Surface structure and roughness of Nickel-Titanium wires

Nickel-Titanium wires not only show the well known shape memory effect, but also pseudoelastic behaviour. This means, as the material is bent in a wide range of tension, the stress in the material stays constant. This special behaviour allows the improvement of orthodontic devices. The investigation of wires for this application reveals great differences in the surface roughness of the wires from various suppliers. The roughness causes a loss of spring energy for the orthodontic wire by frictional effects. The investigation of the mechanical behaviour of these materials leeds to differences compared with the ideal behaviour expected for the stress/strain curve. Obviously these differences are caused by surface effects of the wires. To evaluate these questions intensive roughness measurements with profilometry, laser spectroscopy and scanning force microscopy have been performed. Further more defined bending and tensile experiments have been carried out and corresponding roughness and structure analysis with the scanning force microscope has been taken into account. By etching the wires significant surface effects could be found.     

Looking at the micro-topography of polished and blasted Ti-based biomaterials using atomic force microscopy and contact angle goniometry

Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter.     

Looking at the micro-topography of polished and blasted Ti-based biomaterials using atomic force microscopy and contact angle goniometry

Surface topography of polished and blasted samples of a Ti6Al4V biomaterial has been studied using an atomic force microscope. Surface RMS roughness and surface area have been measured at different scales, from 1 to 50 μm, while at distances below 10 μm the surface RMS roughness in both kinds of samples is not very different, this difference becomes significant at larger scanning sizes. This means that the surface roughness scale that could have a main role in cell adhesion varies depending on the size, shape and flexibility of participating cells. This consideration suggests that in cell–material interaction studies, surface roughness should not be considered as an absolute and independent property of the material, but should be measured at scales in the order of the cell sizes, at least if a microscopic interpretation of the influence of roughness on the adhesion is intended. The microscopic information is contrasted with that coming from a macroscopic approach obtained by contact angle measurements for polar and non-polar liquids whose surface tension is comprised in a broad range. Despite the very large differences of contact angles among liquids for each surface condition, a similar increase for the blasted surface with respect to the polished has been found. Interpretation of these results are in accordance with the microscopic analysis done through the use of a functional roughness parameter, namely the valley fluid retention index, evaluated from the AFM images, which has been shown not to correlate with the RMS roughness, one of the most commonly used roughness parameter.     

Physical properties (thermal,thermomechanical, magnetic,and adhesive) of some smart orthodontic wires

Thermal, thermomechanical, and caloric properties of commercial orthodontic wires (produced by Natural Orthodontics Corp., USA) with cylindrical and rectangular geometry were studied. Depending on the applied forces, there were identified the range of elasticity, the elasticity–viscoelasticity coexistence domain and the domain in which a maximum force of 18 N is applied, for the orthodontic wires. When increasing the thickness of orthodontic wires, deformation decreases. The Controlled Force Module, in the tension mode, was used for the determination of the orthodontic wires elongation at application of the stretching forces from 0 to 13 N, at 35 °C, maintaining each static force value for 3 min. The increase in the cross-sectional area of the orthodontic wires disfavors the process of elongation of the sample, at the same applied static force. Using the Multi-Frequency–Strain–Stress modulus, in the tension mode, DMA cyclic heating–cooling measurements were performed. The measured physical quantities for orthodontic wires were Storage Modulus, Loss Modulus, Tanδ and Stiffness, at heating and cooling. Thus, the characteristic temperatures of the phase transitions (A_s, A_f, M_s, M_f), of all the studied orthodontic wires were identified. Also, the values of the elasticity modulus (Young’s Modulus) of the orthodontic wires were calculated at 35 °C. With the DSC Q200 device, using temperature-modulated differential scanning calorimetry method, a multi-step temperature variation program, was applied to a rectangular wire, in three stages (cooling–heating–cooling). Through the interpretation of heat fluxes (reversible, irreversible and total), the phase transitions in the formation of martensite, austenite, but also of the rombohedral phase (R-phase), were identified. Formations of austenite and martensite were also evidenced by the classical DSC method, but the classical DSC method also enabled the R-phase identification. The adherence of some food dyes on the orthodontic wires, as well as the modification of the surface roughness of the orthodontic wire after the deposition of the food dye, was also studied. By magnetic measurements, it was established that the orthodontic wires had paramagnetic properties at room temperature, and nitinol was a mixture of 49.2% austenite and 50.8% martensite.

