Dimple Density Research Articles

Tribological behavior of laser-textured NiCrBSi coatings

This work presents and discusses the tribological behavior of NiCrBSi coatings obtained by diode-laser cladding and with good high-temperature antiwear behavior, textured by aNd-YAG laser to achieve good friction behavior. Different densities and distances between dimples were tested by obtaining their coefficients of friction at different sliding speeds under a load of 100 N with a CETR UMT-3 tribometer and comparing results with a non-textured NiCrBSi coating. The study led to the following conclusions: surface texturing, carried out according to the contact area, reduces the coefficient of friction throughout the speed range tested; to obtain tribological improvement, it is necessary to focus on the relationships between density, dimple diameter and contact area; smaller diameters allow higher numbers of dimples per unit of area, improving the tribological behavior; incorrect dimple density values lead to disruptive behavior, due to both high and low dimple ratios.

Performance Analysis of Full-Film Textured Surfaces With Consideration of Roughness Effects

Abstract A mass-conservative algorithm that implements the Jakobsson–Floberg–Olsson cavitation theory is used to predict the performances of seal-like structures and thrust bearings with a dimpled surface texture. The results of a series of simulations for load-carrying capacity, film thickness, dimple depth, dimple density, cavitation pressure, leakage, and friction force are presented, and the relationship between these performance parameters is studied. It is shown that, under the conditions simulated, surface roughness can improve the load-carrying capacity, but its effect is limited.

Numerical Simulation on the Lubrication Performance of Surface Textured Piston Rings

Surface texturing has been successfully employed in some tribological applications in order to diminish friction and wear. This technology may be used in a piston ring to decrease the friction and wear of the contact between a piston ring and cylinder liner. A numerical simulation of lubrication between a surface textured piston ring and cylinder liner based on the hydrodynamic lubrication theory was conducted. The influence of surface texture parameters on piston ring lubrication performance was obtained by solving the mathematical equations with a multi-grid method. The results show that under the micro-dimple area density of 5%-40% the minimum oil film thickness increases and the dimensionless friction force decreases with the increasing of it. Under the dimple area density of 40%-60%, the minimum oil film thickness and the dimensionless friction force change slightly. Under various dimple area densities the optimum dimple depth at the given working condition in this paper is about 5µm.

Experimental investigation of tribological performance of laser textured stainless steel rings

A series of experiments is conducted to examine the frictional characteristics of laser surface-textured, heat-treated 17-4 PH stainless steel specimens. Two dimple shapes are tested: circular and elliptical. The circular dimples are designed with different sizes, dimple densities and dimple depth-to-diameter ratios. It is found that the cavitation pressure (Pcav) in a circular dimple is strongly affected by the operational speed. The surface textured specimens provide low coefficient of friction compared with plain (dimple-free) surfaces. However, for the material used in the current experiments, the surface texture decreases the surface's resistance to wear.

High strain rate shear deformation and fracture behaviour of biomedical titanium alloy

The rate and temperature dependent shear deformation behaviour of biomedical titanium alloy (Ti–15Mo–5Zr–3Al) is investigated using a torsional split Hopkinson pressure bar at strain rates ranging from 1·2 × 103 to 2·8 × 103 s–1 and temperatures of –150, 25 and 300°C. It is found that both the strain rate and the temperature have a significant effect on the high strain rate shear properties and fracture characteristics of the alloy. As the strain rate is increased or the temperature is reduced, the flow stress, workhardening rate, strain rate and temperature sensitivity increase, but the activation volume and activation energy decrease. The fracture strain increases with both increasing strain rate and increasing temperature. The shear flow behaviour of the Ti–15Mo–5Zr–3Al specimens is accurately described by the Kobayashi–Dodd constitutive equation. Metallographic observations reveal that the failure of the present Ti–15Mo–5Zr–3Al alloy is dominated by intensive localised shearing. Moreover, the SEM fractographs show that the fracture surfaces are characterised by a dimple-like structure. The dimple density increases with an increasing strain rate or temperature. By contrast, the dimple size reduces at higher strain rates and temperatures and gives rise to a significant improvement in the fracture resistance.

Effect of surface laser texture on friction properties of nickel-based composite

Laser surface texturing (LST) was performed on the nickel-based composites by a Nd:YAG pulsed laser and the regular-arranged dimples with diameter of 150 μm were fabricated on their surfaces. The textured surfaces were smeared with molybdenum disulfide powder. The tribological properties of the textured and filled composites were investigated by carrying out sliding wear tests against an alumina ball as a counterface using a high temperature ball-on-disk tribometer. The tests were conducted at a sliding speed of 0.4 m/s and at normal loads ranging from 20–100 N and from room temperature to 600 °C. The friction coefficient of nickel-based composite textured and smeared with molybdenum disulfide was found to reduce from 0.18 to 0.1 at the temperature range from 200 to 400 °C. The texture with a dimple density of 7.1% was observed to prolong wear life of MoS2 film by more than four times in comparison to the texture with other dimple densities. The lubricious oxide particles stored in the dimples reduce friction coefficient at elevated temperatures and compensate for the extra lubricant owing to the degradation of MoS 2 caused by its oxidation at high temperatures.

