Influence of Remaining Oxide on the Adhesion Strength of Supersonic Particle Deposition TiO₂ Coatings on Annealed Stainless Steel

Abstract

The cold spray or Supersonic Particle Deposition technique has great potential for producing ceramic nanostructured coatings. This technique operates at a processing temperature that is low enough to preserve the initial feedstock materials’ microstructure. Nevertheless, depositing ceramic powders using a cold spray can be challenging because of the materials’ brittle nature. The interaction between substrate and particles is significantly influenced by substrate attributes, including hardness, material nature, degree of oxidation and temperature. In this study, the effect of the substrate’s remaining oxide composition on the adhesion strength of an agglomerated nano-TiO₂ coating was investigated. The results showed that the coating adhesion strength increased for hard materials such as stainless steel and pure chromium as the annealed substrate temperature also increased from room temperature to 700 °C, indicating thicker oxide on the substrate surface. TiO₂ particles mainly bond with SUS304 substrates through oxide bonding, which results from a chemical reaction involving TiO₂-OH⁻. Chromium oxide (Cr₂O₃) is thermodynamically preferred in SUS304 and provides the OH⁻ component required for the reaction. SUS304 shows a thermodynamic preference for chromium oxide (Cr₂O₃), and this enables Cr₂O₃ to provide the necessary OH⁻ component for the reaction.

1. Introduction

Cold spray is a deposition technique that operates in a solid state without melting the feedstock. High-velocity particles transfer their kinetic energy to the substrate, resulting in localized heat and interfacial deformation [1,2,3]. This in turn leads to mechanical interlocking and metallurgical bonding [4]. Although the bonding process is not fully understood, optimal temperature, critical velocity and energy levels are needed for effective bonding to take place [5,6]. High strain rate formation and localized heating at the particle-substrate interface result in microscopic protrusions, which can cause metallurgical bonding through material deformation and atomic-level interactions [7,8].

Severe plastic deformation of materials determines interface bonding, resulting in metal-jet formation and adiabatic shear instability (ASI) at the interface [9,10,11,12]. The native oxide layer is broken due to the impact of the high-velocity particles, thus creating contact between particles and substrates and resulting in jet formation via ASI [13,14,15]. As opposed to the results of Hassani-Gangaraj et al. [16], Grujicic et al. [17] and Assadi et al. [5], presented evidence that contradicted the notion that ASI is not a mandatory factor for adhesion in the cold spray process. In response to the statements made by Assadi et al. [18], Hassani-Gangaraj et al. [19] upheld their simulation-driven study, which indicated that ASI was unnecessary for bonding.

The inadequate adhesion and delamination issues of soft and hard or hard and hard interfaces pose significant challenges to industries. Therefore, comprehending the bonding mechanism helps to address these concerns and benefit the advanced manufacturing sector. There is a significant demand for thick copper coatings on steel (SS316L) plates, which are similar to those of bulk copper and show strong adhesion, particularly for application in Tokamaks’ vacuum vessels [20,21]. Singh et al. [7] conducted a study to analyze the bonding process between copper particles and steel substrates (soft-on-hard interface) by modifying substrate conditions and cold spray parameters. Drehmann et al. [22], Wustefled et al. [23] and Dietrich et al. [24] discovered that deformation-induced recrystallization near the particle-substrate interface was responsible for the bonding between aluminum particles and a super-finished monocrystalline sapphire substrate. Metallurgical bonding was facilitated by the development of nanoparticles at the interface, leading to increased adhesion strength between the Al₂O₃ monocrystalline ceramic substrate and the malleable Al particles.

The substrate surface plays a crucial role in achieving strong adhesion in cold-sprayed coatings as it determines the bonding strength between the first layer of TiO₂ particles and the outermost surface. When a metal substrate is used, an active oxide layer with a thickness of several micrometers is formed on the outer surface. Upon contact with the substrate surface, the agglomerated TiO₂ particles disintegrate, leading to particle-substrate contact a few nanometers beneath the oxide layer.

Cold spraying is a common method for creating TiO₂ coatings on metallic objects. Combining a powdered photocatalytic oxide metal with a ductile metal powder allows for plastic deformation upon particle impact. However, the presence of metal oxide particles covering 30–80% of the top surface can hinder the performance of the ceramic TiO₂ coating. Nevertheless, studies have shown that thick, pure agglomerated TiO₂ coatings without additional substances exhibit photocatalytic activity comparable to the raw powder [25]. However, these pure agglomerated coatings have weak interfacial adhesion strength, and the bonding mechanism is not fully understood.

