Tribological Behavior of NiMoAl-Based Self-Lubricating Composites.

The present study focused on the development of NiMoAl-based self-lubricating composites using solid lubricants as the second phase by powder metallurgy. For this, Cr2AlC MAX phase, Cr2AlC-Ag, and MoS2 powders were mixed with the NiMoAl-based matrix and subsequently hot pressed to produce bulk composite samples. The average hardness and wear resistance of the matrix were found to be increased with the addition of MoS2, Cr2AlC MAX phase, and Cr2AlC-Ag powder to the NiMoAl matrix. The addition of Cr2AlC to NiMoAl was more effective in improving the wear resistance than MoS2. The addition of Cr2AlC and Cr2AlC-Ag has increased the hardness by about 75% than that with the addition of NiMoAl alloy. A scanning Kelvin probe system was used to study the surface properties of the tribofilm in detail through work function mapping from the edge area to the wear area (groove). Among all the samples, the one with the addition of Cr2AlC-Ag powder to the NiMoAl matrix possesses the best tribo-mechanical properties. Cr2AlC-Ag composite addition to NiMoAl was found to decrease the wear rate by one-third and to reduce the coefficient of friction by one-fourth, compared to the base NiMoAl alloy. This was attributed to the high-sintered density and formation of strong tribofilms consisting of mixed oxides such as Ag2MoO4 and Al2O3, as confirmed by micro Raman spectra.

Introduction

There is a potential demand for lubricating materials with low coefficient of friction (CoF) and reduced wear rate.[1] However, most of the liquid lubricants lose their properties, if the working temperature is increased slightly, because of the coking and volatilizing processes. To overcome these issues, solid lubricants are often preferred. In this regard, powder metallurgical components with the addition of solid lubricants were found to have huge potential because of their improved tribological properties,[2,3] and some of their applications are in gas turbine seals, gears, transmission parts, brake bands, lining, and so forth.[4] Conventional solid lubricants, such as molybdenum disulfide (MoS2), graphite, inorganic fluorides, noble metals, and a few metal oxides possess lubricating effects under limited ambient conditions.[1,3,5,6] MoS2 is an attractive material because of its extremely low CoF and wear rate in the presence of other materials. However, the presence of humidity may decrease the performance of MoS2.[7] Graphite exhibits outstanding lubricating properties in a humid atmosphere.[7] Hexagonal boron nitride (h-BN) has been an excellent solid lubricant for high-temperature applications because of its graphite-like lamellar structure. However, the poor sinterability and nonwettability of h-BN limit its applications.[8] It appears that there is hardly any single lubricant that meets both, low-temperature and elevated temperature requirements, that is, having a low CoF and wear rate in a wide range of temperature. To overcome these issues, the use of composite lubricants has been suggested by many researchers.[1−3,9]

In the past years, nickel (Ni)-based metal matrix composites (Ni–TiC, Ni–Al–SiC, Ni–BN, and Ni–Cr–graphite) were reported to exhibit good tribological properties.[7] Most of these composites are produced through the powder metallurgy route.[8,10−14] Nickel–molybdenum (Ni–Mo)-based alloys have been introduced in the automotive sector to avoid scuffing-related problems. Ni–Mo alloys provide self-lubrication under dry conditions and also improve thermal conductivity.[15] However, less run-in wear behavior and the abrasive nature are the main problems of Ni–Mo-based alloys. The performance of the Ni–Mo alloy was improved by the addition of aluminum (Al). The addition of Al strengthens the Ni–Mo alloy by the formation of tightly adherent alumina at high temperature that resists attack by oxidation, carburization, and chlorination.[16−18] To further improve the wear resistance and high-temperature lubrication efficiency (to reduce CoF), additional solid phases, such as graphite, h-BN, MoS2, and Ag, are used with the Ni-based matrix.[7,8,15] NiMoAl has been widely used in the automotive sector, and it exhibits a relatively high CoF of around 0.80 at room temperature.[19,20] Sliney[21] has reported that the addition of 10–20 wt % of a solid lubricant phase (graphite and MoS2) to the matrix (polyimide-bearing materials) improves the lubricating properties significantly without compromising its strength. Dangsheng reported that the addition of 20 wt % MoS2 in Ni–Cr (Ni–20Cr)-based alloys prepared by hot pressing shows the reduction in CoF from 0.46 to 0.21 at room temperature without compromising the hardness.[22]

