Friction Layer Formation Research Articles

Dry sliding behavior and friction layer formation in copper-free barite containing friction materials

The tribological behavior of materials used for vehicular car brakes is investigated using pin-on-disc tests. In particular, the disc was made of a commercial gray cast iron, used for real brake discs. The pins were made of newly developed formulations of friction materials, whose thread-line is the elimination of copper. This element was present in the selected bench-mark friction material, which is a low-metallic material, in which copper is replaced by re-arranging the overall composition and by adding increasing amounts of barite, up to a total maximum concentration of 33.5wt%. Wear products, namely the friction layers that formed on the mating surfaces of the pin materials, have been characterized to identify the main phenomena involved and influencing wear rate and friction coefficient. The main outcome of the research is the beneficial influence of barite, the compositional range that is important to target, the possible routes to be pursued, to improve the results achieved so far.

Structural transformation of carbon/carbon composites for aircraft brake pairs in the braking process

Amorphous structure discovered from both the wear debris and the friction layer of the carbon/carbon composites after the braking tests was attributed to the breakdown of crystallites in the graphitized bulk material. Graphite layers produced through interlayer shear deformation were found to be the basic units of the pyrocarbon during sliding while carbon fiber tended to be worn into small fragments along the fiber axis in the primary stage of rubbing. The transition of graphitic pyrocarbon in the friction process was described as four stages. Nano-scale carbon particles of onion-like structure were found in the fiction layer, which were likely to participate in the formation of friction layer and lubricate the friction process.

Formation of Friction Layers in Graphene-Reinforced TiAl Matrix Self-Lubricating Composites

The friction layer structure has been proved to be formed during severe plastic deformation and markedly improves the tribological properties of material. The dry friction and wear performance of graphene-reinforced TiAl matrix self-lubricating composites (GTMSC) at different sliding velocities are systematically researched. GTMSC show the best tribological properties and special friction layer structure containing a wear-induced layer and a grain refinement layer with a nanocrystalline (NC) structure under surface after sliding at a sliding speed of 1.1 m/s. Nanoindentation results show that the grain refinement layer has a higher hardness and elastic modulus than the wear-induced layer. This special microstructure of friction layers beneath the surface after sliding leads to a low coefficient of friction and high wear resistance of GTMSC. Moreover, it is deduced that the appearance of an NC structure results in hardening of the material. The formation mechanisms of friction layers are researched in detail. It can be concluded that the formation of a wear-induced layer results from frictional heat and fracture of the counterpart. The formation of a grain refinement layer is due to severe plastic deformation and dynamic recrystallization. Severe plastic deformation results in the formation of an NC structure and dynamic recrystallization leads to grain refinement.

Antifriction properties of oils with thickeners based on modified fluoroalkyl-containing silica particles

Covalently modified silica particles were prepared by the sol-gel process from tetraethoxysilane in the presence of (2-iodo-4,4,5,5,6,6,7,7,7-nonafluoroheptyloxy)methyloxirane under the conditions of base catalysis. The friction coefficients of formulations of petroleum oil with thickeners based on modified SiO2 particles (5 and 10 wt %) at loads of 10, 20, 30, and 60 N were determined. The formulation with the lowest thickener content under the conditions of low loads demonstrates the most pronounced decrease in the friction coefficient. The probable mechanism of the improvement of antifriction properties is the formation of friction layers based on iron compounds.

Synergistic effect of tungsten disulfide and cenosphere combination on braking performance of composite friction materials

Tungsten disulfide (WS2/TDS) based cenosphere (Cn) filled friction composites with varying cenosphere to WS2 ratio (Cn/TDS) were fabricated by compression molding of phenolic resin based dry formulation mix and evaluated for their thermal, thermo-mechanical and tribological performances. The loss and revival of braking friction effectiveness due to heating or cooling of the disc termed as fade and recovery performance have been characterized on a Krauss friction testing machine following ECE R-90 industrial standards. The fade performance remained dependent on Cn/TDS, where enhanced fading could be correlated to lower Cn/TDS value accompanied with broader frictional fluctuations i.e. μmax–μmin. A decrease in the frictional-recovery response ensued with increase in Cn/TDS. Dynamic mechanical analysis revealed an increase in storage modulus till 2.5wt.% of TDS loading followed by consistent decrease whereas two distinct peaks in loss modulus plots that are composition independent have been observed. Scanning electron microscopy revealed the worn surface morphology associated with the dynamics of contact patches formation and deformation vis-a-vis friction layer formation as integrally responsible for the observed friction performance. Energy dispersive analysis of X-rays (EDX) enabled compositional analysis of the friction layer viz. Fe, W, Si, and Al content which may have a mechanistic role in controlling phenomena like, disc rubbing, lubricity, porosity, and hardness of friction layer formed during braking application.

