Surface roughness can be defined as the deviation of a surface’s normal away from the normal of an idealized flat surface or plain. Surface roughness is a critical factor for machined parts and finished goods because of its effect on their mechanical properties, performance out in the field, and failure susceptibility. A high surface roughness can be detrimental because it can increase a part’s tendency to corrode or wear faster due to surface defects or friction. However, high surface roughness can also be beneficial due to its affinity to promote strong bonding between a part and an adhesive. Surface roughness is a material property that needs to be well understood in order to achieve predictable and reliable performance for any component within a finished good.

One factor to consider is which surface roughness parameter needs to be measured. There are over 40 different surface roughness parameters that can be measured for a given surface, and some factors may be more important than others when it comes to specific applications. The most common surface roughness parameters to measure are Ra, Rq, Rz, and Rt. Ra is the Average Roughness, which is the arithmetic average of the absolute values of the profile height deviations from the mean line as recorded within the evaluation length. Rq (or RRMS) is the Root Mean Square Roughness, which is the root mean square average of the profile height deviations from the mean line. Rt is the Maximum Height of the Profile, which is the maximum peak-to-valley height found over the entire measured range. Rz is the Average Maximum Height of the Profile or Ten-Point Mean Roughness, which is the average of the vertical distance from the five highest peaks to the lowest five adjacent valleys within the profile. Each of these parameters can illustrate different aspects of a surface, therefore determining which ones are needed is key.

But, where do you start when it comes to how to measure surface roughness for a particular sample? Surface roughness can be measured using a number of different methods, and Ebatco is capable of using five distinct techniques. Atomic force microscopy (AFM), contact stylus profilometry, scanning electron microscopy (SEM), scanning probe microscopy (SPM), and white light interferometry are all great methods but considerations need to be taken before choosing a particular technique. The key to choosing a method is knowing the answer to three questions: what material is the sample made of, what is the sample’s geometry, and what range for surface roughness is acceptable for the sample’s intended use. Knowing what material a sample is comprised of will narrow down whether the sample can withstand physical contact during the measurement or not. If the material is soft, such as a delicate biological film or polymer, a non-contact measurement method would be preferred. Knowing the geometry of the specimen will also narrow down which method to use because some methods can handle more curved (or less flat) geometries than others. The contact stylus, AFM, and SPM work well for relatively flat or planner specimens but do not work well on highly curved surfaces. SEM and optical profilometry can handle curved surfaces but up to a certain radius of curvature. Knowing what range to expect for surface roughness can be tricky when dealing with an unknown sample, but a few general assumptions can help narrow down the options. In general, an estimate of surface roughness can be given based upon surface finish or how it feels. If a sample feels rough to the touch or light does not reflect off of it, then the sample’s surface roughness could be in the micron range or higher. If the sample has a mirror like finish, the surface roughness could be less than a micron to nanometers. Once all of these questions have been taken into consideration, an informed decision can be made about which surface roughness method is best suited for a particular sample. Please take a look at the following sections to see the pros and cons for each method described above.

Surface Roughness Analysis using AFM

Surface Roughness Analysis using Contact Stylus

Surface Roughness Analysis using SEM

Surface Roughness Analysis using SPM

Surface Roughness Analysis using White Light Interferometry