MAX phase materials have the general formula MAX, where M represents a transition metal, A is a Group III or IV element of the periodic table, X is either C or N, and n ranges from 1 to 3.[3,23] MAX phase has properties of both metals and ceramics. The recent developments on MAX phase compounds such as Cr2AlC and Ta2AlC were reported to show good tribological properties. Their promising properties are attributed to the nanolaminated structure, with “MX” slabs with an interlayer of pure “A” element.[3,23] The MAX phase materials have shown to possess remarkable tribological performance when tested against Ni-based superalloys at ambient temperature and 550 °C. Among all MAX phase materials, the Cr2AlC MAX phase possesses outstanding corrosion properties,[24] which is useful for automotive applications. At ambient temperature, the existing literature demonstrated that the Cr2AlC MAX phase possesses a CoF of 0.65 ± 0.10.[25] Further, the addition of silver to the Cr2AlC MAX phase (i.e., composites of Cr2AlC–Ag) was found to enhance the tribological properties significantly and reduce the CoF from 0.65 to 0.50 at room temperature.[3]

It appears that the tribological properties of NiMoAl alloys, which are widely used in the automotive sector, may be enhanced further by using the Cr2AlC MAX phase and silver-added Cr2AlC MAX phase. Work function mapping using a scanning Kelvin probe (SKP) system is a convenient method to investigate the tribological properties of the materials. The application of SKP techniques in the field of tribology was in great demand over the years. Presently, it is the only technique that is sensitive to both surface and near-surface defects and allows the study of one of the two interacting surfaces during sliding.[26] The Kelvin probe technique exhibits the highest sensitivity to the changes in the surface conditions.[27] Zharin and Rigney made an extensive study of Kelvin probe techniques in tribological applications.[26,28] The electron work function (EWF) from the SKP system is usually referred to the minimum energy required to remove an electron from the interior of a solid to a position just outside the solid. Studies of sliding of metal in contact, wear under ultralow loads, online investigation of changes during surface rubbing, and changes in the contact potential of a hard disk drive in a humid environment are the few main areas of the SKP system for tribological studies.[27] In the present work, attempts have been made to develop NiMoAl-based composites by the powder hot pressing method using Cr2AlC and Cr2AlC–Ag solid lubricants for improved tribological performance. The formation of the tribo film will be evidenced through the SKP system. Further, the known solid lubricant such as MoS2 powder has also been used with NiMoAl to carry out comparative studies on the tribological performance. The surface properties ofNiMoAl–20 wt % Cr2AlC, NiMoAl–20 wt % Cr2AlCAg, and NiMoAl–20 wt % MoS2 were studied for the first time in the present work through SKP measurements. It has been observed that Cr2AlC addition to NiMoAl was more effective in improving wear resistance than MoS2 addition.

Results and Discussions

Figure a–d shows the X-ray diffraction (XRD) patterns of NiMoAl-based hot pressed composites. The peaks corresponding to Ni (JCPDS no. 87-0712) and Mo (JCPDS no. 42-1120) phases were clearly seen, whereas the peak corresponding to Al was hardly visible (Figure a). This could be due to the small weight fraction of Al (∼2 wt %). In the MoS2 composite (Figure b), peaks of MoS2 were clearly seen apart from the base alloy. No decomposition or reaction with the base alloy was observed. Zhang et al.[29] also observed a similar observation with the addition of 10 wt % MoS2 to the NiMoAl matrix (90 wt % Ni–5 wt % Mo–5 wt % Al). Along with peaks of Ni and Mo, peaks of Cr2AlC could be clearly seen in the Cr2AlC-added composite (Figure c). In the silver-added sample (Figure d), some new peaks attributable to Ag2Al and chromium carbide (Cr7C3) were also found, compared to the previous sample (Figure c).

