Lubricant thickness effects on friction between Pd(100) surfaces

Friction measurements have been made between pairs of Pd(100) surfaces prepared in vacuum with adsorbed n-octane films ranging in thickness from 0 to 20 monolayers on each surface. These measurements have been made at lattice misorientation angles of 0° and 45°. Both sets of measurements reveal friction anisotropy at all except the highest n-octane coverages. The friction coefficient drops with increasing n-octane coverage until it reaches a limiting value of _s 0.4. The static friction coefficient reveals a different dependence on n-octane coverage than has been observed for alcohols. The friction coefficient decreases more slowly with increasing n-octane coverage than for increasing alcohol coverage.     

Ionic liquids as lubricants of polystyrene and polyamide 6-steel contacts. Preparation and properties of new polymer-ionic liquid dispersions

Room-temperature ionic liquids (ILs) have been used as external lubricants in polystyrene (PS) and polyamide 6 (PA6)-steel contacts and as internal lubricants in new polymer-IL dispersions. 1−C _n H_2n+1−3−CH₃-imidazolium X⁻ [X=BF₄; n=2 (IL1), 6 (IL2), 8 (IL3). X=PF₆; n=6 (IL4). X=CF₃SO₃; n=2 (IL5). X=(4−CH₃C₆H₄SO₃); n=2 (IL6)] ionic liquids give low friction and extremely mild wear in PS/AISI 316L stainless steel contacts, independently of IL composition. For AISI 52100 steel pins a tribocorrosion reaction produces FeF₂ and increases friction. PS+IL1 (1; 1.35; 3 wt.% IL1) dispersions show lower dry friction and wear against AISI 52100 as IL1 proportion increases, but the lowest friction, with a one order of magnitude reduction with respect to PS, is reached for PS+1%IL1 once the skin layer has been worn out. Increasing IL1 content to 10 wt.% produces an heterogeneous material with non-uniform IL distribution. IL4 reduces friction and wear in PA6+3%IL4 dispersions against AISI 316L, although the lowest values are obtained with IL4 as external lubricant. The cryofracture surfaces of the polymers have been examined and the thermal stability of the polymers in the presence of ILs has been determined.     

Interactions of n-Hexadecane with 52100 Steel Surface Under Friction Conditions

Straight-chain aliphatic hydrocarbons, particularly n-hexadecane, are used as reference fluids in research concerning the AW/EP effectiveness of triboactive additives. It is assumed, however, that under boundary lubrication conditions these apparently non-reactive hydrocarbons might influence the tribochemistry of the additives because aliphatic compounds also undergo chemical reactions. Both the thermochemical and tribochemical oxidation of aliphatic hydrocarbons lead to the formation of triboactive products (e.g. alcohols, aldehydes and carboxylic acids). Interactions of these compounds with rubbing surfaces were examined using GC/MS, XPS and EDS techniques. It has been found that carboxylic acids react with the iron surface generating salts or complex compounds.     

Tribopolymerization of n-butyl acrylate on the steel–steel rubbing surface

Antiwear property of n-butyl acrylate (BA) in hexadecane for steel–steel friction elements was studied. Scanning electronic microscopy (SEM) images of worn scars of balls, and width measurements of worn tracks of disks indicate that BA has good antiwear property.Tribopolymerization (polymerization initiated by the rubbing surface) tests of BA used as lubricant instead of an additive were conducted, to classify antiwear mechanisms. Infrared spectroscopy (IR) of washing solution and thermogravimetry (TG) traces of wear debris confirmed that tribopolymers were generated on steel–steel interface in situ. Also considerable wear products were precipitated from vacuum-condensed worn fluids with methanol as the non-solvent, characterized by IR, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) spectroscopy, showing that the precipitates were poly(n-butyl acrylate) (PBA) with very high molecular weight.To study tribopolymerization mechanisms of BA, effects of sliding velocities and adding 1 wt% n-butyl alcohol to BA fluids on the mass of tribopolymers were investigated, respectively. Expectedly, the mass of tribopolymers dramatically enhanced with the sliding velocity increasing, showing that the tribopolymers were generated just due to friction processes. Additionally, tribopolymerization of styrene (easy to thermopolymerize) was studied. Unexpectedly—but not unreasonably—no substantial worn products were precipitated from vacuum-condensed worn fluids. Based on these experimental results, an exoelectron-radical-tribopolymerization mode, consistent with some Kajdas’ tribochemistry theories, for BA was proposed.     

