Surface roughness is a special material parameter. This name is often shortened to just roughness and is a component of surface texture. It is quantified by deviations of the direction of the vector of the real surface from its ideal shape. If these deviations are large, the surface is rough; if they are small, the surface is smooth. In surface metrology, roughness is usually considered the high-frequency short-wave component of the measured surface. However, in practice, it is often necessary to know both amplitude and frequency to ensure that the surface is suitable for a specific purpose. Surface roughness is a very important parameter in design.
Role and significance
Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces usually wear out faster and have higher friction coefficients than smooth ones. Roughness is often a good predictor of the characteristics of a mechanical component since surface irregularities can form nucleation sites for cracks or corrosion. Roughness, on the other hand, can promote adhesion. Generally speaking, instead of scale descriptors, cross-scale descriptors, such as surface fractality, provide more meaningful predictions of mechanical interactions on surfaces, including contact stiffness and static friction. Surface roughness is a rather complex parameter, details of which can be found below.
High and low values
Although a high roughness value is often undesirable, it can be difficult and expensive to control during manufacture. For example, it is difficult and expensive to control the surface roughness of parts obtained by surfacing modeling (FDM). A decrease in these indicators usually increases the cost of manufacture. This often leads to a compromise between the cost of producing a component and its effectiveness in use.
Measurement methods
The indicator can be measured by manual comparison with a “roughness comparator” (a sample of known surface roughness), but in a more general case, the measurement of the surface profile is carried out using profilometers. They can be a contact type (usually a diamond stylus) or optical (for example, a white light interferometer or a laser scanning confocal microscope).
However, controlled roughness can often be desirable. For example, a glossy surface may be too shiny for the eyes and too slippery for the finger (the touchpad is a good example), so controlled indicators are required. Surface roughness is the case when amplitude and frequency are very important.
Its value can be calculated either by profile (line) or by surface (area). The profile roughness parameter (Ra, Rq, ...) is more common. Area roughness parameters (Sa, Sq, ...) give more meaningful definitions.
Parameters
Each of the roughness parameters is calculated using the surface description formula. The standard references that describe each of them in detail are surfaces and their measurements. Surface roughness is a characteristic.
Profile roughness parameters are included in the British (and worldwide) standard BS EN ISO 4287: 2000, identical to ISO 4287: 1997. The standard is based on the ″ M ″ system (middle line).
There are many different roughness parameters, but the above are the most common, although standardization often occurs for historical reasons, and not for certain merits. Surface roughness is a combination of bumps.
Some options are used only in certain industries or in certain countries. For example, MOTIF parameters are mainly used in the French automotive industry. The MOTIF method provides a graphical assessment of the surface profile without filtering undulations from roughness. MOTIF consists of a portion of the profile between two peaks, and the final combination eliminates “minor” peaks and retains “significant” ones. The surface roughness in the drawing is the presence of bulges, captured and carefully measured on it.
Since these parameters reduce all the information in the profile to one number, care must be taken when applying and interpreting them. Small changes in how raw profile data is filtered, how the midline and measurement physics are calculated, can greatly affect the calculated parameter. On modern digital equipment, scanning can be evaluated to make sure there are no obvious failures that distort the values.
Features of parameters and measurements
Since for many users it may not be obvious what each measurement actually means, the modeling tool allows the user to adjust key parameters by visualizing surfaces that are clearly different from the human eye, different in measurements. For example, some parameters cannot distinguish between two surfaces, where one consists of peaks and the other consists of troughs with the same amplitude.
By convention, each 2D roughness parameter is an uppercase letter R, followed by additional characters in the subscript. The subscript defines the formula that was used, and R means that the formula was applied to the 2D roughness profile.
Different capital letters mean that the formula has been applied to a different profile. For example, Ra is the arithmetic mean for the roughness profile, Pa is the arithmetic average for the unfiltered rough profile, and Sa is the arithmetic average for the three-dimensional roughness.
Amplitude parameters
The amplitude parameters characterize the surface based on the vertical deviations of the roughness profile from the midline. For example, the arithmetic mean value of the filtered roughness profile, determined from deviations from the center line within the evaluation length, may be related to the range of collected points of this roughness. This value is often used to indicate surface roughness.
Arithmetic average roughness is the most widely used one-dimensional parameter.
Research and observation
The mathematician Benoit Mandelbrot pointed out the relationship between surface roughness and fractal dimension. The description provided by the fractal at the micro roughness level may allow controlling the material properties and the type of chip formation. But fractals cannot provide a full-blown representation of a typical machined surface affected by tool feed marks; they ignore the geometry of the cutting edge.
A little more about measurement
Surface roughness parameters are defined in the ISO 25178 series. Resulting values: Sa, Sq, Sz ... Many optical measuring instruments can measure surface roughness by area. Area measurements are also possible using contact systems. Multiple, closely spaced 2D scans are taken from the target area. Then they are stitched digitally using appropriate software, resulting in a three-dimensional image and the corresponding roughness parameters.
Soil surface
Soil surface roughness (SSR) refers to the vertical changes present in the micro- and macrorelief of the soil surface, as well as their stochastic distribution. There are four different classes of SSRs, each representing a characteristic vertical length scale:
- the first class includes changes in the microrelief from individual grains of soil to aggregates of the order of 0.053–2.0 mm;
- the second class consists of variations of soil lumps from 2 to 100 mm;
- the third class of roughness of the soil surface is the systematic differences in elevation due to tillage, called oriented roughness (OR), in the range from 100 to 300 mm;
- the fourth class includes planar curvature or macroscopic topographic features.
The first two classes explain the so-called microroughness, which, as has been shown, significantly affects the event and the seasonal scale depending on the amount of precipitation and tillage, respectively. Micro roughness is most often quantified using random roughness, which, in essence, is the standard deviation of the data on the elevation of the surface of the layer around the average height value after correction for the slope using the best fit plane and eliminating the effects of soil cultivation in individual height readings. Exposure to precipitation can lead to a deterioration or increase in micro roughness, depending on the initial conditions and soil properties.
On rough ground surfaces, the effect of separation of rain spray tends to smooth the edges of the roughness of the soil surface, which leads to an overall decrease in RR. However, a recent study in which the reaction of smooth soil surfaces to rainfall was studied showed that RR can increase significantly with small initial micro roughness scales of the order of 0-5 mm. It has also been shown that an increase or decrease is consistent between different SSRs.
Mechanics
The surface structure plays a key role in controlling the mechanics of contact, that is, the mechanical behavior that manifests itself at the interface between two solid objects when they approach each other and pass from contactless conditions to full contact. In particular, normal contact stiffness is determined primarily by roughness structures (surface tilt and fractality) and material properties.
From the point of view of engineering surfaces, roughness is considered harmful to the characteristics of the part. As a result, most production prints set an upper roughness limit, but not a lower one. An exception is the cylinder bore, in which the oil is stored in the surface profile and the minimum surface roughness (Rz) is required.
Structure and fractality
The surface structure is often closely related to its frictional and wear-resistant properties. A surface with a higher fractal dimension, a large or positive value will usually have a slightly higher friction and wear out quickly. Peaks in the roughness profile are not always contact points. Shape and waviness (i.e. both amplitude and frequency) should also be considered, especially when processing surface roughness.