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Article

The Role of Nitrogen Doping in Enhancing the Thermal Stability and Wear Resistance of AlSi Coatings at Elevated Temperatures

by
Qunfeng Zeng
Key Laboratory of Education Ministry for Modern Design and Rotor-Bearing System, Xi’an Jiaotong University, Xi’an 710049, China
Coatings 2025, 15(11), 1296; https://doi.org/10.3390/coatings15111296
Submission received: 4 October 2025 / Revised: 28 October 2025 / Accepted: 3 November 2025 / Published: 6 November 2025
(This article belongs to the Special Issue Advanced Tribological Coatings: Fabrication and Application)
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Abstract

AlSi and nitrogen-doped AlSi (AlSiN) coatings were deposited onto 22MB5 steel, while h-BN coatings were applied to H13 steel using the magnetron-sputtering method. The thermal stability and tribological properties of the AlSi and AlSiN coatings were systematically investigated from room temperature to 800 °C in ambient air. The results indicate that the AlSiN coatings with an FeAl transition layer exhibited outstanding wear resistance and high thermal stability behaviors at elevated temperature because the FeAl layer can inhibit the diffusion of Al and absorb Fe, forming iron-rich intermetallic compounds with a high bonding strength. The FeAl layer plays a critical role in enhancing the coating’s performance. Analysis of the wear mechanisms revealed that the AlSiN coating primarily underwent adhesive wear, while the AlSi coating suffered from abrasive and oxidative wear. These findings offer valuable insights for developing protective coatings for the hot-stamping-forming process.

1. Introduction

The use of high-strength steel is essential to achieve lightweight and safety in the automotive industry, with hot stamping being used for forming high-strength auto parts. During hot stamping, the die continuously contacts, reciprocates, and impacts the hot workpiece, leading to high-temperature friction and wear. Therefore, protective coatings are necessary to prevent oxidation of the hot stamping steel and extend the die’s service life. AlSi coatings are widely used for this purpose in the automotive industry [1]. However, the formation of brittle compounds in these coatings [2,3] can accelerate part failure during the hot-stamping process [4]. Doping has been employed to enhance coating properties, offering potential for further optimization in microstructural optimization, solid solution strengthening, enhanced oxidation resistance, reduction in internal stress, and tribological performance optimization [5]. The thermal stability and tribological behaviors of coatings are critical factors for the friction pair between high-strength steel and die steel [6]. Therefore, it is necessary to study the thermal stability and tribological behaviors of AlSi coatings. Pelcastre et al. demonstrated that hard phases form in AlSi coatings during heating [7]. Recent studies have focused on AlSiN coatings, where nano-crystalline AlN is embedded in an amorphous Si3N4 matrix [8,9,10,11,12]. These coatings exhibit high hardness and strength, effectively preventing the oxidation of high-strength steel during austenitization. However, AlSiN coatings tend to have a high coefficient of friction (CoF) at elevated temperatures, prompting the need to explore solid lubricant coatings for hot-forming applications [13,14,15,16,17,18,19,20,21].
In this study, AlSi and nitrogen-doped (AlSiN) coatings were prepared using magnetron sputtering. Al-Si is a proven but tribologically weak oxidation-resistant system. Nitrogen was used to dope and transform it into a hard, wear-resistant AlSiN nanocomposite, and magnetron sputtering was employed as the most suitable method to precisely synthesize this advanced coating and directly address the research gap in high-temperature tribological performance for hot-stamping applications. The thermal stability and tribological properties of the AlSi and AlSiN coatings were investigated from room temperature to 800 °C in ambient air. A friction pair was developed to enhance the anti-friction and wear resistance of 22MnB5 steel under harsh hot-stamping conditions due to the oxidation of 22MnB5 during hot stamping and Al-Si coatings as a solution to industry uses, causing wear on dies [22,23,24]. Additionally, the wear mechanisms of the coatings at elevated temperatures were systematically analyzed.

2. Experimental Details

2.1. Deposition of AlSi Coatings and N Doping

High-strength 22MnB5 steels were used. The sizes of the disc samples were 30 mm in diameter and 3 mm in thickness. The steels were polished, cleaned, and dried, respectively. The coatings were prepared on steel by magnetron-sputtering technology. Initially, the chamber was evacuated to 10 Pa, and after introducing argon, the vacuum reached 6 × 10−5 Pa. The gas flow rate was 20 sccm for the ratio 1:1 of Ar–N2. There was an Fe target, an Al target, a Si target, and a Ti target. The purity of the targets was about 99.9%. The power of the coating deposited was 200 W. FeAl coatings were first prepared as the transition layer, and then Al coatings and AlSiN coatings were deposited, respectively. The AlSi coating was composed of three parts: a pure Al layer for the outer layer, an FeAlSi ternary phase for the middle layer, and an FeAl intermetallic compound for the inner layer. During austenitization, the FeAl layer absorbed Fe, forming tough FeAl and Fe3Al phases, thereby enhancing the coating’s performance [25].
h-BN exhibits the friction-reducing and self-lubricating properties, which enhance the wear resistance of AlSiN coatings. Therefore, we explored the potential for hybrid coating systems combining AlSiN and h-BN and provided a comparison between AlSiN and h-BN coatings. The h-BN coatings were deposited on an H13 steel pin by a radio-frequency magnetron-sputtering system in an Ar atmosphere with an h-BN target (Φ76.2 mm × 3 mm, with a purity of 99.9%; Quanzhou Qijin New Material Technology Co., Ltd., Quanzhou, China) [13]. The h-BN coatings were deposited for 180 min. The power was 300 W. The thickness of the h-BN coatings was about 560 nm.

