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Article

Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms

by
Yafei Zhao
1,
Yanping Gao
2,
Zhaoyang Zhai
2,
Yanchao Zhang
2 and
Dongya Zhang
2,*
1
AECC Sichuan Gas Turbine Establishment, Chengdu 610500, China
2
School of Mechanical and Precision Instrument Engineering, Xi’an University of Technology, Xi’an 710048, China
*
Author to whom correspondence should be addressed.
Coatings 2024, 14(10), 1233; https://doi.org/10.3390/coatings14101233
Submission received: 23 August 2024 / Revised: 12 September 2024 / Accepted: 19 September 2024 / Published: 24 September 2024
(This article belongs to the Special Issue Wear-Resistance and Corrosion-Resistance Coatings)
Browse Figures
Review Reports Versions Notes

Abstract

:
The increasing demand for high-performance aircraft engines has led to a greater emphasis being placed on advanced sealing coating technologies. Developing long-life, self-lubricating, and wear-resistant coatings is of significant research value. This study focuses on the fabrication of a novel self-lubricating and wear-resistant NiCoCrAlY-Cr2O3-AgMo composite coating. This coating was deposited onto a GH4169 substrate utilizing plasma spraying. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD) methods were employed to characterize the elemental composition and microstructure of the fabricated NiCoCrAlY-Cr2O3-AgMo composite coating. Microhardness measurements across the coating cross-section indicated a gradual increase in hardness from the GH4169 substrate to the NiCoCrAlY-Cr2O3-AgMo coating. The average hardness of the GH4169 substrate was 413.92 HV0.2, while the CoNiCrAlY bonding layer region exhibited an average hardness of 467.60 HV0.2. The NiCoCrAlY-Cr2O3-AgMo coating itself demonstrated an average microhardness of 643.22 HV0.2. Room temperature friction tests indicated that the average coefficient of friction (COF) of the GH4169 substrate was 0.665. In contrast, the NiCoCrAlY-Cr2O3-AgMo coating exhibited a significantly lower average COF of 0.16, representing a 75.94% reduction compared to the uncoated GH4169 substrate. High-temperature friction tests were conducted at 400 °C, 500 °C, and 600 °C, indicating average COF values of 0.438, 0.410, and 0.268, respectively, for the NiCoCrAlY-Cr2O3-AgMo coating. Specifically, at 600 °C, the formation of a lubricious NiMoO4 tribofilm on the coating surface was observed. This tribofilm effectively reduced the wear rate of the GH605 pin to 2.78 × 10−6 mm3/N·m, highlighting the potential of the NiCoCrAlY-Cr2O3-AgMo coating to reduce wear in high-temperature sliding contact applications.

1. Introduction

The ever-increasing performance demands placed on advanced aircraft engines necessitate enhanced sealing capabilities in their most critical components. These components operate under increasingly harsh conditions, demanding higher temperature resistances, superior sealing performances, and extended operational lifespans from the sealing interfaces. Existing research highlights leakage and wear as major limiting factors hindering the realization of high-performance aircraft engines [1]. Therefore, enhancing the wear resistance of seals is paramount to extending their service life and improving the overall stability of the sealing system. Proctor’s study [2] demonstrated that applying a hard chromium carbide coating to the rotor runway surface effectively extends the lifespan of finger seals. This finding suggests that incorporating wear-resistant coatings at the contact interface between the runway and the finger seal offers a promising avenue for reducing wear and prolonging seal longevity.
Soft metals, such as Ag, Au, Pb, and Cu, have relatively low shear strengths and exhibit excellent self-lubricating properties, making them promising candidates for enhancing friction and wear behavior across a wide temperature range. Under the combined influence of frictional heat and ambient temperature, these soft metals can be drawn from the substrate to the friction surface. The applied shear forces then induce plastic deformation, leading to the formation of a lubricating film on the surface, thereby reducing friction and enhancing wear resistance [3,4]. Similarly, NASA developed the PS series of high-temperature coatings, which leverage the unique properties of a BaF2/CaF eutectic composite lubricant and soft metals. At temperatures exceeding 400 °C, a brittle-to-ductile transition occurs with this combination, enabling effective lubrication due to the relatively low shear strength of the resulting material [5,6,7].
Niu [8] fabricated a composite coating by incorporating Ag, BaF2/CaF, and Mo into a Ni3Al matrix utilizing vacuum hot pressing sintering. This coating exhibited excellent lubrication properties from room temperature to ~800 °C. Specifically, at 1000 °C, the synergistic effect of molybdates was seen on the friction surface, along with Ag and BaF2/CaF, resulting in a low coefficient of friction (COF) of 0.24. Jia [9] incorporated silver molybdate and graphite into the coating. High-temperature friction tests demonstrated that silver molybdate was readily regenerated on the friction surface, achieving favorable tribological characteristics over a wide temperature range. Chen [10,11] developed a NiMoAl-Ag coating by adding hard Cr3C2 and soft metal Ag to NiMoAl. The molybdates generated at high temperatures offered self-lubricating effects, reducing both the COF and wear. Li [12] prepared a Ni3Al-Cr2O3-Ag, BaF2/CaF2 eutectic composite coating through ion spraying. In a temperature range of 20~800 °C, this coating exhibited a low COF ranging from 0.14 to 0.42, with wear rates remaining in the order of 10–4 mm3/N·m, indicating an excellent lubrication performance. Xin [13] fabricated a composite coating on a GH4169 substrate, utilizing Cr3C2 (NiCr) as the strengthening phase, Cu/MoO3 as the lubricating phase, and NiCrAlY as the bonding phase. The results demonstrated that the synergistic lubrication of copper molybdate and copper oxide generated at 600~800 °C offered a coating with good wear resistance and self-lubricating properties. Wang [14] employed hot pressing sintering to prepare a NiCr matrix composite containing silver and molybdenum. Similar to the previous studies, silver molybdate and nickel chromate were generated at high temperatures, offering synergistic lubrication and enhancing the coating’s high-temperature friction performance. The COF ranged from 0.19 to 0.2. Finally, a NiCoCrAlY-Cr2O3-AgMo composite coating was prepared by plasma spraying [15,16]. This coating displayed an outstanding lubrication performance from room temperature to 800 °C, with wear rates in the order of 10−5 mm3/N·m. Above 600 °C, the synergistic lubrication from the nickel molybdate and silver molybdate formed on the friction surface effectively reduced high-temperature wear.
The dynamic sealing that occurs in the gas path of aircraft engines represents a type of high-temperature dry friction. Utilizing solid lubricant coatings to reduce friction and wear between the seal and its mating surface represents a crucial approach to enhancing the reliability and service life of these seals under extreme operating conditions. To achieve consistently low friction and wear across a broad temperature range, materials with low shear strengths are employed at lower temperatures to ensure lubrication at the friction interface, whereas at high temperatures, lubrication is primarily offered by tribo-chemical reaction products, such as molybdates and vanadate, formed from metallic trioxides [17]. Recent advancements in aircraft engine seal technology have demonstrated the significant role of surface coatings in reducing wear. Specifically, finger seals, operating at high temperatures and speeds, impose stringent performance requirements on these coatings. Applying surface coatings to the mating surface of contact-type finger seals has been proven to be effective in minimizing wear, enhancing wear resistance, and finally extending seal lifespan. This advancement holds significance for the research and development of aircraft engines.
This study focuses on the fabrication of a NiCoCrAlY-Cr2O3-AgMo coating on a GH4169 alloy substrate utilizing the plasma spraying method. The coating’s characteristics are evaluated utilizing methods such as energy-dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Then, the coating’s tribological performance is evaluated through pin-on-disk tests conducted under various load conditions at both room temperature and high temperatures. A comparative analysis is performed between the coated and uncoated GH4169 substrate, analyzing the compositional makeup, microstructure, and friction and wear behavior of both the GH4169 substrate and the GH605 counterface material.

