Self-lubricating Materials Research Articles

Influence of ceramic coating pores on the Tribological performance of PEO–PTFE composite coatings on the Ta–12W alloy

Self-lubricating composite coatings with an intermeshing structure on the Ta–12W alloy were fabricated using plasma electrolytic oxidation (PEO) and polytetrafluoroethylene (PTFE) impregnation sintering. Porous PEO ceramic coatings were formed in alkaline silicate–phosphate electrolytes using NaF and NaOH as the pore morphology adjustment agents. The tribological performances of the PEO ceramic and PEO–PTFE composite coatings were examined under 10–70-N loadings using a ball-disk wear tester. An optimized basic PEO ceramic coating with a thickness of 25 μm, a porosity of 23.8 %, and a hardness of 1410 HV is obtained in an electrolyte containing 5 g/L of NaF and 5 g/L of NaOH. A soft PTFE lubricating layer is fixed on the hard PEO ceramic coating, forming an intermeshing structure that can effectively reduce the coefficient of friction (COF) over time while enhancing the load capacity of the composite coating. In addition, the synergistic effect of the PEO ceramic coatings and self-lubricating material is proposed. The PTFE material stored in the pores will be constantly transferred to the contact interface to achieve a reduced COF. The PEO ceramic coating with a suitable porous structure acts as an “armor” because it can withstand a large load and provide physical support to the PTFE material during the friction process. Thus, the composite coating achieves a low COF with a low wear rate and a high load-bearing capacity.

Improving tribological performance of porous oil-impregnated GO/PA6 composites with double lubrication structure

Using carbon nanomaterials to construct self-lubricating polymer materials with porous self-lubricating structures has become an effective way to improve the tribological performance of polymer composites. The synergistic effect between 1.0 wt% graphene oxide (GO) and lubricating oil resulted in excellent tribological properties of polyamide 6 (PA6) composites under different loads and speeds. It indicated that the introduction of GO in porous PA6 matrix (GO/PA6) would provide enhanced tribological performance with the friction coefficient reduction by 23.4% and wear resistance improvement by 36.2%. This was explained by the lubricating oil stored in the pores of PA6 material would be released to the surface of the matrix material under the dual effect of load and temperature during the friction process. The released lubricating oil would formed a solid-liquid double lubrication structure with the well-dispersed GO in the matrix, thus the porous oil-immersed GO/PA6 composite materials demonstrates excellent tribological performance. This work provides a solid-liquid double lubrication strategy for enhancing the tribological properties of self-lubricating polymers, which would promote the self-lubricating polymer composites more widely applied in the field of friction materials.

Preparation and Tribological Properties of Self-Lubricating Epoxy Resins with Oil-Containing Nanocapsules.

The design and development of self-lubricating materials aid in enhancing the tribological performance and prolonging the service life of aero self-lubricating spherical plain bearings. Herein, monodisperse hollow mesoporous carbon nanospheres (HMCNs) with thin shell thickness and uniform particle size were synthesized. Afterward, oil-containing nanocapsules (Oil@HMCNs) were obtained by the impregnation method, which was an innovative approach to prepare nanoscale capsules. The Oil@HMCNs were monodispersed, and the oil content rate could reach 45 wt %. The nanocapsules were applied as additives to achieve the self-lubricating properties of epoxy resin (EP). It was revealed that the coefficient of friction (COF) and the wear rate of EP in dry friction can be decreased from 0.56 to 0.084 and from 50.15 × 10-6 to 0.27 × 10-6 mm3/N/m, respectively. The tribological properties of EP composites in dry friction are better than those of EP with external oil. According to the analysis of friction pairs, the antifriction performance was contributed by the released automatic transmission fluid 6HP (ATF6) from nanocapsules. Meanwhile, the presence of ATF6 effectively could inhibit fatigue wear, and the decrease of compressive strength and surface hardness led to plastic deformation and mechanical polishing. Both resulted in a lower wear rate of EP. In conclusion, this work explored the application possibility of hollow mesoporous materials in lubrication. The prepared EP composites exhibited excellent tribological performances and self-lubricating properties, which were expected to be applied to self-lubricating spherical plain bearings and other mechanical parts.

