Graphene-Based Lubrication for Tribological Applications: Nanolubricants and Self-lubricating Nanocomposites

In this work, the effects of graphene nanoplatelets (GNPs) additives on tribological properties of aluminum are investigated. The objective of this research is to investigate and explain the enhancement mechanisms of GNPs at the contact surface during tribological testing. The graphene nanoplatelets are studied both as an oil additive (Chapter I) and as a reinforcement (Chapter II) experimentally. The coefficient of friction (COF) and wear rate were identified using a pin-on-disk test setup. Mineral, organic, and synthetic oils are not always efficient enough to satisfy the demands of a high-performance lubricant; therefore, mixing additives with base fluids is an approach to improve the lubrication ability and to reduce friction and wear. In chapter I, GNPs are used as lubricant additives to make nanolubricants. Then, the combined effect of the material’s variables (GNPs loading, size, and dispersion stability) and tribo test’s variable (applied normal load) are investigated on COF and wear rate of aluminum. Tribological studies are all carried out in the boundary lubrication regime. Three-dimensional surface metrology is performed using an optical profilometer. Various surface analyses, including Scanning Electron Microscope (SEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Raman Spectroscopy are performed to assess the chemical elements on the tested surfaces. The experimental and theoretical analyses show that GNPs are effective in reducing friction and wear, although, this positive effect is more influential at higher loads. Also, it is demonstrated that there is a critical concentration of GNPs, below which a reduced wear rate is not sustained. The proposed mechanism to describe the effect of GNPs in boundary lubrication condition is “reduced direct metal-metal contact area” at the contact surface. In other words, a material which has low shear strength layers sits between two contacting surfaces and separates the two sliding metal surfaces with no actual contact between them. This means that there is less formation of asperity junctions between the two surfaces. Although liquid-based lubricants are efficient enough in most tribological applications, there are circumstances, such as extreme environmental conditions such as high or low temperatures, vacuum, radiation, and high contact pressure in some aerospace applications, where no liquid lubricants can be present. In addition, interminable providing of lubricant at the contact surface is another challenge ahead. In order to respond to these challenges of using liquid oil at extreme environmental conditions, in chapter II of this dissertation, the synthesis and performance of self-lubricating aluminum matrix nanocomposite are evaluated (Chapter II). Aluminum powder is mixed with varying concentrations of GNPs and alumina nanoparticles to form a hybrid metal matrix nanocomposite. High-energy ball milling is conducted at room temperature while powders are immersed and protected by benzene bath. Degassing is accomplished by heating to 135oC. Consolidation of the powders is conducted by single action cold compaction and single action hot compaction. Pin-on-disk experiments are conducted to investigate the tribological behavior of aluminum matrix composites reinforced by GNPs and compare them with unreinforced aluminum. Then, the combined effect of material’s variables (reinforcement type and loading) and tribo test’s variable (applied normal load) were investigated on COF and wear rate of aluminum. SEM and EDX were performed to assess the stoichiometry of the elements on the tribo surfaces. In addition, Raman Spectroscopy and Transmission Electron Microscopy (TEM) were also performed to identify the bonding/interactions between the phases on the surface. Results imply that the COF and wear rate of composites decrease by embedding graphene nanoparticles due to reduction the real contact area between the mating surfaces by forming the lubricant. Besides, the addition of alumina particles in Aluminum/GNPs composites can further improve COF and wear rate because of rolling effect of alumina nanoparticles. Increasing the loading of GNPs reduces the COF, while there is an optimum concentration of GNPs, above and below which the wear rate is increased. In addition, the COF and wear of all composites decreases by increasing normal load. Based on the observations, multiple mechanisms are proposed to describe the improved tribological behavior of the synthesized self-lubricating nanocomposites. In addition to the reduced direct metal-metal contact area at the contact surface, the fact that the layered GNPs structure is exposed to at the contact surface keeps the surface lubricated. In other words, under sliding conditions, the transfer layer formation of the GNPs on the tribo surfaces acts as a solid lubricant film, which prevents direct contact between the mating surfaces. Additionally, it is experimentally confirmed that GNPs prevent the surface from oxygen diffusion, thereby reducing the amount of oxides which are harder and more abrasive at the contact surface. “Load bearing” of added alumina nanoparticles, in addition to the increased hardness of the matrix, is another proposed mechanism of wear resistance enhancement. It has been shown that an effective lubricant layer forms when the solid lubricant has a strong adhesion to the bearing surface; otherwise, this lubricant layer can be easily rubbed away and tends to have a very short service life. Raman data confirms the formation of Al4C3 bonds on the tribo layer under certain test conditions