2. Experiment and Details

2.1. Preparation of NiCoCrAlY-Cr2O3-AgMo Coating

A GH4169 superalloy with dimensions of 200 mm × 100 mm × 5 mm was selected as the substrate material (its chemical composition is demonstrated in Table 1). The raw materials utilized were spherical powders of NiCoCrAlY, Cr2O3, Ag, and Mo elements with particle sizes of 45~150 μm. The ratio of NiCoCrAlY, Cr2O3, Ag, and Mo was 11:16:2:1. Prior to the coating process, the surface of the GH4169 substrate was cleaned with alcohol to remove any oil contaminants. Then, the surface was sandblasted to eliminate the oxide film and create a wave-like groove morphology. This morphology enhances the bonding strength between the coating and the substrate by promoting mechanical interlocking.
The coating was deposited utilizing a PRAXAIR7700 plasma spraying system with an SG-100 plasma spraying gun. The plasma spraying parameters are listed in Table 2. Initially, a 100 μm thick CoNiCrAlY bond coat was deposited onto the GH4169 substrate. Then, a 100 μm thick NiCoCrAlY-Cr2O3-AgMo coating was applied. The coated samples were then polished utilizing 400, 800, 1000, and 1200 grit sandpaper. Finally, the samples were polished with W2.5 diamond spray polishing agent to achieve a surface roughness of Ra0.182 μm, resulting in a NiCoCrAlY-Cr2O3-AgMo coating with a total thickness of 200 μm.
The NiCoCrAlY-Cr2O3-AgMo-coated samples were cut into standard specimens with dimensions of 10 mm × 10 mm × 5 mm utilizing a wire cutting machine. The phase composition of the NiCoCrAlY-Cr2O3-AgMo coating was analyzed by XRD utilizing an XRD-6000 X-ray diffractometer with Cu Kα radiation. The XRD analysis was performed at an operating voltage of 40 kV, an operating current of 30 mA, a scanning range of 20°–120°, and a scanning speed of 4°/min. The microhardness of the NiCoCrAlY-Cr2O3-AgMo coating was measured utilizing an HVS-1000A micro-Vickers hardness tester with an applied load of 200 g and a dwell time of 10 s.

2.2. Friction and Wear Tests

Two friction and wear testers, the TriboLab UMT-2 and the Rtec MFT-5000, were employed to conduct dry sliding wear tests on the coatings at both room and high temperatures. To ensure the accuracy of the test, each friction test was repeated three times, including the wear rate measurement.
Room Temperature Friction Tests: a consistent load ranging from 5 N to 15 N (corresponding to a contact pressure of 1.6 MPa to 4.8 MPa) and a rotational speed of 2000 rpm (equivalent to a linear velocity of 2.5 m/s) were maintained. The test duration was set to 1 h.
High-Temperature Friction Tests: A constant load of 5 N (corresponding to a contact pressure of 1.6 MPa) and a rotational speed of 2000 rpm (equivalent to a linear velocity of 2.5 m/s) were maintained. The tests were conducted at three temperatures: 400 °C, 500 °C, and 600 °C, each for a duration of 30 min.
GH605 cylindrical pins, with the chemical composition detailed in Table 3, were utilized as the upper specimens and were mounted on a load sensor fixture. The lower specimens consisted of the coated components and a specialized fixture, which were then fixed to a rotating platform. For the high-temperature tests, a specialized furnace was utilized to subject the contact area to the designated high temperatures. Considering that the NiCoCrAlY-Cr2O3-AgMo coating exhibited a higher hardness compared to the GH605, wear was primarily observed on the GH605 cylindrical pins. Pre- and post-test measurements of the GH605 pin lengths were taken to determine the linear wear. Then, the volumetric wear rate of the GH605 pin was calculated. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilized to analyze the surface morphology and elemental distribution of the wear tracks on the coatings, offering insights into their tribological performance and lubrication mechanisms.

