Scratch hardness
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 rapid 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, and uses the pencil hardness (manual operation) at which the coating film is not damaged by plowing, or the next level of plowing damage to the coating film. Pencil hardness (instrumental test) of hardness was 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 as it is for writing, but is ground vertically to a flat end with sandpaper. 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. my country's national standard GB6739-86 "Measurement Method of Pencil Hardness of Coating Film" stipulates that there are 13 grades of pencils from 6H to 6B, 6H is the hardest, and 6B is the softest. As the referee test, the instrument test method should be used. The general instrument model is the QHQ pencil scratch Hardness Tester.
The needle scratch method uses the tip of the instrument to scratch the coating film. When the 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 penetrated by the needle under the specified load, or the minimum load required to penetrate the coating by the needle. There are two kinds of instruments in use now: automatic type and manual type. The automatic type can directly display the results from the electrical instrument by virtue of the conductivity. my country's national standard GB9279-88 "Paint and Varnish Scratch Test" stipulates that the automatic type instrument should be used as the referee test instrument.
Scratch hardness reflects a combination of modulus, tensile strength, and adhesion. When measuring hardness, the coating film is not only affected by pressure, 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.
