Trans. Nonferrous Met. Soc. China 31(2021) 744-752

Self-lubricating behavior of Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀ high-entropy alloy matrix composites

Wei-cheng XIAO, Kun LI, Liu-liu HAN, Yong LIU

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Received 8 May 2020; accepted 5 November 2020

Abstract: Eutectic high entropy alloys (EHEAs) have high temperature stability, good mechanical properties, and are promising for tribological applications at high temperatures. To study the high temperature lubrication behavior, Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀-(BaF₂/CaF₂)_x (x=3-20, wt.%) composites were prepared by spark plasma sintering (SPS), with BaF₂/CaF₂ eutectic powder used as solid lubricant. The lubrication behavior and mechanical properties were studied at both room and high temperatures. With the increase of the content of BaF₂/CaF₂ eutectic powder, the friction coefficients and the wear rates of the composites at 600 and 800 °C decrease significantly. The composites with eutectic powder content of 15 and 20 wt.% have the best lubricating performance at 600 °C, with low friction coefficient and wear rates, mainly due to the good mechanical properties of EHEA matrix, the lubrication effect of BaF₂/CaF₂ phase and the oxides formed on the worn surface.

Key words: eutectic high entropy alloy; self-lubricating composite; BaF₂/CaF₂ eutectic powder; tribological properties; microstructure

1 Introduction

Self-lubricating materials are important in industry, such as petrochemical, automotive, mining, aerospace and other fields. However, under severe working condition, the operating temperatures sometimes are so high that conventional lubricating systems cannot serve [1,2]. Therefore, the high temperature behavior of self-lubricating materials is very crucial [3-5].

In 1930s, powder metallurgical metal-based self-lubricating composites were developed [6], generally composed of metallic matrix and ceramic or mineral solid-lubricating phases [7]. The matrix phase can provide sufficient mechanical properties and high temperature oxidation resistance. The most commonly used matrix for high temperature self-lubricating composites are Ni- and Co-based superalloys. The lubricating phases include inorganic fluorides, metal oxides and other ternary compounds. However, the chemical properties between solid lubricants and metals are quite different, and the addition of solid lubricants will reduce the sintering ability of the composites and mechanical properties [8,9]. Therefore, it is important to achieve a balance between the mechanical properties and the tribological properties when designing self-lubricating composites.

High-entropy alloys (HEAs) are becoming more and more important in metallic materials [10-13]. Although HEAs generally contain at least four principal elements, they have a simple structure and excellent properties [14-16]. Recently, LU et al [17] proposed a new kind of HEAs, eutectic high entropy alloys (EHEAs). EHEAs have several features, such as good high-temperature creep resistance, stable microstructures and high hardness [18]. Therefore, EHEAs are promising for using as high temperature self- lubricating composites. However, there are few studies on EHAs or EHEAs with self-lubrication phases. HAN et al [19] prepared Fe-16.4Mn- 4.8Ni-9.9Al-xC (wt.%) composites by spark plasma sintering (SPS), and showed a significant improvement in tribological performance due to  the self-lubricating effect of graphite. ZHANG et al [20,21] used CoCrFeNi as matrix, and different solid lubricants such as Ag, Cu and fluorides, and showed a better performance of lubrication and higher wear resistance than those of pure HEAs.

In our previous study, FeCoCrNiTa EHEA has excellent mechanical properties and high- temperature stability. It consists of face-centered- cubic (FCC) and Laves phases, and it can be a promising candidate for engineering applications at elevated temperatures [22,23]. In this work, Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀ EHEA matrix composites were prepared by SPS. BaF₂/CaF₂ eutectic was selected to be solid lubricant because its excellent lubrication performance and high temperature stability. The microstructural evolutions and lubrication properties of the EHEA composites were investigated.

2 Experimental

Gas-atomized Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀ powder and BaF₂/CaF₂ eutectic powder were mixed and filled in a graphite die with a diameter of 28 mm. The preparation method of BaF₂/CaF₂ eutectic powder was reported in Refs. [24,25]. The proportions of the raw materials are listed in  Table 1. The powder was sintered at 1373 K in the SPS Equipment (HPD25/3), and held for 15 min at 30 MPa, followed by furnace cooling.