3. Results and Discussion

3.1. Morphology and Compositional Analysis of NiCoCrAlY-Cr2O3-AgMo Coating

Figure 1 depicts the surface and cross-sectional morphologies of the NiCoCrAlY-Cr2O3-AgMo coating. The as-sprayed coating surface, depicted in Figure 1a, exhibits a rough morphology due to the nature of the plasma spraying process. Therefore, polishing is required to obtain a smoother surface, as seen in Figure 1b. Figure 1c displays the cross-sectional morphology of the coated sample after cutting. From bottom to top, the image indicates the GH4169 substrate, the CoNiCrAlY bond coating, and the NiCoCrAlY-Cr2O3-AgMo coating. A relatively clear interface exists between the GH4169 substrate and the CoNiCrAlY bond coat, whereas a smooth transition is observed between the CoNiCrAlY bond coat and the NiCoCrAlY-Cr2O3-AgMo coating, lacking a clear boundary. This gradual transition, characteristic of graded coatings, enhances the bonding strength between the two materials [16]. Figure 1d offers a magnified SEM image of the polished coating surface. The dark areas correspond to Cr2O3, while the lighter regions represent NiCoCrAlY. The remaining bright spots indicate the presence of Ag and Mo. Figure 1e provides the distribution of coating and surface elements. It can be seen from the figure that the degree of the bonding between the substrate and NiCoCrAIY-Cr2O3-AgMo coating is good.
Figure 2 presents the XRD pattern and EDS spectrum of the NiCoCrAlY-Cr2O3-AgMo coating. XRD was employed to detect the surface phase composition of the coating, and we identified that the XRD peaks are the most evident due to the presence of the highest content of NiCoCrAlY and Cr2O3. The high-temperature environment during the plasma spraying process (above 500 °C) likely led to the oxidation of Mo, resulting in the formation of a small amount of MoO3, which can also be seen in the XRD pattern. The absence of obvious peaks for Ag and Mo in the XRD pattern suggests their relatively low concentration in the coating. Therefore, an EDS analysis was employed to verify their presence. The EDS spectrum confirms the presence of Al, Cr, Co, and Ni as major components, while also indicating trace amounts of Ag and Mo on the coating’s surface.
Figure 3 illustrates the cross-sectional hardness distribution of the NiCoCrAlY-Cr2O3-AgMo coating. A gradient increase in hardness is observed from the GH4169 substrate to the NiCoCrAlY-Cr2O3-AgMo coating, finally exceeding 600 HV0.2 in the NiCoCrAlY-Cr2O3-AgMo layer. The average hardness of the GH4169 substrate is 413.92 HV0.2, while the CoNiCrAlY bonding layer exhibits an average hardness of 467.60 HV0.2. The NiCoCrAlY-Cr2O3-AgMo coating itself demonstrates an average cross-sectional hardness of 643.22 HV0.2. This enhanced hardness can be attributed to several factors. Firstly, the dispersion of Cr2O3, a ceramic material known for its high hardness and wear resistance, acts as a strengthening phase in the composite coating. These hard ceramic particles function as a microscopic “skeleton,” increasing the material’s rigidity and reducing plastic deformation under applied stress, thereby effectively improving both hardness and wear resistance [18,19]. Secondly, the NiCoCrAlY binder phase, with its excellent high-temperature oxidation and corrosion resistance, offers not only robust mechanical properties and thermal stability but also ductility and toughness. This ability to absorb and disperse external loads prevents stress concentration between the hard ceramic particles, further enhancing the coating’s overall hardness and crack resistance [16,20,21]. Thirdly, the incorporation of Ag, with its low shear modulus at high temperatures, can be employed as a solid lubricant, reducing friction both in the coating and between the coating and the counterface material [16,22]. Simultaneously, Mo, characterized by its high melting point and hardness, further enhances the coating’s hardness and wear resistance. The synergistic effect of silver and molybdenum facilitates the formation of a lubricating layer on the coating surface, minimizing wear while preserving a high level of hardness. Finally, the utilization of plasma spraying technology ensures a dense coating, minimizing defects such as pores and microcracks. This densification strengthens the adhesion between the coating and the substrate, reducing the probability of delamination and fracture under load. In conclusion, the relationship between these mechanisms results in the NiCoCrAlY-Cr2O3-AgMo coating exhibiting superior hardness while retaining a degree of toughness.