This research aimed to investigate how substrate oxidation levels affect the adhesion of coatings to surfaces. Stainless steel (SUS304) was chosen as the substrate to prevent the development of substrate metal jets caused by particle impact. The study considered factors including substrate oxide thickness, chemical composition, and atomic composition.

2. Equipment and Procedures

2.1. Spray Process

Throughout all coating experiments, the spray parameters used are shown in

Table 1. Spray parameters.

Cold Spray Machine	De-Laval 24TC Nozzle
Gas carrier	N2
Operating temperature	500 °C
Operating pressure	3 MPa
Traverse speed	10 mm/s
Spray distance	20 mm
Powder feed rate	3 g/min
Pass number	1

below.

2.2. Materials

Agglomerated TiO₂ powder was used as a feedstock (TAYCA Corporation, Osaka, Japan). It had an anatase crystalline form; its average particle size was 7.55 µm (Figure 1).

The study utilized two substrates: pure chromium (Cr) and stainless steel (SUS304). To explore the influence of surface oxide at varying temperatures, the substrates were initially subjected to grit-blasting, which was then followed by annealing. The annealing process was conducted under atmospheric conditions using an electric furnace that was set to two different temperatures, specifically 400 °C and 700 °C. The substrates were heated gradually at a rate of 15 °C per 5 min and then soaked for 5 min before being cooled down to room temperature inside the furnace.

2.3. Testings

2.3.1. Testing Tensile-Strength

The study adhered to JIS H 8402 standards and utilized specimens with dimensions of Ø 25 mm × 10 mm to test the adhesion strength. The fracture load value, indicating adhesion strength, was measured using a universal testing machine (Autograph AGS-J, Shimadzu, Kyoto, Japan). For every spraying condition, five specimens were tested, and the average fracture load value was recorded. Furthermore, the fractured coating surface underwent EDX analysis to examine its composition.

2.3.2. Coatings Characterizations

To analyze the TiO₂ coating cross-sectional microstructures on annealed substrates, a scanning electron microscope (SEM: JSM-6390, JEOL, Tokyo, Japan) was employed. A sample measuring 25 mm × 10 mm was embedded in a hardenable resin for observation. The embedded sample underwent a grinding process using silica papers until a #3000 grit size was achieved. Subsequently, it was polished using 1 µm and 0.3 µm alumina suspension.

2.3.3. Micro-Vickers Hardness

The correlation between the adhesion strength of the TiO₂ coating on the annealed substrate and the hardness of the substrate’s surface was investigated using a micro-Vickers hardness tester (HMV-G, Shimadzu, Kyoto, Japan). The substrate’s hardness was measured with a 98.07 mN test load and a 10 s dwell time applied to the cross-sectional sample. The measurement revealed a hardness value of HV 0.1.

2.3.4. Substrate Oxide Evaluations

The substrate oxide thickness was measured using X-ray photoelectron spectroscopy (XPS) with the ULVAC-PHI Quantera SXM-CI, Kanagawa, Japan. XPS analysis involved using a monochromatic Al Kα source with a current of 15 mA and a voltage of 10 kV. Narrow scans ranging from 0 to 1000 eV were performed for Fe 2p, Cr 2p, and O 1s in order to analyze the different annealed substrates. The measured binding energies were corrected using C 1s at 285.0 eV. Pre-sputtering, which could potentially alter the sample surface and affect the measurements, was not conducted. Detailed XPS analysis conditions for substrate oxide analysis can be found in

Table 2. XPS parameter for substrate oxide layer analysis [26].

Regions Measured	Fe 2p, Cr 2p, O 1s
X-ray output	10
Probe diameter	50
Time per step	30
Pass energy	140
Cycle	30

.

2.3.5. Wipe Test

To investigate the deformation of a single TiO₂ particle on different substrates, a wipe test was performed. Prior to deposition, the substrates were prepared by grinding and polishing to achieve a smooth, mirror-like surface. The spraying process was carried out using nitrogen as the process gas at a temperature of 500 °C and a pressure of 3 MPa. The distance between the nozzle and substrate was maintained at 20 mm, and the process was conducted at a traverse speed of 2000 mm/s. Before spraying, the substrates were cleaned with acetone. The deposition of a single TiO₂ particle on a mirror-polished annealed substrate was observed using the FEI Helios Dual Beam 650 field emission scanning electron microscope (FESEM, FEI, Hilsboro, OR, USA) and focused ion beam (FIB, FEI, Hilsboro, OR, USA) microscope from FEI, based in Oregon, USA.