Figure 1

X-ray diffraction patterns of hot pressed samples (a) NiMoAl, (b) NiMoAl–20 wt % MoS2, (c) NiMoAl–20 wt % Cr2AlC, and (d) NiMoAl–20 wt % Cr2AlCAg.

The relative sintered density of the composites has been shown in Figure a (for this purpose, the theoretical densities were calculated through the rule of mixture for reference). The relative sintered density of NiMoAl, NiMoAl–20 wt % MoS2, NiMoAl–20 wt % Cr2AlC, and NiMoAl–20 wt % Cr2AlCAg was found to be 94, 95, 93, and 96%, respectively. It could be seen that the silver-added composite seems to have a slightly better density than the others. The microhardness of the sintered samples before wear testing has been shown in Figure b. The most conventional way of enhancing the wear resistance of any material is increasing the hardness of the material.[30] The average microhardness was found to be 363 ± 10, 410 ± 10, 651 ± 10, and 635 ± 10 HV for NiMoAl, NiMoAl–20 wt % MoS2, NiMoAl–20 wt % Cr2AlC, and NiMoAl–20 wt % Cr2AlCAg samples, respectively. The addition of a hard reinforcing phase to the matrix enhances the overall hardness of the composite. The MoS2-based composite shows about 12% increase in the hardness compared to the base alloy (363 ± 10 HV). The addition of 20 wt % Cr2AlC and Cr2AlC–Ag to the NiMoAl matrix enhances the hardness by more than 75%. The hardness of pure phase MoS2, Cr2AlC, and Cr2AlCAg are 4 GPa,[31] 5.2 GPa,[32] and ≈5–6 GPa,[3] respectively.

Figure 2

(a) Relative sintered density and (b) the hardness of the prepared composites.

The micrograph [back-scatter detector (BSE)] of the NiMoAl-based hot pressed composites has been shown in Figure a–d. In the base alloy (Figure a), there seems to be a phase separation where two distinct regions, that is, Mo-rich and Ni-rich, could be clearly seen. The Ni-rich phase (the gray region) is in the form of a continuous matrix on which the Mo-rich (the white region) discontinuous phase is dispersed along with the fine grains of Ni3Al. Mo-rich grains are typically in the size range of 100–300 μm. No segregation of Al could be seen. When MoS2 was added (Figure b), the composite seems to have a relatively fine microstructure compared to others. In this sample (MoS2-added), Ni-rich (the gray region) seems to be in the form of a continuous matrix and Mo (the white region) of spherical shape and size in the range of 60–80 μm is distributed uniformly. MoS2 is distributed near to the grain boundary and within the matrix. Zhang et al.[29] reported that the addition of 10 wt % of MoS2 to the NiMoAl matrix (80 wt % Ni–5 wt % Mo–5 wt % Al–10 wt % MoS2) shows reduction in the hardness than the base alloy (90 wt % Ni–5 wt % Mo–5 wt % Al), and it was attributed to the segregation of MoS2 near to the grain boundary, which further weakens the grain boundary sharply and reduces the strength. Cr2AlC reinforcement has been found to change the microstructure significantly (Figure c). The grains of Mo (the white region), Ni (the gray region), and Cr2AlC phase are clearly distinguishable. Grain sizes of Mo-rich and Ni-rich phases are relatively smaller as shown in Figure a. In the silver-added composite (Figure d), the grain around Cr2AlC seems to be more fused, and the overall microstructure seems to be highly dense. Silver-rich or Ag2Al regions were clearly seen, which are very fine in size.

Figure 3

BSE image of hot pressed (a) NiMoAl, (b) NiMoAl–20 wt % MoS2, (c) NiMoAl–20 wt % Cr2AlC, and (d) NiMoAl–20 wt % Cr2AlCAg samples.