High-Pressure synthesis of Al2O3-cBN-hBN Self-lubricating ceramic

Al2O3-based self-lubricating ceramics are cutting tool materials that meet the extreme working conditions of dry cutting. However, due to the soft lubricating components, the mechanical properties of composites are dramatically reduced. In this paper, Al2O3-cBN-hBN self-lubricating material was designed. High density, fine grain ceramics were synthesized by high-pressure technology. The influence of thermodynamic condition, cBN/hBN addition on mechanical and thermal properties of sintered composites was analyzed. Friction and wear tests were carried out. It is found that the addition of cBN particles can significantly improve the mechanical and thermal properties of the composites. However, the truss effect caused by excessive volume fraction of cBN can reduce the pressure transfer efficiency, and then affect the properties of sintered composites. The different hBN content can change the wear mechanism of the composites. The release efficiency of hBN particles in composites was enhanced due to the large modulus difference between cBN and hBN. The well-sintered Al2O3-cBN-hBN bulks present an excellent mechanical property. The hardness of the composites is improved at least by 54% compared with the traditional self-lubricating material on the premise of ensuring fracture toughness and lubrication performance.

Metallurgical reactions and tribological properties of self-lubricating Al-WS2 composites: Laser powder bed fusion Vs. spark plasma sintering

Self-lubricating aluminium-based composites reinforced with solid lubricants promise to meet the demand for lightweight materials in green tribological applications. The design advantages granted by additive manufacturing (AM) processes coupled with their capacity for in-situ production of composite materials are yet to be exploited in the realm of Al-transition metal dichalcogenides composites. In this work, laser powder bed fusion (LPBF) was deployed for the in-situ fabrication of Al-WS2 composites for the first time, elucidating the process-structure–property relationships in comparison to reference spark plasma sintering (SPS) samples. The WS2 response to the respective fabrication technique was also firstly investigated through a holistic characterisation. The formation of new phases (W for LPBF, Al5W and Al12W for SPS) provided the potential for microstructural tailoring for optimal tribological performance. For tribological properties, LPBF Al-WS2 exhibited a coefficient of friction (COF) 0.55 ± 0.01 and specific wear rate 3.4 ± 0.3 × 10−3 mm3/N∙m, slightly better than the SPS counterpart (COF 0.57 ± 0.02, specific wear rate 3.6 ± 0.3 × 10−3 mm3/N∙m). Furthermore, a novel methodology for studying the evolution of worn surfaces is proposed and validated, by which a tribo-layer formed at lower friction cycles was observed for the LPBF samples, meaning that AM will also be advantageous for the performance aspect of self-lubricating materials.

Wear in-situ self-healing polymer composites incorporated with bifunctional microcapsules

Self-lubricating materials inevitably suffered mechanical damage such as wear, micro-scratch and cracks, which will shorten the service life of materials. The highlight of this work is to improve the wear resistance of the material by coupling self-lubricating and self-healing. Herein, we investigated the ability of a polymer composite incorporated with bifunctional microcapsules to heal mechanical damage. Thermally robust microcapsules containing organic solvent dibutyl phthalate (DBP) and linseed oil (LO) were synthesized by in situ polymerization of urea and formaldehyde in an oil-in-water emulsion. The microcapsules were filled in an epoxy (EP) resin matrix and cured to form composite materials. Multiple characterization methods such as 3D scratch instrument, UMT-5 machine and Atomic Force Microscope (AFM) were adopted to investigate the healing performance and healing mechanism of materials. The composite materials incorporated with bifunctional microcapsules presented excellent scratch self-healing ability (the self-healing efficiency was higher than 96% by volume) and wear resistance (the friction coefficient has been reduced by 92% and the wear rate has been reduced by three orders of magnitude). It was demonstrated that the combination of lubrication enhancement and wear scar self-healing could greatly prolong the service life of lubricating materials.

