Analysis of the Causes of Defects in Thermal Spray Coatings
With the rapid development of modern industry, the application of thermal spraying is currently widely used in many fields. So if there is something wrong with the coatings during thermal spray operation, what will be the causes? Here we are to learn it!
1. Coating shelling:
(1) There is visible oil and water in the compressed air.
(2) The surface of the substrate is not rough enough or there is dirt and others attached on the substrate, making the coating adhesion reduced.
(3) The workpiece contains grease, which spilled when spraying.( especially ductile iron crankshaft).
(4) The spray gun is far away from the workpiece so that the plasticity is reduced before the metal particles reach the workpiece, making it not fully bonded.
(5) Interval time between turning and pulling, pulling and different process of results in surface oxidation.
(6) Using alumina grinding wheel results in local overheating and expansion.
(7) The spray gun sparks are not concentrated and the flame deflects when spraying, making metal particles not strong enough to adhere to the surface of the tool.
(8) The workpiece line speed and gun movement speed is so slow that the inclusions when spraying float on the surface and reduce the adhesion strength.
2, Coating cracking:
(1) The spray gun moves too slowly during spraying so that the coating is too thick and overheated.
(2) The spraying distance is too close and overheats the coating .
(3) The spray materials’ shrinkage is too high or contain many elements that lead to warm cold crushing (such as sulfur, phosphorus, etc.).
(4) The current is too high during EFI. Using oxidizing flame when gas spraying, making the coating excessively oxidized.
(5) The workpiece after finishing spraying is excessively quenched and cracked.
(6) Water vapor and oil mist in compressed air reduce the bonding strength of the coating.
(7) The rotation center of the workpiece is inaccurate. Spraying sparks are skewed on one side, making the second coating layer thick and thin and nonuniform contracted.
3, Coating layering:
(1) Not finishing the spraying and wait for too long when using intermittent spraying, so the coating will produce delamination when polishing.
(2) The oil and water brought out by the compressed air during spraying are splashed on the surface of the workpiece.
(3) The spraying site is unclean. There is a large amount of dust adsorbed on the surface of the workpiece after spraying each layer so that there will be some foreign objects isolated or partly isolated between the layers.
4, Coating non-wearable
(1) The spray gun is too far away from the workpiece when spraying, making the metal particles cool earlier, and they become loose coating after spraying on the workpiece. When the metal thermal spraying coating is working, the particles partially fall off and rub the friction surface.
(2) There is a large amount of wheel shavings embedded in the coating while grinding, which scratch the surface.
(3) The wire feed speed is too fast, making the particles flaky.
(4) Metal materials are not suitable because of the not high hardness and non-wear resistance (for example, carbon content of steel wire is low, and the coating is too soft).
(5) The air pressure is too low and the spraying distance is too far, which reduce the bonding strength.