Table 1 Compositions of EHEA composites

The X-ray diffractometer (XRD, Rigaku D/MAX-2250, Japan) using a Cu K_α radiation was used to analyze the phase compositions. Scanning electron microscope (SEM, Quanta FEG 250) equipped with an energy dispersive X-ray spectroscopy (EDS) analyzer was used to observe the microstructures of the self-lubricating composites. To further identify the chemical compositions of phases, electron probe micro- analyses (EPMA) was used. The Archimedes method was used to measure the density of the composites. The Vicker’s hardness instrument with a load of 0.5 N was used to measure the micro-hardness.

The dry wear tests at room temperature, 600 °C and 800 °C were carried out on a rotational ball-on-disk high temperature tribometer in air. The ball used was Si₃N₄ with a size of 6 mm. The self-lubricating composites were cut in the form of disks with a size of d28 mm × 5 mm. The rotation diameter, sliding speed, loads and sliding time of tests were 4 mm, 0.28 m/s, 20 N and 30 min, respectively. Each sample was tested three times, with each time of 30 min. After testing, the worn surfaces were observed by using SEM. A 3D surface profilometer (NT9100, Veeco) was used to measure the worn volume of composites. A micro- beam XRD diffractometer (Rigaku Rapid IIR) was used to analyze the phase compositions of the worn surface.

3 Results

3.1 Material characterizations

Figure 1 shows the typical morphologies of the EHEA powder and the fluorides powder. The EHEA powder is spherical, with a particle size less than 50 μm.

Figure 2 shows the XRD patterns of self- lubricating composites. The FCC phase, Laves phase, fluorides phase (BaF₂ and CaF₂) can be obviously observed. With the increase of the content of BaF₂/CaF₂ eutectic powder, there are stronger peaks corresponding to BaF₂ and CaF₂ phases in the XRD patterns. This result suggests that the BaF₂/CaF₂ eutectic powder does not react heavily with the matrix during SPS.

Fig. 1 Morphologies of EHEA powder (a) and BaF₂/CaF₂ eutectic powder (b)

Fig. 2 XRD patterns of different composites

Figure 3 demonstrates the microstructures of the composites, and Fig. 4 shows the compositional profiles. The grey phase is rich in Co, Cr, Fe and Ni, the white phase rich in Ta, and the dark phase rich in Ba, Ca and F. This indicates that the grey phase is FCC phase, the white phase is Laves phase, and the dark phase is of fluorides. From the results of XRD and microstructure of Sample S₃, the fluorides uniformly distribute and have chemical stability in the matrix.

3.2 Mechanical properties

According to Table 2, the density of the S₁ to S₄ composites is in the range of 9.11-7.31 g/cm³, above 99% theoretical value of all composites. When the content of BaF₂/CaF₂ eutectic powder is increased to 20 wt.% (Sample S₅), the relative density decreases.

The hardness of the composites gradually decreases with the increasing content of BaF₂/CaF₂ from 545 HV to 399 HV. The microhardness values of the FCC matrix phase and the Laves phase are higher than those of fluorides phase.

Fig. 3 Microstructures of Sample S₃ sintered at 1100 °C

3.3 Friction and wear behavior

The coefficient of frictions (COFs) with the testing time is shown in Fig. 5. Compared to Samples S₁ and S₂, the high-temperature COFs of Samples S₃, S₄ and S₅ decrease obviously. And the COFs of Samples S₄ and S₅ are more stable.

The wear rates and averaged COFs of composites at different temperatures are shown in Fig. 6. The COFs of composites decrease at high temperature, and the COFs of Samples S₃, S₄ and S₅ decrease dramatically. As the content of BaF₂/CaF₂ increases to 9 wt.%, the averaged COF of Sample S₃ decreases to 0.22 at 600 °C, and the wear rate maintains to be 5.9×10^-5 mm³/(N·m). Further increasing the content of BaF₂/CaF₂, the COF becomes more stable and maintains around 0.22. The wear rates of the composites are in the magnitude of 10^-5 mm³/(N·m). With temperature increasing, the wear rate decreases.

