Zno solid lubricants

Journal of Alloys and Compounds 821 (2020) 153494

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Improving the tribological properties of AISI M50 steel using Sns/Zno solid lubricants Ammar H. Elsheikh a, Jingui Yu b, Ravishankar Sathyamurthy c, M.M. Tawfik d, S. Shanmugan e, F.A. Essa d, * a

Production Engineering and Mechanical Design Department, Faculty of Engineering, Tanta University, Tanta, 31527, Egypt School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan, 430070, China Department of Automobile Engineering, Hindustan Institute of Technology and Science, Chennai, 603103, Tamil Nadu, India d Mechanical Engineering Department, Faculty of Engineering, Kafrelsheikh University, Kafrelsheikh, 33516, Egypt e Research center for Solar Energy, Department of Physics, Koneru Lakshmaiah Education Foundation, Green Fields, Vaddeswaram, Guntur (DT), Andhra Pradesh, 522502, India b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2019 Received in revised form 19 December 2019 Accepted 20 December 2019 Available online 23 December 2019

Motivated by enhancing the tribological behavior of M50 steel (M), two nanoscale self-lubricants (SnS and/or ZnO) have been incorporated with M50 matrix to form two new matrix composites: M50 þ 10% SnS (MS) and M50 þ 10% SnSþ10%ZnO (MSZ). The samples were fabricated using spark plasma sintering machine (SPS). The fabricated samples were tested against silicon nitride ceramics ball on a pin-on-disk tribometer under wide ranges of loads (3e12 N), sliding speeds (0.2e0.8 m/s), and temperatures (RT e 450  C) to investigate the tribological behavior of the samples. FESEM, EDS, EPMA, XPS, and XRD tests were carried out to explore the morphology of worn surface and recognize the anti-wear and frictionreduction mechanisms of the samples. MSZ recorded the least friction (0.22) compared to MS (0.31) and M (0.58) under severe conditions (12 N load and 0.8 m/s sliding speed at temperature of 450  C). The synergistic action between SnS and ZnO as well as produced lubricious oxides, molybdenites, and carbonates on interacting surfaces is the main reason to enhance the tribological characteristics of MSZ samples. © 2019 Elsevier B.V. All rights reserved.

Keywords: Powder metallurgy Zinc oxide Tin sulfide M50 steel Solid lubricant Dry friction

1. Introduction M50 steel has shown promising applications in modern engines due to its superior performance at high temperature in terms of stability, contact fatigue, toughness, and strength [1]. It has been used in manufacturing of machine components which subjected to high temperature such as shaft bearing of gas-turbine engines and aircraft gears [2e5]. However, M50 steel suffers from its catastrophic scuffing failure during rolling-sliding loading under extreme conditions which accompanied by large friction and high temperature [6e8]. Therefore, the need to investigate the tribological behavior of M50 steel under severe conditions arises. Self-lubricating materials have been proposed as a promising alternative to conventional materials that subjected to extreme

* Corresponding author. E-mail addresses: [email protected] (F.A. Essa).

(J.

https://doi.org/10.1016/j.jallcom.2019.153494 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Yu),

[email protected]

operating conditions such as heavy load, cryogenic, high temperature, irradiation environment, and corrosive media [9e12]. The tribological behavior of hybrid M50 steel composites incorporated by WS2 and ZnO under a wide a range of temperatures has been investigated [13]. The results indicated that: the triblogical properties of M50/WS2 are enhanced at low temperature and impaired at high temperatures (over 400  C); while M50/ZnO exhibited contrary behavior to M50/WS2 (the triboligical properties are enhanced at high temperatures and are impaired at low temperatures). M50/ZnO-WS2 has excellent tribological behavior over a wide range of temperature as compared with other investigated alloys (M50, M50/WS2, and M50/ZnO). To investigate the effect of ZnO concentration on the tribological behavior of M50/ZnO, the developed alloy was tested under different operating loads [14]. The mechanical and tribological properties of the developed alloy were increased with concentration increase till reaching their optimal values at concentration of 20% after which they were declined. In another study, Essa et al. [15] have investigated the tribological behavior of hybrid M50 steel composites incorporated

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by MoS2 and ZnO under a wide a range of temperatures. Good lubricating behavior was observed at low temperatures (up to 400  C) for M50/MoS2 and at high temperatures (higher than 400  C) for M50/ZnO. To obtain the optimal tribological behavior, the hybrid lubricants (ZnOeMoS2) should be used due to their synergic effects over a wide range of temperatures. Huang et al. [16] proposed WS2 (5 wt%) as a nanoscale solid lubricant to augment the tribological behavior of WC-Ni-Cr composite. The composite prepared using pulsed electric current sintering technique. The increase in sintering pressure resulted in increasing the hardness and density of the composites. Mai et al. [17] synthesized selflubricating Ti3C2 nanosheets/copper composite using a facile electrodeposition technique. The fabricated samples exhibit low WR and coefficient of friction which are 19 times and 46% lower than their Ti3C2-free counterpart, respectively. That is due to the formation of compact tribolayer of Ti3C2 on the worn surface, which reduces the bad effects of the direct metal-to-metal. The tribological performance of M50/MoS2 were analyzed under wide range of temperatures (150e450  C) [18]. MoS2 and FeS (generated during the sintering process) improved the tribological properties due to their synergistic effect. Liu et al. [11,19] investigated the tribological behavior of M50 composite containing 10 wt%(50Sn40Ag10Cu) and compared it with those of pure M50 alloy and surface-scattered composite (with C60 microparticles). Results indicated that surface-scattered composite shows the best wear and friction performance under different loads and temperatures while scattered-free composite lose their lubricating effects at high loads greater than 12 N. Intermetallic compounds of Ag3Sn, Cu6Sn5, FeSn, FeSn2 and oxides boost the lubricating layer strength and reduce the distortion of the lubricating layer. Moreover, FeSn and FeSn2 augment the bonding between the load-bearing layer and the surface lubricating layer which prohibit premature shedding of the lubricating layer. Deng et al. [20] conducted tribological tests on M50/Ag matrix composite with different Ag concentrations (0, 2.5, 5.0, and 7.5 wt%) and wide range of temperatures (25  Ce450  C). The best tribological behavior was observed at Ag concentration of 5.0%. The WR decreased by about 50%, when the test temperature changed from 25  C to 450  C. The sliding speed, applied load, and operating temperature influences on the mechanical and tribological properties of metal composites [21e23]. Reduction of coefficient of friction (COF) and wear rate (WR) at high operating temperature of tribo-components is highly desirable to improve the energy efficiency and components lifetime [24e26]. Solid lubricants have been proven to be an excellent choice for tribo-components that operate in severe conditions with high temperatures (above 350  C), contact pressures, corrosive environment, vacuum, and radiation; where liquid lubricants could be subjected to rapid thermal degradation [27e29]. Tin sulfide SnS has an orthorhombic structure [30], in which each tin atom is surrounded by three adjacent sulfur atoms to produce anisotropic crystal structure. There are many solid lubricants that provide excellent antifriction characteristic at high temperatures due to their easy shearing characteristics such as metal oxides (MoO3, ZnO, etc.) [31,32], alkaline halides (CaF2, BaF2, CeF3) [33,34], and noble metals (Ag, Au, etc.) [35,36]. Nevertheless, these lubricants work effectively only through small range of temperatures and some of them are brittle at RT. To overcome this problem, many researchers suggested that incorporation between multiple solid lubricants in metal matrix composites. These hybrid lubricants are used to reduce the friction at wide temperature ranges [37e39]. The influences of elevated temperatures on the lubrication features of WS2eZnO were investigated [40]. Thus, ZnO/SnS are expected to enhance the tribological properties of M50 alloy. Moreover, they are nontoxic, and abundant with reasonable cost. The previous work focused mainly on the effect of temperature on the friction and

