Abstract
This study used a unique, low-power (2.8 kW) gas tunnel plasma spray and γ-Al2O3 feedstock powder to deposit a thin Al2O3 coating (<2 µm) on mild steel substrate. By optimising the plasma gas flow rate, powder feed rate, spraying time, and distance, a dense and homogeneous Al2O3 coating was obtained at a very high deposition rate (150 nm/s). During plasma coating, the high energy density plasma transforms the γ-Al2O3 powder into a highly crystalline α-Al2O3 as the predominant microstructure of the coating. The performance of the Al2O3 coating on mild steel was then evaluated preliminary using a linear sliding wear test against a chrome-steel ball, with the results indicating an 18–26% reduction in wear when compared to the uncoated substrate.
Introduction
Ceramic coatings are widely used to protect metallic materials due to their excellent corrosion resistance properties. Among the various types of ceramic coatings, alumina (Al2O3) is particularly popular as it also enhances wear resistance. However, the conventional methods used to obtain thin and uniform Al2O3 coatings, such as physical or chemical vapour deposition (PVD or CVD), can be time-consuming due to their very low deposition rates, which are generally around 1 nm/s [1,2]. There are several crystal forms of alumina, including θ-Al2O3, γ-Al2O3, and α-Al2O3, with the latter known to be the most stable and hardest structure among them [3].
Unlike PVD or CVD, gas tunnel-type plasma system (GTPS) is capable depositing hard and dense ceramic coatings at very high speeds. Compared to other plasma spraying techniques, GTPS utilises a gas vortex chamber to generate a highly focused and dense plasma jet. This results in a plasma jet with increased energy, velocity, and precision. Previous studies have shown the effectiveness of GTPS in depositing ceramic coatings, including alumina, with thicknesses ranging from a few microns to hundreds of microns at speeds of 1–5 µm/s, depending on deposition parameters [4–6]. Compared to high-velocity oxy-fuel (HVOF) coating, GTPS can deposit ceramic materials of higher quality [7] due to the higher temperature generated by its high energy density plasma.
Kobayashi [4–6,8] originally designed the GTPS, with the aim of rapidly depositing ceramic coatings with thicknesses of several hundred microns using plasma power ranging from 20 to 30 kW. Recently, a smaller-scale GTPS has been developed, which allows for operation at significantly lower input power (typically below 5 kW). While high-power GTPS is preferred for thick coatings on large components, the low-power version is suited for coating small or delicate parts due to its ability to reduce the risk of thermal damage and provide better control over coating formation. However, the low-power GTPS has received relatively little attention, and identifying the key parameters involved remains a challenge for controlling the process.
As a result, this paper investigates the effectiveness of low-power GTPS in the fabrication of thin ceramic coatings using alumina. Mild steel (MS) was chosen as the metallic substrate because it is prone to wear and tribological degradation, particularly in shafts, bearings, and gears. For this purpose, we utilised γ-Al2O3 powder as the feedstock for coating alumina on MS substrate using a GTPS gun operating at a plasma power of 2.8 kW with argon as the plasma gas. This study demonstrated the effectiveness of GTPS in heating and melting the sprayed powder. It is found that the low-power GTPS system can transform γ-Al2O3 into dense α-Al2O3 through high temperature-energy density plasma without the need for additional heat treatment.
Experimental details
Materials and characterisation methods
An ASTM A36 mild steel rod (25 mm × 10 mm) was prepared as the substrate. The surface was polished with 500, 800, and 1200 grit SiC papers, followed by polishing with 1 μm Al2O3 paste. Gamma-alumina (γ-Al2O3) was used as the feedstock powder, with the particles have d50 of 0.1–1.0 μm and a purity of 99.95% (Sigma Aldrich Pte Ltd, Singapore). Microstructural analysis was carried out using a scanning electron microscope (SEM-Hitachi SU3500, Japan) and Field Emission SEM – Zeiss-Supra 55VP coupled with energy dispersive X-ray spectroscopy (EDX, Oxford Instrument, UK). Mitutoyo Surftest SJ 410 was used to measure the average surface roughness (Ra) of the substrate. XRD analysis (Rigaku SmartLab, Japan) was used to study crystallographic structures using Cu-K radiation at 40 kV and 30 mA, and the coating is measured at a grazing angle (1.0°).