2.2. Microstructure

The surface morphology and the coating thickness were analyzed using a scanning electron microscope (SEM, MALA3 LMH, Brno, Czech Republic) and energy-dispersive spectroscopy (EDS). The microstructure of the coatings was characterized by an X-ray diffractometer (XRD, D8-Advance, Bruker, Saarbrücken, Germany) at a scanning speed of 8°/min and a scanning range of 20°~90°. The targeted metal in the X-ray tube used was copper. The surface roughness of the coatings was measured by atomic force microscopy (AFM). An optical microscope (CX40M, Sainyu Optical Technology Co., Ltd., Ningbo, China) was used to examine the widths and morphologies of the wear tracks of the experimental samples and the surfaces of the experimental samples after high-temperature oxidation. The optical microscope was equipped with 5×, 10×, 20×, 50×, and 100× objective lenses.

2.3. Friction and Wear Behaviors

A pin of H13 steel with a 6 mm diameter and a 12 mm length was used for the tribotests. The testing parameters were a load of 1 N and a sliding speed of 0.05 m/s. The contact pressure of the friction pair of the coatings on the 22MnB5 steel and H13 steel counterbody was almost equal to the contact pressure of actual hot-stamping conditions. The testing temperatures were room temperature (RT), 400 °C, 600 °C, and 800 °C, with each condition repeated three times using three separate samples.
The hardnesses of the substrates and coatings were measured. The hardness of the steel substrate was about 294.8 HV, while the AlSi coating showed a higher hardness of ~521.2 HV. The hardnesses of the AlSiN coatings and AlSiN coatings were about 896.5 HV and 913.2 HV. The AlSiN coatings with the FeAl layer demonstrated even greater hardness, measuring ~896.5 HV and ~913.2 HV, respectively.