wear of M50-steel. So, logically, the temperature was investigated on a large scale as can as possible (RT-800  C). Furthermore, the applied temperature is varying according to the application. In this study, the role of SnS/ZnO solid lubricants in enhancing the tribological properties of M50 alloy under different operating conditions is experimentally investigated. The experimental objectives can be reported into these points; 1. Three samples were investigated: pure M50 steel (M), M50 þ 10 wt% SnS (MS) matrix composite, and M50 þ 10 wt% ZnOþ10 wt% SnS (MSZ) matrix composite. 2. Friction tests of the prepared samples were carried out on a pinon-disk tribometer against Si3N4 ball. 3. The friction tests were conducted under different operating loads (3e12 N with step of 3 N), different sliding speeds (0.2e0.8 m/s with step of 0.2 m/s) and different temperatures (RT, 150e450  C with step of 100  C) to discover the tribological performance of M50 steel alloy incorporated by solid lubricants. 4. Mechanisms of wear and friction were realized to demonstrate the tribological and mechanical behavior of the developed composites.

2. Procedures of experimentations 2.1. Manufacturing the samples Table 1 presents the composition of M50 alloy by element. Microparticles (35e65 mm) with a high purity of 99.9 wt% have been used in this study. Firstly, the powders with the composition listed in Table 2 are mixed in polytetrafluoroethylene (PTFE) vials subjected to a vibrating millings under a frequency of 45 Hz [41]. After that, the three prepared samples (M, MS, and MSZ) are fabricated using spark plasma sintering technique (SPS) in cylindrical graphite dies (26 mm diameter) in a protective Ar atmosphere. The SPS machine used in this study is a D.R.Sinter® SPS3.20 (SPS Syntex Inc.). Sintering process is executed using a heating rate of 100  C per min to preserve the samples homogeneity and to avert the undesirable melting of the mixed particles. The heating process is scheduled as follows: the temperature was fixed at 800  C for 8 min and at 850  C for 30 min. Sintering Temperature was set at 1000  C for 5 min at 35 MPa. A high temperature scopes was used to monitor the sintering process through an observation hole to pursue the sintering process. The measured hardness of M50 sample ranged between 60.2 and 61.4 HRC which matches the standard values. Undesirable graphite layers on M50 samples’ surfaces were removed by surface finishing and mechanical polishing. The mechanical polishing had two main processes: wet polishing and dry polishing using 0.05 mm diamond pastes and grit emery papers with a fine grit grade of 1200, respectively. The tribological and mechanical tests were conducted on cylindrical M50 sample (8 mm thickness and 25 mm a diameter). 2.2. Density and hardness measurements Sample density is an important physical property that used to calculate the WR. The samples densities have been measured using Archimedes’ principles based on ASTM B962-08 standards [42].

Table 1 Composition of M50 alloy (wt.%). Mo

Cr

V

C

Cu

Si

Ni

Mn

Fe

4.00

3.72

1.00

0.72

0.30

0.20

0.10

0.05

Balance

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Table 2 Densities and hardness of the tested samples. Specimen

Density (g/cm3)

Hardness (HV1)

M MS MSZ

7.27 ± 0.036 7.05 ± 0.035 6.87 ± 0.034

468.85 ± 0.094 485.73 ± 0.098 439.32 ± 0.086

Three tests were performed for every specimen and the density of a sample is calculated as the average of the three experimental tests. The hardness tests was employed on HVS-1000 Vickers’ hardness tester based on ASTM E92-82 standard [43]. Five randomly selected measuring points on each sample side were used. The pyramid indentation was used under 1000 g load for 10 s. The sample hardness is calculated as the average of the ten readings. The measured densities and hardness of the tested samples are listed in Table 2. 2.3. Friction and wear tests The friction tests have been performed based on ASTM G99-95 standard using a pin-on-disk high-temperature HT-1000 tribometer (Zhongke Kaihua Technology Development Corp., China) [44]. Silicon nitride (Si3N4) balls with 3 mm radius (Vickers hardness 16.7 GPa, surface roughness (Ra) 10 mmm) were used as the counterpart during friction tests. Firstly, the samples and the balls are cleaned ultrasonically using acetone, and then they subjected to warm air to dehydrate acetone from their surfaces. Three different experimental sets were conducted on a tribometer to figure out the influences of the solid lubricant type on M50 as tabulated in Table 3. Each experiment has three replicates. The first experimental set was carried out by varying applied loads (3, 6, 9, and 12 N) while sliding speed and temperature were kept at constant values. The second experimental set was carried out by varying sliding speed (0.2, 0.4, 0.6, and 0.8 m/s) while applied load and temperature were kept at constant values. The third experimental set was carried out by varying temperature (RT, 150, 250, 350, and 450  C) while applied load and sliding speed were kept at constant values. Specimens were fixed in the center of a furnace using four screws. Then, the temperature set button was used to raise the temperature regularly from RT to test temperature. Heating rate was set as 20  C per min, while the sliding distance was set as 480 m. Friction values were instantaneously recorded using built in software in the tribometer. The WR (WR) could be calculated by:

 W mm3
 v WR mm3 N1 m1 ¼ Ds ðmÞ:P ðNÞ

(1)

Where (W v ), (Ds ), and (P) denote the wear volume, sliding Table 3 Design of experiments. Experiment No.1

Applied load (N)

Sliding speed (mm/s)

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11

3 6 9 12 12 12 12 12 12 12 12

0.2 0.2 0.2 0.2 0.4 0.6 0.8 0.8 0.8 0.8 0.8

RT RT RT RT RT RT RT 150 250 350 450

Fig. 1. XRD patterns of fabricated and tested samples: (a) M50 before, (b) M50 after, (c) M50 þ SnS before, (d) M50 þ SnS after, (e) M50 þ SnS þ ZnO before, and (f) M50 þ SnS þ ZnO after friction tests.

distance, and load, respectively. On the other hand, the wear volume should be computed, instead of WR, for worn surfaces with lost material less than 3 mg [45]. The wear volume could be calculated by:

    W v mm3 ¼ LðmmÞ,A mm2

(2)

Where (L) and (A) denote the perimeter and the cross-sectional area of the wear track measure using surface profilometer, respectively. 2.4. Microstructure analyses To characterize the surfaces of the samples, XRD tests were conducted on as received and tested samples with Cu Ka radiation at 40 mA and 30 kV with scanning speed of 0.01 s1 to identify the phases’ structure. To obtain the wear scar morphologies and compositions, Energy Dispersive Spectroscopy (EDS) and Electron Probe Micro-Analysis (EPMA) were used for all tested samples to figure out the wear mechanisms. To describe the morphologies of the cross-sectional of the contact surfaces, EDS and Field Emission Scanning Electron Microscope (FESEM) were used. Additionally, the formed phase compositions on the worn surfaces were analyzed and nominated using the X-ray photoelectron spectroscope (XPS). Using these characterization techniques, the wear and friction mechanisms as well as effects of different investigated solid lubricants on the tribological behavior of M50 matrix composite could be figured out. 3. Results and discussion 3.1. Hardness, density, and XRD of samples The average measured densities and hardness of the three investigated samples are listed in Table 2. M samples have a high hardness of 468.85 ± 0.094 HV1 which makes it well-suited for many applications. The hardness of MS is enhanced by about 16.88 HV1. In contrast, the hardness of MSZ is decreased by about 29.53

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Table 4 Chemical compounds observed in XRD patterns. Chemical compound

Dominant peaks

Diffraction angles (2q)

a-Fe

(112), (101), (250), and (114) (100), (101), (102), (110), (103), and (104) (210), (212), (081), and (190) (200) and (118) (303) and (532) (202), (311), and (404) (002) and (221) (112), (132), and (311) (022) (122), (311), (331), (422), and (521)

38 , 44.4 , 80.5 , and 98 2q ¼ 31.9 , 36.2 , 47.4 , 56.6 , 63 , and 81.8 42.506 , 64.004 , 71.394 , and 80.253 49.554 and 71.463 42.401 and 63.651 , 30.05 , 35.4 , and 62.5 42.611 and 49.013 42.6 , 63.54 , and 63.93 42.401 . 34.195 , 41.724 , 55.768 , 63.443 , and 63.822

ZnO SnS FeS FeSO3 Fe3O4 SnO SnO2 SnSO4 ZnSO4

HV1 due to the more porosities formed in MSZ compared with MS or M. Porosity is created due to interfacial reactions and shrinkage throughout sintering which results in serious material defects, which consequently decrease the mechanical properties of the samples [46e48]. The porosities formation is highly affected by the additives’ fraction; as they increase with increasing the additives’ fraction [46,49]. The densities of MS and MSZ samples are less than that of M samples. The density of MS and MSZ is decreased from 7.27 ± 0.036 g/cm3, in case of M, to 7.05 ± 0.035 g/cm3 and 6.87 ± 0.034 g/cm3, respectively. Fig. 1 shows XRD patterns of M, MS, and MSZ before and after friction test and different chemical compounds observed in XRD patterns according to JCPDS cards standards as well as their dominant peaks according to miller indices (hkl) are tabulated in Table 4. Briefly, the main components of M, MS, and MSZ (martensite, carbides phases, and oxides) were observed as chief tops in Fig. 1. Some of these compounds are formed after friction tests such as Fe3O4, SnO, SnO2 SnSO4, and ZnSO4; which formed during fabrication and/or friction process [50]. 3.2. Effect of applied loads on tribological behavior 3.2.1. Friction and wear of samples Friction tests have been conducted under four different applied loads of 3 N, 6 N, 9 N, and 12 N at 0.2 m/s and RT. The WR and COF of the investigated samples (M, MS, and MSZ) are shown in Fig. 2. The significant effect of the lubricant type and the applied load on the

wear and friction of M can be observed in these figures. The wear and friction have the same trends for all examined loads. M has the highest wear and friction compared with MS and MSZ for all investigated loads, except the WR at applied load of 12 N at which the WR of MSZ is higher than that of M as shown from Fig. 2. The use of solid lubricants could reduce WR and COF of M, while increasing the applied load could decrease WR (up to 9 N) and COF of all investigated samples. The COF of M is decreased by about 32.57% when the applied load increased from 3 N to 12 N. Moreover, M samples exhibited the greatest WR compared with MS and MSZ, where the WR is increased by about 6.67% when the applied load increased from 3 N to 12 N. MSZ exhibited lower WR and COF compared with that of MS and M. On the other hand, the COF of MS and MSZ samples are decreased by about 38.37% and 39.81%, respectively, when the applied load increased from 3 N to 12 N. Among all investigated samples, MSZ has the minimum COF under different applied loads. The superior tribological behavior of MSZ is attributed to the magnificent effect of hybrid solid lubricant. 3.2.2. Worn surfaces under different loads The morphology of the worn surfaces of the tested samples (M, MS, and MSZ) is shown in Fig. 3 and Fig. 4 via EDS and EPMA mapping images. As observed from this figure, the applied load has a significant effect on the morphology the worn surface. M samples have a scratchy wear and rough worn surface with three main wear mechanisms. The first is abrasion wear mechanism results from the abrasion between grits and the samples surfaces which produces long and deep grooves on the wear track. The second is scuffing characterized by the adhesive adjunctions. Delamination resulted

