Evaluation of the Resistance of APS-Developed Woka-Diamalloy Carbide Coatings to High-Temperature Damage

Abstract

This study was conducted to evaluate the high-temperature protection performance of new hard coating systems. Woka 7202 (Cr₃C₂-NiCr) and Diamalloy 2002 (WC-NiCrFeBSiC) powders were coated onto 316L stainless steel substrates using the atmospheric plasma spraying (APS) method and subjected to isothermal oxidation (5–100 h) and hot corrosion (55% V₂O₅ + 45% Na₂SO₄, 1–5 h) tests. Although the coatings exhibited a laminar microstructure and some pores, cracks, and oxide-containing regions, they did not show any flaking or structural integrity deformations during the tests. Microstructural changes, oxide layer morphology, and the phases formed were examined in detail. The findings demonstrate that these coating systems not only provide chemical and structural stability against existing high-temperature environments, but also meet the requirements of next-generation thermal protection needs. In this regard, the study provides directly applicable information for the coating design and performance optimization for turbine blades, energy production equipment, and similar industrial components exposed to high-temperature oxidation and hot corrosion.

1. Introduction

Rapid industrialization and population growth worldwide have made sustainable production strategies essential due to the limited availability of natural resources [1,2,3]. Critical raw materials, such as metals and alloys, are indispensable in both industrial and technological development processes [1,4]. However, the extraction, processing, and utilization of these materials pose significant economic and environmental challenges [5,6]. The depletion of raw material reserves and the continuous increase in demand for these materials necessitate more efficient resource utilization and the development of long-lasting solutions. In this context, strategies supporting the sustainable economy play a vital role in ensuring effective resource management and minimizing waste [7,8]. Modern industrial policies aim not only for economic growth but also for optimizing production processes with environmental sustainability in mind. Strengthening material surfaces with protective coatings not only reduces raw material consumption but also enhances system performance, contributing to industrial sustainability [1,9]. This approach supports life-cycle management as an essential component of a smart economy and reduces the environmental impact of production processes by extending the service life of materials [5,6]. Solutions offered by coating technologies are becoming increasingly important to extend the lifespan of materials used in industrial applications and reduce costs. Coatings improve surface properties and enhance resistance to damage mechanisms such as wear, corrosion, oxidation, and erosion [10,11,12,13]. At the same time, these technologies contribute to reducing the carbon footprint by enabling environmentally friendly production processes. Among these technologies, thermal spray coating methods are widely used, especially in sectors such as energy, aerospace, automotive, and defense, by creating high-performance protective layers on material surfaces [14,15]. Thermal spray coating methods refer to a family of technologies that enable coating powders to adhere to the substrate surface through high-temperature and velocity spraying [16,17]. These methods stand out for their adaptability to different material combinations and application scenarios. Among thermal spray coating technologies, the atmospheric plasma spraying (APS) method has emerged as a prominent coating technology due to its superior properties and extensive material processing capacity. The APS method is based on the principle of melting or semi-melting coating materials in powder form using a high-temperature plasma jet and then spraying them onto the substrate surface under atmospheric conditions [18,19,20]. APS allows for the deposition of a wide variety of materials, including ceramics, metals, metal-ceramic composites, and certain polymers. While the APS method offers advantages in the application of high-temperature coatings, it naturally has limitations such as porosity and oxide content. These pores and oxide phases can limit the mechanical strength and wear resistance of the coating to a certain extent. Additionally, using homogeneous coatings in large-scale industrial applications using APS can present scalability challenges, as it requires precise control of process parameters.