3. Results and Discussion

3.1. High-Temperature Thermal Stability of 22MnB5 Steel and AlSi Coatings

Figure 1 presents the SEM images and EDS analysis of the coatings’ cross-sectional morphologies. The coatings exhibited well-defined interfaces with the substrate, with no cracks or delamination observed on the bonding surfaces. The bonding between the FeAl transition layer and the AlSiN coatings, and the FeAl transition layer and the 22MnB5 steel, was compact. The thicknesses of the AlSi coating, AlSiN coating, and AlSiN coating with an FeAl layer were about 1.7 μm, 0.8 μm, and 0.91 μm, respectively. The thickness of the FeAl transition layer was about 250 nm. This range of 0.8–1.7 µm for the coating thickness was chosen on the basis of a balance between achieving optimal mechanical and tribological performance while maintaining cost-effectiveness and feasibility for industrial-scale deposition processes. Thinner coatings may not provide sufficient wear resistance, while thicker coatings could lead to increased residual stresses and adhesion issues, compromising durability [26,27,28].
Each element has its own characteristic X-ray wavelength, and the magnitude of the characteristic wavelength depends on the characteristic energy released during the energy level transition process. An energy spectrometer conducts component analysis by taking advantage of the different characteristic energies of X-ray photons of different elements. The characteristic X-rays generated when an electron beam interacts with substances are utilized to provide information on the chemical composition of the sample. The chemical components of the coatings were measured using mapping on the coated area. Before measuring, each sample was sprayed with a gold layer to enhance the conductivity or reduce the charging effect, which caused Au signals to appear in the energy spectrum analysis. In the EDS spectra of the AlSi coatings, the atomic ratio of A1 and Si was 8.5:1.5. For the AlSiN coatings, the atomic ratio of the N, A1, and Si elements was approximately 6:3:1, confirming that the sputtering parameters were well optimized. Minor traces of Na and Fe were also detected in the AlSiN coating, likely due to sample-handling contamination. Specifically, direct skin contact during preparation may have transferred salts (e.g., NaCl) onto the sample surface [29].
(a)
High-temperature thermal stability of AlSi coatings
During the heating process, there is serious decarburization and the oxidation peeling phenomenon on the surface of steel for traditional hot-stamping parts without coatings. This decarbonization results in reducing the strength of the steel. Oxides increase the friction between the steel plate and the mold, reducing the service life of the mold and the production efficiency. The surface morphologies of 22MnB5 steel and AlSi coatings after a high-temperature oxidation experiment were observed by OM, and the high-temperature thermal stability of the AlSi coatings was analyzed. The oxidation surfaces of the AlSi coatings at different temperatures were analyzed by EDS.
Figure 2 shows a comparison of the oxidation surface morphologies of 22MnB5 steel and the AlSi coatings at different temperatures. It is shown that the surfaces of the 22MnB5 steel and AlSi coatings were smooth at RT. At 400 °C, there was slight oxidation and decarburization on the surface of the 22MnB5 steel due to a few oxidation black points, while there were no obvious oxidation black spots on the surface of the AlSi coating. At 600 °C, it was shown that there was relatively serious oxidation of the 22MnB5 steel. The AlSi coating was slightly oxidized, and there were more black spots on the surface of the coating. At 800 °C, it was found that the surface of the 22MnB5 steel was completely covered by iron oxides, and there was obvious oxidation and decarburization. The AlSi coating partially failed due to oxidation. The steel substrate was bare and oxidized, and the AlSi coating still existed in other areas, but was severely oxidized, and an obvious dendrite structure could be seen. Compared with 22MnB5 steel, it is shown in the results that AlSi coatings have good high-temperature thermal stability behaviors, but their high-temperature thermal stability behaviors need to be improved [30].
EDS spectra of the oxidized surfaces of the AlSi coatings at different temperatures were obtained by SEM to analyze the high-temperature thermal stability behaviors of the AlSi coatings and assess their elemental composition changes and oxidation behaviors. These techniques provided a more comprehensive and convincing analysis of the thermal stability of the coatings. Table 1 shows the EDS spectra of the AlSi coatings. It was found that the content of the O element on the AlSi coatings’ oxidized surfaces increased with the increase in temperature. At 600 °C, the atomic percentage of the O element was 47.72%, and at 800 °C, the atomic percentage of the O element was 50.56%, meaning that the coatings almost failed.
Compared with 22MnB5 steel, the thermal stability behavior of the AlSi coatings at high temperatures was excellent, but at 600 °C, the AlSi coating was seriously oxidized, and at 800 °C, the surface was mostly covered by iron oxide, and the AlSi coating was completely oxidized.
(b)
High-temperature thermal stability of AlSiN coatings
The surface morphologies of AlSiN coatings after high-temperature oxidation tests were observed by an optical microscope. Figure 3 shows the oxidized surfaces of AlSiN coatings without an FeAl layer and with an FeAl layer at different temperatures.
It was seen that from RT to 600 °C, there was no oxidation phenomenon on the surface of the AlSiN coating without an FeAl transition layer, but at 800 °C, there was slight oxidation and an obvious crack phenomenon on the surface of the coating. The AlSiN coating with an FeAl transition layer was almost not oxidized at all temperatures. It was found that compared with the AlSi coating, the thermoformed steel with AlSiN coatings had good oxidation resistance at high temperatures, and the AlSiN coatings without an FeAl transition layer had good thermal stability behavior at high temperatures, while the FeAl transition layer had no oxidation cracking phenomenon at high temperatures. It was shown that the FeAl transition layer improved the high-temperature thermal stability behavior of the AlSiN coatings of the thermoformed steel because the FeAl layer was helpful to inhibit the diffusion of Al and absorb Fe, forming iron-rich intermetallic compounds with good toughness, so the bonding interface between the coating and substrate was high [31].
The EDS spectra of AlSiN oxidized surfaces at different temperatures were obtained to verify the high-temperature thermal stability behavior of AlSiN coatings at different temperatures. Table 2 and Table 3 show the EDS spectra of the surfaces of AlSiN coatings with and without an FeAl layer at different temperatures. It was seen that the content of the O element on the oxidized surface of the AlSiN coatings increased slightly with the increase in temperature, indicating that the high-temperature thermal stability of the AlSiN coatings was better than that of the AlSi coatings. In addition, the content of the O element on the oxidized surface of the AlSiN coatings with an FeAl transition layer was higher than that of the AlSiN coatings without an FeAl transition layer. In summary, the high-temperature thermal stability behavior of AlSiN coatings was better than that of AlSi coatings, and the FeAl transition layer improved the high-temperature thermal stability behavior of the AlSiN coatings. The difference in oxygen may have been caused by the coating deposition of magnetron sputtering [32].
The brittleness of the AlSiN coating was attributed to the microstructure and composition of the AlSiN coating. AlSiN coatings are composed of aluminum, silicon, and nitrogen elements. The brittleness of AlSiN coatings may be explained by the following: The microstructure of AlSiN coatings has a direct effect on their brittleness. AlSiN coatings, particularly when deposited by physical vapor deposition (PVD) techniques, often exhibit a columnar microstructure. This growth pattern creates inherent weaknesses along the column boundaries, which can act as easy pathways for crack propagation. Consequently, the coatings become more susceptible to cracking and fracture under mechanical or thermal stress [33]. A primary factor contributing to this brittleness is the presence of nitrogen. While the incorporation of nitrogen is essential for forming the hard (Al,Si)N solid solution and significantly improves the coating’s hardness and wear resistance, it also increases its intrinsic brittleness [34]. This compromise in toughness manifests as a tendency for the coating to form micro-cracks and undergo spallation when subjected to impact or repetitive loading. The mechanical properties of the coating are a direct result of its microstructure and composition. High crystallinity, often associated with increased microhardness, typically comes at the expense of fracture toughness. This creates a classic trade-off in hard coatings: as hardness increases, the material’s ability to plastically deform and absorb energy diminishes, making it more brittle and prone to catastrophic failure under applied forces [35]. Therefore, the brittleness of an AlSiN coating is not governed by a single factor but by the complex interplay between its columnar growth morphology, nitrogen content, and the resulting mechanical properties of hardness and toughness.

