Self-lubricating Materials Research Articles

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.

Friction and wear behavior of Ti(C,N) self-lubricating cermet materials with multilayer core-shell microstructure

In order to promote the friction and wear properties of cermet materials, an advanced Ti(C,N) self-lubricating cermet materials with the addition of ceramic coated nano-sized solid lubricant powders (CaF2@Al2O3) was developed. The friction and wear properties of Ti(C,N) self-lubricating cermet materials were studied by spherical disk matching. Results showed that the optimal content of CaF2@Al2O3 powders was 10 vol%, and the addition of CaF2@Al2O3 powders significantly reduced the friction coefficient of cermet. Meanwhile, the multilayer core-shell microstructure can significantly improve the wear resistance of Ti(C,N) based cermet. The higher temperature on the friction contact area made CaF2 change from brittle state to plastic state, which was evenly coated on the friction surface to form the self-lubricating film. On the one hand, as the load increased, the friction coefficient of cermet with multilayer core-shell microstructure decreased, but the wear rate increased. On the other hand, as the friction speed increased, the friction coefficient first increased slightly, then decreased gradually, and then continues to increase after decreasing to the minimum value. Besides, the wear rate first decreased and then increased with the increase of speed. The formation of multilayer core-shell microstructure has good wear-resistance effects on Ti(C,N) cermet, and the decrease of friction coefficient is mainly due to the addition of CaF2.

Surface modification of polysulfone/PAO40 microcapsules via polydopamine to improve thermal stability and used to prepare polyamide 6-based self-lubricating composite

Herein, the modification of polydopamine (PDA) surface improved the high temperature stability of polysulfone (PSF)/PAO40 microcapsules ( D -microcapsules), which achieved the potential application of microcapsules in polyamide 6 (PA6). The D -microcapsules in PA6 for lubricative application has been investigated by ball-on-disc friction tests. In particular, the inclusion of 9 wt% D -microcapsules yielded the best tribological properties where the value of COF (0.088) was largely decreased as compared with that of the neat PA6(0.41). Moreover, the wear rate was found to be decreased from 4 × 10 −3 mm 3 /Nm to 4.3 × 10 −6 mm 3 /Nm. It has been demonstrated that better antifriction performance was associated with the lubricating film generated by PAO40. Therefore, the prepared PA6-based self-lubricating composites can be further applied in the field of self-lubricating materials.

Self-lubricating ceramic tool materials synergistically toughened by nano-coated particles and silicon carbide whiskers

Nano-coated particles and whiskers were used as reinforcing additives for ceramic tool materials to take advantage of synergistic toughening and enhancement mechanisms. An Al2O3/TiC/CaF2@SiO2/SiCw ceramic tool material was fabricated by hot-pressing. Self-lubricating ceramic tool materials toughened by nano CaF2@SiO2 coated particles and SiC whiskers were studied. When the addition content of nano CaF2@SiO2 coated particles was 10 vol% and the content of SiC whiskers was 20 vol%, ceramic tool materials with the best comprehensive properties were obtained. The results showed that the hardness of the ceramic tool material was 16.52 GPa, the flexural strength was 698 MPa, and the fracture toughness was 6.89 MPa m1/2. Compared with the ceramic tool material with 10 vol% nano CaF2 particles and 20 vol% SiC whiskers, the above properties were improved by 7.06%, 31.74% and 19.06%, respectively. The SiC whiskers played a toughening role, while nano CaF2@SiO2 coated particles reduced the porosity, the compactness and also filled between the grain boundaries of the ceramic tool materials during sintering. This improved the interfacial bonding between ceramic grains, which increased the incidence of transgranular fracture in the ceramic tool materials.

Sintered 316L/Cu/h-BN composites

Sintered 316L/h-BN composites show high potential as self-lubricating materials, which can be applied as moving parts where lube oils and greases are not applicable and inaccessible for maintenance. The past production of such composites by sintering, faced two problems. Firstly - interaction between 316L matrix and h-BN resulting in loss of h-BN content and its lubricity. And secondly - poor 316L matrix integrity. This work employed two approaches to solve such problems. The first approach was the use of nitrogen-containing atmosphere for sintering, to retard 316L matrix and h-BN interaction. And the second approach, was copper addition to promote sintering of 316L powder particles. Sintered self-lubricating 316L/Cu/h-BN composites were produced from mixtures of 3 different 316L/Cu matrices (made by additions of 2.0, 4.0 and 6.0 wt.% copper powder to 316L powder) and 15 vol.% of h-BN powder. Green compacts of powder mixtures were sintered under cracked ammonia (75 vol.% hydrogen + 25 vol. % nitrogen) at 1300°C for 60 minutes. The sintered composites were cooled in a Linn high temperature sintering furnace. It was found that copper additions led to the decrease of densities of sintered 316L/Cu/h-BN composites, compared to that of sintered 316L/h-BN composite, due to porosity left behind by copper powder melting and dissolution. Tensile strengths of sintered 316L/Cu/h-BN composites were improved when copper contents were 4 and 6 wt.%. There was no intergranular phase, the evidence of 316L matrix and h-BN interaction in all experimental sintered composites. Microstructural observation by scanning electron microscopy (SEM) also revealed that h-BN flakes still existed in pores of sintered composites. The unreacted h-BN flakes are expected to provide lubricity of sintered composites.