Fig. 4 Microstructure and corresponding elemental mappings of Cr, Fe, Co, Ni, Ta, Ba, Ca and F in Sample S₃

Table 2 Density and hardness of EHEA matrix composites

In order to investigate the wear mechanism, the worn surface of the composites was analyzed, and the results are shown in Fig. 7 and Table 3. Some plowing grooves can be observed on the worn surfaces, suggesting that slight abrasive wear happens. Adhesive wear also occurs during the wear process, evidenced by the formation of patches and the transfer of materials. According to the change of O content in the EDS result, severe oxidation occurs at high temperatures.

The COF of Sample S₂ is about 0.35 at 600 °C, which is the same as that of Sample S₁. According to Fig. 7, the wear mechanisms of S₂ are adhesive wear, abrasive wear and oxidation wear. With the increase of BaF₂/CaF₂ content, the worn surface of S₂ forms a discontinuous and broken glassy layer, which may lost the self-lubricating ability.

The worn surfaces of Sample S₃ tested at RT have similar morphology with Samples S₁ and S_2, showing adhesive wear and slightly abrasive wear. With temperatures increasing to 600 and 800 °C, the worn surfaces of the composites become  smooth, and the continuous glassy layers form on the worn surfaces. According to the EDS results, the Ba, Ca and F contents of the glassy layer (Region 8) are higher than those of the rough surface (Region 9). This result is mainly due to the fact that the glassy layer is rich in fluorides.

Fig. 5 COFs with sliding time of different composites

Fig. 6 Wear rates and averaged COFs of different composites with temperature

With further increasing of BaF₂/CaF₂ content, the whole worn surface of Samples S₄ and S₅ is similar to those of Sample S₃ after testing at RT, 600 °C and 800 °C. The glassy layers form on the worn surface at high temperatures, which are rich in fluorides. In summary, the major wear behavior of Samples S₄ and S₅ is almost the same with that of Sample S₃.

To further study the lubricating behavior of Sample S₃ at high temperatures, the cross-sectional microstructure and EDS results of the worn surface at 600 °C are shown in Fig. 8 and Table 4. The Ba, Ca and F contents of the glassy layer (Region A) are higher than those of the composite (Region B). The thickness of the continuous glassy layer is about 7 μm, which is bonded with the composite closely.

Figure 9 shows the micro-beam XRD patterns of Sample S₃ after testing at RT, 600 °C and 800 °C. At RT, FCC phase, Laves phase, BaF₂ and CaF₂ are observed, and no peaks of oxides appear on the worn surface. At 600 and 800 °C, the glassy layer mainly consists of different metal oxides (Cr₂O₃, Fe₂O₃ and other oxides) and BaF₂/CaF₂ eutectic phase.

4 Discussion

4.1 Microstructures

In this work, the EHEA matrix composites were prepared by SPS with microstructures consisting of FCC phase, Laves phase, BaF₂ and CaF₂. Fluoride phases distribute homogeneously in the EHEA matrix, and have good chemical stability. This distribution makes the microstructure uniform, and also makes it easier to form a complete and continuous lubrication layer during the wear process.

Fig. 7 Morphologies of worn surface and marked areas for EDS

Table 3 Compositions of regions on worn surfaces marked in Fig. 7

Fig. 8 Microstructures of cross-sections of worn Sample S₃ after testing at 600 °C

Table 4 Compositions of regions on worn surface marked in Fig. 8

Fig. 9 Micro-beam XRD patterns of Sample S₃ after testing at different temperatures

The melting point of BaF₂/CaF₂ eutectic phase is lower than the sintering temperature, so the fluoride phases mostly distribute in the pores of the matrix. The liquid phase sintering can further improve the density of the composites [26], but excessive liquid phase during the sintering process will have an adverse impact on the mechanical properties of the composites.