3.2. High-Temperature Tribological Properties of Coatings

Comparative studies were conducted between AlSiN coatings with and without an FeAl interlayer to investigate the influence of the FeAl transition layer on the tribological performance of the AlSiN coatings. Tribological tests were performed, and the wear track morphology was analyzed to elucidate the high-temperature wear mechanisms of the AlSiN coatings. The high-temperature tribological properties of the AlSiN coatings were compared with those of AlSi coatings.
(a)
High-temperature tribological properties of AlSi coatings and AlSiN coatings
(1)
Room temperature
Figure 4 shows the coefficients of friction (CoFs) of the AlSi coatings and AlSiN coatings at room temperature. It was found that the CoF curves of the AlSiN coatings fluctuated relatively compared with the AlSi coating. The CoF of the AlSiN coating without the FeAl transition layer rose from 0.23 to about 0.55 and then slowly decreased to about 0.3. Finally, the CoF was about 0.3, and the CoF of the AlSiN coating with the FeAl transition layer was about 0.2 at the beginning. After 25 s, the CoF increased rapidly to about 0.5, and then the CoF fluctuated sharply, and the CoF was about 0.4. It was shown that the stable CoF of the AlSiN coatings was high at room temperature. At room temperature, the stable CoF of the AlSi coating was about 0.23.
Figure 5 shows the wear morphologies of the AlSiN coatings and the pin at room temperature. It was found that the wear mechanism of the AlSiN coatings was abrasive wear. The difference was that the wear surface of the AlSiN coating without the FeAl transition layer was completely covered by iron oxide, and no wear marks were seen, while the wear debris of the AlSiN coating with the FeAl transition layer was few.
A relatively obvious circular wear area was formed on the pin, and it can be seen that the wear chips generated during the sliding process adhered to the front of the pin. At the same time, there were many wear chips with small particles in the wear area. Therefore, the wear mechanism of the AlSi coating was abrasive wear. The width of the wear mark on the flat surface was 545.74 μm, and the diameter of the circular wear area on the pin was about 551.63 μm. The pin and AlSiN coating with the transition layer pair had little grinding debris. The AlSiN coatings appeared to flake off compared with the AlSi coating because the AlSiN coatings doped with the N element had high hardness. The difference was that the AlSiN coating with the FeAl transition layer had the coatings on the worn surface parallel to the direction of the worn surface, while the AlSiN coating without the FeAl transition layer had only scattered small pieces of peeling. The FeAl transition layer was composed of brittle iron–aluminum intermetallic compounds (i.e., the Fe2Al5 phase and FeAl3 phase), which made the AlSiN coating with the FeAl transition layer brittle. This also explains why the AlSiN coating with the FeAl transition layer had few wear debris on the worn surface because the coating was removed during the sliding, and the CoF of the AlSiN coating with the FeAl transition layer was volatile because there was no uniform wear debris on the surface. The wear mechanisms of the AlSiN coatings were abrasive wear and spalling wear, and the AlSiN coating with the FeAl transition layer was prone to brittle fracture at room temperature.
(2)
Temperature of 400 °C
Figure 6 shows the CoFs of the AlSiN coatings and the AlSi coating at 400 °C. The CoF curves of the AlSiN coating with the FeAl layer and the AlSiN coating without the FeAl layer were rough, but the CoF curve of the AlSiN coating was stable. The initial CoFs of the AlSiN coatings were low. The CoFs of the AlSiN coatings were high, at about 0.5, in the stable stage.
Figure 7 shows the wear morphology of the AlSiN coatings and pin at 400 °C. There was still some coating on the worn surface of the AlSiN coating without an FeAl layer, and long strip traces of the coating peeling and the coating fragments were seen on the worn surface. The coating fragments adhered to the worn surface. This indicates that the AlSiN coating without the FeAl layer had slight adhesion. The wear area of the pin adhered to the wear debris, so there was abrasive wear. At 400 °C, the initial CoF of the AlSi coating was relatively high at 0.73, and the CoF fluctuated and gradually dropped to 0.38.
The wear area on the pin was irregular and long; there was a small amount of wear debris; and there was slight oxidation on the surface of the pin. In addition, observing the wear marks on the disk, it can be seen that there were relatively obvious trenches in the wear marks, and there was the phenomenon of large pieces of the coating being peeled off, and a large amount of coating debris was accumulated. This is because the AlSi coating was brittle and fractured under the force, deformed and compacted under the combined action of temperature and pressure, and then acted as an abrasive particle on the frictional contact surface, which aggravated the wear. Therefore, the CoF curve of the AlSi coating fluctuated sharply, and the CoF was large.
The wear mechanisms of the AlSi coatings were abrasive wear and furrow wear. The width of the wear mark was 802.55 μm. A few peeling coating fragments were attached to the worn surface of the AlSiN coating with the FeAl layer, and grooves were found. There was a small amount of wear debris on the worn surface, and the AlSiN coating with the FeAl layer was observed on the wear area of the pin. There was a small amount of wear debris. The wear mechanism of the AlSiN coating without the FeAl layer was adhesive wear with abrasive wear, while the wear mechanism of the AlSiN coating with the FeAl layer was abrasive wear with adhesive wear.
(3)
Temperature of 600 °C