3.2. Room Temperature Tribological Performance

Figure 4 depicts the COF curves of both the GH4169 alloy and the NiCoCrAlY-Cr2O3-AgMo coating at room temperature. The GH4169 alloy exhibits a high initial COF of 0.75 with significant fluctuations. After 300 s, the COF value rapidly decreases to 0.65, followed by a gradual increase. Stabilization is achieved around 600 s, with an average COF of 0.665 throughout the test. In contrast, the NiCoCrAlY-Cr2O3-AgMo coating demonstrates a rapid initial increase in the COF, reaching 0.175. This is followed by a steady decline to 0.135 after 100 s, which remains stable for the majority of the test. After 2100 s, a slight upward trend appears with minor fluctuations. The average COF for the coating is 0.139, representing a 79.1% reduction compared to the GH4169 alloy. This superior performance at room temperature can be attributed to the lubricating properties of the chromium oxide and AgMo phases in the coating. These phases effectively reduce friction and wear during the tribological process. During the friction process, the AgMo phase is disrupted. Metallic silver forms a lubricating film on the coating surface, further minimizing the amount of friction that occurs. Moreover, molybdenum particles, likely dispersed throughout the coating, contribute to its hardness and wear resistance. This dispersed molybdenum may also play a role in load distribution, further reducing wear [23,24].
Figure 5 illustrates the wear results for GH4169, the NiCoCrAlY-Cr2O3-AgMo coating, and the GH605 cylindrical pins under tribological testing. Figure 5a depicts the wear morphology of the GH605 pin after being subjected to wear tests against both GH4169 and the NiCoCrAlY-Cr2O3-AgMo coating. Due to the higher surface hardness of both GH4169 and the NiCoCrAlY-Cr2O3-AgMo coating compared to GH605, the GH605 pin experienced more evident wear. However, it is evident that the wear scar length on the GH605 pin is significantly shorter when tested against the NiCoCrAlY-Cr2O3-AgMo coating compared to GH4169. Figure 5b presents the volumetric wear rates of the GH605 pin. When paired with GH4169, the GH605 pin exhibits a wear rate of 3.12 × 10−4 mm3/N·m, whereas, when tested against the NiCoCrAlY-Cr2O3-AgMo coating, the wear rate significantly decreases to 1.3 × 10−6 mm3/N·m, representing a 99.58% reduction, highlighting the significant enhancement in wear resistance offered by the NiCoCrAlY-Cr2O3-AgMo coating to the GH605 substrate. Figure 5c offers a comparison of the wear profiles of the GH4169 and NiCoCrAlY-Cr2O3-AgMo surfaces after the tribological tests. The profile of the NiCoCrAlY-Cr2O3-AgMo coating remains relatively unchanged, indicating minimal wear. In contrast, the GH4169 surface exhibits an evident wear scar, with a maximum depth exceeding 4 μm and a wider wear track.
Figure 6 illustrates the COF values of NiCoCrAlY-Cr2O3-AgMo coatings under various loads: 5 N, 8 N, 10 N, and 15 N. At a 5 N load, the initial COF sees a sharp increase to 0.125, after which it maintains a stable state with almost no fluctuation. Under a normal load of 8 N, the initial COF rises rapidly to 0.175. After 100 s, it gradually decreases to 0.135 and remains stable. However, after 2100 s, an upward trend appears with minor fluctuations. The average COF throughout the entire process is 0.139. With a 10 N load, the initial COF climbs swiftly to 0.125 and then stabilizes with minimal fluctuation. Finally, under a 15 N load, the initial COF also increases rapidly to 0.125, followed by a slow upward trend. However, after 2100 s, there is a dramatic increase in the COF, indicating coating failure. A comparative analysis indicates that the AgMo coating exhibits a relatively low and stable COF under a 10 N load, suggesting that at this specific load, AgMo can promote the formation of a lubricating film, thereby reducing friction [23]. Increasing the load to 15 N, however, results in the destruction of the coating’s microstructure due to the significantly larger contact stress. In severe cases, this can lead to complete coating failure and a consequential increase in the COF.
Figure 7 illustrates the wear rates of GH605 cylindrical pins under various loads. At a load of 5 N, the wear rate of GH605 is merely 1.2 × 10−6 mm3/N·m. When the load increases to 8 N, the wear rate rises to 1.3 × 10−6 mm3/N·m, representing an 8.3% increase compared to the 5 N load. At 10 N, the wear rate reaches 5 × 10−6 mm3/N·m, a 3.17-fold increase from the 5 N load. In addition, under a load of 15 N, the wear rate climbs to 5.78 × 10−6 mm3/N·m, marking a 3.82-fold surge compared to the 5 N load. Evidently, the wear rate of the GH605 cylindrical pin exhibits an upward trend with increasing load. However, it is crucial to note that this wear rate remains significantly lower than the volumetric wear rate observed when GH4169 is subjected to friction against the base material (3.12 × 10−4 mm3/N·m). This contrast highlights that when paired with a NiCoCrAlY-Cr2O3-AgMo coating, the wear of GH605 can be significantly reduced, thereby extending its service life.