Experimental investigation on martensitic transformation and fracture morphologies of austenitic stainless steel

Formation and nucleation mechanisms (i.e. γ → ε, γ → α′, γ → deformation twins → ε → α′ and γ → ε → α′) of deformation- induced martensite (DIM) have been studied through analytical transmission electron microscopy (TEM) after tensile deformation of AISI 304LN stainless steel at various strain rates (SR) at room temperature (RT). Quantitative metallography has been employed extensively to assess martensitic transformation (MT) as function of strain and SR. It has been observed that the enhancement of SR during tensile deformation promotes the early formation of DIM, while suppressing its saturation value at fracture. Fracture surface morphologies and dimple geometries (i.e. dimple density, dimple diameter and dimple size distribution) have been quantified through image processing (IP) of tensile fractographs. It is noted that at lower SR, dimple density is high while dimple diameter is smaller, and vice versa. Concomitantly, the strength is noted to be low and ductility is high at lower SR, and vice versa. DIM has been found to be responsible for high dimple density at low SR. At high SR, MT is suppressed and hence low dimple density. The variation in SR dependent MT accounts for the variation in dimple metrics vis-à-vis tensile properties.

Unipolar radiofrequency treatment to improve the appearance of cellulite

Background: Previous studies suggest that radiofrequency (RF) energy may be effective as a treatment for cellulite. Objective: This bilateral paired blinded comparative study assesses the efficacy and safety of a unipolar RF device for improving the appearance of cellulite using a new quantitative cellulite grading system. Methods: In this randomized, blinded, split‐design study, 10 individuals (aged 32–57 years) with a clinically observable excess of subcutaneous fat and cellulite (minimum grade 2 out of 4) on the thighs received up to six unilateral treatments (number of treatments at the investigator's discretion) at 2‐week intervals with unipolar RF. The untreated side of the thigh served as an internal control. Treated thighs were randomly assigned by alternate allocation. Results were evaluated using study participant questionnaires and by two blinded evaluators (JSD, KAA) using photographs and the author's (MAA) cellulite grading scale at each treatment visit and at 1‐month and 3‐month follow‐up visits after the final treatment. A novel quantitative four‐point cellulite grading system is presented and applied, which separately grades dimple density, dimple distribution, dimple depth, diameter and contour. Results: All participants responded to treatment (mean of 4.22 and range of three to six treatments). The blinded evaluations of photographs using the cellulite grading scale demonstrated the following mean grading scores for the treated leg versus the control leg: dimple density of 2.73 vs 3.18 (11.25% mean improvement), dimple distribution 2.89 vs 3.32 (10.75% mean improvement), dimple depth 1.47 vs 1.54 (2.5% mean improvement), and mean score of 2.36 (SEM 0.45) vs 2.68 (SEM 0.57) (8.00±2.84% mean improvement). The treatment was painless and side effects included minimal to moderate erythema which resolved within 1 to 3 hours. No crusting, scarring or dyspigmentation was observed. Conclusions: This randomized, blinded, split‐design, controlled study employing a quantitative four‐point grading scale demonstrated that this unipolar RF device is safe for the treatment of cellulite. Clinically visible and quantified improvement which did not achieve statistical significance but showed a trend toward improvement was observed in all patients following a mean of four treatments at 2‐week intervals.

DNS of low Reynolds number turbulent flows in dimpled channels

Direct numerical simulation (DNS) is performed to study turbulent flows over dimpled surfaces in a channel. Results on mean field and second-order quantities are obtained. ‘Horseshoe’ vortices can be observed in the dimples of sparse arrays. As inter-dimple separation is reduced, the ‘feet’ of the horseshoe vortices are gradually lifted off the dimple surface, and the resulting flow structures in the cavities become flattened and stretched to become something akin to two-dimensional separation bubbles. At the higher dimple density, the stream traces near the surface also develop a distinct formation similar to what had been observed in earlier Reynolds-averaged Navier–Stokes (RANS) simulations (Isaev, S.A., Leont'ev, A.I. and Baranov, P.A., 2000, Technical Physics Letters, 26, 15; Lin, Y.L., Shih, T.I.-P. and Chyu, M.K., 1999, ASME paper, 99-GT-263; Lin, Y.L. Shih, T.I.-P., 2001, International Journal of Transfer Phenomena, 3, 1). Regions of high turbulence intensity are found above the downstream half of the dimples and along their side edges. These regions coincide with the locations of vortex shedding found in the experiments of Ligrani et al. (2001, Physics of Fluids, 13, 3442) and the locations of vorticity concentrations observed in Park et al. (2004, Numerical Heat Transfer, Part A (Applications), 45(1), 1) and Won and Ligrani (2004, Numerical Heat Transfer, Part A (Applications), 46(6), 549). For a fixed mean pressure gradient, it is observed that the flow rates through the channels are reduced by the presence of dimples. This indicates that the dimpled channels we have studied so far have larger drag than flat-wall channels. Computed friction coefficients for dimpled channels also confirmed the conclusion.

