Micro-dimpled Surface Research Articles

The oil lubricated fretting wear behavior under different sliding displacements and frequencies of Ti6Al4V titanium alloy as affected by micro-shot peening

The fretting wear behavior under oil lubricants is of great practical significance since oil is usually used to regulate fretting regime for minimizing wear. In this study, micro-shot peening (MSP) was performed on Ti6Al4V titanium alloy, in which 0.6 MPa injection air pressure, 60 s shot time and high-speed steel particles with an average diameter of 50 μm were used. Surface topography, microstructure, hardness and compression residual stress of test samples were examined. Oil lubricated fretting wear tests were then conducted between titanium alloy sample and GCr15 counterbody under different sliding displacements and frequencies. The test results show that the surface structure of MSP treated samples experienced severe plastic deformation, the surface hardness was increased from 310.8 HV to 433.5 HV, and a compression residual stress of −303.5 MPa was introduced within surface layer. Moreover, MSP treatment reduced the coefficients of friction through producing a micro-dimpled surface. Thereby, the fretting wear resistance of MSP treated sample under oil lubricated condition was improved. By comparison, at higher sliding displacement and frequency, the ability of MSP treatment to resist fretting wear was improved more significantly since more better lubricating effect can be obtained. Fretting damage of untreated sample under oil lubrication was mainly attributed to fatigue wear, delamination wear, abrasive wear and oxidation wear, while that of MSP treated sample was mainly regional peeling off and abrasive wear.

Feasibility of Inducing Superhydrophobicity on Laser-Textured Surfaces: Development of Mathematical Model and Experimental Investigations

Hydrophobicity is a prominent characteristic of a surface that governs its applications in domains such as wear reduction by lubrication retention, self-cleaning surfaces, fluid drag reduction, viscosity testing, development of oleophobic coatings, etc. A superhydrophobic surface exhibits a water contact angle (CA) of 150∘ or larger. High surface energy of nontextured surface limits its wettability. Texturing of a surface imparts low surface energy which proves to be favorable for enhancing the overall surface hydrophobicity. Research and analysis done to fathom an optimum method by which surfaces accomplish superhydrophobicity is still miniscule. It is challenging to fabricate superhydrophobic surfaces by micro-machining due to the expansive range of the features involved. To minimize the exorbitant costs incurred due to trial-and-error-based experimentation, a mathematical model with [Formula: see text]90% accuracy has been developed in this study, which would help determine the closest ranges of values of parameters like micro-dimple diameter and areal density responsible for inducing superhydrophobic properties on a micro-dimpled specimen. The exceptionality of this study lies in the fact that though mathematical models are available for textures like micro-grooves and micro-pillars, but miniscule research is available for micro-dimpled surfaces with hardness greater than 55 HRC.

Geometric influence on friction and wear performance of cast iron with a micro-dimpled surface

Geometric influences on the tribological performances of cast iron with a micro-dimpled texture are researched experimentally. The friction coefficients are measured on four different dimpled shapes with various geometrical parameters with the changing of operating load and rotation speed by a ring-on-ring test rig. Finally the wear characteristics of different surfaces are also obtained to make comparisons. Results show that the effects of micro dimples improve the critical load and reduce the critical rotational speed and expand the range of hydrodynamic lubrication, meanwhile maintain a similar minimum of friction coefficient compared with a smooth surface. Geometrical parameters of micro dimples have significant influences on the friction coefficient.

Micro‑dimpled surface atelocollagen maintains primary human hepatocytes in culture and may promote their functionality compared with collagen coat culture.