One of the key advantages of this method is its ability to produce thick, dense, and highly adhesive coatings. These advantages support smart manufacturing systems by enabling more efficient production processes and minimizing material losses. The long-term performance of coating systems is directly related to understanding the damage mechanisms that may occur on the surface and designing coatings to resist these mechanisms. Coating materials are exposed to various damage mechanisms under high-temperature operating conditions, such as oxidation, hot corrosion, thermal shock, and solid particle erosion [21,22,23]. Oxidation, one of these damage mechanisms, is a process that begins with the formation of oxide layers on the material surface and gradually weakens the chemical and mechanical properties of the material. Oxidation occurring at high temperatures is a major factor limiting the service life of coatings [24,25,26]. Hot corrosion damage, on the other hand, typically begins with the accumulation of molten salts such as sulfates and vanadates on the surface, causing chemical reactions at high temperatures and resulting in material loss. Developing coatings resistant to such damage mechanisms enables a more environmentally friendly and sustainable approach in energy generation systems [27,28,29]. Additionally, wear and solid particle erosion create damage on the coating surface through mechanical interactions, limiting the functionality of the protective layer. This study aims to develop coating systems that provide superior resistance to the damage mechanisms. Within the scope of the study, a mixture of 50% Diamalloy 2002 powder and 50% Woka 7202 powder was applied onto a 316L stainless steel substrate using the APS method. Diamalloy 2002 offers high hardness and wear resistance, while Woka 7202 is known for its resistance to hot corrosion. These coatings were tested under different oxidation and hot corrosion conditions and analyzed using advanced characterization techniques. Oxidation tests were conducted at different time intervals to investigate the structure and thickness of oxide layers formed by the reaction of coatings with oxygen, while hot corrosion tests were designed to evaluate the effects of a 45% Na₂SO₄ and 55% V₂O₅ salt mixture. The analysis of these processes not only provides innovative solutions to improve material performance but also ensures the achievement of environmental sustainability goals through reduced natural resource usage and minimized waste. The obtained results demonstrate the potential of coating technologies to enhance raw material savings and offer more durable solutions for industrial applications, contributing to the literature. In this context, the study presents a significant step toward developing sustainable production technologies in line with the principles of the smart economy and aims to optimize the balance between industry and the environment.

2. Materials and Method

2.1. Substrate and Coatings Preparation

The study used commercial 316L stainless steel from Bircelik company (Istanbul, Türkiye) as the substrate material. It is a material characterized by significant oxidation and corrosion resistance owing to its elevated chromium and nickel content in the chemical composition. The chemical composition of the basic material consists of 16% Cr, 12% Ni, 2% Mo, 1.8% Mn, 0.6% Si, 0.2% Cu, 0.08% C, 0.04% P, 0.05% V, 0.03% S, and 0.005% Ti, with the remainder being Fe. Coatings applied on 316L stainless steel are mainly used on industrial components exposed to high temperature, abrasive and corrosion, such as gas turbine blades, power generation turbines, chemical reactor surfaces and marine equipment. These coatings both protect the metal substrate, extending its service life, and increase its surface resistance to abrasion and oxidation. A CNC apparatus (Yamazaki Mazak, Oguchi-cho, Japan) machined the substrate material to dimensions of 25 mm × 25 mm × 4 mm. Subsequent to the cutting operation, the substrate’s surface was grit-blasted to guarantee that the produced material demonstrates the requisite performance throughout the coating procedure. During the sandblasting procedure, Al₂O₃ particles with mesh sizes ranging from 50 to 65 were projected onto the surface at a 90° angle and a pressure of about 2–3 mbar. Subsequent to grit-blasting, ultrasonic cleaning was conducted using ethyl alcohol (C₂H₅OH) to eliminate residues, including oil and dirt, from the surface. Consequently, the adherence of the powders intended for application to the substrate surface prior to coating was enhanced. After the waste material was prepared for the coating process, a powder mixture of 50% Woka 7202 + 50% Diamalloy 2002 was prepared for application to the substrate surface. Woka 7202 powder mixture is known as hard phase Cr₃C₂-NiCr and its chemical composition by weight is 69.9Cr-20Ni-9.6C-0.5Fe and Diamalloy 2002 powder mixture is known as matrix phase WC-Co and its chemical composition is %50 (88WC-12Co) and %50 (66Ni-18Cr-7Fe-4B-4Si-1C). The prepared powder mixture was coated on the substrate surface by APS technique. The parameters used in the APS technique are given in

Table 1. Deposition parameters of the APS technique.

Current	500 A
Voltage	65 V
Ar flow	55 (L·min−1)
H2 flow	8 (L·min−1)
Powder feed rate	25 g/min
Stand-off distance	200 mm

.

