Optimization of the surface texture for silicon carbide sliding in water

Surface texturing has been recognized as an effective means to improve the tribological performances of sliding surfaces. Usually, generation additional hydrodynamic pressure to increase the load carrying capacity is regarded as the most significant effect of surface texture. In the case of silicon carbide sliding against identical material in water, the experimental results indicate that surface texture is also helpful to improve the running-in progress to smooth the contact surfaces, showing another reason to result in low friction. Based on the consideration of enhancing the generation of hydrodynamic pressure and improving running-in progress, a surface texture pattern, which was combined with large (circle, 350 μm in diameter) and small (rectangular, 40 μm in length) dimples, was designed to maximize the texture effect on the load carrying capacity of SiC surfaces sliding in water. The friction coefficient of such textured surface was evaluated and compared with that of untextured and those only with large or small dimples only. The friction reduction mechanisms of the patterns with different dimples in size are discussed.     

Microstructure and tribological properties of TiCu2Al intermetallic compound coating

TiCu₂Al ternary intermetallic compound coating has been in situ synthesized successfully on pure Ti substrate by laser cladding. Tribological properties of the prepared TiCu₂Al intermetallic compound coating were systematically evaluated. It was found that the friction coefficient and wear rate was closely related to the normal load and sliding speed, i.e., the friction coefficient of the prepared TiCu₂Al intermetallic compound coating decreased with increasing normal load and sliding speed. The wear rate of the TiCu₂Al intermetallic compound coating decreased rapidly with increasing sliding speed, while the wear rate first increased and then decreased at normal load from 5 to 15 N.     

Influence of surface roughness and contact load on friction coefficient and scratch behavior of thermoplastic olefins

To study the effects of surface roughness and contact load on the friction behavior and scratch resistance of polymers, a set of model thermoplastic olefins (TPO) systems with various surface roughness (R_a) levels were prepared and evaluated. It is found that a higher R_a corresponds to a lower surface friction coefficient (μ_s). At each level of R_a, μ_s gets larger as contact load increases, with a greater increase in μ_s as R_a level increases. It is also observed that with increasing contact load and increasing R_a, the μ_s tend to level off. In evaluating TPO scratch resistance, a lower μ_s would delay the onset of ductile drawing-induced fish-scale surface deformation feature, thereby raising the load required to cause scratch visibility. However, as the contact load is further increased, the μ_s evolves to become scratch coefficient of friction (SCOF) as significant sub-surface deformation and tip penetration occur and material displacement begins, i.e., ploughing. No dependence of R_a and μ_s on the critical load for the onset of ploughing is observed. In this work, the distinction between μ_s and SCOF will be illustrated. Approaches for improving scratch resistance of polymers via control of R_a are also discussed.     

Contact mechanics and rubber friction for randomly rough surfaces with anisotropic statistical properties

In this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory.     

Friction between α-Al2O3(0 0 0 1) surfaces and the effects of surface hydroxylation

Molecular dynamics simulations were performed to study the friction between hydroxylated α-Al₂O₃(0 0 0 1) surfaces at the temperature of 300 K. Effects of the degree of surface hydroxylation and sliding velocity have been discussed. Results indicate that the friction coefficient decreases with increased degrees of hydroxylation. For all degrees of surface hydroxylation, the friction law crosses over from thermal activation to viscous damping at sliding velocity of 80 m/s.     

A Monte Carlo method for simulating fractal surfaces

A Monte Carlo method is presented for simulating rough surfaces with the fractal behavior. The simulation is based on power-law size distribution of asperity diameter and self-affine property of roughness on surfaces. A probability model based on random number for asperity sizes is developed to generate the surfaces. By iteration, this method can be used to simulate surfaces that exhibit the aforementioned properties. The results indicate that the variation of the surface topography is related to the effects of scaling constant G and the fractal dimension D of the profile of rough surface. The larger value of D or smaller value of G signifies the smoother surface topography. This method may have the potential in prediction of the transport properties (such as friction, wear, lubrication, permeability and thermal or electrical conductivity, etc.) on rough surfaces.     