Experimental Investigation of Partial Laser Surface Texturing for Piston-Ring Friction Reduction

An experimental study is presented to evaluate the effect of partial laser surface texturing (LST) on friction reduction in piston rings. In a previous study, 30% friction reduction was obtained with full LST where the full width of the piston ring is textured with a very large number of microdimples that act individually as microhydrodynamic bearings. In partial LST, only a portion of the piston-ring width is textured with high dimple density, producing a “collective” effect of the dimples that provides an equivalent converging clearance even with nominally parallel mating surfaces. Experimental results obtained with flat and parallel test specimens with partial LST are presented, confirming a previously published theoretical model and the advantage of partial over full LST. Friction reduction by LST with actual production-crowned piston rings and cylinder liner segments is not straightforward and needs further investigation. Presented at the STLE Annual Meeting in Las Vegas, Nevada May 15-19, 2005 Review led by Gray Barber

04/02166 Thermal modelling and experimental validation of ground temperature distribution in greenhouse: Ghosal, O. et al. International Journal of Energy Research, 2004, 28, (1), 45–63

We present an experimental study into the impact of surface texturing on crank shaft bearing friction. This was achieved by applying a series of laser-etched patterns to the surface of shell components and comparing friction measurements - obtained using a journal bearing test rig - to those of a non-texture reference. Tests were performed under a range of applied loads, speeds and lubricant viscosities in order to simulate the operating conditions encountered in an engine. Three texture configurations were assessed with varying dimple density and area coverage. The lubricant film temperature was also measured in order to remove shear heating effects and deduce accurate friction values.The bearing shell with a texture pattern applied to the whole of its circumference showed substantial reduction in friction (approximately 18%) when compared with the non-textured reference. This reduction in friction increases as the film thickness increases (i.e.: moving right on the Stribeck curve) and is most pronounced under full film hydrodynamic conditions. Friction reductions of up to 13% were observed using bearing shells that were only textured outside the loaded region. This is attributed to a reduction in shear rate due to an increased gap without alteration of the pressure generation and is important since this texture configuration maintains its effectiveness, even after the heavily loaded region becomes worn. These reported hydrodynamic friction reductions have practical implications for automotive bearings.

A comparison of the dynamic shear properties and fracture behaviour of continuous and pulsed current plasma arc weldments of 304L stainless steel

The dynamic shear behaviour and fracture characteristics of continuous current (CC) and pulsed current (PC) plasma arc weldments of 304L stainless steel are studied and compared by means of torsional split-Hopkinson bar and SEM metallographic techniques under strain rates ranging from 9×10 2 s -1 to 3.3×10 3 s -1 at room temperature (25°C). Results show increasing strain rates increase flow stress, fracture strain and work hardening rate of 304L weldments. For all tested strain rates, strain rate sensitivity increases as work hardening stress increases, but activation volume decreases. Shear stress and strain rate sensitivity of PC weldments are all higher than CC weldments at the same strain rate, but activation volume is lower. SEM fracture surfaces are found to contain fusion zone dimples, heat affected zone dimples and smooth shallow dimples. With increasing strain rate, FZ and HAZ dimple density and depth increase while smooth shallow dimples decrease, in all cases more significantly in PC weldments than in CC weldments. Constitutive equations for both weldments are proposed and successfully describe the dependence of dynamic shear properties on applied strain rate. The significantly superior performance of the PC weldments under high strain rate conditions should be taken into consideration when designing structural elements containing 304L SS.

