What properties of coating materials to understand when designing coating materials

       The chemical composition and physical properties of the spray material are one of the most important factors affecting the final performance of the coating.

    1) Chemical composition

    The material used for spraying shall be tested without decomposition and sublimation in the flame. Any material that is fully meltable and has a low vapor pressure can be used for spraying. High temperature resistant oxide ceramics and some refractory metals are the most stable in the flame. The stability of the particles in the flame depends not only on the nature of the spray material, but also on the composition of the working gas. When argon is used as the working gas to spray titanium, a coating composed of titanium can be obtained; when nitrogen is used as the plasma gas, titanium nitride is formed in the coating; when sprayed in the atmosphere, titanium is easily oxidized.

2) Physical properties

Melting point, density, thermal expansion coefficient, body structure, phase transition phenomenon and porosity are several basic properties that determine whether a material can be used as a coating.

(1) The melting point of the material is the most important basic property in high temperature coatings. As the material approaches its melting point, its mechanical strength decreases, its chemical activity increases, and its vapor pressure increases. Some materials drop sharply in strength and stability near the melting point, and even cannot be used.

(2) When spraying aerospace parts, the density of the coating material is of great significance. Often the lightest is selected from the same technical characteristics.

(3) The thermal expansion coefficients of the coating and the base material should be close so that their expansion with temperature and change is also similar. The smaller the difference between the thermal expansion coefficients of the coating and the substrate, the higher the bonding strength between them.

(4) The crystal structure has a significant impact on the coating quality, for example, the coating composed of cubic yarn crystals has high stability. Because this material is isotropic and has no internal stress, cracks are not prone to occur during sharp temperature transitions and local changes.

(5) Many substances can produce phase transitions when temperature changes. Generally, phase transitions are often accompanied by volume changes. Therefore, materials that undergo phase transitions during spraying should not be selected as coating materials. Because the volume change often produces cracks, the matrix is easy to corrode. Therefore, materials that are prone to phase transition must be stabilized before they can be used as coating materials.

(6) Porosity is also a very important physical property index of the coating. The favorable role of pores is to provide a lower elastic modulus and absorb the stress of different thermal expansion. In wear-resistant coatings, pores can store oil and hold tiny abrasive particles. Conversely, in coatings that require oxidation and corrosion resistance, porosity is a disadvantage.

(7) Bonding strength is the basic requirement for selecting coating materials. Without sufficient bonding strength, the coating will fall off under the action of mechanical load stress, thermal stress and stress caused by different thermal expansion.