Figure 8 shows the CoFs of the AlSiN coatings and the AlSi coating at 600 °C. The CoF curve trends of the AlSiN coating with the FeAl layer and without the FeAl layer were almost the same. The CoFs of the AlSiN coatings in the stable stage were about 0.38, and the curves of the CoFs of the AlSiN coatings fluctuated smoothly.
Figure 9 shows the wear morphologies of the AlSiN coatings and the pin at 600 °C. The wear area on the pin was irregular in shape, and an obvious adhesion phenomenon appeared in the wear area. Both sides of the wear mark were smooth, but there were deep grooves in the middle area, and the wear debris had accumulated. The surface of the pin was seriously oxidized. The AlSi coating peeled off during the movement and then acted as abrasive particles on the friction contact surface. There was adhesion between the friction pairs due to high temperature, which aggravated the wear. The CoF curve fluctuation was more severe than that at 400 °C, and the CoF was large. The wear mechanisms of the AlSi coating were mainly adhesive wear and furring wear. The width of the wear mark was 1099.49 μm. There was the existence of AlSiN coating on the worn surface, which was caused by abrasive wear. There was relatively obvious adhesion on the worn surface of the AlSiN coatings. The wear debris adhered to the wear areas of the coatings. The grinding debris was compacted under pressure and thermal stress. There was obvious oxidation on the worn surface of the pin, and the AlSiN coating without the FeAl layer had obvious transfer of the coating material onto the worn surface of the pin, while the AlSiN coatings with the FeAl layer only stuck the wear debris to the pin surface. The wear mechanisms were adhesive wear and abrasive wear, but the adhesive wear of the AlSiN coating without the FeAl layer was serious. At 600 °C, the CoF of the coating increased from 0.25 to about 0.58.
The FeAl interlayer primarily enhances the adhesion between the AlSi coating and the substrate, which is critical for improving wear resistance and preventing delamination under mechanical stress. Additionally, the FeAl layer contributes to the load-bearing capacity, reducing the direct stress on the AlSi coating and improving its overall durability. In terms of thermal stability, the FeAl interlayer acts as a diffusion barrier, mitigating interdiffusion between the substrate and the AlSi coating at elevated temperatures. This helps maintain the structural integrity and mechanical properties of the AlSi coating under thermal cycling or high-temperature conditions. The FeAl layer also exhibits good oxidation resistance, which further supports the thermal stability of the coating system.
(4)
Temperature of 800 °C
Figure 10 shows the CoFs of the AlSiN coatings and the AlSi coating at 800 °C. The CoF curves of the AlSiN coatings with the FeAl layer and without the FeAl layer were almost the same. The CoFs were about 0.4. The CoF curve of the AlSiN coating without the FeAl layer fluctuated greatly compared with the AlSiN coating with the FeAl layer. This is because the bonding surface between the AlSiN coating and steel, without the FeAl layer, formed brittle iron and aluminum metal compounds, which had a certain influence on the stability of friction. The CoF of the AlSi coating increased from 0.18 to about 0.38 with time.
Figure 11 shows the wear morphologies of the AlSiN coatings and the pin at 800 °C. The oxidation on the surface of the ball was more serious. The wear area was irregular, and there was obvious adhesion in the wear area. The wear marks on the AlSi coating were narrow, and there was a micro-ploughing phenomenon in the wear area, which was because the AlSi coating was seriously oxidized. Aluminum oxide films were formed on the surface. However, because the aluminum oxide films were very thin, the films broke and failed quickly. The steel matrix was directly exposed to severe oxidation. The wear mechanisms of coatings were adhesive wear and oxidation wear. The width of the wear mark on the AlSi coatings was 603.69 μm. It was found that it was similar to that at 600 °C, and no obvious adhesion existed on the worn surface at 800 °C. The AlSiN coating with the FeAl layer had obvious transfer films of the coating onto the wear surface of the pin. However, the AlSiN coating without the FeAl layer led to the wear surface of the head pin in the moving direction. The wear mechanism of the AlSiN coatings at 800 °C was adhesive wear.
The microstructures of the AlSiN coatings were observed, as shown in Figure 12 and Figure 13. The surface morphologies of the worn surfaces of the AlSiN coatings with or without the FeAl layer were different. At RT and 400 °C, the coatings were more serious, and the abrasive chips accumulated on the worn surface. The pressed coatings exhibited brittle fracture due to extrusion under the load. At 600 °C and 800 °C, there was a very smooth compacted layer on the worn surface, but at 600 °C, there was a small amount of wear debris. The AlSiN coating with the FeAl layer did not exhibit brittle fracture at 800 °C. The worn surface was flat, and there was a compacted layer formed on the surface, which explained why the CoF fluctuated. From room temperature to 800 °C, the wear mechanisms of the AlSiN coatings were abrasive wear, plough wear, and adhesive wear. AlSiN coatings have high wear resistance. The doping of N with Al and Si elements formed AlN crystals with smaller grains and amorphous Si3N4, producing the high hardness, high thermal stability, and high wear resistance of the AlSiN coatings [36,37,38,39,40]. The wear mechanisms of the AlSiN coatings were abrasive wear and peeling wear; the wear mechanism was adhesive wear at 400 °C; and the wear mechanisms were abrasive wear and slight adhesive wear at 600 °C. There was abrasive wear and adhesive wear at 800 °C.