Figure 1(a) shows the SEM image of the polished MS substrate, exhibiting a mirror finish (inset picture), with a measured final surface roughness ranging from 0.11 to 0.17 μm. The XRD pattern of the MS substrate showed three main peaks, corresponding to the (110), (200), and (211) planes of the body-centered cubic (b.c.c) structure of the ferritic (α-Fe) phase (JCPDS card no. 04-014-0360). Meanwhile, Figure 1(b) shows the SEM image of an individual particle and XRD pattern of the feedstock powder. The XRD pattern reveals the three main reflections corresponding to the γ-Al2O3 phases (JCPDS card no. 00-010-0425), observed as broad peaks at 37°, 46°, and 66°, indicating the (311), (400), and (440) planes. The broadening of these peaks suggests the presence of other metastable alumina phases and indicates the partially weak crystalline nature of γ-Al2O3 due to small crystallite sizes [9].
SEM image and XRD spectra of (a) MS substrate (small inset picture show mirror-polished substrate) (b) γ-Al2O3 feedstock powder.
Plasma coating and linear reciprocating sliding wear test
The schematic layout of the low-power GTPS gun used in this study is shown in Figure 2(a). The system has a water-cooled copper body, a 600 mm gun length, and a 10 mm nozzle. Connecting the anode to the substrate holder produces a more stable plasma arc and more efficient energy transfer, resulting in a uniform coating thickness. The γ-Al2O3 powder was fed into the plasma jet vertically by gravity flow using an external powder feeder. This method allows for more efficient powder delivery, uniform melting, and prevent deposits onto electrode. The GTPS is designed so that the plasma jet is constrained within a narrow annular nozzle so that the spray stream is more focused and collimated. This enables a more efficient particle heating and melting, resulting in a rapid deposition of in-flight particles onto substrate. Table 1 shows the coating parameters used in this study.
(a) Schematic configuration of the GTPS system (b) Ball-on-flat linear reciprocating sliding wear test. GTPS coating parameters.
As shown in Figure 2(b), the coefficient of friction (CoF) and wear mass loss of the coating were measured using a ball-on-flat type tribometer with a linear reciprocating wear module (DUCOM). A 10 mm-diameter AISI 52100 bearing chrome-steel ball was used as the counter body. The sliding test was conducted at different sliding speeds (2, 4, 6 Hz) under dry sliding conditions with a constant load of 50 N. The worn surfaces were analysed using SEM.
Results and discussions
Characterisation of Al2O3 coating on MS substrate
A cross-sectional SEM image of the Al2O3 coating obtained after deposition is shown in Figure 3(a). The coating has a uniform thickness with an average thickness of about 1.45 ± 0.05 µm. In this experiment, the estimated deposition rate for Al2O3 coating is 150 ± 50 nm/sec. EDX line scan was used to assess the coating-substrate interface based on Al and O elements intensity. By using a short spraying distance of 10 mm in this study, the feedstock particles were effectively confined to the hot plasma zone, resulting in efficient vaporisation and uniform deposition on the substrate. Figure 3(b) shows a higher magnification SEM image of the same region of interest as Figure 3(a), but tilted slightly (10°) to capture both the cross-sectional and surface morphology of the coating layer. The coating appears mostly uniform, but there are some noticeably large bumps caused by unmelted particles, as shown in the image. This occurs when the plasma jet is cut before the powder flow stops completely. The powder that adheres to the hot substrate surface form large unmelted particles.
SEM images showing (a) cross-sectional image of Al2O3 coating with EDX line scan (b) tilted image (10°) of the top surface and cross-section of Al2O3 coating (c) surface microstructure of Al2O3 coating at high magnification and (d) XRD spectra of as-deposited Al2O3 coating.
