Graphite-based Lubricants Research Articles

A qualitative understanding of high intensity mechanical shearing and exfoliation of graphite nanoplatelets in a three-body contact using molecular dynamic simulations

Graphite and graphene-based lubricants are increasingly investigated for manufacturing processes such as forming and machining. Their effectiveness depends on trapping graphite particles in between the tribologically interacting surfaces and the consequent shearing and exfoliation of the graphitic layers. The mechanism by which such shearing and exfoliation occurs is however difficult to elucidate through experiments. To address the aforementioned difficulty, we have carried out molecular dynamics studies of graphene exfoliation by simulating a three-body rubbing contact scenario, with graphite nanoplatelet trapped between iron and diamond. The iron is given simultaneous normal and shear velocities towards the stationary diamond surface. High intensity continuous rubbing, simulated via large sliding distances and diminishing gaps between the iron and diamond surfaces, is used to understand the extent of exfoliation and the damage incurred in the exfoliated layers. Simulations show that under certain conditions, layer shearing and exfoliation result in graphene layers being produced. Increasing normal velocity and decreasing the shear velocity increases the number of fragments produced. Several phenomena observed in the simulations, such as locking-in of layer orientations, tearing of edges, amorphization, and iron contamination, agree qualitatively with experimental observations. The insights from this study can be used to optimize and control the process parameters of friction induced mechanical exfoliation processes and also to understand the usable life of graphite-based lubricants.

Effects of lubrication, specimen preparation and tool coating on the friction behavior of commercially pure titanium at elevated temperature

Titanium forming processes are often limited by severe adhesive wear as a result of poor friction conditions. This can be partially remedied with careful selection of lubricant, billet preparation and tool coating, but the optimal combination of these factors is not known. A full factorial ring compression experiment, with grade 2 commercially pure titanium rings deformed at 300 °C, was conducted to study the effect of each of these factors over three levels. The change in internal diameter was compared to a set of calibration curves generated by an FEA simulation of the process in order to determine the friction coefficient during each trial. A robust statistical analysis methodology was used to isolate and evaluate the effect of varying each factor. The choice of lubricant was found to be the most statistically significant factor by a considerable margin, followed by the method of billet preparation, with tool coating found to be insignificant. Of the lubricants tested, the graphite-based lubricant resulted in the lowest friction, followed by the WS2- and MoS2-based lubricants. Sandblasted billet surfaces resulted in similar friction to as-machined surfaces, whereas those subjected to micro-arc oxidation performed notably worse. For reducing friction during warm forming of titanium, a graphite-based lubricant is therefore recommended, with tool coating and billet surface preparation unlikely to provide significant further improvement.

Effect of graphite and MoS2 based solid lubricants for application at wheel-rail interface on the wear mechanism and surface morphology of hypereutectoid rails

The effectiveness of two graphite-based and one MoS2-based solid lubricants subjected to high creepages were experimentally assessed. This research correlated the evolution of coefficient of traction (CoT) to in-depth wear surface morphology and microstructure. The lubricating potential of these solid lubricants to reduce wear and thereby extend the operation life of rail components was comprehensively evaluated. The graphite-based lubricants were able to reduce the CoT and improve the wear resistance considerably. Furthermore, the graphite-based lubricant with lower carbon content was found to be effective at all tested conditions, offering 30%–55% reduction in CoTs, longer retentivity, and better surface morphology. However, the MoS2-based lubricant did not offer significant reduction in the CoT and resulted in substantial abrasive wear at higher creepages.

Friction characterization when combining laser surface texturing and graphite-based lubricants

The present work analyzes the friction capabilities at room temperature of three types of lubricants (denoted as A, B and C) with a graphite concentration of 5%. To do that, the standard pin-on disc test is deployed to study the variation of the friction coefficient when combining these graphite-based lubricants with surfaces made by grinding and different laser surface textures. These lubricants are characterized by measuring the percent of the chemical elements, the average size of the graphite particles and the kinematic viscosity. The experiments show that the lubricant B combined with a higher density of LST presents the lowest friction coefficient of about 0.24. Additionally, assuming a hydrodynamic regime for the textured surfaces, the fluid dynamics simulations carried out as part of the study showed, in agreement with the experimental measurements, the lowest friction coefficient value for a textured surface with the highest dimple density. This seems to be associated to the combined effect of an increase of the hydrodynamic pressure with a weak vortex formation within the dimples, due to the low distortion of the streamlines which, ultimately, attenuates the friction coefficient between the surfaces.