3.3. High-Temperature Tribological Performance

Figure 8 illustrates the COF of NiCoCrAlY-Cr2O3-AgMo coatings at high temperatures of 400 °C, 500 °C, and 600 °C. At 400 °C, the COF initially stabilizes around 0.625, exhibiting minor fluctuations in the first 300 s. Then, the COF demonstrates a rapid decline to 0.5, followed by a gradual reduction, finally reaching 0.4 after 1800 s of operation. When the temperature is increased to 500 °C, the initial COF is observed to be 0.75. This is followed by a swift decrease to 0.4 in the first 200 s. The COF remains relatively stable for the following 400 s. However, after 600 s, the average COF rises to 0.425, accompanied by more significant fluctuations. At a temperature of 600 °C, the initial COF is 0.625. The COF then experiences significant fluctuations, eventually stabilizing after 400 s with an average value of 0.26. As evident from these results, the COF exhibits a decreasing trend with increasing temperature, accompanied by enhanced stability. This phenomenon can be attributed to the formation of a self-lubricating transfer film at high temperatures. The hardness of this film decreases as the temperature rises, leading to a reduction in the COF.
Figure 9 illustrates the surface morphology of the NiCoCrAlY-Cr2O3-AgMo coating after wear testing at high temperatures. At 400 °C, the coating surface exhibits a significant amount of pitting, adhesive wear debris, and plowing scratches. The wear track profile indicates a maximum depth of 10 μm with both the wear depth and width being relatively clear at this temperature. This significant amount of wear is attributed to the breakdown of the wear-resistant coating during the friction process. Wear debris generated during the test adheres to the scratched areas and is repeatedly detached, exacerbating the wear process. At 500 °C, the wear behavior of the coating demonstrates improvement. Fewer pits and plowing grooves are observed on the surface. The wear track profile indicates a reduced local wear depth of 5 μm, and the wear track width is significantly smaller compared to the 400 °C test. This improvement suggests that the coating’s wear resistance is enhanced at this temperature. At 600 °C, the wear surface of the coating becomes relatively smooth, indicating a significant improvement in wear resistance. This enhancement is attributed to the formation of a new tribo-film, NiMoO4, on the friction surface at this temperature. The formation of NiMoO4 requires a specific temperature and normal load to occur and function effectively as a solid lubricant, thereby enhancing the wear resistance of the coating.
Figure 10 demonstrates the XRD results of the NiCoCrAlY-Cr2O3-AgMo coating’s friction surface at various temperatures. The XRD patterns at 400 °C and 500 °C are identical to those observed at room temperature. However, a new substance, NiMoO4, appears at 600 °C. The formation of NiMoO4 at this temperature plays a crucial role in reducing friction. It acts as a lubricant, effectively reducing wear and tear on the friction surface [25,26], whereas at lower temperatures (400 °C and 500 °C), the absence of such a lubricating substance leads to more significant wear due to adhesive wear mechanisms, particularly compared to room temperature and 600 °C [27].
Figure 11 illustrates the wear rate of the GH605 pin during high-temperature friction tests. As the temperature increases to 400 °C and 500 °C, the wear rate of the GH605 pin rises from 1.38 × 10−6 mm3/N·m at room temperature to 5.056 × 10−5 mm3/N·m and 7.446 × 10−5 mm3/N·m, respectively. This increase is primarily attributed to the difficulty in forming effective lubricating substances at lower temperatures. However, at 600 °C, the formation of NiMoO4 on the NiCoCrAlY-Cr2O3-AgMo coating surface (Figure 10) offers lubrication, effectively reducing the wear rate of the GH605 pin to 2.78 × 10−6 mm3/N·m. Specifically, the wear rate of the GH605 pin remains lower than that observed when paired with a GH4169 counterpart at room temperature across the entire temperature range from room temperature to 600 °C. This suggests that the coating is particularly well-suited for finger seal applications, effectively reducing wear on the GH605 pin in a temperature range of room temperature to 600 °C.

3.4. Wear Mechanism Analysis

Figure 12a,b present the SEM images of the wear surface morphology of the GH4169 substrate and the NiCoCrAlY-Cr2O3-AgMo coating under an 8 N load. The GH4169 substrate exhibits significant grooving, a characteristic feature of abrasive wear. In addition, evidence of adhesive wear is apparent, indicated by material detachment and plastic deformation (Figure 12a). Under the same load, the NiCoCrAlY-Cr2O3-AgMo coating demonstrates a reduced extent of wear. Figure 12b indicates fewer wear scars compared to the GH4169 substrate, with the majority of the friction surface resembling the unworn surface. This observation highlights the coating’s remarkable ability to minimize wear, primarily through mild abrasion. Figure 12c illustrates the wear morphology of the NiCoCrAlY-Cr2O3-AgMo coating under a 10 N load. The wear tracks in the worn region are shallow, and a significant portion of the friction surface remains similar to the unworn state. Specifically, the coating surface remains intact, devoid of any detachment or cracks. However, upon increasing the load to 15 N, an increase in abrasive wear grooves and the appearance of fine wear debris become evident, signifying an increase in abrasive wear with increasing load. This phenomenon can be attributed to the increased stress and temperature at the coating surface resulting from the higher load. These factors influence the internal stress distribution, thermal stability, and thermal expansion behavior of the coating. At a 15 N load, the microstructure of the coating is severely damaged or even fails. Therefore, abrasive and adhesive wear mechanisms are exacerbated during the friction process.
The surface wear of GH4169 is more severe than that of the wear-resistant coating, which is consistent with the significant fluctuations observed in its COF curve. The application of the NiCoCrAlY-Cr2O3-AgMo coating significantly enhances the wear resistance. In this coating, Cr2O3, a hard ceramic phase, increases the coating’s hardness, thereby improving its wear resistance. The NiCoCrAlY alloy, acting as a metallic bonding phase, offers good toughness and plasticity for absorbing and dispersing stress during the friction process and reducing both crack initiation and propagation. Additionally, the addition of AgMo facilitates the formation of a lubricating film, which further minimizes friction and wear on the coating surface.
Figure 13 presents SEM images of the wear surfaces of GH605 cylindrical pins subjected to different loads at room temperature. Figure 13a depicts the friction morphology of a GH605 pin after being rubbed against a GH4169 substrate under an 8 N load. The surface exhibits slight adhesive wear and abrasive wear along the edges of the contact area, reflected by furrows, pits, and a relatively large wear scar. In Figure 13b, the GH605 pin was rubbed against a wear-resistant coating under the same 8 N load. Here, the wear condition significantly improves, indicating a reduced presence of wear debris and shallower furrows compared to Figure 13a. As the load increases to 10 N (Figure 13c), the number of wear particles and furrows on the GH605 pin surface increases significantly, accompanied by an increase in furrow depth and the appearance of minor pitting. Further increasing the load to 15 N (Figure 13d) intensifies these features, resulting in deeper furrows and a more significant presence of pits. This progression clearly demonstrates that increasing the load exacerbates the wear of the GH605 cylindrical pin.