Figure a shows the wear behavior of different samples as a function of sliding distance. The base alloy (NiMoAl) shows a continuous increase in wear loss with increasing distance. Wear loss has been reduced slightly by adding MoS2 in the NiMoAl alloy. Upon the addition of the Cr2AlC MAX phase, the wear resistance has further increased. The carbide phases in a metal alloy have been credited for excellent wear resistance because they act as protective barriers and resist the delamination of the surrounding matrix.[30] The silver-added composite shows the best performance and very good wear resistance among all (Figure a). Compared to the NiMoAl alloy, the Cr2AlCAg composite shows almost one-third reduction in wear loss. The trends observed during wear (Figure a) have also been reflected in the CoF (Figure b). Among all the samples, the NiMoAl alloy exhibits a very high CoF. The addition of 20 wt % of MoS2, Cr2AlC, and Cr2AlC–Ag to the NiMoAl matrix exhibits significant reduction in the CoF by 52.18 ± 3, 55.43 ± 2, and 70.65 ± 4%, respectively. The CoF of MoS2 (0.44)- and Cr2AlC (0.41)-added composites are comparable. When the silver was added to the NiMoAl matrix, there is a considerable drop in the CoF as compared to the base alloys, and also, the curve was very smooth, compared to other composites (Figure b). Gupta et al.[3] observed that the addition of silver (20 vol %) to the Cr2AlC MAX phase exhibits a reduction in the CoF by 23%. However, in the presence of the NiMoAl matrix, the addition of 20 wt % of Cr2AlCAg (preparation of Cr2AlCAg is given in the Experimental Section) shows 34.15% reduction in CoF at room temperature as compared to 20 wt % of Cr2AlC. The addition of solid lubricants to the NiMoAl matrix reduces the CoF and decreases the wear rate without compromising the hardness (Figure b).

Figure 4

(a) Wear characteristics of different samples and (b) CoF characteristics of different samples.

Figure a–f shows the field emission scanning electron microscopy (FESEM) images, micro Raman spectra of the wear-tested samples, elemental analysis of the hot pressed (before wear testing) and wear-tested (after wear testing—) samples surfaces. The NiMoAl sample shows surface deformations and large grooves; indicating the chipping-off of a large chunk of surface layers (Figure a). The formation of debris along with microcracks is observed on the worn surface. Chipping of the materials on the surface indicates the formation of a nonprotective tribofilm. Zhang et al.[29] also observed the formation of shallow grooves and microcracks on the worn surface of hot pressed NiMoAl (90 wt % Ni–5 wt % Mo–5 wt % Al) sample. The elemental analysis displays that all the prepared samples before wear testing is without any oxygen content (Figure f). However, after the wear testing, the samples contain the excess additional elements of oxygen (O), iron (Fe), and carbon (C). It is associated with the tribochemical reaction of the counter materials and the sample. Carbon and oxygen play an important role in the formation of a friction film, and it is related to the C–O adsorption layer.[33] The tribolayer that is rich in carbon is also advantageous for the reduction in the wear.[30] The micro Raman analysis shows the peaks corresponding to NiO and NiMoO4 (Figure e). The formation of NiO and NiMoO4 on the worn surface is associated with the oxidation of the NiMoAl sample by the tribochemical reaction on the rubbed surface during the friction process. The formation of NiMoO4 on the worn surface can provide lubrication for the sample as it is an effective high-temperature lubricant with low shearing strength.[20,34] Deformation levels on the surface and grooves’ sizes have decreased significantly by the addition of MoS2 (Figure b). Deformation valleys are smaller and not continuous, unlike the NiMoAl alloy (Figure a). The distribution of in situ formed patchy oxide (NiO) was observed as an island on the worn surface along with the chipping of the material and the microcracks (Figure b). It appears that the formation of patchy NiO on the surface reduces the tribofilm integrity and causes spallation. An additional phase of MoO2 was also detected on the worn surface by micro Raman analysis (Figure e). The dioxide of molybdenum exhibits nearly similar lubrication performance as that of molybdenum disulfide[35] and thereby reduces the CoF and wear rate. The recent report[29] exhibits that the addition of MoS2 to the NiMoAl matrix does not play the expected lubricating role. It could be because of the fact that the MoS2 particles are agglomerated and do not form a continuous lubricating film. The appearance of chipping and microcracks on the MoS2 added composites in the present investigation is well in agreement with the reported study[29] (Figure b). The addition of Cr2AlC has decreased these surface defects and the size of grooves (Figure c). The Cr2AlC particles acting as a reinforcing phase are closely implanted on the worn surfaces, which effectively enhance the wear resistance of the sample. The surface has smaller grooves, which is distributed almost uniformly across the surface. The sample surface exhibits delamination behavior along with the abrasive wear. The micro Raman analysis shows the presence of Fe2O3, NiO, NiMoO4, and Al2O3 phases (Figure e). The formation of Fe2O3 and NiMoO4 are providing adequate lubrication properties along with adhered Al2O3 on the surface. Formation of Al2O3 during the wear testing strengthens the tribofilm and improves the tribological properties. The possible tribochemical reaction of the Cr2AlC grain is as follows

