Solid FeS lubricant: a possible alternative to MoS2 for Cu–Fe-based friction materials

Molybdenum disulfide (MoS2) is one of the most commonly used solid lubricants for Cu–Fe-based friction materials. Nevertheless, MoS2 reacts with metal matrices to produce metal sulfides (e.g., FeS) and Mo during sintering, and the lubricity of the composite may be related to the generation of FeS. Herein, the use of FeS as an alternative to MoS2 for producing Cu–Fe-based friction materials was investigated. According to the reaction principle of thermodynamics, two composites—one with MoS2 (Fe–Cu–MoS2 sample) and the other with FeS (FeS–Cu2S–Cu–Fe–Mo sample), were prepared and their friction behaviors and mechanical properties were compared. The results showed that MoS2 reacted with the Cu–Fe matrix to produce FeS, metallic ternary sulfides, and Mo when sintered at 1050°C. The MoS2–Cu–Fe and FeS–Cu2S–Cu–Fe–Mo samples thereby exhibited similar characteristics with respect to phase composition, density, hardness, and tribological behaviors. Micrographs of the worn surfaces revealed that the stable friction regime for both composites stemmed from the iron sulfides friction layers rather than from the molybdenum sulfides layers.


Introduction
Cu-Fe-based powder metallurgy (P/M) friction materials are now widely used in vehicle brake pads and industrial clutches because of their stable coefficient of friction (COF), good heat resistance, and high strength [1][2][3][4][5]. MoS 2 is one of the most commonly used solid lubricants for Cu-Fe-based friction materials; it can be used to stabilize friction and improve the wear resistance of composites [6][7][8][9]. However, numerous studies have shown that MoS 2 reacts with Cu or Fe matrices when sintered at temperatures above 950°C [10][11][12][13][14][15][16][17][18][19][20][21]; our recent study revealed that the lubricity of MoS 2 is attributable to the resultant formation of metal sulfides such as FeS [21]. Because of its layered structure, good thermal stability, and low cost, FeS may be a suitable alternative to MoS 2 lubricants in the production of Cu-Fe-based friction materials.
In this work, we investigated the use of FeS as an alternative to MoS 2 for Cu-Fe-based P/M friction materials. A sample with FeS (FeS-Cu 2 S-Cu-Fe-Mo system, abbre-viated FS) was designed to feature constituents consistent with the friction material containing MoS 2 (MoS 2 -Cu-Fe system, abbreviated MS). The two samples were compared by phase analyses, microscopy observations, and tests of their mechanical and friction properties.

Component design
The designed systems were MS and FS. MS embodied the common compositions of Cu-Fe-based braking materials, and FS served as a reference material for comparison. Fe (spherical), Cu (rod-shaped), MoS 2 (lamellar), FeS (irregular-shaped), Cu 2 S (granular), and Mo (ellipsoidal) powders were used as the main ingredients in the experiments (Fig. 1). Given the accuracy of X-ray detection, 10wt% of MoS 2 was added to the MS system. The dosages of FeS and Cu 2 S were selected on the basis of potential chemical reactions [19]. Detailed compositions are listed in Table 1.

Sample preparation
All powders were well blended proportionally according to Table 1 in a V-type mixer for 2 h and then cold pressed under 300 MPa. The compacted samples were sintered at (1050 ± 5)°C (the common sintering temperature for Cu-Fe-based composite materials) in a box furnace under a nitrogen-hydrogen (N 2 :H 2 = 3:1 by volume) atmosphere for 30 min and were then cooled to room temperature.

Performance testing
X-ray diffraction (XRD) was used to analyze the phase composition of the samples before and after sintering. Scanning electron microscopy (SEM) in conjunction with energy-dispersive X-ray spectrometry (EDS) was used to observe the microstructure of the samples and to analyze the constituents of the materials, respectively. The density and porosity of the sintered samples were measured by the Archimedes method. The hardness of samples was determined using a Vickers hardness tester (model MH-6) operated with a force of 200 N for 10 s. Compression strength was tested on a universal testing device (model WDW-50) using cylindrical specimens measuring φ7.8 mm × 10 mm. The average result of five tests is reported in this work.
Dry sliding wear tests were carried out using a pin-on-disc testing device (Plint TE-92 multifunction friction tester); the pin and disc featured three-point contact. The disc was made of forged steel (30CrSiMoVA) with a hardness of HRC 55-60, a surface roughness of 0.05 μm, and a friction diameter of 60 mm. The sintered composite materials were cut into cylindrical pins with a diameter of (7.9 ± 0.1) mm and a height of (20 ± 0.2) mm. The dry sliding tests were carried out at a sliding speed of 0.94 m/s (300 r/min) under a normal pressure of 0.35 MPa and a total duration of 60 s. Before each test, the pins were abraded with the disc under a normal pressure of 0.28 MPa and a rotating speed of 300 r/min to ensure uniform contact with the disc. Because the wear tests were conducted under relatively mild conditions, the wear loss of the samples was minimal; the wear-loss results are therefore not described in the text.

