Thermal Barrier Coatings: An Insight into Conventional Plasma Spray and Water-Stabilized Plasma Spray

Abstract

Thermal barrier coating (TBC) systems have presented an ongoing design issue in bids to improve the lifespan of coatings. A TBC can support an extended lifespan by repairing cracks between interfacial layers during high thermal exposure while at the same time increasing coating thickness. Two deposition techniques, atmospheric plasma spray and water-stabilized plasma spray (WSP), have been distinguished to understand mechanical and thermal performance based on their contrasting torch systems and microstructural characterization. This insight paper will underline the superiority of WSP coating and the need to leverage existing technology by optimizing better deposition parameters for future fatigue-resistant TBC production.

1. Introduction

Since the 1970s, new thermal barrier coating (TBC) developments have progressed through revolutionary deposition techniques. The criteria for novel materials to acquire low thermal conductivity with erosion and thermal shock resistance have also been explored [1]. The TBC system utilized for turbine engine components is made up of integral layers consisting of a bond coat and a topmost ceramic layer. These layers protect against harsh environments and exposure to constant high temperatures. The multilayer coatings are designed carefully to accentuate the temperature gradient property, subsequently extending the lifespan of the coatings. The design requirements for TBC and fabrication methods have been evolving and continuously in need of modification in tandem with the increasing thrust-to-weight ratio criteria, as depicted in Figure 1 [2]. As these multilayers within a TBC system can perform at their optimal requirements, the benefits of TBC do not only protect its lifespan against heat and corrosion. It took many engineering design procedures to optimize engine efficiency—the addition of a 7 wt.% Y₂O₃-stabilized ZrO₂ (7YSZ)-coated turbine component permits the maximum allowable hot combustion gases without compromising the superalloy substrate. The discovery of this new material has been a significant step of improvement in tackling thermal efficiency while reducing fuel costs [3]. Hence, it is also paramount to explore the optimal deposition techniques employed in 7YSZ coating, which will be interchangeably referred to as TBC throughout this paper.

This paper aimed to compare conventional plasma spray to water-stabilized plasma spray by selectively focusing on atmospheric plasma spray (APS). The APS system is the most common deposition technique employed in the aerospace and orthopedic fields and for manufacturing tooling parts [4,5]. Fundamentally, the APS technique relies on a source of a gas torch to induce plasma stabilization; however, recent developments have shown that a water-stabilized plasma spray (WSP) is a more promising coating deposition technique. Although it was described more than 70 years ago by Gerdien and Lotz, it was only recently utilized for industrial-scale applications [6].

As illustrated in Figure 2, the TBC system comprises ceramic layers of 7YSZ, a thermally grown oxide (TGO), and a bond coat. The topcoat 7YSZ has a thickness range from 0.1 to 3 mm, providing adequate room for porosity to yield strain compliance [3]. The low-thermal-conductivity layer also hinders fast heat transfer activity into the superalloy substrate of the gas engine and protects against corrosion and erosion beneath it. Between the substrate and topcoat is a bond coat layer with a thickness range from 30 to 100 µm [3]. This intermediary layer supports adhesion in the TBC system. It also systematically controls the interdiffusion of elements between the topcoat and substrate to enhance the creep and thermal fatigue resistance. When the temperature exceeds 1000 K, a thin oxidation layer forms on the bond coat known as TGO [3]. The TGO is a 0.1 to 10 µm thick layer and governs by internal oxidation from the product of the hot gas that forms aluminum oxide (Al₂O₃). Due to the low-oxygen ionic diffusivity and its chemical stability, it aids as a protective alumina layer between the topcoat and substrate [7].

2. Arc Plasma Methods

Thermal spraying through APS has been amongst the common coating deposition preferences in industries that rely upon a gas torch source to induce plasma. Generally, this technique has numerous benefits: a single-step manufacturing technology, coating large-area components irrespective of geometrical shapes, forming graded composites, low heat input, low cost, and potential for improved thickness [8]. The primary mechanism of a gas torch starts with introducing powders into the gas stream. Consequently, these particles melt through high temperatures and accelerate due to the high velocity of the plasma gas stream. The stream of particles strikes the surface, and the particles turn into flattened splats, which adhere to the substrate texture. A gas torch has been used in APS as a potential spraying technology to produce coatings. However, the drawback of APS is the inability to produce thick deposition coating, which is useful in specific applications such as coating gas turbine parts.

A recent development to substantiate the coating limitation through this technology was further investigated. In place of a gas torch, a high-enthalpy system is introduced through a water torch source, commonly termed as WSP. The advantages of WSP revolve around its ability to provide constant arc pressure, high power, and throughput material while reducing operational costs (water as a working medium); this outcome produces high deposition efficiency [9]. Based on the principle of water stabilization, it can induce extremely high plasma temperature and flow velocity due to the presence of oxygen–hydrogen plasma. The high hydrogen content yields high enthalpy and sound velocity [10]. It was noted that water torches provide close to one order higher spraying rates compared to gas torches [6]. As such, the applicability of water torches extends to even larger-area coatings, expanding their usefulness to turbine blade production. A brief comparison between a gas torch and a water torch is highlighted based on a study by Hrabovsky [6], as summarized in

Table 1. Comparison of plasma arc techniques between a gas torch and a water torch.

	Gas-Stabilized Torch	Water-Stabilized Torch
Mechanism	Heat transfer by convection and radial heat transfer by conduction and radiation
Electric arc stabilization	By gas flow in axial direction with vortex	By liquid vortex in tangential direction
Mass flow rate of plasma	Controlled independently through incoming gas flow rate	Controlled by arc processes
Characteristics of plasma(example of typical deposition parameters)	High arc power (200 kW) against high mass flow rate (5.0 g/s) Low plasma enthalpy (24 MJ/kg) Low temperature (6200 K)	High arc power (133 kW) against low mass flow rate (0.33 g/s) High plasma enthalpy (272 MJ/kg) High temperature (16,200 K)
Spraying rate	One order higher in a water torch (coating thickness of up to 20 mm achievable) than in a gas torch

.

