Study on the Mechanical and Tribological Properties and the Mechanisms of Cr-Free Ni-Based Self-Lubricating Composites at a Wide Temperature Range

: A Cr-free Ni-based self-lubricating composites with MoS 2 and Ag as lubricants were fabricated by the powder metallurgy method. The microstructures were examined. The mechanical properties and tribological behaviors of the composites were evaluated from room temperature to 800 °C. The fractography was observed and the fracture mechanisms were analyzed. The morphologies and the phase compositions of worn surfaces were determined and the wear mechanisms were elaborated. The results indicate that MoS 2 did not completely decompose after sintering, and the NiMoAl-MoS 2 -Ag composite has the best tribological properties (0.22, 1.68 × 10 − 5 ) at 800 °C. The main wear mechanisms are micro-ploughing and plastic deformation. The improvement of tribological properties was attributed to the formation of the lubricating film consisting of NiO, Mo oxides, various molybdates, and Ag particles. The reactions resulting in these compositions are proposed. The mechanical properties degrade with the rise of temperature and the addition of lubricants. Both NiMoAl and NiMoAlAg alloys exhibit micro-void accumulation fracture while the composites with MoS 2 reveal intergranular fracture. Tests were conducted under a test Before The which come from composites. Raman analysis shows the formation


Introduction
Metal matrix composites (MMCs) have been widely used in many fields such as the automobile industry, the nuclear power industry, and the aerospace industry due to their excellent mechanical properties, wear resistance, and the small thermal expansion coefficient [1][2][3]. As the working temperature of advanced engines increases continuously, the parts need to endure the high temperature and large temperature variation (start-stop and run), and, at the same time, most machine parts need to work under complex stress for a long time. The strength and lubrication of moving parts have become a key factor affecting the reliability and life of the entire system. Therefore, the unification of mechanical and lubricating properties over a wide temperature range (RT~800 °C) will become an important trend for the development of bulk materials [4,5]. Nickel (Ni) has a series of advantages such as high strength, satisfactory oxidation, hot corrosion resistance, and good microstructure stability. In addition, Ni can become a solid solution made of other metal elements and does not precipitate in a deleterious phase. Thus, Ni is an excellent matrix metal and has been widely applied in advanced engines. Introducing eligible solid lubricants into strengthened nickelbased alloys is an effective way to achieve the unification of high mechanical properties and good tribological behaviors, which has attracted the attention of researchers at home and abroad [6][7][8][9][10][11][12][13].
Graphite and molybdenum disulfide (MoS2) are a common solid lubricant and have low shear strength due to the lamellar structure. However, they will oxidize or decompose and lose lubrication at temperatures about 400 °C [14]. Silver (Ag), which is a soft metal, has often been added into composites as a lubricant serving at medium and low temperatures due to its low shear strength and large diffusion coefficient during sliding [15]. Metal oxides, fluoride, and oxysalt are high temperature lubricants [16][17][18]. Molybdates have gained increasing attention due to the lower friction coefficients at high temperatures [16,[19][20][21]. Adding low and high temperature lubricants to the metal matrix can achieve continuous lubrication in a wide temperature range, but may lead to a significant decline in the comprehensive mechanical properties of the composite. Thus, an alternative approach of taking advantage of the synergistic effect of the extrinsic low temperature lubricants and the in-situ formed high temperature lubricants generated by a tribo-chemical reaction was proposed [22,23]. For instance, Aouadi et al. [22] studied the tribological properties and mechanisms of Mo2N/MoS2/Ag coatings and found that the coatings exhibited lower friction coefficients (0.1~0.4) and wear rate (10 −5~1 0 −7 mm 3 /Nm) from room temperature (RT) to 600 °C, which is due to the synergistic effects of Ag and lubricious silver molybdate phases during the wear test. The anti-friction mechanism of silver molybdate at high temperatures was attributed to the lamellar structure [24]. Furthermore, NiMoAl-Cr2O3-based composites containing MoS2 and Ag