The volume wear rate at each temperature was calculated based on the cross-sectional area [41,42,43]. The wear rate of the AlSi coating was 1.50 × 10−3 mm3(Nm)−1 at room temperature; at 400 °C and 600 °C, the wear rates of the AlSi coatings were 6.61 × 10−3 mm3(Nm)−1 and 9.02 × 10−3 mm3(Nm)−1, respectively; and at 800 °C, due to the serious oxidation of the worn surface, the wear rate was very low, at 8.91 × 10−5 mm3(Nm)−1.
At RT, the wear rate of the AlSi coating was higher than that of the AlSiN coatings. The wear rates of the AlSiN coating and the AlSiN coating with the FeAl layer were 4.50 × 10−4 mm3(Nm)−1 and 4.52 × 10−4 mm3(Nm)−1, respectively. At 400 °C, the wear rates of the AlSiN coating and the AlSiN coating with the FeAl layer were 4.5 × 10−4 mm3(Nm)−1 and 8.2 × 10−4 mm3(Nm)−1, respectively. At 600 °C, the wear rates of the AlSiN coating and the AlSiN coating with the FeAl layer were 2.3 × 10−3 mm3(Nm)−1 and 4.24 × 10−3 mm3(Nm)−1, respectively. At 800 °C, the wear rates of the AlSiN coating and the AlSiN coating with the FeAl layer were 2.6 × 10−4 mm3(Nm)−1 and 6.1 × 10−4 mm3(Nm)−1, respectively. The wear rate of the AlSiN coating was much lower than that of the AlSi coating. The wear rate of the AlSiN coating decreased by 70%, 93.2%, and 74.5% from RT to 600 °C. The wear rate of the AlSiN (with FeAl) coating decreased by 79.8%, 87.6%, and 53%.
At room temperature, the AlSi coating and the AlSiN coating exhibited low friction at the initial stage, which then decreased over the following time because there may have been an absorption layer on the surface of the coatings, which provided slight lubrication and high hardness. At 400 °C, there was no glaze layer because the coatings were not oxidized enough, causing high initial friction. At 600 °C and 800 °C, there was a glaze layer of oxide because the coatings were oxidized enough at the high temperatures, causing low initial friction.
(b)
High-temperature tribological properties of h-BN coatings
According to the above experimental results, AlSiN coatings have poor tribological properties, and the CoF of AlSiN coatings is greater than that of AlSi coatings. To improve the high-temperature tribological properties of AlSiN coatings, the tribological properties of the friction pair of an AlSiN coating with an FeAl layer and an h-BN coating, which are known for their excellent lubricating properties, were studied.
The cross-section of the h-BN coating is shown in Figure 14. The microstructure of the coating was dense, and it was well-bonded with the 22MnB5 steel. The thickness of the h-BN coating was 0.56 μm. The characteristic peaks of h-BN at 27.7° and 32.6° are the typical peaks of h-BN. The 44.68°, 64.88°, and 82.2°of 2θ are the peaks of iron [44]. The atomic ratio of the B element to the N element in the h-BN coating was 4.7:5.3.
(1)
Room temperature
Figure 15 shows the CoFs of the AlSiN coating and the steel and h-BN coating on the steel pin at room temperature. The CoFs of the AlSiN coating and the h-BN coating at the initial stage were low; moreover, the CoFs of the AlSiN coating and the h-BN coating were close to super-low friction between 30 and 40 s, and the CoF was about 0.003. After 60 s, the CoF increased sharply to about 0.58 and then slowly decreased and finally stayed at about 0.3. It was low compared with the steel pin and AlSiN coating pair.
(2)
Temperature of 400 °C
Figure 16 shows the CoFs of the h-BN coating and the AlSiN coating at 400 °C. In the first 60 s, the CoF curves of the AlSiN coating and the h-BN coating were almost the same as that of steel. Then, the CoFs decreased and were low compared with steel, and were finally about 0.3.
(3)
Temperature of 600 °C
Figure 17 shows the CoF of the friction pair at 600 °C. The CoF of the friction pair was high at the initial stage and about 0.42. During the friction process, the CoF curve of the friction pair basically coincided with that of the steel, and the CoF fluctuation was high. In the stable stage, the CoF was about 0.3.
(4)
Temperature of 800 °C
Figure 18 shows the CoF of the friction pair at 800 °C. The CoF of the friction pair at the initial stage was high and about 0.6, and the CoF gradually decreased and reached about 0.18.
Figure 19 shows the SEM images of the wear surfaces. The CoF of the AlSiN coating with the h-BN coating at RT was low compared with the steel. Especially at the initial stage, the anti-friction effect was obvious. The lubrication mechanism occurred in the layer structure of the h-BN coatings [45]. The CoF was low in the initial stage, and the AlSiN coatings were broken in the later stage, resulting in a high CoF.
At 400 °C, the Co of the AlSiN coating with the h-BN coating was not significantly different from that of the steel. It was considered that at the initial stage, the coatings on the surface of the disk and pin were broken under load, which weakened the inter-layer slip during the friction process. At 600 °C, the CoF of the friction pair of the h-BN coating and the AlSiN coating was similar to that of steel. There were no coatings on the worn surface of the disc. The abrasive debris on the worn surface was compacted. At 800 °C, the CoF at the initial stage was high, and there was no anti-friction effect.
