Wen Shizhu

Mechanical and tribological properties of non-hydrogenated DLC films synthesized by IBAD

Abstract Diamond-like carbon (DLC) films have been deposited on Si(100) substrates by an ion beam assisted deposition (IBAD) system. The effect of ion bombardment energy on surface roughness, mechanical and tribological properties of the DLC films has been studied over the ion bombardment energy range of 100–800 eV. The measured results of atomic force microscopy (AFM) showed that the DLC films are very smooth and the surface roughness is lower than that of the Si substrate at certain ion bombardment energy. Elasticity and nanohardness of the DLC films were studied by a nanoindentation technique. From nanoindentation load-displacement curves, it was concluded that most of the DLC films have high elasticity. Comparing the nanohardness with the conventional knoop microhardness of the same series of samples, we saw that the values of knoop microhardness are much higher than those of nanohardness. The correlation of the sp 3 bond fraction in the films estimated from Raman spectroscopy with surface roughness, elasticity and hardness has been established. Tribological behavior of the DLC films sliding against the ZrO 2 ball was evaluated using a pin-on-disk tribometer. It was found that the surface roughness and the hardness have an effect on the friction coefficients.

Superlubricity of a Mixed Aqueous Solution

A super-low friction coefficient of 0.0028 is measured under a pressure of 300 MPa when the friction pair (the silicon nitride ball sliding on the silicate glass) is lubricated by the mixed aqueous solution of glycerol and boric acid. The morphorlogies of the hydroxylated glass plate are observed by an atomic force microscope (AFM) in deionized water, glycerol, boric acid and their mixed aqueous solution. Bonding peaks of the retained liquids adhered on the surface of the sliding track are detected by an infrared spectrum apparatus and a Raman spectrum apparatus. The mechanism of the superlubricity of the glycerol and boric acid mixed aqueous solution is discussed. It is deduced that the formation of the lubricant film has enough strength to support higher loads, the hydration effect offering the super lower shear resistance. Key words: superlubricity, water based lubricant, ultra-low friction

Advances in research on a multi-channel on-line ferrograph

This paper introduces the basic principle, functions and test results of a multi-channel on-line ferrograph. The instrument catches wear debris with an electromagnet, detects wear debris with a photoelectric sensor, and controls sampling and data processing with an 8098 single-chip microprocessor which can communicate with a master computer. The instrument has four sampling channels, which can monitor not only one machine but also four machines one by one. The software of the instrument includes five modules which are a main program, a keyboard control program, a floating point operation program, a serial communication program and a self-checking program. The results of experiments on a gear box show that increasing average values detected by the instrument correspond to increasing load, so the instrument can meet the need for on-line monitoring of the wear condition of machines.

The inspection of the sliding surface and subsurface of plasma-sprayed ceramic coatings using scanning acoustic microscopy

Abstract The microstructures of the sliding surfaces and subsurfaces of some plasma-sprayed ceramic coatings (Cr2O3∗, TiO2, Al2O3, Al2O350%TiO2, WC18%Co) have been inspected using scanning acoustic microscopy (SAM). The results show that SAM is an effective tool for detecting non-destructive subsurface flaws and deformations which could not be seen by traditional optical microscopy or scanning electron microscopy (SEM). SAM can, therefore, be used to develop understanding of the relationship between tribological behaviour and surface and subsurface changes. It is found that propagated flaws may be found just below the deformation region of the surface. The Cr2O3∗ coating has a very strong tendency to cohesion, which may increase resistance to wear. Some relationships between TiO2, Al2O3, Al2O350%TiO2, WC18%Co coatings and their tribological changes have been built up.

Atomistic structural change of silicon surface under a nanoparticle collision

This study investigates the effect of the incident angle on the trajectory of a nanoparticle and the damaged region on a silicon surface, by molecular dynamic simulation of the collision and recoil of a nanoparticle with a monocrystlline silicon surface. With the change of the inci-dent angle, the recoil angle of the particle changes in a large range from an obtuse angle to an acute angle. The incident angle determines which part of the particle is in contact with the surface when the particle penetrates into the deepest position. Furthermore, it is the contacting part of the particle that the released elastic deformation energy of the surface acts on. These lead to the phenomenon that the recoil angle is sensitive to the incident angle in the collision process at a nanoscale. A depressed region is formed on the surface after the collision. The shape of the damaged region changes from a deep scoop to a flat arc, which is consistent with the trajec-tory of the particle. Some silicon atoms on the surface are extruded out by the incident particle, and form a pileup at the rim of the depressed region.