Fig. 2. Average COF and WR of specimens under various loads.

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Fig. 3. EPMA of frictional surface of M, MS, and MSZ under different loads.

from plastic deformation is the third mechanism. EDS results shown in Fig. 4 indicates that the worn surface is filled by oxide particles of wear debris. Abrasive substances such as silicon carbides, oxides, and nitrides as well as vanadium oxides are formed as depicted in XRD pattern (Fig. 1) and EDS (Fig. 4). The deep and long grooves as well as delamination pits are decreased by increasing the applied load as shown in Fig. 3. Furthermore, the COF is dramatically decreased by increasing the applied load due to the creation of lubricious molybdenites and oxides. For MS samples, the iron sulfide is formed during sintering process under high temperature as a reaction product of sulfide with iron [51]. MS has smoother worn surfaces compared with that of M as shown Fig. 3. Moreover, the amount of abrasive particles and grits are decreased with increasing the applied load. Adhesion is the dominated wear mechanism of MS especially at high loads at which continuous SnS tribo-layers are created. From the images of EMPA test, smooth islands are observed on the worn surfaces which contain SnS solid lubricant as affirmed by EDS test as shown in Fig. 4. In addition, severe abrasion and furrows have been diminished contrary to M. The distribution of SnS becomes more homogenous with increasing the applied load. The formation of solid lubricant tribo-films that contains SnS decreases the COF and the WR of MS compared with M samples as shown from Fig. 2. Furthermore, with increasing the applied load, some lubricating oxides, molybdenite, and carbonates are formed from MS or from

the Si3N4 ball which help in decreasing COF and WR. Nitride (N) is another element that transfers from the Si3N4 ball as depicted in EDS chemical analysis (Fig. 4). Based on the aforementioned discussion, the COF of MS was decreased by 39.37% when the applied load increased from 3 to 12 N as shown in Fig. 2. The COF of MS is less than that of M by about 3.72% and 13.48% at applied loads of 3 N and 12 N, respectively. Furthermore, the WR of MS is less than that of M by about 33.91% and 31.51% at applied loads of 3 N and 12 N, respectively. Consequently, MS shows better tribological behavior compared with M samples. Morphologies of the worn surfaces of MSZ under different loads are shown in Fig. 3 which may help in understanding the potential synergic merits of hybrid SnS/ZnO lubricant. The morphology structure of MSZ is better than that of M and MS as shown in Fig. 3. However, the former suffers from the existence of surface abrasion and micro cracks caused by the created wear debris. In addition, the adhesion and delamination wear mechanisms were also detected at high applied loads due to the existence of additive solid lubricants. The morphology of the MSZ worn surface has discontinuous smooth tribo-films, small delaminated layers, and some surface propagated micro-cracks. While, discontinuous tribo-films contain ZnO and SnS especially at higher applied loads as affirmed by EDS analysis (Fig. 4). Another reason to the tribological behavior enhancement of MSZ under high loads is the existence of lubricating oxides, molybdenites, carbonates formed from the tested or

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WR decreases when the sliding velocity increased from 0.4 m/s to 0.6 m/s at which the minimum value of WR is recorded. After that, WR increased again when the sliding velocity is increased from 0.6 m/s to 0.8 m/s. M samples had the greatest WR among all investigated samples ever; the WR is increased by about 25.72% when the sliding speed is increased from 0.2 m/s to 0.6 m/s, and decreased by about 13.04% when the sliding speed is increased from 0.6 m/s to 0.8 m/s. On the other hand, the WR is decreased from 2.73E-5 to 2.13E-5 in case of MS and from 3.68E-5 to 2.70E-5 in case of MSZ when the sliding speed is increased from 0.4 m/s to 0.6 m/. Finally, the minimum COF was 0.5 for MSZ under 0.8 m/s, while the lowest WR (2.13E-5) was for MS under 0.6 m/s.

Fig. 4. EDS of frictional surface of M, MS, and MSZ under different loads.

from the mated Si3N4 ceramic ball. The formation of nitride (N) on MSZ worn surfaces results in decreasing friction between the samples and the ball. The COF of MSZ is less than that of M by about 18.18% and 26.96% at applied loads of 3 N and 12 N, respectively. The WR of MSZ is less than that of M by about 23.63% at applied load of 3 N and is higher than that of M by about 13.50% at applied load of 12 N. WR increment of MSZ at higher applied loads may be resulted from the decrease in hardness of MSZ samples as depicted in Table 2. 3.3. Effect of sliding speeds on tribological behavior 3.3.1. Friction and wear of samples To figure out the role of sliding speeds on the tribological behavior of the synthesized samples, friction tests were carried out under 0.2, 0.4, 0.6, and 0.8 m/s at 12 N and RT. M samples have higher COF & WR compared to MS and MSZ. For all examined samples (M, MS, and MSZ), the highest and lowest COF were observed at 0.4 and 0.8 m/s, respectively. COF of M increased by about 7.86% when the sliding speed was increased from 0.2 m/s to 0.4 m/s. Then the friction decreased by about 22.91% and 5.40% when the sliding speed was increased from 0.4 m/s to 0.6 m/s and from 0.6 m/s to 0.8 m/s, respectively. The COF of MS reached its peak value (0.90) at a sliding speed of 0.4 m/s, and decreased by about 38.88% when the sliding speed increased by about 7.69 to 0.8 m/s. The COF of MSZ was increased by about 7.69% with changing the speed of 0.2e0.4 m/s at which it reached its peak value (0.70), and the COF is decreased by about 28.56% when the sliding speed reached 0.8 m/s at which the minimum COF (0.5) is recorded. The WR was increased with increasing the sliding speed from 0.2 m/s to 0.4 m/s at which WR reached its peak value. Then,