Three-parameter approach for elastic–plastic fracture of the semi-elliptical surface crack under tension

Systematic three-dimensional elastic–plastic finite element analyses are carried out for a semi-elliptical surface crack in plates under tension. Various aspect ratios (a/c) of three-dimensional fields are analyzed near the semi-elliptical surface crack front. It is shown that the developed J–Q annulus can effectively describe the influence of the in-plane stress parameters as the radial distances (r/(J/σ₀)) are relatively small, while the approach can hardly characterize it very well with the increase of r/(J/σ₀) and strain hardening exponent n. In order to characterize the important stress parameters well, such as the equivalent stress σ_e, the hydrostatic stress σ_m and the stress triaxiality R_σ, the three-parameter J–Q_T–T_z approach is proposed based on the numerical analysis as well as a critical discussion on the previous studies. By introducing the out-of-plane stress constraint factor T_z and the Q_T term, which is determined by matching the finite element analysis results, the J–Q_T–T_z solution can predict the corresponding three-dimensional stress state parameters and the equivalent strain effectively in the whole plastic zone. Furthermore, it is exciting to find that the values of J-integral are independent of n under small-scale yielding condition when the stress-free boundary conditions at the side and back surfaces of the plate have negligible effect on the stress state along the crack front, and the normalized J tends to a same value when φ equals about 31.5° for different a/c and n. Finally, the empirical formula of T_z and the stress components are provided to predict the stress state parameters effectively.     

A new approach to generation of ruled surfaces and its applications in engineering

Based on the Study displacive theory that there is a one-to-one correspondence between an oriented straight line in three-dimensional Euclidean space (E ³) and a dual point on the surface of a dual unit sphere (DUS) in three-dimensional dual space (D ³), this paper presents a new methodology for the generation of ruled surfaces by planning (fitting or interpolating) the motion locus of a point on the surface of a DUS and discusses its applications in engineering. Some geometric problems and an optimal generation model for ruled surfaces are dealt with in detail. The correctness and effectiveness of this methodology are illustrated by examples and application in practice.     

Computer modeling of the continuous annealing furnace

A computer program to calculate the strip temperature heated in the continuous annealing furnace was developed, using the zone method for radiative heat transfer analysis with the measured gas temperature in the furnace. Using theF _E Operator, the present study considered the effects of soot and transient species, in addition to the H₂O−CO₂ gas mixture on the gas radiative heat transfer. The predicted strip temperature distribution forF _E=1.05 represented well the measured data. The maximum difference in the heat flux transfered to the strip from the combustion gas forF _E=1.0 (without soot and transient species gas radiation) and 1.05 (with soot and transient species gas radiation) was about 15%. The present study also investigated the effects of line speed and thickness variations on the strip temperature, establishing the bases for the on-line computer model.     

Biofilm formation in Haas palatal expanders with and without use of an antimicrobial agent: an in situ study

Orthodontic appliances causes specific alterations in oral environment, including reduction of pH, increase of dental biofilm and elevation of salivary microbial levels, causing an increased risk for dental caries. This study evaluated, using microbial culture and scanning electron microscopy (SEM), the in situ contamination by mutans streptococci (MS) of different surfaces of Haas palatal expanders with and without use of chlorhexidine gluconate mouthrinses (CHX). Thirty‐four patients were randomly assigned to two groups (n = 17/group), using placebo (Group I) and 0.12% CHX (Group II—Periogard^®) mouthrinses twice a week. After 4 months, appliances were submitted to microbiological processing and after fragments were analyzed by SEM. Mann–Whitney U test (α = 5%) was used to assess differences between groups on the appliances' different surfaces and to compare the contamination on the free and nonfree surfaces of these components. There was no difference (p = 0.999) between groups regarding the number of MS colonies/biofilms on the nonfree surfaces, which showed intense contamination. However, free surfaces of Group II presented less contamination (p < 0.001) than those of Group I in all appliances' components. Results of the microbial culture were confirmed by SEM. Use of 0.12% CHX was effective in reducing the formation of MS colonies/biofilms on free surfaces of Haas expanders, in situ.     

