Strength and Young's Modulus Behavior of a Partially Sintered Porous Alumina

The strength of alumina materials that had been subjected to varying degrees of densification was determined. Significant increases in strength were obtained for these materials even when there was minimal densification. For the most porous materials, the Weibull modulus values were similar but showed a significant increase for materials that were close to the theoretical density. Young's modulus data were found to be similar to previous work, in that significant increases occur during the initial stage of sintering, showing a strong correlation with the strength data. A simple modi fication to a previous theory allowed the Young's modulus data to be fitted with excellent accuracy.     

Young's Modulus, Flexural Strength, and Fracture of Yttria-Stabilized Zirconia versus Temperature

The flexural strength and elastic modulus of cubic zirconia that was stabilized with 6.5 mol% yttria was determined in the temperature range of 25°–1500°C in air. Specimens were diamond machined from both hot-pressed and sintered billets that were prepared from alkoxy-derived powders. The flexural strength of the hot-pressed material decreased, from }300 MPa at 25°C to 50 MPa at 1000°C, and then increased slightly as the temperature increased to 1500°C. The flexural strength of the sintered material decreased, from 150 MPa at 25°C to 25 MPa at 750°C, and then appeared to increase slightly to }1500°C. Flexural strengths were comparable to other fully stabilized zirconia materials. The overall fracture mode was transgranular at low temperatures, mixed mode at }500°–1000°C, and intergranular at higher temperatures. Pores or pore agglomerates along grain boundaries and at triple points were fracture origins. The value of the porosity-corrected Youngs moduli was 222 GPa at 25°C, decreased to }180 GPa at 400°C, and then was relatively constant with increasing temperature to 1350°C.     

Effect of Porosity on Structure,Young's Modulus,and Thermal Conductivity of SiC Foams by Direct Foaming and Gelcasting

The study demonstrates the aqueous processing of solid‐state‐sintered SiC foams by gelcasting technique. Aside from increasing strength of green bodies, gelcasting monomers were the source of carbon additive which helped in sintering of SiC foams. Sintered foams with the relative density (RD) between 0.44 and 0.11 were processed by direct foaming of SiC slurries followed by gelcasting and sintering. Structural analysis by X‐ray tomography showed the presence of spherical pores with bimodal pore size distribution and the proportion of large size cell and their interconnectivity increased in low RD foams. SEM study revealed that decreased RD resulted in gradual changes in the strut microstructure from the grains with faceted interface to smooth interfaced grains. The analysis of changes in Young's modulus and thermal conductivity with RD were in agreement with the Ashby model for open cell foams.     

Bulk Modulus and Young's Modulus of Nanocrystalline γ-Alumina

Compression measurements were performed for the first time on nanocrystalline γ-alumina utilizing a diamond anvil cell (DAC) and the energy dispersive X-ray diffraction method. The cubic unit cell ( a = 0.7924 nm) for γ-alumina was found to have a volume compression of about 2.4% over the pressure range from ambient to 3.8 GPa at room temperature under both hydrostatic and nonhydrostatic conditions. Using the first-order Bridgman equation and the Birch equation of state, the isothermal bulk modulus ( B ₀) was determined to be 162 ± 14 GPa and Young's modulus ( E ) was estimated to be 253 ± 22 GPa assuming a Poisson's ratio for γ-alumina of 0.24 ± 0.2.     

Determination of Young's Modulus of Thin Films

An improved vibrating-reed method is described for determining Young's modulus of thin films deposited on both sides of a substrate. This technique consists of measuring the resonant frequency of a cantilever composite beam obtained by coating both sides of a substrate. The calculation procedure is presented to evaluate the film modulus from sample geometry, material density, and mechanical resonant frequency. For accurate determination of resonant frequency, the phase angle between the exciting and vibration signals is analyzed. Using the proposed technique, Young's modulus of ZrO₂ thin films is calculated, obtaining a value in agreement with literature data.     

High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

Determination of Young's Modulus in Chemically Vapor-Deposited SiC Coatings

Young's modulus of CVD and SiC coatings was measured in situ (on the substrate) using strain gage/flexure and dynamic resonance techniques. The average Young's moduli of CVD SiC from these techniques were 446 and 415 Gpa, respectively, and were considered to be in good agreement. The determination of a representative coating thickness and cross section is critical in order to accurately estimate Young's modulus of coatings.     

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II, Strength Method

An examination is made of the sharp-indentation technique of strength-test precracking for toughness evaluation. The experimental approach follows that proposed by other workers but the theoretical analysis contains one vital new feature; the residual-stress term discussed in Part I of this study is now introduced explicitly into the strength formulation. This modification overcomes a major systematic discrepancy evident in the previous models and at the same time, by virtue of attendant changes in the nature of the crack stability prior to attaining a failure configuration, eliminates the need for frac-tographic measurements. Other advantages are also apparent, notably an insensitivity to postindentation radial crack extension. The main disadvantage is that only one result is obtained per specimen. Indentation/strength data from ceramics listed in Part I confirm the essential features of the theory and provide a suitable calibration factor. The method has special application to those materials which do not necessarily produce a well-defined radial crack pattern, in which case an "effective" K_c appropriate to fracture properties at the flaw level is obtained.