Abstract

In order to achieve thick coatings on thin substrates (e < 1 mm) which grit blasting is difficult and can even be impossible, a deposit achieved by using two successive techniques might be the solution. For example it could consist in depositing first by plasma enhanced CVD (PECVD) a thin alumina coating (~ 2 tm thick) and then by plasma spraying a thick (300 - 500 µm) alumina coating. To study such a possibility a systematic study of splats morphology on various substrates was performed. The substrates were either plasma sprayed (- 0.8 - 0.9 mm thick) or PECVD deposited (2 pm thick on mirror polished 304L stainless steel). The plasma sprayed substrates were annealed or not in order to get a columnar, a granular or y columnar structures and then polished (Ra -- 0.4 pm) white PECVD coatings exhibiting an Ra - 6 nm and a columnar amorphous structure, were sprayed as deposited. Splat studies show the drastic influence of alumina substrates crystalline structure and entrapped gas release on splat formation. For plasma sprayed coatings disk shaped splats with an excellent adhesion of corresponding coatings (up to 40 MPa) were obtained on hot (673-873 K) y alumina while on a alumina splat shape and morphology were irregular and resulting coatings adhesion very poor (below 3 MPa). For PECVD substrate gas release upon impact of molten particles modified drastically the wettability and their thermal contact with substrate resulting in very porous splats or in splats were most of alumina is in the splat rim.

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