2.3.6. TEM Testing

To examine the oxide layer post high-velocity cold spraying with various pressures impacting the substrate surface, TEM (transmission electron microscopy) testing was performed. The sample was prepared by creating a thin film, which was then analyzed using field emission gun (FEG) electron microscopy. The analysis was conducted on an EOL JEM-2100F, Tokyo, Japan field emission transmission electron microscope instrument in scanning mode at 200 kV.

3. Result

3.1. Adhesion Strength Testing and Fracture Surface Analysis on Annealed Substrates

Figure 2 illustrates the adhesion strength of TiO₂ coating on pure chromium (Cr) and annealed stainless steel (SUS304). The adhesion strength of both substrates increased as the annealing temperature increased, with values ranging from 0.51 to 2.63 MPa for SUS304 and 0.71 to 1.44 MPa for pure Cr. Figure 3 displays the fracture surface of SUS304 and TiO₂ coating after tensile strength testing, confirming that the fracture occurred at the interface between the substrate and coating and indicating a strong cohesive bond between TiO₂ particles during the coating formation process.

Figure 4 depicts the cross-sectional microstructure of SUS304 and TiO₂ coatings at room temperature and 700 °C, representing the conditions with the lowest and highest adhesion strengths, respectively. The coating exhibited a thickness of 200 to 300 µm, indicating the achievement of the critical velocity of particles during the spraying process. Figure 5 and Figure 6 present the results of EDS analysis and the spectrum of the fractured TiO₂ coating on pure chromium substrates annealed at room temperature and 700 °C, respectively. Figure 6 demonstrates a notable presence of chromium in the fractured coating of the substrate annealed at 700 °C compared to the room-temperature substrate. This suggests that chromium oxide may influence the bonding mechanism, potentially contributing to the observed increase in adhesion strength in Figure 2. However, further investigation is required to fully comprehend the role of chromium oxide in the bonding process.

3.2. Depth Profile of Oxide Layer

Variations in the levels of oxygen and chromium within the substrates concerning their depths are illustrated in Figure 7a–c. The results demonstrated that there is a correlation between annealing temperature and the concentration of oxygen within the near-surface area. As the temperature of the substrate increased, the thickness of the pure chromium oxide layer also increased, leading to an increase in the adhesion strength of the TiO₂ coating upon pure chromium annealed substrate from room temperature to 700 °C annealed.

Additionally, Figure 8a–c, which depicts the analysis of the oxide layer on SUS304, indicated that the thickness of the stainless-steel oxide layer also increased with the annealing temperature of the substrate. According to Ko et al. [27], when Cu/AlN and Al/ZrO₂ bonding couples are formed, the oxide layer becomes amorphous and atomic intermixing occurs at the interface due to chemical adhesion.

The results of this study indicated that the thicker oxide layers on the surface and higher temperatures led to increased adhesion strength of TiO₂ coating upon SUS304 and pure Cr. The bonding mechanism was influenced by the characteristics of Cr₂O₃ oxide between room temperature and 700 °C. Hence, further research would be conducted on the oxide’s chemical composition.

3.3. Evaluation of the Chemical State of Iron, Chromium and Oxygen in the Oxide Layer

Figure 9a,b depict the oxygen content and chemical state of chromium, respectively. Figure 9a shows that the oxide layer on pure chromium at room temperature exhibits a dominant peak for chromium metal at 574.0 eV. In contrast, for pure chromium substrates annealed at 400 and 700 °C, Figure 9a illustrates the presence of a peak at 576.0 eV, corresponding to Cr₂O₃, across the outermost surface of the oxide layer. This indicates that Cr₂O₃ is the primary component of the oxide layer at these annealing temperatures.

Figure 9b highlights the presence of hydroxide (OH⁻) at the outermost surface of the oxide layer for pure chromium substrates annealed from room temperature to 700 °C. The red-dotted line represents a peak position of 531–532 eV, indicating the presence of hydroxide. The study findings suggest that the presence of hydroxide in the oxide layer, ranging from room temperature to 700 °C annealing temperatures, contributes to the observed trend of increasing adhesion strength of the coating with higher annealing temperatures.

