Reciprocating sliding tribology of brake oil treated carbon fiber reinforced ceramic matrix composites

The effect of brake oil on sliding behavior of carbon/carbon(C/C) and carbon/carbon-silicon carbide(C/C-SiC) composites was investigated with the variation of laminate orientation and surface conformity. The partial and low conformity contacts with the normal and parallel orientations of laminates were considered. The normal load was varied from 50 to 90 N in a step of 10 N. The friction and wear behavior was investigated under reciprocating sliding conditions. The results showed that friction coefficient and wear loss of composites with normal orientation of laminates were larger as compared to those of composites with parallel orientation of laminates. C/C composites with normal orientation of laminates yielded the highest value of friction coefficient. Wear loss decreased by a maximum of 78%, and friction coefficient decreased by a maximum of 49% in low conformity contacts as compared to partial conformity contacts. The presence of brake oil reduced the adhesion tendency of compacted wear debris because the formation of friction film was difficult, and thus, friction behavior was affected. The wear debris retention between the contact surfaces due to confined area motion in reciprocating sliding depicted the tribological behavior.

Two Dimensional Nanoscale Reciprocating Sliding Contacts of Textured Surfaces

Detailed behaviors of nanoscale textured surfaces during the reciprocating sliding contacts are still unknown although they are widely used in mechanical components to improve tribological characteristics. The current research of sliding contacts of textured surfaces mainly focuses on the experimental studies, while the cost is too high. Molecular dynamics（MD） simulation is widely used in the studies of nanoscale single-pass sliding contacts, but the CPU cost of MD simulation is also too high to simulate the reciprocating sliding contacts. In this paper, employing multiscale method which couples molecular dynamics simulation and finite element method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. Four textured surfaces with different texture shapes are designed, and a rigid cylindrical tip is used to slide on these textured surfaces. For different textured surfaces, average potential energies and average friction forces of the corresponding sliding processes are analyzed. The analyzing results show that ＂running-in＂ stages are different for each texture, and steady friction processes are discovered for textured surfaces II, III and IV. Texture shape and sliding direction play important roles in reciprocating sliding contacts, which influence average friction forces greatly. This research can help to design textured surfaces to improve tribological behaviors in nanoscale reciprocating sliding contacts.

Nanoscale Reciprocating Sliding Contacts of Textured Surfaces:Influence of Structure Parameters and Indentation Depth

Textured surfaces are widely used in engineering components as they can improve tribological properties of sliding contacts, while the detailed behaviors of nanoscale reciprocating sliding contacts of textured surfaces are still lack of study. By using multiscale method, two dimensional nanoscale reciprocating sliding contacts of textured surfaces are investigated. The influence of indentation depth, texture shape, texture spacing, and tip radius on the average friction forces and the running-in stages is studied. The results show that the lowest indentation depth can make all the four textured surfaces reach steady state. Surfaces with right-angled trapezoid textures on the right side are better for reducing the running-in stage, and surfaces with right-angled trapezoid textures on the left side are better to reduce wear. Compared with other textured surfaces, the total average friction forces can be reduced by 82.94%–91.49% for the case of the contact between the tip with radius R = 60rand the isosceles trapezoid textured surface. Besides,the total average friction forces increase with the tip radii due to that bigger tip will induce higher contact areas. This research proposes a detailed study on nanoscale reciprocating sliding contacts of textured surfaces, to contribute to design textured surfaces, reduce friction and wear.

Self-lubrication of tribologically-induced oxidation during dry reciprocating sliding of aged Ti–Ni51.5 at% alloy

The tribological behaviors of Ti–Ni51.5 at%alloy strengthened by finely dispersed Ni_(4)Ti_(3) particles in reciprocating sliding against GCr15,Al_(2)O_(3),and ZrO_(2) at room temperature were studied.Interestingly,the coefficient of friction(COF)suffered a sheer drop(from 0.9 to 0.2)when the aged alloy slid against GCr15 at a frequency of 20 Hz under a 20 N load without lubrication.However,severe‐mild wear transition disappeared when a solutionized alloy was used.Moreover,the COF stabilized at a relatively high level when Al_(2)O_(3) and ZrO_(2) were used as counterparts,although their wear mechanisms showed signs of oxidation.Scanning electron microscopy(SEM)and X‐ray element mappings of the wear scars of the counterparts clearly indicate that the formation of well‐distributed tribo‐layer and material transfer between the ball and disk are pivotal to the severe‐to‐mild wear transition in the aged Ti–Ni51.5 at%alloy/GCr15 friction pair.The higher microhardness and superelasticity of the aged alloy significantly accelerate the material transfer from GCr15 to the disk,forming a glazed protective tribo‐layer containing Fe‐rich oxides.

Experimental investigation of granular friction behaviors during reciprocating sliding

Granular friction behaviors are crucial for understanding the ubiquitous packing and flow phenomena in nature and industrial production.In this study,a customized experimental apparatus that can simultaneously measure the time history of normal and tangential forces on the inside-shearing unit is employed to investigate the granular friction behaviors during a linear reciprocating sliding process.It is observed that the evolution behaviors of two normal forces distributed separately on the shearing unit can qualitatively reflect the effects of the force chain network.During the half-loop of the reciprocating sliding,the total normal force,which indicates the load-bearing capacity of the granular system,experiences the following typical stages:decreases abruptly and stabilizes momentarily,further decreases significantly to the minimum,gradually increases to the maximum,and then remains stable.These stages are associated closely with the relaxation,collapse,reconstruction,and stabilization of the force chain,respectively.Interestingly,the coefficient of friction(COF)can reach a stable value rapidly within the initial sliding stage and subsequently remain constant.The average COF within stable ranges decreases significantly with the external load G in the power function form,G^(-0.5).Meanwhile,the COF increases slightly with the sliding velocity.Finally,a complete illustration of the dependences of the granular COF on the external load and sliding velocity is provided.Our study contributes to granular friction research by providing an innovative experimental approach for directly measuring the COF and implicitly correlating the evolution of the force chain network.

Subloading-friction model with saturation of tangential contact stress

The incorporation of the saturation of the tangential contact stress with the increase of the normal contact stress is required for the analysis of the friction phenomenon of solids and structures subjected to a high normal contact stress,which cannot be described by the Coulomb friction condition,in which the tangential contact stress increases linearly with the increase of the normal contact stress.In this article,the subloading-friction model,which is capable of describing the smooth elastic-plastic transition,the static-kinetic transition,and the recovery of the static friction during the cease of sliding,is extended to describe this property.Further,some numerical examples are shown,and the validity of the present model will be verified by the simulation of the test data on the linear sliding of metals.