Phase Lubrication Research Articles

Improvement of wear resistance of polyarylene ether nitrile composites through synergistic effect of PTFE and ZnO

Semi-crystalline polyarylene ether nitrile (PEN) has good thermal and mechanical properties, but its wear resistance needs to be improved, which limits its application in extreme environments. In this work, PEN was used as a matrix resin, disc-shaped zinc oxide (ZnO) was used as a wear-resistant reinforcing filler and nucleating agent, and polytetrafluoroethylene (PTFE) solid lubricating phase was introduced to fabricate the PEN/ZnO/PTFE composites. The lowest coefficient of friction and wear rate of PEN-based composites were observed when filled with 7.5 wt% of PTFE. To further improve the hardness and elastic modulus, the PEN/ZnO/PTFE composites were subjected to heat treatment, and the wear rate was decreased by 26.1 %. In addition, the properties of the composites were investigated under different loading conditions and the results showed a significant improvement in the tribological properties. This study further confirms that the synergistic effect of the rigid filler (ZnO) and solid lubricant (PTFE) can substantially reduce friction and wear. Thus, it is a significant advancement in the development of high-performance wear resistant PEN- based composites.

Study on the effect of TiC/B on the performance of laser cladding in-situ fabricating wear-resistant self-lubricating coatings

To improve the wear resistance of IN718, coatings containing the self-lubricating phase h-BN were prepared and the coating properties were analyzed in relation to the variation of laser power. The results show the TiC added to the cladding powder partially decomposes and in-situ fabricates new reinforcing phases TiN, B4C, etc. The lubricating phase h-BN was in-situ fabricated by adding B into the powder. As the laser power increased from 1.6 kW to 2 kW and 2.5 kW, eutectic microstructures of high melting point reinforcing and lubricating phases were observed. Due to the decomposition of TiC and in-situ fabricated of TiC2, TiN and B4C concentrations changing with the laser power, the surface hardness of the coatings prepared at 2.5 kW was the highest at 391.03HV0.5. Which is 1.096 and 1.108 times of 1.6 kW and 2 kW. At room temperature, the coating fabricated by 2 kW has the highest average friction coefficient of 0.537. This is because the self-lubricating phase h-BN is a high-temperature lubricating phase and the wear surface has a low oxide content, and is influenced by the hardness of the coating. At 500°C, the coating fabricated with 2.5 kW has the lowest average friction coefficient due to the best synergistic effect of the lubricating and reinforcing phases at 0.376. The 2.5 kW coating has the best wear resistance, with wear rates 0.69 and 0.54 times at room temperature and 0.82 and 0.8 times at 500°C of 1.6 kW and 2 kW. The wear mechanism at room temperature is abrasive wear, and at 500°C is mainly a combination of h-BN and oxide synergistic lubrication.

Topographical hard protective coating for joint replacement implants

Joint replacement surgery, essential for managing joint diseases, requires improvements in tribocorrosion performance to ensure surgical success and longevity of joint implants. Transition-metal light-element (TMLE) compound coatings, known for their high hardness and chemical stability, have been extensively researched and applied for surface protection of joint implants. However, these coatings typically lack a lubrication phase, leading to high friction coefficients and severe corrosion wear, which makes long-term effective protection challenging. A promising approach is to utilize the natural lubricating proteins present in body fluids, which are continuously available and can thus address long-term service issues of TMLE coatings. In this work, we utilized micro-arc oxidation (MAO) technology to develop an underlying morphology, then conformally deposited a TiB2 layer, resulting in a cratered dual-layer TiB2/MAO coating. This unique cratered dual-layer structure not only preserves the high hardness and wear resistance of TiB2 but also aims to (1) absorb wear particles to prevent abrasive wear and (2) increase surface energy to optimize protein lubrication capacity. Consequently, the TiB2/MAO coating exhibits low friction coefficients and wear rates in protein-containing simulated body fluids. Furthermore, the dual-layer TiB2/MAO coating demonstrates excellent corrosion resistance and biocompatibility. This dual-layer coating design synergistically combines the superior intrinsic properties of material with unique structural construction, while also harnessing continuously available external proteins as lubricants to further optimize performance, thereby introducing an advanced strategy for developing protective coatings for implant materials.