Fast mobility induced self-lubrication at metallic glass surface

The enhanced surface mobility in metallic glasses (MGs) has been a constant source of fascination due to its unique mechanical properties. We show experimentally that the mobile surface layer of MGs functions as a lubricating layer in friction experiments, which is evidenced by a reduction of a friction coefficient of 50% or less and suppression of dissipative stick-slip behavior with decreasing scratch depth down to nanoscale in the various MGs. The lubrication mechanism could be attributed to easier shearing of the mobile surface layer induced by homogeneous plastic flow. Importantly, the thickness of the self-lubricating layer is inversely proportional to glass transition temperature with a higher homologous temperature yielding a larger thickness. These results extend the fundamental understanding of the ubiquitous MG surface and present a path for the rational design of self-lubricating materials.

Surfactant assisted and in situ formed micro liquid metal as excellent lubricant additive in polyimide coating

Liquid metal (LM) can be well distributed in polyimide (PI) coatings by ultrasonic shear into 1–2 μm diameter. The LM incorporated PI (PI-LM) coatings show remarkably enhanced tribological properties when the coating slide against GCr15 steel ball at room temperature, while the mechanical properties are slightly reduced. When the filling amount is 8%, the wear is just 10% of pure PI and the friction coefficient can be reduced by 31.6%. During friction process, the LM microspheres are squeezed to the frictional interface and form a transfer film together with PI debris, achieving lower wear and friction coefficient. These results show that PI-LM coatings have high potential as advanced self-lubricating materials.

Mechanical properties, microstructure and crack healing ability of Al2O3/TiC/TiB2/h-BN@Al2O3 self-lubricating ceramic tool material

To prepare a self-lubricating ceramic tool with excellent mechanical properties and crack healing ability, the h-BN@Al2O3 solid lubricant with a core-shell structure was prepared by the heterogeneous nucleation method, and was added into Al2O3/TiC ceramic matrix together with TiB2. Examination of the micro-morphology and phase composition of h-BN@Al2O3 found that the surface of the solid lubricant h-BN was coated with Al(OH)3 at first, and after calcination at 1200 °C, a dense and uniform Al2O3 shell was formed. At the same time, the effects of the h-BN@Al2O3 addition on the ceramic tool material were studied. The mechanical properties of the ceramic tool material with h-BN@Al2O3 were improved relative to than those with h-BN. The optimal content of h-BN@Al2O3 solid lubricant was 5 vol%. In the friction and wear test, h-BN in h-BN@Al2O3 particles can be stripped to form a solid lubricating film on the material surface, which plays a good role in reducing wear and resisting wear. The crack healing ability of the self-lubricating ceramic tool material was studied by prefabricating cracks with a length of 400 μm on the surface. After a heat treatment at 700 °C for 60 min, the surface cracks of the ceramic tool material were completely healed, and the flexural strength was restored to 90% of that of the smooth samples. The oxidation of TiB2 and h-BN in ceramic tool materials is the main reason for crack healing.

Tribological Behaviour of MoS2 and Graphite Reinforced Aluminium Matrix Composites

Tribological behaviour of MoS2 and graphite reinforced aluminium matrix composites were investigated in this research. In the current scenario, applications in which either metal based alloys or composites are used, requires lubrication to avoid friction. In such cases, self-lubricating materials based aluminium composites can be fabricated to exhibit better tribological properties. These materials can be fabricated by reinforcing self-lubricating materials like graphite (Gr), molybdenum disulphide (MoS2) and so on within the aluminium matrix. In this tribological study, two different solid lubricants (Gr and MoS2) were taken at equal proportions to fabricate aluminium matrix composites individually through stir casting method. Pin on Disc apparatus was utilized to record the rate of wear of the composite specimen by considering the process parameters like sliding distance, load and counter-face disc hardness. Experiments were designed by Taguchi’s approach and analyzed by means of response values and analysis of variance (ANOVA). Results depicted that graphite based aluminium alloy composites possessed better wear resistance than MoS2 reinforced aluminium alloy composites.