     

The role of few-asperity contacts in adhesion

The surface roughness of a few asperities and their influence on the work of adhesion is of scientific interest. Macroscale and nanoscale adhesion data have seemingly given inconsistent results. Despite the importance of bridging the gap between the two regimes, little experimental work has been done, presumably due to the difficulty of the experiment needed to determine how small amounts of surface roughness might influence adhesion data lying in between the two scales. To investigate the role of few-asperity contacts in adhesion, the pull-off force was measured between different sized atomic-force microscope (AFM) tips (with different roughnesses) and sample surfaces that had well-controlled material properties. There were seventeen tips of four different types, with radii from 200 nm to 60 microm. The samples were unpatterned single crystal silicon with a chemical silicon dioxide surface resulting from a standard silicon wafer clean. Some of the samples were treated with a few angstroms of vapor deposited diphenylsiloxane. We observed that the uncorrected (for surface roughness) pull-off force was independent of the radius of the AFM tip, which was contrary to all continuum-mechanics model predictions. To explain this behavior, we assumed that the interactions between the AFM tip and sample were additive, material properties were constant, and that the AFM tip, asperities, and sample surfaces were of uniform density. Based on these assumptions, we calculated a simple correction due to the measured root mean square (RMS) surface roughness of the AFM tips. The simple correction for the RMS surface roughness resulted in the expected dependence of the pull-off force on radius, but the magnitudes were higher than expected. Commercial and heat-treated AFM tips have minimal surface roughness and result in magnitudes that are more reliable. The relative uncertainty for the pull-off force was estimated to be 10%. In this paper, we derive how the cantilever and tip parameters contribute to the measured pull-off force and show how the corrected results compare with theory. Although much work is still needed, the work presented here should advance the understanding of adhesion between the macroscale and nanoscale regimes.     

Roughness of microspheres for force measurements

We have investigated the morphology and surface roughness of several commercially available microspheres to determine their suitability for force measurements using the atomic force microscope. The roughness varies considerably, depending on sphere size and material, ranging from nearly ideally flat up to micrometer-sized features. Because surface roughness significantly influences the magnitude and accuracy of measurement of surface forces, the results presented here should be helpful for colloid physicists and in particular for those performing force measurements.     

Au-Ag template stripped pattern for scanning probe investigations of DNA arrays produced by dip pen nanolithography

We report on DNA arrays produced by dip pen nanolithography (DPN) on a novel Au-Ag micropatterned template stripped surface. DNA arrays have been investigated by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) showing that the patterned template stripped substrate enables easy retrieval of the DPN-functionalized zone with a standard optical microscope permitting multi-instrument and multitechnique local detection and analysis. Moreover the smooth surface of the Au squares ( approximately 5-10 A roughness) allows AFM/STM to be sensitive to the hybridization of the oligonucleotide array with label-free target DNA. Our Au-Ag substrates, combining the retrieving capabilities of the patterned surface with the smoothness of the template stripped technique, are candidates for the investigation of DPN nanostructures and for the development of label-free detection methods for DNA nanoarrays based on the use of scanning probes.     

Nanoscale analyses of modified polypropylene microtubes internal surface: an approach covering topographical and force spectroscopic parameters

A nanoscale characterization of modified and unmodified polypropylene (PP) microtubes internal surface was performed to investigate their structural, chemical, and physical properties. Nanoroughness, stiffness, elasticity, attraction behavior, adhesion forces, and chemical environment were investigated to test some manufacturer statements regarding Axygen MAXYMum Recovery® products. They announced that this class of material presented special features, originated from a modification to the original PP resin and by using a diamond polished mould, providing lower retention and minor interference on laboratorial tests, such as low roughness and little interaction tendency. Then, in this study, modified and control internal surfaces of PP microtubes were compared by atomic force microscopy, scanning electron microscopy, and Fourier transform infrared spectroscopy. Nanoroughness and force spectroscopy parameters assessed by atomic force microscopy showed out as a sensible and high‐resolution technique, crucial to discriminate differences between the surfaces. This type of investigation can be considered as a promising approach that can be applied to other polymeric systems, considering nanoscale properties, physical/chemical modifications, and as an alternative route for quality control checking concerning polymeric surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Oscillatory forces of nanoparticle suspensions confined between rough surfaces modified with polyelectrolytes via the layer-by-layer technique

