Microstructure, mechanical and tribological properties of CrSiC coatings sliding against SiC and Al₂O₃ balls in water

Highlights

• CrSiC coatings with Si content of 2.0–7.4 at.% were deposited via adjusting the TMS flow.

• The amorphous structure in the CrSiC coatings was presented.

• No obvious fluctuations of hardness (about 13 GPa) were observed with TMS flow.

• CrSiC/SiC tribopairs showed better tribological performance than CrSiC/Al₂O₃ tribopairs.

Abstract

CrSiC coatings with different silicon contents were prepared using unbalanced magnetron sputtering via adjusting trimethylsilane (Si(CH₃)₃H) flows. Their phase structure, bonding structure, microstructure and hardness were characterized by X-ray diffraction (XRD), X-ray photoelectrons spectroscopy (XPS), a field emission scanning electron microscope (FESEM) and nano-indenter, respectively. The tribological properties of CrSiC coatings sliding against SiC and Al₂O₃ balls were investigated in water. The results showed that the CrSiC coatings were speculated to be X-ray amorphous. Although the hardness of coatings fluctuated slightly (13.2–13.8 GPa), the coatings showed poor wear resistance due to the decline of the crack resistance and toughness. Moreover, the friction coefficient (0.24–0.31) and the wear rate (2.97–7.66 × 10⁻⁶ mm³/Nm) of CrSiC/SiC trobopairs were lower than those of CrSiC/Al₂O₃ tribopairs.

Introduction

Recently, water-lubrication systems have already been proposed to replace oil-lubrication systems in the fields of food and medicine factories, drainable pumps and hydraulic systems because of pollution caused by oil-lubrication systems. But water lubrication systems present some technical problems for metallic materials, such as lubricity, corrosion and reliability [1]. In view of the above situation, the surface modification of stainless steels via depositing hard coatings was an effective method. At present, carbon-based coatings and transition metal nitride-based coatings are the promising candidates for the sliding parts in water hydraulic systems, the journal bearings and mechanical face seals for water pump [2], [3]. Table 1 in Ref. [4] shows that the CrN-based coatings have been proved to be suitable in water lubrication. However, their low hardness and high friction coefficients would limit their wide application in industry. Although the carbon-based coatings (such as diamond, DLC, a-CN_x and GLC) have exhibited low friction coefficient and low wear rate as sliding against ceramic and steel balls in water [2], [5], [6], [7], [8], their poor adhesion strength to substrate, high internal stress and weak thermal stability also worsen their application life [5], [9], [10]. According to the results in Refs. [11], [12], [13], [14], [16], [17], it is clear that doping transition metal elements (Ti, Cr, etc.) into the carbon-based coatings could effectively reduce the internal stress and improve the adhesion strength of coatings. As seen in Table 1, Refs. [9], [14], [15], [16] have reported that the low friction coefficient (μ = 0.1–0.3) was obtained when the Cr-DLC coatings with low Cr content slid against steel in the 40–50% humidity. Zhou's groups [12], [13] also reported that Cr/a-C films sliding against different mating balls (stainless steel, Al₂O₃, SiC and Si₃N₄) presented good water lubrication characteristics. A low friction coefficient (μ = 0.07–0.3) was obtained with low Cr content (≤4.9 at.%), while the friction coefficient and wear rate all increased with increase of Cr content (12.0–14.1 at.%) due to serious abrasive wear. Similarly, Keunecke et al. [18] also pointed out that the CrC/a-C:H coatings with high Cr contents (Cr > 50 at.%) provided poor wear resistance under oil lubrication. In addition, the influences of crystalline structures (amorphous structure, nanocomposite and crystalline composite) in CrC system on the mechanical and tribological properties of CrC films have been investigated [19], [20], [21], [22]. For example, Jellad et al. [23] have reported the crystalline Cr₃C₂ film (partially crystallized) exhibited a higher hardness (24–25 GPa) than amorphous Cr₃C₂ (18 GPa). Though the corrosion and oxidation resistance of a-CrC/a-C coatings were improved via increasing a-C content to form dense microstructure [19], [20], [21], their corresponding hardness (6.9–13 GPa) usually was lower than that of crystalline CrC coatings (18–25 GPa) [19], [23], [24]. Furthermore, because the high friction coefficient (0.68–1.04) was obtained when the CrC_x coatings slid against steel ball in air [17], the CrC_x coatings could not satisfy the application requirements under extreme conditions. Due to surface graphitization during sliding, the nc-Cr_xC_y/a-C:H coatings displayed low friction coefficient [22], [25], [26], but the friction property of hydrogen containing carbon-based film is susceptible to temperature [10]. Thus, how to further improve the mechanical and tribological properties of CrC-based coatings are key factors to the application of engineering.

