Andrea Scrivani

A comparative study of high velocity oxygen fuel, vacuum plasma spray, and axial plasma spray for the deposition of CoNiCrAlY bond coat alloy

In the aerospace field as well as in the stationary gas turbine field, thermally sprayed coatings are used to improve the surface properties of nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly yttria partially stabilized zirconia). The purpose of the current study was to assess the properties of thermally sprayed bond coat CoNiCrAlY alloys comparing the performance of three different techniques: vacuum plasma spray (VPS), high velocity oxygen fuel (HVOF), and axial plasma spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of microstructural (porosity, oxide concentration, unmelted particles presence) and mechanical (hardness) characteristics. The surface composition and morphology of the coatings were also determined. Specific efficiency tests were performed for the three examined technologies. The highest quality coatings are obtained by VPS, but also high velocity oxygen fuel and AxPS sprayed films have interesting properties, which can make their use interesting for some applications.

Chemical stripping of ceramic films of titanium aluminum nitride from hard metal substrates

We report here the result of a study of different chemical stripping methodologies for ceramic coatings deposited by PVD on hard metal substrates. We show that an approach based on the study of the system by surface science techniques leads to the possibility of developing effective stripping methods as well as guidelines for improving the process. We will report about two methods tested for the stripping of layers of titanium aluminum nitride (TINALOX) deposited by Physical Vapor Deposition (PVD) on hard metal substrates. The methods were based on (i) the combination of hydrogen peroxide and potassium oxalate in alkaline conditions, and (ii) potassium permanganate in a concentrated sulfuric acid solution. Of the two methods, the second (permanganate and sulfuric acid) appears to be the most promising, although further studies will be needed to optimize the process.

Prolonged MCrAlY Coating Lifetime by Means of the LPPS Process

It is state of the art that MCrAlY powder is sprayed by LPPS or alternatively by HVOF either as an environmental protective coating or as a bond coat of the TBC on hot section parts of the turbine. The lifetime of the MCrAlY coating can be predicted by the two different losses of the Aluminum content causing a β-depletion of the β NiAl-phase of MCrAlY: the Al loss by the oxidation on the surface and the Al loss by the interdiffusion into the substrate. These effects were analyzed with a NiCoCrAlYRe and a CoNiCrAlY coating sprayed by LPPS and HVOF. Long term oxidation tests demonstrate: the coatings sprayed by LPPS have less β-depletion by diffusion compared to the coatings sprayed by HVOF. This was evaluated for the tested alloy-coating combinations. The β-depletion caused by oxidation is dependent on the MCrAlY: the NiCoCrAlYRe coating has less β-depletion when sprayed by LPPS while in the CoNiCrAlY coating the β-depletion caused by oxidation is lower in the HVOF sprayed coating.Copyright

Development, Characterization and Testing of MCrAlY HVOF Coating Used as Bond Coat for YPSZ Top Coat

MCrAlY coatings (where M stands for Co, Ni, or CoNi) are used as a single overlay coating or as bond coat for thermal barrier coating (TBC) systems. Low Pressure Plasma Spray (LPPS) or Vacuum Plasma Spray (VPS) MCrAlY coatings are considered nowadays the state of the art, nevertheless High Velocity Oxygen Fuel (HVOF) MCrAlY coating is gaining more and more importance due to its quality and to the possible cost reduction in comparison to the state of the art LPPS/VPS coatings. On the other hand the HVOF process tends to have a relatively low surface roughness, which hinders the possibility to act as bond coat for a TBC coating layer. In this work, a HVOF deposition process was developed to obtain a rough MCrAlY coating which can be used as bond coat for yttria partially stabilized zirconia (YPSZ) top coat. Developed parameters were used to apply a TBC system on a stage 2 blade. TBC sections on the same blade have successfully passed thermal fatigue testing according to the specification of an important OEM.Copyright