A model of a belt chamber cracking based on crack profiles calculations

Abstract

The problem of a Belt chamber matrix cracking is presented. The influence of crack surface quality on the effective values of near crack tip stress is discussed. It is shown that under working conditions of the vessel, the existing shear friction between upper and lower crack surfaces caused by crack surface roughness can prevent the crack surface sliding displacement. Therefore, the control variable for matrix cracking is the value of stress intensity factor K_I corresponding to normal node of loading only. The calculations are performed by finite element method within the range of linear elastic fracture mechanics.     

Tribological properties of silicate materials on nano and microscale

We studied the friction properties of four model silicate materials at the nanoscale and microscale. From nanotribology, we characterized the tribological properties at single asperity contact scale and from microtribology, we characterized the tribological properties at multi asperity contact scale. First, for each material we measured chemical composition by XPS, Young's modulus by acoustical microscopy and roughness σ by atomic force microscopy (AFM). Second, we measured the nanofriction coefficients with an AFM and the microfriction coefficients with a ball probe tribometer, for three hardnesses of the ball probe. We identified one friction mechanism at the nanoscale (sliding friction) and two friction mechanisms at the microscale (sliding friction and yielding friction). Comparison of the nano and microfriction coefficients at the same sliding friction regime shown, that the tribological properties of these materials didn’t depend on roughness.     

Laser surface texturing of gray cast iron for improving tribological behavior

Laser surface texturing process involves creation of microfeatures, e.g., tiny dimples, usually distributed in a certain pattern, covering only a fraction of the surface of the material that is being treated. The process offers several advantages for tribological applications, including improved load capacity, wear resistance, lubrication lifetime, and reduced friction coefficient. In the present study, the surface modification of gray cast iron, using millisecond (λ = 1,064 nm), nanosecond (λ = 1,064 nm) and femtosecond (λ = 800 nm) pulse duration laser irradiation, is adopted to establish a particular geometrical pattern with dimple features and dimensions, to improve wear and friction behavior. The effect of various laser processing parameters, including laser pulse energy, pulse duration and processing speed, on the performance characteristics of the laser-treated samples is investigated. The microtextured surfaces were produced on gray cast iron using different millisecond (0.5 ms), nanosecond (40 ns) and femtosecond (120 fs) laser source with the dimple depth between 3 and 15 μm. The coefficient of friction for the untextured surface was ~0.55, millisecond laser textured ~0.31, nanosecond laser textured ~0.02 and femtosecond laser ~0.01, under normal force of 50 N and sliding speed of 63 mm/s. Surface texturing of the gray cast iron surface using femtosecond pulse duration resulted in significant improvement in wear resistance in comparison to the untextured as well as millisecond and nanosecond laser-textured surface.     

Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

ABSTRACT

Interaction forces between solid surfaces are often mitigated by adsorbed molecules that control normal and friction forces at nanoscale separations. Molecular dynamics simulations were conducted of opposing semi-ordered monolayers of united-atom chains on sliding surfaces to relate friction and normal forces to imposed sliding velocity and inter-surface separation. Practical examples include adsorbed friction-modifier molecules in automatic transmission fluids. Friction scenarios in the simulations had zero, one, or two fluid layers trapped between adsorbed monolayers. Sliding friction forces increased with sliding velocity at each stable separation. Lower normal forces were obtained than in most previous nanotribology molecular simulations and were relatively independent of sliding speed. Distinguishing average frictional force from its fluctuations showed the importance of system size. Uniform velocities were obtained in the sliding direction across each adsorbed film, with a gradient across the gap containing trapped fluid. The calculated friction stress was consistent with measurements reported using a surface forces apparatus, indicating that drag between an adsorbed layer and trapped fluid can account sufficiently for sliding friction in friction modifier systems. An example is shown in which changes in molecular organisation parallel to the surface led to a large change in normal force but no change in friction force.     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

Study on the Tribological Properties of Carbon Fabric Reinforced Phenolic Composites Filled with Nano-Al2O3

Carbon fabric reinforced phenolic (CFRP) composites filled with nano-Al₂O₃ were prepared by a dip-coating and heat molding process and the tribological properties of the resulting composites under different sliding conditions were investigated systematically on a block-on-ring test rig. The worn surfaces were observed in a scanning electron microscope (SEM) to understand the mechanism. Nano-Al₂O₃ particles, as the filler, were effective in reducing the friction coefficient and wear rate of the CFRP composites. The steady state friction coefficient of the CFRP composites filled with 4 wt.% nano-Al₂O₃ particles was about 65.5% of that of unfilled CFRP composites, and the wear rate, in this case, was about 74.7% of that of unfilled CFRP composites. Tribological tests under different sliding conditions revealed that the filled CFRP composites seemed to be more suitable than unfilled CFRP composites for tribological applications under higher sliding speed and load. Moreover, the wear resistances of the unfilled and filled CFRP composites were found to be related to the stability of the transfer film on the counterface.     