were reported to show wear rates (10 −5~1 0 −4 mm 3 /Nm) and friction coefficients (0.18~0.77) from RT to 700 °C when tested against Inconel 718 using a pin-on-disk high temperature tribometer [23] and the in-situ-formed silver molybdate was responsible for the good tribological properties at 700 °C. However, the mechanical properties of these materials in a wide temperature range, especially the high temperature mechanical properties are seldom reported. Most of the strength-related studies of self-lubricating materials only focus on the strength at RT. For example, Xiong et al. [25] investigated the flexural strength and tensile strength of Ni-based composites at RT as well as Ding et al. [26] tested the tensile strength of the PM304 composite at RT. Yang et al. [27] investigated the tensile and compressive properties of NiCrMoAl alloys from RT to 1000 °C, but the study is preliminary and needs further work. Furthermore, the reports on the deformation and fracture mechanisms of lubricating materials are even fewer. Consequently, the mechanical properties, the deformation, and fracture mechanisms of composites at a wide temperature range are attractive to the authors. Accordingly, in this paper, the Nickel-base composites with Ni as the matrix, Mo and Al as the strengthening elements, and Ag and MoS2 as the lubricants were prepared by a powder metallurgy (P/M) method. Cr is an important strengthening element in Ni-base superalloys and composites. However, in this paper, the Cr was excluded deliberately due to the possible complexity induced by CrxSy [28][29][30] on the studies of mechanical and tribological properties. The mechanical and tribological properties of the Cr-free Nickel-based composites from room temperature to 800 °C will be synthetically studied. The fractography of composites at different temperatures will be investigated to elucidate the deformation-fracture mechanisms. Meanwhile, the morphologies and compositions of worn surfaces will be analyzed to understand the wear mechanisms.

Materials and Methods
The Nickel-based composites containing different contents of lubricants (see Table 1) were prepared by a powder metallurgical method. The raw materials were commercial Ni powder (60 μm, 99.50% purity, Jinchang Changqing, Jinchang, China), MoS2 powder (1.5 μm, 98.50% purity, Shanghai Shenyu, Shanghai, China), Mo powder (48 μm, 99.95% purity), Al powder (48 μm, 99.80% purity), and Ag powder (25 μm, 99.99% purity). The latter three powders were purchased from Beijing Xing Rong Yuan Technology Co., LTD. The proportionally weighed powders were first ball-milled in zirconia jars and balls (balls to powders ratio of 10:1) by a planetary ball mill machine (QM-3SP4, Nanjing university instrument plant, China) with a rotational speed of 400 rpm for 20 h. Then the powders after being milled were loaded into graphite die and hot pressed at 1050 °C-1100 °C with holding time of 30 min~1 h under the pressure of 25 MPa in a vacuum hot press sintering furnace (ZT-45-20Y, Shanghai Chen Hua, China) with the dynamic vacuum of about 10 −2 Pa. After cooling, the sintered bulks were machined into the desired samples, and the surfaces were polished with 800 meshes emery paper and ultrasonically cleaned with acetone for the following tests and analyses.
The densities of composites are measured by using the AccuPyc 1330 automatic true density analyzer (AccuPyc 1330, Micromeritics, Atlanta, GA, USA). The hardness tests of the samples were conducted using an MH-5-VM microhardness tester (Shanghai Hengyi Science and Technology Corporation, Shanghai, China) on polished surfaces. For each sample, at least 10 tests were made with a load of 3 N and a hold time of 5 s, and the average values are given in Table 1.
The tensile and compressive tests at different temperatures were conducted in Instron 5582 (Instron Corporation, Norwood, MA, USA) and MTS E45.105 universal testing machines (MTS Systems Corporation, Eden Prairie, MN, USA) under atmospheric conditions. The specimens for the tensile and compressive tests are thin slab with the gauge dimension of 20 mm × 4 mm × 2 mm and the cylinder with a size of Φ 6 mm × 9 mm, respectively. The relevant standards for tensile and compression tests were GB/T 228.1-2010, GB/T 228.2-2015, and GB/T 7314-2007. All mechanical tests were performed at least three times under the same conditions to ensure the repeatability of test results, and the average values were shown. Before the test, the surfaces of each sample were ground to 600 # emery paper and then ultrasonically cleaned with acetone.