The improvement of wear and corrosion resistance of nickel-graphite coating with modified graphite phase size

The corrosion resistance, friction and wear properties of an abradable seal coating are highly dependent on the size and distribution of lubrication phase. In this work, nickel-graphite (Ni-G) coatings were fabricated by a supersonic atmospheric plasma spraying system (SAPS) and a standard atmospheric plasma spraying system (APS), respectively. The effect of size and distribution of graphite phase on the performance of Ni-G coatings was investigated. The results suggested that the SAPS coating had an average single graphite phase area of 35.4 ± 5.1 μm2, which is approximately 35.4 % less than that of the APS coating, while the average graphite phase width and length in SAPS coating decreased by 22.2 % and 16.7 % compared to the APS coating. The fine distributed graphite phase improved the formation of structure with a small cathode and a large anode, which effectively increased the corrosion resistance of SAPS coating. The reciprocating sliding friction facilitated the creation of a uniform and uninterrupted lubricant layer, attributed to the finely dispersed graphite phase of SAPS coating. In comparison to the as-sprayed and subsequent corroded APS coatings, the friction coefficients of the SAPS coating exhibited a reduction of approximately 9.1 % and 13.3 %, respectively. The aforementioned findings showed that the small-sized graphite phase was conductive to the improvement of corrosion resistance, friction and wear properties of Ni-G abradable coatings.

Friction Coefficient of Wet Clutches as a Function of Service Mileage

As a core component for efficient variable speed transmission and energy saving, wet clutches are widely used in the transmission systems of energy-saving and new energy vehicles. However, with an increase in the service mileage of the wet clutch, the friction coefficient undergoes alterations. This leads to a deterioration of the control accuracy of the clutch transmission torque, which ultimately has a negative impact on the dynamic characteristics and driving safety of the entire vehicle. In order to understand the service behavior of the friction coefficient in a wet clutch, wet clutches with different service mileages were investigated experimentally and theoretically. The results show that as the service mileage increased, the hydrodynamic lubrication phase was extended. Analyses of the three-dimensional profile of the friction plate and the theoretical simulation of the friction revealed that the edge ridges of the friction pads were flattened. This increased the clutch engagement force when the asperities on the separator and friction plates came into contact.

Organic-inorganic core-shell materials as reinforced fillers of UHMWPE to improve its tribological and mechanical properties

Generally, modification of materials by adding multiple types of nano particles could cause dispersion problems, generally limited the performance improvement effect. Herein, we report the modification of UHMWPE materials by organic-inorganic core-shell particles with PTFE as the core and Al(OH)3 as the shell, which prepared by in-situ polymerization of sodium bicarbonate and aluminum sulfate solution. Core-shell particles were filled in UHMWPE matrix and sintered into composite by spark plasma sintering (SPS). The mechanical and tribological properties of the composites were characterized by tensile compression testing machine, universal micro tribometer (UMT-5), three-dimensional white light interferometer and scanning electron microscope (SEM). The results showed that the addition of core-shell structural materials effectively improved the tribological properties (friction coefficient decreased by 40%, wear rate decreased by 30%) and mechanical properties (compression strength increased by 2.4 times) of the composites. It was demonstrated that the inorganic framework provided strength to improve the mechanical properties and wear resistance of materials, and the organic phase was used as a lubricating phase to reduce the friction coefficient of materials. Surface treatment of core-shell particles both improved the binding between particles and matrix and decreased the formation of agglomerations, thus significantly reducing ploughing phenomena’s effect produced by abrasive wear of agglomeration

Evolution of the structure and tribological behavior of the organic coatings under gamma-ray irradiation

More reliable and long-term irradiation stability is essential for lubrication protection materials that served in nuclear energy equipment. In this paper, three typical commercially organic bonded solid lubricating coatings were chosen to investigate the influence of gamma (γ)-ray irradiation on microstructure and tribological behaviors. The evolution of microstructure and the changes of tribological properties for these coatings after irradiation were comparatively studied in detail. The results revealed that the crystallinity of the molybdenum disulfide (MoS2) lubricating phase was improved and were partial oxidized in the coatings after γ-ray irradiation, resulting in the surface roughness of the coating with MoS2 as the solid lubricating phase decreased significantly. The polytetrafluoroethylene (PTFE) lubricating phase was degraded from the long chains structure to smaller molecules after γ-ray irradiation. The results of tribological properties of the three coatings after γ-ray irradiation indicated that, for polyamide-imide-based lubricating coatings, the friction coefficient is decreased and the wear rate is increased after irradiation. For phenolic epoxy-based lubricating coating, the cross-linking effect of γ-ray irradiation predominated to improve the wear resistance and prolong the wear life of the coating. But overall, the influence of irradiation on tribological properties of three polymer-based coatings could almost be neglected duo to the changes of the microstructure in nanoscale. This work provides path and direction for design and preparation of the lubricating coatings that served in a nuclear reactor.

