Use a hard object to mark or scratch the coating film on the surface of the paint film to measure the hardness of the coating film. The commonly used methods are the pencil hardness method and the scratch needle method. A commonly used scratch hardness is the pencil hardness, which is a quick test method and is often used in the field for paint films on rigid substrates.
The pencil hardness method has two methods: manual operation and instrument test. It uses a pencil with known hardness to measure the hardness of the coating film. The pencil hardness (instrument test)) is used as the hardness of the coating film. Pencils should be standard high-grade drawing pencils manufactured by specified manufacturers. The pencil lead does not contain lead but graphite and china clay. It is not sharpened like it is for writing, but is ground vertically with sandpaper to a flat end. When testing, it is at an angle of 45° to the sample, and the force is just enough not to break the lead. push. The results are expressed as just hardness without scratching. Experienced people can reproduce the results of ±1 file hardness.
Different countries adopt different pencil hardness grading. CHINA's national standard GB 6739-86 "Measurement Method of Coating Film Hardness Pencil" stipulates that the pencils used are 13 grades from 6H to 6B, 6H is the hardest, and 6B is the softest. As an arbitration test, an instrument test method , and the general instrument model is QHQ type pencil scratch Hardness Tester.
The scratch test method uses the needle tip of the instrument to scratch the coating film. When the scratch needle is pushed on the paint film, the stress generated exceeds the elongation strength of the paint film and breaks. The film hardness is represented by the scratch resistance of the coating film. Whether it is pierced by the needle under the specified load, or the minimum load required for the needle to penetrate the coating. There are two kinds of instruments in use now: automatic type and manual type. The automatic type can directly display the results from electrical instruments relying on conductivity. CHINA's national standard GB 9279-88 "Paints and Varnishes, Scratch Test" stipulates that automatic type instruments should be used as Apparatus for referee tests.
Scratch hardness reflects a combination of modulus, tensile strength and adhesion. When measuring the hardness, the coating film is not only affected , but also by shear force, which also reflects the adhesion of the coating film. Therefore, the hardness characteristics of the coating film measured by it are different from those of the pendulum damping test method. .