4.2 Wear behavior

The smooth and continuous glassy layer formed on the worn surface is the key for decreasing the friction coefficients at high temperatures. When the content of BaF₂/CaF₂ is higher than 9 wt.%, the glassy layers forming on the worn surface can prevent the direct contact between composite disk and SiN₄ ball, which keeps the COF stable and the wear rates low.

The BaF₂/CaF₂ phase is stable at high temperatures, and it has good lubricating properties because of its lower melting point and shear stress. During the sliding test at high temperatures, the BaF₂/CaF₂ eutectic powder can coat on the worn surface easily. Therefore, sufficient and homogeneously distributed BaF₂/CaF₂ eutectic phase in the matrix is the very important for the self-lubricating effect. In addition, the oxides may soften at high temperatures and easily form a continuous layer with the BaF₂/CaF₂ eutectic phase. Therefore, the metal oxides can also improve lubricating properties at high temperatures.

For metallic solid self-lubricating materials, the lubricating phase can decrease the mechanical properties. It is important to find a balance between lubricating properties and mechanical properties. When the composite has 9-15 wt.% lubricants (Samples S₃ and S₄), it has the best combination of lubricating and mechanical properties (Table 5).

Table 5 Wear properties of Samples S₃ and S₄ and other powder metallurgy (PM) HEA self-lubricating materials

Compared with other self-lubricating materials of almost the same type of compositions (Table 5), the composites in this work have lower COFs and wear rates at 600 °C. In summary, the EHEA composites may have potential applications in high temperature service.

5 Conclusions

(1) The phase compositions of Fe₂₂Co₂₆Cr₂₀- Ni₂₂Ta₁₀-BaF₂/CaF₂ composites are of FCC, Laves phase, BaF₂, and CaF₂. The BaF₂ and CaF₂ show high chemical stability with the EHEA.

(2) The hardness of Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀- BaF₂/CaF₂ composite gradually decreases with the increase of BaF₂/CaF₂ eutectic powder content from HV 545 to HV 399.

(3) The EHEA composites show good lubricating performance at high temperatures. When the content of BaF₂/CaF₂ eutectic powder is 9-15 wt.%, both the COF and the wear rate at high temperatures are lower than other FeCoCrNi-based composites. The good lubricating performance is due to the effect of BaF₂/CaF₂ eutectic phase, the formation of complex oxides, and high hardness of the matrix.

Acknowledgments

The authors acknowledge the financial support from the National Natural Science Foundation of China (51671217).

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Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀高熵合金基复合材料的自润滑性能

肖维城，李 昆，韩六六，刘 咏

中南大学 粉末冶金国家重点实验室，长沙 410083

摘  要：共晶高熵合金(EHEAs)具有高温稳定性和良好的力学性能，并有望在高温下应用于摩擦领域。为了研究其高温润滑性能，采用BaF₂/CaF₂共晶粉末作为润滑相，通过放电等离子烧结制备Fe₂₂Co₂₆Cr₂₀Ni₂₂Ta₁₀- (BaF₂/CaF₂)_x (x=3%~20%，质量分数)复合材料，在室温和高温下测试摩擦性能和力学性能。结果表明，随着BaF₂/CaF₂共晶粉末的增加，复合材料在600和800 °C时的摩擦因数和磨损率明显降低。其中，共晶粉末含量为15%和20%的复合材料在600 °C时具有最佳的润滑性能。复合材料的低摩擦因数和低磨损率主要归功于共晶高熵合金基体良好的力学性能、BaF₂/CaF₂相的润滑作用以及在磨损表面上形成的连续氧化物。

关键词：共晶高熵合金；自润滑复合材料；BaF₂/CaF₂共晶粉末；摩擦磨损性能；显微组织

(Edited by Bing YANG)

Corresponding author: Kun LI; Tel: +86-731-88877322; E-mail: kunlee@csu.edu.cn

DOI: 10.1016/S1003-6326(21)65535-8

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press