What is surface roughness

The surface roughness is generally formed by the processing method adopted and other factors, such as the friction between the tool and the part surface during the processing, the plastic deformation of the surface layer metal during chip separation, and the high-frequency vibration in the process system. Due to different processing methods and workpiece materials, the depth, density, shape and texture of the traces left on the machined surface are different.

Surface roughness is closely related to the matching property, wear resistance, fatigue strength, contact stiffness, vibration and noise of mechanical parts, and has an important impact on the service life and reliability of mechanical products. Ra is used for general marking.

Relevant specifications include "Surface texture Profile method Surface roughness parameters and their values" (GB/T 1031-2009) and "Representation of surface texture (GB/T 131-2006 (ISO 1302:2002)".

2、 Effect of surface roughness

For a long time, people have overemphasized the coating and the design and detection of coating thickness in the anti-corrosion treatment of steel structures, while ignoring the quality of steel surface treatment before coating. The treatment of substrate surface before coating is directly related to the anti-corrosion performance and protection life of the entire coating system. Long term practice has proved that many protection systems fail early, more than 70% of which are caused by improper surface treatment.

1. Effect on coating adhesion

There are three main adhesion and bonding modes of coatings: primary valence bond bonding, polar bonding and mechanical bonding. Principal valence bond bonding is the strongest form of bonding, which refers to the chemical bonding between the chemical groups in the coating and the groups on the surface of the substrate; Polarity binding refers to the attraction of positive and negative electrons in the coating relative to the polarity of the substrate; Mechanical bonding refers to the physical connection between the coating and the surface roughness or surface profile of the substrate. The influence of surface roughness on the adhesion of coating is mainly carried out by influencing the way of mechanical bonding, which is roughly as follows:

1) The surface roughness can help the coating adhere to the steel surface to avoid "sliding".

2) The surface roughness increases the surface area of the substrate. Relevant research shows that the surface area of steel increases by 19%~63% after spray derusting under the pressure of 0.7MPa, so the van der Waals force of the base material molecules in the paint film and the polar groups on the metal surface will also increase accordingly, improving the adhesion of the paint film.

3) When the abrasive with edges and corners is used for rust removal by spraying, the steel surface can form a three-dimensional geometric shape, which makes the paint film and the steel surface mechanically occlude, and further improves the adhesion of the paint film.

2. Influence on coating dosage

When the coating is applied on the steel surface after surface treatment, the coating thickness on the wave crest is lower than that on the wave trough. However, the film thickness on the wave crest is the most important factor determining the coating quality. Therefore, it can be considered that those coatings that have no effect on the film thickness on the wave crest are lost in the roughness of the steel surface. The loss of paint caused by different surface roughness is also different. When the steel shot blasted surface is coated with workshop primer, the surface roughness has less impact on the loss of paint. However, the loss of paint on the surface roughness of steel is much higher when the abrasive with sharp edges and corners is used for sandblasting.

3、 Surface roughness measurement

It is one of the most popular and acceptable methods to measure the surface roughness of metal after abrasive blasting by using duplicate tape. Because the copy tape for measuring the surface roughness can be attached to the inspection quality report, anyone can re measure the metal surface roughness copied on the tape.

1) Use a brush to remove dust and residual sand blasting abrasives from the metal surface.

2) Visually evaluate the metal surface roughness, and select the appropriate copy tape. Each tape is in the middle of its measurement range, and the measurement data is the most accurate. The measured value beyond the measuring range is not reliable.

3) Paste the copy tape on the metal surface sprayed with sand and free of dust residue.

4) Grind the sponge foam in the white circle of the duplicating tape with the grinding rod provided in the kit until the sponge foam in the circle changes color completely.

5) Adjust the micrometer and zero it.