Effect of Temperature on Tribological Properties of Tial Based Self-Lubricating Composites

A kind of TiAl based alloy with high temperature self-lubricating property was prepared by vacuum hot pressing sintering technology, and its mechanical and physical properties were tested. The friction and wear experiments were carried out at different temperatures, and the high-temperature tribological properties of the new TiAl based alloy were studied. The results show that the new TiAl based self-lubricating material with TiC, TiB2 and CaF2 can effectively improve the mechanical and physical properties of the matrix, and make it have good mechanical and physical properties as well as good tribological properties above 500 °C.

Frictional and Lubricating Materials with Mineral Components

The results of the tribotechnical studies on frictional materials and lubricating composites in Armenia are presented. The characteristics of developed braking materials, antifriction polymeric composites, and lubricants are given. Recommendations on designing machine parts made of antifriction and friction composites based on polymers, as well as lubricants for friction units using local raw materials and minerals, are developed. Main regularities in the friction and wear of the surface layers of antifriction and friction composites materials are revealed. Environmentally safe and efficient brake materials for motor cars and railroad vehicles are presented. Self-lubricating materials for sliding bearings and novel additives for lubricants have been proposed for industry.

The friction and wear behaviors of self-lubricating composites with two types of warp-knitted fabric structures

Replacing the traditional self-lubricating fabric composites with new type of wear-resistant composite reinforced by warp-knitted fabrics PTFE-Nomex and PTFE-Nomex/Nomex (self-lubricating fiber: PTFE, reinforcing fiber: Nomex), in which the hybrid graphite/graphene particle modified phenolic is used as the adhesive resin. The influence of the fabric structure on the friction and wear properties of self-lubricating fabric composites was evaluated by the ball-on-disk and bearing friction tests. And the wear mechanism of the composites was investigated through scanning electron microscopy (SEM) and mechanical tests. The ball-on-disk friction test shows that the friction and wear properties of two kinds of composites with different structures are different with the increase of friction distance. In addition, the self-lubricating warp-knitted composite materials were prepared as self-lubricating joint bearing liners to evaluate the bearing application. It is proposed that the wear resistance of the two kinds of warp-knitted composites is poor, and the single layer is slightly better. It is related to the friction mode, mechanical and bonding properties of the bearing liners.

Self-lubricating materials for extreme temperature tribo-applications

Abstract Growing demand in the materials working at extreme environmental conditions has raised a serious concern for lubricating mediums in the field of tribology. Limitations of conventional liquid based lubricants to decompose beyond 300 °C and its associated human health and environmental hazards have compelled to look for an alternative. Self-lubricating materials are new generation materials utilizing the benefits of incorporated solid-lubricating compounds to minimize friction and wear over a wide range of temperatures. This paper is an effort to summarize the existing solid-lubricating materials (polymer based, soft metal based, intermetallic composites, metal oxides and fluorides, TMDs, hBN based) for tribological applications under cryogenics (-269 °C) to high temperature (upto 1000 °C) environment. Their lubricating mechanisms and associated reason behind friction reduction is discussed. The paper also introduces the key characteristics of a futuristic ‘smart’ tribo-material.

Preparation and Properties of High-Temperature Self-Lubricating Materials Based on H13 Steel

High-temperature self-lubricating materials based on H13 steel are prepared by the powder metallurgy method. The sintering process, phase addition and high-temperature testing of the material are studied so that the mechanical and wear performance of the composites can be optimized. The results show that the optimum sintering process is 2 h at a temperature of 1250 °C. When the contents of Cr2C3 and CaF2 are 10%, the mechanical and sintering properties of the composites are acceptable. The relative density reaches 80.2% and the bending strength reaches 720.2 MPa. Moreover, CaF2 effectively reduces the friction coefficient and wear rate of the material. The friction coefficient decreases to a minimum of 0.24, but the wear rate increases to a maximum of 7.26 × 10−5 mm2/min at a load of 1065 g. Cr2C3 helps to alleviate the oxidation of the composites, and the precipitation of CaF2 causes network cracks to reduce the thermal fatigue performance.

Preparation and tribological properties of PTFE/DE/ATF6 composites with self-contained solid-liquid synergetic lubricating performance