Synergistic lubrication mechanisms of AISI 4140 steel in dual lubrication systems of multi-solid coating and oil lubrication

In order to improve tribological properties of AISI 4140 steel (AS) for dry/starved lubrication, AS biomimetic self-lubricating material (AS-SACT) was prepared by filling SnAgCu-TiC into biomimetic microtextures. Compared with AS, the friction coefficient of AS-SACT was reduced by 5.34% and wear depth was reduced by 62.39% in starved lubrication. Tribological properties of AS-SACT in starved oil are close to those of full oil lubrication. Under starved lubrication, the precipitated lubricant spreads into a 2.2 µm solid film on contact surface. In addition, the lubricant precipitation area forms a 'storage area' for storing lubricating oil and wear debris. The synergistic lubrication effect of multi-solid coating and oil lubrication makes AS-SACT have excellent anti-friction and wear-resisting effect.

Synthesis and properties characterization of an Al2O3-based ceramic materials with intragranular self-lubricating nanostructure

In this study, a novel intragranular self-lubricating nanostructure was proposed for preparing a potential ceramic cutting tool material. Nano-sized CaF2, which meets the requirements of such a material, was prepared and used in the synthesis of ceramic materials with intragranular self-lubricating nanostructure. Based on the obtained results, an Al2O3/TiC/CaF2 intragranular self-lubricating nanostructure ceramic material was prepared by vacuum hot-pressing sintering. As the nano-sized CaF2 content increased, the flexural strength first increased and then decreased, hardness decreased, and fracture toughness increased. The results indicated that the formation of intragranular nanostructures could significantly improve the friction performance of this material. The friction coefficient of this material against a 40Cr steel ball was 0.12–0.25, and decreased as the CaF2 content increased. The wear rate also decreased as the CaF2 content increased from 3.3–10 vol%, but the friction coefficient decreased only slightly. Furthermore, the wear rate increased when 13.3 vol% CaF2 was added.

How Polytetrafluoroethylene Lubricates Iron: An Atomistic View by Reactive Molecular Dynamics.

The tribochemistry and transfer film formation at the metal/polymer interface plays an essential role in surface protection, wear reduction, and lubrication. Although the topic has been studied for decades, challenges persist in clarifying the nanoscale mechanism and dynamic evolution of tribochemical reactions. To investigate the tribochemistry between iron and polytetrafluoroethylene (PTFE) in ambient and cryogenic environments, we have trained and expanded a ReaxFF reactive force field to describe iron-oxygen-water-PTFE systems (C/H/O/F/Fe). Using ReaxFF molecular dynamics simulations, we find that mechanical shearing of single asperity induced the degradation of PTFE molecules and radicals, showing subsequent oxidation and hydroxylation reactions of the radicals initiated by C-C bond cleavage, in agreement with previous experimental observations. Furthermore, we studied mechanisms of interfacial tribochemical reactions and formation of transfer films. We found that tribochemical wear and Fe-C and Fe-F bonding networks are important mechanisms for anchoring molecular chains to form a transfer film on the iron countersurface. Hydroxyl groups can dehydrogenate to form short and strong chelation bonds with the Fe2O3 countersurface. A friction-induced oriented molecular layer plays a key role in reducing friction, which is responsible for the excellent lubrication property. By varying temperatures in the range of 10-300 K, we found a nonmonotonic change in friction with a maxima at 100 K. At cryogenic temperatures, the molecular mobility was obviously suppressed, while the chain rigidity was enhanced, resulting in the less oriented interface and brittle-like shear interface, which is responsible for nonmonotonic friction. This work elucidates mechanisms of tribochemical reactions and transfer film formation between iron and PTFE at the atomistic level, facilitating design and development of self-lubricating materials, especially under harsh conditions.