Control of crystalline orientation of hot forged Nd-Fe-B magnet

Nd-Fe-B powder obtained by melt spinning was compacted, and then forged to produce a magnet with even magnetic properties. There is a strong correlation between the strain introduced to the magnet while it is being forged and the obtained magnetic properties. In general, such strain is unevenly introduced to the magnet owing to the friction generated between the mold and magnet. Consequently, the obtained magnetic properties also become uneven. We obtained even magnetic properties by defining the optimum shape before forging with the use of finite element method (FEM), choosing the optimum mold material and lubricant, and introducing strain evenly. From the viewpoint of decreasing friction, we used a smooth-surface ceramic mold and a graphite-based lubricant. If a ceramic mold with a normal shape is used, its corners are easily broken during forging because stress is concentrated at these corners. Ceramic molds are also extremely expensive. In this study, we configured a mold composed of ceramic plates in a simple shape. This approach successfully prevented the mold from being broken and kept the mold price low.

Friction capabilities of graphite-based lubricants at room and over 1400 K temperatures

The present work investigates the lubricant capabilities at room and hot-forging temperatures (>1400 K) of three types of lubricants with two different graphite concentrations (8% and 12%). These lubricants are distinguished by measuring the percentage of chemical elements and average size of graphite particles. Later, two standardized methods, i.e., pin-on disc and ring test, are utilized to assess the main friction differences under laboratory and real industry conditions, respectively. The results exhibit that the friction values at room temperature are lower for lubricant B, no matter which type of graphite concentration is used, whereas at hot-forging temperatures, greater percentage of graphite enhances lower frictional values when higher deformations are assessed. Additionally, the ring tests performed at hot-forging temperatures show significant tribology differences when the degree of deformation reaches 50%. Particularly, the lubricant B shows the lowest values of friction coefficients of 0.22 and 0.21 for 8% and 12% of graphite concentration, respectively. Therefore, it can be concluded that the selection of a proper type of lubricant (regarding chemical composition and size of solid suspension) and the graphite concentration are sensitive parameters, when it comes to achieve different bulk deformations combined with extreme temperatures like in hot-forging process.

Structural and Morphological Transformations in Aqueous Graphite-Based Lubricant Subjected to Extreme Tribological Interactions of Low-Speed Intermittent Machining

We study morphological and structural changes in graphite flakes present in aqueous graphite-based lubricants when repeatedly subjected to extreme tribological interactions at the tool–chip interface in low speed, high depth of cut intermittent machining, a process involving combined high normal and shear stress. Orthogonal tube turning experiments are conducted on a specially fabricated mild steel workpiece completely submerged in an aqueous graphite solution. Surface profiler, electron microscopy and Raman spectroscopy studies on initial and sheared graphite flakes indicate crushing followed by layer sliding, rolling, edge curling, tearing and exfoliation of layers. Raman spectroscopy studies highlight formation of low amorphous carbon by structural disorder and edge defects formed while crushing and tearing with no new basal defects in the sheared graphite layers. Some regions of the flake thin down to highly disordered few-layer graphene with a thickness reduction from about 30 micrometres to about 1–2 nanometres with an I2D/IG value of around 0.7. When reused back into the tribological region, these morphological changes can significantly influence the continued lubrication process by the generation of few-layer graphene.

Wear onset in hot stamping of aluminium alloys sheets

In the last two decades, severe regulations on fuel consumption, gas emissions and product recyclability have driven the attention of the automotive industry to the use of high-strength aluminium alloys to manufacture lightweight structural parts of the car body-in-white. To overcome the low formability and springback when these alloys are formed at room temperature, new high temperature forming processes have been developed with a critical impact on the lubrication policies, dies materials, and wear resistance.The paper presents experimental investigations on the tribological performances of solid lubricants for the AA6016 aluminium alloy, by reproducing the different thermal conditions at the blank-die interface typical of the hot stamping process. A novel spraying apparatus was developed to apply the lubricants on the metal blank and capable of accurate control of the deposition parameters (i.e. temperature, pressure, film thickness) and quantity of the depositions. The performances of lubricants based on graphite, molybdenum-disulphide and boron-nitride were investigated, in terms of frictional and wear behaviour, by means of a novel strip drawing apparatus in the temperature range 300–400°C and normal contact pressures up to 15MPa. The surface characteristics of the metal sheets at the wear onset were analysed by means of optical profilometry, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the wear onset was primarily due to a loss of adhesion of the lubricants, and it was delayed when using graphite-based lubricants. It was found that at increasing the process temperature both the friction coefficient and the material adhesion increased, while the higher the sliding speed and pressure, the lower the friction coefficient. In particular, the influence of the pressure was found to be larger than the one of the sliding speed.

Effect of Forging Parameters on Low Cycle Fatigue Behaviour of Al/Basalt Short Fiber Metal Matrix Composites

This paper deals with metal matrix composites (MMCs) of Al 7075 alloy containing different weight percentage (2.5, 5, 7.5, and 10) basalt short fiber reinforcement and unreinforced matrix alloy. The samples were produced by the permanent stir casting technique. The casting ingots were cut into blanks to be forged in single stage and double stage, using MN press and graphite-based lubricant. The microstructures and fatigue properties of the matrix alloy and MMC samples were investigated in the as cast state and in the single and double stage forging operations. The microstructure results showed that the forged sample had a uniform distribution of the basalt short fiber throughout the specimens. Evaluation of the fatigue properties showed that the forged samples had higher values than those of the as cast counterparts. After forging, the enhancement of the fatigue strength of the matrix alloy was so significant and high in the case of 2.5 and 5.0 wt. percentage basalt short fiber reinforced MMC, and there was no enhancement in 7.5 and 10 weight percentages short fiber reinforced MMCs. The fracture damage was mainly due to decohesion at the matrix-fiber interface.

