Raymond A. Cutler

Liquid phase sintering of silicon carbide

Abstract It is shown that the decomposition reactions during the sintering of liquid phase silicon carbide (SiC) can be described well by thermodynamics. This allows for an optimization of the sintering parameters. The use of carbon as a sintering additive, together with, for instance, yttria plus alumina, is of advantage. When C is used, SiO 2 will not occur in the liquid phase during sintering or in the amorphous and crystalline phases after sintering. The microstructure of sintered samples is described.Liquid phase sintering is used to densify silicon carbide based ceramics using a compound comprising a rare earth oxide and aluminum oxide to form liquids at temperatures in excess of 1600° C. The resulting sintered ceramic body has a density greater than 95% of its theoretical density and hardness in excess of 23 GPa. Boron and carbon are not needed to promote densification and silicon carbide powder with an average particle size of greater than one micron can be densified via the liquid phase process. The sintered ceramic bodies made by the present invention are fine grained and have secondary phases resulting from the liquid phase.

The effect of binder thickness and residual stresses on the fracture toughness of cemented carbides

Much of the data on WC-Co cermets show that the fracture toughness,KIc, increases with increasing tungsten carbide grain size at fixed volume fraction of the cobalt binder phase. It is shown that the origin of this effect can be explained on the basis of the plane stress fracture of constrained cobalt phase and the periodic internal stresses arising due to differential thermal contraction of the two phases. Quantitative models have been derived which take these two effects into account. The effect of macroscopic residual stresses, such as those generated by milling WC-Co drilling inserts, on the apparent toughness has also been analysed. It is shown that for the chevron-notched type specimen the macroscopic residual stress affects not only the maximum load but also the length of the crack at which the maximum occurs. A graphical method is presented which permits the evaluation of the true K‡Ic.

Pressureless-sintered Al2O3-TiC composites☆

Abstract Titanium carbide (TiC) (typically 30 wt.% TiC is used in cutting tools) dispersed in an alumina (Al 2 O 3 ) matrix improves the hardness over that of an Al 2 O 3 cutting tool, principally by limiting Al 2 O 3 grain growth. Al 2 O 3 -TiC composites have been traditionally processed by intimately mixing the oxide and carbide powders and subsequently hot pressing at temperatures near 1700 °C and pressures 35 MPa. Pressureless sintering of Al 2 O 3 -30wt.%TiC was accomplished by adding titanium hydride (TiH 2 ) and heating at moderately high rates (40–50°C min −1 ) in a graphite furnace under inert gas. Both free titatnium and a eutectic liquid, formed by the carbothermal reduction of Al 2 O 3 , aided sintering. Conventional heating rates permit free titanium to combine with TiC at temperatures below the melting point of titanium and therefore do not take advantage of the liquid phase. Short sintering times above 1850°C are necessary owing to the high vapor pressure of the phases produced by the carbothermal reduction of Al 2 O 3 . A density greater than 99% of the theoretical density, a hardness in excess of 22 GPa and a fourpoint flexural strenght higher than 600 MPa were measured on pressureless-sintered-and-hot-isostatistically-pressed bars. These properties were comparable with those measured on composites made by hot pressing. There were no observable differences (microstructural or mechanical) between the hot-pressed and the pressureless-sintered parts. Machining tests showed that the performance of the pressureless-sintered composites was comparable with that of conventional tools. It was also shown that Al 2 O 3 -30wt.%TiC powders synthesized by the aluminothermic reduction of titania in the presence of carbon could be pressureless sintered without TiH 2 additions. The mechanical properties of these composites were comparable with those of hot-pressed Al 2 O 3 -TiC composites.

Electrochemical Oxygen Separation Using Solid Electrolyte Ion Transport Membranes

A novel process employing solid electrolyte-based ion-transport membranes enables the production of high-purity oxygen at elevated pressure from a feed stream of ambient pressure air. This technology exploits the theoretically infinite selectivity of oxygen ion migration through a dense ceramic electrolyte membrane under the influence of an externally applied electrical potential. The solid electrolyte is derived from cerium oxides with dopants added to enhance both ion transport and membrane processability. The oxidation and reduction reactions are promoted by the use of porous perovskite electrodes, which together with the electrolyte form an electrochemical cell. Stacks comprising multiple cells in a planar configuration have demonstrated excellent electrochemical performance and stability, mechanical integrity. and the capacity to produce high-purity oxygen over thousands of hours. An oxygen generator based on this technology must incorporate an integrated thermal management system air mover, power supply, and control systems.