Effect of spray parameters on the microstructure, solid lubricant phase retention, and high-temperature tribological performance of plasma sprayed YSZ/BaF2 composite coatings

This report deals with the development of a self-lubricated plasma sprayed YSZ/BaF2 coating taking the following into account: (i) the compatibility of the solid lubricant BaF2 with YSZ ceramic in terms of wettability, (ii) the homogeneous dispersion of lubricant in the ceramic matrix, and (iii) the measurement of weight fraction of lubricant phase finally retained in the coating microstructure using Rietveld refinement. The YSZ/BaF2 mix demonstrated excellent flowability, and homogeneous distribution of BaF2 with a dopant concentration of 15 wt%. The primary purpose of this work is to explore the influence of the spray parameters on BaF2 phase retention in the YSZ/BaF2 coatings, and its tribological performance at a high temperature. Under the given parametric conditions, a maximum of 3 wt% of BaF2 was retained. The YSZ/BaF2 coating reduced the coefficient of friction and wear rate of YSZ by 39% and 38%, respectively at a temperature of 600 °C.

Research on the change in lubrication state and temperature control strategy of high-speed train axle box bearings using non-destructive testing

ABSTRACT The heat generation problem of axle box bearing during churning phase leads to abnormal temperature rise and temperature alarm, which affects the normal operation of high-speed train. Aiming at this problem, the temperature characteristics of bearing initial lubrication phase is studied, and the multi-speed grade cycle running schemes are used in tests to run multiple bearings for multiple times. The temperature data and friction data in tests are collected by using non-destructive testing method, and characteristic extraction is carried out. Under the premise of not destroying the internal structure and lubrication state of the bearing, the lubrication state inside the bearing is reflected by the change of the characteristic values. A 118 min scheme is proposed, and the bearing using this scheme to run on the test rig is used in the actual operation. The results show that the temperature of the bearing with running-in test is lower than that of the bearing without running-in test, which indicates the effectiveness of the proposed temperature control strategy.

Role of the codeposited C and W element on the tribological performance of WS2 coating

WS2 is a widely used solid lubricating material that exhibits applications in various fields, including automotive components, precision instruments, and key parts that require antiwear properties. However, WS2 is highly susceptible to humidity, which significantly limits its practical utility. In order to investigate and enhance the tribological and mechanical properties of WS2 coatings, a method involving codeposition of carbon (C) and tungsten (W) with WS2 was employed using magnetron sputtering, resulting in the successful preparation of W-diamondlike carbon (DLC)/WS2 composite coatings. Subsequent investigations revealed a synergistic effect of C and W through various methods. The addition of W noticeably improved both the nanohardness and Young’s modulus of the W-DLC coatings, thereby further enhancing the mechanical properties of the W-DLC/WS2 composite coatings and improving their wear resistance. However, it was observed that excessive W tended to oxidize, intensifying the abrasive wear of the composite coatings during friction. Moreover, Raman spectroscopy analysis indicated the presence of carbon in the composite coatings in the form of DLC, which served as a lubrication phase. The presence of DLC facilitated the formation of transfer films composed of graphite. Additionally, it was discovered that the number of graphite layers in the transfer films had an impact on the tribological properties of the coatings.

Microstructure and Properties of FeAlC-x(WC-Co) Composite Coating Prepared through Plasma Transfer Arc Cladding

Tungsten carbide (WC) is widely used in wear-resistant parts due to its excellent wear resistance. Iron-based alloys are used in the repair and remanufacturing of engine components due to their good compatibility with iron-based workpieces. In order to enhance the wear resistance of engine components in service under abrasive conditions, composite coatings have been prepared for cast iron engine components by adding WC-Co to iron-based powders. This study investigates the microstructure and wear properties of composite coatings of iron-based alloys reinforced with different contents of WC particles. The composite coatings mainly contained γ-Fe, α-Fe, WC and Fe3W3C. With the addition of the WC-Co strengthening phase, the average hardness of the FeAlC-x(WC-Co) composite coatings increases from 524 HV0.2 to 814 HV0.2. Wear test results showed that when the WC addition was 20%, it had the lowest frictional coefficient of 0.5 and the lowest wear mass loss of 1.3 mg. Compared to the original Fe-based alloy coatings, the WC particle-reinforced FeAlC composite coatings display improved wear resistance on a reduced friction basis, mainly benefiting from the high wear resistance of the graphite solid lubrication phase and carbides in the cladding.

