Tribochemical interactions of Si-doped DLC film against steel in sliding contact

This study concerns the effects of tribochemical interactions at the interface of Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the films and mating balls after tests. Results showed a darkgray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction (∼0.06), the Si-DLC film showed a significant lower coefficient of friction (∼0.022) with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.     

Tribological properties of adsorbed water layer on silicon surfaces

Tribological properties of adsorbed water layer on solid surface in sliding contact have not yet been fully understood. In this regard, it is important to better understand how surface hydrophilicity and humidity influence the tribological behavior of adsorbed water-mediated microcontact. In this study, we investigated the influence of adsorbed water layer and capillary force on friction as a function of relative humidity for silicon surfaces with different water affinity. Friction of the silicon surface with different water affinity was examined under various humid environments in a wearless sliding condition (low contact pressure) against a glass sphere. Numerical analysis was also conducted to calculate capillary force and interfacial shear strength for each surface as a function of relative humidity. The friction of the hydrophobic Si surface was low and stable, and almost independent of relative humidity whereas that of the hydrophilic surfaces were significantly influenced by relative humidity. These behaviors were explained in terms of capillary wetting and the role of confined water layer in the contact area. The influence of confined water layer became more dominant over capillary force as relative humidity increased. There was a good correlation between the calculated shear strength and the measured friction force for all surfaces regardless of their hydrophilicity and humidity condition.     

Tribological properties of nanoporous anodic aluminum oxide film

Friction and wear properties of nanostructured anodic aluminum oxide (AAO)) films were studied in relation to contact load and pore size (pore diameter). Uniformly arrayed nanoporous aluminum oxide films (pores of 28 nm, 45 nm, 95 nm, and 200 nm diameter and 60-100 μm thick) were synthesized by anodization. Reciprocating wear tests using 1 mm diameter steel balls as counterpart were carried out for a wide range of load (from 1 mN to 1 N) at ambient environment. The friction coefficient reduced with the increase of load. The friction coefficient decreased by approximately 30% when the load increased by 3 orders of magnitude. The pore density marginally affected the frictional properties of AAO films. The influence of pore size on the friction coefficient was significant at relatively high loads (0.1 N and 1 N) whereas it was negligible at low loads (1 mN and 10 mN). The worn surface of AAO films tested at low loads did not experience tribochemical reaction and exhibited only mild plastic deformation. Dispersed thick smooth films were formed on the worn surface of all samples at relatively high loads whereas only extremely thin smooth film patches were rarely formed at low loads. These thick smooth films were generated by combined influence of tribochemical reaction at the contact interface and plastic deformation of compacted debris particles as evidenced by energy-dispersive spectroscopy analysis. We suggest that these thick films mainly contributed to the decrease of friction regardless of the pore size.     

Tribological Behavior of Grafted Polymer Gel Nanocoatings

A robust molecular lubrication layer on a silicon surface has been fabricated from a grafted polymer gel with thickness below 10 nm. A functionalized rubber-glassy block-copolymer was chemically grafted to a silicon oxide surface and its tribological performance was enhanced by vapor saturation with a minute amount of alkyl-based paraffinic oil. A combination of tribological measurements and Auger electron spectroscopy was used to monitor the polymer layer wearing behavior. We observed that unlike a dry polymer layer and a classic boundary lubricant, an alkylsilane self-assembled monolayer, the polymer gel coating exhibited a steady friction response, a very low value of the coefficient of friction, and possessed much higher wear-resistance.     

Cavity design method for injection-molded spur gears

Mold cavities of gears should be made larger than the product specification since plastics shrink when changing from a molten to a solid state. For injection molded spur gears, two design methods for the compensation of shrinkage are widely used. One is the module correction method and the other is the pressure angle correction method. Both methods are based on the assumption that shrinkage occurs toward the center of a molded gear. This paper deals with the shrinkage rate and proposes a method of designing gear cavity derived from the measured shrinkage rates which govern the outside diameter, the tooth depth and the tooth thickness of a molded gear. The proposed method imposes no restriction on the shrinkage direction and provides a cavity with all of the fundamental gear design parameters.     

Effects of relative humidity on tribological properties of boron carbide coating against steel

Boron carbide coatings of 100 nm thick were synthesized on silicon substrate by DC magnetron sputtering using B₄C target with a mixture of Ar and methane (CH₄ at 1.2 vol.%) as processing gases. Tribological properties of the coating were studied in relation to the effects of relative humidity (RH). Reciprocating wear tests using 3 mm diameter steel balls as a counterpart were carried out at three relative humidity conditions. Confocal microscopy was used to observe worn surfaces and the wear scars on the steel balls. Elemental composition of the coating and worn surfaces were characterized using X-ray photoelectron spectroscopy and Auger electron spectroscopy. The tribological properties of boron carbide coating slid against steel ball were strongly affected by relative humidity. Lower and steady friction coefficient and higher wear resistance for both the coating and the steel ball were achieved at higher relative humidity. At high RH, tribochemical reaction occurred in the sliding surfaces, forming boric acid and carbon in a graphitic form on the worn surface of coating and a soft layer on the ball surface. The formation of boric acid on boron carbide coating combined with graphite structure led to the low and stable friction of boron carbide coating in medium and high relative humidity conditions. Smooth layer was formed on the worn surface of the steel ball at high relative humidity due to the tribochemical reaction. Low and steady value of friction coefficient and reduction of wear loss of both steel ball and boron carbide coating were attributed to the formation of the soft layer.