Influence of Succinimide Dispersants on Film Formation, Friction and Antiwear Properties of Zinc Dialkyl Dithiophosphate

ZDDP (zinc dialkyldithiophosphate) is arguably the most successful antiwear additive ever employed in crankcase engine lubricants. It was originally used as an antioxidant and shortly afterwards recognized for its antiwear and extreme pressure properties. Unfortunately, another critical additive polyisobutylsuccinimide-polyamine (PIBSA-PAM), which is used as a dispersant in engine oils, is known to be antagonistic to ZDDP in terms of film formation, friction and wear. The mechanisms of this antagonism have been widely studied, but they are still not well understood. Furthermore, in order to protect engine exhaust catalysts from sulphated ash, phosphorus and sulphur (SAPS) and extend drain intervals of engine lubricants, a progressive reduction in ZDDP quantity but a growth in the use of PIBSA-PAM is required. The aim of this study is to explore the mechanisms and practical effects of the antagonism between ZDDP and PIBSA-PAM. Of particular interest is the impact on performance of the ratio of ZDDP to PIBSA-PAM, as measured by P:N ratio. Since ZDDP is a very effective antiwear additive, it produces only very low or "mild" rates of wear. To study this requires a new way to measure mild wear behaviour of formulated oils. Several techniques have been applied in this study to investigate the film formation, friction and wear properties of ZDDP- and/or PIBSA-PAM-containing oils. These include a new mild wear testing method, which is tested and developed using a range of different types of additives. It is found that the ratio of P:N plays a strong role in determining tribofilm formation and friction of ZDDP/PIBSA-PAM blends. However it plays a much weaker role in determining wear behaviour. It is found that some PIBSA-PAMs have considerable friction-reducing properties in their own right. The results suggest that PIBSA-PAM may interfere with the behaviour of ZDDP in several ways: by forming a ZDDP/ PIBSA-PAM complex at the metal surfaces to reduce the local activity of ZDDP; by PIBSA-PAM partially removing the ZDDP film; possibly also by PIBSA-PAM blocking ZDDP from metal surfaces. The newly-developed wear testing method can be used conveniently and effectively to study mild wear properties not just of ZDDP but of a wide range of other additives

High-temperature lubrication mechanism of alkaline borates

Like any other metalworking processes, lubrication plays a crucial role in hot metal forming (e.g. hot rolling). An effective lubrication ensures high energy efficiency, low material loss and optimal product quality. The current study investigates potential lubrication properties of alkaline borates at elevated temperature by extensive experimental work. Advanced microscopy analysis allows insights into working mechanics of the lubricants at different scales which help addressing some fundamental questions arise from the past literatures. Tribological behaviors of sodium borate were thoroughly studied by pin-on-disc testing. With a transition point around 525oC, the material exhibits exceptional lubrication performance over the range of 600oC-800oC on sliding steel pair (GCr15/mild steel). This is demonstrated by remarkable reduction in friction coefficient and wear loss volume on both contact surfaces lubricated by sodium borate compared to the unlubricated case..

Experimental techniques for investigating lubricated, compliant contacts

The study of Tribology between soft or compliant surfaces is not well understood despite its importance to many biological and engineering applications, ranging from synovial joints to rubber o-ring seals. It has also been shown that the science of Tribology and lubrication in compliant contacts is an important factor in the sensory perception and functionality of skin, hair and the oral cavity, and so has an immediate application of the design of consumer products such as skin creams, hair conditioners and foodstuffs. This thesis aims to improve our understanding of thin film lubrication between soft, deformable surfaces under light loading and low-pressure conditions. The primary focus of the thesis is the development of techniques by which to measure the film thickness between compliant surfaces, from the nano- to the micro-scale. Several experimental techniques currently exist for measuring film thickness in hard, metallic contacts and these are widely employed in Tribology research of engineering systems. However they require considerable modification to be applicable to compliant contacts. This thesis describes the development of two such techniques; · a optical interferometric technique; for measuring nano-scale thicknesses in compliant contacts; · a laser induced fluorescence technique; developed to enable measurement of lubricant thickness of relatively thick films in compliant contacts

Wind turbine condition monitoring : technical and commercial challenges.

