Effects of powder particle size, bulk density, fluidity and oxygen content on thermal spraying process

       The advantage of spray powder is that it is not limited by the wire forming process, the production cost is low, the source is wide, and the components can be mixed in any proportion, so that the prepared coating has special characteristics. The spraying powder should meet the composition and performance requirements of the coating and the spraying process requirements, so it should have suitable powder physical properties.



Powder physical properties are one of the important basis for formulating thermal spraying process parameters. The powder physical properties that affect the thermal spraying process mainly include particle size, shape, distribution, specific surface area, adsorption, fluidity, etc. In the powder feeding process of thermal spraying, if the particle size of the powder is too small, the fluidity will be poor, which will make powder feeding difficult; if the particle size is too large, the powder will not be completely melted in the high temperature heat flow. If the distribution is too wide, there must be some powders that will not pass through the high temperature zone of the plasma beam sufficiently, resulting in uneven heating of the powders.



1 granularity



Particle size is a technical index to measure the size of powder particles. The powder particle size and range must be appropriate to ensure that the powder has appropriate bulk density and fluidity, and is ready for high-quality coatings.



Thermal spray powder particles are generally measured by mesh, and ultrafine powders are characterized by μm units. In powder samples, the particle size of the powder is characterized by the sign and the mesh number. For example, -120+300 mesh means that the powder particle size is less than 120 and greater than 300 mesh. The larger the mesh number, the finer the powder. Thermal spray powder should meet certain particle size requirements. The metal powder used for flame spraying is generally less than 120 mesh, and the commonly used one is -180+320 mesh; the particle size of the ceramic powder for plasma spraying is generally -140+320 mesh or -500+300 mesh; the powder particle size suitable for HVOF is generally 15 ~53μm, finer powder particle size is also 5 ~ 25μm, some HVOF equipment can achieve the spraying of ultra-fine powder below 10μm.



2 Loose specific gravity



The loose specific gravity is also called the loose density. Its physical meaning is the ratio of mass to volume after the dry powder flows into the container naturally, and the unit is g/cm3. The size of the loose specific gravity is not only closely related to the composition of the powder, but also related to the shape and surface quality of the powder. For the same alloy powder, the larger the value of the loose specific gravity is, the better the quality is. If the loose specific gravity is too small, it means that there is a loose oxide layer on the surface of the powder particles, or the shape of the powder particles is irregular.



2.3 Liquidity



Fluidity refers to the flowability of powder, and it is one of the important indicators to measure the processability of powder thermal spraying. The powder fluidity is usually measured by a Hall flowmeter, the unit is s/50g, and its physical meaning is the time s required for 50g of a certain powder to flow through a standard funnel with a certain aperture. The smaller the flow value, the better the fluidity of the powder. Good fluidity is easy to achieve uniform powder feeding, and poor fluidity is easy to block the powder feeding channel.



The fluidity of alloy powder is generally good, about 20~45s/50g. The fluidity of ceramic powder is generally poor, about 50~120s/50g, and some even have arching phenomenon in Hall flowmeter, no self-flow, no fluidity, usually achieved by vibration or stirring in thermal spray powder feeder Forced powder feeding.



The thermal spray powder should have good flow ability so as to flow smoothly and evenly in the powder feeding channel. Spherical thermal spray powder is a new type of spray powder, which not only has good fluidity, but also has excellent coating performance. The poor sphericity of powder particles, surface oxidation and surface moisture absorption will reduce the fluidity of the powder. Appropriate powder composition and milling process should be selected to obtain the ideal powder particle morphology.



4 Oxygen content



For alloy powder, oxygen content is also an important physical property index. The oxygen element contained in the powder generally exists in the form of oxide, because the metal element is inevitably slightly oxidized during the manufacturing process of the powder. The more serious the oxidation, the higher the oxygen content. The oxides contained in the powder are harmful components that cause pores and inclusions in the coating. The oxygen content should be strictly limited. The oxygen content of alloy powder is generally less than 0.2%.