Figure 10a–c provides a visual representation of the oxide layer chemical composition in SUS304. The chemical condition of ferum and chromium in SUS304 was identified via the 2P3/2 atomic orbital satellite peak. In Figure 10a, the location of the iron metal peak for stainless steel, which was roughly 706.7 eV [28], was observed on the outer surface of the room-temperature SUS304 substrate (a). When the SUS304 substrate was annealed at 400 °C and 700 °C, the outermost surface of the substrate showed a peak position of hematite Fe₂O₃ (Fe²⁺) at around 709.3 eV [28]. This result suggested that there was a shift within the chemical state of the SUS304 substrate from a Fe metal state at room temperature to the hematite state after annealing at 400 °C and 700 °C.

The oxide layer of SUS304 at room temperature mainly consisted of chromium metal, as indicated by the notable peak at 574.0 eV [29]. For SUS304 substrates annealed at 400 and 700 °C, the peak position of Cr₂O₃ at 576.0 eV was observed across the outer surface of the oxide layer [28], demonstrating a shift from the metal state of chromium to the chromium oxide state. In Figure 10c, the red-dotted line demonstrated the presence of hydroxide, OH⁻, between 531 and 532 eV [28], suggesting that the oxide layer for SUS304 at 400 and 700 °C was a combination of Cr₂O₃, Fe₂O₃ and hydroxide.

The process of cold spraying metal particles onto glass or ceramic substrates involves various factors, including the chemical characteristics of the substrates and the impact particles. In a study conducted by Song et al. [30], they examined the cold spraying of Al particles onto a glass substrate. This resulted in the formation of an 80 nm-thick interlayer between the Al particle and the glass substrate. The interlayer exhibited nanocrystalline grains and an amorphous phase, with a notable presence of sodium enrichment. The formation of this interlayer is believed to be a result of the high temperatures generated during the impact process, which induce a reaction between the Al particle and the glass substrate, leading to the formation of liquid stages at the interface.

The strong adhesion observed between the Al particles and the glass substrate is attributed to the high affinity of Al for oxygen within the substrate. The study findings regarding the adhesion strength of coatings on hard metal substrates align with the results reported by Song et al. They conducted a single particle study or wipe test to further investigate the influence of the chemical composition of the oxide layer on the substrate surface on the bonding mechanism.

3.4. Interface Oxide Layer TEM Analysis between TiO₂ Particle at Room Temperature and 700 °C Annealed Substrates

Figure 11 shows high-magnification images that illustrate the presence of an amorphous stage at the interlayer between annealed SUS304 and a single-particle TiO₂. The interlayer thickness was approximately 10 nm, confirming the existence of an interface oxide layer formed after cold-spraying TiO₂ onto SUS304.

In a study conducted by Kim et al., kinetic spraying was employed to deposit single titanium particles onto mirrored steel substrates. Their findings revealed the presence of a thin amorphous oxide layer at the interface between the particle and the substrate, even after experiencing severe plastic deformation due to particle impacts. This oxide layer left on the substrate surface after cold spraying acted as a bonding agent between the deposited particle and the substrate [31]. The bonding mechanism involved in this process was further investigated through TEM line analysis, focusing on both the room temperature and 700 °C annealed substrates.

The TEM line analysis shown in Figure 12a,b provided insights into the composition of a single-particle TiO₂ on annealed SUS304 at room temperature and 700 °C. The analysis revealed the presence of Ti, O and Cr atoms. The Ti and O atoms were attributed to the TiO₂ coating and hydroxide on the substrate surface, while the Cr atoms indicated the presence of the oxide layer on SUS304. At room temperature, the substrate exhibited Cr metal along with a combination of Fe₂O₃⁺, Cr₂O₃ and OH⁻. Annealing SUS304 at 700 °C resulted in the highest coating adhesion strength. The presence of Cr₂O₃ oxide may influence the bonding mechanism.

In line with these findings, Song et al. [29] highlighted the importance of considering various factors, including the chemical characteristics of substrates and impact particles, when cold spraying metal particles onto glass or ceramics. Similarly, Drehmann et al. [22] investigated the effect of substrate pre-heating on the adhesion strength of Al particles on an AlN substrate with different levels of softness and hardness. They found that annealing the as-sprayed samples improved the adhesion strength. Increasing the temperature of the AlN substrate before spraying resulted in higher adhesion strength, which was attributed to the thermal energy input that triggered atomic mobility at the Al-AlN interface. Deformation-induced recrystallization near the interface also contributed to atomic mobility, reducing the grain orientation mismatch and improving adhesion strength.

