Areal Surface Topography Research Articles

Characterizing the as-built surface topography of Inconel 718 specimens as a function of laser powder bed fusion process parameters

Purpose The ability to use laser powder bed fusion (LPBF) to print parts with tailored surface topography could reduce the need for costly post-processing. However, characterizing the as-built surface topography as a function of process parameters is crucial to establishing linkages between process parameters and surface topography and is currently not well understood. The purpose of this study is to measure the effect of different LPBF process parameters on the as-built surface topography of Inconel 718 parts. Design/methodology/approach Inconel 718 truncheon specimens with different process parameters, including single- and double contour laser pass, laser power, laser scan speed, build orientation and characterize their as-built surface topography using deterministic and areal surface topography parameters are printed. The effect of both individual process parameters, as well as their interactions, on the as-built surface topography are evaluated and linked to the underlying physics, informed by surface topography data. Findings Deterministic surface topography parameters are more suitable than areal surface topography parameters to characterize the distinct features of the as-built surfaces that result from LPBF. The as-built surface topography is strongly dependent on the built orientation and is dominated by the staircase effect for shallow orientations and partially fused metal powder particles for steep orientations. Laser power and laser scan speed have a combined effect on the as-built surface topography, even when maintaining constant laser energy density. Originality/value This work addresses two knowledge gaps. (i) It introduces deterministic instead of areal surface topography parameters to unambiguously characterize the as-built LPBF surfaces. (ii) It provides a methodical study of the as-built surface topography as a function of individual LPBF process parameters and their interaction effects.

Correlational study of multiscale analysis and the metrological characteristics of areal surface topography measuring instruments

With the metrological characteristics, an effective framework for the calibration, adjustment, performance specification and verification of areal surface topography measuring instruments has been introduced. The characteristics have been thoroughly researched and analyzed for their different applications. Each characteristic provides specific information for a certain axis or feature of the measuring instrument. This enables an uncertainty estimation and a comparability of different measuring instruments. In this study, the comparison results of metrological characteristics are correlated with information obtained from multiscale analysis. With this examination it can be shown that both, metrological characteristics and multiscale analysis include significant information about transfer behaviors of surface structures, which can be applied advantageously for a calibration, performance verification and uncertainty determination of areal surface topography measuring instruments.

Quantitative Evaluation of the Batwing Effect on Depth Measurement in Coherence Scanning Interferometry for Rectangular Gratings

Coherence scanning interferometry (CSI) is a widely used non-contact method for measuring areal surface topography. The calculation of groove depth typically employs the ‘W/3 rule’ specified in ISO 5436-1. However, the batwing effect causes overshoots near the groove edges, which can introduce noise singularities in the selected W/3 region for depth calculation, thereby affecting the measurement of rectangular grating depth. This paper introduces the definition of batwing height and width and proposes a simulation model that considers various factors, such as the center wavelength, spectrum of the illuminating light, shadow effect, and numerical aperture, to analyze their influence on depth measurement. The simulation results demonstrate that the batwing width is the primary factor influencing depth measurement for small grating periods. Specifically, a decrease in numerical aperture increases the batwing width, leading to larger depth measurement errors, while a decrease in the center wavelength reduces the batwing width, resulting in smaller depth measurement errors. The influence of the illumination spectrum and shadow effect on depth measurement is found to be minor. Experimental validation using a step standard consisting of rectangular gratings with different periods and depths confirms the agreement between the experimental and simulated results. The proposed method provides a quantitative evaluation of depth measurement accuracy in CSI.

Comprehensive profile and areal calibration by additively manufactured material measures

Abstract The calibration of surface texture measuring instruments is standardized with two distinct types of material measures. ISO 25178-70 categorizes material measures that feature a profile and an areal surface topography. The result is that different types of measuring instruments like profilers on the one hand and areal surface topography measuring instruments on the other hand may require different material measures whose scope of application may be limited to only one of the named instrument types. The reason is that most manufacturing principles allow either a linear or circular extrusion of a geometry limiting the possibilities to manufacture material measures that are suitable for both, profile and areal surface topography measuring instruments. Since a comparability is desired for as many different measuring instruments as possible, we examine to what degree profile and areal material measures of ISO 25178-70 can be adapted and combined to possibly allow a calibration of all types of surface topography measuring instruments. Additive manufacturing with direct laser writing (DLW) is characterized by a high degree of freedom in the design of material measures. An enhancement of structures that can be imaged either in multiple lateral directions or extruded to circular geometries is possible, allowing both, a profile sampling in different directions, just as well as an areal measurement. In the present publication, a modification of the ISO 25178-70 material measures is described including the design process, the manufacturing and the measurement with areal and profile surface topography measuring instruments to practically demonstrate the feasibility of a multifunctional calibration that considers the possible effects of directionality. We show that it is possible to combine different profile and areal geometries by linear and/or areal extrusion of the corresponding profile-based geometry. By aligning multiple material measures onto one sample, it can also be demonstrated that a comprehensive calibration of an optical profiler is enabled with only one measurement.