A comparison of the dynamic shear properties and fracture behaviour of continuous and pulsed current plasma arc weldments of 304L stainless steel

The dynamic shear behaviour and fracture characteristics of continuous current (CC) and pulsed current (PC) plasma arc weldments of 304L stainless steel are studied and compared by means of torsional split-Hopkinson bar and SEM metallographic techniques under strain rates ranging from 9×10 2 s -1 to 3.3×10 3 s -1 at room temperature (25°C). Results show increasing strain rates increase flow stress, fracture strain and work hardening rate of 304L weldments. For all tested strain rates, strain rate sensitivity increases as work hardening stress increases, but activation volume decreases. Shear stress and strain rate sensitivity of PC weldments are all higher than CC weldments at the same strain rate, but activation volume is lower. SEM fracture surfaces are found to contain fusion zone dimples, heat affected zone dimples and smooth shallow dimples. With increasing strain rate, FZ and HAZ dimple density and depth increase while smooth shallow dimples decrease, in all cases more significantly in PC weldments than in CC weldments. Constitutive equations for both weldments are proposed and successfully describe the dependence of dynamic shear properties on applied strain rate. The significantly superior performance of the PC weldments under high strain rate conditions should be taken into consideration when designing structural elements containing 304L SS.

Effect of Micron-Scale Surface Deformation on the Corrosion Behavior of Coil-Coated Aluminum Alloy 5182

Abstract An investigation of the corrosion behavior of coil-coated AA5182 (UNS A95182) can ends revealed directional underfilm corrosion during immersion in 0.6 M sodium chloride (NaCl) solution. Corrosion occurred preferentially within ∼ 10 mm wide bands that were dull in appearance. Microscopic examination revealed that these dull bands were a result of small dimples ≈ 10 μm deep and 100 μm wide. Regions that had a higher dimple density had a lower pitting potential than regions with fewer dimples. Subsequent investigation determined that dimples were introduced into the coil-coated sheet by feed rollers during can end manufacturing. Oxide damage and second-phase precipitation at dislocations resulting from micron-scale surface deformation were proposed as the mechanisms of increased corrosion susceptibility. The present study showed the importance of manufacturing protocol in the determination of corrosion performance of organic coatings on alloy substrates. More specifically, the importance of the sub...

Distribution of dimple sizes on the fracture surface of spheroidized steel in the transition region

Fractography on fractured K IC specimens of spheroidized steel has revealed a statistical distribution of dimple sizes (in the ductile failure region) dependent on the temperature and distance from the pre-crack tip. The statistical distributions of carbide sizes and dimple sizes conform closely to a Weibull distribution. The size parameter of the statistical distribution of dimples increases with the distance from the pre-crack tip, but diminishes as the temperature rises. The shape parameter declines as the distance from the pre-crack tip increases, but grows with the test temperature. The experimental findings have confirmed a probabilistic theory of void nucleation ahead of the pre-crack tip. The dimple density on the fracture surface at a certain distance from the pre-crack tip corresponds to the density per unit area of carbides multiplied by the probability of void nucleation.

Effect of pore sizes and cholesterol-lipid solution on the fracture toughness of pure titanium sintered compacts

Commercial pure titanium has been widely used as an implant material because of its excellent biocompatibility and good ductility. To determine the effect of pore size on the fracture resistance of porous titanium compacts, a series of fracture toughness ( K Q ) tests were performed on commercial pure titanium powder compacted to 0.17 and 0.62 GPa. Pore sizes ranged from 25 to 103 μm, with porosity between 8.5 and 35%. Two sets of fracture toughness tests using disc-shaped compacts (ASTM E 399-90) were performed, the first in air at 37°C and the second with compacts treated in cholesterol-lipid solution at 37°C. The K q value of compacts with a smaller mean pore size ( ca. 50 μm) was approximately twice that of the compacts with a larger mean pore size (100μm). The effect of cholesterol-lipid solution treatment was detrimental, perhaps due to preferential lipid absorption by the titanium oxide and/or the presence of chlorides. For the smaller pore size compacts, the K Q values decreased by up to 20%. For the larger pore size compacts, the effect of cholesterol-lipid solution was less significant. Morphologically, compacts with smaller pore size had a predominantly ductile fracture with significantly higher dimple density than the larger pore size compacts.

Ductile fracture of 1100, 3003 and Al-4%Cu alloys

Ductility of aluminum alloy was studied in correlation with the fracture surface structure observed by electron microfractography. The results obtained are as follows:In 1100 and 3003 alloys, the fracture surface was characterized by deep and large dimples, nucleated at lump shaped inclusions. However, about 10% of inclusions were associated with the dimple nucleation. The dimple density was correlated with the number of large inclusions rather than with the density of whole inclusions. The brittleness in the cold worked state was due to dimple initiation occurred easily.In an Al-4%Cu alloy, the most ductile state was produced by the G. P. zone formation; very large dimples were observed on the fracture surface. G. P. zones were, thus, considered to contribute to hardening of the matrix but not to the dimple nucleat on, while inclusion particles initiated the dimples. The ductility was the lowest when the Θ' precipitation occured; very fine dimples were present on the fracture surface. A large number of weak Θ' precipitates acted as the dimple nuclei. Very fine dimples, nucleated at Θ particles, were observed at the over aged state. The small number of large precipitates resulted in the recovery of ductility.