Tribological Sensitivity of PTFE/Alumina Nanocomposites to a Range of Traditional Surface Finishes

Wear tests were performed with polytetrafluoroethylene (PTFE) + Al ₂ O ₃ nanocomposites on various manufactured surfaces to determine whether or not the wear resistance of these nanocomposites is a strong function of surface preparation. Four different surface finishes of grade 304 stainless steel counterfaces were used: electropolished (R _q = 88 nm), lapped (R _q = 161 nm), wet-sanded (R _q = 390 nm), and dry-sanded (R _q = 578 nm). PTFE + Al ₂ O ₃ nanocomposites made from powders of roughly 2-20 μm PTFE (matrix) and ～44 nm Al ₂ O ₃ (filler) were prepared at filler weight percentages of 0, 1, 5, and 10% and tested on each surface finish. Additionally, 5 wt% 44-nm nanocomposites were compared to identically prepared 5 wt% 80- and 500-nm Al ₂ O ₃ filled PTFE composites on each surface. Friction coefficients were between 0.12 and 0.19 and wear rates decreased from K = 810 × 10^{? 6} mm ³ /(Nm) for the 5 wt% 500-nm alumina-filled PTFE on the dry-sanded surface to K = 0.8 × 10^{? 6} mm ³ /(Nm) for the 5 wt% 80-nm filled composite on the lapped surface. It was found that the minimum wear rate occurred on the lapped counterface for every composite, and the wear rate is a strong function of the transfer film thickness and morphology.     

Development of the delamination evaluation parameters (I) — the delamination aspect ratio and the delamination shape factors —

Although the previous researches evaluated the fatigue behavior of A1/GFRP laminates using the traditional fracture mechanism, their researches were not sufficient to do it : the damage zone of A1/GFRP laminates was occurred at the delamination zone instead of the crack-metallic damages. Thus, previous researches were not applicable to the fatigue behavior of Al/GFRP laminates. The major purpose of this study was to evaluate delamination behavior using the relationship between crack length(a) and delamination width(b) in A1/GFRP laminate. The details of investigation were as follows : 1) Relationship between the crack length(a) and the delamination width(b), 2) Relationship between the delamination aspect ratio(b/a) and the delamination area rate ( (A_d)_n/ (A _d ) a_ll) , 3) The effect of delamination aspect ratio(b/a) on the delamination shape factor (f _s ) and the delamination growth rate (dA _d /da). As results, it was known that the delamination aspect ratio(b/a) was decreased and the delamination area rate ( (A _d ) _n / (A _d ) _All ) was increased as the normalized crack size(a/W) was increased. And, the delamination shape factors (f _s ) of the ellipse-II (f _s3 ) was greater than of the ellipse-1 (f _s2 ) but that of the triangle (f_s1) was less than of the ellipse-I (f _s2 ).     

Low-Friction MoS x Coatings Resistant to Wear in Ambient Air of Low and High Relative Humidity

The investigation of the tribological performance of MoS₂-based coatings in air of high humidity is critical for the future use of such low-friction and high-wear-resistant coatings in ambient air. Sulfur-deficient MoS _x coatings with a basal plane (x = 1.3) and a random (x = 1.8) crystallographic orientation were produced by planar magnetron sputtering. The coefficient of friction and the wear loss of MoS_x coatings in comparison with TiN and amorphous TiB₂ coatings were investigated in bi-directional sliding fretting tests performed in ambient air of different relative humidity. The wear rate expressed as a volumetric loss per unit of dissipated energy was determined. From these results, the best friction and wear performance was achieved with basal-plane-oriented MoS _x coatings tested at a relative humidity in the range of 10-50%. A coefficent of friction of 0.06-0.08 and a wear rate of 4 × 10³ m³J^-1, at a normal load of 1 N and a fretting frequency of 10 Hz, were recorded for that type of MoS _x coatings.     

基于能量优化和细分的参数曲面混合和孔洞填充

提出一种用能量优化法对参数曲面片进行细分的曲面混合及孔洞填充方法。该方法是先根据能量优化原理求出混合(填充)细分曲面的控制顶点，运用边界修改细分法，在正常Catmull-Clark细分之后，将控制网格边界轮廓去除，再进行下一次细分，从而生成一张完整光滑的细分曲面。该方法在保持混合(填充)细分曲面与基曲面在边界处C^2连续的同时，使混合(填充)细分曲面具有较好的光顺性。实例表明，该方法具有较高的效率，且混合(填充)的效果较好。     

Effect of the functional groups in ionic liquid molecules on the friction and wear behavior of aluminum alloy in lubricated aluminum-on-steel contact