About the Load-Carrying Capacity of Elastohydrodynamic Lubrication Film

This paper analytically investigates elastohydrodynamic lubrication (EHL) film load-carrying capacity in pure rolling for different surface speeds and different lubricant temperatures considering the lubricant viscoplasticity. The viscoplasticity assumption, based on experimental results from the literature, provides a much lower lubricant limiting shear stress than that extrapolated from high pressure. The analysis is carried out for speeds up to 10.0 m/s and maximum Hertzian stresses to 4.5 GPa. The results show that, for high speed and/or high viscosity of lubricant, the lubricant viscoplasticity essentially results in inlet zone slip and a much thinner film than the conventional theory prediction. The deviation from conventional theory lessens with speed and/or viscosity reductions.

Effects of external electric fields on frictional behaviors of three kinds of ceramic/metal rubbing couples

Abstract In this paper, effects of external electric fields on frictional behaviors of Al2O3/brass, Al2O3/stainless steel and Al2O3/carbon steel couples under boundary lubricating conditions were studied on a self-made plate/plate type tribotester. Emulsion of 1 wt% zinc stearate dispersed into deionized water was used as lubricant in the experiments. The experiment results have shown that external electric fields affect the friction coefficient and its fluctuation of each rubbing couple substantially. The three couples showed different rates of change in friction coefficient and different tendency of change as the field was turned on and off during the rubbing process. Results acquired from experiments carried out in a wide range of external voltage, from 0 to 110 V, indicating that friction coefficient of Al2O3/brass couple increases monotonously with the intensity of the external electric field, although no linear relationship was found between them.

EHL performance of the lubricant with shear strength : Part I- Boundary slippage and film failure

This paper analytically investigates the isothermal line contact elastohydrodynamic lubrication of three lubricants with much different shear strengths under the nondimensional operating parameters of w = 2.15e-4 and U = 2.53e-10 applying the lubricant ideal viscoplastic rheological model. The boundary slippage of the low-shear-strength lubricant occurring in the EHL inlet zone was found and results in a much thinner film compared to the classical EHL theory prediction. The film boundary slippage and its growth with the slide/roll ratio variation of tile low-shear\- strength lubricant exhibit special phenomena, which are much different from those of the high-shear-strength lubricant. The easy occurrence of film failure in concentrated contact in the case of high sliding speed, heavy load, large slide/roll ratio, and low-shear-strength lubricant was concluded due to the severe friction heating on the surface conjunction and the lubricant thermal desorption on tile lubricant/surface boundary. The EHL film failure mechanism was further recognized.

A lubrication deviation from the classical EHL theory by the lubricant viscoplasticity : Part I - film thickness dependence

This paper studies film thickness in isothermal pure rolling elastohydrodynamic lubrication (EHL) considering lubricant viscoplasticity. The central film thickness formula based on lubricant viscoplasticity regressed out by this study shows a deviation of elastohydrodynamic lubrication from the conventional theory under high rolling speeds and heavy loads. This deviation actually describes the elastohydrodynamic film thickness when the lubricant shear strength and the maximum endurable shear stress on the lubricant-surface interface in EHL inlet zones degrades due to high temperature.

Theoretical study on the lubrication failure for the lubricants with a limiting shear stress

It is observed that the non-slip boundary conditions in the classical lubrication theory are not valid for lubricant with a limiting shear stress. By determining the position of the maximum shear stress and changing velocity boundary condition to stress condition, slip lubrication equations are deduced. With numerical solution of the equations, the location and scale of the boundary slip are analyzed. Finally, after the influences of the slip on the lubrication properties are discussed, results show that a significant decrease of the load carrying capacity causes slip lubrication failure.