Dear Prasad Rao,
Although I do not have an experience in the application of thermal spray coatings on devices for melting non-ferrous metals, I can hypothesize about the interaction between the alumina coating and the substrates you mentioned.
Based on the application you mentioned, I guess there will be thermal cycling involved; right?
Supposing you can successfully spray well-adhered Al2O3 coatings (even by HVOF) on SiC substrates, my supposition would be that the differences of coefficient of thermal expansion (CTE) between Al2O3 (~8 x 10-6/oC) and SiC (~4 x 10-6/oC) would result in high stress/strain levels at the coating/substrate interface during thermal cycling, and it would tend to lead to an early coating failure.
Depending on the type of stainless steel you use, the CTE difference between the Alumina and stainless steel can be minimized; which is "good" for lowering the stresses/strains above mentioned during thermal cycling.
That said, it is known that when Al2O3 is thermally sprayed, there is some degree formation of the metastable gamma-Al2O3 and even some amorphous Al2O3 in the coating structure (from the previous molten particles). Upon subjecting the as-sprayed coating to high temperatures (>800oC), the gamma and amorphous phases would tend to crystalize into the stable alpha-Al2O3 phase. As ceramic materials exhibit negligible plasticity, this crystallization (phase transformation) process would tend to lead unwanted additional stress/stain, thereby leading to coating failure.
Therefore I speculate that as long as (i) the CTE values of Al2O3 and the stainless steel substrate employed can be "nearly matched" (depending on the type of stainless steel) and (ii) if the application temperature does not go about 800oC, this option is better than the other 1st one suggested (i.e., Al2O3 coating on SiC substrate).
I hope it helps!
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Rogerio Lima, PhD
Senior Research Officer
National Research Council of Canada
Fellow of ASM International (FASM)
President of Thermal Spray Society (TSS)
e-mail:
rogerio.lima@nrc-cnrc.gc.ca------------------------------
Original Message:
Sent: 11-21-2023 09:02
From: Prasad Rao
Subject: HVOF spray of Aluminum oxide over Silicon carbide & HVOF spray of Aluminum oxide over SS310 stainless steel
Hi
I have a requirement for HVOF spray of Al203 upon silicon carbide and other option is Al2o3 spray on Stainless steel. Please comment and give your experience on these applications. The end application is for melting nonferrous metals especially Aluminum.
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Prasad Rao
DIRECTOR
METEXPERT METALURGICAL CONSULTANCY PVT LTD
BANGALORE
+917829721305
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