Generating synthetic as-built additive manufacturing surface topography using progressive growing generative adversarial networks

Numerically generating synthetic surface topography that closely resembles the features and characteristics of experimental surface topography measurements reduces the need to perform these intricate and costly measurements. However, existing algorithms to numerically generated surface topography are not well-suited to create the specific characteristics and geometric features of as-built surfaces that result from laser powder bed fusion (LPBF), such as partially melted metal particles, porosity, laser scan lines, and balling. Thus, we present a method to generate synthetic as-built LPBF surface topography maps using a progressively growing generative adversarial network. We qualitatively and quantitatively demonstrate good agreement between synthetic and experimental as-built LPBF surface topography maps using areal and deterministic surface topography parameters, radially averaged power spectral density, and material ratio curves. The ability to accurately generate synthetic as-built LPBF surface topography maps reduces the experimental burden of performing a large number of surface topography measurements. Furthermore, it facilitates combining experimental measurements with synthetic surface topography maps to create large data-sets that facilitate, e.g. relating as-built surface topography to LPBF process parameters, or implementing digital surface twins to monitor complex end-use LPBF parts, amongst other applications.

Characterization of the Maximum Height of a Surface Texture.

Average surface height and maximum amplitude can affect surface functions. In the industry, these parameters can be obtained based on profile measurements. However, variability in maximum profile height is high. A more stable parameter can be obtained from the results of the areal surface topography measurements as the average value of the parallel profiles. The aim of this study is to establish this parameter directly from the result of the areal surface texture by correcting the maximum surface height to material ratios in the range of 0.13-99.87%. This method was tested by measuring 100 surface topographies with a stylus profilometer and a white light interferometer. It can be utilized correctly for deterministic textures and random one- and two-process surfaces for which the correlation between neighboring profile ordinates is not very high. In other cases, the method should be modified. Employing this method, the maximum profile amplitude Pt and parameters characterizing the average profile height Pq, Pa, and the ratios Pq/Pa and Pp/Pt describing the shape of the profile ordinate distribution can be correctly estimated. Pq/Pa and Pp/Pt were more stable than the kurtosis Pku and skewness Psk. The corrected maximum height S±3σ can be adopted as a parameter that characterizes the areal surface texture as more stable than the maximum surface height St. Pq/Pa and Pp/Pt were more steady than kurtosis Pku and skewness Psk.

INVESTIGATION OF 3D SURFACE TOPOGRAPHY FEATURES ON THE POLYCRYSTALLINE DIAMOND TURNING OF ALUMINA

The product’s surface quality and service performance depend on the surface integrity features formed by the machining process. Surface integrity consists of many features including surface topography and others. In this work, 3D (areal) surface topography features, particularly height variations, were investigated on the polycrystalline diamond (PCD) turning of alumina. Variations of the areal unevenness and height distribution parameters, namely arithmetic mean deviation (Sa), maximum peak height (Sz), skewness (Ssk) and kurtosis (Sku), with respect to turning parameters such as spindle speed (1000, 2000 n 3000 rpm), feed rate (0.05, 0.075 & 0.1 mm/rev) and depth of cut (1, 3 & 5 [Formula: see text]m) were studied. The Taguchi technique was carried out in this study based on the L9 standard orthogonal array for optimizing the turning parameters for the generation of better topography features. The error analysis was done with the experimental results and the error variations were noticed to be less than 10%. Based on the experimental results, the best surface figure combination of Sa 0.67 [Formula: see text]m, Sz 3.18 [Formula: see text]m, Ssk 0.0039 and Sku 3.06 was found at the employment of spindle speed of 1500 rpm, feed rate of 0.1 mm/rev, and depth of cut of 1 [Formula: see text]m. The distribution of peaks and valleys formed over the PCD-turned alumina surfaces may influence the different functional properties such as fatigue, friction, wear, etc. Besides, the results were examined through means of responses and analysis of variance (ANOVA). The test results also reported that the depth of cut outperforms other hard turning parameters over the surface topography features of the turned alumina ceramics. At a lower depth of cut, the cutting tool is involved in the shearing action which causes the stable material removal of alumina ceramics through an appreciable chip formation.