3.3.2. Worn surfaces under different sliding speeds Morphologies of the worn surfaces of M samples shown in Fig. 6 reveal the significant effect of the sliding speed on the tribological behavior of the investigated alloy. As observed from this figure, adhesion is the dominant wear mechanism especially at a sliding speed of 0.4 m/s. Hence, the COF and WR of M are increased at this sliding speed. Adhesion has a significant effect on the COF as reported by Ref. [52], as it results in puckering effect which take place at the sliding surface and results in a sharp increase in the frictional forces and hence the mechanical failure may occur. Increasing the sliding speeds results in smoother morphology and shallower pits compared with that at low speeds because of the formation of new tribo-layers that contain metal oxides, molybdenite, and, carbonates as shown in Fig. 7. Conversion of adhesion wear mechanism to fatigue wear mechanism at high speeds and the reduction of delamination are another two causes of forming smoother morphology. Fine oxide debris are also trapped to form a tribo-layer on M surface; this tribo-layer helps in decreasing the solidesolid contact, and consequently adhesion is reduced [53]. Under 0.6 m/ s, knockout deformation, texture refining, and fatigue wear were dominant wear behavior. Finally, sliding action creates some particle refinements on the plastic deformation, which help in dissipation of COF. Under 0.8 m/s, fatigue wear occurs due to the strain hardening of the worn surface. Additionally, the fatigue cracks are extended and the tribo-film is delaminated and consequently the WR is increased. Furthermore, the strain hardening and the deformation of the surface tribo-layer result in reducing COF value. However, M samples exhibited the worst COF and WR compared with MS and MSZ as shown in Fig. 6. MS has smoother worn surfaces compared with that of M due to the iron sulfide formed on the contact surface in case of MS. The predominant mechanisms of wear are adhesion and delamination which increased at 0.4 m/s and result in sharp increase in the COF and WR. Increasing the sliding speeds results in decreasing the delamination and adhesion and consequently delamination pits with a smoother morphology are formed on the worn surfaces. Molybdenite, lubricating oxides, carbonates, and nitride were organized at high speeds from M alloy, the test ball, and/or SnS additive which result in decreasing the COF and WR. SnS decreases the solidesolid contact and acts as a solid lubricant to reduce friction. The COF is decreased by about 28.57% when the sliding speed is increased from 0.2 m/s to 0.8 m/s. The WR in case of MS is decreased by about 31.51% and 22.64% compared with that of M at a sliding speed of 0.2 m/s and 0.8 m/s, respectively. Furthermore, the severe deformation of the surface layer results in decreasing the COF at the same sliding speed. MSZ has softer worn surfaces compared with M and MS as shown in Fig. 6. The three main wear mechanisms occur during testing MSZ samples are delamination, abrasion, and adhesion. The dominant wear mechanism changes with changing the sliding speed which results in changing the tribological behavior of MSZ at different sliding speed. Severe delamination is observed on the worn surface of MSZ at sliding speeds of 0.4 m/s and 0.6 m/s; this is because of the

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Fig. 5. Average COF and WR of specimens under various speeds.

delamination, abrasion, and micro ploughing wear mechanisms. Accordingly, the worst surface structure is found under 0.4 m/s. This increases COF & WR sharply. Abrasion was detected under 0.4 m/s and 0.6 m/s, but it is diminished at 0.8 m/s where adhesion

and delamination were also reduced. Consequently, the worn surfaces of MSZ have smoother morphology at high sliding speeds. Moreover, as observed from Fig. 7, the oxygen (O) and vanadium (V) elements exist on the worn surface of MSZ which indicates the

Fig. 6. EPMA of frictional surface of M, MS, and MSZ under different sliding speeds.

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of M50 sliding against Si3N4 ball at all temperatures except at 200  C, whereas COF is of maxima at 200  C. Therefore, COF of M raises up from 0.70 at RT to 0.75 at 200  C and decreases to 0.58 at 800  C as shown in Fig. 8. While the WR increases from 3.15E-5 mm3N1m1 at RT to 3.96E-5 mm3N1m1 at 150  C, and then decreases to 3.07E-5 mm3N1m1 at 450  C. When using SnS, MS produces a friction increased from 0.55 at RT to 0.60 at 150  C, and then declined to 0.31 at 450  C. Besides, the WR increases from 2.45E-5 mm3N1m1 at RT to 3.06E-5 mm3N1m1 at 150  C, and then decreases to 2.07E-5 mm3N1m1 at 450  C. When using the hybrid lubricating effects of ZnOeSnS, MSZ sample submitted continuous superior self-lubricating parameters at all temperatures and acquired the least friction behavior ever. Incorporating the hybrid two lubricants has the merit of temperature-adaptive reaction, which diminished the friction from 0.50 to 0.22 when raising the temperature from RT to 450  C, respectively. Moreover, the WR is changed from 2.83E-5 at RT to 3.33E-5 at 450  C. As concluded from Fig. 8, MZM provides satisfactory superior friction behavior at all temperatures because of the temperature-adaptive reaction of self-lubricating solid lubricants. This is because SnS provides a good tribological performance at low temperatures. In addition, ZnO plays an important role to reduce the friction at high temperatures [54,55]. Briefly, Fig. 8 illustrate that the minimum COF was recorded by MSZ at 450  C (0.22).

