Key Engineering Materials Vol. 642

Abstract: This study uses the finite element method (FEM) to predict the workpiece surface parameters, including contact area ratio and surface roughness, of asperity flattening in indentation and sliding contact for magnesium alloys sheet during warm isothermal forming. Contact area ratio and surface roughness are investigated for different process and material parameters, such as sliding distance, temperature, normal pressure and bulk strain rate by finite element analysis. The predicted results of the surface parameters from the finite element analysis are in good agreement with the results obtained from experiments.

Abstract: Friction is a crucial issue for the performance of a machine with sliding mechanism. While the sliding surface is often manually scraped to improve its tribological properties, the quality is not stable nor the efficiency of the scraping process. This study investigated the influences of micro-scale circumferential grooves on the frictional characteristics of a sliding surface through theoretical analysis and numeric simulations. The study found that sliding surface with micro grooves can reduce the friction, compared to plain surface. An in-depth analysis indicated that the slider with micro-scale grooves was uplifted when it slides, thus reduced the sliding friction. Moreover, as the amount of lubricant collected on the grooves increased, the friction force decreased with time and approached a stable condition.

Abstract: polishing is an essential process for deriving of the mirror-like surface of the AISI 316L stainless steel to minimize the surface roughness. Sliding, plowing and cutting, the fundamental mechanisms of abrasive machining, are occurred during the polishing and then would decrease the roughness on the surface. By 3MTM TrizactTM structured abrasive belts, the previous stainless steel are polished. Pyramid arrays containing the abrasives and epoxy formed on the belt give the abrasive machining. The glass transition and degradation temperatures of epoxy are about 150 and 250 degrees, respectively. The friction coefficient between the AISI 316L workpiece and 3MTM TrizactTM structured abrasive belt under different rotational speed may affect the polishing efficiency and the cost. The mechanism of the abrasive machining includes the sliding, plowing and cutting while the previous two are preferred during polishing. By 3MTM TrizactTM Structured Abrasive belts, the grit sizes are #800 and #2000, the MIM stainless steel cases are polished. The contact dynamic friction coefficient induces a higher specific polishing energy, the polishing temperature is thus increased. Deriving the friction coefficient is therefore important for stainless steel polishing. Experimental results indicate that the dynamic friction coefficient decreases respect to the rotational speed and the press-on force of workpiece. The coefficients of friction of all the workpieces under different tangential velocity are in a large range of 1.2–5.6, but the friction coefficient of the tangential velocity of 23.56 m/sec shows slightly variation even press-on force in 5 Newtons or in 20 Newtons.

Abstract: Diamond-like carbon (DLC) films is useful in many applications. To improve the tribological properties in DLC, we spin coat the multi-walled carbon nanotubes (CNTs) with different solution on (100) silicon. DLC was deposited by plasma enhanced chemical vapor deposition (PECVD) with C₂H₂ and H₂. The results show that the I_D/I_G ratio is increasing with higher CNTs content while the friction coefficient and critical load are decreasing with larger CNTs contents. The decreasing friction coefficient results from graphitation on the surface due to higher sp² content. The decreasing critical load is attributed to higher internal stress. The effect of friction coefficient and CNT concentration on stress distribution is studied by a nanoscratch finite element analysis. The results indicate that low friction coefficient and high CNT concentration will reduce the stress magnitude in the film. Therefore, the decreasing friction coefficient in CNT doped DLC film with increasing CNT concentration should reduce stress in the film and is good for adhesion. The discrepancy between friction coefficient and critical load is explained in terms of high internal stress during deposition. A surface treatment on CNT before deposition to reduce internal stress is currently under investigation.

Abstract: Band saws machine is the most efficient metal cutting machine. By losing less material, material qualitative, having big saw range and having low cutting costs are superiority of band saws. Band saws are acting with most long route and more limited support point in all cutting tools. In band sawing machine, band saws and structure are easy to produce vibration. Affect cutting life, costs of band saws and material surface topography. So this study focuses on analyzing what is vibration cause of band saws by vibration source and establishes the relationship between band saws natural frequency along and change cutting condition. In this research use the C-430GNC band saws machine, and use S45C material for cutting test. Accelerometers are used with a measurement system to capture sawing vibration data. By the fast Fourier transform to analyze spectrogram transform in order to identified characteristic spectrums, and find out vibration feature with band sawing operating conditions and unusual evens came from wear of blade. Minimum cutting vibration and best surface topography were compared to find out the best cutting condition.

