Eldon D. Case

The influence of the microstructure on the hardness of sintered hydroxyapatite

Abstract The Vickers hardness, H V , was measured for 42 sintered monophase hydroxyapatite (HAP) specimens having volume fraction porosities, P, that ranged from 0.02 to 0.31 and average grain sizes between 1.7 μm and 7.4 μm. The H V versus porosity behavior of the sintered HAP specimens were successfully described by the exponential function of porosity, H V = H 0 exp(– bP ), where the least-squares fit values of the zero-porosity hardness H 0 and the constant b were 6.00±0.7 GPa and b =6.03, respectively. The function H V = H 0 exp(− bP ) is consistent with a minimum solid area model proposed by Rice. In contrast to the strong dependence of hardness on porosity, there was no clear trend of hardness as a function of grain size, for the grain size range included in this study.

CHARACTERIZATION OF FLY ASH FROM COAL-FIRED POWER PLANTS

X-ray analysis shows that mullite and silica are the major crystalline phases in fly ash. The “method of known additions” from X-ray diffraction techniques was used to calculate changes in the significant peak intensities of mullite and silica to determine their weight fractions in fly ash. This furthers the efforts of characterizing fly ash, which are being conducted to supplement the search for applications of this abundant material. The weight fractions of crystalline mullite and silica were determined to be 14.2 and 5.1 wt%, respectively. Thermal gravimetric studies as well as SEM and particle size analysis were also conducted on the fly ash.

Elastic modulus determination of coating layers as applied to layered ceramic composites

Abstract This paper develops relationships for determining the in-plane elastic modulus of a coating by two experimental techniques: (1) dynamic resonance and (2) static bend. Dynamic resonance measurements on model two-layer and three-layer composite beams (consisting of bonded strips of alumina and glass) agree well with the relationships developed. In addition, the dynamic resonance and static bend techniques are applied to a SiC coating/graphite substrate composite, where the two methods give statistically similar results for the elastic modulus of the SiC coatings.

Room-Temperature Mechanical Properties and Slow Crack Growth Behavior of Mg2Si Thermoelectric Materials

Mg2Si is of interest as a thermoelectric (TE) material in part due to its low materials cost, lack of toxic components, and low mass density. However, harvesting of waste heat subjects TE materials to a range of mechanical and thermal stresses. To understand and model the material’s response to such stresses, the mechanical properties of the TE material must be known. The Mg2Si specimens included in this study were powder processed and then sintered via pulsed electrical current sintering. The elastic moduli (Young’s modulus, shear modulus, and Poisson’s ratio) were measured using resonant ultrasound spectroscopy, while the hardness and fracture toughness were examined using Vickers indentation. Also, the Vickers indentation crack lengths were measured as a function of time in room air to determine the susceptibility of Mg2Si to slow crack growth.

Resonant ultrasound spectroscopy measurement of Young's modulus, shear modulus and Poisson's ratio as a function of porosity for alumina and hydroxyapatite

Modulus–porosity relationships are critical for engineered bone tissue scaffold materials such as hydroxyapatite (HA), where porosity is essential to biological function. Resonant ultrasound spectroscopy (RUS) measurements revealed that the Youngs modulus, E, and shear modulus, G, of both alumina and HA decrease monotonically with increasing volume fraction porosity, P, for 0.06 < P < 0.39 (alumina) and 0.05 < P < 0.51 (HA). Although the elastic moduli of porous materials have been measured by a number of different ultrasonic resonance techniques (of which the RUS technique is one example) and over the last decade the elastic moduli of many solids have been measured by the RUS technique, this study is the first systematic RUS study of porous materials. Comparison of E versus P data for alumina (which has been studied extensively) with literature data from several measurement techniques indicates the RUS technique is effective for modulus–porosity measurements. Another key result is that although the HA specimens included in this study have a unimodal pore size distribution, the details of the decrease in E and G with increasing P agree well with literature data for HA with both unimodal and bimodal pore size distributions. In addition, Poissons ratio exhibits a local minimum in the porosity range of 0.2 < P < 0.25 for both HA and alumina, which may be related to the pore morphology evolution during sintering.