Deployment of larger scale wind turbine systems, particularly offshore, requires more organized operation and maintenance strategies to ensure systems are safe, profitable and cost-effective. Among existing maintenance strategies, reliability centred maintenance is regarded as best for offshore wind turbines, delivering corrective and proactive (i.e. preventive and predictive) maintenance techniques enabling wind turbines to achieve high availability and low cost of energy. Reliability centred maintenance analysis may demonstrate that an accurate and reliable condition monitoring system is one method to increase availability and decrease the cost of energy from wind. In recent years, efforts have been made to develop efficient and cost-effective condition monitoring techniques for wind turbines. A number of commercial wind turbine monitoring systems are available in the market, most based on existing techniques from other rotating machine industries. Other wind turbine condition monitoring reviews have been published but have not addressed the technical and commercial challenges, in particular, reliability and value for money. The purpose of this paper is to fill this gap and present the wind industry with a detailed analysis of the current practical challenges with existing wind turbine condition monitoring technology

Investigating the effect of retained austenite on wear and fatigue behavior of AISI 8620 steel

Enhancing the operational life and reliability of drivetrain components via heat treatment routes is a long-studied topic of research. By varying parameters using heat treatment, the resulting attributes of the material such as hardness, residual stress, amount of retained austenite (RA), microstructure, grain size, grain orientation etc. can be varied. While the effect of hardness and residual stress on the resulting tribological properties of the steel is well known, the effect of RA is not as clearly understood yet. In this study, AISI 8620 steel samples were subjected to several heat treatment schemes using conventional and laser-based surface treatment routes to vary the amount of RA. The samples were subsequently used to conduct wear and rolling contact fatigue tests where samples are compelled to fail due to abrasive wear, micropitting, macropitting and white etching crack (WEC) formation. By conducting these systematic experiments, the effect of RA on those failure modes was elucidated using several surface based and microstructure based experimental techniques. It was observed that at higher contact pressures, a higher hardness and higher RA% result in superior wear resistance. This is attributed to the higher hardness resulting from the heat treatment route to generate higher RA% as well as martensitic transformation of austenite during sliding. Generated RA% due to laser treatment was higher at lower scanning velocity and with air and nitrogen as a shielding gas compared to argon but was limited to below 10%. Higher RA had significant effect on micropitting but less significant effect on macropitting since sub-surface cracks observed to have a compelling impact on macropitting life. Higher RA samples showed far more branching WEC networks, and the white etching area adjacent to the crack networks was less defined. The study establishes the role of RA on tribomechanical behavior in the context of other mechanical and microstructural phenomena as well as the correlation of behavior to heat treatment routes for preferred wear and fatigue life. Overall, the findings will provide valuable input for the design and manufacturing of drivetrain components for a wide range of applications ranging from agricultural equipment to the wind energy sectors

Effect of Wiper Edge Geometry on Machining Performance While Turning AISI 1045 Steel in Dry Conditions Using the VIKOR-ML Approach

AISI 1045 can be machined well in all machining operations, namely drilling, milling, turning, broaching and grinding. It has many applications, such as crankshafts, rollers, spindles, shafts, and gears. Wiper geometry has a great influence on cutting forces (Fr, Ff, Fc and R), temperature, material removal rate (MRR) and surface roughness (Ra). Wiper inserts are used to achieve good surface quality and avoid the need to buy a grinding machine. In this paper, an optimization-based investigation into previously reported results for Taguchi’s based L27 orthogonal array experimentations was conducted to further examine effect of the edge geometry on the turning performance of AISI 1045 steel in dry conditions. Three input parameters used in current research include the cutting speed (Vc), feed (f) and depth of cut (ap), while performance measures in this research were Ra, Fr, Ff, Fc, R, temperature (temp) and MRR. The Vise Kriterijumska Optimizacija Kompromisno Resenje (VIKOR) method was used to normalize and convert all the performance measures to a single response known as the VIKOR-based performance index (Vi). The machine learning (ML) approach was used for the prediction and optimization of the input variables. A correlation plot is developed between the input variable and Vi using the ML approach. The optimized setting suggested by Vi-ML is Vc: 160 m/min; ap: 1 mm and f: 0.135 mm/rev, and the corresponding value of Vi was 0.2883, while the predicted values of Ra, Fr, Ff, Fc, R, temp and MRR were 2.111 µm, 43.85 N, 159.33 N, 288.13 N, 332,16 N, 554.4 °C and 21,600 mm3/min, respectively