Type, shape, particle size range, etc. of abrasives for thermal spray blasting

       Abrasive properties include abrasive type, abrasive hardness, shape, particle size, service life, safety and toxicity.



1) The type of abrasive. Abrasives are divided into metals, inorganic non-metals, and natural agricultural products. Metal-type abrasives are mainly chilled cast iron shot, broken steel grit and short fiber steel wire, etc., which are mainly used in mass production of ferrous metal parts.



Inorganic non-metallic abrasives are mainly corundum, silicon carbide, copper slag, slag, garnet, quartz sand, river sand, etc.



Natural agricultural product abrasives include walnut shells, nut shells, grain bran and other sandblasting abrasives for soft metals or for special purposes.



2) The hardness of the abrasive. It should be higher than the hardness of the surface of the substrate, and the hardness of different types of abrasives varies greatly. The abrasive has high hardness. When spraying the surface of the substrate at high speed, its ability to impact, chisel and squeeze the surface of the substrate is large, and the sandblasting efficiency is high. Of course, for soft substrates, embedded phenomena may occur. The surface hardness of the substrate is high, so it is suitable to use high-hardness abrasives; on the contrary, for soft substrates, moderate or soft abrasives should be used.



3) The shape of the abrasive. The bonding between the thermal spray coating and the substrate is mainly physical and mechanical bonding, so higher surface roughness is required than other surface coating technologies, so that the coating and the substrate can obtain greater bonding strength. The abrasives for thermal spraying generally use polygonal, irregular water chestnut sharp abrasives, such as broken steel grit, broken gold steel grit, etc., to obtain the necessary surface roughness. For soft matrix materials, it is also best to use nutshell abrasives with sharp corners that do not embed, or choose abrasives with moderate hardness and blunt edges.



4) The particle size range of the abrasive. Under the same sandblasting process conditions, the coarse-grained abrasive can obtain a large surface roughness, and the unevenness is large. Hard, coarse-grained abrasives wear a lot to blasting equipment; fine-grained abrasives form a relatively small and uniform surface roughness. The selected abrasive should have a certain particle size range, and the particle size should not be very different to ensure a relatively uniform roughening effect.



The selection of the abrasive particle size range is also related to the type of matrix material, the thickness of the matrix and the part to be blasted. For example, the abrasive grain size range for sandblasting of steel matrix is 16#~60# corundum sand (250-1180um), and the thin-wall matrix is 25#~120# corundum sand (125~710um). For sandblasting of non-ferrous metals, soft metals and most plastic substrates, 60#~ 100# (150~ 250um) corundum sand is mostly used. In order to ensure the coarsening effect of sandblasting, the abrasive used for sandblasting should be screened regularly, and the crushed fine sand should be screened out to ensure the particle size range of the abrasive.



5) The service life of the abrasive. The grinding degree and passivation state of the abrasive determine the service life of the abrasive. The service life of the abrasive is mainly related to the type of abrasive and the surface hardness of the substrate. Metal-type abrasives are hard and tough, and their service life is much longer than that of hard and brittle inorganic non-metallic abrasives, so they are mostly used as abrasives for continuous automatic sandblasting machines.



6) Reprocessing and recycling of abrasives. The abrasive for blasting should be regularly screened out fine powder, cleaned (removal of impurities) and dried. The abrasive for blasting should always be kept clean and dry, especially the compressed air for blasting should be anhydrous.



7) The harm of abrasives to human body. It is easy to generate fine dust during sandblasting, which is harmful to the human respiratory system and lungs. Necessary protective measures should be taken.