In recent years, it has been found that the silicon was one of the most effective alloying elements to improve mechanical and tribological properties of coatings [27], [28], [29], [30]. According to Refs. [31], [32], [33], the hydration reaction of silicon-contained compounds was easily occurred to form the Si(OH)_x gel on the friction surface as sliding in water, and the Si(OH)_x gel as self-lubricating transfer layer was beneficial to improve the wear and friction properties of coatings in water [29], [30], [31]. For example, Refs. [27], [28], [29] have reported that the mechanical properties, corrosion resistance and oxidation resistance of CrN coatings were enhanced via introducing silicon into coatings. Furthermore, the friction and wear properties of CrSiN and Si-DLC coatings were also improved due to the tribochemical reaction in water, which was attributed to the formation of silicon nitride or silicon carbide phase [29], [30]. For example, Geng et al. [29] have reported the friction coefficient of CrSiN coatings (μ = 0.35) with Si/(Cr + Si) of 8.9 at.% was lower than that of CrN coatings (μ = 0.46) as sliding against WC balls in water, and their wear resistance was also improved. In our previous reports [4], the CrSiCN with 2.1 at.% Si showed low friction coefficient of 0.11 and wear rate 8.4 × 10⁻⁸ mm³/Nm as sliding against SiC balls in water. Ziegele et al. [34] have compared the friction behavior of single layer SiC, CrC and multilayer CrC/SiC coatings against a fixed SAE 52100 steel ball in air, the friction coefficients of all multilayer coatings kept at around 0.2 and was lower than that of single layer, whilst higher wear was obtained as compared with single layer. Bertóti et al. [35], [36] have studied the composition and chemical structure of CrSiC coatings as a function of silicon content, and reported that the hardness and Young's modulus of ternary CrSiC coatings with the Si content of 12.8–25.7% varied between 13–16 GPa and 120–140 GPa. The CrSiC films presented a significant elasticity and scratch resistance by nanoscratch tests, and the silicide (Cr_xSi) might been formed at relatively high Cr and low Si content [37]. However, as a function of Si content, transition metal nitride or carbide coatings with low silicon content such as CrSiN (C_si = 4.1–6.7 at.%) [27], [38], TiSiC (C_si = 9–16 at.%) [39] and CrSiCN (C_si = 0–3.4 at.%) [4] presented better mechanical properties that were attributed to the solid solution hardening and the formation of nanocomposite microstructure. But with further increasing Si content, superfluous amorphous silicides (a-Si₃N₄, a-SiC) were formed at the grain boundary of nanocrystallites that weaken the grain boundary strength and the wear resistance of coatings. In addition, the compressive stress in the CrSiC coatings reduced with an increase in Si content, while their crack resistance became poor when the Si content was higher than 3.5 at.% [40]. However, the microstructure and tribological behavior of CrSiC coatings with low Si content in water lubrication have not yet been studied.

In here, the CrSiC coatings with low silicon contents were deposited on Si (100) wafers and 316L stainless steel disks using unbalanced magnetron sputtering via adjusting trimethylsilane [(CH₃)₃SiH or TMS] flow, and the influence of TMS flow on the microstructure, mechanical properties and tribological properties of CrSiC coatings in water was outlined.