Phase analysis
The XRD patterns of the mixed powders and the sintered samples are shown in Figs. 2(a) and 2(b), respectively. All of the original components-MoS 2 , Mo, Fe, Cu, FeS, and Cu 2 S, were detected in the powders before sintering ( Fig.  2(a)). The peak intensity of a given powder depends on its concentration and degree of crystallinity, among other factors. In the as-sintered samples, no peak attributable to MoS 2 or Mo was observed in the pattern of either MS or FS. New peaks of phases such as FeS, CuFeS 2 , and Cu 1.84 Mo 6 S 8 were detected in the pattern of the as-sintered MS. Because MoS 2 cannot decompose under the sintering conditions used in this work [22], we speculated that MoS 2 reacted with Fe or Cu matrix to produce these complex sulfides and Mo. The presence of identical components in the patterns of as-sintered MS and FS confirmed this supposition. No Mo peak was observed in the pattern of either sample because of the partial dissolution of this element into the metal matrices [23], which will be discussed later. Fig. 3 shows the backscattered electron (BSE) images of the sintered samples. Six regions, which are marked with different colors, are observed in the image of MS (Fig. 3(a)). On the basis of the corresponding EDS results, the gray regions represent Fe and the light-gray regions represent Cu.

Microstructural analysis
The other three regions are assumed to be associated with the products of reactions between MoS 2 and the matrices. White particles represent Mo, which was produced during the sintering and distributed unevenly in the matrices. The particles represented by blue arrows consisted mainly of Fe and S in an atomic ratio of approximately 1 (Fig. 3(c)) according to the EDS analysis results. Combining these findings with the XRD results, we deduced that these regions were FeS and were preferentially located adjacent to Mo particles. The last regions (red arrows) were determined to contain Cu, Fe, S, and Mo; we therefore identified these regions as metallic ternary sulfides CuFeS 2 and Cu 1.84 Mo 6 S 8 . These sulfides tended to locate at the border of Fe and Cu. Several pores (the sixth region) were also observed in the sample.
The compositions of the six regions in the BSE images of FS were identical to those in the BSE images of MS; however, each region in FS was finer and distributed more uniformly than the corresponding regions in MS. This finding is attributed to the inhibition of mutual contact between the particles as a result of the presence of more additives in the FS system.

Mechanical properties
The porosity (ε), density (ρ), compressive strength (σ), and Vickers hardness (HV) of the sintered samples are illustrated in Fig. 4. The FS displayed a higher porosity than

Fig. 4. Properties of the samples: (a) porosity (ε) and density (ρ); (b) compressive strength (σ) and Vickers hardness (HV).
MS as a result of the presence of more particle interfaces in the FS sample. The densities of the two samples were approximately equal. The MS exhibited a higher compressive strength than FS, whereas their Vickers hardnesses were approximately equal ( Fig. 4(b)). These results indicate that FS exhibits micromechanical properties similar to those of MS, although FS exhibits inferior macromechanical properties because of its higher porosity. In addition, the reaction between MoS 2 and the matrices promotes the diffusion of Mo (from MoS 2 particles) into the iron matrix to form an Fe(Mo) solid solution, which reinforces the composites [24].

Comparison of friction behavior
The constant-speed COF of the samples is illustrated in Fig. 5. The initial sharp increases in the COF of both sam-ples, where the COFs increase linearly until a normal pressure of 0.35 MPa is achieved, represent a loading stage. The COFs of both samples are nearly identical with respect to both the COF values and their fluctuation ranges (0.53-0.59).
To further explain the similarities in the COFs of the two systems, we obtained representative BSE images of the worn surfaces of the MS and FS specimens (Fig. 6). Notably, both samples' surfaces were covered with an integrated and continuous antifriction layer such that little of the matrix was observed. EDS analysis demonstrated that the layers of both samples had similar chemical compositions: Fe, Cu, S, O, and Mo, as shown in Fig. 7. Given the concentrations of these elements, the main components of these layers were assumed to be iron sulfides and oxides. Such sulfide and oxide coatings can be generated by the smear effect or by the oxidation of FeS and metallic ternary sulfides. The anti- friction layer was found to be well adhered to the matrices and played an important role in preventing metal-on-metal contact during the rubbing process, thereby ensuring a stable and low COF. Consequently, we concluded that the two samples abide by the same friction mechanism and that the produced iron sulfides rather than the MoS 2 itself enhances the lubricity.   Fig. 6(a) and (b) box 2 in Fig. 6(b).

Conclusions
The use of FeS as an alternative to MoS 2 for Cu-Fe-based P/M friction materials was investigated. The two samples-MS and FS, were compared on the basis of phase analyses, microscopy observations, and tests of their mechanical and friction properties. The main conclusions deduced from the results of this study are summarized as follows: (1) MoS 2 reacted with the Cu-Fe matrix under the sintering conditions to produce FeS, metallic ternary sulfides, and Mo. As a result, the phase compositions of the MS system are identical to those of the FS system.
(2) The MS displayed a lower porosity but higher com-pressive strength than FS, whereas the densities and Vickers hardness of both samples were approximately equal.
(3) FeS and metallic ternary sulfides were uniformly distributed in the matrices for both the MS and FS systems. During the rubbing process, the worn surfaces of both samples were covered with an integrated and continuous antifriction layer, which led to their similar friction behaviors.
(4) The two samples abide by the same friction mechanism, and the lubricity benefits from the produced iron sulfides rather than from the MoS 2 itself. Given the high cost and lability of MoS 2 , FeS may be a desirable alternative to MoS 2 for producing Cu-Fe-based friction materials.