This paper addresses the systematic study of surface roughness effects on the internal structuring of silica nanoparticle suspensions under confinement. The confining surfaces are modified by physisorption of layers of oppositely charged polyelectrolytes with the so-called layer-by-layer technique. The layer-by-layer technique modifies the surface roughness without changing the surface potential of a multilayer with the same outermost layer, by increasing the number of constituent layers and ionic strength of the polyelectrolyte solutions and by selecting an appropriate pair of polyelectrolytes. The oscillatory forces of nanoparticle suspensions with a particle diameter of 26 nm are measured by a colloidal-probe atomic force microscope (CP-AFM). The characteristic lengths of the oscillatory force, i.e., wavelength, which indicates interparticle distance, and decay length, or particle correlation length, are not affected by the surface roughness. The corresponding reduction in the oscillatory amplitude and the shift in the phase correlate with an increase in surface roughness. Increasing surface roughness further induces a disappearance of the oscillations, and both confining surfaces contribute to the effect of surface roughness on the force reduction. In order to show an oscillatory force, the particles have to show positional correlation over a reasonably long range perpendicular to the surface, and the correlation function should be the same over a larger lateral area. This requires that both the particles and the surfaces have a high degree of order or symmetry; otherwise, the oscillation does not occur. A roughness of a few nanometers on a single surface, which corresponds to about 10% of the nanoparticle diameter, is sufficient to eliminate the oscillatory force.     

Mapping molecular adhesion sites inside SMIL coated capillaries using atomic force microscopy recognition imaging

Capillary zone electrophoresis (CZE) is a powerful analytical technique for fast and efficient separation of different analytes ranging from small inorganic ions to large proteins. However electrophoretic resolution significantly depends on the coating of the inner capillary surface. High technical efforts like Successive Multiple Ionic Polymer Layer (SMIL) generation have been taken to develop stable coatings with switchable surface charges fulfilling the requirements needed for optimal separation. Although the performance can be easily proven in normalized test runs, characterization of the coating itself remains challenging. Atomic force microscopy (AFM) allows for topographical investigation of biological and analytical relevant surfaces with nanometer resolution and yields information about the surface roughness and homogeneity. Upgrading the scanning tip to a molecular biosensor by adhesive molecules (like partly inverted charged molecules) allows for performing topography and recognition imaging (TREC). As a result, simultaneously acquired sample topography and adhesion maps can be recorded. We optimized this technique for electrophoresis capillaries and investigated the charge distribution of differently composed and treated SMIL coatings. By using the positively charged protein avidin as a single molecule sensor, we compared these SMIL coatings with respect to negative charges, resulting in adhesion maps with nanometer resolution. The capability of TREC as a functional investigation technique at the nanoscale was successfully demonstrated.     

Glass fracture surfaces seen with an atomic force microscope

Fracture surfaces of Suprasil 2, Herasil 2, AR glass and Duran glass rods have been studied by an atomic force microscope (AFM) in the contact mode. They could be characterized in the fracture mirror, the mist and the hackle zones. The RMS (root mean square) roughness in the fracture mirror of all glasses investigated increased with growing distance from the origin of the fracture. On several fracture surfaces of different glasses steps have been observed, due to fracture in shear mode. Furthermore changes in the fracture surfaces during scanning have also been observed. They are thought to stem from reactions of the freshly broken glass surface with the surrounding atmosphere and forces between the scanning tip and the soft surface.     

Predicting the shock sensitivity of cyclotrimethylene-trinitramine

We studied the surface and thermal properties of seven different varieties of cyclotrimethylene-trinitramine (RDX) crystalline explosives from five manufacturers using differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The specific varieties of the RDX studied were acquired from the previous Reduced Sensitivity RDX Round Robin program. They were chosen because intensive characterization of the materials already existed including shock sensitivity and cyclotetramethylene-tetranitramine (HMX) impurity levels. AFM scans revealed a diversity of surface defects. To quantify the surface defects on the crystalline surface of the RDX particles, surface roughness measurements were acquired. Statistical analysis was undertaken to correlate the observed surface, HMX impurity levels, and DSC thermal curve properties with the known shock sensitivities of the material. It was determined that a statistically significant relationship exists between surface roughness and the shock sensitivity of the material while no relationship was observed between the DSC thermal properties and either surface roughness or shock sensitivity. The HMX content greatly affected the thermal properties of RDX but was uncorrelated with the shock sensitivity.     