3. Atmospheric Plasma Spray

Many research papers have shown that plasma-sprayed 7YSZ is prone to cracks within the TGO layer and the interfacial layer between TGO/YSZ or TGO/bond coat [4,11,12,13,14]. Due to the different thermal expansion stresses amongst these layers, the microcrack will propagate and unavoidably cause the spallation of the topcoat layer [15]. Other contributing factors also result from the stiffness of TBC due to sintering, creep, and stress relaxation between the bond coat and YSZ layer [15]. However, the major drawback of TBC coating failure through APS is its inability to accommodate strain, where microstructural morphology ranges from 10 to 15% porosity. As strain compliance reinforces porosity and microcracks, a study has shown that the erosion rate resulted in 125 g/kg when impacted at 90°. The outcome indicates poor erosion resistance due to failure around splat boundaries of the microcracks from APS deposition [16]. In addition, the microstructural grains from low-porosity coatings exhibited fine columnar grains due to impact-quenched and molten lamellae. The majority exhibited coarse equiaxed grains of various sizes due to unmelted or resolidified particles, as shown in Figure 3. [17]. After many thermal cycles, the TGO layer nearing its critical failure point will propagate and introduce long delamination cracks into the already-weakened YSZ layer. It was noted that short-range sintering is probably the main factor behind the observed formation of distributed fine porosity, hence favoring delamination crack progression [17].

The microstructural features of TBC are influenced by its failure phenomenon. A lamellar structure forms when splats from APS condense into a structured layer [11]. The occurrence of pores induces fracture resistance and provides strain tolerance, while the splat interface initiates the beginning of TBC failure. A parametric study concluded that the presence of pores improves TBC fracture only when it is maintained within a critical range. In addition, splat interface waviness improves the TBC fracture more than a flat splat interface. The crack length at the flat splat interfaces is more sensitive to the distribution of pores, as simulated in Figure 4 [18].

In hindsight, the traditional TBC system is predominantly fabricated from APS compared to other deposition technologies. Despite resulting in a coating with a shorter lifespan, it is highly promising for producing gas turbine blades, primarily due to its low cost and high production efficiency [19]. However, the coating quality is still restricted by the allowable powder feedstocks, where particle size ranges from 10 to 100 µm. Further improvement to achieve low thermal conductivity and low-density coatings via the process parameters is unlikely [20,21,22,23].

4. Water-Stabilized Plasma Spray

With the increasing demand for large-scale and mass-production coatings, WSP is an ideal choice due to its prospects for extending the lifespan of a TBC coating. Due to its high-power plasma jet, an coating of excellent quality is predicted to result from WSP, which induces an increased powder melting point. As a result, the defects are minimized within the coating [9]. In a study by Ctibor et al., a novel TBC material consisting of strontium zirconate (SrZrO₃) was deposited on a stainless steel substrate through WSP. The results demonstrated a satisfactory quality without cracks or disruptions at the interface of SrZrO₃ coating and substrate [24]. In addition, a porous coating was observed, and porosity is a needed property for plasma-sprayed coatings. The overall coating exhibited lamellar microstructural properties with low thermal conductivity, allowing more extensive thermal and mechanical performance optimization than an APS deposition technology [24].

The WSP effectively deposits ceramic coatings and extends its usage into other protective coatings. Another study was conducted to investigate the fatigue properties of Fe-Al intermetallic coatings. The characteristic study of the Fe-Al layer through WSP demonstrated similar extrinsic and intrinsic properties to a 7YSZ material—splats, pores, and microcracks. Although it has been identified that thermal-sprayed coating can either deteriorate or improve the coating, there is a need to understand the relationship between WSP deposition and fatigue life [25]. In Figure 5, two different types of splats are distinguished in the coatings—those fully molten into flattened splats and unmolten particles formed in the spherical-like microstructure. A large proportion of pores and cracks were also observed within the microstructures. The morphology of WSP and APS differ because of the contrasting deposition conditions. The former governs by a larger diameter of particles in the feedstock, a higher feed rate, and high-power plasma torch output. Therefore, fatigue behavior can be substantially influenced based on the type of testing conditions. This study showed that FeAl-WSP, of similar atomic percent, had better fatigue resistance than APS deposition technology. The reported reason is due to the absence of coating delamination or intrasplat cracking, even when a higher thickness was attributed [26].

5. Future Work and Conclusions

The WSP deposition technique has shown to be a favorable choice for thermal barrier and protective coatings. The high thickness achievable through a high-enthalpy system can improve the overall deposition efficiency. In addition, porosity is vital to induce strain compliance in the turbine gas engine coating, which can be successfully fabricated through WSP. Henceforth, the WSP offers promising prospects in large-scale industrial applications. In this paper, very few findings are covered on the TBC material obtained through WSP as a sole deposition technique. This limited resource is likely due to more emerging technology adopted using the WSP technique’s fundamentals. Currently, progressive technology is commercially available, where it operates using a dual gas and water mode called hybrid water-stabilized plasma (WSP-H). The addition of gas stabilization increases plasma density and provides effective momentum transfer on the fragmented liquid compared to the WSP without hybrid stabilization [27]. Many experimental procedures have been attempted to investigate the results of using WSP-H. The applicability of this hybrid system can be extended to produce stable coatings for aerospace, orthopedic, and dielectric properties [5,27,28]. As such, it is necessary to understand the fundamentals of a WSP technique and its deposition parameters to achieve the desired outcome.