The friction and wear tests were using a ball-on-disk high temperature tribometer (UMT-3, Bruker Corporation, Billerica, MA, USA) under a dry sliding condition in air. The rotating disk samples were the Ni-based composites with a size of Φ 24 mm × 3 mm. The mating balls are Al2O3 (HV 16.5 GPa) with a diameter of 10 mm. Tests were conducted under a normal load of 20 N, a rotational speed of 200 rpm, and a wear track radius of 5 mm. The test temperatures were chosen to be RT, 400 °C, and 800 °C with a sliding distance of 377 m and duration time of 60 min. Before the test, all the specimens were ultrasonically cleaned in acetone. The friction coefficients were continuously recorded by the computer's own software (UMT Test, Bruker Corporation, Billerica, MA, USA) during the test. The volume loss ∆V was obtained by the equation below.
where R was the radius in meter and S was the abrasion profile area. Then the wear rate ω was calculated by Equation (2) below.
where ω was the wear rate (mm 3 /Nm), ∆V was the volume loss (mm 3 ), N was the load (N), and L was the sliding distance (m). The Non-contact optical profilometer (MicroXAM-800, KLA-Tencor Corporation, Milpitas, CA, USA) was used. The surface profile information can be obtained by micro interferometry and the area S of the wear trace profile of the composite can be calculated by software (Origin v8.0773, OriginLab Corporation, Northampton, MA, USA) integration. All the tribological tests were carried out at least two times under the same conditions to ensure the repeatability of the results, and the average results were given. The phase structures and compositions of the samples were determined by X-ray diffraction analysis (D8 Discover high resolution, Bruker, Billerica, MA, USA) with Cu-Ka radiation in the 2θ range of 10°-80°. The microstructures, fractography, and morphologies of the wear surface of the samples were examined by scanning electron microscopy (SEM, Apreo S, Thermo Fisher Scientific, Waltham, MA, USA) equipped with energy dispersive spectroscopy (EDS). The contrast of backscattered electron imaging (BEI) was caused by different atomic numbers, so backscattered electron imaging (BEI) are generally used to distinguish different phases. The phase composition of worn surfaces was further analyzed by Confocal Micro Raman spectrometer (LabRAM HR Evolution, HORIBA Scientific, Paris, France) with a laser wavelength of 532 nm. In the field of tribology, Raman spectroscopy can be used to analyse the phase structure of the local area of the wear surface, which can better reflect the change of wear surface composition with temperature. This reveals the lubrication mechanism. Table 1 lists the hardness and densities of the composites. It can be seen that, compared with the N alloy, adding MoS2 alone (NM) decreases the hardness and density. Introducing Ag alone (NA) significantly reduces the hardness, but increases the density of the material, while additions of Ag and MoS2 together (NMA) lower the density and hardness of the composites, and the density and hardness values of NMA are between that of NA and NM. Figure 1 displays the X-ray diffraction (XRD) patterns of the mixed powders and sintered composites. The XRD results of mixed powders indicate that the peaks of Ni become weaker and broad after milling with the addition of Ag and MoS2 because of the grains refinement and the internal stress of phases [31]. After hot press sintering, the peaks of Al2O3 appeared, and Mo diffraction peaks disappear while hinting the solid solution of Mo into the Ni-based solution. For the MoS2-containing composites (NM and NMA), after hot pressing, the MoS2 peaks can also be detected, which indicates that MoS2 did not decompose completely. This is inconsistent with Li and Xiong's reports [25].     Figure 3 shows the friction coefficients and wear rates of sintered Ni-based composites at different temperatures. It can be seen that, with the rise of temperature, the friction coefficients are lower generally, except the N and NA alloy. Both of them reveal the higher friction coefficients at 400 °C than at RT and 800 °C.

Tribological Properties
At RT, the wear rates of the four composites are favorable and on the order of 10 −5 . While at 400 °C, the wear rates increase significantly and reach 2 × 10 −4 mm 3 /Nm. When the temperature rises to 800 °C, the wear rates sharply drop to below 5 × 10 −5 mm 3 /Nm. The N alloy has the lowest friction coefficient and wear rate at RT, while the NMA composite exhibits decent tribological properties at temperatures above 400 °C.