Four imidazolium-based room temperature ionic liquids containing phosphonyl functional groups, i.e. 1-(3′-O,O-diethylphosphonyl-n-propyl)-3-alkylimidazolium tetrafluoroborates and hexafluorophosphates, were synthesized. The physical properties of the resulting synthetic products were evaluated, and their tribological behaviors as the lubricants for an aluminum-on-steel sliding system were evaluated on an oscillating friction and wear tester, with the emphasis being placed on the effect of the O,O-diethylphosphonyl groups in the ionic liquid molecules on the tribological behaviors. Thus the friction and tests were conducted at a frequency of 25 Hz, a sliding amplitude of 1 mm, and for a duration of 30 min. The worn aluminum surface was analyzed by means of scanning electron microscopy, energy dispersive X-ray analysis, and X-ray photoelectron spectroscopy. It was found that the synthesized ionic liquids had better friction-reducing and anti-wear ability for the aluminum-on-steel system than their nonfunctionalized courterparts (1-ethyl-3-hexylimidazolium tetrafluoroborate, coded as L206, and 1-propyl-3-octylimidazolium hexafluorophosphate, coded as LP308). Especially, they had much better load-carrying capacity than L206 and LP308. The tribological behaviors of the synthetic lubricants were dependent on both the anions and the side-substituted alkyl chains attached to the imidazolium cations. Moreover, physical adsorption and complicated tribochemical reactions were involved during the sliding process of the Al-on-steel system under the lubrication of the synthetic functionalized ionic liquids, which led to the generation of physically adsorbing and chemically reacting films composed of five-member-ring complex compounds, metal fluorides, nitrogen oxide, and FePO₄ on the rubbed Al surface. Those physically adsorbing and chemically reacting films contributed to effectively decrease the friction and wear of the aluminum sliding against steel.     

Micromorphological characterization of zinc/silver particle composite coatings

The aim of this study was to evaluate the three‐dimensional (3D) surface micromorphology of zinc/silver particles (Zn/AgPs) composite coatings with antibacterial activity prepared using an electrodeposition technique. These 3D nanostructures were investigated over square areas of 5 μm × 5 μm by atomic force microscopy (AFM), fractal, and wavelet analysis. The fractal analysis of 3D surface roughness revealed that (Zn/AgPs) composite coatings have fractal geometry. Triangulation method, based on the linear interpolation type, applied for AFM data was employed in order to characterise the surfaces topographically (in amplitude, spatial distribution and pattern of surface characteristics). The surface fractal dimension D_f, as well as height values distribution have been determined for the 3D nanostructure surfaces. Microsc. Res. Tech. 78:1082–1089, 2015. © 2015 The Authors published by Wiley Periodicals, Inc.     

Electrolyte Jet Machining (EJM) of antibacterial surfaces

Electrolyte Jet Machining (EJM) has been performed on stainless steel surfaces with the aim of reducing bacterial retention through the generation of nanoscale surface morphology. Following initial EJM experiments aimed at investigating the influence of machining depth, machining speed and current density on the resulting surface roughness, three characteristic surfaces were produced with a current density of 10–18 A/cm² and a machining speed of 0.8–8 mm/s to obtain an arithmetic mean height (S_a) of 0.5–0.74 μm and a density of peaks (S_pd) of 0.25–1.26 μm⁻¹. Relatively large differences between the three surfaces in terms of S_pd allowed thorough investigation into the effects of surface feature size on bacterial retention to be performed. Reductions in the order of 90% compared to control samples were achieved for gram-positive Bacillus cereus and Staphylococcus aureus across the entire tested parameter range (S_pd = 0.25–1.26 μm⁻¹), while reductions in the order of 99% were achieved for gram-negative Escherichia coli and Pseudomonas aeruginosa for surfaces characterized by S_pd > 1 μm⁻¹. Not only do the results call attention to EJM as an innovative technology for producing antibacterial surfaces, they also highlight important differences in the behavior of gram-positive and gram-negative bacteria in relation to EJM-textured surfaces with nanoscale surface morphology.     