Figure 5

FESEM image of the worn area of (a) NiMoAl, (b) NiMoAl–20 wt % MoS2, (c) NiMoAl–20 wt % Cr2AlC, (d) NiMoAl–20 wt % Cr2AlCAg sample, (e) micro Raman spectra of the wear-tested samples, and (f) elemental analysis of the hot pressed (before wear testing) and wear-tested (after wear testing) samples surfaces.

The worn surface of the Ag-based composite shows a relatively much smoother surface compared to all other samples (Figure d). Deformation and grooves have almost reduced, and the magnitude of surface defects is minimal because of the addition of silver. Silver with good self-lubrication properties can diffuse and accumulate on the rubbing surface to form a lubricating film and finally reduces the CoF. The uniformly distributed debris along with the adhered patchy oxide of Ag2MoO4 was observed on the worn surface (Figure d). The surface wear scratches seem to be formed almost uniformly, indicating better wear resistance and tribological properties. Micro Raman spectra show a large number of mixed oxides, such as NiO, Ag2MoO4, NiMoO4, and Al2O3 (Figure e). It shows that the surface consists of a tribofilm containing a variety of oxides (Figure d). The possible tribochemical reactions have been given below[20]

The formation of composite oxides including Ag–Mo–O and Al–O seems to be very advantageous, which reduces the wear as well as CoF.[36,37] The in situ formation or addition of soft phase in the matrix is advantageous because, during the repeated action of the applied load and the abrasive force, the softer phase will tend to wear out earlier. Further, these worn-out materials tend to fuse with the substrate under the mechanical force. As the wear process continues, the wearing out of the softer phase continues, which forms the tribofilm. This tribofilm, once extensive on the surface, plays a pivotal role in preventing further wear of the composite by reducing the direct contact between the two hard surfaces.[30]

The FESEM–energy-dispersive X-ray analysis (EDAX) elemental mapping of wear tested NiMoAl–20 wt % Cr2AlC–Ag composite surface (as shown in Figure ) shows the uniform distribution of silver oxides throughout the surface. Liu et al.[38] demonstrated that the addition of 20 wt % silver molybdate in NiMoAl (80Ni15Mo5Al) shows outstanding tribological properties at a wide temperature range. In the present study (NiMoAl–20 wt % Cr2AlCAg), the in situ-formed Ag2MoO4 is the key factor for the reduction in the CoF.

Figure 6

FESEM–EDAX mapping of the NiMoAl–20 wt % Cr2AlC–Ag composite surface after the wear test.

