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.     

Characterization of porous graphitic monoliths from pyrolyzed wood

Porous graphitic carbons were obtained from wood precursors using Ni as a graphitization catalyst during pyrolysis. The structure of the resulting material retains that of the original wood precursors with highly aligned, hierarchical porosity. Thermal characterization was performed by means of thermogravimetry and differential scanning calorimetry, and the onset temperature for graphitization was determined to be ~900 °C. Structural and microstructural characterization was performed by means of electron microscopy, electron and x-ray diffraction, and Raman spectroscopy. The effect of maximum pyrolysis temperature on the degree of graphitization was assessed. No significant temperature effect was detected by means of Raman scattering in the range of 1000–1400 °C, but at temperatures over the melting point of the catalyst, the formation of graphite grains with long-range order was detected.     

Reaction–formation mechanisms and microstructure evolution of biomorphic SiC

Biomorphic SiC is fabricated by liquid Si infiltration of a carbon preform obtained from pyrolized wood that can be selected for tailored properties. The microstructure and reaction kinetics of biomorphic SiC have been investigated by means of TEM, SEM, EBSD, and partial infiltration experiments. The microstructure of the material consists of SiC and Si and a small fraction of unreacted C. The SiC follows a bimodal size distribution of grains in the micrometer and the nanometer range with no preferential orientation. The infiltration-reaction constant has been determined as 18 × 10⁻³ s⁻¹. These observations suggest that the main mechanism for SiC formation is solution–precipitation in the first stage of growth. If the pores in the wood are small enough they can be choked by SiC grains that act as a diffusion barrier between Si and C. If that is the case, Si will diffuse through SiC forming SiC grains in the nanometer range.     

Microstructure and room-temperature mechanical properties of Si3N4 with various α/β phase ratios

After polycrystalline silicon nitride samples with various / phase ratios were fabricated by uniaxial hot-pressing under vacuum, their microstructures, -to- transformations, and grain-growth mechanisms were then characterized by electron microscopy. Room-temperature fracture toughness (KIC) increased with increasing -phase content, and a value of 7.0 ± 0.5 MPa m1/2 was obtained for a sample that contained 60 vol% phase. The increase in KIC is the result of an increase in elongated-grain fractions with increasing content, which promotes energy absorption by crack deflection, as well as by grain debonding, pullout, and bridging mechanisms.