A. R. de Arellano-Lopez

Effect of heat treatment on phase stability, microstructure, and thermal conductivity of plasma-sprayed YSZ

The effects of 50-hour heat treatments at 1000°C, 1200°C, and 1400°C on air plasma-sprayed coatings of 7 wt% Y2O3-ZrO2 (YSZ) have been investigated. Changes in the phase stability and microstructure were investigated using x-ray diffraction and transmission electron microscopy, respectively. Changes in the thermal conductivity of the coating that occurred during heat treatment were interpreted with respect to microstructural evolution. A metastable tetragonal zirconia phase, with a non-equilibrium amount of Y2O3 stabilizer, was the predominant phase in the as-sprayed coating. Upon heating to 1000°C for 50 hours, the concentration of the Y2O3 in the t-zirconia began to decrease as predicted by the Y2O3-ZrO2 phase diagram. The c-ZrO2 phase was first observed after the 50-hour heat treatment at 1200°C; monoclinic zirconia was observed after the 50-hour heat treatment at 1400°C. TEM analysis revealed closure of intralamellar microcracks after the 50-hour/1000°C heat treatment; however, the lamellar morphology was retained. After the 50-hour/1200°C heat treatment, a distinct change was observed in the interlamellar pores; equiaxed grains replaced the long, columnar grains, with some remnant lamellae still observed. No lamellae were observed after the 50-hour/1400°C heat treatment. Rather, the microstructure was equivalent when viewed in either plan view or cross-section, revealing large grains with regions of monoclinic zirconia. Thermal conductivity increased after every heat treatment. It is believed that changes in the intralamellar microcracks and/or interlamellar pores are responsible for the increase in thermal conductivity after the 1000°C and 1200°C heat treatments. The increase in thermal conductivity that occurs after the 50-hour/1400°C heat treatment is proposed to be due to the formation of m-ZrO2, which has a higher thermal conductivity than tetragonal or cubic zirconia.

Thermomechanical response of polycrystalline BaZrO3

Thermomechanical properties of fine-grained, 96% dense, polycrystalline BaZrO{sub 3} specimens were studied. The average thermal expansion coefficient of BaZrO{sub 3} at 2511000{sup o}C was 7.72x10{sup -6}{sup o}C{sup -1}. At 25{sup o}C, Youngs modulus was 240.5 GPa and the shear modulus was 97.2 GPa. These values decreased by {approx}10% with heating to 1015{sup o}C. Poissons ratio was 0.237 and was independent of temperature. The BaZrO{sub 3} specimens exhibited a thermal expansion coefficient of about one half, and elastic moduli more than twice, those of polycrystalline high-temperature superconductors. Diffusional creep was observed when BaZrO{sub 3} was compressed at 1300-1400{sup o}C; the activation energy for steady-state deformation was 460{+-}30 kJ/mole. The mechanism was probably grain-boundary sliding, controlled by diffusion of one of the cations along grain boundaries. Deformation rates below 1000{sup o}C were too low to relax thermally generated stresses.

A new generation of bio-derived ceramic materials for medical applications

A new generation of bio-derived ceramics can be developed as a base material for medical implants. Specific plant species are used as templates on which innovative transformation processes can modify the chemical composition maintaining the original biostructure. Building on the outstanding mechanical properties of the starting lignocellulosic templates, it is possible to develop lightweight and high-strength scaffolds for bone substitution. In vitro and in vivo experiments demonstrate the excellent biocompatibility of this new silicon carbide material (bioSiC) and how it gets colonized by the hosting bone tissue because of its unique interconnected hierarchic porosity, which opens the door to new biomedical applications.

Indentation moduli and microhardness of RE–Si–Mg–O–N glasses (RE=Sc, Y, La, Sm, Yb and Lu) with different nitrogen content

Abstract The indentation moduli and microhardness of RE–Si–Mg–O–N glasses (RE=Y, Sc and lanthanides La, Sm, Yb and Lu) containing 20–24 eq.% of nitrogen were found to depend on RE type and nitrogen content as well. The Lu-containing glasses were approximately 15–28% harder than La-glasses with the same nitrogen content and 4% increase in nitrogen content results in 1 to ∼10% increase of microhardness. The effectiveness of the RE elements with regard to their ability to increase selected mechanical properties of the oxynitride glasses rises from La, Y/Sc/Sm to Yb and Lu. The properties studied increase approximately linearly with cationic field strength (CFS) of the corresponding lanthanide cation in the glass network. The properties of Y- and Sc-containing glasses do not follow this dependence. These changes cannot be explained in terms of CFS and additional effects have to be considered to characterize relationships between composition, structure, and properties in the glasses containing non-lanthanide elements.

High temperature mechanical behavior of porous open-cell Al2O3

Abstract Crushing strength and fracture toughness values have been determined for open-cell Al 2 O 3 of density less than 30% of the theoretical density for temperatures up to 1500°C. Room temperature crushing strengths as a function of relative density were found to obey approximately a model for mechanical properties based on bending of individual struts within a cell. Fracture toughness values showed some deviation from the model, which was attributed to the presence of closed cells and microstructural inhomogeneities. Strength and toughness increased at about 900–1100°C but declined above 1200°C. Compressive creep of the open-cell Al 2 O 3 has been measured for temperatures of 1200–1500°C. Creep occured by diffusional flow for strain rates between 10 −8 and 10 −6 s −1 for stresses in the range 20–100 kPa. The activation energy for steady state creep was 504 kJ mol −1 , which is typical for creep of dense Al 2 O 3 . The onset of tertiary creep was associated with the formation of creep cracks in the struts subjected to bending.