There was no wear debris on the wear surface of the grinding disc at room temperature. The worn surface of the pin with the h-BN coating was attached to wear debris. The worn surface of the pin was composed of micro-furrows because of the high hardness of h-BN. The wear mechanisms of the AlSiN coating and h-BN coating were spalling wear and abrasive wear. This is why the CoF between the AlSiN coating and h-BN coating was ultra-low. The reason is that h-BN coatings have high hardness and are thin, and the coatings were broken under load, resulting in the accumulation of multi-layer h-BN debris on the worn surface [46,47,48]. The h-BN reduced the CoF, but the breaking and spalling of the AlSiN coating caused a high CoF. There were slight scratches on the worn surface of the disc, while there were only coatings on the outer area at 400 °C. The wear mechanism of the friction pair was abrasive wear. However, the coatings were broken quickly under load, forming layers of h-BN debris in the worn area. The AlSiN coatings were broken and formed on the worn surface, reducing the CoF. There was no coating in the worn area of the disc, and there was an obvious ploughing phenomenon on the worn surface at 600 °C. There was wear debris on the surface of the h-BN coating. The wear mechanisms were plough wear and abrasive wear. There were no coatings on the worn surface of the AlSiN coating at 800 °C. The worn area on the disc was rough, and there was still compacted wear debris at the interface. The wear mechanisms were plough wear, abrasive wear, oxidation wear, and adhesive wear. TiAlN coatings are widely recognized for their excellent hardness, high-temperature oxidation resistance, and wear resistance. TiAlN coatings typically exhibit a CoF ranging from 0.4 to 0.6 under dry sliding conditions [49]. The wear mechanisms are often dominated by abrasive and oxidative wear at high temperatures [50]. CrN coatings are known for their good corrosion resistance, moderate hardness, and excellent adhesion to substrates [51]. CrN coatings generally exhibit a CoF of 0.5 to 0.7 [52]. The wear mechanisms often involve adhesive and oxidative wear, especially at higher temperatures [53,54]. AlSi and AlSiN coatings are gaining attention for their unique combination of high hardness, thermal stability, and oxidation resistance. They are particularly promising for applications in hot-stamping and -forming processes, where high-temperature performance is critical. The typical temperature range for the hot-stamping and -forming process of coated boron steel is between 600 °C and 800 °C [55]. This range ensures the steel is adequately softened for forming in the die before the rapid quenching that creates its final martensitic, high-strength microstructure. AlSiN coatings, especially those with FeAl transition layers, exhibit superior wear resistance and lower CoFs (0.1 to 0.3) compared with TiAlN and CrN coatings. The wear mechanisms of AlSiN coatings are primarily adhesive, while AlSi coatings experience abrasive and oxidative wear. AlSiN coatings demonstrate exceptional thermal stability up to 800 °C, outperforming many TiAlN and CrN coatings, which often degrade at lower temperatures due to oxidation or phase transformations. AlSiN coatings inherently exhibit excellent thermal stability and oxidation resistance up to 800 °C. This makes them particularly suitable for demanding applications like hot stamping. The incorporation of nitrogen and FeAl transition layers in AlSiN coatings significantly improves their wear resistance. The novelty of AlSi and AlSiN coatings lies in their unique combination of high-temperature stability, enhanced wear resistance, and cost-effectiveness, making them a promising alternative to traditional coatings like TiAlN and CrN. These advantages are particularly relevant for applications in hot stamping and forming, where performance at elevated temperatures is critical.
A h-BN coating functions primarily as a solid lubricant [56,57,58]. Its layered structure provides excellent anti-stick and low-friction properties, effectively preventing steel from adhering to the die surface [59,60]. An AlSi coating is the industrial standard for a bare boron steel sheet itself. The coating forms a protective Al-Fe-Si layer during austenitization that prevents scale formation on the blank [61,62,63]. An AlSiN coating is a hard ceramic coating applied directly to die surfaces. It is highly wear-resistant, thermally stable, and acts as a diffusion barrier. Its extreme hardness and chemical inertness at high temperatures (900 °C) significantly reduce abrasive wear and prevent the transfer of aluminum from the AlSi-coated blank onto the die. The role of nitrogen doping in AlSi coatings is possibly shown in the following aspects: Nitrogen doping can enhance the hardness and wear resistance of AlSi coatings by improving mechanical properties [9,64]. For instance, an aluminum nitride (AlN) coating possibly forms covalent bonds by combining nitrogen atoms with aluminum atoms, enhancing the stability of the coating’s crystal structure and enabling a high hardness value. Another aspect is optimizing thermal conductivity performance. Nitrogen doping can improve the heat conduction path and enhance thermal conductivity [65]. However, the performance of an AlSiN coating is heavily dependent on the presence of an intermediate bond layer, with FeAl proving to be superior. When AlSiN is deposited directly onto hot work tool steel, a significant problem arises due to the mismatch in thermal expansion coefficients. The tool steel expands and contracts more than the hard AlSiN coating during the rapid heating and quenching cycles of production. This mismatch generates high internal stresses, leading to the formation of micro-cracks and eventual delamination or spallation of the coating. Its service life is limited. An AlSiN coating with an FeAl interlayer creates a functionally graded material system. For a durable and cost-effective hot stamping die solution, the optimal choice is a multilayer coating system: an FeAl bond layer topped with a hard AlSiN ceramic layer. This combination provides the necessary adhesion and thermal stress resistance (from the FeAl) along with the extreme wear resistance, anti-galling, and diffusion barrier properties (from the AlSiN). While h-BN is useful as a lubricant, the AlSiN + FeAl system is the superior technological solution for protecting dies in high-volume production against the aggressive wear from AlSi-coated boron steel.