Characterisation of roof tile coating degradation using nano-indentation test and surface profilometry

Nano-science and nano-technology have been extensively explored to develop functional and smart coating systems. In this context, it is important to investigate novel characterisation methodologies for surface feature measurement and evaluation of diverse coating systems at nano and micrometrical scales which cannot be implemented by conventional tests. In this study, nano-indentation test have been carried out to characterise the environmental degradation of a polymer coating on a building material substrate. Multiple locations were selected for nano-indention for analysis and evaluation. It has shown that the coating weathering led to a reduction in deformation depth and an increase of hardness and modulus. Furthermore, the weathering resulted in surface quality degradation, which was characterised by hugely reduced surface homogeneity in terms of standard deviations in deformation, hardness and modulus. The coating degradations were also characterised by surface roughness measurement by profilometry. There were significant differences in surface roughness at the micrometrical scale for coatings with varying environmental degradations. They were quantified in terms of surface roughness statistics. The micro morphologies of the coating were observed by SEM which showed a good agreement with results from nano-indentation test and surface roughness measurement. This study demonstrates that nano-indentation test and surface roughness measurement are viable methodologies to evaluate specialised polymer coating systems and to characterise their degradations at the nano and micrometrical scales.     

Functionalization of Biodegradable Polyester for Tissue Engineering Applications

Summary: Chemical modification of polymer surface may potentially be used to create smart materials that can guide cellular adhesion, proliferation and maintenance of specific expression of molecules. The microbial polyester poly (3-hydroxybutyrate) (PHB) has been attracted attention as promising material for applications in tissue engineering. In this work, a wet-chemical method, base ethylenediamine aminolysis, was performed to improve the adhesion of chondrocytes isolated from human articular cartilage to PHB films. The effects of chemical treatment on PHB films was evaluated by following changes in morphology and surface chemical composition using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. While the effect on cells morphology was studied by scanning electron microscopy (SEM). The treatment with ethylenediamine did not change significantly the morphology of the structures of PHB films surface. However, the roughness of the aminolyzed films was slightly higher. The introduction of nitrogen-containing groups was confirmed by XPS. In vitro experiments indicated that the surface modification did not have toxic effects in cells, since they could adhere and proliferate on modified PHB films. It was observed that long-time treatment improved ability of PHB films to support cell growth, which could be accounted to physicochemical and topological effects.     

Adhesion between coating layers based on epoxy and silicone

The adhesion between a silicon tie-coat and epoxy primers, used in marine coating systems, has been studied in this work. Six epoxy coatings (with varying chain lengths of the epoxy resins), some of which have shown problems with adhesion to the tie-coat during service life, have been considered. The experimental investigation includes measurements of the surface tension of the tie-coat and the critical surface tensions of the epoxies, topographic investigation of the surfaces of cured epoxy coatings via atomic force microscopy (AFM), and pull-off tests for investigating the strength of adhesion to the silicon/epoxy systems. Calculations for determining the roughness factor of the six epoxy coatings (based on the AFM topographies) and the theoretical work of adhesion have been carried out. The coating surfaces are also characterized based on the van Oss-Good theory. Previous studies on the modulus of elasticity of the polymers involved have also been considered. It was found that adhesion problems might be due to inadequate wetting, the significantly different topographies, and differences in the mechanical strengths of the epoxies. Acid-base interactions calculated from the van Oss-Good theory were found useful in explaining the enhanced adhesion for some epoxy/silicon surfaces.     

Infrared reflection studies of ceramics: characterization of SiC layers on graphite substrates

Summary Technical CVD-grown silicon carbide (-SiC) layers on graphite substrates have been studied by infrared spectroscopy. Specular reflectance spectra were measured at oblique incidence, and for the same experimental conditions such spectra were simulated starting from oscillator parameters which basically determine the optical behaviour of the material. Both the roughness of the surface and the roughness of the interface within the coated material, have considerable influence on the spectra. Modelling these zones as gradient layers on the basis of effective-medium approximations, convincing agreement between experiment and simulation can be obtained. In this way the effective dielectric function of the layer, as well as its thickness, are derived. Certain features within the reststrahlen band can be assigned to the excitation of surface phonons which strongly depend on the morphology of the layer.     