The schematic mechanism of the SKP measurement and the work function difference plots are shown in Figure a–g. The schematic diagram (Figure a,b) summarizes the interaction of surfaces and the vibrating gold tip. The SKP instrument has a high sensitivity to changes in surface potential with high spatial resolution. Investigation of tribological behavior by SKP has great potential and has been demonstrated previously.[27] As for tribology, the EWF from SKP is a useful parameter for studying surface deformation features, surface renewal, gas and lubricant adsorption, oxidation, phase transformations, redistribution of alloy components, and so forth.[28] It is possible to study the evaluation hot spots or spots of damages on the rubbing surface with the time of friction. The contact potential difference (CPD) measurements by using the SKP technique was used for two major purposes: (1) to determine the critical points with respect to changes in normal load, with relevance to the selection and optimization of the material and (2) to determine the kinetics of friction processes, including periodic changes which may be related to those in fatigue.[28]

Figure 7

(a) Schematic diagram of the samples for SKP measurement, (b) schematic interaction of surfaces and the vibrating gold tip, (c) SKP measurements of NiMoAl, (d) NiMoAl–20 wt % MoS2, (e) NiMoAl–20 wt % Cr2AlC, (f) NiMoAl–20 wt % Cr2AlCAg, and (g) EWF difference plot.

The work function of the edge area of NiMoAl shows 4.477 eV. However, after the wear testing, the work function of the grooved area is increased to 4.675 eV (Figure c); that is, the difference was about 0.198 eV. The obtained work function differences were found to be 0.157, 0.165, and 0.131 eV, respectively, for NiMoAl–20 wt % MoS2, NiMoAl–20 wt % Cr2AlC, and NiMoAl–20 wt % Cr2AlCAg samples (Figure g). These differences in work function (Δϕ) from the edge area (ϕ1) to the grooved area (ϕ2) clearly give an idea about the formation of the tribofilm in the grooved area. The presence of surface defects, their character, chemical composition, and density will influence Δϕ.[26] The NiMoAl alloy possesses high CoF (Figure b) and high EWF difference (Δϕ) (Figure g). The increase in ϕ2 (work function in the worn area) could be due to the formation of damaged surface film (NiO and NiMoO4) (Figures b and 5e). During dry wear tests, the surfaces having higher differences in EWFs showed higher CoF. A similar behavior is observed in the reported study,[39] which shows that when the surface film is damaged, the broken atomic bonds on the surface become active and lead to higher friction.[40] Compared to the NiMoAl alloy, the MoS2-added composite shows less surface damage and less Δϕ (Figure d), which is attributed to the formation of the MoO2 lubricating[35] conductive phase[41] (Figures b and 5e). The Cr2AlC MAX phase-added composites show a further reduction in the surface defects as compared to the above two samples. Even though the NiMoAl–20 wt % Cr2AlC composite (Figure e) exhibits a slightly higher Δϕ as compared to the NiMoAl–20 wt % MoS2 composite, it is associated with the formation of an adhered nonconductive Al2O3 phase[42] along with other oxides on the worn surface of NiMoAl–20 wt % Cr2AlC (Figures b and 5e). The NiMoAl–20 wt % Cr2AlCAg composite (Figure f) shows the least CoF (Figure b) and lowest EWF difference (Figure g), and it could be due to the formation of the highly conductive adhered Ag2MoO4 phase[43] on the surface along with the Al2O3 phase (Figures b and 5e). The well-adhered tribofilm is protecting the surface from damage. The difference in the surface potential associated with the sliding leads to changes in the structure and the deformation influences the energy levels of the solid. Generally, for metals, the work function differences are reported because of the occurrence of plastic deformation. When conductive and soft metals such as silver and gold undergo significant plastic deformation, it leads to a decrease in the work function.[44] As compared to the NiMoAl base alloy, the increases in the initial ϕ1 (before wear testing) (Figure c–f) are related to the addition of different solid lubricant materials. However, after the wear testing, the changes in the ϕ2 are dependent on the surface properties (conductivity of the tribofilm and the severity of surface defects), which stabilizes the friction regime. The obtained Δϕ in the present investigation depends on the conductivity of the tribofilm and the severity of surface defects, which is in agreement with the results reported by Zharin and Rigney.[28]