Fig. 7. EDS of frictional surface of M, MS, and MSZ under different sliding speeds.

formation of erosive vanadium oxides (V2O5). These formed vanadium oxides increase WR. Moreover, some carbonates, molybdenites, nitrides, and lubricating oxides could be formed under high speeds which may help in decreasing COF and WR. Lubricating tribo-films contain SnS and ZnO are observed in EDS analysis (Fig. 7). The homogeneity and stability of these lubricating tribofilms are enhanced by increasing sliding speeds. So, the COF of MSZ against Si3N4 ball is decreased by about 23.07% with raising speed of 0.2e0.8 m/s as shown in Fig. 5. The low hardness of MZM, Table 2, is another reason for increasing its wear rate especially at high speeds. The WR of MSZ is decreased by about 13.50% and 7.94% compared with that of M at sliding speeds of at 0.2 m/s and 0.8 m/s, respectively. The COF of MSZ is decreased by about 28.57% compared with that of M at 0.8 m/s. This enhancement is resulted from the formation of ZnO/SnS rich tribo-layer which contains the lubricious oxides, molybdenites, and, carbonites. Finally, it can be concluded that MSZ samples shows better friction behavior compared with other samples. 3.4. Effect of different temperatures on tribological performance of M50 3.4.1. Friction and wear of samples The friction experiments were conducted under different temperatures (RT, 150, 250, 350, and 450  C) under constant load of 12 N and constant sliding speed of 0.8 m/s. The average COF of M50 steel sliding against an Si3Ni4 ball under different temperatures (RT, 150, 250, 350, and 450  C) under 12 N and 0.8 m/s are illustrated in Fig. 8. It can be concluded that the temperature affects significantly the friction and wear of M50. Also, the friction and wear of samples have similar trends along the entire temperature range. As revealed from Fig. 8, M sample records the worst friction compared to MS or MSZ. Besides, raising the temperature reduces the wear and friction

3.4.2. Worn surfaces under different temperatures EPMA with EDS analyses of M frictional surfaces when sliding against Si3N4 under different temperatures can be seen in Fig. 9 and Fig. 10. The temperature, as the applied load and sliding speed, is a main parameter affecting strongly the morphology of M frictional surface. Generally speaking about the morphology of M under different temperatures, M exhibits the worst tribological behavior, and it has scratchy and rough worn surfaces. In addition, the dominant wear mechanisms are the abrasion coming from the harsh ploughing grits which lead to deep and long furrows, the scuffing with highly adhesive junctions, and the delamination with large plastic deformed layers. As a result, these different types of severe wear mechanisms coincide with the high values of friction and wear of M under different temperatures. Wear debris of oxides are found by EDS as shown in Fig. 10. From Fig. 10, the delamination pits and furrows are slashed, and some lubricious oxides and molybdenites form discontinuous tribo-films especially at elevated temperatures as suggested by EDS analyses, Fig. 10. Consequently, the COF of M is reduced remarkably from 0.70 to 0.58 with raising the temperature from RT to 450  C. This is maybe due to the created lubricious molybdenites and oxides which are the main reason of decreasing the COF of M regarding the temperature as illustrated from Fig. 8. Generally, the friction of M is decreased with raising the temperature except at 150  C, where the greatest value of the friction is obtained at this temperature throughout all investigated temperatures. The formation of some dis-lubricious carbides and oxides like silicon and iron carbides (SiC, FeC, and Fe3C) and silicon oxide (SiO2) as suggested by XRD (Fig. 1), EDS, and XPS (Fig. 13) is the possible reason for the high friction at this temperature, where they work against the lubrication direction. The chemical activity and shear modulus are two considerable parameters having a main effect on the metal’s friction and wear. Whenever the chemical activity of a metal is increased, and the shear resistance is reduced, the abrasive/adhesive friction of this metal is increased. Hence, oxides and carbides are created, which obtain a scratchier surface wear scars. Consequently, high friction values are attained [56]. As a result, the friction is increased slightly at 150  C due to forming the abrasive carbides and oxides. Furthermore, the delamination pits are bigger at 450  C than that of lower temperatures due to the adhering junctions influenced by high temperatures. Finally, the

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Fig. 8. Average COF and WR of specimens under various temperatures.

COF and WR of M sample are greater than that of the other specimens as shown in Fig. 8. This is due to the adhering junctions and the formed wear debris grits on the frictional surface resulting in highly delaminated layers and high abrasion, and hence high friction is attained [57].

Fig. 11 illustrates EPMA and EDS of contact surfaces of MS sample sliding against Si3N4 ball under different operating temperatures from RT to 450  C. As aforementioned, the synthesis of iron (the main substrate element) with SnS (the solid lubricant) produces the iron sulfide regarding the chemical reaction between

Fig. 9. EPMA of frictional surface of M, MS, and MSZ under different temperatures.

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Fig. 10. EDS of frictional surface of M, MS, and MSZ under different temperatures.

the iron and sulfur. Moreover, continuous tribo-films contain mainly SnS were found on MS frictional surfaces (Fig. 10 ¼ EDS) and (Fig. 14 ¼ XPS), and deep and long grooves are no longer exist. SnS lubricating tribo-films are distributed homogeneously on the wear contacts especially at high temperatures. Furthermore, MS provides its worst friction at 150  C (0.60). At this temperature, the worn surface is characterized by highly delaminated layers with rough oxide particles formed on MS surface with sliding. As well, the wear surface is scratchy and covered with wear grits as shown in Fig. 11. The formation of SnS-enrich tribo-films on MS worn surfaces at low temperatures is the main reason for decreasing the friction and wear of the same specimen as illustrated from Fig. 8. XRD (Fig. 1), EDS (Fig. 10), and XPS (Fig. 14) analyses affirmed that the frictional surfaces of MS at low temperatures have compacted tribo-films enriched by SnS solid lubricant. While, at high temperatures, there are two opposite directions affecting the tribological behavior of MS. First is the dis-lubricious effect resulting from losing SnS its lubricious effect and forming wear oxide grits helping to enlarge the friction. While, the second direction comes from producing some lubricating oxides (created from the original composites or transferred from the mated ball), molybdenites, carbonates, and iron sulfide, and hence the COF is diminished under high temperatures. As a result, the COF of MS is diminished from 0.55 to 0.31 when increasing the temperature from RT to 450  C, respectively. On the other hand, the WR is declined from 2.45E-5 mm3N1m1 to 2.07E-5 mm3N1m1 via changing the temperature from RT to 450  C, respectively. Results revealed that the COF of MS could be decreased by about 46.55% compared with that of M at 12 N, 0.8 m/ s, and 450  C. This is due to the organized tribo-layers containing SnS, oxides, molybdenites, and carbonates that reduce MS COF. Subsequently, it can be concluded that SnS is a good solid lubricant to be used with M50 under different temperatures from RT to 450  C. Moreover, MS sample provides a tribological behavior better than that of pure M50 without any lubricant.