Tailoring Cu nano Bi self-lubricating alloy material by shift-speed ball milling flake powder metallurgy

A novel method for preparing lead-free Cu nano Bi (CNB) alloy materials by nano Bi powder and shift-speed ball milling (SSBM) flake powder metallurgy was proposed. To improve the mechanical and tribological properties of the alloy materials, the spherical CuSn10 powder was firstly extruded and sheared into flakes by low-speed ball milling (LSBM) in the SSBM system. Meanwhile, the nano Bi powder was evenly dispersed and coated on the CuSn10 flakes. The mechanical alloying between mixed powders and the cold welding of flake CuSn10 flakes containing nano Bi into layered particles were then realized through short-time high-speed ball milling (HSBM) in the SSBM system. While promoting the uniform dispersion of lubricating phase Bi, the surface area of matrix CuSn10 flake was increased so as to achieve better combination of CuSn10 matrix in the material. Results show that the mechanical properties of the CNB material prepared through nano Bi powder and SSBM flake powder metallurgy improved compared with the Cu-Bi material prepared by HSBM. In addition, the antifriction and wear resistance improved by 30% and 80%, respectively.

INFLUENCE OF TFE EXPLOITATION TEST CONDITIONS ON FRICTION AND WEAR OF POLYMER COMPOSITES

The creation of new composite self-lubricating polymer materials with the use of Armenian minerals, and the study of the filler influence on tribological processes and working parameters of tribojoints is an actual problem in machine and device building.The developed composites are characterized by increased wear resistance, improved strength characteristics, and a reduced coefficient of friction, which expands the range of possibilities of their application in modern friction units of machines.The processes of friction and wear of antifriction self-lubricating polymer materials are significantly determined by the phenomenon of friction transfer, which is an important aspect of the adhesion mechanism of polymer wear and plays a significant role in the mechanism of self-lubrication of metalpolymer friction pairs. The analysis of tribological tests of composite materials depending on test conditions is carried out. The tribological behavior of polyphenilene sulfide filled with molybdenum concentrate and polytetrafluoroethilene sliding against a steel counterface is investigated. The influence of the filler proportions, sliding speed and counterface roughness on the wear rate and coefficient of friction of the self-lubricating polymer-based composites has been studied. For this purpose pin-on-disk configuration and the design of experimental approach utilizing Taguchi’s orthogonal arrays are used, allowing to make a reasonable choice of composites with minimum wear.

Wear characteristics of sintered Cu-Sn/graphite lubricants composites

The incorporation of finely dispersed particles in a metal matrix by a suitable process led to new generation composite that enables the production of large range of composites with unique properties. Copper based sintered composites produced by powder metallurgy processes are now widely used in engineering parts such as bearings and bushes. This kind of bearings used in aerospace industries. Composites based on copper tin alloys containing a solid lubricant like graphite/Tungsten disulfide/PTFE has been developed as self-lubricating material. In this work, sintered Cu-Sn-Graphite composite were prepared with various compositions.The presence of particles of dry lubricant powder was identified with the help of SEM. Wear characterization such as co efficient of friction, wear rate and hardness test were conducted scratch tester is used to find the coefficient of friction and wear of the sintered composites. The wear rate has been decreased from 25.85% to 10.39% when grain size is 5 to 10 µm and 15 vol% of graphite is added. The percentage of decrease of wear rate 15.5%. The co efficient of friction is decreased from 0.4 to 0.25 when grain size is 5 to 10 µm and 25 vol% of graphite is added. The percentage of decrease of Co efficient of friction is 37.5%. Comparing wear rate and co efficient of friction grain size of 5 to10-micron graphite can be added till 25 vol% for obtaining better result.

Fretting study of an impregnated graphite bearing/nitrided stainless steel in a dry contact submitted to severe thermal-vibratory loading

Some industrial applications require the use of self-lubricating materials when fluid lubrication cannot be used. Carbon-based materials and in particular graphites are usually a used solution. However, the application of these materials is limited at high temperature; these materials are exposed to significant degradation and to high friction due to their high sensitivity to the air humidity and to the desorption phenomenon. This study determines the influence of a specific metallic impregnation on the graphite bearing, which is submitted to a severe thermo-vibratory loading by fretting against a stainless steel surface. The stainless steel surface has undergone a nitriding treatment by plasma. During the fretting contact, a strong transfer of the impregnant takes place from the impregnated graphite bearing to the steel conterface by adhesion; this deposit film allows a significant improvement in the tribological properties of the contact surfaces at high temperature.

