Oxidation Behavior of NiCoCrAlY Coatings Deposited by Vacuum Plasma Spraying and High-Velocity Oxygen Fuel Processes

Abstract

To reduce the formation of detrimental complex oxides, bond coatings in the thermal barrier coatings for gas turbines are typically fabricated using vacuum plasma spraying (VPS) or the high-velocity oxygen fuel (HVOF) process. Herein, VPS and HVOF processes were applied using NiCoCrAlY + HfSi-based powder to assess the oxidation behavior of the bond coatings for both coating processes. Each coated sample was subjected to 50 cyclic heat treatments at 950 °C for 23 h and cooling for 1 h at 20 °C with nitrogen gas, and the weight change during the heat treatment was measured to evaluate the oxidation behavior. After the oxidation test, the coating layer was analyzed with X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The VPS coating exhibited faster weight gain than the HVOF coating because the alumina particles generated during the initial formation of the HVOF coating inhibited oxidation and diffusion. The VPS coating formed a dense and thick thermal growth oxide (TGO) layer until the middle of the oxidation test and remained stable until the end of the evaluation. However, the HVOF coating demonstrated rapid weight loss during the final 20 cycles. Alumina within the bond coat suppressed the diffusion of internal elements and prevented the Al from being supplied to the surface. The isolation of the Al accelerated the growth of spinel TGO due to the oxidation of Ni, Co, and Cr near the surface. The as-coated VPS coating showed higher hardness and lower interfacial bonding strength than the HVOF did. Diffusion induced by heat treatment after the furnace cyclic test (FCT) led to a similar internal hardness and bonding strengths in both coating layers. To improve the quality of the HVOF process, the densification of the coating layer, suppression of internal oxide formation, and formation of a dense and uniform alumina layer on the surface must be additionally implemented.

Investigation on the Microstructure and Micro-Mechanical Properties of Thermal-Sprayed NiCoCrAlY High Entropy Alloy Coating

“…The spraying gun was Diamond Jet 2700 plasma sprayer (Sulzer Metco, Switzerland), equipped with IRB 2400/16 automatic controller (ABB, Zurich, Switzerland). The optimal spraying parameters are as follows: 0.7 m 3 /h hydrogen flow rate, 0.7 m 3 /h argon flow rate, 40-50 g/min powder feed rate, 800 mm/s gun speed, 3 mm inter-pass spacing, 12 cm spraying distance, 38-40 bar of pressure, and with stand-off distance of 20-60 mm, and the substrate temperature was <150 • C. These parameters were selected based on the information available in the literature [4,7,13,28,29], as well as on the 'trial and error' approach to fine tune the parameters, as required by the currently employed deposition instrument.…”

“…This layer has low thermal conductivity and allows the growth of aluminium oxide in service and thus, provides an effective barrier against high temperature [5]. The following layer is a 'thermal growth oxide (TGO) layer', and they find their place between the 'top coat' and 'bond coat', and form via a diffusional process, to that of the elements from the bond coat [6][7][8]. The third layer is the 'bond coat'.…”

Study on High-Temperature Oxidation Behavior of Platinum-Clad Nickel Composite Wire

“…Scientists have produced a significant amount of research on high-temperature protection coatings of nickel and nickel alloys, including the selection of coating materials [7,8], coating preparation [9,10], coating modifications [11,12], properties [13,14], and oxidationresistance mechanisms [15,16]. These research results have promoted the development of high-temperature protection fields and also provided guidance for the development of this project.…”

A Direct Laser Sintering Approach for the Electrophoretic Deposition Overlay of Yttria-Stabilized Zirconia on the Surface of a Thermal Barrier Coating System

“…Thermal-sprayed ceramic coatings are widely used in gas turbine and other heat engine components as thermal barrier coatings (TBCs) [3]. TBCs are used to protect metallic components from hot corrosion and oxidation and to reduce heat transmission [4]. YSZ is a material that is frequently employed in the design of sophisticated gas turbine engine systems [5].…”

“…Many approaches, such as fine-grained powders, microwave or irradiation treatment, and reaction bonding technology, can be employed to overcome the problems of high sintering temperatures [22,23]. The use of EPD coupled with the reaction bonding method can result in lower-than-usual sintering temperatures [4,24]. Furthermore, by controlling and selecting the correct sintering cycle, it is possible to obtain refined microstructures of the sintered bodies, as observed in spark plasma sintering, two-step sintering, and laser sintering [25].…”