1. Introduction
With the continuous development of aerospace technology, the extreme performance of the equipment continues to make breakthroughs. Bearing is one of the key basic components of modern mechanical equipment. The ideal performance requirements of spindle-bearing materials are high surface hardness, suitable toughness, high corrosion resistance, and high temperature resistance. The third-generation CSS-42L bearing steel is one of the preferred bearing steel materials, which is comprised of high strength, high toughness, and relatively suitable corrosion resistance [
1]. However, its wear resistance and corrosion resistance need to be further improved to accommodate the complex and harsh working conditions. Several surface modification methods have been attempted. Qiu et al. [
2] found that the corrosion resistance and mechanical properties of CSS-42L-bearing steel could be modified by the duplex treatment of ions implantation and Cr coating. The corrosion rate of the (Ti + C) co-implanted samples was more than 10 times lower than that of the untreated samples. Yang et al. [
3] studied the comprehensive properties of CSS-42L specimens after carburizing and nitriding at different quenching temperatures. The results indicated that nitriding treatment can not only improve the hardness and wear resistance of CSS-42L carburized steel but also improve its corrosion resistance to a certain extent. However, the application of chemical heat treatment techniques is limited due to the high requirements for specific experimental conditions. Available research has indicated that the higher experimental temperature (generally above 600 °C) would deteriorate the mechanical properties of the bearing matrix [
4].
High-entropy alloy (HEA) is one sort of novel multi-element alloy with higher mixing entropy, consisting of five or more main elements in equal or unequal molar ratios, discovered by Ye et al. [
5] in 2004. HEA film developed on the basis of HEA with low-dimensional morphology exhibits properties similar to those of bulk high-entropy alloys, such as thermodynamic high-entropy effects, kinetic slow diffusion effects, severe distortion effects of lattice structures, and performance cocktail effects, which can be used for hard wear-resistant coatings for cutting tools, corrosion-resistant coatings for components service in a corrosive environment, diffusion barrier layers in the field of Cu interconnects, etc. [
6]. Moreover, some properties of HEA coating are even superior to its bulk material, such as hardness [
7]. Some researchers have suggested that the introduction of interstitial elements such as C, N, and O can further improve the mechanical properties of HEA films [
8,
9,
10]. Braic et al. [
8] co-sputtered (CuSiTiYZr)C films with different flow ratios in Ar + CH4 atmosphere. The experimental results showed that (CuSiTiYZr)C coatings with a carbon/metal ratio of about 1.3 had the highest hardness and the best wear and corrosion resistance. Zhang et al. [
11] successfully fabricated a FeCrNiCoBx HEA cladding layer on 304L stainless steel by laser cladding method. The in situ formed orthorhombic (Cr, Fe)
2B phase improves the microhardness and corrosion resistance in 3.5 wt.% NaCl solution when x ≤ 1. However, when x = 1.25, the borides transform into tetragonal (Fe, Cr)
2B phase, contributing to the decreased corrosion resistance. Hwai-Te et al. [
12] investigated the effect of nitrogen content and substrate bias on the mechanical properties and corrosion performance of HEN thin films (AlCrSiTiZr)
100−xN
x. They found that the (AlCrSiTiZr)
100−xN
x deposited film had the best corrosion resistance in 0.1 mol/L H
2SO
4 when RN = N
2/(N
2 + Ar) = 30%, and its Icorr is almost only 10% of the 6061 aluminum alloy matrix, exhibiting excellent corrosion resistance potential. More and more scholars are interested in the study of high-entropy nitride alloy films due to their high hardness, high strength, high wear resistance and high corrosion resistance [
10].
Comparison of the current methods commonly used to prepare HEA films, including magnetron sputtering (MS) [
13,
14], laser cladding [
15,
16], spraying [
17], electrodeposition [
18], plasma-transferred arc cladding [
19], etc. Among the technologies mentioned above, MS has become the most commonly used technology to process HEA films due to its advantages of higher deposition rate, a more uniform structure, and easier to introduce gas elements in alloys to synthesize nitride, oxide, or carbide. The HEA films obtained by MS are mostly dense in structure and have a significant “quick quenching” effect [
20]. The element diffusion and nucleation of intermetallic compounds are inhibited. FCC or BCC solid-solution phase or amorphous phase may be generated, which is beneficial to wear resistance and corrosion resistance. Dou et al. [
14] deposited FeAlCoCuNiV HEA coatings by DC magnetron sputtering and tested its electrochemical behavior in various corrosive media. The experimental results displayed that the coating was composed of a single FCC solid solution and exhibited more superior corrosion resistance than 201 stainless steel in acidic, alkaline, and salt corrosive media. Zheng et al. [
13] prepared VAlTiCrSi HEA films with excellent corrosion resistance in artificial seawater by DC magnetron sputtering. Diffraction of X-rays (XRD) and Transmission Electron Microscope (TEM) results indicated that HEA film presented a uniform amorphous structure, thereby enhancing the barrier properties of the substrate. In addition, MS can flexibly control sputtering parameters to tune the properties of thin films.
