Pad Surface Research Articles

Impact sites representing potential bruising locations associated with bed falls in children

Bruising can occur as a result of accidental or abusive trauma in children. Bruises are an early sign of child abuse and their locations on the body can be an effective delineator of abusive trauma. Since falls are often reported as false histories in abuse, the ability to predict potential bruising locations in falls could be valuable when attempting to differentiate between abuse and accident. In our study we used an anthropomorphic test device (ATD), a surrogate representing a 12 month old child, adapted with a custom developed force sensing skin to predict potential bruising locations during simulated bed falls. The sensing skin is made of custom resistive force sensors integrated into a conformable skin, adapted to fit the contours of the ATD. The sensing skin measured and displayed recorded force data on a computerized body image mapping system when sensors were activated. Simulated bed fall experiments were performed from two initial positions (FF – facing forward and FR – facing rearward) and two fall heights of 61cm (24 in) and 91cm (36 in) onto a padded carpet impact surface. Findings indicated potential bruising primarily in two planes of the ATD body. The majority of contact regions and greater forces were recorded in one plane, with fewer regions of contact and decreased force exhibited in an adjoining second plane. Additionally, no contact was recorded in the two planes opposite the impact planes. Differences in contact regions were observed for varying heights and initial position. Limitations of ATD biofidelity and soft tissue properties must be considered when interpreting these findings.

Characterization of holding brake friction pad surface after pin-on-plate wear test

This article concerns the metrological characterization of the surface on a holding brake friction material pin after a pin-on-plate (POP) wear test. The POP test induces the formation of surface plateaus that affect brake performances such as wear, friction, noise and heat. Three different materials’ surfaces have been characterized after wear from data obtained with a focus variation 3D microscope. A new surface characterization approach with plateau identification is proposed, using the number of plateau on the surface, equivalent diameter, length and breadth as measurands. The identification method is based on determining and imposing ISO 27158-2 lower plateau limit (LPL) in material probability curves; and on applying a combined criterion of height segmentation threshold and equivalent diameter threshold. The method determines the criterion thresholds for each material since LPL appears typical by material. The proposed method has allowed quantifying the surface topography at two different levels of wear. An expanded measurement uncertainty of 3.5 µm for plateau dimensions in the range 50–2000 µm and one of 0.15 µm for plateau heights up to 10 µm have been documented.

Improvement of Thrust Bearing Calculation Considering the Convectional Heating within the Space between the Pads

A modern thrust bearing tool is used to estimate the behavior of tilting pad thrust bearings not only in the oil film between pad and rotating collar, but also in the space between the pads. The oil flow in the space significantly influences the oil film inlet temperature and the heating of pad and collar. For that reason, it is necessary to define an oil mixing model for the space between the pads. In the bearing tool, the solutions of the Reynolds equation including a cavitation model, the energy equation and the heat transfer equation are done iteratively with the finite volume method by considering a constant flow rate. Both effects—laminar/turbulent flow and centrifugal force—are considered. The calculation results are compared with measurements done for a flooded thrust bearing with nominal eight tilting pads with an outer diameter of 180 mm. The heat convection coefficients for the pad surfaces mainly influence the pad temperature field and are adjusted to the measurement results. In the following paper, the calculation results for variable space distances, influence of different parameters on the bearing behavior and operating condition at high load are presented.

Grooved-structure design for improved component damping ability

Abstract To investigate the influence of the different structures of elastic damping components on vibration performance and tribological characteristics, different-angled grooved structures were designed and manufactured on the damping components and damping ability tests were carried out on the developed experimental setup. Results showed that the system with grooved-structure damping components exhibited a lower vibration amplitude and better contact conditions for a pad surface than the system without a damping component. To provide a possible explanation of why the improvement effect of different damping components on damping abilities exhibited discrepancy, a hammer impact test was carried out and the contact pressure distribution on a pad surface and the deformations of different grooved damping components were simulated using the finite element method.

Improvement of dynamical and tribological properties of friction systems by introducing parallel-grooved structures in elastic damping components

This paper presents the influence of introducing elastic damping components having different parallel-grooved structures in friction systems. Experimental and numerical studies are conducted to evaluate the effects on the friction-induced vibration, and friction and wear. A number of parallel-grooves structures are designed and manufactured on damping components. The experiments are performed using a purpose-built experimental test bench. The effects of different damping components on the dynamical and tribological properties are estimated by comparing the dynamic vibration performance, and friction and wear characteristics of the contact interface. The experimental results show that the system with damping components could effectively suppress the friction-induced vibrations, and improve the wear characteristics. Consequently, this helps stabilize system, especially for the damping components having the structure of two parallel grooves. The contact stress distribution of pad surface and the deformations of different damping components are evaluated through numerical studies. The observations and explanations are supported by experimental results.