In situ microscopy of crack healing in borosilicate glass

In situ environmental scanning electron microscopy studies of semi-macro indent cracks in borosilicate glass indicate that crack healing occurred at humidities as low as 8% relative humidity and at temperatures as low as about 400°C. The crack morphology changes observed in situ include slow crack regression (at low temperatures) and multiple crack pinch-off (at higher temperatures). Subsurface crack morphology changes were observed using conventional scanning electron microscopy of the fractured healed specimens. Subsurface healing included pinch-off into voids which appear quasi-circular in cross-section. In addition, crack debris were observed to hinder the crack healing process, which has important implications for fatigue of ceramic materials, where debris generation is frequently reported in the literature.

Part II: Fracture strength and elastic modulus as a function of porosity for hydroxyapatite and other brittle materials

Part I of this paper discussed the Weibull modulus m, versus porosity P behavior of brittle materials, including HA. While the Weibull modulus m deals with the scatter in fracture strength data, this paper (Part II) focuses on two additional key mechanical properties of porous materials, namely the average fracture strength , and Youngs modulus E, for P in the interval from P≈ zero to P≈P(G) (the porosity of the unfired compacts). The versus P data for HA from this study and the literature data for alumina, yttria stabilized zirconia (YSZ) and silicon nitride are described well by functions of ϕ, where ϕ=1-P/P(G)= the degree of densification. A similar function of ϕ applies to the versus P behavior of HA from this study and data from the literature for alumina, titanium and YSZ. All of the data analyzed in this study (Part II) are based on partially and fully sintered powder compacts (excluding green powder compacts), thus the /σ(0) versus ϕ and /E(0) versus ϕ relationships may apply only to such specimens.

The effect of quenching media on the heat transfer coefficient of polycrystalline alumina

Surface heat transfer coefficient values were measured for polycrystalline alumina quenched into water, into oil, and into liquid nitrogen. Since the measurements of the surface heat transfer coefficient h for alumina (and ceramics in general) are very limited, we compare our measurements with calculations of h for the water quench and with measurements of h on non-ceramic materials for the oil and liquid nitrogen quenches.

Cyclic thermal shock in SiC-whisker-reinforced alumina composite

Abstract Cyclic thermal shock damage in SiC whisker-alumina ceramic-ceramic composites was monitored non-destructively via elastic modulus and internal friction measurements. Thermal-shock-damage-induced changes in internal friction were found to be a linear function of a crack damage parameter (which in turn is a function of crack density and crack size). The linear relation between internal friction changes and the damage parameter is explained if one assumes that internal friction is proportional to the integrated surface area of damage-induced cracks. Thermal shock damage, as measured by both elastic modulus and internal friction, was observed to saturate as a function of an increasing cumulative number of thermal shock cycles. This saturation damage level increases in proportion to ΔT p , where ΔT is the shock temperature difference. The exponent p has a value of approximately 6 for the range of ΔT included in this study. This power-law relation in ΔT implies a fatigue-like power-law relation in stress (or alternatively, a power-law relation in stress intensity K).

Temperature-dependent elastic moduli of lead telluride-based thermoelectric materials

In the open literature, reports of mechanical properties are limited for semiconducting thermoelectric materials, including the temperature dependence of elastic moduli. In this study, for both cast ingots and hot-pressed billets of Ag-, Sb-, Sn- and S-doped PbTe thermoelectric materials, resonant ultrasound spectroscopy (RUS) was utilized to determine the temperature dependence of elastic moduli, including Youngs modulus, shear modulus and Poissons ratio. This study is the first to determine the temperature-dependent elastic moduli for these PbTe-based thermoelectrics, and among the few determinations of elasticity of any thermoelectric material for temperatures above 300 K. The Youngs modulus and Poissons ratio, measured from room temperature to 773 K during heating and cooling, agreed well. Also, the observed Youngs modulus, E, versus temperature, T, relationship, E(T) = E 0(1–bT), is consistent with predictions for materials in the range well above the Debye temperature. A nanoindentation study of Youngs modulus on the specimen faces showed that both the cast and hot-pressed specimens were approximately elastically isotropic.