Effects of silicon carbide in semi-metallic brake materials on friction performance and friction layer formation

The research presented in this paper is focused on the effect of silicon carbide (SiC) on friction–wear properties of semi-metallic friction composites (FC). Semi-metallic FC with increasing content of silicon carbide (SiC: 0, 3.4, 5.6, 9 and 14.6 vol.%) were prepared and slid against cast iron disc and their friction–wear properties were evaluated. The friction coefficient ( μ) was observed to increase with SiC content, nonetheless the highest content (14.6 vol.% of SiC) did not significantly increase its value. The volume wear rate ( V) of tested friction composites slightly increases with SiC content and temperature. The character of friction layer was analyzed using scanning electron microscopy (SEM) with energy dispersive X-ray microanalysis (EDX) and the topography of friction surface was studied using atomic force microscopy (AFM). Two types of films assigned as film I and film II appearing on the friction surfaces of samples after friction test were observed. Film I originate on the friction surface of sample without SiC and is composed of debris of iron, iron oxide, stibnite and carbon. Film II is formed on the friction surface of composites containing SiC and in contrast to film I contain additional debris of SiC.

Formation of Friction Layers and Wear Resistance of Steels under Conditions of Boundary Friction

We studied the influence of the lubricant compositions “Grafitol” with ∼10% graphite (1), “Aerol” containing ∼17% MoS2 (2), “Limol” containing ∼60% MoS2 (3), “Limol” + 10% chlorine-paraffin (4), and “Limol” + 10% copper powder (5) on the wear rate and formation of the fine structure of surface friction layers of structural steels. We established a correlation between the tribological characteristics of steels and lubricants. The abrasive wear of 40KhFA steel was minimum if it was lubricated with “Limol” + 10% copper powder. In this case, its wear was smaller by a factor of 10, 2, 1.25, and 7.25 as compared with lubricant compositions 1–4, respectively. In the course of minimum wear of 40KhFA steel, in the surface friction layers, we observed the minimum values of second-kind distortions (Δa/a) and of the true size of domains of X-ray coherent scattering (D) as well as the minimum difference between the crystal lattice constants (a) of steel before and after friction.

A combinatorial approach for automotive friction materials: Combined effects of ingredients on friction performance

AbstractThe combined effects of ingredient A and C in A + B + C systems where A and C are Al2O3, graphite, MoS2, steel wool and Twaron (aramid pulp), and B is a binder (benzoxazine) on friction performance were studied. Ray design with Golden Section was used for the formulated A + B + C ternary composites. Two combined factors, morphological combinations including fiber/fiber, fiber/filler and filler/filler and combinations by means of material nature including abrasive/abrasive, abrasive/lubricant and lubricant/lubricant, and their composition dependence were considered. Synergetic effects, defined as a minimum wear in wear‐composition relationships, were found in morphological combinations of fiber/filler (steel wool/Al2O3/B and Twaron/Graphite/B systems). Synergetic effects producing maximum values in friction coefficient (μ)‐composition curves were not found in all combinations. All combinations of lubricant/lubricant and abrasive/abrasive reduce μ and the effects of the combinations of abrasive/lubricant are dependent upon their abrasive or lubricant efficiency. Strong abrasive (Al2O3) plays an important role on the formation of friction layer that the transferred iron from disc to friction materials in combinations of Al2O3/lubricant/B systems, and there is no such a transfer of iron from disc to friction materials on the surface of lubricant/lubricant combinations. The S type friction transition was observed in MoS2/steel wool/B and graphite/steel wool/B systems and was caused by adhesion friction mechanism.

Tribological behavior of sliding diamond-like carbon films under various environments

Tribological behaviors of amorphous diamond-like carbon (DLC) films were experimentally evaluated under various environments using a steel ball-on-disk wear-rig at dry sliding surfaces. The DLC films were prepared on Si wafer by r.f. PACVD method using benzene (C 6H 6). Every test was performed under the normal load 4.9 N and sliding velocity of 0.05 m/s, and the coefficients of friction of DLC films were measured with the contact cycle. When the test was performed under a vacuum, the coefficient of friction of DLC films showed a significant fluctuation with the contact cycles, remarkably accompanied with roll-shaped polymeric wear debris. Another test using thermally-baked DLC films showed that the friction was lower and more stable. Under a humidity controlled air, friction of DLC films was also stable. Tests with dry O 2 and N 2 gas were extended to find any other environmental factors which could affect the tribological behavior of DLC films. It was found that tribological behavior of DLC films was dependent on the formation of friction layers which were mainly affected by the environment. In the final discussion of this work, effects of environment on the tribological behavior of DLC films were combined and discussed in terms of tribo-chemistry of DLC films.