Because the experimental conditions such as load, method of wear testing, and so forth, can affect the tribological behavior, the CoF of the NiMoAl-based composites obtained in this work is not directly comparable with the other existing literature. Possibly, there can be two effects on the field of tribology: (1) on a micron level, the damaged surfaces with oxide fragments tend to be rougher, which can reduce the overall contact area and hence imparting lower friction; (2) the friction can be further reduced because of the rotation of debris formed during the wear testing. The obtained results are significant to the material selection and the tribological aspects in the field of tribology. The MoS2 and Cr2AlC added composites shows enhancement in the tribological properties. In the presence of MoS2 added NiMoAl composite, the surface exhibit the formation of spallation along with the segregation of patchy oxide (NiO), which reduces the integrity of the tribofilm. The Cr2AlC added composite shows Fe2O3 pick up from the counter material, exhibits abrasion wear, and its CoF is comparable to the MoS2 added composite. For automotive applications, the worn surface should possess the least possible surface delamination and abrasion wear. Among all the prepared NiMoAl-based composites, the NiMoAl–20 wt % Cr2AlCAg composite exhibits least CoF, maximum reduced wear rate, remarkable hardness, and least surface defects after the wear testing. The present study exhibits that the addition of the Cr2AlC–Ag solid lubricant as the second phase is the most suitable for the enhancement of tribological properties of the NiMoAl matrix under ambient conditions for automotive applications.

Conclusions

This present work successfully modified the wear and tribological properties of NiMoAl-based materials by using Cr2AlC and Cr2AlC–Ag composite solid lubricants and compared this with MoS2. The addition of 4 wt % silver (equivalent weight of silver in NiMoAl–20 wt % of the Cr2AlCAg composite) to the NiMoAl matrix shows almost a one-third reduction in the wear loss and a considerable drop in the CoF of the NiMoAl matrix. The work function differences from the edge area to the grooved area clearly give an idea about the formation of the tribofilm in the grooved area, which is confirmed through SKP measurements. Among all the prepared samples, the NiMoAl–20 wt % Cr2AlCAg composite exhibits excellent tribomechanical properties as compared to all the systems studied. A strong composite tribo film of mixed oxides was found to provide good lubrication in the Cr2AlC–Ag added sample. Based on the obtained results, it can be concluded that the NiMoAl–20 wt % Cr2AlCAg composite is beneficial as a lubricating material with low CoF and reduced wear loss, especially for automotive applications.

Experimental Section

Preparation of NiMoAl-Based Composites

For the present work, nickel (Ni), molybdenum (Mo), and aluminum (Al) powders of about 50–65 μm and 99.2% purity were obtained from Powder Alloy Corporation (Loveland, USA). MoS2 powder (99.2% purity, −325 mesh) was procured from Loba Chemie, and Ag powder (>99% pure, ∼325 mesh) was procured from SRL India. This work synthesized various phases separately: (a) synthesis of the base alloy (NiMoAl), (b) synthesis of the Cr2AlC MAX phase powder, (c) synthesis of the Cr2AlC–Ag composite powder, and (d) synthesis of NiMoAl-based composites with different solid lubricants. Ni, Mo, and Al powders were mixed using a turbo mixer (room temperature, spin speed: 50 rpm, MXM 2, Insmart, and India-make) for about 2 h, in the weight percentage of 54, 44, and 2%, respectively. This mixed powder is henceforth designated as NiMoAl.

Preparation of Cr2AlC MAX Phase and Cr2AlCAg Powders

The Cr2AlC MAX phase powder was prepared in-house, for the present study, using the method reported earlier with the same precursors and experimental apparatus.[45] The resultant product was crushed and sieved using a −325 mesh, to obtain the final Cr2AlC MAX phase powder. Further, the Cr2AlC powder was mixed with the 20 vol % Ag powder by using a turbo mixer for 3 h (room temperature, spin speed: 50 rpm, MXM 2, Insmart, and India-make). Then, the mixed powders were used to make pellets by a cold compaction press (∼70 MPa). The pellet (Cr2AlC–Ag) was sintered at about 1100 °C for 1 h under an argon atmosphere (flow rate 15 °C/min). The sintered sample was crushed and sieved (−325 mesh) to obtain the composite lubricant powder of Cr2AlC–Ag.[3]