Scanning Probe Microscopy and Spectroscopy of CVD Diamond Films

 The surface morphology and electronic properties of as-deposited CVD diamond films and the diamond films which have been subjected to boron ion implantation or hydrogen plasma etching have been systematically studied by high resolution scanning probe microscopy and spectroscopy techniques. AFM and STM image observations have shown that (a) both the as-deposited CVD diamond films and the boron ion implanted films exhibit similar hillock morphologies on (100) crystal faces and these surface features are formed during the deposition process; (b) boron ion implantation does not cause a discernible increase in surface roughness; (c) atomic flatness can be achieved on crystal faces by hydrogen plasma etching of the film surface. Scanning tunnelling spectroscopy analysis has indicated that (a) the as-deposited diamond films and the hydrogen plasma etched diamond films possess typical p-type semiconductor surface electronic properties; (b) the as-deposited diamond films subjected to boron implantation exhibit surface electronic properties which change from p-type semiconducting behaviour to metallic behaviour; (c) the damage in the boron implanted diamond films is restricted to the surface layers since the electronic properties revert to p-type on depth profiling.     

Scanning force microscopy experiments probing micromechanical properties on polymer surfaces using harmonically modulated friction techniques II. Investigations of heterogeneous systems

Applying a high-frequency lateral vibration between tip and sample in a scanning force microscope (SFM), a harmonically modulated lateral (friction) force image can be obtained using lock-in techniques. Harmonically modulated lateral force microscopy (HM-LFM) offers several advantages compared with standard lateral force microscopy (LFM). After a brief investigation of the scan velocity dependence of LFM and HM-LFM, two samples were investigated. First, the surface of a poly(acrylonitrile-co-styrene)/polybutadiene blend (ABS) was used to demonstrate the ability of the new technique to decrease the stick effects of the SFM tip. Second, an interface between two chemically very similar polymers was prepared by melting polypropylene (PP) and poly(propene-block-ethene) (PP-block-PE) films on each other. After cutting, the surface roughness of this sample was very high. It is shown that only HM-LFM clearly resolves the local micromechanical properties without artefacts.     

Characterization of Plasma Polymerized Hexamethyldisiloxane Films Prepared by Arc Discharge

Herein, we present a simple method for fabricating plasma polymerized hexamethyldisiloxane films (pp-HMDSO) possessing superhydrophobic characteristics via arc discharge. The pp-HMDSO films were deposited on a soda–lime–silica float glass using HMDSO monomer vapor as a precursor. A detailed surface characterization was performed using scanning electron microscopy and atomic force microscopy. The growth process of the pp-HMDSO films was investigated as a function of deposition time from 30 to 300 s. The non-wetting characteristics of the pp-HMDSO films were evaluated by means of contact angle (CA) measurements and correlated with the morphological characteristics, as obtained from microscopy measurements. The deposited films were found to be nano-structured and exhibited dual-scale roughness with the static CA values close to 170°. Fourier transform infrared spectroscopy analysis was carried out to investigate chemical and functional properties of these films. Methyl groups were identified spectroscopically to be present within the pp-HMDSO films and were proposed to result in the low surface energy of material. The synergy between the dual-scale roughness and low surface energy resulted in the superhydrophobic characteristics of the pp-HMDSO films. A possible mechanism for the pp-HMDSO film formation is proposed.     

Roughness assessment and wetting behavior of fluorocarbon surfaces

The wetting behavior of fluorocarbon materials has been studied with the aim of assessing the influence of the surface chemical composition and surface roughness on the water advancing and receding contact angles. Diamond like carbon and two fluorocarbon materials with different fluorine content have been prepared by plasma enhanced chemical vapor deposition and characterized by X-ray photoemission, Raman and FT-IR spectroscopies. Very rough surfaces have been obtained by deposition of thin films of these materials on polymer substrates previously subjected to plasma etching to increase their roughness. A direct correlation has been found between roughness and water contact angles while a superhydrophobic behavior (i.e., water contact angles higher than 150° and relatively low adhesion energy) was found for the films with the highest fluorine content deposited on very rough substrates. A critical evaluation of the methods currently used to assess the roughness of these surfaces by atomic force microscopy (AFM) has evidenced that calculated RMS roughness values and actual surface areas are quite dependent on both the scale of observation and image resolution. A critical discussion is carried out about the application of the Wenzel model to account for the wetting behavior of this type of surfaces.