Investigation of friction in warm forging of AA6082

This paper presents an experimental investigation of friction in hot forging of AA6082, which is a standard forging alloy in automotive engineering, mechanical engineering and in naval architecture, by employing a modified ring-on-disc test. The experiments were performed with three different commercial graphite-based lubricants and at various loads, sliding velocities and specimen surface conditions. In addition, some tests were performed without lubrication.

Performance of Graphite and Boron-Nitride-Silicone Based Lubricants and Associated Lubrication Mechanisms in Warm Forging of Aluminum

Although water/oil-graphite emulsions are widely used in warm forging processes, they carry environmental concerns. In an attempt to replace graphite-based lubricants in warm forging of aluminum alloys, two variants of boron-nitride-silicone lubricants were formulated. The two variants were made by dispersing boron nitride powder in polydimethyl siloxane oil at concentrations of 1% and 8%. The formulated lubricants were initially tested for their thermal degradation characteristics using a thermogravimetric analyzer and compared to the thermal degradation behavior of graphite and silicone oil lubricants. Ring compression tests were then carried out at 260°C and 370°C. Boron-nitride-silicone lubricant variants did not show significant difference in performance as die temperature was increased from 260°Cto370°C. This is in contrast to graphite, which performed much better at 260°C than at 370°C, due to thermal oxidation. On the other hand, silicone oil exhibited the worst performance at 260°C and the best performance at 370°C. In both boron nitride lubricant variants, the polydimethyl siloxane facilitated hydrostatic/hydrodynamic lubrication at 260°C, with boron nitride acting as a barrier film that reduced friction. However, the lubrication mechanisms changed at 370°C, where the depolymerization of polydimethyl siloxane led to formation of silica due to thermal oxidation. Silica, together with boron nitride, acted as a film barrier with low shear strength. The dual lubrication mechanisms make boron-nitride-silicone lubricants suitable for a wide range of aluminum forging temperatures.

熱間継目無鋼管圧延時の潤滑特性に及ぼすスケールの影響

It is well known that an iron oxide layer formed on a surface of work piece plays an important role of lubricating properties in hot rolling. However, a few papers fully reported the influence of the iron oxide layer on lubricating properties. Therefore, we investigated the influence of compositions and thickness of the iron oxide layer on lubricity by means of a simulation test of semless pipe hot rolling.First, some iron-oxide layers of various compositions and thickness were formed on three different steel materials by oxidizing treatment under different conditions: where the work pieces were exposed to air until the oxide layer grew to the set thickness. Then, the hot work piece was rolled between a cylindrical roll and a flat tool lubricated with graphite-based lubricant. The friction coefficient was measured during rolling, and the flat tool surface was observed after rolling.Summaries of the results are shown below.① In case the rolled material was carbon steel, scoring didn't occur regardless of its elongation in the test condition. On the other hand, in case the rolled material was 10% Cr steel or 18%Cr–8%Ni stainless steel, scoring occurred when the elongation was high.② The abovementioned results are thought to be caused that the scale of FeO generated in the case of carbon steel has high plasticity at elevated temperature.

Frictional behaviour of Al359/SiC/20p composite under isothermal and non-isothermal hot-working conditions as a function of surface roughness

The frictional behaviour of 359/SiC/20p composite, using a graphite-based lubricant, has been studied by means of ring-compression tests performed under both isothermal and non-isothermal conditions. It was observed that the decrease of the inner diameter increases with increasing surface roughness of the ring samples. Furthermore, with greater surface roughness, the decrease in the inner diameter is affected by both temperature and strain rate, whilst a lower roughness value makes the interfacial friction almost insensitive to the working conditions. A comparison between the experimental data and calibration curves has enabled the determination of the shear friction factor as a function of surface roughness, temperature and strain rate.

Graphite: its formation and manufacture

The effectiveness of two graphite-based and one MoS2-based solid lubricants subjected to high creepages were experimentally assessed. This research correlated the evolution of coefficient of traction (CoT) to in-depth wear surface morphology and microstructure. The lubricating potential of these solid lubricants to reduce wear and thereby extend the operation life of rail components was comprehensively evaluated. The graphite-based lubricants were able to reduce the CoT and improve the wear resistance considerably. Furthermore, the graphite-based lubricant with lower carbon content was found to be effective at all tested conditions, offering 30%–55% reduction in CoTs, longer retentivity, and better surface morphology. However, the MoS2-based lubricant did not offer significant reduction in the CoT and resulted in substantial abrasive wear at higher creepages.