John J. Petrovic

A comparative overview of molybdenum disilicide composites

Abstract MoSi 2 -based composites possess significant potential to meet the demands of advanced high temperature structural applications in the range 1200–1600 °C, in oxidizing and aggressive environments. These materials constitute an important new class of “high temperature structural silicides”. The intermetallic compound MoSi 2 possesses properties which make it a very desirable matrix for high temperature composites, and these properties are described and compared with those of other high melting point silicides. The developmental history of composites based on MoSi 2 is traced from its beginnings to the present. Mechanical property improvements derived from SiC and ZrO 2 reinforcements, as well as matrix alloying, are described, and properties of current MoSi 2 -based composites compared with those of silicon-based structural ceramics. Finally, important research and development directions for the continued improvement of MoSi 2 -based composites and their use as high temperature structural components are discussed.

Growth of beta-silicon carbide whiskers by the VLS process

Beta-silicon carbide whiskers are being grown by a vapour-liquid-solid (VLS) process which produces a very high purity, high strength single crystal fibre about 6μm in diameter and 5–100 mm long. Details of the growth process are given along with a general explanantion of the effects of the major growth parameters on whisker growth morphology.

Key developments in high temperature structural silicides

Significant progress has been made in the past few years in both the scientific understanding of high temperature structural silicides and in their technological development. This overview highlights key aspects of this structural silicide research and development, in the areas of materials, composites, and applications. Silicide materials discussed are MoSi2, Mo5Si3, Mo<5Si3C<1, Ti5Si3, and C40 type silicides. Results on MoSi2–Si3N4, MoSi2–SiC, MoSi2–Al2O3, and MoSi2–Si3N4–SiC composites are summarized. Finally, selected applications in glass processing, wear resistance, diesel engine glow plugs, and aerospace components are described.

Review Mechanical properties of ice and snow

The mechanical properties of ice and snow are reviewed. The tensile strength of ice varies from 0.7–3.1 MPa and the compressive strength varies from 5–25 MPa over the temperature range −10°C to −20°C. The ice compressive strength increases with decreasing temperature and increasing strain rate, but ice tensile strength is relatively insensitive to these variables. The tensile strength of ice decreases with increasing ice grain size. The strength of ice decreases with increasing volume, and the estimated Weibull modulus is 5. The fracture toughness of ice is in the range of 50–150 kPa m1/2 and the fracture-initiating flaw size is similar to the grain size. Ice-soil composite mixtures are both stronger and tougher than ice alone. Snow is a open cellular form of ice. Both the strength and fracture toughness of snow are substantially lower than those of ice. Fracture-initiating flaw sizes in snow appear to correlate to the snow cell size.

Mechanical behavior of MoSi2 and MoSi2 composites

Abstract MoSi 2 is a key member of a new class of high-temperature structural silicide materials. Important features of the mechanical behavior of MoSi 2 and MoSi 2 composites are reviewed. The mechanical properties of Mosi 2 single crystals, polycrystalline MoSi 2 , and MoSi 2 -based composites are discussed in association with properties such as elevated temperature deformation and low-temperature fracture toughness. Interrelationships between single crystal, polycrystal, and composite mechanical behavior are identified.

Tensile mechanical properties of SiC whiskers

An initial evaluation has been made of the tensile mechanical properties of SiC whiskers synthesized by a vapour-liquid-solid process. A micro-tensile tester and associated testing techniques were developed for this purpose. The SiC whiskers exhibit an average tensile strength of 8.40 GPa (1 220 000 psi) and an average elastic modulus of 581 GPa (84 300 000 psi), and were considerably stronger and stiffer than continuous, polycrystalline SiC fibres. These results indicate the significant potential of SiC whiskers as short-fibre reinforcement elements for ceramic matrix composites.

Synthesis of molybdenum disilicide by mechanical alloying

Abstract Considerable interest and effort are being directed towards developing molybdenum disilicide (MoSi2) alloys with low oxygen content. During alloy synthesis, oxygen combines with Si to form glassy SiO2 precipitates at the MoSi2 grain boundaries, resulting in a degradation of its mechanical properties. We have used mechanical alloying, a high-energy ball-milling process, to synthesize alloy powders of MoSi2, MoSi2-27 mol.% MoSi3, MoSi2-50 mol.% Mo5Si3 and MoSi2-50 mol.% WSi2 starting from elemental powders. The processing of the powders, as well as the loading of the powders in graphite dies, was performed under high-purity argon inside a glovebox. The finer grain and particle size of the mechanically alloyed powders enabled us to hot-press them at 1500 °C, which is 300 °C lower than the temperature currently used for hot-pressing commercial powders. We have been successful in reducing the oxygen content in our alloys to about 310 ppm by weight, as measured by nuclear (d,p) reactions. We report the formation of metastable phases in the mechanically alloyed powders and their characterization by X-ray diffraction and differential thermal analysis. We also report the characterization of the hot-pressed alloys by optical and transmission electron microscopy, and the measurement of high-temperature mechanical properties.

Partially stabilized ZrO2 particle-MoSi2 matrix composites

A 30 vol % partially stabilized ZrO2 particle-MoSi2 matrix composite was synthesized by hot pressing to 96% theoretical density at 1700°C. No chemical reactions between the PSZ and MoSi2 were observed after hot pressing, indicating thermodynamic stability of these species. The composite formed an adherent and coherent glassy-appearing oxidation layer after oxidation at 1500°C. The room temperature indentation fracture toughness of the composite was 2.5 times that of pure MoSi2. These results demonstrate the feasibility of PSZ particle-MoSi2 matrix composites.

Creep of molybdenum disilicide composites

Abstract The creep deformation behavior of MoSi 2 , MoSi 2 , with SiC and WSi 2 alloy, and the alloy with SiC reinforcement was determined under compression in the temperature range 1100–1450 °C. The effects of reinforcement and alloying were evaluated. Existing theories of composite strengthening were examined in relation to the present results. Models based on creeping matrix with elastically deforming fibers appear to predict the observed behavior better than any other models. Activation energies and volumes were evaluated to identify the creep mechanisms. Based on the stress exponents and the energies, it is concluded that creep progress with increasing temperature and stress from newtonian viscous flow involving diffusion processes to power-law creep involving dislocation climb.

ZrO2 and ZrO2SiC particle reinforced MoSi2 matrix composites

Abstract ZrO 2 MoSi 2 and (ZrO 2 SiC)MoSi 2 composites were fabricated by hot pressing-hot isostatic pressing at 1700 °C. No reactions between ZrO 2 , SiC and MoSi 2 were observed. An amorphous silica glassy phase was present in all composites. Composites with unstabilized ZrO 2 particles exhibited the highest room temperature fracture toughness, reaching a level three times that of pure MoSi 2 . Both the room temperature toughness and 1200 °C strength of (ZrO 2 SiC)MoSi 2 composites were higher than those of ZrO 2 MoSi 2 composites, indicating beneficial effects of combined reinforcement phases. Low strength levels were observed at 1400 °C as a result of the presence of the silica glassy phase. Elimination of glassy phases and refinements in microstructural homogeneity are processing routes important to the optimization of the mechanical properties of these types of composites.