No abstract

The solution-precursor plasma spray (SPPS) method is a new process for depositing thick ceramic coatings, where solution feedstock (liquid) is injected into a plasma. This versatile method has several advantages over the conventional plasma spray method, and it can be used to deposit nanostructured, porous coatings of a wide variety of oxide and non-oxide ceramics for a myriad of possible applications. In an effort to understand the SPPS deposition process, key diagnostic and characterization experiments were performed on SPPS coatings in the Y2O3-stabilized ZrO2 (YSZ) system. The results from these experiments show that there are multiple pathways to SPPS coating formation. The atomized precursor droplets undergo rapid evaporation and breakup in the plasma. This is followed by precipitation, gelation, pyrolysis, and sintering. The different types of particles reach the substrate and are bonded to the substrate or the coating by sintering in the heat of the plasma. The precursor also reaches the substrate or the coating. This precursor pyrolyzes in situ on the substrate, either after it reaches a “cold” substrate or upon contact on a “hot” substrate and helps bond the particles. The coating microstructure evolves during SPPS deposition as the coating temperature reaches approximately 770 °C.

The effect of starting SiC powder (␤-SiC or ␣-SiC), with simultaneous additions of Al 2 O 3 and Y 2 O 3 , on the microstructural evolution of liquid-phase-sintered (LPS) SiC has been studied. When using ␣-SiC starting powder, the resulting microstructures contain hexagonal platelike ␣-SiC grains with an average aspect ratio of 1.4. This anisotropic coarsening is consistent with interface energy anisotropy in ␣-SiC. When using ␤-SiC starting powder, the ␤ 3 ␣ phase transformation induces additional anisotropy in the coarsening of platelike SiC grains. A strong correlation between the extent of ␤ 3 ␣ phase transformation, as determined using quantitative XRD analysis, and the average grain aspect ratio is observed, with the maximum average aspect ratio reaching 3.8. Based on these observations and additional SEM and TEM characterizations of the microstructures, a model for the growth of these high-aspect-ratio SiC grains is proposed.

Effects of N2 sintering atmosphere and the starting SiC powder on the microstructural evolution of liquid‐phase‐sintered (LPS) SiC were studied. It was found that, for the β‐SiC starting powder case, there was complete suppression of the β→α phase transformation, which otherwise goes to completion in Ar atmosphere. It was also found that the microstructures were equiaxed and that the coarsening was severely retarded, which was in contrast with the Ar‐atmosphere case. Chemical analyses of the specimens sintered in N2 atmosphere revealed the presence of significant amounts of nitrogen, which was believed to reside mostly in the intergranular phase. It was argued that the presence of nitrogen in the LPS SiC helped stabilize the β‐SiC phase, thereby preventing the β→α phase transformation and the attendant formation of elongated grains. To investigate the coarsening retardation, internal friction measurements were performed on LPS SiC specimens sintered in either Ar or N2 atmosphere. For specimens sintered in N2 atmosphere, a remarkable shift of the grain‐boundary sliding relaxation peak toward higher temperatures and very high activation energy values were observed, possibly due to the incorporation of nitrogen into the structure of the intergranular liquid phase. The highly refractory and viscous nature of the intergranular phase was deemed responsible for retarding the solution–reprecipitation coarsening in these materials. Parallel experiments with specimens sintered using α‐SiC starting powders further reinforce these arguments. Thus, processing of LPS SiC in N2 atmosphere open the possibility of tailoring their microstructures for room‐temperature mechanical properties and for making high‐temperature materials that are highly resistant to coarsening and creep.