Primary human hepatocytes (PHHs) are the gold standard for drug development procedures; however, maintaining functional PHHs in vitro is challenging in conventional collagen-coated cultures. In the present study, we developed a new scaffold comprising high amounts (≥1 mg/cm2) of atelocollagen exposed to ultraviolet radiation to induce cross-linking and improve stability. Scanning and transmission electron microscopy revealed a micro-dimpled surface (MDS) scaffold composed of randomly arranged atelocollagen fibrils. The scaffold was therefore designated as MDS atelocollagen. PHHs cultured on MDS atelocollagen were round with a compact cytoplasm and exhibited enhanced levels of albumin (ALB) secretion and cytochrome P450 (CYP) 3A4 activity. The expression of hepatocyte-related genes, such as serum proteins, drug metabolism-related CYPs, and nuclear receptors, was enhanced in cells cultured on MDS atelocollagen, but not in those cultured on conventional atelocollagen. Moreover, the abnormal gene expression of cell adhesion molecules observed in conventional atelocollagen culture was suppressed when the cells were grown on MDS atelocollagen, thereby suggesting a cell behavior similar to that of in vivo hepatocytes. These results suggest that MDS atelocollagen functionally preserves PHHs while conserving the simplicity of conventional PHH atelocollagen-coated cultures.

Influence of tool material and geometry on micro-textured surface in radial ultrasonic vibration-assisted turning

Surface texturing has been widely used to fabricate functional surface, which can improve the friction performance or achieve suitable wettability for different mechanical products. In order to fabricate micro-textured surface in an efficient and cost-effective way, a surface texturing method of radial ultrasonic vibration-assisted turning (RUVT) was proposed. The texturing generation mechanism of RUVT was firstly analyzed through a theoretical model. Then, the influence of tool geometry on micro-texture, including clearance angle and nose radius, was further characterized by a surface topography simulation. The simulated and experimental results show that the clearance angle and nose radius have important effect on the shape and width of micro-dimples of the textured surface, respectively. Compared with a cemented carbide tool, the diamond tool is more preferable to be used to generate well shaped and distributed micro-dimples, because of its sharp cutting edge and less wear during the texturing process. The experimental textured surfaces, fabricated under different clearance angles of 0°, 7° and 20°, also have shown different wetting properties through wettability tests. It is verified that the RUVT can be a cost-effective texturing way for fast generation of micro-dimpled surfaces by choosing appropriate tool parameters.

Friction reduction performance of microstructured surfaces generated by nonresonant modulation cutting

Microstructured surfaces can reduce friction force between two contact pairs in relative motion under hydrodynamic contact. In this study, we evaluate the friction reduction performance of microdimpled surfaces with different cross-sectional profiles generated by nonresonant modulation cutting. Computational fluid dynamics simulation was conducted to simulate the friction reduction effect between surfaces with lubricants. An elliptical modulation texturing method is introduced based on a two-dimensional nonresonant vibration tool, which could generate adjustable high-frequency elliptical tool trajectories. Different dimpled surfaces were generated using three types of tool trajectories. Their influence on the friction reduction and load-carrying capacity were experimentally studied and evaluated by comparing the simulation and experimental results.

Mechanical micro-texturing of Ti-6Al-4V surfaces for improved wettability and bio-tribological performances

The present work focuses on the improvement in wettability and bio-tribological performance of Ti-6Al-4V titanium alloy using mechanically micro-textured surfaces. Three types of mechanical micro-tools, namely, micro flat-end mill, micro ball-end mill and micro-drill having equal diameter of 200 μm each, are used to fabricate circular micro-dimples with flat, semi-hemispherical and conical end geometries, respectively, on Ti-6Al-4V surface. The effects of micro-textured geometries on surface roughness, microhardness, surface wettability and coefficient of friction (COF) are estimated. Surface wettability is measured in terms of pure water contact angle, whereas, COF is estimated under in vitro condition of a hip joint prosthesis. Among different micro-dimpled surfaces, semi-hemispherical end micro-dimpled surface is found to be more suitable for hip joint prosthesis due to its extremely good surface wettability and lower COF. Results of this study also indicate that the mechanical micromachining processes are efficient methods for the fabrication of functional surfaces on titanium alloys and similar biomaterials.