Synergistic effect of V and Ag diffusion favored the temperature-adaptive tribological behavior of VAlN/Ag multi-layer coating

Surface coating technology offers several benefits (excellent wear resistance, low coefficient of friction-COF, better corrosion durability etc.) for key components used in harsh aero-engines. However, the development of temperature-adaptive lubricant coating is still the crucial issue. In this study, VAlN/Ag multi-layer coatings with various thickness ratios were fabricated by a hybrid deposition system composing of DCMS and HiPIMS techniques. Results showed that all coatings presented Ag layers with growth twins, and fcc-VAlN layers with columnar structure. Increasing the temperature from 25 to 650 ℃ led to a decrease of COF from 0.47 to 0.22 in coatings, where the lowest COF and wear rate was particularly achieved at 0.08 and 4.3 × 10−5 mm3N−1m−1 under higher rotation speed, respectively. Based on the microstructural characterizations on wear track of coating and wear scar of counter ball, the synergistic diffusion of V and Ag elements at elevating temperature stimulated the in-situ formation of distinctive Al2O3/AgVO3-Ag3VO4/Ag microstructure, which encompass lubricant phases for lower COF and high wear resistance. The reason behind of this phenomena could be attributed to the easily broken of Ag-O bonds in lamellar silver vanadium oxides at high temperature, accompanying the remarkable lubrication of Ag on top layer.

Self-lubricating and wear-resistant epoxy resin coatings based on the “soft-hard” synergistic mechanism for rapid self-healing under photo-thermal conditions

Mechanical components made of alloy steel are susceptible to severe surface wear after long periods of operation. The construction of protective coatings with high wear resistance and self-healing properties provides references for solving this problem. Herein, a highly wear-resistant coating (named as C-EP/PFW) that can rapidly self-healing under photothermal conditions was constructed by introducing both a novel phase-change material (polytetrafluoro-wax) and polydopamine-functionalized carbon nanotubes (PDA/CNTs) into the epoxy resin. The melt-phase transition of PFW at approx. 120 °C induces intermolecular phase migration and repair of the damaged area. The PDA/CNTs can act as microscopic bearings to enhance the load-bearing capacity of the composite coating, they can also construct thermal conductivity networks to enhance the thermal conductivity and repair efficiency of the coating. Based on the perfect combination of CNTs' excellent thermal conductivity and PDA's photothermal conversion capability, the average repair rate of the composite coating was increased by 20.7 % than pure EP/PFW coating. Additionally, the C-EP/PFW coatings in both dry and wet friction environments (seawater and sunflower oil) exhibited excellent self-lubricating properties. This is attributed to the synergistic of the soft lubricating phase on the repair of the lubrication transfer film and the microscopic slipping layer constructed by the CNTs at the tribofilm. The C-EP/PFW coatings, which are capable of rapid self-healing in thermal environments, provide a reference for the construction of highly wear-resistant protective layers on the surfaces of steel mechanical components.

High temperature self-lubricating Ti-Mo-Ag composites with exceptional high mechanical strength and wear resistance

