Non-equilibrium Microstructure and Thermal Stability of Plasma-sprayed Al–Si Coatings

Abstract

A splat-quenched, thick Al-Si deposit was manufactured by low-pressure plasma spraying (LPPS) and investigated in terms of microstructural inhomogeneity, Si solid solubility in ␣-Al, formation of metastable phases, and thermal stability. The LPPS Al-Si deposit had an inhomogeneous, layered microstructure consisting of splat-quenched lamellae and the incorporation of unmelted or partially melted particles. The splat-quenched Al-Si lamellae were formed by deposition of a fully liquid droplet and had an almost featureless microstructure at relatively low magnifications. There was a significant reduction in the ␣-Al lattice parameter in the LPPS Al-Si deposit because of extended Si solubility in the ␣-Al matrix. Transmission electron microscopy investigations showed that the splat quenching of liquid Al-Si droplet led to (i) columnar grain growth of ␣-Al(Si), (ii) formation of nano-sized Si precipitates in the Al matrix which was supersaturated with Si; and (iii) formation of amorphous Si phase embedded in the crystalline Al matrix. On reheating, the amorphous Si transformed into fine crystalline Si by interdiffusion of Al and Si atoms. Simultaneously, Si precipitation occurred in the supersaturated ␣-Al matrix. The overall activation energy for the Si crystallization/precipitation was estimated as ∼81 kJ/mol from a modified Kissinger analysis.

Synthesis of In Situ ZrB2-SiC-ZrC Coating on ZrC-SiC Substrate by Reactive Plasma Spraying

“…It should be noted that the splats increase gradually with the increase in spraying currents and distances. This might be related with an increase in particle temperature, since an increase in spraying current increases the heat energy of the plasma jet and an increase in spraying distances causes the longer dwell time in plasma jet [ 31 ]. Additionally, a few splash structures were also observed, and the morphology of solidifying droplets was formed, as depicted in Figure 9 d. This splash structure resulted from the fact that in-flight particles, on impact with the substrate, fragmented into smaller multiple droplets, and then solidified rapidly.…”

Effects of Plasma-Sprayed La0.7Sr0.3MnO3Coating on Thermally Grown Oxide Scale and Electrical Conductivity of Fe-Cr Interconnect for SOFCs

“…4) were connected to one another generating a three dimensional pore network which would serve as a potential penetration path of oxidizing gases. This pore network was an inherent characteristic of the plasma-sprayed coatings, [26][27][28][29] and was not eliminated completely without subsequent sintering at elevated temperatures above 1200 • C. The plasma-sprayed LSM coating using the small-sized powder had a low gas leakage rate which would be sufficient to protect the underneath SS430 substrate against oxidation environment.…”

“…Plasma spray coating is generally deposited through a rapid splat cooling process, and a high cooling rate over 10 5-8 K/s usually results in the formation of metastable phase in the coating. 28,29 The broadening of XRD peaks of the perovskite LSM phase implied that fine crystalline grains were formed in the as-sprayed LSM coating. After heat-treatment at 800 • C for 1 h, the XRD diffraction peaks of LSM phase became more pronounced due to recrystallization and/or grain growth.…”

“…[23][24][25][26][27][28] A high temperature plasma jet heats feedstock powders to be molten, and directs the liquid droplets to the substrate to form a coating. Since the plasma sprayed ceramic coating has commonly contained a large amount of microstructural defects such as pores, intersplat porous layer and micro-sized cracks, [26][27][28][29] systematic study on its application for SOFC interconnector has yet been reported.…”