As expected that using the hybrid system of ZnO and SnS solid lubricants for M50 composites could enhance the tribological behavior of M50/ZnOeSnS composites over a wide temperature range because of the temperature-adaptive action, the MSZ sample is investigated under different temperatures from RT to 450  C. So, EDS and EPMA are carried out for MSZ under different temperatures after friction test under different temperatures as shown in Fig. 12. It can be seen from the figure that the frictional surfaces of MSZ are enclosed by smooth tribo-films with surface micro-cracks’ propagation, small delaminated layers, and slight long furrows. In contrast, at 150  C, MSZ has a morphology characterized by scratchy and rough surface with high deformation, parallel grooves, and surface cracks as observed from Fig. 12. Therefore, the COF of MSZ at 150  C is recorded as the greatest between all investigated temperature points. This is referred to forming wear debris grits which furrow the worn surface and produce parallel long notches as can be seen from Fig. 12. Moreover, some lubricating layers are partially deformed due to the adhesive bonding between Si3N4 ball and real contact surface area of MSZ. So, the COF of MSZ is enlarged at 150  C compared to that at RT as obtained from Fig. 8. On the other hand, slight notches and delamination are the major wear mechanisms of MSZ frictional surface at 250  C, Fig. 12. Furthermore, the contact surface of MSZ is enclosed by a tribo-layer film distributed homogenously over the frictional surface with fine wear grits. Moreover, the delamination mechanism leads to create micro-cracks on the deformed layers and shear lips. As the temperature reaches 350  C, MSZ has a worn surface morphology enriched by continuous and homogenously distributed islands of lubricating oxides, carbonites, and molybdenites as revealed by Fig. 12. As well, as cleared from Fig. 12, the frictional surface of MSZ at 450  C is characterized by diffused continuous tribo-films with slight delamination pits. In short, the adhesive wear, abrasive wear, and delamination are the major wear mechanisms provided by the tribo-pair system of Si3N4/MSZ. For more declaration, XPS, EDS, and XRD results obtained that the frictional surfaces of MSZ are enriched by SnS at low temperature degrees. Whilst, at 150  C, the friction is increased again due to forming the dis-lubricating abrasive oxides. As the temperature continues raising, the sulfide amount (S) is obtained to decrease dramatically in the tribo-layers films as examined by EDS test. As well, the same lubricating film on the wear contact surface is enriched by oxygen (O). Furthermore, SnS is partially oxidized with increasing the temperature, and then, ZnSnO4 component is formed as a final product of the tribo-chemical reaction between SnS and ZnO. Thus, the friction of MSZ at 450  C is diminished due to the friction-reduction effect of the hybrid components of SnS and ZnSnO4 as obtained from Figs. 1 and 15. As well, at the high temperatures, XRD (Fig. 1), EDS (Fig. 10), and XPS (Fig. 15) tests attained that SnS is oxidized and vanished to react with ZnO and produces ZnSnO4. As a consequence, the friction-reduction mechanism at high temperatures is due to the high adaptive-temperature lubricants of ZnSnO4 and ZnO. So, the COF of MSZ is reduced from 0.50 to 0.22 when increasing the temperature from RT to 450  C, respectively. Besides, the specific WR is increased from 2.83E-5 mm3N1m1 to 3.33E-5 mm3N1m1 with raising the temperature from RT to 450  C, respectively. It can be concluded from the experiments that using the hybrid lubricants of SnS and ZnO could decrease the COF of M50 steel by 62.07% compared with that of M sliding under 12 N and 0.8 m/s at 450  C, Fig. 8. In addition, increasing the WR of MSZ is referred to the hardness decline of the sample as presented in Table 2. 3.5. FESEM and XPS tests on the tribo-film For more elaboration for the anti-wear and friction-reduction

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Fig. 11. FESEM (a) and EDS (b) of cross-sectional wear scars of M at 450  C.

mechanisms, FESEM and EDS point, surface, and mapping tests are carried out to investigate the cross-sectional wear scars of M and MSZ at 450  C as revealed in Figs. 11 and 12. The morphology photos are for the cross-sectional area of wear contact perpendicular to the sliding direction. It is observed from Figs. 11(a) and 12(a) that the substrate composites are covered by a tribo-layer on the top of contact surface. As illustrated from Fig. 11(b), the lubricating tribo-film covering the substrate composites of M is enriched by lubricating oxides, carbonates, and molybdenite with a thickness of 279 nm. Whereas, as illustrated from Fig. 12(b), the lubricating tribo-film covering the substrate composites of MSZ is enriched by some lubricating oxides, carbonates, molybdenite, and ZnOeSnS as a main component of the tribo-layer film with a thickness of 450 nm. Furthermore, these tribo-films layers are the leading reasons to diminish the COF of the specimens as shown in Fig. 8. EDS results, Fig. 12(b), agree with the results of XPS and XRD, which affirm the effect of the temperature-adaptive reaction of SnSeZnO on M50 tribological properties. Correspondingly, the high concentration of ZnO and SnS lubricants obtained by EDS test is a strong pointer of the tribo-film. It should be declared that the lubricating tribo-film layer is befallen to the top frictional surface of MSZ. Through sliding processes, MSZ provides less friction because of the relative slippage occurs between the lubricating layer and the