1-N-alkyl -3-methylimidazolium ionic liquids as neat lubricants and lubricant additives in steel–aluminium contacts

The influence of the alkyl chain length and of the anion on the lubricating ability has been studied for the room-temperature ionic liquids (IL) 1-n-alkyl-3-methylimidazolium X⁻ [X = PF₆; n = 6 (L-P106). X = BF₄; n = 2 (L102), 6 (L106), 8 (L108). X = CF₃SO₃; n = 2 (L-T102). X = (4-CH3C6H4SO3); n = 2 (L-To102)]. Neat IL have been used for AISI 52100 steel-ASTM 2011 aluminium contacts in pin-on-disk tests under variable sliding speed. While all IL give initial friction values lower than 0.15, real-time sharp friction increments related to tribochemical processes have been observed for L102 and L-P106, at room-temperature and at 100 °C. Electronic microscopy (SEM), energy dispersive (EDS) and X-ray photoelectron (XPS) spectroscopies show that wear scar surfaces are oxidized to Al₂O₃ and wear debris contain aluminium and iron (for L102) fluorides. For L-P106, the steel surface is covered with a P-containing tribolayer. A change of anion (L-T102; L-To102) reduces friction and wear, but the lowest values are obtained by increasing the alkyl chain length (L106; L108). When the more reactive L102 and L-P106 are used as 1 wt.% base oil additives at 25 °C, tribocorrosion processes are not observed and a friction reduction (69–75% for 1 wt.% L102) and a change from severe (10⁻³ mm³ m⁻¹) to mild wear (10⁻⁴ to 10⁻⁶ mm³ m⁻¹) is obtained with respect to the neat IL. 1 wt.% IL additives also show good lubricating performance at 100 °C.     

Development of cleavage fracture toughness locus considering constraint effects

In this paper, the higher order terms in the crack tip stress fields are investigated macroscopically for more realistic assessment of structural material behaviors. For reactor pressure vessel material of A533B ferritic steel, effects of crack size and temperature have been evaluated using 3-point SENB specimens through a series of finite element analyses, tensile tests and fracture toughness tests. The T-stress, Q-parameter andq-parameter as well as theK andj-integral are calculated and mutual relationships are investigated also. Based on the evaluation, it has proven that the effect of crack size from standard length (a/W=0.53) to shallow length(a/W=0.11) is remarkable whilst the effect of temperature from - 20°C to-60°C is negligible. Finally, the cleavage fracture toughness loci as a function of the promising Q-parameter orq-parameter are developed using specific test results as well as finite element analysis results, which can be applicable for structural integrity evaluation considering con-straint effects.     

Advanced Tribometer for In Situ Studies of Friction, Wear, and Contact Condition—Advanced Tribometer for Friction and Wear Studies

An advanced ball-on-disk tribometer was developed for in situ studies of friction, wear, and contact condition during sliding. Kinetic friction force, contact resistance (R _c), acoustic emission (AE), ball position perpendicular () to the plane of the disk (ball and disk wear), and disk surface reflectance (disk wear) were all measured simultaneously during sliding experiments. Metal (440C steel) balls were slid against ceramic (n-doped polysilicon) wafers at light load (10g) and short test duration (2.5min). Significant changes in measured parameters were observed as sliding progressed. These changes are discussed, and when considered together provide new insights into friction and wear mechanisms not readily obtainable from more standard tribometers. The effects of disk run-out (effective surface waviness) on and R _c were also investigated. Friction and R _c were periodic with a period equal to the period of disk rotation. The behavior was complex, but generally going up a hill increased and decreased R _c, with the opposite behavior going down a hill. We established a critical link between low-frequency friction oscillations (LFFO) and the nature of the contact between sliding surfaces (R _c measurement). The geometric ratchet mechanism was ruled out as a cause of LFFO, as the surface slope was too small to explain the large friction oscillations. Coating the balls and wafers with lower friction materials resulted in negligible LFFO, which makes it unlikely that LFFO were simply a result of an oscillating normal force created by dynamic effects. LFFO likely have their origins in the complex nature of the contact between rubbing surfaces.     

Multi-scale roughness transfer in cold metal rolling

We report on a comparative Atomic Force Microscope (AFM) multi-scale roughness analysis of cold rolled Al alloy and steel roll, in order to characterize the roughness transfer from the steel roll to the workpiece in cold strip rolling processes. More than three orders of length-scale magnitudes were investigated from 100 microns to 50 nanometers on both types of surfaces. The analysis reveals that both types of surfaces are anisotropic self-affine surfaces. Transverse and longitudinal height profiles exhibit a different roughness exponent (Hurst exponent) ζ=0.93±0.03 and ζ=0.5±0.05 Different length-scale cut-offs are obtained in each direction l^sup=50μm, l^sup>100μm. Height and slope distributions are also computed to complement this study. The above mentionned self-affine characteresitics are found to be very similar for the roll and the strip surfaces, which suggest that roughness transfer takes place from the macroscopic (100 μm) to the very small scale (50 nm).