Plasma Spray‐Physical Vapor Deposition (PS‐PVD) of Ceramics for Protective Coatings

Abstract

In order to generate advanced multilayer thermal and environmental protection systems, a new deposition process is needed to bridge the gap between conventional plasma spray, which produces relatively thick coatings on the order of 125-250 microns, and conventional vapor phase processes such as electon beam physical vapor deposition (EB-PVD) which are limited by relatively slow deposition rates, high investment costs, and coating material vapor pressure requirements. The use of Plasma Spray -Physical Vapor Deposition (PS-PVD) processing fills this gap and allows thin (< 10 m) single layers to be deposited and multilayer coatings of less than 100 m to be generated with the flexibility to tailor microstructures by changing processing conditions. Coatings of yttria-stabilized zirconia (YSZ) were applied to NiCrAlY bond coated superalloy substrates using the PS-PVD coater at NASA Glenn Research Center. A design-of-experiments was used to examine the effects of process variables (Ar/He plasma gas ratio, the total plasma gas flow, and the torch current) on chamber pressure and torch power. Coating thickness, phase and microstructure were evaluated for each set of deposition conditions. Low chamber pressures and high power were shown to increase coating thickness and create columnar-like structures. Likewise, high chamber pressures and low power had lower growth rates, but resulted in flatter, more homogeneous layers. INTRODUCTIONThermal and environmental barrier coatings (TBCs, EBCs) are necessary for the protection of metal and ceramic components in high temperature turbine engine environments. To meet everincreasing temperature demands for improving efficiency, these coatings have become increasingly complex in composition and architecture. Thermal spray technology has been a longstanding processing method for depositing TBCs and EBCs.1 In traditional air plasma spray (APS), coatings are formed by the buildup of molten ceramic material as a torch traverses the substrate. Electron beamphysical vapor deposition (EB-PVD) has been used to create smooth and more strain tolerant TBCs by growth of columnar grains. In this process, vaporized ceramic material condenses onto the hot substrate surface under high vacuum conditions (<10 -4 torr). The resultant coating has a microstructure that is well suited for turbine airfoil components.2 However, these processes both exhibit limitations, as APS methods produce rough coatings on the order of 125-250 microns thick (a single pass is 50-80 microns), and conventional EB-PVD processing is limited by slower growth rates, high equipment investment costs, and coating material vapor pressure requirements.A new processing technology, known as Plasma Spray -Physical Vapor Deposition (PS-PVD) has been developed in order to bridge the gap between conventional APS and PVD techniques to create unique microstructures.3,4 The PS-PVD system at the NASA Glenn Research Center is one of only four systems currently available at this time. Conventional low pressure plasma spray (LPPS) is a...

Microstructure of vapor deposited coatings on curved substrates

“…[1][2][3] Numerous methods have been proposed for the deposition of these coatings including liquid droplet processes such as air or vacuum plasma spray, 4 and electron beamphysical vapor deposition (EB-PVD) 5 concepts that create an atomically dispersed vapor plume, which is condensed on the substrate surface. More recently, hybrid techniques have been also proposed including plasma spray-PVD (PS-PVD), [6][7][8] which uses a high power plasma to evaporate liquid droplets in a high-pressure gas jet, and electron beamdirected vapor deposition (EB-DVD) 9,10 in which an electron beam is used to evaporate a source material located in the throat of a helium gas jet forming nozzle.…”

“…23 A similar capability has been reported for the gas flow sputtering method [28][29][30] using a deposition chamber pressure of 47 Pa, and the PS-PVD method, which operates with chamber pressures in the 100-1000 Pa range. [6][7][8] Vapor condensation onto a nonplanar substrate geometry affects both coating growth behavior during deposition and in-service performance of the ensuing coating. For example, the delamination resistance of a TBC appears to depend on the curvature of the substrate to which it is applied.…”

Thermally grown oxide in water vapor on coated and uncoated SiC

“…Plasma Spray‐Physical Vapor Deposition (PS‐PVD) (Oerlikon Metco) was used to apply Yb 2 Si 2 O 7 (YbDS) EBCs on the surfaces of separate Hexoloy ® samples as has been done previously 11‐13 . The processing conditions are shown in Table 3.…”

“…Additional steam cycling exposures were performed on Plasma Spray‐Physical Vapor Deposited (PS‐PVD) Yb 2 Si 2 O 7 (YbDS) EBC on α‐SiC (Hexoloy ® ) to understand the effect the EBC is having on the oxidation of the underlying SiC. The PS‐PVD process has been described more fully in previous work 11‐13 …”

Enhanced oxidation resistance of SiC/SiC minicomposites via slurry infiltration of oxide layers

“…These coatings typically consist of Y or Yb-containing rare-earth silicates as the protective layer and mullite as a bond coat between the EBC layer and the SiC matrix and are applied via an atmospheric plasma spray approach 8 .…”