Mechanical Properties
The tensile and compressive strengths of the four composites at RT, 400 °C, and 800 °C are shown in Figure 4. With the increase of temperature, the strengths of the composites decrease, except for a slight increase in compressive strength of NM at 400 °C. Clearly, the Ni-based alloy without any lubricants has superior mechanical properties in the whole temperature range demonstrating that the lubricants decrease the mechanical strengths. Then, compared with other composites, the NA composite with Ag reveals relatively high mechanical properties in the range of RT~800 °C. The tensile and compressive strengths of the four composites at 1000 °C are also tested (not displayed in Figure 4), but the composites containing MoS2 (NM and NMA) are so weak at this temperature that the strengths are negligible.

Microstructures of the Composites
From the XRD patterns ( Figure 1) and SEM micrographs (Figure 2), it can be observed that, after sintering, an appreciable MoS2 phase still exists. Few of them decompose during sintering, which is somewhat surprising since, in the previous report, most MoS2 decomposed during the sintering process and transformed into sulfide [28]. The main reason may be the absence of Cr in this study. In the previous study [32,33], the MoS2 react with Cr as follows: Cr + MoS2 → CrxSx+1 + Mo (x = 3,5) [25], and the element Cr acts as a "catalysis" and accelerate the decomposition of MoS2. However, in this study, without the "catalysis", the sintering temperature of 1050-1100 °C is not high enough for the decomposition of MoS2, which begins to decompose above 1370 °C.

Tribological Properties and Wear Mechanisms
The worn surface morphologies and the XRD patterns of the four composites after wear tests at different temperatures are displayed in Figures 5 and 6. At room temperature, for the N alloy, the shallow grooves and micro-cracks appear on the worn surface, and the main wear mechanism is micro cutting and micro-crack (Figure 5a1). For NM, many spalling and delamination cover the worn surface (Figure 5b1), and the wear mechanism is adhesive wear. For NA, grinding debris and spalling appear on the worn surface (Figure 5c1), and the wear mechanism is dominated by abrasive wear. For NMA, the spalling and cracks of the worn surface can be clearly observed, and the wear mechanism is dominated by adhesive wear and a micro-crack (Figure 5d1). The XRD patterns demonstrate no new phases emerge during the RT friction test. The tribological properties and wear mechanisms should be explained by the intrinsic microstructures of composites. For the N alloy, the micro-surface of the sintered material is relatively compact (Figure 2a) and results in slight wear, which corresponds to a lower friction coefficient and wear rate (Figure 3). For NM, it seems that the remaining MoS2 does not play the expected lubricating role, which can be explained that the MoS2 particles are agglomerated and do not form a continuous lubricating film, and another explanation could be that the lubrication effect of MoS2 is less than that of surface roughness caused by its addition. A similar explanation has been proposed by Ramalho [34]. For NA, the reason may be that the Ag phase distributed in the whole sample and, on the surface, it is not enough to form a continuous lubrication film. For NMA, the agglomerated MoS2 and scattered Ag cannot provide the effective lubrication.
At 400 °C, for the N alloy, the worn surface of the former becomes rough and many spalling pits and delamination appear, and the wear mechanism is adhesive wear (Figure 5a2). For NA, the wear scar is wide and deep on the worn surface and produces a lot of wear debris alongside the peeling pits, and the wear mechanism is dominated by abrasive wear (Figure 5c2). For NM, the grooves with wear debris occur on the worn surface, which indicate the wear mechanism is dominated by abrasive wear (Figure 5b2). For NMA, the grooves together with large pieces of spalling exist on the worn surface, and the wear mechanism is adhesive wear (Figure 5d2). XRD patterns show the new oxides (NiO and MoO2) appear. The sharp increase of wear rates at 400 °C is attributed to the decrease of hardness and the fragmentary oxides do not form continuous film but act as abrasives instead.