Influence of squeezing and interface slippage on the performance of water‐lubricated tilting‐pad thrust bearing during start‐up and shutdown

AbstractThe film thickness varied because of the swing of the tilting‐pad thrust bearing, which introduced the squeezing effect in the start‐up and shutdown periods. The interface slippage might occur between the water and the pad surface, because of the effects of water polarity and surface energy of the material. To study the performance of the water‐lubrication tilting‐pad thrust bearing during start‐up and shutdown, a transient model considering the effects of squeezing and interface slippage was established. The effects of squeezing and interface slippage on film‐thickness distribution, nondimensional minimum film thickness, circumferential angle, and friction torque were studied. The theoretical friction torque was consistent with the experimental result quite well. The result demonstrated that the effect of squeezing was higher in the opening process than in the closing process; the interface slippage effect influenced the start‐up and shutdown significantly, blocking the formation of the water film. The friction torque of the bearings increased if the interface slippage existed, because the solid contact friction‐force increase was higher than the fluid shear‐force decrease in the mixed lubrication. When both effects worked simultaneously, the interface slippage dominated in the start‐up period. During the shutdown situation, as the rotational speed sustained a loss, the interface slippage was abated, where the influence of the squeezing effect dominated gradually.

Effect of CVD-Coated Diamond Discs on Pad Surface Micro-Texture and Polish Performance in Copper CMP

The effect of pad surface characteristics on the thermal, tribological and kinetic attributes of copper CMP was investigated. Three CMC D100 pads with very different surface micro-textures were generated using three very different CVD-coated diamond conditioning discs. Pad samples were collected after polishing and analyzed for their surface contact area and topography using confocal microscopy. The contact area, contact density, and asperity height increased with increasing conditioner aggressiveness. Copper removal rates and pad surface temperatures increased with increasing polishing pressure and sliding velocity for all pads albeit in a non-Prestonian manner. The pad generated by the most aggressive disc caused the highest removal rates yet it showed the lowest overall coefficient of friction (COF). The tall asperities, open pores, and adequate contact of this pad produced a greater removal rate than pad surfaces with shorter asperities and glazed pores produced by the less aggressive discs. Trends in COF, temperature and removal rate were successfully simulated using a two-step modified Langmuir-Hinshelwood model which also yielded values for the chemical and mechanical rate constants. The simulation results indicated that the process was chemically limited for all polishing conditions, and that the process became even more chemically limited as P × v increased.

Effect of Conditioner Type and Downforce, and Pad Break-In Time, on Pad Surface Micro-Texture in Chemical Mechanical Planarization

The impact of conditioner types and downforces during pad break-in, and the resulting effects on the evolution of pad surface micro-texture was investigated. Two different conditioning discs were used with 3 possible downforces to break-in pads. Pad samples were extracted throughout the break-in process and confocal microscopy was used to analyze the samples for their surface micro-topography and pad-wafer contact. Results showed that all experimental cases resulted in similar trends of mean summit height. More importantly, each case resulted in a different evolution of summit height distribution. Comparing the two discs used, one disc was observed to be more sensitive to changes in downforce compared to the other. The differences in the behavior of the two discs is explained by the differences in cutting mechanics, which is due to the different characteristics of the two discs. Both discs generated large amounts of pad fragments, which were shown to cause pore obscuration on the pad surface. In 4 out of 5 cases, the pad surface micro-texture stabilized within 30 minutes of break-in and all cases stabilized within 60 minutes. This work demonstrated the importance of understanding how different conditioner types react to changes in downforce when breaking-in a pad.

Real-Time Shear and Normal Force Trends in Tungsten Chemical Mechanical Planarization with Different Conditioning Discs

In-situ conditioning is known to increase material removal rate (RR) during chemical mechanical planarization (CMP), because it roughens the pad surface that is in contact with the wafer. Previous studies have shown that the conditioning disc changes the pad surface micro-texture and affects the thermal, kinetic and tribological attributes of the CMP process. This study focuses on two different diamond discs for in-situ conditioning during tungsten CMP, and shows how real-time force data improves our understanding of the effects from using one disc versus the other. Shear and normal forces were analyzed through spectral analysis, variance and force scatter plots. Frequency signatures that appear regardless of the type of disc used were likely attributed to vibrations of the polisher and the harmonics of these vibrations. A new parameter, directivity (Δ), was employed, which highlighted the significant differences in shear force variance between the two discs. Above all, this study confirmed that high Δ correlated with high RR. Compared to a related study on copper CMP with these discs, the trends between Δ and RR seemed to continue to hold.