Titanium alloys are of keen interest as lightweight structural materials for aerospace and automotive industries. However, a longstanding problem for these materials is their poor tribological performances. Herein, we designed and fabricated a multiphase Ti-Mo-Ag composite (TMA) with heterogeneous triple-phase precipitation (TPP) structure by spark plasma sintering. A lamellar α-phase (αL) precipitates from the β-phase under furnace cooling conditions and maintains a Burgers orientation relationship (BOR) with β-matrix. An active eutectic transition also occurs in the titanium matrix, resulting in TiAg phase. The intersecting acicular TiAg and lamellar αL cut β grains into fine blocks and the primary equiaxed α phase also provides many interfaces with β phase, which together effectively impede dislocation movement and increase strength. Compared with other titanium composites, TMA with TPP microstructure gets an excellent combination of strength (yield strength 1205 MPa) and toughness (fracture strain 27%). Furthermore, the TPP structure endows TMA with strong cracking resistance, which aids in reducing abrasive debris at high temperatures during sliding and obtaining a low wear rate. Simultaneously, Ag particles distributed at grain boundaries will easily diffuse to the wear surface, in situ forming the necessary lubricating phase Ag2MoO4 with Mo-rich matrix debris via oxidation. TMA possesses excellent tribological properties with especially low wear rate of 8.0 × 10−6 mm3N−1m−1 and friction coefficient (CoF) of merely 0.20 at 600 °C. Unlike other self-lubricating composites with high volume fraction of soft ceramic lubricants, which inevitably sacrifice their mechanical strength and ductility, the composite TMA possesses well-balanced strength, toughness and self-lubricating properties. It holds important implications to design other metal matrix self-lubricating composites (MMSCs) used for load-bearing moving parts.

One-step fabrication of amorphous/ITO-CNTs coating by plasma electrolytic oxidation with particle addition for excellent wear resistance

As moving machine parts, aluminum alloys suffer from poor wear resistance, however, the combination of amorphous materials and anti-friction lubricant phases has rarely been investigated to improve tribological performance. The amorphous coatings were fabricated by one-step plasma electrolytic oxidation (PEO) with ITO and CNTs addition, of which CNTs were used as lubricant additives to enhance the anti-friction properties of aluminum alloy. It is found that the amorphous phases in the coatings increase with ITO concentration in the electrolyte. As well as, when the concentration of ITO in the electrolyte reaches 40 g/L, XRD cannot detect Al2O3 phase, but only ITO and amorphous phase. The formation of the amorphous coating is due to the fact that In3+ can be used as a network modifier to improve the glass-forming ability of molten oxides and the rapid cooling after discharge. Further, the amorphous coating exhibits a higher hardness (1472 HV0.5) compared to single Al2O3 coatings (267 HV0.5) and its main wear mechanism is adhesive wear with a high friction coefficient (∼0.68). In contrast, with the incorporation of CNTs, the amorphous coating exhibits slight abrasive wear after 6000 sliding cycles under dry friction conditions, with a stable frictional coefficient of around 0.14. Of which, the amorphous coating (1704 HV0.5) serves as a load-bearing and CNT acts as a lubricant phase during friction and wear. Hence, through the synergistic effect of the CNTs additive and the amorphous coating, the tribological properties of aluminum alloy are significantly improved, with a low friction coefficient and shallow wear trace.

Tribological behavior of a novel Si- and WC- co-reinforced a-C multilayer coating at 25– 500 °C

To improve the applicability of a-C coatings in a wide temperature range, a novel Si- and WC- co-reinforced amorphous carbon coating (SiC-nWCC), consisting of Si-doped a-C nanolayers and WC/a-C nano-multilayers, is proposed in this paper. A series of SiC-nWCC coatings with different modulation ratios (0.49–1.98) were fabricated by a closed field unbalanced magnetron sputtering system. The influences of modulation ratio on the microstructure, mechanical properties together with thermal stability of SiC-nWCC coatings were explored. The tribological behaviors of SiC-nWCC coatings against M50 steel balls were investigated at room temperature to 500 °C in ambient air. The results show that the SiC-nWCC coating with the modulation ratio of 1.98 achieved superior wide-temperature applicability, featuring the average friction coefficient lower than 0.17 and the wear rate less than 9.84 × 10−6 mm3/N·m in the whole temperature range of 25–500 °C. The excellent wide-temperature tribological performance of the SiC-nWCC coating are mainly attributed to the good thermal stability and the formation of temperature adaptive tribological layers. At 25 °C, sp2-rich carbon is the main lubrication phase of tribological layers, and at 200– 400 °C, the lubrication performance of coating relies on both sp2-rich carbon and Si-rich oxides, and at 500 °C, the self-lubrication phases of Si-rich oxides and WO3 are account for the outstanding lubricating performance of coating. This work provides a profound insight for developing the wide-temperature applicable a-C based coatings.