4. Conclusions

The high-temperature thermal stability and tribological properties of AlSi coatings and AlSiN coatings on 22MnB5 steel and h-BN coatings on H13 steel were investigated. The conclusions are listed as follows:
(1)
The CoF of the AlSi coatings was relatively high compared with that of uncoated 22MnB5 steel. The wear mechanisms of the coatings varied with temperature, exhibiting abrasive wear at RT, adhesive wear at 400 °C, and adhesive wear at 600 °C and 800 °C.
(2)
The role of nitrogen doping in AlSi coatings is helpful, possibly, to enhance the hardness and wear resistance of the AlSi coatings and optimize the thermal conductivity performance of the coatings. The AlSiN coatings with an FeAl transition layer exhibited the best thermal stability behaviors. The FeAl layer significantly enhanced the performance of AlSiN-coated steel.
(3)
The friction pair of N-doped AlSi coatings and h-BN coatings demonstrated a lower CoF compared with the friction pair of AlSiN coatings and steel, showing a pronounced anti-friction effect.

Funding

The present work was supported by the National Key Research and Development Program of China (2024YFC3015803) and the Henan Key Laboratory of High-Performance Bearings (ZYSKF202302).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Cross-sectional SEM images and EDS of AlSi and AlSiN coatings.
Figure 1. Cross-sectional SEM images and EDS of AlSi and AlSiN coatings.
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Figure 2. The surface morphologies of 22MnB5 steel and AlSi coatings at different temperatures.
Figure 2. The surface morphologies of 22MnB5 steel and AlSi coatings at different temperatures.
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Figure 3. The oxidation surface morphologies of AlSiN coatings with and without FeAl coatings at different temperatures.
Figure 3. The oxidation surface morphologies of AlSiN coatings with and without FeAl coatings at different temperatures.
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Figure 4. CoFs of the coatings at room temperature.
Figure 4. CoFs of the coatings at room temperature.
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Figure 5. Surface morphologies of the wear surfaces of coatings at room temperature.
Figure 5. Surface morphologies of the wear surfaces of coatings at room temperature.
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Figure 6. CoFs of the coatings at 400 °C.
Figure 6. CoFs of the coatings at 400 °C.
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Figure 7. Surface morphologies of the wear surfaces of AlSiN coatings at 400 °C.
Figure 7. Surface morphologies of the wear surfaces of AlSiN coatings at 400 °C.
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Figure 8. CoFs of the coatings at 600 °C.
Figure 8. CoFs of the coatings at 600 °C.
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Figure 9. Surface morphologies of the wear surfaces of AlSiN coatings at 600 °C.
Figure 9. Surface morphologies of the wear surfaces of AlSiN coatings at 600 °C.
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Figure 10. CoFs of coatings at 800 °C.
Figure 10. CoFs of coatings at 800 °C.
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Figure 11. Surface morphologies of the wear surfaces of AlSiN coatings at 800 °C.
Figure 11. Surface morphologies of the wear surfaces of AlSiN coatings at 800 °C.
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Figure 12. SEM images of the wear surfaces of AlSi coatings without an FeAl layer at different temperatures.
Figure 12. SEM images of the wear surfaces of AlSi coatings without an FeAl layer at different temperatures.
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Figure 13. SEM images of the wear surfaces of AlSi coatings with an FeAl layer at different temperatures.
Figure 13. SEM images of the wear surfaces of AlSi coatings with an FeAl layer at different temperatures.
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Figure 14. SEM image and XRD of h-BN coating.
Figure 14. SEM image and XRD of h-BN coating.
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Figure 15. CoFs of h-BN coating and AlSiN coating at room temperature.
Figure 15. CoFs of h-BN coating and AlSiN coating at room temperature.
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Figure 16. CoFs of h-BN coating and AlSiN coating at 400 °C.
Figure 16. CoFs of h-BN coating and AlSiN coating at 400 °C.
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Figure 17. CoFs of h-BN coating and AlSiN coating at 600 °C.
Figure 17. CoFs of h-BN coating and AlSiN coating at 600 °C.
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Figure 18. CoFs of BN coating and AlSiN coating at 800 °C.
Figure 18. CoFs of BN coating and AlSiN coating at 800 °C.
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Figure 19. SEM images of the worn surfaces of the AlSiN coatings.
Figure 19. SEM images of the worn surfaces of the AlSiN coatings.
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Table 1. EDS spectra of AlSi oxidation surfaces at different temperatures.
Table 1. EDS spectra of AlSi oxidation surfaces at different temperatures.
Temperature (°C)Atomic Percentage (at.%)
COAlSiFe
RT22.786.4934.1112.160.85
40013.2515.5156.7913.480.98
60020.3742.7231.283.671.96
80018.5650.6525.145.652.81
Table 2. EDS spectra of the surfaces of AlSiN coatings without FeAl layers at different temperatures.
Table 2. EDS spectra of the surfaces of AlSiN coatings without FeAl layers at different temperatures.
Temperature (°C)Atomic Percentage (at.%)
NOAlSiFe
RT42.386.4928.1112.1610.86
40040.438.3127.2012.5411.52
60037.567.4928.0213.1113.82
80037.4411.2124.3311.3015.72
Table 3. EDS spectra of the surfaces of AlSiN coatings with FeAl coatings at different temperatures.
Table 3. EDS spectra of the surfaces of AlSiN coatings with FeAl coatings at different temperatures.
Temperature (°C)Atomic Percentage (at.%)
NOAlSiFe
RT45.0913.2623.939.947.77
40043.2615.4823.209.548.52
60041.1314.8724.159.1610.15
80039.7915.3523.5410.4110.91
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Zeng, Q. The Role of Nitrogen Doping in Enhancing the Thermal Stability and Wear Resistance of AlSi Coatings at Elevated Temperatures. Coatings 2025, 15, 1296. https://doi.org/10.3390/coatings15111296

AMA Style

Zeng Q. The Role of Nitrogen Doping in Enhancing the Thermal Stability and Wear Resistance of AlSi Coatings at Elevated Temperatures. Coatings. 2025; 15(11):1296. https://doi.org/10.3390/coatings15111296

Chicago/Turabian Style

Zeng, Qunfeng. 2025. "The Role of Nitrogen Doping in Enhancing the Thermal Stability and Wear Resistance of AlSi Coatings at Elevated Temperatures" Coatings 15, no. 11: 1296. https://doi.org/10.3390/coatings15111296

APA Style

Zeng, Q. (2025). The Role of Nitrogen Doping in Enhancing the Thermal Stability and Wear Resistance of AlSi Coatings at Elevated Temperatures. Coatings, 15(11), 1296. https://doi.org/10.3390/coatings15111296

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