A new kind of PTFE composite with self-contained solid-liquid synergetic lubricating effect is prepared by the combination of PTFE, porous diatomite (DE), and immersed lubricating oil (ATF6). The tribological and mechanical properties of composites with different DE contents and varying cold-pressed pressures were investigated systematically. Due to the oil storage effect of DE particles, the maximum oil content of PTFE/DE composite is 16.4 wt%. With the increase of DE content, the surface hardness, elastic modulus and yield strength can be increased from 3.2 to 3.6 HV, 37.8–60.2 MPa and 8.7 MPa–12.8 MPa, respectively. With the addition of DE, the wear rate of PTFE decreases but the coefficient of friction (COF) increases. Meanwhile, with the addition of DE and ATF6, both the COF and wear rate of PTFE decrease. The COF can be reduced by 68% from 0.121 (specimen 0%-10 MPa-dry) to the minimum value of 0.039 (composite 10%-10 MPa-ATF6), and the wear rate can be reduced by 82% from 1.95 × 10−4 mm/Nm (0%-50 MPa-dry) to the minimum value of 0.35 × 10−4 mm/Nm (10%-50 MPa-ATF6). The enhanced antifriction performance of the composite PTFE/DE/ATF6 is contributed by the synergistic lubricating effect of self-contained liquid lubricant of ATF6 and the solid lubricant of PTFE. The prepared PTFE/DE/ATF6 composites exhibit excellent tribological performance as well as the mechanical properties, which can be used to fabricate a new kind of self-lubricating materials with high performance and long durability.

Influence Factors on Mechanisms of Superlubricity in DLC Films: A Review

As a kind of self-lubricating material, diamond-like carbon (DLC) film is famous for its excellent tribological properties. Superlubricity state with nearly-vanishing friction achieved with DLC film has enormous potential applications in future mechanical systems. It is pointed out that its superlubricity state is highly related to both the inherent properties of the DLC film and external sliding conditions. Moreover, the underlying mechanisms of the superlubricity are complicated, posing uncertainties on their engineering application. This review provides an overview of the influence factors, including film composition, ambient, temperature, normal load and sliding velocity and their correlations with the anti-friction behaviors of DLC films. These understandings will enable a more effective engineering application of self-lubricating carbon films with excellent tribological properties.

Prediction of Wear Characteristics of AA2219-Gr Matrix Composites Using GRNN and Taguchi-Based Approach

Aluminium matrix composites are widely used in many applications due to its numerous advantages. Experimental investigation of wear characteristics and prediction of wear is the order of the day. The present study examines the aluminium alloy AA2219-graphite (Gr) composites in terms of its wear characteristics in dry sliding condition. The conventional stir casting method was adopted to fabricate the composites with reinforcements in proportion of 1.5, 3 and 4.5 wt% to determine its capability as self-lubricating material in dry sliding conditions. Taguchi Method was utilized to study each parametric influence on the responses of wear test. A generalized regression neural network (GRNN) is used to predict the wear characteristics of AA2219-Gr composites at different factors and levels based on Taguchi L27 orthogonal array optimized outcomes. The GRNN extrapolations were confirmed by the responses of the study and the predictions were on par with the obtained results.

Effect of in situ phases on microstructure and properties in Al–Bi immiscible alloy

Al–Bi immiscible alloy is of particular interest as potential self-lubricating wear materials with a homogeneous distribution of minority phase. However, it is difficult to obtain a homogeneous microstructure by conventional casting methods due to liquid phase separation of Al–Bi immiscible alloy. We have developed a new strategy to restrain liquid phase separation and improve the properties of Al–Bi immiscible alloy by in situ phases. The in situ AlB2 phase acts as heterogeneous nucleation site to accelerate the nucleation and slow down the velocity of the Bi-rich droplet, resulting in a significant size reduction and a homogeneous microstructure of Al–Bi immiscible alloy. The self-lubricating wear resistance of Al–Bi immiscible alloy can be further enhanced by in situ Al2Cuphase.

Effect of micrometer sized ceramic particles on the tribological properties of Polytetrafluoroethylene based composites

Polytetrafluoroethylene (PTFE) is a self-lubricating matrix material with low friction coefficient and corrosion resistance but high wear rate. Ceramic particle fillers could effectively improve the tribological properties of PTFE-based composites, reduce its wear rate. However, there has been much debate about anti-wear mechanisms for particle fillers, which mainly focus on wear resistance micro-mechanism and mechanochemistry approach. In this paper, four kinds of PTFE-based composites filled with micrometer-sized ceramic particles (SiC, Si3N4, SiO2 and h-BN) were prepared and tested to explore their tribology properties using a pin-on-disk tribometer. The filler proportions ranged from 5 to 20 wt% in 5 wt% increments were added. The worn surfaces of the composites and transfer films attached on the steel disk were examined by Scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) and 3D topographies. Ultralow wear was observed on the PTFE/SiO2 and PTFE/Si3N4 composites in wear tests, which was attributed to tribochemistry. There were protection films formed on the worn surface, which hindered further transfer of the composites. Meanwhile, adhesion of the transfer film was enhanced by metal-chelate salts of perfluorinated carboxylic acids formed by tribo-chemical reactions. The wear rate of PTFE decreased by three orders of magnitude (from 6.25 * 10−4 to 2.21 * 10−7 mm3/(Nm)) after addition of 20 wt% SiO2 particles. No chemical reaction was observed on the worn surface of PTFE/SiC and PTFE/h-BN composites.