Tribological behavior and lubrication mechanism of h-BN/ceramic composites: Effects of h-BN platelet size and ceramic phase

The h-BN platelet size causes significant changes in the efficiency of extruding the lubricant phase from the material interior onto the friction surface, affecting the lubricating films forming and friction coefficient. The highest h-BN release efficiency was achieved when h-BN size in the h-BN/SiO2 composite was 7 µm, causing the lowest friction coefficient (0.20). In addition, the binding force of h-BN and ceramic phases are negatively correlated with their friction coefficients. The high binding force between the two phases contributes to the h-BN release efficiency and the "trapping" of abrasive debris, resulting in the continuous lubricating films and low friction coefficient. The highest binding force between Si3N4 and h-BN, resulting in high h-BN release efficiency and better friction coefficient (0.21).

In situ formation of spherical MoS2 particles on high-entropy alloy coating for low friction

Introductionof lubricant phase into thermally sprayed coating while avoids high-temperature ablation is still a challenge. Here, vacuum impregnation and hydrothermal reaction, acted as a post-treatment process, was applied to synthesize MoS2 phase on and within high-entropy alloy (HEA) coating. The as-synthesized MoS2 phase with high purity and spherical shape distributed in the pores and finally formed a thin layer on coating surface. Compared to the single HEA coating, coefficient of friction (COF) and wear rate (Q) of the HEA/MoS2 composite coatings possessed a significantly reduction, i.e 87.2% and 75.6% reduction in COF and Q at 2 N, respectively. Besides, owing to the formation of lubricating film during the sliding process, the composite coating had a more stable sliding process. At the same time, the doped MoS2 played a positive role in reducing abrasive- and adhesive wears for both coating and coupling ball. Since the lubricating film can be continuously replenished and repaired, the HEA/MoS2 self-lubricating coatings maintained a good chemical stability and provided a relatively good wear properties.

Effect of Ti3SiC2 mass fraction on microstructure and tribological performance of laser cladded NiCr coating at high temperature

PurposeThis paper aims to investigate the effect of Ti3SiC2 on the high-temperature tribological behaviors of NiCr coating, which was beneficial to improve the friction-wear performance of hot work mold.Design/methodology/approachNiCr-Ti3SiC2 coatings were prepared on H13 steel substrate by laser cladding. The microstructure, phases and hardness of obtained coatings were analyzed using a super-depth of field microscope, X-ray diffraction and microhardness tester, respectively, and the tribological performance of obtained coatings at 500°C was investigated using a high-temperature tester.FindingsThe results show the NiCr-Ti3SiC2 coatings are comprised of γ-Ni solid, solution, TixNiy, TiC and Ti3SiC2 phases, and the coating hardness is increased with the increase of Ti3SiC2 mass fraction, which is contributed to the fine-grain and dispersion strengthening effect by the addition of Ti3SiC2. The NiCr-Ti3SiC2 coatings present excellent friction reduction and wear resistance by the synergetic action of Ti3SiC2 lubricant and hard phase, and the wear mechanism is predominated by abrasive wear and oxidation wear.Originality/valueTi3SiC2 phase was used to reinforce the tribological performance of H13 steel at high temperature, and the roles of friction reduction and wear resistance were discussed.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2023-0004/

Molecular Probing of the Stress Activation Volume in Vapor Phase Lubricated Friction.

When two solid objects slide over each other, friction results from the interactions between the asperities of the (invariably rough) surfaces. Lubrication happens when viscous lubricants separate the two surfaces and carry the load such that solid-on-solid contacts are avoided. Yet, even small amounts of low-viscosity lubricants can still significantly lower friction through a process called boundary lubrication. Understanding the origin of the boundary lubricating effect is hampered by challenges in measuring the interfacial properties of lubricants directly between the two surfaces. Here, we use rigidochromic fluorescent probe molecules to measure precisely what happens on a molecular scale during vapor-phase boundary lubrication of a polymer bead-on-glass interface. The probe molecules have a longer fluorescence lifetime in a confined environment, which allows one to measure the area of real contact between rough surfaces and infer the shear stress at the lubricated interfaces. The latter is shown to be proportional to the inverse of the local interfacial free volume determined using the measured fluorescence lifetime. The free volume can then be used in an Eyring-type model as the stress activation volume, allowing to collapse the data of stress as a function of sliding velocity and partial pressure of the vapor phase lubricant. This shows directly that as more boundary lubricant is applied, larger clusters of lubricant molecules become involved in the shear process thereby lowering the friction.