How to Design Coating Thickness

       It seems that the coating thickness can theoretically be determined according to the needs, for example, the wear-resistant coating thickness is determined according to the wear rate, the predetermined life and the necessary machining allowance; the thickness of the thermal insulation coating is selected according to the thermal conductivity of the coating. But in fact, both the coating and the substrate may generate various types of stress during the spraying process. These stresses will affect the bond strength of the coating and the stress distribution of the coating after quenching. In general, the coated surface is under tensile stress. The greater the thickness, the greater the stress value, and even the coating peels off from the substrate or the coating itself cracks. There is often compressive stress at the interface of the substrate bonded to the coating. Depending on the shape of the substrate, the force acts differently. The spherical matrix is the most favorable, followed by the cylindrical matrix, whose compressive stress enhances the bond strength. If the substrate is planar, the compressive stress at the interface tends to weaken the bond between the coating and the substrate. The most unfavorable is the inner hole surface, which promotes the release of the coating from the substrate.

   The bond strength of the coating decreases as the thickness of the coating increases. The thickness of the plasma coating in practical application is generally about 0.5mm. Some heat-resistant coatings can be alternated with metal layers according to the needs of thermal insulation, which can reach about 2mm. In the multi-layer composite coating, the metal bottom layer is generally within 0.05mm to 0.01mm, and the insulating ceramic layer is about 0.2mm to 0.3mm.