Comparison of microstructure and oxidation behavior of CoNiCrAlY coatings produced by APS, SSAPS, D-gun, HVOF and CGDS techniques

Highlights

• CoNiCrAlY coating systems were successfully produced by on Inconel 718 substrate using APS, SSAPS, D-gun, HVOF and CGDS techniques.

• Changes in the dimensions of TGO layer, microstructural properties and chemical compatibilities of CoNiCrAlY coatings were comparatively investigated as functions of time and temperature.

• It was determined that CoNiCrAlY coatings with produced by CGDS technique exhibit superior properties as compared to other techniques.

Abstract

MCrAlY coatings are widely preferred in materials used at high temperature applications exposed to oxidation and hot corrosion. The coating process used in the production of thermal spray coatings directly affects the microstructure and lifetime of the coating. In this study, a CoNiCrAlY metallic bond coat was deposited on Inconel 718 superalloy substrate material using APS, SSAPS, D-gun, HVOF and CGDS spraying techniques. The produced coatings were subjected to isothermal oxidation tests at 1000 °C for different exposure times. Porosity, surface roughness, XRD, SEM, EDX-elemental mapping analysis were used to investigate and determine the microstructures and the changes in the dimensions of thermally grown oxide (TGO) layer as a function of time and temperature. The obtained data were compared before and after the oxidation tests, and the findings were evaluated considering other related studies within the literature. The results show that CGDS CoNiCrAlY coatings exhibited better oxidation resistance than the coatings produced by the other techniques.

Introduction

MCrAlY type coatings have been commonly used especially in aerospace industry and aero engines for protection of superalloy based substrate components of gas turbines from failures emerging as a result of high temperature effects such as oxidation and hot corrosion [[1], [2], [3]]. As a bond coat material, MCrAlY is particularly preferred in thermal barrier coating systems to provide sufficient bond strength between the ceramic top coat and the superalloy substrate, to prevent rapid oxidation of the base alloy [[4], [5], [6], [7], [8]]. In a MCrAlY system, M denotes the metallic content in the coating's composition, which is generally nickel, cobalt, chromium or a combination of both. MCrAlY type coatings are generally applied in the form of CoNiCrAlY, NiCoCrAlY and NiCrAlY on super-alloy substrates [[3], [4], [5],9,10]. Chromium and aluminum are included in the coating content to increase its oxidation and hot corrosion resistance [5,6,[11], [12], [13], [14]]. The role of yttrium (Y), which is generally used in low amounts, is to increase the bond strength of the oxide layer with the MCrAlY coating [1,3,[15], [16], [17], [18]]. MCrAlY type coatings are generally produced with thermal spray coating techniques such as atmospheric plasma spraying (APS), low pressure plasma spraying (LPPS), supersonic plasma spraying (SSAPS), detonation gun (D-gun), high velocity oxygen fuel (HVOF) and cold gas dynamic spraying (CGDS) [1,[19], [20], [21], [22], [23], [24], [25], [26]].

In APS technique, MCrAlY is melted in a high energy plasma environment and sprayed onto a superalloy substrate in a molten or semi-molten form in open atmosphere [[20], [21], [22]]. This conventional thermal spraying technique is mainly preferred for its low operational costs whereas its implementation in open atmosphere constitutes a major drawback as it leads to formation of a porous structure with high oxide content [[20], [21], [22],27]. In SSAPS technique, the production of coatings is achieved at supersonic particle velocities [[20], [21], [22],[27], [28], [29], [30]]. Generally as compared to conventional APS coating with the same composition, this method allows obtaining a more dense and thinner grain structure than APS coatings [4,[28], [29], [30], [31]]. In this process, the stream of high velocity molten particles can be obtained in the high-energy plasma jet, which is exposed to the atmosphere. In this technique, a coating structure with lower oxide content due to higher particle velocities can be produced compared to other plasma spray coating techniques [5,29,30,32]. D-gun technique is a traditional thermal spray technology as HVOF although it is different in terms of the deposition process. It is a discontinuous spraying process while HVOF has a continuous spraying procedure. Both D-gun and HVOF techniques are widely used in the production of wear and corrosion resistant coatings [33,34]. In this technique superior bond strength in addition to a coating surface with lower porosity and higher resistance against compressive loads is obtained, owing to the high operating temperatures and velocities [[33], [34], [35], [36], [37]]. HVOF technique is also known as an alternative and novel deposition technique in the literature [19,38,39]. In this technique, since the velocity of the particles is higher compared to the APS technique, the oxide content of the produced coating is lower [3,25,33]. Coatings deposited by HVOF technique, therefore, have a low porosity and oxide content and a dense microstructure, which provides high wear and oxidation resistance [4,16,36,40,41]. In the CGDS technique, deposition is implemented at relatively lower particles temperatures compared to the other thermal spray techniques. The coating materials are sprayed onto the substrate surface at supersonic velocities in a non-molten state, and the bonding of the particles on the surface is achieved via plastic deformation [3,11,39]. This technique is mainly favored for deposition of coatings with low oxide and porosity content. The main principle of the CGDS system is to ensure that powder material to be coated with severe plastic deformation adheres onto the selected substrate surface [39]. With this high acceleration process, gases such as air, nitrogen and helium are firstly sent from the coating nozzle to the substrate surface after being heated and compressed by pressure [2,9,25,42,43]. The particles coming out of the nozzle are in contact with each other and the substrate, providing metallurgical bonding [19,23,39,44,45].