Rubber friction on smooth surfaces

We study the sliding friction for viscoelastic solids, e.g., rubber, on hard flat substrate surfaces. We consider first the fluctuating shear stress inside a viscoelastic solid which results from the thermal motion of the atoms or molecules in the solid. At the nanoscale the thermal fluctuations are very strong and give rise to stress fluctuations in the MPa-range, which is similar to the depinning stresses which typically occur at solid-rubber interfaces, indicating the crucial importance of thermal fluctuations for rubber friction on smooth surfaces. We develop a detailed model which takes into account the influence of thermal fluctuations on the depinning of small contact patches (stress domains) at the rubber-substrate interface. The theory predicts that the velocity dependence of the macroscopic shear stress has a bell-shaped form, and that the low-velocity side exhibits the same temperature dependence as the bulk viscoelastic modulus, in qualitative agreement with experimental data. Finally, we discuss the influence of small-amplitude substrate roughness on rubber sliding friction.     

Two-dimensional friction oscillator: group-preserving scheme and handy formulae

With vibration isolation of buildings and turbomachinery blades in mind, we study the dynamic behaviour of a single-mass two-degree-of-freedom oscillator with dry friction damper, viscous damper and elastic spring connected in parallel. The mass is mounted on an elastic supporting plate allowing movement in two directions on a plane. We formulate a multi-dimensional friction model, from which the sliding conditions and the sticking conditions of the mass are derived. For calculations we develop a group-preserving scheme, which preserves the projective proper orthochronous Lorentz group PSO_o(2,1) symmetry of the model in the sliding phase so as to satisfy automatically the sliding conditions at each time step without iteration at all. The oscillator is then subjected to simple harmonic excitations, and the responses are displayed. According to the simple harmonic balance method together with a circular orbit assumption on displacements, we derive closed-form formulae for handily estimating the steady state responses, which are then compared with the results calculated by the group-preserving scheme to confirm the applicability of the formulae. We also derive formulae specifically for a two-dimensional friction oscillator with rigid base support, which include an exact formula of the magnification factor and a simple formula for estimating the minimum driving force amplitude (or the maximum friction force bound) to avoid sticking.     

Tribological characterization of chromium nitride coating deposited by filtered cathodic vacuum arc

CrN coatings were prepared by filtered cathodic vacuum arc (FCVA) technique. The influence of the deposition parameters (nitrogen partial pressure PN₂, substrate bias voltage V_s and preheating of the substrate) on the structural, mechanical and tribological properties of the FCVA CrN coatings was investigated. Further, the FCVA CrN coating was compared in dry reciprocating sliding with commercial multi-arc ion plating (MAIP) CrN coating as to friction and wear properties. Profilometer, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) were used to evaluate the wear scars and the wear mechanisms were discussed. The results showed that the structural, mechanical and tribological properties of the FCVA CrN coatings were significantly dependent on the deposition parameters. The FCVA CrN coating deposited with PN₂ of 0.1 Pa, V_s of −100 V and without preheating exhibited the optimal mechanical and tribological properties. The FCVA CrN coating exhibited much better anti-abrasive and anti-spalling properties than the MAIP CrN coating, which was resulted from significant reduction of macroparticles and pitting defects by the FCVA technique. The MAIP CrN coating suffered severe concentrated wear by a combination wear mechanisms of delamination, abrasive and oxidative wear when high normal load was applied, while for the FCVA CrN coating the wear mechanisms were ultra-mild abrasive and oxidative wear.     