Sintering behaviour and properties of SiCAION ceramics

Silicon carbide (SiC), aluminium oxycarbide (AI2OC), and aluminium nitride (AlN) all have the same wurtzite crystal structure and can be processed so as to form SiCAION, an acronym for the solid solution. This paper describes processing of SiC-Al2OC ceramics by pressureless reactive sintering and gives mechanical property data on the same. Experiments showed that densification occurred by a liquid-phase sintering mechanism. Both alpha and beta SiC up to a particle size of 5μm were used to form the solid solution, boron additions were not necessary to promote densification, and densities greater than 97% of the theoretical were achieved by pressureless sintering. SiC-Al2OC ceramics, containing minor amounts of AIN, were fabricated from conventional raw materials. Phase identification by X-ray diffraction and metallography showed that the materials consisted of two phases: SiCAION and SiC. Mechanical property data were obtained on pressureless sintered and hot-pressed materials. Hot-pressed materials had room-temperature strengths in excess of 600 MPa, hardness greater than 25 G Pa, and fracture toughness greater than 4 M Pa m−1/2. Pressureless sintered bars had bend strengths in excess of 300 M Pa.

High temperature creep of SiC densified using a transient liquid phase

Silicon carbide-based ceramics can be rapidly densified above approximately 1850 {degree}C due to a transient liquid phase resulting from the reaction between alumina and aluminum oxycarbides. The resulting ceramics are fine-grained, dense, and exhibit high strength at room temperature. SiC hot pressed at 1875 {degree}C for 10 min in Ar was subjected to creep deformation in bending at elevated temperatures between 1500 and 1650 {degree}C in Ar. Creep was thermally activated with an activation energy of 743 kJ/mol. Creep rates at 1575 {degree}C were between 10{sup {minus}9}/s and 10{sup {minus}7}/s at an applied stress between 38 and 200 MPa, respectively, resulting in a stress exponent of {approx}1.7.

Ceria–lanthanum strontium manganite composites for use in oxygen generation systems

Abstract Lanthanum strontium manganite (LSM) can be thermal expansion-matched with undoped ceria (CeO 2 ) or Gd 2 O 3 -doped ceria (CGO) by incorporating equimolar amounts of the A-site cations La and Sr. Such high Sr levels, however, can result in low elastic modulus and room-temperature plastic deformation, as well as low strength that would be typical of a microcracked ceramic. The addition of a continuous ceria network to such LSM compositions allows the composite material to behave mechanically like ceria, while still maintaining substantial electrical conductivity. The LSM and ceria in these composites show minimal reactivity after sintering, regardless of whether the ceria is doped or undoped. In addition, there is little penalty for sintering the composites relative to the monolithic materials. The composite materials are two to three times stronger than LSM, with the plastic deformation decreasing as the strength increases. Conductivity considerations are discussed for both mixed ionic–electronic conducting (MIEC) devices and interconnects for oxygen generation.

Polycrystalline t′-ZrO2(Ln2O3) formed by displacive transformations

ZrO2 (3 mol% Y2O3) tetragonal and t′-ceramics (displacively formed ceramics) were compared with ZrO2 ceramics (3 mol% Ln2O3, where Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Er, or Yb) processed in an identical manner. Sintering at 1500 °C for 2 h produced mainly tetragonal polytypes for the dopants with smaller ionic radii than Dy(i.e., Er, Y and Yb) but when ZrO2 was reaction sintered with dopants with larger ionic radii excessive monoclinic phase transformation and associated microcracking resulted. High-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime was used to fabricate t′-composites of ZrO2 doped with Y, Yb, Er or Dy. Room-temperature fracture toughness and strength values are explained on the basis of a ferroelastic-cubic-to-tetragonal transformation. The domain structure was viewed by transmission optical microscopy (TOM) or transmission electron microscopy (TEM).

Solid particle erosion of SiC-Al2OC ceramics

Erosion rates of SiC-Al2OC ceramics, with Al2OC content varying from 5 to 75wt%, were assessed using 240-grit alumina abrasive particles accelerated to a velocity estimated at 120msec−1 and impacting the target at normal incidence. The target ceramics varied in hardness from 27.1 GPa for SiC-5wt% Al2OC to 10.8 GPa for SiC-75wt% Al2OC, but the fracture toughness was essentially independent of composition (Klc ã 3.5 MPa m1/2). The erosion weight loss varied linearly with the test duration for all the ceramics and the erosion rate decreased systematically with increasing target hardness; the hardness dependence of the erosion rate was, however, much greater than the predictions of the currently available erosion models.

Fabrication and Characterization of Slip-Cast Layered Al2O3-ZrO2 Composites

Monolithic and three-layered Al2O3-15 vol. % ZrO2 composites were fabricated by slip casting aqueous slurries. The outer and inner layers of three-layer composites contained unstabilized and partially stabilized ZrO2, respectively. Transformation of part of the unstabilized ZrO2 led to surface compressive stresses in the outer layers. Strain gage, xray, indentation crack length, and strength measurements were used to determine the magnitude of residual stresses in the composites. The strength of the three-layer composites (1.1 to 1.2 GPa) was 500–700 MPa higher than that of the monolithic outer layer composites at room temperature and 350 MPa higher at 750°C. The strength differential decreased rapidly above the monoclinic to tetragonal transformation temperature. Three-layered composites showed higher Weibull modulus and excellent damage resistance. Cam follower rollers were fabricated to demonstrate the applicability of this technique for making automotive components.