Investigation on the scuffing resistance of ductile cast iron as affected by fine particle bombardment to produce surface hardened layer and micro-dimpled surface

The objective of this paper is to investigate the influence of fine particle bombardment (FPB) on the scuffing behavior of ductile cast iron. Ductile cast iron samples were processed by FPB treatment, in which three injection air pressures (0.2MPa, 0.4MPa and 0.6MPa) were used for given particle flux (8–10g/s), treatment time (15s), nozzle diameter (9mm) and standoff distance (100mm). Surface morphology, microstructure and hardness of these samples were examined. Step–loading scuffing tests were then performed on a block-on-ring machine, in which the FPB treated ductile cast iron was used for the block and an alloy cast iron without FPB treatment was used for the ring. The test results show that FPB treatment improves the surface hardness of ductile cast iron by nanocrystallization. An increment of injection air pressure would increase the surface hardness through reducing the grain size within the nanocrystalline layer. FPB treatment decreases the friction coefficient through reducing the surface roughness and producing a micro–dimpled surface. The friction coefficient is decreased when increasing the injection air pressure. The scuffing resistance of ductile cast iron depends on surface hardness and friction coefficient; it would be improved through using a higher injection air pressure during the FPB treatment.

Influence of a hybrid treatment consisting of fine particle bombardment and powder impact plating on the scuffing behavior of ductile cast iron

The objective of this research was to determine the influence of applying a hybrid treatment, consisting of fine particle bombardment (FPB) and powder impact plating (PIP), on the scuffing behavior of ductile cast iron. Ductile cast iron samples were processed by an FPB treatment, a PIP treatment, and a hybrid treatment of both FPB and PIP. Scuffing behavior of these samples was investigated using block-on-ring tests. A piston alloy of ductile cast iron was used for the block and a cast iron cylinder alloy was used for the ring. Results showed that scuffing resistance depends on surface hardness and friction coefficient for the test samples. FPB treatment of block samples increases their surface hardness by nanocrystallization and reduces the friction coefficient of the tribological pair by reducing the surface roughness and producing a micro-dimpled surface. Thus, the scuffing resistance of FPB treated block is improved. The scuffing resistance is also improved by PIP treatment, which reduces the friction coefficient by producing a soft Sn film on the contact surface. After a hybrid treatment of both FPB and PIP, the surface hardness of the ductile cast iron substrate is increased and the friction coefficient of the tribological pair is further reduced relative to either single treatment. Therefore, hybrid treatments such as the ones described here can improve the scuffing resistance of cast irons.

Tribological investigation of diamond-like carbon coated micro-dimpled surface under bovine serum and osteoarthritis oriented synovial fluid

Osteoarthritis-oriented synovial fluid (OASF), i.e., that typical of a patient with osteoarthritis, has different physical and biological characteristics than bovine serum (BS), a lubricant widely used in biotribological investigations. Micro-dimpled and diamond-like carbon- (DLC) coated surfaces are key emerging interfaces for orthopedic implants. In this study, tribological performances of dimpled surfaces, with and without DLC coating, have been investigated under both BS and OASF. The friction tests were performed utilizing a pin on a disk tribometer, whereas contact pressure, speed, and temperature were simulated to a ‘medium walking gait’ of hip joint conditions. The mechanical properties of the specimen and the physical properties of the lubricant were characterized before the friction test. Raman analysis was conducted to identify the coating condition both before and after the test. The DLC-coated dimpled surface showed maximum hardness and residual stress. A DLC-coated dimpled surface under an OASF lubricated condition yielded a lower friction coefficient and wear compared to those of plain and dimpled specimens. The higher graphitization of coated materials with increasing load was confirmed by Raman spectroscopy.

Study on the micro-dimpled surface in terms of drag performance

The main objective of the present study is to assess the feasibility of a micro-dimpled surface on the drag performance. Flat plates and 3-D cylindrical bodies are prepared to study the effectiveness of the micro-dimpled surface. Micro-sized dimples are carved into the metal surface directly using an ultrasonic nano-crystal surface modification method. The turbulent boundary layer over the dimpled-surface and the wake behind the trailing edge of the flat plates are measured by flow measurement techniques and revealed the augmentation of axial momentum in the boundary layer. The dimpled surface with submicron-scaled roughness affected the hydrophobicity of the surface. It was eventually found that the dimple manufacturing provides better surface finish which causes the reduction in drag force.