At 800 °C, for the N alloy, on the worn surface, there are large pieces of spalling pits and delamination, and the wear mechanism is mainly adhesive (Figure 5a3). For NM, the worn surface appears as a relatively smooth film with some laminations, and the wear mechanism is mainly adhesive and plastic deformation (Figure 5b3). For NA, the worn surface is relatively smooth, which is covered by a continuous glaze layer with some detachments. Thus, the main wear mechanism is plastic deformation (Figure 5c3). For the NMA composite, the worn surface emerges a discontinuous glaze layer, and spalling and significant plastic deformation occur. The wear mechanism is mainly micro plow and plastic deformation (Figure 5d3).
XRD patterns ( Figure 6) display that for the N alloy except for the Ni-based solution and Al2O3, only the NiO can be detected, while, for NM, three new phases (MoO2, MoO3, NiMoO4) appear. For NA, MoO2, and Ag2MoO4 occur. For NMA, besides some oxides (NiO, MoO2, MoO3) and molybdates (NiMoO4, Ag2MoO4), a new phase Ag2O emerges. The metal oxides and molybdates are good high temperature lubricants, and the fewer lubricants in N alloy may explain the higher friction coefficient and wear rate at 800 °C. After an 800 °C friction test, the diffraction peaks of MoS2 vanish, which means MoS2 had oxidized. Previous studies [35,36] have reported that composite coatings containing MoS2 and Ag required sulfur as a catalyst for the formation of silver molybdate. However, the results in this paper show that the S element is not necessary, and only Mo and Ag can also form silver molybdate at high temperatures, which is consistent with previous research results [23].  In order to further analyze the compositions of the wear surface and explore the mechanisms of tribo-chemical reaction at 800 °C, the Raman analyses inside and outside the wear track were applied, and the results are given in Figure 7. For the N alloy, MoO2 and NiO appear both inside and outside the wear track. The following reactions may occur.

Mo+O2→MoO2 (4)
For NM, outside the wear track, only NiO and MoO2 can be detected, while, inside the wear track, NiO, MoO2, MoO3, and NiMoO4 exist, which suggests the tribo-chemical reaction causes the formation of MoO3 and NiMoO4. It seems that, except reaction (3) and (4), the following reactions also occur.
For NA, Raman spectra reveal Ag2MoO4 only appear inside the wear track, which means it is generated by the tribo-chemical reaction. The most likely reactions are shown below. 4Ag+O2→2Ag2O (7) Mo+O2→MoO2 Mo+2Ag+2O2→Ag2MoO4 (11) For NMA, besides the above-mentioned oxides and molybdates, a new molybdate Ag2Mo2O7 appears, which can form by the following reactions.
2MoS2+7O2→2MoO3+4SO2↑ (12) 4Ag+O2→2Ag2O (13) 2MoO3+Ag2O→Ag2Mo2O7 (14) 4Ag+2MoS2+8O2→Ag2Mo2O7+Ag2O+4SO2↑ (15) NiO, MoO3, and molybdates are easily deformed or sheared during high-temperature friction and, thus, have a low friction coefficient and wear rate [16,37]. The synergistic effects of NiO, MoO2, MoO3, NiMoO4, Ag2MoO4, and Ag2Mo2O7 are responsible for the excellent tribological properties of this composite.  Figures 8 and 9 shows SEM of the counter balls of N and NMA composites at different temperatures. It can be seen that they are similar with the wear trace of the disk sample. At RT, for the N alloy, relatively narrow material transfer widths indicate lower wear rates. For NMA, the material transfer is relatively wide and there are many substances, which indicates that the tribological properties are poor. At 400 °C, for N and NMA composites, material transfer increases and no continuous tribolayer is formed. At 800 °C, for the N alloy, on the transfer material, there are spalling pits and a thin tribolayer. For NMA, a continuous and thick tribolayer appears on the transfer material. Furthermore, the Raman and EDS analysis (Figure 10) of the transfer materials are present on the counter ball of NMA at 800 °C. The results show that the transferred material is mainly rich in Ni, Mo, Al, Ag, and O elements, which come from composites. Raman analysis shows the formation of NiO and molybdate lubricant phases. The influence of the two composites on the counter balls is almost the same, which are both caused by the material transfer on the counter balls. Thus, this reduces the counter balls' wear. At high temperatures, the formation of a transfer film between the worn surfaces impede the direct contact of the friction pairs, which improves the tribological properties of the NMA composite and then avoids damage to the counter material.