original composites or the tribo-film itself. So, the shearing forces are reduced regarding this lubricating mechanism, and then the friction forces are declined. Consequently, the hybrid lubricant of SnS/ZnO is an effective lubricant to decrease the friction of M50 steel-based composites sliding against silicon nitride ceramic ball under different loads, sliding speeds, and operating temperatures due to the temperature-adaptive reaction. Hence, MSZ provides the best friction behavior compared to M or MS even under different loads, speeds, or temperatures. To understand the anti-wear and friction-reduction mechanisms correctly, XPS test is performed to investigate the frictional surfaces of all specimens to study the chemical structure of the tribo-film layers formed on the worn surfaces. Fig. 13 plots the survey scan and binding energies of Fe2p, Cr2p, O1s, V2p, N1s, C1s, Mo3d, and Si2p, respectively. This survey scan is obtained for the wear contact of the pure M50 (M). Additionally, Fig. 14 plots the survey scan and binding energies of Fe2p, Cr2p, O1s, V2p, Sn3d, N1s, C1s, Mo3d, S2p, and Si2p, respectively. This survey scan is obtained for the worn surface of MS. Moreover, Fig. 15(a) obtains the survey scan and binding energies of Zn2p, Fe2p, Cr2p, O1s, V2p, Sn3d, N1s, C1s, Mo3d, S2p, P2p, and Si2p, respectively. This survey scan is obtained for the worn surface of MSZ. The Fe2p peaks at 709.96 eV and 710.82 eV indicate the oxidization of the iron into

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Fig. 12. FESEM (a) and EDS (b) of cross-sectional wear scars of MSZ at 450  C.

Fig. 13. XPS spectra (survey scan) of elements on the worn surfaces of M at 450  C.

Fig. 14. XPS spectra (survey scan) of elements on the worn surfaces of MS at 450  C.

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FeO and Fe2O3, respectively as found in Fig. 15(b). This coincides the XRD results. The Zn2p peaks at 1022.1 eV and 1045.5 eV reveal that ZnO is stacked on the wear contact (Fig. 15(c)). Also, this result agrees with the EDS and XRD results. As well, the intensities of ZnO peaks become strong and sharp via increasing the temperature. The S2p peaks at 161.5 eV and 168.6 eV denote the existence of the

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metal sulfides on the frictional wear track (Fig. 15(d)). The S2p peaks at 486 eV and 486.6 eV denote the existence of the tin oxides (Fig. 15(e)). The C1s peak at 286 eV reveals the presence of carbides and carbonates (Fig. 15(f)). The Cr2p peak at 577.03 eV affirms the existence of chromium oxide (Cr (III) oxide) as observed from Fig. 15(g). When raising the temperature, Mo3d peaks at 233.67 eV

Fig. 15. XPS spectra of elements on the wear contacts of MSZ at 450  C: (a) survey scan, (b) Fe2p, (c) Zn2p, (d) S2p, (e) Sn3d, (f) C1s, (g) Cr2p, (h) Mo3d, (i) Si2p, (j) N1s, (k) O1s, (m) P2p, and (n) V2p.

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Fig. 15. (continued).

and 234.1 eV confirm the creation of molybdate (MoO3) on the frictional surface (Fig. 15(h)). Besides, this affirms that a tribochemical reaction was conducted to produce MoO3 as obtained from the figure. The Si2p peak at 103.77 eV confirms the existence of silicon oxide (SiO2) as obtained from Fig. 15(i). The N1s peak at 401.27 eV confirms the existence of Oxynitride (NiSi2O and NiSiO2) as can be obtained from Fig. 15(j). In addition, this result confirms that a surface layers of the silicon nitride ceramic ball were

transferred to the worn surface of the samples. The O1s peaks at 529e530.97 eV reveal the existence of metal oxides and carbonates (Fig. 15(k)). Moreover, the existence of phosphate in the tribo-film was confirmed by XPS spectrum, which showed a binding energy of P2p as shown in Fig. 15(m). It is well recognized that the phosphate boundary tribo-film was important for improving the tribological characteristics. The V2p peak reveal the existence of vanadium oxides (Fig. 15(n)). Therefore, tribo-chemical reactions

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were happened under these parameters. The declaration of EDS and EPMA was boosted by the XPS results. Finally, based on the XPS findings of MSZ, ZnO and SnS lubricants are the main components of the tribo-layer films with creating new carbonates, carbides, molybdenite and oxides.

[5] [6]

[7]

4. Conclusions This study presents an experimental investigation on the tribological behavior of M50-steel composited by different solid lubricants (SnS and ZnO) at different loads (3e12 N), sliding speeds (0.2e0.8 m/s), operating temperatures (RT e 450  C). The pin on disk friction tests have been performed on all investigated samples at different temperatures using Si3N4 ceramic ball. Different material characterization techniques have been used to figure out the effect of incorporating of solid lubricants with M50 matrix. The mechanisms of wear and friction were recognized for all samples. The following conclusions can be drawn on the basis of the obtained results: 1. Micro-hardness of MSZ is lower than that of M and MS. 2. Tribological behavior of MSZ is better than that of M and MS due to the synergic action between ZnO and SnS for wide range of applied loads, sliding speeds, and temperatures. 3. COFs of MS and MSZ are decreased by 13.48% and 26.97%, respectively compared with that of M under 12 N, 0.2 m/s, and RT. 4. Better tribological behavior is observed at higher sliding speeds; COF of MS and MSZ are decreased by 21.43% and 28.57%, respectively compared to that of M under 12 N, 0.8 m/s, and RT. 5. Effects of temperature-adaptive reactions of SnS and ZnO offer excellent friction behaviors of MSZ composites from RT to 450  C compared to other tested samples. 6. MSZ and MS WRs are reduced by 62.07% and 46.55%, respectively with comparison of M at 450  C because of the synergistic action between SnS and ZnO, and the formed lubricating molybdenites, carbonites, and oxides. 7. MSZ specimen exhibited the best recorded friction at 12 N, 0.8 m/s, and 450  C, whereas it reached its lowest value of 0.22.

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Statement of originality The work is new and original and it is not being considered elsewhere. The article has been written by the stated authors who are ALL aware of its content and approve its submission. No conflict of interest exists. If accepted, the article will not be published elsewhere in the same form, in any language, without the written consent of the publisher. Author contribution section The work is conducted by equal contribution between all authors. The article has been written by the stated authors who are ALL aware of its content and approve its submission. No conflict of interest exists. References

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