How the coating is combined

       The bonding of the coating includes the bonding of the coating to the substrate surface and the cohesive bonding of the coating. The bond strength of the former is called cohesion, and the bond strength of the latter is called cohesion. It is generally believed that the bonding between the coating and the surface of the substrate and the bonding between the coating particles are the same as physical-chemical bonding, including the following types.



    (1) Mechanical bonding, the particles that collide into a flat shape and undulate with the surface of the substrate are combined with the uneven surface (that is, the anchor effect) to form a mechanical bond, so the mechanical bonding and the surface roughness of the substrate are combined. related. The surface of the substrate is roughened by sandblasting, rough turning, threading, brushing or chemical etching to improve the bond strength of the coating. The roughening of the substrate surface stops the shrinkage strain of the powder as it solidifies locally. Reducing the influence of internal stress can also improve the bonding strength of the coating.

    Generally speaking, the particles are basically mechanically bonded, so the main body of the bonding of the spray coating is mechanical bonding.

    (2) Metallurgical-chemical bonding, which is a type of bonding when diffusion and micro-alloying occurs on the surface of the coating and substrate, including the formation of intermetallic compounds or solid solutions on the bonding surface. For example, oxy-acetylene flame powder spraying makes molybdenum as the bottom layer. Because of the high melting heat and specific heat capacity of molybdenum, a large amount of heat energy can be absorbed in the spraying flame. The local surface of the substrate reaches a high temperature instantaneously, and due to the high binding energy of molybdenum and oxygen, molybdenum is oxidized in the flame during spraying, and when it impacts the surface of the substrate, molybdenum oxide cracks and volatilizes. The fresh molybdenum surface is exposed, which can be combined with the oxides on the surface of the substrate. This cleaning effect makes the molybdenum particles and the surface of the substrate have good metal contact, diffusion and welding to each other, and a high enthalpy is generated. Metallurgical reaction with the surface of the base material, showing a micro-metallurgical bond.

    At present, exothermic nickel-clad aluminum composite powder is commonly used. This metal powder produces an exothermic reaction on the surface of the substrate, generates intermetallic compounds, and releases the maximum heat, so that the contact surface between the substrate and the powder reaches a high temperature instantaneously. The mobility of atoms on the contact interface is increased, so that the atoms of the powder and the surface of the substrate diffuse into each other. Because the diffusion time is short and the thickness of the diffusion layer is only a few microns, it is called micro-metallurgical bonding.

    Some coatings, such as plasma spraying Ni, Cr, Mo, Ta and W on aluminum or carbon steel substrates, react with the substrate to form intermetallic compounds such as Fe3Ni, Fe7W6, δ-FeMo, Fe2Ta, etc. This combination is called For chemical bonding, in enamel production, metal oxides such as cobalt oxide, manganese oxide, and tungsten oxide are added to the formula of the base glaze, and these oxides are melted at high temperature. Oxidation-reduction reaction occurs on the surface of the metal workpiece, which oxidizes iron and reduces itself into a metal film-like substance attached to the surface of the workpiece. This transition layer (metal film) has a good combination with both (glaze layer and workpiece). The adhesion is greatly improved. This bond is also a chemical bond.

    (3) Diffusion bonding, the bonding between the coating and the surface of the substrate is due to the mutual diffusion of molecules in the two materials, the interface in the middle disappears, and "interweaving" with each other to form a diffusion layer to closely connect the coating and the substrate. For example, when thermally spraying a metal on another workpiece (metal), it is not uncommon for SEM analysis to show that interdiffusion of the two metals occurs between the coating and the base. If molybdenum is sprayed on the steel substrate, the thickness of the diffusion layer can be several microns, so it is called micro-metallurgical bonding.

    (4) Physical bonding, the bonding of particles to the substrate surface is formed by van der Waals forces or secondary valence bonds.

    Oxidation of the substrate surface or spray particles will affect the bonding of van der Waals forces, so the substrate surface is required to be very clean. Sandblasting can make the substrate appear abnormally clean and highly active fresh metal surface. If sprayed immediately, it has a great effect on improving the van der Waals force and can improve the bonding strength of the coating.

    Generally speaking, the bonding between the coating and the substrate surface is mainly mechanical bonding. The existence of residual stress in the coating is one of the characteristics of thermal spray coatings. When the molten particles collide with the surface of the substrate, they deform and solidify under the effect of chilling, resulting in a microscopic shrinkage effect. The residual stress of the coating is caused by the thermal spraying conditions and the difference in the physical properties of the sprayed material and the base material. During the instantaneous cooling process, the inconsistency of the temperature between the edge and the core of the particle causes the flat sheet to shrink and curl, and the thermal stress in the sheet cannot be completely released, resulting in cracks in the coating. It affects the quality of the coating and limits the thickness of the coating, and measures should be taken in the process to eliminate and reduce residual stress.