2.2. Investigations on Oxidation and Hot Corrosion

At high operating temperatures, the degradation rates become more pronounced, and the service life is reduced by limiting the amount of coating layer used. Damage rates in hot weather conditions, characteristics such as automotive, aviation, marine, and steam turbines operating at high temperatures, and exposure to the harmful effects of salts formed by the combustion of low-quality fuels. Among these salts, impurities such as Na₂SO_4, V₂O₅, NaCl, and KCl are particularly prominent. These salts melt at high levels of access and cause damage by infiltrating the inner layers of the protective coating layers, disrupting their stability. This success can be obtained and maintained from the Bircelik company. The powder mixture known as 50% Woka 7202 + 50% Diamalloy 2002 was coated on a stainless-steel substrate by APS methods. For oxidation and hot temperature tests, 5, 25, 50, and 100 h of oxidation tests and 1, 3, and 5 h of hot temperature tests were carried out in a high-temperature furnace (Protherm, PLF 130/12, Protherm, Ankara, Türkiye) at 900 °C, respectively. In addition, to investigate hot air exchange damages, a 50% Woka + 50% Diamalloy coating system with a strong corrosive structure of 55% V₂O₅ (99.8% purity) + 45% Na₂SO₄ (99.8% purity) hot air ratio was operated and weighed at 2.3 ± 0.2 mg/cm² in a precision balance system. After the tests, characterization formation was continued to analyze the deformation behavior on the coating surface and the permanent morphological and microstructural changes in the coating after hot temperatures. XRD (Rigaku Dmax 2200 PC, CuKα radiation, Rigaku, Tokyo, Japan, and Match phase analysis software, 4 September, 2025, 4.2 Build 334) was used to scan the crystal/phase structures, while scanning electron microscope-SEM (Zeiss EVO LS10, Zeiss, Oberkochen, Germany) and energy dispersive spectroscopy-EDS (Oxford Instruments, Oxford, UK) were used. Xmax 50 (Oxford Instruments, Oxford, UK) and elemental mapping analysis were used to monitor the oxidation behavior and microstructural changes.

3. Results and Discussion

3.1. Characterization of As-Sprayed Coating System

Figure 1 presents cross-sectional SEM images of coating systems deposited on a 316L stainless steel substrate using the APS method. These coatings reflect the characteristic lamellar microstructure of the APS technique, providing the required mechanical strength, oxidation resistance, and surface quality for high-temperature applications. The images obtained before the tests show that the coating layers, approximately 200 µm thick, exhibit a lamellar structure, a dense morphology, and a controlled level of porosity. The presence of lamellar structures in the coating microstructure enhances the thermal barrier effect of APS coatings while improving their thermal expansion compatibility at high temperatures. Additionally, the APS technique creates layer-by-layer coatings by melting the coating powders in a high-temperature plasma jet and depositing them at high velocities onto the substrate surface. This feature supports the formation of a homogeneous microstructure and enhances resistance to thermal shock and other damage mechanisms. The approximately 200 µm thick coating layers promote the formation of a lamellar microstructure on the surface while also limiting the formation of oxide phases. This situation, in turn, increases the adhesion strength between the coating and the substrate, providing long-term mechanical and thermal durability. Furthermore, coatings produced with carbide-based materials such as Woka 7202 and Diamalloy 2002 offer the necessary protection in abrasive and corrosive environments, demonstrating that the APS method delivers long-lasting solutions under high-temperature conditions. The lamellar structure of these coatings produced by the APS method helps maintain their microstructural integrity, protecting against thermal and mechanical damage in industrial applications, and enhancing the overall performance of the material.

Figure 2 shows the cross-sectional images of the elemental mapping analyses of the as-sprayed 50% Woka 7202 + 50% Diamalloy 2002 powder content coating system obtained before oxidation and hot corrosion tests. In Figure 2, for the coating made from the 50% Woka 7202 + 50% Diamalloy 2002 powder mixture, in addition to Ni and Cr, W and Co elements are also found in high concentrations in the upper layer of the coating. These elements were found predominantly in the coating, while Fe was again observed intensely in the substrate material. The presence of W and Co elements provides the coating with superior mechanical strength, particularly under abrasive and corrosive conditions, and these elements are distributed homogeneously in the coating, consistent with the microstructural characteristics of the APS coating technique. Both coating samples show dense, oxide-free, and porosity-free microstructures, which are characteristic of the APS technique. The dense structure of these coatings provides high resistance to oxidation, as the coating material is sprayed onto the surface at high speeds. The even distribution of elements that enhance resistance to wear, oxidation, and hot corrosion improves the mechanical stability of the surface. The homogeneous elemental distribution in the microstructure of the coatings, which offer a reliable, long-lasting, and cost-effective surface protection solution for industrial applications, increases the service life in corrosive and high-temperature conditions.