OPTIMIZATION OF TRIBOLOGICAL PROPERTIES OF AN EPOXY HYBRID POLYMER COMPOSITE REINFORCED WITH ZrB2 AND PTFE PARTICLES USING RESPONSE SURFACE METHODOLOGY FOR HIGH-TEMPERATURE TRIBOLOGICAL APPLICATIONS

Hybrid PTFE/epoxy composites are widely used as materials for self-lubricating spherical bearing which are used in a high-temperature environment. In the present work, zirconium diboride (ZrB2) particles are incorporated to enhance high-temperature tribological properties of PTFE/epoxy composites. Pin on disc experiment is conducted with the aid of design of experiments (DOE) using central composite-response surface methodology (RSM). Under a load of 40 N and 1.25 m/s sliding speed, the optimum content 5.95 vol% of PTFE and 5.05 vol% of ZrB2, yields an ultralow coefficient of friction (COF) in conjunction with a low wear rate of the composite. The addition of ultra-high-temperature ceramic ZrB2 particles and solid lubricant PTFE is found to enhance the thermal conductivity and improve the heat transfer thereby reducing contact temperature. The use of optimum composition of the composite is capable of reducing the wear rate and high local temperature due to friction, implying its potential use as a self-lubricating spherical bearing liner material.

Dual-Phase Nanocomposite TiB2/MoS1.7B0.3: An Excellent Ultralow Friction and Ultralow Wear Self-Lubricating Material.

Proper lubrication is essential to the reliable and efficient operation of mechanical systems ranging from the industrial to the nanoscale. Self-lubricating materials that can self-generate and sustain concurrent ultralow friction and ultralow wear in harsh environments open up a unique realm that is unattainable by traditional external lubrication mechanisms, but developing such extraordinary materials has been a long-standing grand challenge. Here, we devise an unconventional strategy to construct a dual-phase nanocomposite (DPNC) that comprises a wear-resistant phase (TiB2) and an internal lubricant source (MoS1.7B0.3). Tribological tests demonstrate simultaneous ultralow friction coefficient (∼0.03) and ultralow wear rate (∼10-10 mm3·N-1·m-1) of the synthesized DPNC specimen in ambient environments; these superb properties remain intact after the specimen has been annealed at 400 °C in air. First-principles energetic and stress-strain calculations elucidate atomistic mechanisms underpinning DPNC TiB2/MoS1.7B0.3 as an ultimate self-lubricating material. This accomplishment solves the classic lubricity-durability tradeoff dilemma, enabling advances to meet the most challenging lubrication needs.

The synergistic effect of SiC/R-GO composite on mechanical and tribological properties of thermosetting polyimide

The effective means to solve material wear is to develop self-lubricating composite materials with excellent tribological, thermal, and mechanical properties. Herein, the composites of reduced graphene oxide (r-GO) nanosheet decorated with Silicon Carbide (SiC) were facilely prepared with employing a silane coupling agent, and the corresponding r-GO/SiC/thermosetting polyimide (r-GO/SiC/TPI) nanocomposite films were obtained by in situ polymerization method. The mechanical, tribological, and thermal properties of these nanocomposite films were investigated. When the content of r-GO/SiC was at 1.0 wt%, the compression strength and compression modulus of the composite increased by 37.7% and 47.3%, respectively, which were much higher than that of TPI composites addition of r-GO or SiC alone. Furthermore, r-GO/SiC/TPI composites also exhibited the lowest wear rate and friction coefficient in these reinforced TPI nanocomposites. When the content of r-GO/SiC was 0.8 wt%, particularly, the friction coefficient and wear rate of r-GO/SiC/TPI decreased by 22.8% and 79.8% compared to pure TPI, respectively. Additionally, trace amount r-GO/SiC hybrids also significantly enhance the thermal stability of TPI matrix. Compared to the polyimide composites reinforced by common nano-scale inorganic fillers, the outstanding mechanical and tribological properties of this r-GO/SiC/PI composites could be attributed to the ball on plane structure of GO/SiC, which lead to crack reflection, strength increment. These r-GO/SiC/TPI composites demonstrate the promising potential to be used as high-performance tribological materials in industry applications.