Abstract: This study investigated the combined influence of stochastic surface roughness patterns and a magnetic field produced by an infinitely long wire on the distribution of hydrodynamic pressure in journal bearings lubricated with ferrofluids. According to our results, the magnetic field can raise the dimensionless pressure, improve the dimensionless load capacity and reduce the modified friction coefficient, especially at high eccentricity ratio. However, these bearing Characteristics are also influenced by surface roughness pattern depend on the length to diameter ratio λ, and the critical value of λ equal to 0.6, 0.95 and 1.8 in this study.

Abstract: A finite element procedure is implemented for the elastic-plastic analysis of carbon steels subjected to reciprocating fretting contacts. The nonlinear kinematic hardening rule based on Chaboche model is used to model the cyclic plastic behavior in fretting contacts. The results show that accumulation of plastic strains, i.e. ratchetting, may occur near the contact edge while elastic shakedown is likely to take place in substrate.

Abstract: Surface modification of polytetrafluoroethylene (PTFE) fiber by plasma grafting polymerization with acrylamide (AAm) was studied to improve the compatibility between polyacetal (POM) and PTFE. The best modified PTFE manufactured by self-designed plasma polymerization was investigated and denoted as AAm (20wt%)-g (plasma 30min)-PTFE. The grafting polymerization is successful and shown by ESCA spectra of modified PTFE fibers. It is also proved by SEM photographs and the improvement of properties of the POM/PTFE composites. As a result, the friction coefficients, Taber wear factors, and impact strengths of composites with 85wt% of POM and 15wt% of a variety of PTFEs were investigated. The frictional coefficient of POM/ AAm (20wt%)-g (plasma 30min)-PTFE deceased to 0.162; while that of POM/PTFE was 0.174. Taber wear factor of the composite significantly decreased from 5.2 to 1.5 when the PTFE was replaced by AAm-g (plasma)-PTFE. Meanwhile, the impact strength increased from 6.1 kg-cm/cm (POM/PTFE) to 9.3kg-cm/cm (POM/ AAm-g (plasma)-PTFE). Besides, the multi-yield behaviors were found in the Stress-Strain curves of composites with modified PTFE fibers.

Abstract: To understand the effects of tribo-film formation derived from ZnDTP (zinc dialkyl ditio phosphate) on friction behavior, friction tests were carried out with varying sliding time. The sliding surfaces were analyzed using Fourier transform infrared spectroscopy (FT-IR) and X-ray Photoelectron Spectroscopy (XPS). On the basis of the surface analytical results of the tribo-film, the absorption bands in the region of 1300-1000 cm^-1 assigned as polyphosphate appeared in all IR spectra after the sliding. The friction behavior of ZnDTP was related to the intensity change of the IR spectra peak at 1200 cm^-1.

Abstract: Tribological behaviour of polymer and hard coating films is complicated with bio-lubricant. Contacting and kinematic conditions of a knee joint when a person in running was simulated in the study. Substrates of specimens are Ti₆Al₄V and 316LVM stainless steel. Two kinds of polymer films, PCL-PEG-PCL and mPEG-PCL-mPEG, were used to simulate the tissue of cartilage. The silicon nitride film, SiN_xH_y prepared by PECVD, was used as a protecting film, and it’s thickness about 1000 nm under the polymer film. The testing device was developed, upper specimen is rotated reversely and the lower one is moved linearly forward and backward for 6 mm. A frequency vibration is applied under the lower specimen to simulate the vibration caused from walking, which is sated 2 Hz. Two torque meters are used for friction measurement in rotational and sliding directions. Experiments simulated vertical vibration, rotary and reciprocating motion. Comparing friction coefficient with different substrates with SiN_xH_y film, mixture of PCL-PEG-PCL and bovine serum can effectively decrease friction but useless for mPEG-PCL-mPEG. If adhesion of PCL-PEG-PCL polymer can be improved, it has potential in the application of artificial joint.