Anisotropic Surface Roughness Research Articles

Anisotropic resistivity of thin films due to quantum electron scattering from anisotropic surface roughness

Platinum thin films with thickness in the range of 0.4–12nm were formed by filtered vacuum arc plasma deposition on a substrate with anisotropic roughness. The electrical resistivity was measured in directions parallel and perpendicular to the surface modulation as a function of film thickness, and the resistivity was found to be anisotropic with the degree of anisotropy increasing with decreasing film thickness. The very small thickness of the films calls for a quantum model for film resistivity, and it is shown that the measured resistivity can be interpreted as due to quantum electron scattering by the thin film anisotropic surface roughness.

Relating microwave backscatter azimuth modulation to surface properties of the Greenland ice sheet

AbstractAzimuth dependence of a normalized radar cross-section (σº) over the Greenland ice sheet is modeled with a simple surface scattering model. The model assumes that azimuth anisotropy in surface roughness at scales of 3–300 m is the primary mechanism driving the modulation. To evaluate the contribution of azimuth anisotropy in surface roughness to the radar backscatter, the model is compared to models based on isotropic surface roughness. The models are inverted to estimate snow surface properties using σº measurements from the C-band European Remote-sensing Satellite advanced microwave instrument in scatterometer mode. Results indicate that the largest mesoscale rms surface slopes are found in the lower portions of the dry snow zone. Estimates of the preferential direction in surface roughness are highly correlated with katabatic wind fields over Greenland, which is consistent with wind-formed sastrugi as the dominant mechanism causing azimuth modulation of σº. The maximum improvement of the azimuth modulation surface model compared to its isotropic counterparts occurs in the lower regions of the dry snow zone where the azimuth variability of σº is the largest. In regions with azimuth modulation over 1 dB, the mean root-mean-square error estimate of the azimuth-dependent surface scattering model is 0.46 dB compared with 0.70 dB for similar models using isotropic roughness.

Measuring and Quantifying the Surface-Roughness Anisotropy of Modeled and Industrially Produced Surfaces

‘Three-dimensional surface profilometry’ when used for analysis and product specification reports roughness parameters that provide an average surface description over a relatively large area. Many commercial sheet steels are produced with special textured surfaces for tribological benefits or appearance benefits. These surfaces, as well as others, may demonstrate high levels of roughness anisotropy that is not quantifiable by simple three dimensional surface parameters. This anisotropy can play an important role in the surface appearance of the finished product and in the tribological behaviour during forming. The current work presents a method for quantifying surface-roughness features as a function of angular orientation with respect to rolling direction. The measurement methodology was applied to several model surfaces and one industrially produced electron-beam textured-surface (EBT). This methodology extracts multiple surface-height profiles of the same angular orientation from a single surface and calculates an average roughness parameter for the orientation angle based on the multiple profiles. Particularly interesting results were the large number of profiles necessary to obtain repeatable values for the roughness variation with respect to direction and the strong influence of surface feature size on the repeatability of said results. These results indicate that care must be taken when using a single extracted profile to represent a ‘three-dimensional’ surface.

Effect of Rolling Texture on the Anisotropy of Surface Roughness in the Single-Point Diamond Turning of Polycrystalline Aluminum(Ultra-precision machining)

Effect of Rolling Texture on the Anisotropy of Surface Roughness in the Single-Point Diamond Turning of Polycrystalline Aluminum(Ultra-precision machining)

Influence of Lubricant Additive and Surface Texture on the Sliding Friction Characteristics of Steel under Varying Speeds Ranging from Ultralow to Moderate

A newly developed tribometer that undergoes significant changes in sliding speed, ranging from ultralow (5 μm/s) to moderate (17 cm/s), was used to study the lubricated friction characteristic of steel. In this study, the friction characteristics of stearic acid-formulated oil were studied to clarify the effects of surface roughness or surface roughness texture on friction. Several kinds of specimens having isotropic and anisotropic surface roughness with different textures were used. For an isotropic surface, a rougher surface resulted in low friction under low-speed conditions. The same surface produced high friction under high-speed conditions, where macroscopic hydrodynamic action was predominant. Remarkably less friction was observed in the transverse than in the longitudinal direction when the specimen had anisotropic roughness. This difference was particularly notable under ultra-low-speed conditions. Two other parameters of skewness and kurtosis of roughness distributions show that low friction was obtained when surface roughness distribution approached normal. It appears that the low friction observed with a rough surface or a transverse roughness direction could be explained by the microscopic hydrodynamic action of fluid together with the lubricity of the adsorbed molecular layer.