Meanwhile, the high-magnification FE-SEM image in Figure 3(c) shows that the surface of the coating is free from cracks and defects, with partially and fully melted alumina particles seen in the surface morphology. The partially melted particles consisted of densely packed fibrous grains of γ-Al2O3, while the fully melted regions revealed well-structured crystals, which believed as α-Al2O3 grains. The XRD results in Figure 3(d) confirm the presence of two distinct alumina phases in the coating. The high intensity and sharp diffracted peaks are consistent with stable α-Al2O3 crystals (JCPDS card no. 00-46-1212), indicating a high degree of crystallisation whereas the peaks assigned to metastable γ-Al2O3 (JCPDS card no. 00-010-0425) are broad and low. The high-temperature plasma and rapid cooling rates during GTPS are believed to promote the phase transformation of γ-Al2O3 to α-Al2O3 [8–10]. The α-Al2O3 possesses superior properties compared to other metastable alumina based on higher melting point, hardness, and denser structure [8–10].
Sliding wear test
The pristine MS substrate and the as-deposited alumina coating underwent dry sliding wear tests using a chrome-steel ball as the counter body. Figure 4(a,b) compare the coefficient of friction (CoF) variations between uncoated and coated samples at different sliding speeds of 2, 4, and 6 Hz. At a constant sliding load of 50 N, it can be observed that the CoF decreases as the sliding speed increases for both uncoated and coated samples. This is due to shorter adhesion time between the contacting surfaces. Thermal softening of the coating may also contribute to a decrease in CoF at higher sliding speeds.
Friction and wear behaviour of uncoated and coated MS (a) CoF for uncoated MS (b) CoF for alumina-coated MS (c) Average CoF and (d) Average wear mass loss at different sliding speeds. SEM micrographs of worn surfaces (e) uncoated MS (50 N, 2 Hz) and (f) coated MS (50 N, 2 Hz).
Meanwhile, distinct CoF fluctuations can be observed between uncoated and coated samples, with the uncoated MS exhibiting an increase in CoF during running-in due to smoothening of surface asperities before reaching steady-state [11]. In contrast, the coated MS had an unstable CoF during running-in, indicating possible coating wear or damage and resulting in CoF fluctuations [12]. The average CoF values measured during steady-state state, as plotted in Figure 4(c), show that the CoF values for the coated sample are lower and more stable, ranging from 0.084 to 0.077, compared to the pristine MS sample, which ranges from 0.1 to 0.08. The wear mass loss (mg) for uncoated and coated samples at various sliding speeds is shown in Figure 4(d). As the sliding speed increased, the wear mass loss decreased. In comparison to pristine MS, the deposition of Al2O3 coating with a thickness of 1.45 ± 0.05 µm could reduce wear mass loss by 18–26%.
Finally, Figure 4(e,f) show the SEM image of the worn surface for uncoated and coated samples after a sliding wear test at 2 Hz. The worn surface of the uncoated MS in Figure 4(e) showed deep abrasive grooves along the wear path, indicating severe adhesion and wear. The relatively low hardness of the MS substrate compared to the counter body explains this severe wear. On the contrary, the thin Al2O3 coating could effectively functioned as tribo-coating, as evidenced by SEM image in Figure 4(f). The shallow plough grooves indicate that the coating experienced less severe abrasive and adhesive wear from the counter body. The accumulated wear debris seen on the surface is likely from detached semi-molten alumina particles. These particles have poor crystal structure and weak adhesion to the surface, hence easily detached.
Conclusions
In this study, γ-Al2O3 particles were deposited onto mild steel using low-power gas tunnel plasma spray operated at 2.8 kW. The as-deposited coating was examined, and the following is a summary of the findings:
Microscopy analysis reveals a dense, homogeneous alumina coating with a relatively uniform thickness of 1.4 ± 0.5 µm and good adhesion to the mild steel substrate. X-ray diffraction analysis reveals that the coating mainly consists of γ-Al2O3 and α-Al2O3 phases, with a greater presence of highly crystalline α-Al2O3 phases. Sliding wear tests against steel counter-body show that the coating improves mild steel resistance to abrasive and adhesive wear, resulting in 18–26% less wear than the uncoated substrate.
Footnotes
Acknowledgements
This project was supported by Universiti Teknologi Malaysia through Collaborative Research Grant National (CRG 26.3 4B537) and Professional Development Research University (PDRU 06E17).
Disclosure statement
The authors have no conflicts of interest to declare.