Oxidation behavior of IN738LC in situ deposited with SiO2 during a burner rig test

Abstract

A burner rig was utilized for in situ depositing SiO2 on IN738LC and simultaneously for testing its oxidation behavior. For the SiO2 deposition, a mixture of TEOS and methanol was steadily fed into the combustion chamber of the burner rig during processing. The exhaust flame temperature was set at 1374°C, and the oxide precursors in the exhaust gas were carried vertically toward the pin-type specimens of IN738LC infixed to a carousel, 10cm away from the exhaust gas nozzle. A thermal cycle consisting of a 54min run and subsequent a 6min pause was repeated four times, and a processed specimen was analyzed in terms of the deposition and oxidation of the substrate.All over the processed specimen was deposited with a Si-rich oxide layer. The surface morphology of the layer varied depending on the surface temperature: from broccoli-shaped grains of several micrometers in diameter at the location of the highest surface temperature of 1018°C to finely dispersed particles of ≤1μm at the cold (≤800°C) peripheral regions of the specimen. Underneath the deposited layer, oxide scales were formed to be ≥10μm in thickness, being identified as outer Cr-rich and inner Al-rich one. Al-rich oxide formed a continuous layer (~3μm) at the location of the highest surface temperature, which became particles broadly distributed near the surface on going to the cold peripheral regions. Meanwhile, Cr-rich oxide formed a continuous layer (3–5μm) at the cold regions, which became narrower and eventually disappeared at the hottest region. The changeover of the oxidation modes accorded with the behavior of the EDX intensity of Cr as a function of that of Al in the oxide scale; the Cr intensity decreased, while the Al intensity increased toward the hotter region. The EDX Si signal for the deposited layer increased with that of O with an increase in the surface temperature along the specimen, suggesting that the layer became denser at the location of higher surface temperatures. It was discussed that the deposited porous layer was densified during the process to a degree depending on the surface temperature of the specimen, which rendered the layer compacter at the location of higher surface temperatures. The oxygen diffusion through the compacter layer was suppressed more effectively, resulting in the preferred oxidation of Al over Cr in the alloy, namely better protection of the metal substrate in terms of the oxidation.