In Figure 3, as-sprayed XRD patterns of Woka %50 + Diamalloy %50 coating system are given. Woka %50 + Diamalloy %50 coating sample, in addition to the Cr₃C₂ and NiCr phases, the WC phase, which increases hardness, and Co-containing phase structures, which contribute to oxidation resistance, were also identified. The combination of these phases further strengthens the wear and high temperature resistance of the coating, positively impacting the performance of the system, especially in industrial applications.

3.2. Oxidation Effect on Coating Systems

Figure 4 shows the cross-sectional SEM images of the coating system with 50% Woka 7202 + 50% Diamalloy 2002 powder produced using the APS technique at 900 °C after isothermal oxidation tests performed for 5, 25, 50 and 100 h. In these tests, the samples were exposed to high temperatures for 5, 25, 50, and 100 h. One of the primary changes observed in the initial oxidation periods is the closure of pores initially present in the structure due to the sintering effect induced by high temperatures. Sintering is a process that enhances the density of the material by closing pores and microcracks within the structure; while this process helps maintain the structural integrity of the coating layer in the early stages, it also sets the stage for new damage mechanisms in long-term oxidation. As oxidation time progresses, surface deformations resembling pitting corrosion begin to appear in the structure. Pitting corrosion is a type of localized corrosion damage that creates small pits on the coating surface. This process is particularly triggered by the local weakening of the coating layer due to reactions with oxygen and other oxidizing agents. With prolonged exposure to oxidation, regional spallation associated with oxidation was observed in the coating layers. These spallation’s occur as a result of internal stresses that develop beneath the oxide layer and the progression of oxides toward the surface of the structure. Additionally, the growth of the oxide layer, along with the volumetric expansion of the coating and the resulting thermal stresses, also contributes to these spallations. Such mechanisms impact the durability and performance of the coating in high-temperature environments, gradually reducing the functionality of the structure over time. These observations contribute to understanding the microstructure changes that high-temperature oxidation tests can induce in coatings and emphasize the damage mechanisms that should be considered when evaluating the long-term durability of these types of coatings.

The cross-sectional elemental mapping analyses of the Woka %50 + Diamalloy %50 coating system after 100 h isothermal oxidation tests are presented in Figure 5. In the coating sample with 50% Woka + 50% Diamalloy content, it was noted that compounds containing W (tungsten) were also present alongside these oxide structures. The tendency of W to oxidize at high temperatures leads to the formation of W compounds within the coating layer. These compounds are among the critical factors that can influence the mechanical strength and thermal resistance of the coating. Furthermore, the Co element present in the Diamalloy 2002 powder content was not detected after oxidation. This indicates a spallation mechanism resulting in the separation of co-containing compounds from the coating surface during oxidation. The removal of Co from the system as oxide compounds may have triggered pitting-like damage on the coating surface. These pitting-like formations were observed in areas where the coating layer weakened locally, allowing oxygen to penetrate the coating through these vulnerable points. Additionally, the depletion of Co from the surface disrupts the homogeneity of the coating, potentially negatively affecting the overall durability of the structure.

Figure 6 shows the XRD analysis patterns of the coating system with 50% Woka + 50% Diamalloy powder content before and after isothermal oxidation tests. According to the obtained results, (Ni card number: 96-210-2252), (NiO card number: 96-432-0494), and (W card number: 96-901-2434) phases are initially present in high concentrations. After the oxidation tests conducted at high temperatures, the formation of (NiWO₄ card number: 96-231-0620), (Cr₂O₃ card number: 96-900-8096), and (Cr₂NiO₄ card number: 96-200-9227) phases within the structure was observed.

