For electrical or thermal insulation, the porosity of an air plasma sprayed (APS) coating is an important property to control. Moreover in aggressive environment the interconnected porosity is responsible for the substrate corrosion. To solve, at least partially, this problem, deposition by mutitechniques (APS and a PECVD) was used to close interconnected and opened porosities. In this study, titanium alloy (TA6V) substrates were coated by alumina using either one or both deposition processes. Electrochemical characterization technique was used to evaluate the open porosity in alumina coatings. It consists of evaluating the polarization resistance of the reference sample surface (uncoated substrate) and to compare it to coated ones. After different tests for selecting the electrolyte solution, the influences of different parameters (thickness and relative position) of each deposition process on coating porosity were examined.

Thick alumina coatings produced by Air Plasma Spraying have an interconnected porosity, thus the use of these coatings in oxidizing or corrosive environment is not suitable. In this paper, a study is developed in order to limit this problem on metallic substrates. It consists in using two successive techniques: APS and PECVD. Two parameters have been shown to be important: the roughness and the preheating temperature. Two types of duplex (PECVD coating as top coat or as bond coat) have been achieved on two substrates (TA6V and stainless steel 316L). The optimization of each process has shown that the substrate has to be grit blasted and preheated (360°C for PECVD and 250°C for APS). This study has revealed that a good (36 ± 5 MPa) APS coating adhesion was obtained on smooth TA6V substrates (due probably to a chemical reaction between TiO 2 and alumina) while for stainless steel substrates, the Ra has to be at least 2µm to achieve 66 ± 5. When observing the first APS splats sprayed on the PECVD alumina smooth layer, they exhibited a specific appearance: low flattening degree (about 2 against 5 on metallic substrates) with most of the alumina in the splat rim or some sort of lace morphology. However, as a whole, the adhesion of the APS coating on the PECVD one was excellent: 60 + 4. An electrochemical method has shown that the PECVD layer on APS coating has reduced drastically its open porosity.

This paper presents the results of a study of the morphology of alumina splats deposited on stainless steel and alumina substrates. The substrates were either plasma sprayed or coated via plasma enhanced CVD. Substrates that were plasma sprayed were annealed if necessary to get specific phase structures, then polished to around 0.4 μm (Ra). CVD-coated substrates with an Ra ~6 nm and a columnar amorphous structure were sprayed as deposited. Splat studies show that the crystal structure of alumina substrates and the release of entrapped gas have a major influence on splat formation. For plasma sprayed coatings, disk-shaped splats with excellent adhesion properties were obtained on hot γ alumina, while on α alumina, splat shape and morphology were irregular and adhesion very poor. The effect of entrapped gas, on the other hand, can be seen in the splats that formed on the CVD-coated substrates. These splats were very porous and, in many, most of alumina flowed out to rim. As the paper explains, this is the result of gas release upon impact of molten particles, which reduces wettability and thermal contact between the splat and substrate.