Chau-Chang Chou

The response surface method and the analysis of mild oxidational wear

Abstract Instead of using the conventional oxidation theory to depict a disk’s wear rate as a function of contact temperature, the response surface method (RSM) is herein introduced to relieve the one-factor-at-a-time defect in portraying tribological characteristics. By means of a central composite design (CCD) technique, fewer operating conditions are needed to establish expressions for the wear rate parameter, the contact temperature and the friction coefficient as a function of sliding speed and applied load. A second degree polynomial was used to represent a curved surface which fits the experimental data. In addition to results for the designated operating conditions, wear rate parameters and contact temperatures obtained from the polynomials were compared with the experimental results. The activation energy in the wear rate expression can thus be derived as a function of sliding speed, applied load and contact temperature. The experimental data for the wear rate parameter can be expressed by smooth curves, instead of two different straight lines in two temperature subdivisions.

Tribological effects of roughness and running-in on oil-lubricated line contacts

Abstract A gear/cam adapter was employed to study various aspects of line-contact lubrication, using oil with extreme-pressure additive at two different concentrations. The effect of run-in on the tribological performance of rollers with two different surface roughnesses was investigated in terms of friction coefficient, wear loss, oil temperature, specimen roughness and electrical resistance. The relation between roller wear loss and the time rate of electrical resistance change was established. Electrical resistance versus oil temperature underwent a dramatic change during testing, a period of rising oil temperature and relatively constant low electrical resistance abruptly becoming a period of relatively constant oil temperature and rising electrical resistance, low wear rates correlating strongly with the second period. The run-in effect on roller wear loss in smooth rollers (Ra = 0.076 μm) is opposite to that in rough rollers (Ra = 0.463 μm). The asperity height of the smooth rollers was increased by wear testing irrespective of run-in; however, run-in enhanced the increase in surface roughness. The extreme-pressure additive concentration instead of run-in was the decisive factor in electrical resistance. Friction coefficients during testing showed a strong positive relation with composite surface roughness.

A New Approach to the Effect of EP Additive and Surface Roughness on the Pitting Fatigue of a Line-Contact System

A gear-cam adaptor was used to investigate the effect of an extreme pressure (EP) additive in the base oil and rollers surface roughness on the pitting life of a disk specimen under oil lubrication with rolling-sliding line contacts. Incorporating the mean friction coefficient, increases in the tickers microhardnesses within the layer of strain hardening can explain the influence of the EP additive on the material toughness of the ratchetting layer. The pull-off forces obtained from the tip/samples force-distance curve of an atomic force microscopy (AFM) can further reveal the ever-growing characteristic of the boundary layer, which is benefit to retard the growth of cracks. Weibull plots for disks pitting life are provided for three lubrication cases. The use of the EP additive in the base oil can expand the range of pitting-life distribution; on the other hand, increasing the rollers surface roughness can also elevate the pitting life of the specimen lubricated by the same oil. From the study of contact subsurface, the distributions of four material responses: boundary layer, ratchetting layer, shakedown layer, and elastic layer, can be found. The EP additive in the base oil affects the behavior of the first two layers and rollers surface roughness, the dominant factor, even does the third one.

Thermal Analysis of a Medium-Carbon Steel Tribo-System and Its Application to Oxidational Wear

Wear behavior of AISI 1045 carbon steel has been studied on a washer-on-disk wear machine at bulk disks temperatures of 25°C and 200°C. An analytical approach to the temperature solutions was developed in terms of a parabolic temperature polynomial for the frictional contact area ; the maximum contact temperature T c is thus obtained by using several measured temperatures as boundary conditions. The tribological properties of carbon steel in sliding contact were studied to investigate the oxidational wear rate associated with the tribo-chemical reaction arising on the worn surface due to frictional heating. A theoretical approach to predict the oxidational wear rate of this carbon steel at 25°C and 200°C was also established.

The effects of strain hardened layer on pitting formation during rolling contact

The role of the strain hardened layer playing in pitting formation during rolling contact is analyzed using the fracture mechanics approach. The other governing factors are the initial crack length, crack angle, contact force, friction on all contact surfaces, and the hydraulic pressure of trapped fluid acting on the crack surfaces. Mode I and II stress intensity factors and the strain energy density factors are calculated by application of the two-dimensional finite element method. The strain energy density theory is applied to show the coupling effects between the hardened layer and the other factors. All analytical results predicted in this paper agree well with the experimental observations.

