Effective Parameters in Axial Injection Suspension Plasma Spray Process of Alumina-Zirconia Ceramics
Suspension plasma spray (SPS) is a novel process for producing nano-structured coatings with metastable phases using significantly smaller particles as compared to conventional thermal spraying. Considering the complexity of the system there is an extensive need to better understand the relationship between plasma spray conditions and resulting coating microstructure and defects. In this study, an alumina/8 wt.% yttria-stabilized zirconia was deposited by axial injection SPS process. The effects of principal deposition parameters on the microstructural features are evaluated using the Taguchi design of experiment. The microstructural features include microcracks, porosities, and deposition rate. To better understand the role of the spray parameters, in-flight particle characteristics, i.e., temperature and velocity were also measured. The role of the porosity in this multicomponent structure is studied as well. The results indicate that thermal diffusivity of the coatings, an important property for potential thermal barrier applications, is barely affected by the changes in porosity content.
Relationships between the microstructure and properties of thermally sprayed deposits
Thermally sprayed deposits have layered structure composed of individual splats. The individual splats have quenching microstructure of quasi-stable preferred fine grains. However, this fine-grained microstructure of the deposits is usually not reflected by improved performance of the deposits because a layered structure with two-dimensional voids occurs between lamellar interfaces. The microstructure of the thermal spray deposits with the emphasis on the layer structural parameters is reviewed. Conventionally, one of the most common quantitative parameters used to characterize the microstructure of the thermally sprayed deposits is the porosity, measured by different methods. However, it is illustrated that the relationships between properties and porosity for bulk porous materials processed by conventional processes cannot be applied to thermally sprayed deposits owing to the two-dimensional characteristics of voids. The total porosity in the deposits is not meaningful from the viewpoint of prediction of the deposit properties. An idealized structural model and related parameters, instead of porosity, are proposed to characterize quantitatively the microstructure of the thermally sprayed deposit. The relationships between the properties and the structural parameters are presented for the plasma-sprayed ceramic deposits based on the proposed microstructure model. The properties include the Young’s modulus, fracture toughness, erosion resistance, and thermal conductivity of the plasma sprayed ceramic deposits. The correlations of theoretical relationships with reported experimental data are discussed. An agreement of theoretical with observed values suggests that the lamellar structure of the deposit with limited interface bonding is the dominant factor controlling the performance of the deposit.
Performance of YSZ electrolyte layer deposited by atmospheric plasma spraying for cermet-supported tubular SOFC
Abstract Yttria-stabilized zirconia (YSZ) was deposited by plasma spraying to develop a cost-effective processing method for the fabrication of electrolyte layer in solid oxide fuel cells (SOFCs). A densification process to plasma-sprayed YSZ coating was conducted with the aim of achieving a dense coating of necessary gas-tightness and an improved ionic conductivity. The densification of ZrO2–4.5 mol% Y2O3 electrolyte, which was produced by atmospheric plasma spray, was performed by impregnating yttrium and zirconium nitrate solution into the coating followed by heat-treatment. The results show that the YSZ layer of the gas permeability comparable to that of fully sintered one can be formed through plasma spraying followed by impregnation densification. The test cell yielded an OCV of 1.04 V and a maximum output power density of 101 mW/cm2 at 930°C. Those results show that the combination of plasma spraying with post-impregnation is a feasible process to fabricate YSZ electrolyte layer used in SOFC.