Mechanical Properties and Fracture Mechanisms
The fractography of N, NA, and NMA after different temperature tensile tests are indicated in Figure 11. For the N (NiMoAl) alloy, the fracture surfaces are characterized by a clear micro-void accumulation fracture (Figure 11a), which can be explained as follows. The sintered microstructures consist of fcc Ni-base solid solution and a few in-situ formed Al2O3 oxide. Under the tensile stress, first, the dislocations generate and glide in {111} face. Then, at the grain boundary or at the Ni/Al2O3 interface, the dislocations will pile up, and the continuous pile up give rise to the stress concentration.
When the stress concentration reaches a certain degree, the micropores could appear. Under the stress, these micropores would enlarge in size and the cracks may initiate and propagate at the micropores until the final rupture. The micro-void accumulation fracture is plastic fracture microscopically, but macroscopically, the NiMoAl alloy is brittle. For the NA alloy (NiMoAlAg), the fracture mechanisms are similar with that of N and reveal the micro-void accumulation fracture (Figure 11b). For the NM (not shown in Figure 11) and NMA composites (Figure 11c), the fracture surfaces are rather coarse and display the intergranular rupture. The difference of strengths and fracture mechanisms should be attributed to the microstructure. For the N alloy, the microstructures consist of a Ni-base solid solution and in-situ formed Al2O3, and the refractory oxides segregate at the grain boundary and play the strengthening role in the alloy. Therefore, the strong grain boundaries impede the movement of dislocations and heighten the strength. For the NA alloy, because the solubility of Ag in Ni is very limited, the microstructures still contain silver aside from Ni-base solid solution and Al2O3 oxides. Due to the low melting point of Ag and the high fluidity at a high temperature, the Ag distributes uniformly in the matrix (Figure 2c). When the dislocations reach at Ag, it is easy for them to cut the soft Ag. Hence, the tensile and compressive strengths of NA are lower than those of N. Nevertheless, for the NM and NMA composite, the agglomerated molybdenum disulfide segregated at the grain boundary (Figure 2b,d), which weaken the grain boundary sharply and results in the intergranular fracture and the dramatical degradation of strength. The rise of test temperature did not change the fracture mechanism and the variation tendency of tensile and compressive strengths.
For now, in this study, it seems that it is difficult to achieve the unification of high mechanical properties and good continuous lubricating behaviors at a wide temperature range. The effect of mixed oxides on the mechanical properties of materials needs further study. Since MoS2 is dramatically decreasing to the mechanical properties as well as its lubricating effect is not as expected, we exclude MoS2, and introduce another strengthening phase, such as tantalum (Ta) and dispersed strengthened Al2O3. The mechanical and tribological properties are investigated, and the results will be reported later.

Conclusions
In this paper, the mechanical and tribological properties and related mechanisms of Cr-free Nibased self-lubricating composites with and without Ag/MoS2 were studied from room temperature to 800 °C. The main conclusions can be drawn as follows.
(1) The addition of metal Ag decreased the hardness and increased the density, whereas the addition of MoS2 has the opposite effect. In the MoS2-containing composite, the sintered microstructures consist of appreciable undecomposed MoS2, which means that the MoS2 did not decompose completely.
(2) With the temperature rising, the friction coefficients of the composites reduce generally, and the wear rates increase abnormally at 400 °C, and then drop to lower than 5 × 10 −5 mm -3 /Nm. The NiMoAl-MoS2-Ag composite shows the best tribological properties at 800 °C (with a friction coefficient of 0.22 and a specific wear rate of 1.68 × 10 −5 mm -3 /Nm), which was attributed to the formation of a lubricating film composed of NiO, Mo oxides, Ag2O, various molybdates, and Ag particles at high temperature. (3) With the addition of lubricants, the tensile and compressive strengths reduce. The NiMoAl and NiMoAlAg alloys exhibit micro void accumulation fracture, while the NiMoAl-MoS2 and NiMoAl-Ag-MoS2 composites reveal an intergranular fracture.