4. Conclusions

This study evaluated the physical properties of NiCoCrAlY-Cr2O3-AgMo coatings, such as hardness, adhesion, and composition. The tribological mechanism of the wear-resistant coating friction pair was studied through a pin-on-disk wear test. The results exhibit the following:
  • The cross-sectional hardness distribution of the NiCoCrAlY-Cr2O3-AgMo coating demonstrated a gradient structure. The hardness gradually increased from the GH4169 substrate to the NiCoCrAlY-Cr2O3-AgMo coating. The average hardness of the GH4169 substrate was 413.92 HV0.2, the average hardness of the CoNiCrAlY bonding layer region was 467.60 HV0.2, and the average microhardness of the NiCoCrAlY-Cr2O3-AgMo coating cross-section was 643.22 HV0.2.
  • At room temperature and under 8 N load, the average COF of the coating was 0.139, which is a 77.13% reduction compared to the COF of the uncoated GH4169 substrate (0.608). The wear scar of the wear-resistant coating was smooth, and its main wear forms were abrasive wear and adhesive wear. At the same time, the wear rate of the GH605 pin against the NiCoCrAlY-Cr2O3-AgMo coating was reduced by 99.58% compared to that of the GH4169, indicating good wear resistance.
  • The high-temperature tribological test results under 400~600 °C demonstrated that the COF of the NiCoCrAlY-Cr2O3-AgMo coating decreased with the increase in temperature. At 600 °C, NiMoO4 was generated on the friction surface, which played a lubricating role and reduced the COF from 0.438 at 400 °C to 0.268. The wear rate of the GH605 pin was reduced to 2.78 × 10−6 mm3/N·m. The wear rate of the GH605 pin was lower than that of the uncoated GH4169 at room temperature in the temperature range of room temperature to 600 °C.

Author Contributions

Y.Z. (Yafei Zhao): Conceptualization, Methodology, Writing—Original draft preparation. Y.G.: Visualization, Editing. D.Z.: Investigation, Supervision. Z.Z. and Y.Z. (Yanchao Zhang): Reviewing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (52075436), Key Research and Development Program of Shaanxi (2024GX-YBXM-284), Shaanxi Provincial Department of Education service local special project (No.23JC057), China Postdoctoral Science Foundation (2023M743246).

Institutional Review Board Statement

Not Applicable.

Informed Consent Statement

Not Applicable.

