to the fracture toughness of ceramics which contain strong small diameter whiskers. Results show that these processes can indeed yield high toughness in SiC whisker reinforced ceramics.

Ceramic composites with a ductile Ni3Al binder phase

Abstract Composites of B-doped ductile Ni3Al alloys with both non-oxide (WC, TiC) and oxide (Al2O3) ceramic powders were produced by hot-pressing. The Ni3Al alloys wet the non-oxide ceramic powders well and form a semi-continuous intergranular phase. However, the Ni3Al alloys do not wet the oxide powders well and tend to form discrete “islands” of the metallic phase. Mechanical property testing showed the flexural strength is retained to temperatures of at least 800 °C. The fracture toughness and hardness were found to be equal to or higher than comparable Co-based hardmetal systems. Initial corrosion tests showed excellent resistance to acid solutions.

Microstructural Contributions to the Fracture Resistance of Silicon Nitride Ceramics

To achieve toughening by the crack bridging process the introduction of large elongated grains by fracture resistance is necessary but not sufficient. While increasing the diameter of the elongated grains can increase the toughening effect, this requires that fracture occur along grain interfaces rather than through the grains. This interface debonding process appears to be modified by the chemistry of the oxynitride glass at the grain boundaries. Experiments show that increasing the yttria to alumina ratio or decreasing the ntirogen content of Si-AI-O-N glasses promotes interfacial debonding. The crack bridging contributions to the R-curve behavior is also a function of the content and size of the bridging reinforcement as noted in whisker-reinforced ceramics. Thus, control of micrstructure and interfacial phases is critical to the development of toughened silicon nitiride ceramics.

Fatigue and fracture behavior of nickel–titanium shape-memory alloy reinforced aluminum composites

Abstract A shape-memory alloy, nickel–titanium (NiTi), has been distributed throughout an aluminum matrix, using powder-metallurgy processing, in the hope of using the shape-memory effect to achieve strengthening and improve the fatigue resistance, as compared with the aluminum matrix. The shape-memory effect was activated by cold rolling the samples at −30°C. Upon reheating to the austenite phase, the NiTi was expected to return to its original shape, while embedded in the aluminum matrix. This action created residual, internal stresses around each particle, which strengthened the material. The yield and ultimate strengths, and fatigue lives of the NiTi reinforced aluminum composites, have been improved considerably, as compared with the unreinforced material. The cross-sectional microstructures of the composites, as well as, the modes of crack growth, have been examined with scanning electron microscopy (SEM) to identify fatigue and fracture mechanisms.

The effects of Cr2O3 and Fe2O3 additions on the thermal conductivity of Al2O3

Thermal conductivity data for a series of Al2O3, Al2O3‐Cr2O3, Al2O3‐Fe2O3, and Al2O3‐Cr2O3‐Fe2O3 samples are reported. The results, which span the temperature range 300–360 K, show that grain size has an important effect on the thermal conductivity of Al2O3, and this effect is larger than would be anticipated from standard theories. Results for seven Al2O3‐based solid solutions were examined in terms of a Callaway formula, and it was found that the point defect relaxation times were principally dependent on solute content and the mass difference between the solute and solvent. The data support a theory that suggests that a heavy atom in a light matrix scatters less effectively than a light atom in a heavy matrix. The data do not show that isolated point defects are more effective phonon scatterers than point defects in a concentrated solution.Thermal conductivity data for a series of Al/sub 2/O/sub 3/, Al/sub 2/O/sub 3/-Cr/sub 2/O/sub 3/, Al/sub 2/O/sub 3/-Fe/sub 2/O/sub 3/, and Al/sub 2/O/sub 3/-Cr/sub 2/O/sub 3/-Fe/sub 2/O/sub 3/ samples are reported. The results, which span the temperature range 300--360 K, show that grain size has an important effect on the thermal conductivity of Al/sub 2/O/sub 3/, and this effect is larger than would be anticipated from standard theories. Results for seven Al/sub 2/O/sub 3/-based solid solutions were examined in terms of a Callaway formula, and it was found that the point defect relaxation times were principally dependent on solute content and the mass difference between the solute and solvent. The data support a theory that suggests that a heavy atom in a light matrix scatters less effectively than a light atom in a heavy matrix. The data do not show that isolated point defects are more effective phonon scatterers than point defects in a concentrated solution.

Theoretical and experimental analysis of the toughening behavior of whisker reinforcement in ceramic matrix composites

A brief summary of recent analytical solutions which accurately describe the experimentally observed toughening behavior in a variety of whisker reinforced ceramics is presented. These results are found to provide important insights into the whisker, matrix, and interface properties required to obtain further increases in toughness by whisker reinforcement of ceramics.

Graphite Foams for Lithium-Ion Battery Current Collectors

Graphite open-cell foams, with their very high electronic and thermal conductivities, may serve as high surface area and corrosion resistant current collectors for lithium-ion batteries. As a proof of principle, cathodes were prepared by sintering carbon-coated LiFePO4 particles into the porous graphite foams. Cycling these cathodes in a liquid electrolyte cell showed promising performance even for materials and coatings that have not been optimized. The specific capacity is not limited by the foam structure, but by the cycling performance of the coated LiFePO4 particles. Upon extended cycling for more than 100 deep cycles, no loss of capacity is observed for rates of C/2 or less. The uncoated graphite foams will slowly intercalate lithium reversibly at potentials less than 0.2 volts versus lithium.

SiC Whisker Reinforced Alumina

SiC whisker reinforced alumina composites exhibit significant improvements in mechanical properties, such as strength and fracture toughness. These composites are typically densified by pressure-assisted sintering (i.e. hot-pressing) with SiC whisker contents ranging from 10 to 30 vol.%. Cutting tools for high nickel alloys are the major application, but other wear and structural uses are also being developed.

Sintered reaction-bonded silicon nitride by microwave heating

Sintered silicon nitride has many desired properties; however, for most applications these materials are too expensive to compete with metal parts. Sintered fraction-bonded silicon nitride (SRBSN) is more economical, with raw material costs <27% those of comparable high-purity materials, making it competitive with metal parts. Conventional processing of SRBSN requires long nitridation times and a two-step firing process. Microwave (M) heating reduces the reaction times and is performed in a one-step process, thereby simplifying the operation. The flexural strength of the M-SRBSN is equivalent to the strength of some materials made from higher-cost powders. Thus, these materials maybe appropriate for a number of applications.

Characterization of sintered reaction-bonded silicon nitride processed by microwave heating

Sintered reaction-bonded silicon nitride (SRBSN) tiles were fabricated by using microwave and conventional heating. Materials from both processes were analyzed at various stages in their fabrication. Microwave processing resulted in a SRBSN material of higher density and strength than the conventionally processed material.