Friction and wear behaviour of cp Ti and Ti6Al4V following nitric acid passivation

Nitric acid passivation increases the thickness of the TiO₂ passive film formed at the cp Ti and Ti6Al4V surface. The TiO₂ oxide, which has a lubricating nature, reduces the wear rate. A linear ball-on-disc friction test has been carried out at room temperature in ambient air, NaCl 3% and Ringer's solutions, with a sliding velocity of 4 mm s⁻¹ and a normal load of 1 N. Friction coefficient curves obtained from ball-on-disc wear test, as well as following the optical microscopy observation of ball trace, indicated the presence of periodic phenomenon. One period can be divided into four stages. This can be clearly seen for cp Ti (accommodation stage, creation of wear particles, adherence of particles layer on the alumina ball and ejection of this layer). In order to confirm this observation and understand the chemical interfacial phenomena, open circuit potential (OCP) measurements were carried out in the same time of friction tests.     

Influence of crystalline diamond nanoparticles on diamond-like carbon friction behavior

Crystalline diamond (CD) particles have been incorporated in diamond-like carbon (DLC) film structure in order to improve DLC electrochemical corrosion resistance. This paper shows the investigation of CD-DLC friction behavior according to the CD average sizes and concentration. The films were growth over 304 stainless steel using plasma enhanced chemical vapor deposition. The response surface methodology was used to develop a mathematical modeling of friction for these films, using the experimental results, in order to identify parameters that control friction and construct tribological maps according to the CD average sizes. The presence of bigger CD particles (250 and 500 nm) increased the film roughness. Films with CD particles of 4 nm presented the most homogeneous friction map, with minor variation in friction coefficient with the increase/decrease of load and sliding speed even when the CD concentration increase. This result suggests that in CD-DLC films containing CD particles of 4 nm average size, the nanoparticles are better incorporated in DLC structure due to its average size (4 nm) that is near than DLC grain size and could occupy the nanospaces between DLC grains.     

A multiscale molecular dynamics approach to contact mechanics

The friction and adhesion between elastic bodies are strongly influenced by the roughness of the surfaces in contact. Here we develop a multiscale molecular dynamics approach to contact mechanics, which can be used also when the surfaces have roughness on many different length-scales, e.g., for self affine fractal surfaces. As an illustration we consider the contact between randomly rough surfaces, and show that the contact area varies linearly with the load for small load. We also analyze the contact morphology and the pressure distribution at different magnification, both with and without adhesion. The calculations are compared with analytical contact mechanics models based on continuum mechanics.     

Friction noise technique for the measurement of surface roughness of papers

Surface roughness is one of the most important attributes of paper and its measurement is normally used in quality control applications. Currently, measurement of surface roughness is done by using air leakage, optical, or subjective methods. In this article, an acoustical method for the determination of surface roughness of papers is described. The method is applied off-line and uses the friction noise recorded when a small brush slide over the surface of the paper. The analysis is done consider the friction noise ranging from 300 Hz to 10 kHz. Considering that friction noise is associated to surface roughness, a signal analysis algorithm was developed to take advantage of this fact. The algorithm consists of a frequency-domain feature extraction routine and an artificial neural network that represents the variation of the spectral features in a roughness scale. The results show that an affordable and efficient friction noise based system can be used for measuring surface roughness of tissue papers and other soft surfaces as an alternative to other methods.     

Hysteresis from dynamically pinned sliding states

We report a surprising hysteretic behavior in the dynamics of a simple one-dimensional nonlinear model inspired by the tribological problem of two sliding surfaces with a thin solid lubricant layer in between. In particular, we consider the frictional dynamics of a harmonic chain confined between two rigid incommensurate substrates which slide with a fixed relative velocity. This system was previously found, by explicit solution of the equations of motion, to possess plateaus in parameter space exhibiting a remarkable quantization of the chain center-of-mass velocity (dynamic pinning) solely determined by the interface incommensurability. Starting now from this quantized sliding state, in the underdamped regime of motion and in analogy to what ordinarily happens for static friction, the dynamics exhibits a large hysteresis under the action of an additional external driving force F_ext. A critical threshold value F_c of the adiabatically applied force F_ext is required in order to alter the robust dynamics of the plateau attractor. When the applied force is decreased and removed, the system can jump to intermediate sliding regimes (a sort of “dynamic” stick-slip motion) and eventually returns to the quantized sliding state at a much lower value of F_ext. Hysteretic behavior is also observed as a function of the external driving velocity.