Data Availability Statement

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Braun, M.J.; Pierson, H.M.; Kudriavtsev, V.V. Finger Seal Solid Modeling Design and Some Solid/Fluid Interaction Considerations. Tribol. Trans. 2003, 46, 566–575. [Google Scholar] [CrossRef]
  2. Proctor, M.P.; Kumar, A.; Delgado, I.R. High-Speed, High-Temperature Finger Seal Test Results. In Proceedings of the AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit (38th), Indianapolis, Indiana, 7–10 July 2002. [Google Scholar]
  3. Voevodin, A.A.; Muratore, C.; Aouadi, S.M. Hard coatings with high temperature adaptive lubrication and contact thermal management: Review. Surf. Coat. Technol. 2014, 257, 247–265. [Google Scholar] [CrossRef]
  4. Hu, J.J.; Muratore, C.; Voevodin, A.A. Silver diffusion and high-temperature lubrication mechanisms of YSZ–Ag–Mo based nanocomposite coatings. Compos. Sci. Technol. 2007, 67, 336–347. [Google Scholar] [CrossRef]
  5. Dellacorte, C.; Sliney, H.E. Tribological properties of PM212—A high temperature, self-lubricating, powder metallurgy composite. Lubr. Eng. 1990, 47, 298–303. [Google Scholar]
  6. Dellacorte, C.; Edmonds, B.J. NASA PS400: A New Temperature Solid Lubricant Coating for High Temperature Wear Applications. NASA/TM 2009-21567,, 1 August 2009. [Google Scholar]
  7. Stanford, M.K.; Yanke, A.M.; DellaCorte, C. Thermal Effects on a Low Cr Modification of PS304 Solid Lubricant Coating. NASA/TM 2004-213111, 1 June 2004. [Google Scholar]
  8. Niu, M.; Bi, Q.; Yang, J.; Liu, W. Tribological performance of a Ni3Al matrix self-lubricating composite coating tested from 25 to 1000 °C. Surf. Coat. Technol. 2012, 206, 3938–3943. [Google Scholar] [CrossRef]
  9. Liu, E.; Bai, Y.; Gao, Y.; Yi, G.; Jia, J. Tribological properties of NiAl-based composites containing Ag3VO4 nanoparticles at elevated temperatures. Tribol. Int. 2014, 80, 25–33. [Google Scholar] [CrossRef]
  10. An, Y.; Chen, J.; Hou, G.; Zhao, X.; Zhou, H.; Chen, J.; Yan, F. Effect of silver content on tribological property and thermal stability of HVOF-sprayed nickel-based solid lubrication coating. Tribol. Lett. 2015, 58, 34. [Google Scholar] [CrossRef]
  11. Chen, J.; Zhao, X.; Zhou, H.; Chen, J.; An, Y.; Yan, F. HVOF-sprayed adaptive low friction NiMoAl–Ag coating for tribological application from 20 to 800 °C. Tribol. Lett. 2014, 56, 55–66. [Google Scholar] [CrossRef]
  12. Giersch, C.P. Goddess on the Frontier: Religion, Ethnicity, and Gender in Southwest China. Pac. Aff. 2018, 91, 149–151. [Google Scholar]
  13. Xin, B.; Yu, Y.; Zhou, J.; Wang, L.; Ren, S. Effect of copper molybdate on the lubricating properties of NiCrAlY laser clad coating at elevated temperatures. Surf. Coat. Technol. 2017, 313, 328–336. [Google Scholar] [CrossRef]
  14. Wang, J.-Y.; Shan, Y.; Guo, H.; Li, B.; Wang, W.; Jia, J. Friction and wear characteristics of Hot-pressed NiCr–Mo/MoO3/Ag self-lubrication composites at elevated temperatures up to 900 °C. Tribol. Lett. 2015, 59, 48. [Google Scholar] [CrossRef]
  15. Zhang, T.; Huang, C.; Lan, H.; Du, L.; Zhang, W. Tribological properties and lubrication mechanisms of a Ag–Mo composite. Lubr. Sci. 2016, 28, 141–156. [Google Scholar] [CrossRef]
  16. Zhang, T.; Lan, H.; Huang, C.; Du, L.; Zhang, W. Formation mechanism of the lubrication film on the plasma sprayed NiCoCrAlY-Cr2O3-AgMo coating at high temperatures. Surf. Coat. Technol. 2017, 319, 47–54. [Google Scholar] [CrossRef]
  17. Li, J.; Xiong, D.; Huang, Z.; Kong, J.; Dai, J. Effect of Ag and CeO2 on friction and wear properties of Ni-base composite at high temperature. Wear 2009, 267, 576–584. [Google Scholar] [CrossRef]
  18. Su, W.; Niu, S.; Huang, Y.; Wang, C.; Wen, Y.; Li, X.; Deng, C.; Deng, C.; Liu, M. Friction and wear properties of plasma-sprayed Cr2O3–BaCrO4 coating at elevated temperatures. Ceram. Int. 2022, 48, 8696–8705. [Google Scholar] [CrossRef]
  19. Sun, H.; Yi, G.; Wan, S.; Kong, C.; Zhu, S.; Bai, L.; Yang, J. Effect of Cr2O3 addition on mechanical and tribological properties of atmospheric plasma-sprayed NiAl-Bi2O3 composite coatings. Surf. Coat. Technol. 2021, 427, 127818. [Google Scholar] [CrossRef]
  20. Ray, A.K.; Roy, N.; Kar, A.; Ray, A.K.; Bose, S.C.; Das, G.; Sahu, J.K.; Das, D.K.; Venkataraman, B.; Joshi, S.V.; et al. Mechanical property and characterization of a NiCoCrAlY type metallic bond coat used in turbine blade. Mater. Sci. Eng. 2009, 505, 96–104. [Google Scholar] [CrossRef]
  21. Chen, Y.; Zhao, X.; Dang, Y.; Xiao, P.; Curry, N.; Markocsan, N.; Nylen, P. Characterization and understanding of residual stresses in a NiCoCrAlY bond coat for thermal barrier coating application. Acta Mater. 2015, 94, 1–14. [Google Scholar] [CrossRef]
  22. Chen, J.; An, Y.; Yang, J.; Zhao, X.; Yan, F.; Zhou, H.; Chen, J.J.S. Tribological properties of adaptive NiCrAlY–Ag–Mo coatings prepared by atmospheric plasma spraying. Surf. Coat. Technol. 2013, 235, 521–528. [Google Scholar] [CrossRef]
  23. Aouadi, S.M.; Singh, D.P.; Stone, D.; Polychronopoulou, K.; Nahif, F.; Rebholz, C.; Muratore, C.; Voevodin, A.A. Adaptive VN/Ag nanocomposite coatings with lubricious behavior from 25 to 1000 °C. Acta Mater. 2010, 58, 5326–5331. [Google Scholar] [CrossRef]
  24. Bai, L.; Yi, G.; Wan, S.; Wang, W.; Sun, H. Comparison of tribological performances of plasma sprayed YSZ, YSZ/Ag, YSZ/MoO3 and YSZ/Ag/MoO3 coatings from 25 to 800 °C. Wear 2023, 526, 204944. [Google Scholar] [CrossRef]
  25. Behera, N.; Ramesh, M.; Rahman, M.R. Elevated temperature wear and friction performance of WC-CoCr/Mo and WC-co/NiCr/Mo coated Ti-6Al-4V alloy. Mater. Charact. 2024, 215, 114207. [Google Scholar] [CrossRef]