Novel, Non-aqueous Bioconversion Systems Using Fungal Spores.

Two novel types of non-aqueous bioconversion systems using fungal spores, either adsorbed on the surface of a filter pad or entrapped in calcium alginate beads, were constructed and applied for a model reaction: reduction of benzil to benzoin by Aspergillus sojae NBRC 32074. The spores adsorbed on a filter pad catalyzed the reduction in some toxic organic solvents, such as methylcyclohexane (log P: 3.61) and din-butyl ether (3.21). For the relationship between the reduction activity and the log P value of the organic solvent, a highly positive correlation (R2: 0.815) was observed. Surprisingly, the reduction proceeded in the more hydrophilic and toxic tert-butyl acetate (log P: 1.76). Glycerol was selected as the best hydride source. The higher the glycerol content, the more the benzoin was produced. While the production of benzil by spores was lower than that by mycelia in harmless di-n-hexyl ether (log P: 5.12), mycelia could not catalyze the reduction in the toxic tert-butyl acetate. In contrast, the spores entrapped in the calcium alginate beads could catalyze the reduction. Although the reduction by alginate-entrapped spores could be stably repeated 5 times in di-n-hexyl ether without a decline in the reduction activity, it was observed that the reduction activity of the spores gradually decreased after repeated reduction in tert-butyl acetate.

Real-Time Shear and Normal Force Trends in Copper Chemical Mechanical Planarization with Different Conditioning Discs

The effect of two different commercially-available CVD-coated diamond conditioning discs on copper CMP was investigated tribologically and kinetically with the intent of correlating pad surface micro-texture to polish performance. Data analyses were particularly focused on in-situ shear and normal force measurements taken during polishing at several pressures (P) and sliding velocities (V), including Fast Fourier Transform (FFT) analysis of the data. One of the two discs consistently resulted in greater variances of shear force with increasing P and V indicative of increased pad-slurry-wafer stick-slip events. This trend correlated to higher values of coefficient of friction (COF) and removal rate (RR). By dividing the shear force variance by the normal force variance, a new parameter, termed directivity, was used to further emphasize the improved performance of the above-mentioned disc. Lastly, the tribological and kinetic data, combined with data regarding the pad's micro-texture and its extent of contact with the wafer, showed that at all values of P and V, higher values of directivity correlated to more contacting pad asperities, a greater asperity density and a thinner micro hydrodynamic lubrication layer, thus highlighting the potential importance of directivity in future studies.

Insights into the Tribological and Kinetic Attributes of Retaining Rings in Chemical Mechanical Planarization

We have successfully applied the Stribeck+ curve to determine the tribological mechanism involved in the ring-slurry-pad interface during CMP at various pressures and velocities. Using two types of slurries (one for copper and another for inter-layer dielectric polishing) and two differently designed rings made of polyether ether ketone (PEEK), one with sharp slots and the other with rounded slots. We find “boundary lubrication” as the initial dominant lubrication mechanism followed by a transition to “mixed lubrication” as pseudo-Sommerfeld numbers increase. Coefficient of friction (COF) values for the ring with sharp slots are larger than their rounded counterparts. COF values are also higher for inter-layer dielectric (ILD) processes as compared to copper. In all cases, COF values are directly correlated with average pad surface temperature. The trend in Stribeck+ curves are explained by plotting a new parameter, referred to as directivity, which shows the cluster shape evolution to be a real phenomenon (and not an optical illusion due to the log-log nature of the curve, nor the inherent dependence of the two axes on one another) caused by force fluctuations at the extreme ranges of velocity and pressure. Raw data from the Stribeck+ curves are used to construct a new “kinetic” plot to help infer the relative wear rate of the ring thus allowing one to choose process parameters that balance it against wafer removal rate. Validation studies involving extended wear of eight differently formulated retaining rings show a correlation between wear rate and the “inferred” wear rate which renders credibility to our approach regarding the above-mentioned “kinetic” plots.

