Development of CVD overaluminising process on different CoNiCrAlY bond coats deposited by Vacuum Plasma Spray (VPS), Low Pressure Plasma Spray (LPPS) and High Velocity Oxygen Fuel (HVOF).
G. Rizzi, A. Scrivani
ABSTRACT
Coating technology is progressing at a steady rate with continuous significant improvements in the coatings performance. In the aerospace field, as well as in the stationary gas turbine field, coatings deposited by different processes (thermal spray, CVD, EBPVD) play an important role in order to increase the performances of the engines.
In particular, in order to improve the resistance to oxidation and corrosion at high temperature, aluminium is deposited by several processes (pack aluminising, above the pack and CVD) in alternative or addition to thermal spray coatings (mainly MCrAlY alloys where M stands for Co, Ni or CoNi).
These MCrAlY coatings are generally deposited by Low Pressure Plasma Spray (LPPS) or Vacuum Plasma Spray (VPS), but also by High Velocity Oxygen Fuel (HVOF) and Air Plasma Spray (APS), which may also be required for their lower cost, even though it is commonly held that the quality of MCrAlY coatings deposited by HVOF or APS is lower because of the partial oxidation that the materials undergoes while being sprayed.
This paper addresses the study of aluminium coatings deposited by CVD on CoNiCrAlY bond coats deposited by Low Pressure Plasma Spray (LPPS), Vacuum Plasma Spray (VPS) and High Velocity Oxygen Fuel (HVOF). The aim was to determine if the CoNiCrAlY coating structures obtained by these processes with different content of oxides and porosity could affect the deposition rate and quality of the Al coatings.
The obtained samples have been characterized from the metallographic point of view in order to determine porosity, thickness and structure of both CoNiCrAlY and Al coatings by means of metallographic and scanning electron microscopy. Al coating thickness has been taken as parameter in order to define the Al coating deposition rate on the different CoNiCrAlY coatings. Surface roughness and morphology of the samples has been studied on CoNiCrAlY surface before and after Al deposition in order to determine how the Al coating affect the surface morphology. Oxidation test has been performed on overaluminised CoNiCrAlY coatings.
From the metallographic characterisation the content of oxygen in the different CoNiCrAlY coating does not vary in significant way, the values of the CVD Al thickness are quite similar independently by the process used for the deposition of CoNiCrAlY. The low difference in oxygen content in the CoNiCrAlY coating does not affect substantially the deposition of Al by CVD. The oxidation tests show how the aluminium coatings “protects” the underlaying CoNiCrAlY coatings: no significant differences are notable in the oxide growth after the oxidation test.
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