Characteristics of powder materials for thermal spraying

       In addition to meeting the functional requirements of the coating, powder materials for thermal spraying must also meet the needs of the spraying process, that is, they can be uniformly, fluently and stably transported into the jet flame to ensure stable and uniform thermal spray coating. layer. Therefore, the basic characteristics of the powder, such as the shape, particle size and particle size distribution, bulk density, fluidity and surface quality of the powder for thermal spraying, are an important part of the performance of the ceramic material for thermal spraying.

(1) Powder particle morphology mainly refers to the geometric shape and surface characteristics of powder particles. The geometric shape can be evaluated by measuring the ratio (statistical value) of the short axis to the long axis of the elliptical spherical particles. The higher the degree of spheroidization, the better the solid fluidity of the powder. Since the degree of powder spheroidization is not only related to the atomization powder milling method and atomization milling process parameters, but also to the chemical composition of the powder itself. Therefore, the spheroidization degree of different types of powders is also different, but it should be ensured that the spraying process can be Smooth and even powder feeding.

There are sometimes holes of different sizes inside the metal powder particles for thermal spraying formulated by the atomization method, some holes penetrate to the surface, and some holes are closed inside the particles. If the spraying process is improper, it will have a direct impact on the quality of the coating. To observe such holes, an optical metallographic microscope is usually used. Surface features refer to surface color, smoothness, and the like.

(2) The particle size of the powder. The selection of powder particle size and its range is mainly determined by the spraying process method and spraying process specification parameters. Even if the powder particle size range is the same, the proportion of its particle size grade composition is not necessarily the same, for example: although the powder particle size is in the range of 125μm~50μm (-120 mesh ~ +320 mesh), but the proportions of powders of three different particle size grades of 100 μm~125 μm, 80 μm~100 μm, 50 μm~80 μm are not the same. The powder particle size range and its particle size grade composition have a direct impact on the coating quality, powder bulk density and fluidity.

(3) Powder bulk density. Powder bulk density refers to the mass per unit volume of powder when it is loosely packed. Since the powder bulk density is related to the spheroidization degree of the powder, the size and quantity of the pores inside the powder particles, the powder particle size composition and other parameters, it also affects the quality of the spray coating.

(4) The fluidity of powder The final fluidity is the time required for a specified amount of powder to freely flow through a standard funnel with a specified aperture. The spraying process and spraying efficiency have a certain impact.