With the continuous development of society and the progress of science and technology, material bonding technology plays an important role in the fields of aerospace, automotive electronics and biomedicine. However, materials such as stainless steel are difficult to be bonded because of their surface characteristics, so they need to be surface treated to achieve the purpose of bonding. Surface treatment technology precisely regulates the surface of metal materials through a series of physical, chemical or electrochemical means, aiming to improve their corrosion resistance, wear resistance, and adhesion to other materials. Especially in bonding applications, the cleanliness, roughness, chemical composition and surface energy of the metal surface have a decisive influence on the bonding strength. Surface contaminants ( such as grease, oxide layer, residual stress, etc. ) can weaken the interaction between the adhesive and the metal substrate, resulting in a decrease in the strength of the bonding interface, and even lead to bonding failure. Therefore, the surface treatment of the material plays an important role in the bonding process. In order to improve the bonding strength of the stainless steel surface, the plasma surface treatment technology can be used before bonding to enhance the wettability and adhesion between the adhesive and the stainless steel.

Plasma surface treatment technology

The plasma is a state of existence in which matter is highly ionized at high temperature or under certain conditions. It is composed of atoms, ions, electrons and excited atoms and ions. It is generally electrically neutral and chemically neutral. It is the fourth state of the existence of matter. Plasma surface treatment technology is a technology that uses high-energy plasma to clean, activate and modify the surface of materials to improve their physical and chemical properties. This technology can improve the wettability, roughness and chemical activity of the material surface, thereby improving the bonding strength and durability between the materials.

Effect of plasma treatment on water contact angle of stainless steel surface

The change of water contact angle on the surface of stainless steel before and after plasma surface treatment

Through the above diagram, it can be seen that the water contact angle of the surface of the stainless steel plate decreases after the plasma surface treatment. The diagram is left without plasma surface treatment, and the diagram has plasma surface treatment. The smaller the water contact angle of the stainless steel surface, the larger the surface energy, and the easier the stainless steel surface is wetted. Because one of the conditions for the glue to firmly bond the stainless steel surface is that the primer has good wettability on the stainless steel surface, the bonding performance of the stainless steel after plasma treatment will be improved.

In summary, the plasma surface treatment has significantly improved and improved the adhesive strength of stainless steel plates. The main reason is that the plasma has a large number of positive and negative ions, free radicals, excited state molecules and other active ions. The atoms on the surface of the ultra-thin stainless steel plate or the atoms on the surface of the attached material are knocked off by the impact to remove the original pollutants and impurities on the surface, and the etching effect will be generated. A small pit is formed on the surface of the sample, which increases the specific surface area of the sample and improves the wettability of the surface of the sample. At the same time, the impact of various active ions will cause the original chemical bond on the surface of the sample to break. Some polar functional groups in the plasma form new chemical bonds with the elements on the surface of the stainless steel plate, and the surface free energy of the stainless steel plate is increased, which plays a role in modifying and activating the surface of the stainless steel plate, thus significantly improving the shear strength of the bonded joint.