The influence of process parameters on the preparation of CaF2@Al(OH)3 composite powder via heterogeneous nucleation

Abstract The use of self-lubricating materials not only reduces friction coefficients and alleviates effects of wear and tear, but also helps to maintain the mechanical properties of materials. By using coating technology in the application of self-lubricating materials, it is possible to effectively improve the bonding strength between a solid lubricant and substrate material and additionally to protect the solid lubricant against oxidation and decomposition during the sintering process. This study uses the heterogeneous nucleation method, where aluminum hydroxide (Al(OH)3) is coated on the surface of calcium fluoride (CaF2) to prepare a CaF2@Al(OH)3 composite powder. It was observed, through analysis of the preparation process parameters, that the Al3+ concentration, pH value, and reaction temperature were the factors which most influence coating quality. Optimum process parameters were found to be an Al3+ concentration of 0.15 mol · L–1, pH value of 7.5, and reaction temperature of 75 °C. Comparing and analyzing the micro-morphology of sample composite powders proved the effectiveness and favorable performance of the CaF2@Al(OH)3 composite we produced under optimized process parameters.

Self-Lubricating Materials for Extreme Condition Applications

Lubrication for extreme conditions, such as high temperature, cryogenic temperature, vacuum pressure, high load, high speed, and corrosive environments, is a continuing challenge among tribologists and space engineers due to the inadequate friction and wear properties of liquid lubricants. As a result, tremendous research effort has been put forward to study lubrication mechanisms for various machine elements under challenging conditions over the past two decades. Self-lubricating materials have been most widely used for adequate lubrication in extreme conditions in recent years. This review paper presents state-of-the-art of materials for lubrication in extreme condition applications in aerospace, automotive, and power generation areas. More specifically, solid lubricants dispersed in various matrices for lubrication application were analyzed in-depth under challenging conditions. This study also reports the self-lubricating materials and their lubrication mechanisms. Finally, various applications and challenges of self-lubricating materials were explored.

Acoustic emission monitoring of wear in aerospace self-lubricating bearing liner materials

Self-lubricating composite bearing liners are used in a range of aerospace systems including fixed wing and rotary wing applications such as a helicopter pitch links. In aerospace applications these bearings are subject to periodic replacement, irrelevant of wear condition, with manual inspections made at set intervals. There is a drive in many areas of aerospace to move from a scheduled replacement plan to an as required approach to increase asset availability and reduce maintenance costs whilst maintaining safety. This has led to a clear research and commercial drive to develop smart bearings that can identify the onset of critical damage and alert an operator of the need for replacement, effectively a Structural Health Monitoring (SHM) system. This paper presents a novel investigation of composite bearing liners in conjunction with Acoustic Emission (AE). The developed test programme and subsequent analysis clearly demonstrate that it is possible to monitor the complex wear process of the liner material using AE within a laboratory environment. Furthermore, a new approach to analysing AE signals is presented that could be integrated into a SHM system for asset management.

Wear resistance of nanostructured metal-polymer self-lubricating powder composites

Tribotechnical tests and microstructural studies were carried out. Wear mechanism of nanostructured metalpolymer self-lubricating composite materials has been established. This mechanism involves in the formation of separating polymer layers on the friction surface, which reduces the coefficient of friction and running-in period of parts of friction units. Carbon nanoparticles move along the friction surface, hinder the development of seizure processes during the interaction of microroughnesses of the contacting surfaces of the material and the counterbody during the destruction of the separating polymer layers. It was found that the polymer filler is displaced from the friction zone, carbon nanoparticles are pressed into the open areas of the surface of the copper matrix of the composite when the pressure in the tribocontact is higher than 1.5 MPa. The temperature in the tribocontact increases, the polymer filler degrades, the carbon nanoparticles are removed from the friction zone, the strength properties of the composite decrease, the friction coefficient and the wear rate increase at a sliding speed above 1.5 m/s. The obtained research results can be used in mechanical engineering, transportation industry and power engineering.