Anisotropic characterization of rock fracture surfaces subjected to profile analysis

The mechanical parameters of a rock fracture are dependent on its surface roughness anisotropy. In this Letter, we show how quantitatively describe the anisotropy of a rock fracture surface. A parameter, referred to as the index for the accumulation power spectral density psd ∗ , is proposed to characterize the anisotropy of a rock fracture surface. Variation of psd ∗ , with orientation angle θ of sampling, is also discussed.

Anisotropy of surface roughness in diamond turning of brittle single crystals

This paper deals with an investigation of the effect of crystallographic orientation and process parameters on the surface roughness of brittle silicon single crystals in ultraprecision diamond turning. The process parameters involve the depth of cut, feed rate, and spindle speed. Experimental results indicate that anisotropy in surface finish occurs when the cutting direction relative to the crystal orientation varies. There exists a periodic variation of surface roughness per workpiece revolution, which is closely related to the crystallographic orientation of the crystals being cut. Such an anisotropy of surface roughness can be minimized with an appropriate selection of the feed rate, spindle speed, and depth of cut. The findings provide a means for the optimization of the surface quality in diamond turning of brittle silicon single crystals.

Anisotropy of Two-Dimensional Electron and Hole Mobilities in (0 0 1) GaAs/Al x Ga 1− x As Heterostructures Under Uniaxial Stress

Transport properties of two-dimensional electron and hole gas at (0 0 1) GaAs/Al x Ga 1 m x As heterointerface in [1 1 0] and [1 m 1 0] directions have been investigated for the first time under in plane uniaxial compression up to 5 kbar. Resistivity, Shubnikov-de Haas oscillations and Hall effect were measured and carrier densities and mobilities were determined. The in-plane uniaxial compression significantly modifies the energy spectrum of p-GaAs/Al x Ga 1 m x As that leads to strong anisotropy of hole mobility under compression. In the case of the n-type heterostructure uniaxial compression causes only change of the carrier concentration and the corresponding small change of the initial mobility anisotropy, determined by anisotropic surface roughness scattering.

Grazing incidence X-ray reflectivity of coumarin side-chain polymers used for liquid crystal photoalignment layers

Specular X-ray reflectivity measurements are used to examine photoinduced surface roughness of side-chain coumarin polymers that form liquid crystal photoalignment layers. Thin films were irradiated with polarised or unpolarised ultraviolet light and the X-ray reflectivity at grazing incidence was measured for samples irradiated with different ultraviolet fluences. Kiessig fringes are observed over a large dynamic range and a comparison of the measured and calculated reflectivity gives a value for the surface roughness. The photoinduced roughness increases with ultraviolet fluence, from 5.5 to 20 Å. Negligible photoinduced roughness is detected for a film, which was previously irradiated using conditions that give strong planar liquid crystal alignment. Specular reflectivity data taken with the plane of incidence of the X-ray parallel and perpendicular to the polarisation direction of the ultraviolet light are identical within experimental uncertainty. Hence in-plane anisotropic surface roughness is not observed.

Anisotropic surface morphology of azopolymer films generated by polarized UV light irradiation

X-ray reflectivity measurements reveal anisotropy of the vertical surface roughness caused by exposure to linearly polarized UV (LPUV) light in the films of two azopolymers. The irradiated surface is found to have higher roughness in the direction parallel to the direction of polarization than in the orthogonal direction. The photo-modification of the surface morphology is caused by spatial changes induced in polymer films by LPUV irradiation. The important role of surface roughness anisotropy in determining the alignment of liquid crystals is discussed.

Anisotropy of Surface Roughness on Aluminium Sheet Studied by Atomic Force Microscopy.

Atomic force microscope images of aluminium sheet are used to calculate the mean roughness Ra and the root mean square roughness Rq for different directions (theta) across the samples. The angle determined values of Ra(theta) and Rq(theta) are significantly lower in the rolling direction than in any other direction. The results provide a clear demonstration of the vector nature of surface roughness. Local features in an angle determined Ra(theta) profile are also compared with the positions of peaks and valleys in the matching topographical cross-section of the surface running at right angles to theta. It is further found that the distribution of z heights for an imaged sample is usually not Gaussian in shape unless the z heights are measured about mean lines constructed across the surface parallel with the rolling direction. The results are relevant for controlling lubrication performance in the aluminium rolling process.

Surface roughness in InGaAs channels of high electron mobility transistors depending on the growth temperature: Strain induced or due to alloy decomposition

InAlAs/InGaAs/InP based high electron mobility transistor devices have been structurally and electrically characterized, using transmission electron microscopy and Raman spectroscopy and measuring Hall mobilities. The InGaAs lattice matched channels, with an In molar fraction of 53%, grown at temperatures lower than 530 °C exhibit alloy decomposition driving an anisotropic InGaAs surface roughness oriented along [11̄0]. Conversely, lattice mismatched channels with an In molar fraction of 75% do not present this lateral decomposition but a strain induced roughness, with higher strength as the channel growth temperature increases beyond 490 °C. In both cases the presence of the roughness implies low and anisotropic Hall mobilities of the two dimensional electron gas.

