Surface Roughness Tester Evaluates the Impact of Substrate on Adhesion

This article primarily explores how the surface roughness of substrates affects the adhesion of coatings or bonds. Appropriate roughness can increase surface area, provide mechanical anchoring points, and enhance bonding strength; however, excessive roughness may lead to stress concentration or incomplete coating coverage, thereby reducing adhesion. The article introduces key measurement parameters of surface roughness instruments, such as Ra, Rz, and Rsk, and explains how experimental design can be used to analyze the relationship between these parameters and adhesion, thereby identifying the optimal roughness range. Finally, it emphasizes that in practical applications, relevant standards must be referenced to ensure accurate measurements and provide guidance for surface treatment processes.

Surface roughness and adhesion

In coating, coating, or bonding processes, the surface roughness of the substrate is one of the key parameters that affect the bonding properties of the interface. Surface roughness directly or indirectly regulates the adhesion strength by changing the actual surface area, mechanical mating effect, and interfacial stress distribution of the substrate. When the roughness is suitable, the surface microstructure can provide effective mechanical anchorage points and promote the physical bonding between interfaces. However, excessive roughness can lead to stress concentrations at the spikes or prevent the coating from fully covering the valley floor, creating defects that weaken adhesion. Therefore, quantitative evaluation of roughness parameters is of great significance for predicting and optimizing adhesion properties.

Measurement parameters of the surface roughness meter

Modern surface roughness meters are typically based on contact probes or optical non-contact principles, enabling precise measurements of a range of characterization parameters. When evaluating the effects of adhesion, the following parameters are often focused:

Ra (Arithmetic Mean Deviation): Describes the arithmetic mean of the profile deviating from the mean line over the length of the sample and is a general indicator of macroscopic roughness.

Rz (maximum height): The vertical distance between the peak and valley lines of the contour over the length of the sample, reflecting the extreme of surface relief.

RSK (Deflection): Characterizes the symmetry of the profile height distribution. Negative skew means that the surface is dominated by valleys, which may be conducive to coating filling; Positive skew is dominated by peaks, which may enhance mechanical locking.

These parameters can be calculated using the following formula (using discrete sampling as an example):

Ra = (1/n) Σ|y_i|

Rz = max(y_pi) - min(y_vi)

where y_iis the height of each point deviating from the average, y_piand y_viThey are peak height and valley depth.

Experimental design

In order to systematically study the effect of substrate roughness on adhesion, control experiments can be designed. Substrate samples of the same material are selected and a series of surfaces with gradient roughness are obtained through different processing methods such as sandblasting, polishing, and etching. Use a surface roughness meter to accurately measure Ra, Rz, Rsk and other parameters of each sample. Subsequently, a uniform coating or adhesive is applied under standard environmental conditions and adhesion tests are performed according to relevant standards (e.g., ASTM D4541, ISO 4624), commonly used by drawing or scrating.

In the data analysis stage, the correlation analysis or modeling of adhesion values (such as tensile strength) and roughness parameters can be performed to find out the parameters that have a significant impact and possible optimization intervals. It is important to note that the optimal roughness range varies depending on the coating system, substrate material, and stress method.

Data examples

The following is the simulation data, showing the roughness of different metal substrates and the general trend of the corresponding tensile adhesion. In practical applications, it needs to be verified according to the specific process.

Substrate type and treatment	Ra (μm) / Adhesion (MPa)
Mild steel, finely polished	0.1 / 8.2
Mild steel, lightly sandblasted	2.5 / 15.3
Mild steel, heavily sandblasted	12.8 / 9.7
Aluminum alloy, chemical etched	1.8 / 12.1
Aluminum alloy, mechanically polished	3.4 / 14.0

It can be seen from the trend that the adhesion does not increase monotonously with roughness, and there is often an intermediate range that maximizes the adhesion. Excessive roughness may result in an increase in the measured Ra and a decrease in adhesion.

Technical considerations

When evaluating with a surface roughness meter, it is important to ensure that the measurement conditions are consistent and representative. The sampling length, evaluation length and filtering conditions should be determined according to ISO 4287, ASME B46.1 and other standards. Clean the surface before measuring to avoid dust or grease. For heterogeneous or anisotropic surfaces, multiple measurements in different directions are required to obtain statistically reliable data. In addition, the probe radius, resolution and other parameters of the roughness meter itself need to be matched with the surface feature scale to avoid measurement errors.

Finally, the combination of roughness data and adhesion test results can provide quantitative guidance for the surface pretreatment process in specific application scenarios, which is an effective way to achieve reliable interface bonding.

References

ASTM D4541-22, Standard Test Method for Determining the Pull-Off Strength of Coatings Using a Portable Adhesion Tester.

ISO 4287:1997, Technical Specification for Product Geometry (GPS) — Surface Structure: Contour Method — Terminology, Definitions and Surface Structure Parameters.

ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Texture).

Related Surface Science & Adhesion Journal Research Papers.