Effect of Conditioning Downforce and Pad Break-In Time on Pad Surface Micro-Texture

The effect of conditioning downforce during pad break-in, and its impact on the evolution of pad surface micro-texture was investigated. Two different conditioning downforces were used to break-in pads. Pad samples were extracted after certain break-in times and analyzed for their topography and contact area using confocal microscopy. Results showed that the pad-wafer contact area and contact density decreased with conditioning downforce. Break-in at the higher conditioning downforce helped in reaching faster stabilized values of the analyzed micro-texture parameters. The evolution of these parameters was different for the two different downforces, however as break-in time increased past 30 minutes, mean summit height and mean summit curvature began to approach approximately the same value. These results indicated that, for the particular disc used in this study, a change in the magnitude of downforce during pad break-in caused a change in break-in time and stable values for some micro-texture parameters. However, a change in the magnitude of downforce during break-in only resulted in a change in break-in time for other micro-texture parameters. The work underscored the need for conducting such tests with different conditioners in order to develop a better understanding of the role downforce plays on the evolution of the micro-texture.

A Mathematical Model for the purpose of analysing the Thermal Stress Characteristics Of PKS-Based Brake Pad with MATLAB

The automotive brake system has significant requirements to achieve adequate braking power as well as effective heat dissipation. Surface cracks and permanent distortions are a number of unfavorable conditions due to thermal stresses and high temperatures on brake friction surfaces. Contact pressure and temperature on the friction surfaces are often affected by frictional heating, thermal deformation and elastic contact in sliding contact systems. In order to improve the thermal quality of palm kernel shell based brake pads, a mathematical model which can be used to simulate thermal stress behaviour pattern in the brake pads is developed. The developed model is in the form of a partial differential equation with the temperature at a position in the lining varying inversely with the cube root of its distance from the surface where braking force is applied. The said model follows the partial differential equation format usually required for simulation in Matlab environment and can be used to predict the surface temperature, heat flux, planar deformation and shear stress on the brake pad surface during application.

Study on pad cutting rate and surface roughness in diamond dressing process

Diamond dressing process has an important role in the elimination of scrap materials on the pad surface and regeneration of the pad surface topography before and after Chemical Mechanical Planarization/Polishing (CMP) process for integrated circuit (IC) fabrication. During diamond dressing process, distributions of cutting locus and overlap cutting of diamond grits usually work as a datum surface for pad surface planarization and uniform pad asperities. This paper proposes a novel view to predicting the distribution of pad cutting rate (PCR) and surface roughness based on cutting length and overlap cutting locus. The geometrical model of diamond dressing on the pad has been built and CMP tool parameters are used for the simulation of cutting length and overlap distribution. The distribution of PCR and predict surface roughness is calculated on each pad zone. Simulation results presented a high PCR and roughness is observed in center zones. The verified experiment is in agreement with simulation results where high PCR and roughness is near the center zones, which cause the shape of the pad surface to be concave. The results of this study can be further applied for prediction and optimization of diamond dressing design and improvement of dressing process.

Study on quasi-orthogonal machining of elastomer pad by single-point diamond tool

Chemical mechanical polishing (CMP) process has been a popular wafer and thin film planarization process for semiconductor fabrication. In CMP process, a diamond dresser with well-distributed diamond grits is usually applied for regenerating the pad surface topography to maintain the pad polishing capability. This paper describes the fundamentally quasi-orthogonal diamond dressing process by pyramid single-point diamond tools at different grit angles under the fixed down pressure and slow dressing speed for elastomer pad conditioning. Experiments of single-point diamond dressing by both face direction dressing (FDD) and edge direction dressing (EDD) have been performed to investigate the normal force profile and pad surface topography. Experimental results show that FDD generates a higher quality of pad surface with lesser plowing volume and relatively stable pad cutting rate (PCR). Moreover, diamond grit with grit angle of 90° has been found to be most suitable while shifting between EDD and FDD during actual diamond dressing process. Results of this study can be applied to diamond dresserv design and optimization of the pad surface topography uniformity in diamond dressing process for CMP of integrated circuit (IC) production.

Reduction of the Adhesive Friction of Elastomers through Laser Texturing of Injection Molds

It is well known that elastomers usually possess poor dry sliding friction properties due to their highly adhesive character. In order to overcome this problematic behavior in industrial applications, interfacial materials such as oils, greases, coatings, or lacks are normally used in order to separate or to functionalize the contact surfaces of elastomers. Alternatively, the high adhesion tendency of elastomers may be explicitly reduced by modifying the elastomer composition itself or by enabling a reduction of its effective contact area through, for example, surface laser texturing. This second approach, i.e., the reduction of the adhesive character of elastomers through laser structuring, will be the main topic of the present study. For this purpose, different micro-sized grooved structures were produced on flat injection molds using an ultra-short pulsed laser. The micro-structured molds were then used to produce injection molded micro-ridged Liquid Silicone Rubber (LSR) sample pads. The investigations consisted firstly of determining the degree of replication of the mold micro-structures onto the surface of the LSR pads and secondly, to ascertain the degree of reduction of the friction force (or coefficient of friction) of these micro-ridged LSR pads in comparison to the benchmark (unstructured LSR pads) when tested under dry conditions against Aluminum alloy (Al-6082) or PA6.6-GF30 plates. For this second part of the investigation, the normal force (or contact pressure) dependency of the coefficient of friction was determined through stepwise load increasing friction tests. The results of these investigations have shown that the production of micro-ridged surfaces on LSR pads through laser structuring of the injection molds could be successfully achieved and that it enables a significant reduction of the friction force for low normal forces (or contact pressures), where the component of adhesion friction is playing an important and determining role in the overall friction behavior of the LSR elastomer.

