M.I. Osendi

TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Mechanical properties of mullite materials

Abstract The mechanical behaviour of two 3Al2O3 · 2SiO2 dense mullite materials with the same level of impurities but different in nature has been studied. Microstructure has been characterized by SEM and TEM. Toughness, bend strength and Youngs modulus have been determined from room temperature up to 1400 °C. Dependence of toughness on strain rate has been investigated. Special attention has been paid to correlate the trend of the mechanical parameters to fractographic observations by SEM.

Thermal conductivity of ceramics in the ZrO 2 -GdO 1.5 system

Low thermal conductivity ceramics in the ZrO 2 -GdO 1 . 5 system have potential in structural (refractories, thermal barrier coatings, thermal protection) and nuclear applications. To that end, the thermal conductivities of hot-pressed x GdO 1 . 5 . (1 - x)ZrO 2 (where x = 0.05, 0.15, 0.31, 0.50, 0.62, 0.75, 0.89, and 1.00) solid solutions were measured, for the first time, as a function of temperature in the range 25 to 700 °C. On the ZrO 2 -rich side, the thermal conductivity first decreased rapidly with increasing concentration of GdO 1 . 5 and then reached a plateau. On the GdO 1 . 5 -rich side, the decrease in the thermal conductivity with increasing concentration of ZrO 2 was less pronounced. The thermal conductivity was less sensitive to the composition with increasing temperature. The thermal conductivity of pyrochlore Gd 2 Zr 2 O 7 (x = 0.5) was higher than that of surrounding compositions at all temperatures. A semiempirical phonon-scattering theory was used to analyze the experimental thermal conductivity data. In the case of pure ZrO 2 and GdO 1 . 5 , the dependence of the thermal conductivity to the absolute temperature (T) was less than 1/T. Therefore, the minimum thermal conductivity theory was applied, which better described the temperature dependence of the thermal conductivity of pure ZrO 2 and GdO 1 . 5 . In the case of solid solutions, phonon scattering by cation mass fluctuations and additional scattering by oxygen vacancies on the ZrO 2 -rich side and by gadolinium vacancies on the GdO 1 . 5 -rich side seemed to account for the composition and temperature dependence of the thermal conductivity.

Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO2 composites with increasing amount of ZrO2 (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheKIC values increase from 2.1 to 3.2 MN m−3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO2 increases to reach ∼0.1 in the sample with 0.2 volume fraction of ZrO2. The presence of ZrO2 enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO2 composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (∼0.5 wt %) of ZrO2 in mullite.

Thermal conductivity of Al2O3/SiC platelet composites

The thermal conductivity of hot-pressed Al2O3/SiC platelet composites is determined as a function of the platelet content, from 0 to 30 vol.% of SiC. Existing heat conduction models are employed to discuss the experimental data. Data agree with the presence of an interfacial thermal resistance at the Al2O3/SiC grain boundaries, which precludes the effect of percolation on the thermal conductivity for the higher percentage of SiC platelets. The observed orientation effect on the thermal conductivity due to an alignment of the platelets is also modelled using the Hasselmans approach. The thermal conductivity of the SiC platelets is calculated from the effective thermal conductivity of the composites.

Joining mechanism in Si3N4 bonded with a Ni–Cr–B interlayer

Abstract A commercially available Ni–Cr brazing alloy foil with B infused at its surface layer was used to join silicon nitride ceramics. The joining region showed a diffusion zone at the middle of the interlayer and a reaction layer at the silicon nitride/brazing alloy interface. The reaction layer mainly consisted of CrN and BN while the diffusion layer was formed by Ni [Cr, Si] solid solution with some CrN precipitates. A reaction mechanism is proposed to explain the microstructure and phases observed in the bonding interlayer.

Microstructure and mechanical strength of Si3N4/Ni solid state bonded interfaces

Joining of hot pressed silicon nitride using Ni interlayers was done by diffusion bonding. Results were compared for two bonding temperatures: 1050 and 1150°C. For the lower joining temperature, no reaction was found but a limited diffusion of atomic Si into the Ni lattice was observed while for the higher temperature a thin reaction layer was found. Some nickel silicides and precipitates of a Y-rich phase were detected within this layer and an extensive diffusion zone of Si into Ni was measured. The mechanical properties of the joints were evaluated by shear and bend tests varying the thickness of the Ni interlayer. Bending strengths of 235 MPa were achieved at room temperature.

Thermal diffusivity of porous cordierite ceramic burners

The applicability of the laser flash method for measuring the thermal diffusivity of highly porous cordierite materials is investigated. Due to the surface roughness of the samples, some indetermination in the sample thickness measurement is produced, which induces errors in the thermal diffusivity calculation. This problem was partially overcome by attaching two thin Cu layers to both surfaces of the samples. The thermal diffusivity and conductivity values of two porous cordierite materials (40 and 50 vol % of porosity) are reported using this procedure and results are discussed comparing with data for three-layer models.

Oxidation behaviour of mullite-SiC composites

The oxidation kinetics of three mullite-SiC composites were studied. The materials were all processed with the same amount of SiC, 10% by volume; in two of the composites the second phase was added as whiskers, and in the third as powders. The sintered composites were exposed to high temperatures (1200 to 1500 ° C) during variable time periods (maximum 122 h) under the oxidizing furnace atmosphere. The nature of the reaction layer formed has been analysed specifying the oxidation rate constants for each composite. The influence on the bend strength of the composites for one of the isothermal oxidizing treatments has also been measured.

Correlation between microstructure and toughness of hot pressed Si3N4 ceramics seeded with β-Si3N4 particles

Abstract Silicon nitride materials with distinct microstructures have been prepared by hot-pressing mixtures of α-Si 3 N 4 powders plus oxide additives and β-Si 3 N 4 seeds. These seeds were β crystals obtained by the SHS technology. The presence of these β-Si 3 N 4 particles retarded the densification rate. The influence of the holding time at the sintering temperature on the microstructure and mechanical properties has been analysed in the β-seeded Si 3 N 4 materials, in comparison with the non-seeded materials. These β-seeds were effective in enhancing α→β transformation and the general coarsening of the microstructure. The toughness has been discussed as a function of microstructural parameters, such as average grain size, average aspect ratio and area fraction of elongated grains.