Ultra high temperature ceramics for hypersonic vehicle applications.

HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems

Tape casting of magnesium oxide.

A tape casting procedure for fabricating ceramic magnesium oxide tapes has been developed as a method to produce flat sheets of sintered MgO that are thin and porous. Thickness of single layer tapes is in the range of 200-400 {micro}m with corresponding surface roughness values in the range of 10-20 {micro}m as measured by laser profilometry. Development of the tape casting technique required optimization of pretreatment for the starting magnesium oxide (MgO) powder as well as a detailed study of the casting slurry preparation and subsequent heat treatments for sintering and final tape flattening. Milling time of the ceramic powder, plasticizer, and binder mixture was identified as a primary factor affecting surface morphology of the tapes. In general, longer milling times resulted in green tapes with a noticeably smoother surface. This work demonstrates that meticulous control of the entire tape casting operation is necessary to obtain high-quality MgO tapes

Investigation of the Effect of Microstructure on the R-Curve Behavior of Metal-Ceramic Composites

An investigation was made into the effect of microstructure on the peak toughness and shape of the crack growth resistance curves for two ceramic-metal composites. An Al{sup 2}O{sup 3}/Al composite formed by Reactive Metal Penetration was used along with an AlN/Al composite formed using a reactive infiltration technique. The results indicate that the toughness increases with an increase in the volume fraction of the metal phase for a particular composite composition, and the peak toughness and shape of the R-Curve also depend on the composite microstructure and metal composition

Al₂O₃-Ni Composites with High Strength and Fracture Toughness

Al2O3-Ni composites were prepared by the reactive hot pressing of Al and NiO. The composites had a two-phase, interpenetrating microstructure and contained ∼35 vol% Ni. They exhibited an impressively high combination of strength and toughness at room temperature; the four-point bending strength was in excess of 600 MPa with a fracture toughness of more than 12 MPa·m1/2. Examination of fracture surfaces showed that Ni ligaments underwent ductile deformation during fracture. SEM analysis revealed knife-edged Ni ligaments with a limited amount of debonding around their periphery (i.e., at the Ni-Al2O3 interface), indicating a strong Ni-Al2O3 bond

Ceramic-metal and Ceramic-intermetallic Composites by Reactive Processing

Ceramic-metal and ceramic-intermetallic composites were prepared by reactive hot pressing. The ceramic to metal ratio of each composition was set by the stoichiometry of a thermodynamically favorable displacement reaction. The powder mixing method and the heating rate during hot pressing were found to have a profound influence on the final microstructure and properties of hot pressed composites. The best composites, judged by mechanical properties, were produced by hot pressing attrition milled precursors using l°C/min heating rates. The four-point bend strength, fracture toughness, Young\u27s modulus, and hardness were measured for Al2O3-MoSi2, Al2O3-Ni, and Al2O3-Nb composites

Reactive Hot Pressing of Alumina-molybdenum Disilicide Composites

Al2O3-MoSi2 composites were prepared by reactive hot pressing using molybdenum, aluminum, and mullite powders as precursors. The Gibbs free energy was highly negative for the composite-forming reaction, which indicated that the products were stable relative to the reactants. After the reaction, the composites had high relative density, ∼96%. Based on the composite-forming reaction, the composites should have contained 18 vol% MoSi2 in an Al2O3 matrix. Scanning electron microscopy revealed that the MoSi2 inclusions were elongated, with an average thickness of ∼5 μm and inclusion lengths that ranged from 5 to 50 μm. Average composite strength was 467 MPa, and toughness was 3.7 MPa·m1/2

Strength and Toughness of Ceramic-metal Composites Prepared by Reactive Hot Pressing

Metal-reinforced A1203-matrix composites were prepared using reactive hot pressing. The volume fraction of the reinforcing phase was controlled by the stoichiometry of the particular displacement reaction used. Dense Al2O3-Ni and Al2O3-Nb composites were fabricated using this technique. The best combination of strength, 610 MPa, and toughness, 12 MPam1/2, was found for the Al2O3-Ni composites. Indentation cracks and fracture surfaces showed evidence of ductile deformation of the Ni phase. The Al2O3-Nb composites had high strength, but the toughness was lower than expected due to the poor bonding between the Nb and Al2O3 phases

Reactive Processing, Microstructure, and Mechanical Properties of Al₂O₃-MoSi₂ Composites

Composites of Al2O3 and Mo intermetallic compounds, specifically MoSi2, Mo(Si,Al)2, and Mo3Al8, were fabricated from Mo-Al-aluminosilicate powder mixtures by reactive hot pressing. The ceramic-to-intermetallic ratio and the composition of the intermetallic phase were varied systematically by controlling the molar ratio of the reactant powders or by using different aluminosilicate precursors. Composite microstructures were characterized by scanning electron microscopy. Mechanical behavior was evaluated by determining 4-point bend strength, Young\u27s modulus, hardness, and coefficient of thermal expansion

Reactive Metal Brazing of Aluminum Nitride

The addition of titanium to eutectic braze compositions causes these alloys to readily wet and bond to AlN ceramics. Electron microscopic characterizations of the metal-ceramic interfaces reveal the presence of TiN, along with other Ti- and Al-containing phases. The formation of such interfacial reaction products is an additional thermodynamic driving force for the creation of useful metal-ceramic bonds

Strength and toughness of ceramic-metal composites prepared by reactive hot pressing

Metal-reinforced Al{sub 2}0{sub 3}-matrix composites were prepared using reactive hot pressing. The volume fraction of the reinforcing phase was controlled by the stoichiometry of the particular displacement reaction used. Dense Al{sub 2}0{sub 3}-Ni and Al{sub 2}O{sub 3}-Nb composites were fabricated using this technique. The best combination of strength, 610 MPa, and toughness, 12 MPam{sup 1/2}, was found for the Al{sub 2}O{sub 3}-Ni composites. Indentation cracks and fracture surfaces showed evidence of ductile deformation of the Ni phase. The Al{sub 2}O{sub 3}-Nb composites had high strength, but the toughness was lower than expected due to the poor bonding between the Nb and A1{sub 2}0{sub 3}phases