Influence of the steel plate on the friction behavior of automotive wet disk clutches

AbstractFriction behavior is the key parameter for the design of automotive wet disk clutches. On the one hand, the Coefficient of Friction (CoF) level should be high to transmit torque efficiently. On the other hand, the clutch requires a positive slope of the CoF over sliding speed for good controllability, high comfort, and a low tendency to shudder. Clutches used in automatic or dual-clutch transmissions mostly use organic friction lining. These friction systems tend to have low CoF at low sliding speeds due to their high requirements regarding shifting comfort. Nevertheless, they show high values of CoF at high sliding speeds.This study investigates the influence of different steel plate finishes on friction behavior in different application-relevant operation modes such as brake shift, unsteady slip, and micro slip. Each of these operation modes requires an accurate CoF measurement at different sliding speed ranges. Therefore, we use different test rig setups. We characterize the steel plates by their areal surface topography measured with an optical system using focus variation. We discuss differences in the friction behavior of the corresponding tribological systems at different operating conditions.Results show an influence of the steel plate surface finish on the CoF level. Therefore, the surface finish of the steel plate influences the functional behavior of wet disk clutches and engineers should consider the surface finish in the early design phase. We discuss the correlation between commonly used surface parameters and friction parameters.

Characterization and filtering of profile and areal surface topography by combining the discrete Legendre and cosine transforms

In the standardized processing of surface topography data, the form removal and filtering operations are clearly separated. This is reflected in the current ISO standards concerning profile surface texture and areal surface texture, the ISO 21920 and the ISO 25178 series respectively. When the scale-limited surface texture is significant compared to the form to be removed, for example with additive manufactured surfaces, the dependence of the surface Fourier spectrum on the removed form and orientation may become significant. This may lead to interaction between the form removal and filtering operations. To counter this interaction, in this paper, the lower-order discrete Legendre polynomials that describe the form are combined with cosine functions that describe the surface texture. This set of base functions is orthonormalized using a Gram-Schmidt procedure. This results in a set of orthonormal functions that allow an independent parameterization of both form and texture. The concept and the related theory are given and illustrated using examples of filtering profiles and areal topographies, description of cylinders and treatment of missing data. The examples show that the concept as presented in this paper is useful for filtering surfaces with a dominant form and can be used in the parametrization of surfaces and cylindrical geometries. Also, the methods presented here can be used for filtering and parametrization in the case of missing points in the data, actual holes in the profile or non-rectangular surfaces.

Characterization of measurement and instrument noise in areal surface topography measurements by the Allan deviation

Measurement and instrument noise as proposed in ISO 25178-600 and defined in ISO 25178-700 are assumed to be both stationary and randomly distributed. This paper presents the temporal and lateral Allan deviation as useful tools for deeper noise analysis in surface topography measurements. The temporal Allan deviation may be used to identify and qualify drift and spikes in the mean topography, which makes averaging over longer times no longer beneficial to reduce measurement noise. The lateral Allan deviation may reveal the application of implicit and/or explicit filtering or other correlations on surface topography measurements.

Feature-Based Characterisation of Turned Surface Topography with Suppression of High-Frequency Measurement Errors.

Errors that occur when surface topography is measured and analysed can be classified depending on the type of surface studied. Many types of surface topographies are considered when frequency-based errors are studied. However, turned surface topography is not comprehensively studied when data processing errors caused by false estimation (definition and suppression) of selected surface features (form or noise) are analysed. In the present work, the effects of the application of various methods (regular Gaussian regression, robust Gaussian regression, and spline and fast Fourier Transform filters) for the suppression of high-frequency measurement noise from the raw measured data of turned surface topography are presented and compared. The influence and usage of commonly used available commercial software, e.g., autocorrelation function, power spectral density, and texture direction, which function on the values of areal surface topography parameters from selected (ISO 25178) standards, are also introduced. Analysed surfaces were measured with a stylus or via non-contact (optical-white light interferometry) methods. It was found that the characterisation of surface topography, based on the analysis of selected features, can be crucial in reducing measurement and data analysis errors when various filters are applied. Moreover, the application of common functions can be advantageous when feature-based studies are proposed for both profile and areal data processing.