Improved friction and wear performance of micro dimpled ceramic-on-ceramic interface for hip joint arthroplasty

The purpose of this study is to investigate the tribological effect of micro-dimpled surface textures for application in ceramic-on-ceramic hip prostheses. A rectangular array of circular dimples was selected as the texture pattern. A CNC micro drilling machine was used to fabricate these dimples on a flat ceramic substrate. Tribology tests were performed by using a pin-on-disk method with selected hertzian contact pressures and speeds based on normal gait of a hip joint, and results compared with non-dimpled surface. A dimpled surface with large dimple diameters and a high dimple density (Ø400µm and density 15%) showed significant tribological performance gains, including a nearly 22% friction and 53% wear reduction.

Microstructuring of Surfaces by Two-stage Vibration-assisted Turning

Microstructuring is one suitable method to improve the tribological properties of surfaces. It leads to a reduction in friction and wear, especially in highly loaded tribological contacts. There are several machining techniques to generate microstructures, such as laser beam removal, cold forming, etching or fly-cutting. This paper describes a new method to produce microstructured surfaces more economically, based on a two- staged vibration-assisted turning process. During the first step, a specific microstructure is realized by ultrasonic vibration assistance of the turning tool in the direction perpendicular to the workpiece surface. In the subsequent step, the peaks of the obtained microstructured surface are removed by precision turning without vibration. This results in a plateau-shaped and microdimpled surface. The two-staged vibration- assisted turning process offers the advantage that no additional process is required after finishing because the microstructuring process is already part of the finishing operation. The investigated process can easily be integrated into large batch production, e. g., for tribological optimization of powertrain components.

Force modeling and applications of inclined ball end milling of micro-dimpled surfaces

Functional micro surfaces have been recognized for their vital roles in a wide range of advanced applications. The fabrication of surface structures at the microlevel can be used to influence tribological, optical, and many other surface characteristics. To take advantage of the benefits of functional surfaces, industry and researchers have begun focusing on finding more sustainable and efficient manufacturing processes. The inclined micro ball end milling technique has become a fast and efficient method for creating micropatterned surfaces. With the right adjustments, the spindle speed and feed rate can be set so that the flutes of the cutter create periodic dimpled patterns onto a workpiece surface. This micromachining technique is an ideal method for fabricating dimpled surfaces, especially for metallic alloys such as dies and molds. Developing surface pattern algorithms for generating different dimple geometries can promote a sustainable future for a variety of novel products and lead to accurate manufacturing of surface characteristics. Accurate modeling of cutting forces is important in order to generate desired surface patterns without causing tool breakage and excessive tool deflection. In this study, a mechanistic force model for inclined ball end milling has been proposed and verified for generating micro-dimpled surface. The micro-dimple machining technique is also applied to microinjection molds to create polymeric components with micropatterns. Frictional aspects of both dimpled and inverted dimple surfaces have been investigated. The results indicate that micro-dimple machining combined with microinjection molding is a viable method of producing polymeric components with functional surfaces for advanced technological applications.

Fabrication of micro-dimpled surfaces through micro ball end milling

Industry and researchers have begun to shift their focus towards more sustainable and eco-friendly manufacturing processes in recent years. They have recognized the vital role of functional micro surfaces for a wide range of advanced applications to address this issue. By physically altering the surface structure of a material in micrometric scale, tribological, optical, fluidic properties and many other surface characteristics can be altered. Several researchers have reported that micro surface patterns can reduce friction between lubricated sliding surfaces, which in turn alleviate energy consumption and increase service life of components. Micro ball end milling is another viable technique for creating patterned surfaces, especially for metallic parts. By tilting the spindle and tool at an inclined angle, the spindle speed and feed rate can be adjusted so that the flutes of the cutter create periodic patterns in a workpiece surface. Machining is an efficient and versatile manufacturing technique, making the micro dimple machining technique an ideal method to fabricate dimpled surfaces. In this study, the fabricated surfaces are evaluated at a tribological level to illustrate their effectiveness at reducing friction. The development of efficient methods to produce micro patterns onto large surface areas can promote a sustainable future for a variety of novel products. The development of efficient surface pattern algorithms for generating different dimple geometries is also a focus; and, trends in cutting forces are identified by changing different machining parameters. Depth of cut and dimple shape, spacing and arrangement are crucial parameters, all of which factor into the performance of a functional surface. The results of this study strongly indicate micro dimple machining as an environmentally sustainable method of producing functional surfaces for advanced technological applications.