3.3. Hot Corrosion Effect on Coating System

Figure 7 shows the cross-sectional SEM images of the coating system with 50% Woka + 50% Diamalloy powder content after hot corrosion tests. During the tests conducted at the high temperature of 900 °C, the molten corrosion salts not only followed reactions described in Equations (1)–(6) but also interacted with W and Co elements present within the system. As indicated by the microstructural changes in the upper part of the coating layer, phase transformations have occurred. Other possible reactions that may occur during the tests are as follows [30,31,32,33]:

CoO + V₂O₅ → CoV₂O₆

(1)

CoO + Na₂SO₄ → CoSO₄ + Na₂O

(2)

WC + 5/2O₂ → WO₂ + CO₂

(3)

WC + Na₂SO₄ + O₂ → WO₃ + Na₂O + CO₂

(4)

Cr₂O₃ + 3Na₂SO4 + 3/2O₂ → Cr₂(SO₃) + 3Na₂O

(5)

NiO + Na₂SO₄ + 1/2O₂ → NiSO₄ + Na₂O

(6)

Cr₃C₂ + V₂O₅ → 3CrO + 2VO + 2C

(7)

Cr₂O₃ + V₂O₅ → 2CrVO₄

(8)

NiO + V₂O₅ → NiV₂O₆

(9)

During hot corrosion, reactions occurring on the coating surface with Na₂SO₄ and V₂O₅ salts disrupt the protective oxide layers of the coating, adversely affecting corrosion resistance. Figure 8 shows the top surface SEM images of the coating system with 50% Woka + 50% Diamalloy powder content. In the images obtained after hot corrosion tests, the presence of needle-like rod structures can be observed from the initial to the final stages of corrosion. Unlike the other coating samples, the amount of needle-like rods on the surface increases as the corrosion process progresses. Figure 9 illustrates the elemental distribution of the coating sample with 50% Woka + 50% Diamalloy content.

Figure 10 shows the point EDS analysis obtained from the upper surface of 50% Woka + 50% Diamalloy coating system after isothermal hot temperature tests. In the 50% Woka + 50% Diamalloy coating system; however, different phase structures with elongated rod-like formations are present. Additionally, these elongated rod-like structures appear to be spread throughout the entire surface [34,35,36].

Figure 11 shows the XRD analysis of the coating system with 50% Woka + 50% Diamalloy powder content obtained before and after hot temperature tests. Before the corrosion tests, phase structures containing (W card number: 96-901-2434), (Ni card number: 96-210-2252), (Cr card number: 96-901-1599), and (Co card number: 96-901-1626) were observed to be predominant. After the hot corrosion tests, phases such as (Cr₂O₃ card number: 96-900-8085), (CrVO₃ card number: 96-154-0143), (CoCr₂O₄ card number: 96-591-0125), and (CoV₂O₆ card number: 96-200-2874) were detected in the system. The needle-like structures present in the system, which are visible in the top surface images, are believed to be composed of these phases.

4. Conclusions

In this study, 50% Woka 7202 powder and 50% Diamalloy 2002 powder mixtures were coated on 316L stainless steel substrate using APS process. Oxidation and hot corrosion behaviors of coating system were investigated, and the failure mechanisms were examined in detail. The results obtained as a result of the study are summarized as follows:

• Although the 50% Woka 7202–50% Diamalloy 2002 coatings obtained with APS provide sufficient integrity under high temperature conditions, porosity and oxide contents can limit mechanical strength and wear resistance. This suggests that process parameters and densification methods should be optimized in coating design.
• Oxidation tests at 900 °C show that oxide phases (NiO, NiWO₄, Cr₂O₃, Cr₂NiO₄) are formed in coatings with increasing oxygen penetration. This reveals that the coatings provide chemical stability by forming protective oxide layers at high temperature, but excessive oxidation may limit the mechanical strength.
• Hot corrosion tests at 900 °C show that corrosive salts leach out of the coating surface over time, leading to oxide formation and cracking. This finding suggests that the mechanical integrity and corrosion resistance of coatings under long-term service conditions may be limited and additional protective measures may be required in design.
• The formation of Cr₂O₃, CrVO₃, CoCr₂O₄ and CoV₂O₆ phases in the coating during hot corrosion indicates that the coating provides chemical stability in corrosive environments. However, needle and rod-like brittle structures can lead to localized stress accumulation and crack formation, limiting mechanical strength, which may affect long-term service performance.
• The coating system maintained its structural integrity at high temperature and showed no flaking or lifting, indicating that it can be used as a reliable protective surface solution in high temperature applications such as turbines and power generation equipment.