Apart from these techniques, arc spray coating, flame spraying and suspension solution plasma spray (SSPS) coating techniques are also available in the literature [46,47]. In the arc spray technique, the arc created as a result of electrical energy is used as the heat source. This technique is also termed as wire arc spraying or arc spraying in literature. The difference between each other is the feedstock form of coating materials. The heat formed by the arc melts the wires and melted or semi-melted coating materials with a compressed air are sent to the substrate material and the coating is thus produced. When the temperature increases, pressurized air is sent to the substrate surface, thus the microstructure of the coatings produced by this technique includes oxide and porosity [[46], [47], [48], [49]]. Flame spraying technique, the oldest of the traditional thermal spray techniques, was discovered in the early 1900s [50,51]. In the system, the high combustion temperatures obtained as a result of the combustion of the combustible gas, the acetylene gas, provide the melting of the coating materials [[52], [53], [54]]. Coating materials sent to the substrate surface in a molten or semi-molten form have a low particle rate. For this reason, particles during the flight are exposed to more open-air. The microstructure obtained after production is an oxidized and porous structure [[54], [55], [56], [57]]. The SSAPS technique is a kind of thermal spray technique, just like the APS technique. In this technique, instead of the conventional powder feedstock used in the APS technique, an aqueous chemical precursor feedstock is used to deposit the coating [58]. The materials in the solution are subjected to both physical and chemical reactions when they are sent together with plasma on a hot pad. Using solution instead of powder feedstock reduces the time and effort required for the coating process [58,59].

Oxidation as a failure mechanism occurs on TBCs at high service temperatures in air [5,7,11,25,37,42,43]. A slow-growing oxide layer is formed on the metallic coating surface after the high temperature exposure of MCrAlY coatings, which is generally called thermally grown oxide layer (TGO) [25,40,42,43,60,61]. Alpha alumina (α-Al₂O₃) is the primarily desired constituent among the oxides forming in the TGO layer [6,12,41,42,60]. The amount of aluminum within the coating is generally sufficient for the formation of a stable alumina layer in MCrAlY coatings. β-NiAl phases are aluminum-rich regions that are formed by the effect of high temperature during the oxidation process. These precipitates of the NiAl phase make it even easier to selectively oxidize aluminum [24,25,39]. At high temperatures for a long time, the depleted β-NiAl phases in MCrAlY coating may lead to the formation of other phases instead of the alumina layer [5,13,37]. Nickel, chromium and cobalt oxidize to form a metal oxide (MO) or react with each other to form a spinel oxide (M(Cr,Al)₂O₄) [4,9,62]. These types of oxides cause a higher growth rate and may result in early spallation of the TGO layer [5,7,40].

In the present study, powders consisting of CoNiCrAlY were deposited by APS, SSAPS, D-gun, HVOF and CGDS thermal spray techniques on Inconel 718 superalloy substrates with a coating thickness of about 100 μm. The produced coatings were subjected to high temperature oxidation tests to determine their high temperature performance, and to make a comparison with each other.