  26. Zhang, P.; Zhao, G.; Wang, W.; Wang, B.; Shi, P.; Qi, G.; Yi, G. Study on the Mechanical and Tribological Properties and the Mechanisms of Cr-Free Ni-Based Self-Lubricating Composites at a Wide Temperature Range. Metals 2020, 10, 268. [Google Scholar] [CrossRef]
  27. Yang, J.; Jia, J.; Li, X.; Lu, C.; Feng, X. Synergistic lubrication of Ag and Ag2MoO4 nanoparticles anchored in plasma-sprayed YSZ coatings: Remarkably-durable lubricating performance at 800 °C. Tribol. Int. 2021, 153, 106670. [Google Scholar] [CrossRef]
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Figure 1. Surface morphologies of the NiCoCrAlY-Cr2O3-AgMo coating: (a) as-sprayed, (b) polished, (c) cross-section, (d) polished (local area), (e) element distribution at the junction.
Figure 1. Surface morphologies of the NiCoCrAlY-Cr2O3-AgMo coating: (a) as-sprayed, (b) polished, (c) cross-section, (d) polished (local area), (e) element distribution at the junction.
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Figure 2. XRD pattern and EDS spectrum of the NiCoCrAlY-Cr2O3-AgMo coating.
Figure 2. XRD pattern and EDS spectrum of the NiCoCrAlY-Cr2O3-AgMo coating.
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Figure 3. Cross-sectional hardness distribution of the NiCoCrAlY-Cr2O3-AgMo coating.
Figure 3. Cross-sectional hardness distribution of the NiCoCrAlY-Cr2O3-AgMo coating.
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Figure 4. COF of GH4169 and NiCoCrAlY-Cr2O3-AgMo (normal load of 8 N, 2000 rpm, room temperature).
Figure 4. COF of GH4169 and NiCoCrAlY-Cr2O3-AgMo (normal load of 8 N, 2000 rpm, room temperature).
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Figure 5. Wear results for GH4169, NiCoCrAlY-Cr2O3-AgMo coating, and GH605 cylindrical pins: (a) wear image of the pins, (b) wear rate of the pins, (c) Wear profiles of GH4169, NiCoCrAlY-Cr2O3-AgMo. (Normal load of 8 N, 2000 rpm, room temperature).
Figure 5. Wear results for GH4169, NiCoCrAlY-Cr2O3-AgMo coating, and GH605 cylindrical pins: (a) wear image of the pins, (b) wear rate of the pins, (c) Wear profiles of GH4169, NiCoCrAlY-Cr2O3-AgMo. (Normal load of 8 N, 2000 rpm, room temperature).
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Figure 6. COF of NiCoCrAlY-Cr2O3-AgMo coatings under different loads (2000 r/min, 1 h, room temperature).
Figure 6. COF of NiCoCrAlY-Cr2O3-AgMo coatings under different loads (2000 r/min, 1 h, room temperature).
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Figure 7. Wear rates of GH605 cylindrical pins under different loads.
Figure 7. Wear rates of GH605 cylindrical pins under different loads.
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Figure 8. The COF of the NiCoCrAlY-Cr2O3-AgMo coating at various temperatures (1.6 MPa).
Figure 8. The COF of the NiCoCrAlY-Cr2O3-AgMo coating at various temperatures (1.6 MPa).
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Figure 9. The morphology and profile of the wear area for the NiCoCrAlY-Cr2O3-AgMo coating under high-temperature conditions.
Figure 9. The morphology and profile of the wear area for the NiCoCrAlY-Cr2O3-AgMo coating under high-temperature conditions.
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Figure 10. The XRD results of the friction surface of the wear-resistant coating at different temperatures.
Figure 10. The XRD results of the friction surface of the wear-resistant coating at different temperatures.
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Figure 11. The wear rate of the GH605 pin during high-temperature friction tests.
Figure 11. The wear rate of the GH605 pin during high-temperature friction tests.
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Figure 12. Surface morphology of wear scars on GH4169 substrate and NiCoCrAlY-Cr2O3-AgMo coating at room temperature.
Figure 12. Surface morphology of wear scars on GH4169 substrate and NiCoCrAlY-Cr2O3-AgMo coating at room temperature.
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Figure 13. SEM images of wear surfaces on GH605 cylindrical pins under different loads at room temperature.
Figure 13. SEM images of wear surfaces on GH605 cylindrical pins under different loads at room temperature.
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Table 1. Chemical composition of GH4169.
Table 1. Chemical composition of GH4169.
ElementCCrNiNbMoAlTi
Wt. (%)≤0.06017.0~21.050.0~55.05.0~5.502.80~3.300.30~0.700.75~1.15
FeTaCoMnSiSP
Balance≤0.10≤1.00≤0.35≤0.35≤0.0020≤0.015
Table 2. Plasma spraying parameters.
Table 2. Plasma spraying parameters.
Process ParameterValue
Voltage60 V
Current450 A
Powder feed rate17 g/min
Spray distance110 mm
Gas flow Ar40 L/min
Gas flow H21.2 L/min
Table 3. Chemical composition of GH605.
Table 3. Chemical composition of GH605.
ElementCCrNiWMn
Wt. (%)0.05~0.1519.0~21.09.0~11.014.0~16.01.0~2.0
FeSiPSCo
≤3.0≤0.40≤0.040≤0.030Balance
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MDPI and ACS Style

Zhao, Y.; Gao, Y.; Zhai, Z.; Zhang, Y.; Zhang, D. Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms. Coatings 2024, 14, 1233. https://doi.org/10.3390/coatings14101233

AMA Style

Zhao Y, Gao Y, Zhai Z, Zhang Y, Zhang D. Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms. Coatings. 2024; 14(10):1233. https://doi.org/10.3390/coatings14101233

Chicago/Turabian Style

Zhao, Yafei, Yanping Gao, Zhaoyang Zhai, Yanchao Zhang, and Dongya Zhang. 2024. "Plasma Spraying NiCoCrAlY-Cr2O3-AgMo Coatings: Fabrication and Tribological Mechanisms" Coatings 14, no. 10: 1233. https://doi.org/10.3390/coatings14101233

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