Si/SiGe quantum wells grown on vicinal Si(001) substrates: Morphology, dislocation dynamics, and transport properties

Compositionally graded, strain relaxed Si0.72Ge0.28 buffers were grown on vicinal Si(001) substrates by gas source molecular beam epitaxy. Misfit dislocations are shown to run along intersections of the {111} glide planes with the (11n) interface. X-ray diffraction studies demonstrate a relative tilt of the epilayer to the substrate in a direction which depends on the interplay between substrate orientation related preferential dislocation nucleation rates and surface contamination induced heterogeneous nucleation. Atomic force microscopy (AFM) images reveal an anisotropy in surface roughness on the μm scale related to reduced growth rates on vicinal surfaces. Transport properties at 0.4 K in two dimensional electron gases grown on these relaxed SiGe buffers show anisotropic scattering times similar to interface roughness scattering which can be correlated to terrace configurations in the nm range determined by AFM.

Micro/nanotribology using atomic force microscopy/friction force microscopy: State of the art

Atomic force microscopy/friction force microscopy (AFM/FFM) techniques are increasingly used for tribological studies of engineering surfaces at scales ranging from atomic and molecular to microscales. These techniques have been used to study surface roughness, adhesion, friction, scratching/wear, indentation, detection of material transfer and boundary lubrication and for nanofabrication/nanomachining purposes. Micro/nanotribological studies of materials of scientific and engineering interest have been conducted. Commonly measured roughness parameters are found to be scale dependent, requiring the need of scale-independent fractal parameters to characterize surface roughness. Measurement of atomic-scale friction of a freshly cleaved highly orientated pyrolytic graphite exhibited the same periodicity as that of corresponding topography. However, the peaks in friction and those in corresponding topography were displaced relative to each other. Variations in atomic-scale friction and the observed displacement have been explained by the variations in interatomic forces in the normal and lateral directions. Local variation in microscale friction is found to correspond to the local slope, suggesting that a ratchet mechanism is responsible for this variation. Directionality in the friction is observed on both micro- and macroscales which results from the surface preparation and anisotropy in surface roughness. Microscale friction is generally found to be smaller than macroscale friction as there is less ploughing contribution in microscale measurements. Microscale friction is load dependent and friction values increase with an increase in the normal load, approaching the macrofriction at contact stresses higher than the hardness of the softer material. The wear rate for single-crystal silicon is negligible below 20 μN and is much higher and remains approximately constant at higher loads. Elastic deformation at low loads is responsible for negligible wear. The mechanism of material removal on a microscale is studied. At the loads used in the study, material is removed by the ploughing mode in a brittle manner without much plastic deformation. Most of the wear debris is loose. Evolution of the wear has also been studied using AFM. Wear is found to be initiated at nanoscratches. AFM has been modified to obtain load-displacement curves and for measurement of nanoindentation hardness and Young's modulus of elasticity, with the depth of indentation as low as 1 nm. Hardness of ceramics on the nanoscale is found to be higher than that on the microscale. Ceramics exhibit significant plasticity and creep on the nanoscale. Scratching and indentation on nanoscales are powerful ways to screen for adhesion and resistance to deformation of ultra-thin films. Detection of material transfer on the nanoscale is possible with AFM. Boundary lubrication studies and measurement of lubricant-film thickness with a lateral resolution on a nanoscale have been conducted using AFM. Self-assembled monolayers and chemically bonded lubricant films with a mobile fraction are superior in wear resistance. Friction and wear on micro- and nanoscales at low loads have been found to be generally smaller compared to that at macroscales. Therefore, micro/nanotribological studies may help define the regimes for ultra-low friction and near-zero wear.

Microtribological studies of doped single-crystal silicon and polysilicon films for MEMS devices