Hybrid surface characterisation of intra thin-walled Ti6Al4V surfaces produced by laser powder bed fusion technology

The proposed work investigates the hybrid surface characterisation of intra thin-walled Ti6Al4V surfaces fabricated using laser powder bed fusion technology. The thin-walled samples were characterised using scanning electron microscopy and Opto-digital microscopy techniques. The fractal dimensional analysis was performed using ImageJ software integrated with an open-source MultiFrac plug-in. The surface microscopy analysis revealed satellites powder particles, partially melted powder particles, spherical balling, and pores on the thin-walled surface. The fractal dimension establishes a correlation between the surface roughness values. The surface areal surface parameters analysis suggested variation along the build direction of the thin-walled Ti6Al4V sample. The development of sharp peaks and thus higher Ra, Sku and Ssk values were found along the build direction of the intra thin-walled samples. Therefore, the combination of areal surface topography analysis and fractal dimension approach can be a promising methodology towards surface characterisation of additively manufactured complex thin-walled surfaces.

Honing process parameters influence on surface topographies

Honing is a superfinishing process that generates fine surface topographies with interesting tribological properties. However the control of the process requires highly experienced operators. The selection of proper stones (abrasive grit size and type) is decisive for reaching the target topography. Furthermore, especially in the case of a feed controlled process, the stone expansion must be in adequacy with the selected stone. This paper proposes to investigate the effect of the abrasive grit size for different stone expansions on surface topographies generated during the honing process. Honed samples are produced by a specific test bench reproducing the honing kinematics and recording process induced forces. In total, two hundred forty areal surface topography measurements are analyzed and discussed alongside with the force measurements. A systemic analysis of a wide range of roughness parameters is performed. Results demonstrate that the grit size has a significant impact on the final topography signature. Furthermore, even though expansion speed was doubled, no obvious influence was observed on the surface roughness parameters except one (isotropy Str). Based on these observations it is suggested that the expansion speed has a limited impact on the number of grooves generated.

International comparison of flatness deviation in areal surface topography measurements

An international comparison of surface topography flatness deviation was carried out. The comparison involved twelve optical surface topography instruments (focus variation instruments, confocal microscopes and coherence scanning interferometers) from six European research laboratories. The results demonstrated the following: (i) The flatness varies orders of magnitude between instruments, dependent on the measurement principle. (ii) The ISO proposed procedure gives meaningful results, especially for instruments based on the confocal principle. (iii) The selection of the measurement artefact is especially important for focus variation measurements, where the flatness deviation is influenced by the surface texture of the used artefact. (iv) A tilt in the used optical flat can significantly influence the measured flatness deviation.

Development of a compact focus variation microscopy sensor for on-machine surface topography measurement

On-machine areal surface topography measuring instruments are required for fast and accurate measurement of parts inside production machines without reducing production rates. This paper presents the design and development of a compact focus variation microscopy sensor that can be integrated into various types of machine tools. The paper focuses on the development of the linear stage of the sensor, which was the major engineering challenge. The overall developed sensor has dimensions of 78 mm diameter and 200 mm length, with a 20 mm travel range. Simulations of tolerance stack-ups for the sensor assembly were performed before the manufacturing of the sensor’s linear motion components to assure they can be appropriately assembled. The linear motion accuracy of the sensor is 2 μm, calibrated using laser interferometry. From measurement in a controlled laboratory, the measurement noise of the sensor is 0.4 μm. Finally, demonstrations of calibrated artefact measurements with the sensor are presented. A single image field measurement with the sensor requires less than 20 s.

Analysis of dimensional accuracy for micro-milled areal material measures with kinematic simulation

The calibration of areal surface topography measuring instruments is of high relevance to estimate the measurement uncertainty and to guarantee the traceability of the measurement results. Calibration structures for optical measuring instruments must be sufficiently small to determine the limits of the instruments.Besides other methods, micro-milling is a suitable process for manufacturing areal material measures. For the manufacturing by micro-milling with ball end mills, the tool radius (effective cutter radius) is the corresponding limiting factor: if the tool radius is too large to penetrate the concave profile details without removing the surrounding material, deviations from the target geometry will occur. These deviations can be detected and excluded before experimental manufacturing with the aid of a kinematic simulation.In this study, a kinematic simulation model for the prediction of the dimensional accuracy of micro-milled areal material measures is developed and validated. Subsequently, a radius study is conducted to determine how the tool radius r of the tool influences the dimensional accuracy of an areal crossed sinusoidal (ACS) geometry according to ISO 25178-70 [1] with a defined amplitude d and period length p. The resulting theoretical surface texture parameters are evaluated and compared to the target values. It was shown that the surface texture parameters deviate from the nominal values depending on the effective cutter radius used. Based on the results of the study, it can be determined with which effective tool radius the measurands Sa and Sq of the material measures are best met. The ideal effective radius for the application considered is between 50 and 75 μm.