Diamond polishing of crystalline materials for optoelectronics

A study of the mechanism of mechanical polishing of optoelectronic components made of crystalline materials has demonstrated that the removal rate in polishing decreases with increasing workpiece material’s bond energy and energy of transfer and grows with increasing thermal conductivity coefficient of the workpiece material, sliding distance of a workpiece surface element against the polishing pad surface, and Lifshitz force. The ratio between the volumetric wear coefficient and thermal diffusivity of the workpiece material is shown to depend on specific heat capacity and energy of transfer.

The natural history of oviposition on a ginkgophyte fruit from the Middle Jurassic of northeastern China.

A distinctive pattern of oviposition lesions occurs on a ginkgoalean seed, Yimaia capituliformis, which likely was inflicted by a kalligrammatid lacewing with a long, sword-like, plant-piercing ovipositor. This newly recorded oviposition type, DT272, occurs in the 165 million-year-old Jiulongshan Formation, of Middle Jurassic age, in Northeastern China. DT272 consists from three to seven, approximately equally spaced lesions with surrounding callus tissue, the fabricator of which targeted fleshy outer and inner tissues of a ginkgophyte fruit. This distinctive damage also is known from the fleshy attachment pad surfaces of basal bennettitalean bracts. Examination of the life history of this probable ginkgoalean-kalligrammatid oviposition interaction indicates that the spacing of the eggs in substrate tissues disfavored inter-larval contact, but little can be said of defense and counterdefense strategies between the plant host and the newly hatched immatures.

Listeria monocytogenes persistence and transfer to cantaloupes in the packing environment is affected by surface type and cleanliness

Investigation of the 2011 U.S. listeriosis outbreak associated with consumption of contaminated cantaloupes (Cucumis melo L) revealed that transfer of Listeria monocytogenes from equipment surfaces to melons in the packing facility was a potential route of contamination. The study presented here examined L. monocytogenes attachment and persistence on soiled materials and different types of equipment surfaces, and transfer to melons in a simulated-packing environment. Attachment involved the exposure of roller conveyors (polyvinyl chloride), conveyor belts (polyvinyl chloride, polyurethane, and nitrile rubber), and brush filaments (nylon and polyethylene) to ca. 7 log10 CFU/mL of a L. monocytogenes multi-strain inoculum for 10 and 60 min. L. monocytogenes showed similar (p > 0.05) attachment to all types of surfaces, and populations attached to surfaces following exposure for 10 min were not significantly different (p > 0.05) from that attached to surfaces after 60 min. Clean and soiled surfaces of nylon-brush filaments, conveyor belts, and foam pads were each inoculated with a L. monocytogenes multi-strain mixture (4.5 log10 CFU/mL) and subsequently stored at 25 °C, and L. monocytogenes populations on the surfaces were recovered for up to 15 days. L. monocytogenes populations on clean surfaces decreased from 4.5 log10 CFU/surface on day 0 to the detection limit by day 14; L. monocytogenes was still present on clean surfaces following enrichment on day 21. However, L. monocytogenes populations remained unchanged on soiled surfaces from day 0–14. Multivariate ANOVA revealed that soiled surfaces significantly (p < 0.05) promoted L. monocytogenes persistence compared to clean surfaces. Comparison of clean surfaces revealed that foam pads and brushes significantly (p < 0.05) promoted L. monocytogenes persistence compared to conveyor belt materials. Conveyor belts and foam pads were spot-inoculated (2.5 log10 CFU/surface), and 15 consecutive wet melons were manually rolled over each spot (n = 6 replicates), and melons were tested for the presence/absence of L. monocytogenes. Foam pads were found to contaminate significantly (p < 0.05) more melons than conveyor belts (polyvinyl chloride, polyurethane, and nitrile rubber). Soiled surfaces supported the survival of higher populations of L. monocytogenes, and surface type affected L. monocytogenes contamination transferred to specific melons.