Aydar Akchurin

Thermal properties of carbon nanofiber reinforced high-density polyethylene nanocomposites

Reinforcing polymers with the appropriate nanofillers is an effective way to obtain a variety of enhanced material properties. In this paper, high-density polyethylene nanocomposites reinforced with either pristine or silane-treated carbon nanofibers at various weight percentages (0.5 wt%, 1 wt%, and 3 wt%) were fabricated through melt-mixing and compressive processing. Silane coatings with two thicknesses, 2.8 nm and 46 nm, were applied on the oxidized carbon nanofibers to improve the interfacial bonding between the carbon nanofibers and the matrix. Scanning electron microscopy and transmission electron microscopy demonstrated the dispersion of carbon nanofibers and the strongly improved interfacial adhesion between the carbon nanofibers and high-density polyethylene matrix due to the silane coating. The thermal properties of high-density polyethylene / carbon nanofiber nanocomposites were characterized and compared with those of the neat high-density polyethylene. The measurement results showed that the thermal conductivity of the high-density polyethylene /carbon nanofiber nanocomposites increased with the carbon nanofiber loading. The enhancement of thermal conductivity was not only due to the high thermal conductivity of carbon nanofibers but also due to the interfacial quality between the carbon nanofibers and the high-density polyethylene matrix. The interfacial thermal contact resistance between the carbon nanofibers and the matrix was determined to be in the range of 0 . 88 × 10 - 6 m 2 K / W to 3 . 89 × 10 - 6 m 2 K / W . Furthermore, the coefficient of thermal expansion of the nanocomposite was found to be reduced by the incorporation of carbon nanofibers.

Wear and Friction of Carbon Nanofiber-Reinforced HDPE Composites

New applications of carbon-based materials have been continuously developed in recent years. Carbon nanofibers (CNFs) with silane coatings were added into high density polyethylene (HDPE) to improve the tribological properties of the nanocomposite material. The nanocomposites were fabricated with various weight percentages of carbon nanofibers (0.5 wt.%, 1 wt.% and 3 wt.%) that were treated with different silane coating thicknesses (2.8 nm and 46 nm) through melt-mixing and compressive processing. The wear and friction tests were performed on a pin-on-disc tribometer under phosphate buffered saline lubricated condition. Compared with the neat HDPE, the friction coefficients of the nanocomposites were reduced in all samples, yet only a couple of nanocomposite samples showed lower wear rates. Micro-hardness measurements of the nanocomposites were carried out and CNFs were found to be capable of increasing the material’s micro-hardness. The effects of concentration and silane coating thickness of CNFs on the tribological properties of the resulting nanocomposites were analyzed and the wear mechanisms of the HDPE/CNF nanocomposites were discussed.

A stress-criterion-based model for the prediction of the size of wear particles in boundary lubricated contacts

In this paper, the formulation and validation of a model for the prediction of the wear particles size in boundary lubrication is described. An efficient numerical model based on a well-established BEM formulation combined with a mechanical wear criterion was applied. The behavior of the model and particularly the influence of the initial surface roughness and load was explored. The model was validated using measurements of the wear particles formed in steel–steel and steel–brass contacts. In the case of steel–steel contact, a reasonable quantitative agreement was observed. In the case of steel–brass contact, formation of the brass transfer layer dominates the particles generation process. To include this effect, a layered material model was introduced.

Mechanical properties, tribological behavior, and biocompatibility of high-density polyethylene/carbon nanofibers nanocomposites:

Carbon nanofibers (CNFs) with silane coatings were used as the reinforcement to enhance the mechanical and tribological properties of high-density polyethylene (HDPE) at different loading levels (0.5 wt% and 3 wt%). To improve the interfacial bonding between the CNFs and HDPE matrix, two types of silane coating thicknesses, 2.8 nm and 46 nm, were applied onto oxidized CNFs. Mechanical properties of the HDPE/CNF nanocomposites including Young’s modulus, ultimate stress, strain at fracture, as well as work of fracture, were investigated through tensile testing. The wear tests were performed on a pin-on-disk tribometer under the bovine serum lubricated condition. The coefficient of friction of the materials in contact with steel balls was monitored over the duration of the wear test. The addition of CNFs not only decreased the coefficients of friction of the nanocomposites, but also reduced their wear rates. The thicker silane-treated CNFs were found to be more effective in reducing the coefficients of friction and elongating the strain of fracture compared with the pristine CNFs and thinner silane-treated ones. The biocompatibility of the nanocomposites against a mouse osteoblast precursor cell line was also evaluated. Among the several types of nanocomposites, the one reinforced with the thicker silane-treated CNFs (46 nm) at 0.5 wt% loading level yielded the highest strain at fracture, the best wear resistance with a wear rate reduction of nearly 57.9% compared to the neat HDPE, and good biocompatibility, making it a promising material for biomedical applications.

A Deterministic Stress-Activated Model for Tribo-Film Growth and Wear Simulation

A new model was developed for the simulation of growth and wear of tribo-chemical films by combining a boundary element method-based contact model and a stress-activated Arrhenius tribo-film growth equation. Using this methodology, it is possible to predict the evolution and steady-state thickness of the tribo-film (self-limitation) at various operating conditions. The model was validated using two cases for which experimental data were available in the literature. The first case is a single microscopic contact consisting of a DLC-coated AFM tip and an iron-coated substrate. The second case is a macroscale contact between a bearing steel ball and disk. Subsequently, mild wear (wear after running-in) was modeled by assuming diffusion of the substrate atoms into the tribo-film.

A Mixed-TEHL Analysis of Cam-Roller Contacts Considering Roller Slip: On the Influence of Roller-Pin Contact Friction

In this work, a mixed lubrication model, applicable to cam-roller contacts, is presented. The model takes into account non-Newtonian, thermal effects, and variable roller angular velocity. Mixed lubrication is analyzed using the load sharing concept, using measured surface roughness. Using the model, a quasi-static analysis for a heavily loaded cam-roller follower contact is carried out. The results show that when the lubrication conditions in the roller-pin contact are satisfactory, i.e., low friction levels, then the nearly “pure rolling” condition at the cam-roller contact is maintained and lubrication performance is also satisfactory. Moreover, non-Newtonian and thermal effects are then negligible. Furthermore, the influence of roller-pin friction coefficient on the overall tribological behavior of the cam-roller contact is investigated. In this part, a parametric study is carried out in which the friction coefficient in the roller-pin contact is varied from values corresponding to full film lubrication to values corresponding to boundary lubrication. Main findings are that at increasing friction levels in the roller-pin contact, there is a sudden increase in the slide-to-roll ratio (SRR) in the cam-roller contact. The value of the roller-pin friction coefficient at which this sudden increase in SRR is noticed depends on the contact force, the non-Newtonian characteristics, and viscosity–pressure dependence. For roller-pin friction coefficient values higher than this critical value, inclusion of non-Newtonian and thermal effects becomes highly important. Furthermore, after this critical level of roller-pin friction, the lubrication regime rapidly shifts from full film to mixed lubrication. Based on the findings in this work, the importance of ensuring adequate lubrication in the roller-pin contact is highlighted as this appears to be the critical contact in the cam-follower unit.

Erratum to: Analysis of Wear Particles Formed in Boundary-Lubricated Sliding Contacts

Due to unfortunate misinterpretation of dynamic light scattering (DLS) measurement results, Figures 10–12 and 17 contain errors. This also affects the discussion based on the comparison of atomic force microscopy (AFM) and DLS results (Fig. 17). The DLS measurements indicate smaller particles compared to AFM (Figs. 10, 11, 12). The calculation of the number of particles per sliding distance and per unit load was based on the DLS data and therefore is incorrect. The correct numbers are: 1200–6000 particles mm N for steel and approximately 12,800–15,200 particles mm N for brass. These corrections do not change the final conclusions. Load, N 2 4 6 8 10 R ad iu s, n m