Introduction

Surface roughness measurement is the measurement of the small-scale variations in the height of a physical surface. This contrasts with larger-scale variations such as form and waviness which are typically part of the geometry of the surface.

Surface roughness measurement can be characterized using either quantitative or qualitative methods. Qualitative techniques include optical appearance such as the reflectivity of a surface or the strength of the machining lay as well as dragging a thumbnail across the surface as a crude tactile sensor [1]. Quantitative analysis has evolved from simple two-dimensional profilometry to more advanced three-dimensional area analysis where information regarding surface structure can be easily obtained. In order to quantify a surface profile, it is essential to remove the unnecessary wavelength components and establish a datum from which the parameters can be calculated. Stylus-based surface roughness measurement system is shown in Fig-1.

 

Fig-1 Stylus-based surface roughness measurement [1]

Measurement methods

There are two different methods are known for surface roughness measurement which are contact-based and non-contact-based measurement.

Contact-based measurement: Stylus measurement

The contact method involves the dragging of a diamond stylus whose tip dimension is such that it can penetrate the detailed geometry of the surface. The stylus is mounted onto an arm with a transducer at the other end as shown in Fig-2. Any change in height of the stylus due to the surface features corresponds to a change in the signal detected and amplified by the transducer and the subsequent electronics. The most common type of transducer is based on the inductance principle and offers a large range-to-resolution as well as being of robust construction.

Fig-2 Stylus based measurement [2]

Advantages and disadvantages of contact based:

Contact-based surface roughness measurement is one of the most widely used methods for measuring surface roughness. However, it has its own advantages and disadvantages in measuring the surface roughness profile, the advantages include the clear wave profile; since the stylus tip is in contact with the surface of the specimen the obtained roughness profile is very clear and a replica of the surface. It is also capable of long-distance measurements, which is not possible in the case of optical measurements.

The disadvantages of the contact-based measurement include mainly the stylus tip wear caused due to the contact between the sharp tip of the stylus with the surface roughness. The sample’s surface will be scratched because of the movement of the stylus tip on the surface during the measurement and the measurement is limited to the radius of the stylus tip. This method is not used in measuring roughness on the viscous samples, also it is a very time-consuming process. Initially, measurement requires the proper positioning of the stylus on the suitable measuring point which is difficult in this method, and sometimes it also requires cutting the samples for tracing by the detector.

Non-contact measurement: Optical Interferometry

Optical interferometers work on the principle of exposing the surface to be characterized to monochromatic or white light and observing the interference fringes produced using an optical flat tilted through a small angle. The fringe patterns are produced by splitting the light beam and the interference patterns are produced due to interference between reflections from the tilted optical flat and the surface to be measured. The fringe patterns are analyzed by a computer program incorporating the appropriate algorithms to give an unfiltered representation of the surface. The data may be statistically processed and filtered to provide parametric values. Fig-2 shows the optical interferometry.

Fig- 2 Representation of Optical Interferometry [3]

Advantages and disadvantages of the non-contact-based measurement:

Developments in optical microscopy have led to the implementation of optics in various applications, one such example is optical interferometry. Optical interferometry is used to overcome the disadvantages of contact-based surface roughness measurements and it is one of the successful technologies used. The scan quality of these measuring instruments is mainly dependent on the optical properties of the materials tested. The highly reflective materials produce artificial spikes in the roughness profiles. The main disadvantage with this technique is the materials with low reflective properties cannot be measured using these devices.

Research on surface roughness measurement:

Surface roughness is one of the important properties that must be considered in any material to determine tribological properties. Researchers have studied various techniques to determine these properties using various principles, Liu Jian et.al., studied the surface roughness measurements using a color distribution statistical matrix. This method is based on the overlap degree of the color image which has relatively high accuracy and a relatively wide measurement range to a certain degree of the brightness to the light source and the texture direction [4]. In a different study F. Luk et.al., studied another method based on a microcomputer-based vision system to analyze the pattern of scattered light from the surface to derive a roughness parameter. The roughness parameters were obtained for a few tool-steel samples which were ground to different roughness [5].

Reference:

[1] Korzeniewska, E., Sekulska-Nalewajko, J., Gocławski, J., Rosik, R., Szczęsny, A. and Starowicz, Z., 2020. Surface morphology analysis of metallic structures formed on flexible textile composite substrates. Sensors, 20(7), p.2128.

[2] https://www.keyence.com/ss/products/microscope/roughness/equipment/line_01.jsp

[3] Toulfatzis, A.I., Pantazopoulos, G.A., David, C.N., Sagris, D.S. and Paipetis, A.S., 2018. Machinability of eco-friendly lead-free brass alloys: Cutting-force and surface-roughness optimization. Metals, 8(4), p.250.

[4] Liu, J., Lu, E., Yi, H., Wang, M. and Ao, P., 2017. A new surface roughness measurement method based on a color distribution statistical matrix. Measurement, 103, pp.165-178.

[5] Luk, F., Huynh, V. and North, W., 1989. Measurement of surface roughness by a machine vision system. Journal of physics E: Scientific instruments, 22(12), p.977.