Erosion and strength degradation of biomorphic SiC

Abstract Solid-particle-erosion studies were conducted on biomorphic SiC based on eucalyptus and pine, reaction-bonded (RB) SiC, and hot-pressed (HP) SiC. The erodents were angular SiC abrasives of average diameter 63, 143, or 390 μm and the impact velocity was 100 m s −1 . Impact occurred at normal incidence. Material loss in all targets occurred by brittle fracture. The biomorphic specimens eroded by formation of both lateral and radial cracks and their erosion rates were higher than both conventional SiCs. The RB SiC eroded as a classic brittle material, by formation and propagation of lateral cracks. The HP SiC, the hardest target, was the most erosion resistant. In erosion of the HP SiC, the abrasive particles, especially the largest ones, fragmented upon impact. The resulting dissipation of energy led to relatively low erosion rates. Flexural strength before and after erosion was measured for the biomorphic eucalyptus, RB SiC, and HP SiC. Erosion damage reduced the flexural strengths of all of the specimens. The relative strength reductions were lowest for the biomorphic eucalyptus and highest for the HP SiC. The hot-pressed SiC responded as predicted by accepted models of impact damage in brittle solids. The responses of the biomorphic and reaction-bonded SiC specimens were modeled as if they consisted of only SiC and porosity. This approximation agreed reasonably well with observed degradations of strength.

Compressive creep of mullite containing Y2O3

Compressive creep of mullite and mullite containing 5 and 9 wt% Y{sub 2}O{sub 3} has been investigated in the temperature range of 1300-1400 C over stresses between {approx}0.6 and 40 MPa in air. The nominally single-phase mullite deforms by diffusional flow with a stress exponent of 1 (for higher stresses) and an activation energy of 385{+-}20 kJ/mol. It is likely that the rate-controlling diffusing species is oxygen. Creep of the Y{sub 2}O{sub 3}-containing specimens was similar to that of the pure mullite at 1300 C. Near and above the temperature at which melting was observed in DTA, the Y{sub 2}O{sub 3}-containing specimens crept significantly faster than the pure mullite. Models of creep of materials that contain a glass phase can explain most, but not all, of the observed behavior. Creep rates were not significantly affected by partial crystallization of the glass to Y{sub 2}Si{sub 2}O{sub 7}, but the crystallized specimens exhibited cavitation at larger strains.

Mechanical properties and microstructure of an Al2O3–SiC–TiC composite

The mechanical properties and microstructure of a hard, electrodischarge-machinable composite, Al{sub 2}O{sub 3}-SiC-TiC, were studied. The material was fabricated by hot pressing 46.1 vol.% Al{sub 2}O{sub 3} powder, 30.9 vol.% SiC whiskers and 23.0 vol.% TiC powder. Significant reaction occurred between the Al{sub 2}O{sub 3} and SiC during processing. The resultant composite consisted of nearly unreacted TiC particles, Al{sub 2}O{sub 3}, plus smaller concentrations of SiC, mullite and possibly a mixture of Al-Si-O-C. The composite exhibited at room temperature an elastic modulus of 409.6{+-}0.5 GPa, microhardness values of 19-32 GPa, indentation fracture toughness (KIC) of 9.6{+-}0.6 MPa(m)0.5, compressive strength as high as 2.8 GPa and fracture strength in bending of {approx}680-825 MPa.

Diffusion-controlled plastic deformation of YBa2Cu3Ox

Abstract Steady-state creep experiments have been performed on polycrystalline YBa 2 Cu 3 O x in the temperature range of 850–970°C, as a function of oxygen partial pressure ( P O 2 ). The results, when combined with a reanalysis of previous deformation studies, indicate that contrary to the previous conclusion that the activation energy was a function of P O 2 , there are two different temperature regimes characterized by P O 2 -independent activation energies of 675 and 1100 kJ/mol for low and high temperatures, respectively. This is interpreted in terms of two independently operating rate-controlling mechanisms: grain-boundary diffusion and lattice diffusion. Diffusion coefficients have been derived by comparing experimental data and theoretical models.

Compressive creep of YBa 2 Cu 3 O x

YBa 2 Cu 3 O x was deformed from 850 to 980 °C in oxygen partial pressures of 10 3 to 10 5 Pa. Steady-state creep rate, , for P (O 2 ) from 10 4 to 10 5 Pa could be expressed as = Aσ 1.0 (GS) −2.8±0.6 exp −(970 ± 130 kJ/mole)/R T , where A is a constant, σ the steady-state stress, GS the average grain size, and R and T have their usual meanings, For P (O 2 ) from 10 3 to 3 ⊠ 10 3 Pa, the activation energy decreased to about 650 kJ/mole and for a given temperature creep kinetics were much faster. The data and microscopic observations indicated that creep occurred by diffusional flow. Comparisons with diffusion data for YBa 2 Cu 3 O x suggested that Y or Ba may be rate-controlling diffusing species.