Stainless steel substrate of thick film heating element The performance of the substrate directly affects the quality of the thick film heating element. The metallographic structure, composition and surface roughness of the stainless steel substrate have a greater impact on the electrical performance of the thick film heating element. Stainless steel is roughly divided into three types according to the metallographic structure: austenitic, special element and martensitic stainless steel. The stainless steel with austenitic structure at room temperature contains about 18% Cr, 8%-10% Ni, and about 0.1% C, and it has a stable austenite structure. In use, the ferrite-based stainless steel has a chromium content of 11% to 30% and has a body-centered cubic crystal structure. This type of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. It has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance.
Martensitic stainless steel is a stainless steel whose mechanical properties can be adjusted through heat treatment. In layman's terms, it is a kind of hardenable stainless steel. Due to the relatively stable chemical properties of austenite and ferrite, and strong corrosion and oxidation resistance, they are often used as substrates for thick film heating elements. The content of chromium in stainless steel directly affects its corrosion resistance. Generally, the content of chromium exceeds 12% to have good corrosion resistance. Steels with a chromium content of less than 12% do not have the function of rust prevention. The content of other elements in stainless steel will affect the coefficient of thermal expansion (TCE) of stainless steel. The coefficient of thermal expansion directly affects the adhesion performance of the insulating medium. If the TCE of the stainless steel substrate is inconsistent with the TCE of the insulating medium, during high-temperature sintering or heating, The insulating dielectric layer may crack or fall, which will seriously affect the performance of the thick film heating element. The stainless steel substrate of the thick film heater is generally selected from 304 and 430 steel, and the material composition is shown in Table 1. The TCE of 304 is about 18ppm/℃, and the TCE of 430 is about 12ppm/℃. Due to the different TCEs of the two materials, the corresponding insulating medium slurry is different. The roughness of the stainless steel surface and the cleanliness of the surface have a great influence on the insulation performance of the thick film heating element. An empirical value is reached through the consultation of thick film heater manufacturers and the scientific research conclusions of some thick film heaters: when the surface roughness of the stainless steel plate is 25-45um, the insulating medium of the heater can play a good insulation role, and Too rough or too smooth will affect the insulation effect of the product. Some well-known foreign manufacturers such as ESL, DuPont and other manufacturers mentioned the pretreatment of stainless steel substrates in some technical materials. Oil and fingerprints need to be removed. Clean steel plates need to be taken with silicone gloves. Contaminated stainless steel substrates After sintering, black spots and cavities are prone to appear on the surface of the dielectric, which affects the density of sintering and thus affects the insulation effect.