3.5. Oxide Composition Evaluation at Interface of Coating Using X-ray Photoelectron Spectroscopy

Figure 13 provides a schematic diagram of the fracture coating surface on the 700 °C annealed substrate, as analyzed using the XPS method. The purpose of this analysis was to determine the chemical states of the elements present after the cold spray process. Both wide and narrow scan analyses were performed, and the results are presented in Figure 14a–c.

In Figure 14a, the wide-scan analysis of the coating interface reveals the presence of peaks corresponding to titanium, oxygen, carbon, and sodium elements. However, no traces of iron or chromium elements were detected during the wide-scan analysis. Moving to Figure 14b, the narrow analysis of the iron element shows the absence of an elemental peak at the coating interface. Conversely, Figure 14c demonstrates the detection of a chromium elemental peak at a position of 577 eV, indicating the presence of chromium hydroxyls (CrOOH). This suggests that the chemical state of chromium changed from Cr2O3 to CrOOH as a result of the reaction between the chromium oxide (Cr₂O₃) present on the oxide layer of the substrate surface and water molecules in the atmosphere during the cold spray process. It has been confirmed that the oxide layer’s thickness increased as the substrate annealing temperature increased, resulting in a thicker layer of Cr₂O₃ on the substrate surface. This, in turn, contributes to the increase in coating adhesion strength. The highest coating adhesion strength was observed at 700 °C annealed substrates, with values of 2.63 MPa for SUS304 and 1.44 MPa for pure Cr. This finding indicates the importance of oxide thickness in influencing the increase in coating adhesion strength for SUS304.

Delamination and weak adhesion strength at interfaces between different materials pose significant challenges in various industries. Understanding the bonding mechanism is crucial to improving adhesion and overcoming these issues. The adhesion strength of cold spray coatings is influenced by factors such as adiabatic shear instability, static recrystallization, mechanical interlocking, and plastic deformation of colliding materials. The state of the substrate plays a significant role in these factors, affecting the bonding properties and characteristics of the cold-sprayed coatings.

In a study conducted by Salim et al. [32], it was found that varying spraying parameters had minimal impact on the adhesion strength of coatings, indicating that mechanical interlocking and substrate shear instability were not the primary bonding mechanisms. Instead, the adhesion strength was influenced by the hardness and oxidizability of the substrates. Modifying the surface chemistry of the substrates could enhance the adhesion strength of TiO₂ coatings. Chemical or physical bonding mechanisms were identified as the primary bonding mechanisms for ceramic coatings, supported by evidence of chemical bonding among TiO₂ particles in TEM images. Additionally, preheating increased the oxidizability of the substrate, which could decrease the adhesion strength of coatings, aligning with the chemical bonding mechanism.

According to Yamada et al., the agglomerated powder of TiO₂ consisted of main particles on the nanoscale with nanoporosity, resulting in a fractured surface with a dangling bond structure. Upon impact, the particles broke apart and then re-bonded, forming a more stable surface and enabling the bonding of newly impacting particles. The formation of the coating involved an interoxide reaction between TiO₂-OH⁻ and the chromium oxide mixture (Cr₂O₃ + Fe₂O₃ + OH⁻) on the top layer of annealed stainless steel. It was observed that the TiO₂ coating on the annealed SUS304 substrate increased in adhesion strength as the annealing temperature increased from room temperature to 700 °C. The adhesion of the impact can be influenced by the passivation layer, including its chemistry, thickness and structure. The impact bonding mechanism between cold-sprayed TiO₂ and SUS304 could be significantly influenced by the thickness of the passivation layer, which might seem unexpected at just 3 nm of growth [33,34,35,36,37,38].

This observation could be explained by considering the localized deformation of the interface that resulted in bonding, which is shown in the schematic diagram (Figure 15).

4. Conclusions

In this study, the influence of chromium oxide on the bonding mechanism between pure ceramic titanium dioxide and a SUS304 substrate was investigated. The characteristics of the pure chromium substrate were examined after annealing in an electric furnace at temperatures ranging from room temperature to 700 °C. The key findings can be summarized as follows:

• No detectable traces of iron were found at the coating interface, while a small amount of chromium hydroxyls was observed.
• The chemical state of chromium transformed from Cr₂O₃ to CrOOH due to a reaction between the pre-existing chromium oxide (Cr₂O₃) on the oxide layer of the substrate and water molecules present in the atmosphere during the cold spray process.