Consolidation of NiMoAl-Based Composite Powders

In the next step, three different types of composite mixtures using 20 wt % of different solid lubricants were prepared by mixing for 2 h: (a) NiMoAl + Cr2AlC, (b) NiMoAl + (Cr2AlC–Ag), and (c) NiMoAl + MoS2. Composite mixtures were consolidated by hot pressing (Vacuum Hot Press, VB Ceramics, India) using a graphite die and punch (15 mm dia.). The hot pressing was carried out at 1100 °C for 30 min of holding at a pressure of 50 MPa in a vacuum level of 10–3 mbar. To avoid sticking of the powders to the punch, boron nitride spray was used. The relative densities of the sintered samples were calculated by the geometric and Archimedes principles.

X-ray Diffraction Study

Phases on various samples were analyzed using an X-ray diffractometer (XRD, PANalytical, Netherlands, with a Cu Kα radiation of wavelength 1.54 Å).

Morphological Study

Surface morphology and composition of composites were analyzed by FESEM (FEI Quanta FEG 200), and EDAX (Flash Detector 610m: Bruker Nano GmbH), respectively.

Mechanical Characteristics

Microhardness (HV—Vickers scale of hardness) was determined by employing 0.3 N force (Matsuzawa, VMT-X) for 10 s. The tests were repeated ten times on the same sample, and the average values were reported.

Tribological Study

The wear behavior was investigated using a pin-on-disc tester with a wear & friction monitor (TR-20L-PH200—DUCOM, India) against a hard-counter material (hardened steel disc). The test parameters adopted were 2 kg load, a speed of 300 rpm, a sliding distance of 3350 m, and a track diameter of 60 mm. All the wear tests were performed at room temperature according to the ASTM G99 standard with a relative humidity of 55–60% under dry-sliding conditions, and an average of three measurements was reported. Worn surfaces were characterized using FESEM and EDAX. Micro Raman analysis (HORIBA France, LABRAM HR Evolution, wavelength: 633 nm, magnification: 50×) was performed on the worn surfaces to understand the oxide layer formation.

SKP System

To determine the tribo film (in situ formed oxide), CPD measurements were carried out using a 2 mm diameter vibrational gold tip at a operating frequency of 78.3 Hz in the SKP system (SKP5050, KP Technology Ltd., UK). Work function differences (Δϕ = ϕ1 – ϕ2) from the edge area (ϕ1) to the wear/groove area (ϕ2) were analyzed. Then, between the conductive gold tip surface and the sample surface, the CPD was measured. An AC voltage, Vac(ω), with a vibrational frequency of 78.3 Hz was applied to the gold tip above the sample surface. Then, an electrostatic force, Fω, given by[46]is sensed, when the gold tip comes near to the sample surface. The CPD was calculated by the surface potential (Vs) which is nullified by the outer voltage (Voff) through a feedback loop. As a consequence, the electrostatic force Fω between the sample surface and the gold tip was counterbalanced. The gold tip was calibrated through standard gold surface measurement for each NiMoAl composite’s measurement. All the experiments were carried out at ambient temperature. Further, the obtained CPD were converted to work function by using the following eq as[46]where 5100 is the actual work function of the gold tip in meV, CPDAu is the CPD between the gold tip and the gold reference surface, and CPDSample is the CPD between the NiMoAl composite surfaces and the gold tip. To have an average value of work function for the better interpretation, the Kelvin probe tip scanned the edge area to the wear area (groove) on the surface of NiMoAl composites with an area of 19.36 mm2 area (raster scan), and the relative variation in the CPD was measured. The area of a single-pixel of SKP raster scan is 48 400 μm2 (x step = 220 μm, y step = 220 μm, and total scan area = 19.36 mm2).