Hydrodynamic Simulation of an Orbital Shaking Test for the Degradation Assessment of Blood-Contact Biomedical Coatings

Biomedical coatings are used to promote the wear resistance and the biocompatibility of a mechanical heart valve. An orbital shaking test was proposed to assess the durability of the coatings by the amount material eroded by the surrounding fluid. However, there is still a lack of understanding with regards to the shaker’s rotating conditions and the corresponding physiological condition. This study implemented numerical simulations by establishing a fluid dynamic model to evaluate the intensity of the shear stress under various rotating speeds and diameters of the shaker. The results are valuable to conduct in vitro tests for estimating the performance of biomedical coatings under real hemodynamic conditions and can be applied to other fluid-contact implants.

Application of the Response Surface Method to the Tribological Analysis of a Medium-Carbon Steel Under Mild-Oxidational Wear

Instead of using the conventional oxidation theory to depict a disks wear rate as a function of contact temperature, the response surface method (RSM) is herein introduced to relieve the one-factor-at-a-time defect in portraying tribological characteristics. By means of a central composite design technique, fewer operating conditions are needed to establish expressions for the wear rate parameter, the contact temperature and the friction coefficient as a function of sliding speed and applied load. A second degree polynomial was used to represent a curved surface which fits the experimental data. In addition to results for the designated operating conditions, wear rate parameters and contact temperatures obtained front the polynomials were compared with the experimental results. The activation energy in the wear rate expression can be derived from a function of sliding speed, applied load, and contact temperature. The experimental data for the wear rate parameter can be described by smooth curves, instead of two different straight lines in two temperature divisions.

Processing and Crystal Microstructure of Porous High Pressure Crystallized Nanodiamond/UHMWPE Biomedical Nanocomposite

A novel processing technique using a series of mixing/refluxing procedures and high-pressure crystallization (HPC) to fabricate nanodiamond/ultra high molecular weight polyethylene (ND/UHMWPE) nanocomposites for biomedical application was examined. For better mimetic lubrication, a porous structure was implemented in this study. Vitamin E as an anti-oxidation additive was also incorporated in selected porous specimens. The morphology of the specimens was investigated by transmission electron microscopy. The phase and crystal characteristics were revealed by Raman spectroscopy and X-ray diffraction. Shore D hardness was used to study the effect of the material’s porous structure and particle-induced crystallization on the bulk mechanical property. The dispersion of NDs in the UHMWPE matrix can significantly promote the crystallinity of the HPC specimens, even with a porous structure. However, the bulk hardness does not reveal this improvement in crystal microstructure.

Fractal dimension and surface topography on the diamond deposition of seeded WC–Co substrates

Diamond thin films were deposited on WC–Co substrates by hot filament chemical vapor deposition to improve the tribological performance. The influence of the substrate surface topography was found to play an important role during the nucleation stage and the later growth rate as well. In this study, we systematically investigated the relation between substrate surface irregularity, which was evaluated by fractal dimension as well as statistical roughness parameters and the quality of the later deposited diamondfilm. Preseeding processes, in diamond acetone suspensions with two particle diameters, by supersonic vibrator were also implemented to investigate the effect of particular size on diamondnucleation. The original surfaces were measured with a stylus profiler and contact-mode atomic force microscopy. The diamond deposited substrates were examined by scanning electron microscopy, x-ray diffractometry, Raman spectroscopy, and Rockwell-C indentation to study substrate topography, crystalline structure of the coating, the composition of diamondfilms, and adhesion between deposited layers and substrates, respectively. The synergetic influence of the substrate’s fractal dimension and the particular size of pre-seeding diamondsuspension were studied and addressed. The deposited film of a WC–Co substrate with higher surface fractal dimension ( > 2.50 ) , preseeded by fine diamondsuspension (4–12 nm particle size) in advance, has a high diamond-rich composition and adhesion strength.

Models for Temperature-Kinetic Aspect of Friction and Wear in Oil Lubrication

Experiments were conducted using a gear-cam adapter to simulate line-contact lubrication and wear in a lubricant with various additive concentrations. In this study, the growth of an adsorption film and a chemical film in oil lubrication can be detected by the electrical contact resistance (ECR) apparatus of a wear testing machine. A constant current source was applied; the electrical voltage is thus obtained if the ECR is available. Roller wear rate and wear loss are formulated in terms of roller-lower specimen contact temperature and several relevant parameters. In addition, wear loss is also related to the rise of electrical voltage during the period of lubrication. The experimental data for wear rate and wear loss can be regressed by the models proposed for various additive concentrations. The rollers wear rate can be predicted through the investigation of the combined effect of growth in electrical voltage and its commencement. For lowering wear rate, the optimum additive concentration in a lubricating oil is determined under various operating conditions.