Microelectromechanical systems (MEMS) devices are made of doped single-crystal silicon, LPCVD polysilicon films and other ceramic films. Very little is understood about tribology and mechanical characterization of these materials on micro- to nanoscales. Atomic force microscopy/friction force microscopy (AFM/FFM) have been used to study surface roughness, friction, scratching/wear and indentation hardness, and Young's modulus of elasticity of p-type (lightly boron-doped) single-crystal silicon (referred to as ‘undoped’), p +-type (boron-doped) single-crystal silicon, polysilicon, and n +-type (phosphorus-doped) LPCVD polysilicon films. Undoped silicon samples are very smooth (r.m.s. = 0.08 nm). Surface-roughness values for p +-type silicon and lapped polysilicon bulk samples are comparable. As-deposited polysilicon films are rougher than undoped and doped single-crystal silicon, unlapped and lapped polysilicon bulk samples. Microscale friction measurements show that the friction values for undoped silicon, p +-type silicon and lapped polysilicon bulk samples are comparable. Polysilicon films exhibit the highest coefficient of friction as compared to other samples, due to the roughness effect. Macroscale friction values for undoped silicon and polysilicon bulk are comparable with those of p +-type silicon sample having the highest value and polysilicon films having the lowest values. The microscale friction values for all samples are lower than the macroscale friction values as there is less ploughing contribution in microscale friction measurements. Local variation in microscale friction is found generally to correspond to the local surface slope. Directionality in the friction is generally observed on the microscale which results from the surface preparation and anisotropy in surface roughness. Microscratching/wear studies indicate that scratch and wear resistance of p +-type silicon is poor as compared to other samples used in this study. Nanoindentation studies indicate that the p +-type silicon sample exhibits lowest hardness and Young's modulus as compared to the other samples, which is responsible for high macrofriction and poor microscratch and microwear behavior of the p +-type silicon sample. Microtribological behavior and nanohardness and Young's modulus values for polysilicon bulk and n +-type polysilicon film are comparable.

Structural anisotropy of normal fault surfaces

Precise description of natural fault surfaces is indispensable to understanding the geometry, mechanics and fluid transport properties of faults. Profiles of fault surfaces in the Wasatch fault zone and Oquirrh Mountains, Utah, are measured at 30° increments within the fault plane to determine the directional anisotropy of surface roughness at wavelengths between 10 −3 m and 30 m, and then compared with profiles of larger-scale fault surfaces. Surface anisotropy and an increasing ratio of surface amplitude to wavelength are consistent with self-affine fault topography at wavelengths between 1 mm and approximately 5 km. Fractal dimension of surface profiles generally decreases systematically as the angle to the slip direction increases. Directional anisotropy is described by an azimuthal scaling function γ φ = K sin( φ) + γ 0 or AF φ = ( AF max −1) sin( φ) + 1, where γ φ and AF φ are the amplitude to wavelength ratio and anisotropy factor respectively at azimuth φ, measured clockwise relative to slip direction within the fault surface, and γ 0 is the amplitude to wavelength ratio parallel to slip direction. K = ( γ 90 − γ 0) is an anisotropy coefficient and increases systematically with spatial wavelength on the fault surface. Characterization of natural fault surfaces provides parameters such as fractal dimension ( D), intercept (log( C)) of power spectra, profile variance, and variation in anisotropy factor ( AF), which are needed to generate fractal models of natural fault surfaces using spectral synthesis. We generate sample models which illustrate the differences between fault surfaces characterized by constant versus azimuthally varying fractal dimension. The latter model surfaces contain low amplitude corrugations superimposed on elongate ridges which parallel slip direction. This surface texture resembles that of natural fault surfaces that refract across lithologic layering or are cut by secondary faults such as R and R′ shears.

The anisotropy of surface roughness measured using digital photogrammetric technique : M. W. Jessell, S. J. D. Cox, P. Schwarze & W. L. Power, in: Fractography: fracture topography as a tool in fracture mechanics and stress analysis, ed M.S. Ameen, (Geological Society, London; Special Publication, 92), 1995, pp 27–37

The anisotropy of surface roughness measured using digital photogrammetric technique : M. W. Jessell, S. J. D. Cox, P. Schwarze & W. L. Power, in: Fractography: fracture topography as a tool in fracture mechanics and stress analysis, ed M.S. Ameen, (Geological Society, London; Special Publication, 92), 1995, pp 27–37

The anisotropy of surface roughness measured using a digital photogrammetric technique

The anisotropy of surface roughness measured using a digital photogrammetric technique

The anisotropy of surface roughness measured using a digital photogrammetric technique

The anisotropy of surface roughness measured using a digital photogrammetric technique

Anisotropic Surface Roughness in Strain Relaxed In0.40GA0.60As on Gaas with a Step-Graded InxGA1-xAs Buffer Layer

AbstractWe report the structural characterization of the 3-D relaxation morphology of In0.4Ga0.6As grown on a step-graded InxGa1-xAs buffer layer on GaAs. Scanning electron microscopy showed “grooves” spaced on the order of microns running only in the [110] direction. Each groove was observed with cross-sectional transmission electron microscopy to mark the location of a vertical low-angle tilt and/or twist boundary. The veiy rough layer morphology may be the result of island coalescence or severe surface roughness that created the grain boudnaries as the layer grew. Strain relaxation in the In0.4Ga0.6As layer was much reduced in the [101] in-plane direction. The asymmetry in residual in-plane strains in the In0.3Ga0.7AS layer and/or the increased In composition may be responsible for the development of an anisotropic surface roughness. X-ray microanalysis revealed a periodic variation in layer composition which correlated with a fine contrast modulation presumably the result of phase segregation.