딤플을 적용한 평판에 대한 항력 감소 연구

The main objective of the present study is to investigate the roles of the micro-dimpled surface on the drag reduction. To investigate the effectiveness of the micro-dimpled surface, the flat plates are prepared. The micro-size dimples are directly carved on the metal surface by ultrasonic nano-crystal surface modification (UNSM) method. Momentum of the main flow is increased by the dimple patterns within the turbulent boundary layer (TBL), however, there is no significant change in the turbulence intensity in the TBL. The influence of dimple patterns is examined through the flow field survey near the flat plate trailing edge in terms of the profile drag. The wake flow velocities in the flat plate are measured by PIV technique. The maximum drag reduction rate is 4.6% at the Reynolds number of <TEX>$10^6{\sim}10^7$</TEX>. The dimples tend to increase the drag reduction rate consistently even at high Reynolds number range.

Micro-dimpled surface by ultrasonic nanocrystal surface modification and its tribological effects

It is well known that dimpled surface can improve tribological characteristics. The fundamental aim of the present research is to demonstrate the process of making micro-dimples by ultrasonic nanocrystalline surface modification (UNSM) technology and to evaluate its effects on tribological characteristics. Ball-on-disk test was performed in order to reveal the influence of mechanism and micro-dimples. UNSM-treated surface is compared to ground surface and friction coefficient of the UNSM-treated surface has been reduced by about 25%, which is a significant phenomenon in improving tribological properties of mating surfaces. Wear volume loss of UNSM-treated surface has also been reduced by about 60%. Scanning electron microscopy (SEM) and surface roughness profile measurement were utilized to study the worn surface.

Controllable and switchable capillary adhesion mechanism for bio-adhesive pads: Effect of micro patterns

Some insects and animals, such as bugs, grasshoppers and tree frogs, realize their efficient adhesion mechanism to glass surface, wall and ceiling by injecting a wetting liquid thin film into the pad-substrate contact area. Their ability to control adhesion (attaching or detaching from a surface) is in many cases connected to the contact geometry and surface patterns of their attachment pads. This paper focuses on the dependence of the capillary adhesion (wet adhesion) on the micro patterns of the bio-adhesive pads. The objective is to reveal the possible mechanism for a bio-adhesive pad to control capillary force through adjusting its micro-scale surface pattern and topography. A capillary adhesion force model is built up taking account of the combined role of micro-dimple geometry as well as the wetting behavior of the confined liquid thin film. Calculated results of the apparent contact angle on the regularly micro-dimpled surfaces are compared with and in good agreement with the experimental measurements. Simulation of the capillary adhesion force reveals that it is controllable in a large magnitude by adjusting a dimensionless surface pattern parameter k defined as a/(a+b), where a is the diameter of micro dimple, and (a+b) is the side length of one pattern cell. When adjusting the parameter k more than 0.75, the capillary adhesion force could be switchable from attractive to repulsive. This effect of micro patterns on the interfacial capillary force is proved to be dominant when the pad-substrate clearance decreases to the nano/micrometer scale. These results indicate that a controllable and switchable capillary adhesive mechanism might be utilized by a living insect or animal to realize its stable adhesion and quick releasing movement through adjusting the micro-pattern topography of its bio-adhesive pad.

Study on long life of artificial joints by investigating optimal sliding surface geometry for improvement in wear resistance

The effective life of artificial joints is approximately 15 years. A smooth metal sliding surface is presumably the most suitable when manufacturing artificial joints; however, the relationship between the characteristics of metal sliding surface and ultra high molecular polyethylene (UHMWPE) wear has not been confirmed. Further, there is no apparent proof that a smooth surface is the optimal option for the improvement in the wear resistance of artificial joints. In this study, we investigated the mechanism of UHMWPE wear and proved that scratch marks caused by a sliding motion against the metal surface are the prime cause of UHMWPE wear. Furthermore, we used a micro-dimpled surface as an effective sliding surface to reduce the UHMWPE wear. A 2-axes pin-on-plate sliding test proved that the life of artificial joints can be extended to approximately 35 years by using a micro-dimpled surface with 1-μm deep dimples.