TiAlMoNbW is a novel high-entropy alloy system. A number of previous works have shown that metallic elements, such as Al, Ti, Nb, Mo, and W, have an obvious function in promoting the mechanical properties of different HEAs. In addition, the Al element can also improve the corrosion resistance of the HEAs, which would be facilitated by Nb and W elements [
21,
22]. Furthermore, Al, Ti, Nb, and Mo have a strong affinity for nitrogen. The forming nitrides possess higher hardness and chemical inertness, which may further contribute to the wear resistance and corrosion resistance of HEA films [
23]. Thus, it is promising to introduce TiAlMoNbW HEA film and its HEN film to improve the wear and corrosion resistance of CSS-42L steel. As far as we know, there is a lack of systematic research on the mechanical and electrochemical properties of coated steel substrates with TiAlMoNbW HEA and HEN films.
In this study, the TiAlMoNbW HEA film and its nitrid film were deposited on the third-generation aerospace-bearing steel CSS-42L by reactive MS technology. The microstructure and surface modification effect of TiAlMoNbW HEA film and HEN film for CSS-42L steel were evaluated. The improvement effects of TiAlMoNbW HEA film and HEN film on the mechanical properties and corrosion resistance of CSS-42L steel were also deeply discussed.
2. Materials and Methods
2.1. Sample Preparation
The composition of CSS-42L steel used in this research is shown in
Table 1. The tested steel was firstly prepared by a vacuum induction melting furnace using Fe-4.3C, Fe-20Si, and Fe-50Mn intermediate alloys. Then the CSS-42L steel ingots were obtained according to the ratio, and the matrix specimens of 10 mm × 10 mm × 4 mm were prepared. Before the deposition process, the base steel was ultrasonically cleaned in ethanol and then dried in a drying oven.
The TiAlMoNbW target was prepared by using the powder metallurgy method. Primarily, the 99.99 wt.% purity Ti, Al, Mo, and Nb blocks were used to produce Ti-25Al-25Mo-25Nb (in at.%) master alloy by using a skull vacuum furnace after being cleaned and dried with acetone. Then, the TiAlMoNb alloy powder was obtained by using the rotary spray method with a ~200 mesh particle size. After being mixed uniformly with 99.995% purity, 2000 mesh tungsten powder, the disc target with a diameter of 4 inches and thickness of 3 mm was produced ultimately by using the vacuum hot-pressing method (parameters are shown in
Table 2). The CSS-42L samples deposited with TiAlMoNbW alloy (HEA) film and TiAlMoNbW nitrid (HEN) film were abbreviated to S-HEA and S-HEN, respectively. The synthesis of HEA film and HEN film were conducted by using M600 magnetron sputtering system in an atmosphere of high-purity gas with a working pressure of 2.5 × 10
−3 Pa. The samples were subjected to a 15 min bias cleaning to remove surface contaminants and oxide layers prior to deposition. The specific preparation parameters of the film are shown in
Table 3.
2.2. Sample Characterization
The crystal structure of the films was characterized by grazing-incidence-angle X-ray diffraction (GIXRD, D/MAX-2500/PC, Rigaku, Tokyo, Japan) with a scanning range of 20°–90°. The film thickness and surface topography were observed using a field emission scanning electron microscope (FE-SEM, S-4800, HITACHI, Tokyo, Japan). The surface roughness and arithmetic mean deviation of the profile (Ra) of each coating were measured using an atomic force microscope (AFM, FM-NanoviewOp, FEISHIMAN, Shaanxi, China) in tapping mode. A nano-indenter (Nano Indenter G200, Agilent, Santa Clara, CA, USA) equipped with a Berkovich diamond probe tip was performed to measure the hardness and elasticity modulus of films using continuous stiffness mode (CSM). Seven points were tested for each film and averaged. The indentation depth was controlled to be 500 nm. The wear resistance of the films was tested by a reciprocating friction and wear apparatus (UMT-3, CETR, Billerica, MA, USA). The frequency is 5 Hz, the load is 5 N, and the friction pair is Si3N4 with a diameter of 6 mm. The wear track length and sliding time were 5 mm and 30 min, respectively. The cross-sections of the wear profile were measured by a surface profilometer (Alpha-Step IQ, Milpitas, CA, USA). An electrochemical potentiostat (PARSTAT MC, AMETEK, Berwyn, IL, USA) was used to conduct the corrosion experiments of HEA films and CSS-42L substrates in different corrosive reagents. A three-electrode connection was used to connect a saturated calomel reference electrode (SCE), a platinum counter electrode, and a working electrode.