International comparison of noise in areal surface topography measurements

An international comparison of surface topography measurement noise was carried out. The comparison involved twelve optical surface topography instruments (focus variation instruments, confocal microscopes and coherence scanning interferometers) from six European research laboratories. The purpose of this comparison was to perform a practical test of the ISO definition and procedures for determining measurement noise on a variety of instruments, and to provide good practice guidance to the user community for quantifying, specifying and interpreting measurement noise. Despite taking steps to enable an optimal comparison by supplying a variety of specimens, where the ‘best’ could be selected and the measurement area was clearly defined, an appropriate comparison of the measurement noise beyond the actual measurement conditions was complex, as the measurement time, speed and lateral resolution were difficult to establish for most of the instruments. The results that could be obtained were: (i) the measurment noise N M can be reliably estimated from two measurements, (ii) noise may vary considerably between instruments, even when they have the same measurement principle, (iii) the noise in a practical measurement may be considerably higher than the noise with a levelled flat, and (iv) the ‘rms repeatability’ has little correlation to the noise determined according to the ISO specification standard, and may lead to values that are more than a hundred times smaller.

The instrument transfer function for optical measurements of surface topography

For optical measurements of areal surface topography, the instrument transfer function (ITF) quantifies height response as a function of the lateral spatial frequency content of the surface. The ITF is used widely for optical full-field instruments such as Fizeau interferometers, confocal microscopes, interference microscopes, and fringe projection systems as a more complete way to characterize lateral resolving power than a single number such as the Abbe limit. This paper is a comprehensive review of the ITF, including standardized definitions, ITF prediction using theoretical simulations, common uses, limitations, and evaluation techniques using material measures.

Application of image processing methods for the characterization of selected features and wear analysis in surface topography measurements

In this paper, the different methods of processing of surface topography image (STI) were proposed for the characterization of selected features from the surface texture. The proposed analysis was performed to improve the functionality of the image assessment in the characterization of material contact properties (e.g. wear-resistant, lubricant retention, sealing or friction) according to the results of surface topography measurement. Various type of surfaces from manufactured details was considered, e.g. turned, ground, honed, laser-textured or isotropic. They were measured by the white light interferometer. Results were analyzed with areal surface topography maps (image) characterization. It was found that wavelet filtering approaches might be valuable in assessments of the selected surface texture features, such as dimple (size or distributions) or treatment trace distances, received by an application of STI processing methods. Additionally, proposed in the paper, the image digital extension method might be crucial in the definition of feature size (depth, width), especially when they are located on the edge of analyzed (measured) detail. Moreover, STI might be advantageous in the reduction of some surface texture measurement errors (a selected type of noise).

A novel material measure for characterising two-dimensional instrument transfer functions of areal surface topography measuring instruments

A new material measure for characterising two-dimensional (2D) instrument transfer functions (ITF) of areal surface topography measuring instruments has been designed, manufactured, calibrated and applied. Several innovative ideas are implemented in its design. Firstly, the material measure is designed with circular structure patterns. Such rotational symmetric patterns are advantageous for characterising the ITF in different angular directions, thus for characterising angular-dependent asymmetries of instruments. Secondly, different types of patterns are designed: circular chirp (CC) pattern and circular discreate grating (CDG) pattern. They represent different kinds of spatial signals applicable for characterising ITFs. A material measure consists of 25 different circular patterns with radii from 30 μm to 300 μm, and wavelengths from 0.1 μm to 150 μm. These patterns can be applied complementary and combinedly, offering high application flexibility for calibrating a variety of instruments, e.g. with different optical objectives from 5× to 100× and with different sizes of field of view (FOV). Material measures with heights of 8 nm, 16 nm and 32 nm, respectively, have been manufactured using state-of-the-art e-beam lithography technique. The feature heights are far less than λ/4, thus they are suitable for characterising the ITF of optical tools which can be approximated as linear systems. A metrological large range atomic force microscope (AFM) has been applied in the calibration of the developed material measure, showing good feature quality. The calibrated material measure has been successfully applied in research and industry.