How to Solve the Problems of High Substrate Temperature and Poor Coating Bonding by Inner Hole Spraying

       When plasma spraying is performed on the inner hole, the spraying distance to obtain the best bonding strength should be controlled within the range of 50mm~100mm. Since the hole in the cylinder is generally small, a satisfactory coating can be sprayed only if the hole diameter is the diameter of the spray gun body plus a spraying distance of at least 50mm. For this reason, the plasma spray gun is required to be precisely machined into a slender rod shape, and the circuit, gas path, water path, powder path, etc. are all closely concentrated in the rod body of the gun. Due to the short spraying distance, the heat of the plasma arc is compressed in the smaller inner hole space, so the substrate is easily overheated, and the outer casing of the spray gun is also heated. For this purpose, the barrel jacket must be able to withstand temperatures up to 250 °C. Cooling air should be introduced to the opposite side of the spraying direction of the plasma jet at the spray gun mouth to cool the substrate, and the outside of the substrate should also be cooled by effective blowing. The substrate rotates at a linear speed greater than 50m/min, so that the temperature of the substrate is lower than 150°C. When high-power spraying is used, the substrate can even be forcedly cooled with CO2 liquefied gas. The plasma spray gun should be maintained at 11℃~13℃ by circulating cooling to keep the performance of various parts in the gun body, especially the seals, stable and reliable.



    Thinner coatings (for example, coatings as thin as 0.3mm thick) are prepared on the inner surface without special problems compared to sprayed flat and outer surfaces. When preparing a coating of materials with a high shrinkage rate of more than 0.5mm in the inner hole, the following measures should be taken:

    1) Generally, a symmetrical thread groove can be processed on the surface of the sprayed inner hole, or a zigzag thread groove can be cut. Align the thread groove when spraying so that the spray material will completely fill the thread groove.

    2) Appropriately increase the preheating temperature of the workpiece, so that the matrix structure can be pre-expanded to compensate for the shrinkage of the coating when it cools and improve the bonding strength of the coating and the matrix. In operation, a quick, multiple-sweep spray method is used to cover the substrate before the surface is heavily soiled, while avoiding hot spots.

    3) When the inner hole is sprayed, the dust is blown and ventilated, and the outer surface is effectively blown to cool down. In addition to the possible problems of poor coating bond due to shrinkage, the problems associated with spraying the bore surface are overheating and dust contamination. When spraying the surface of the inner hole, due to the small space and poor ventilation, and the possibility of additional cooling is often limited, the heat is often concentrated in a small area, especially when spraying thermal insulation materials such as ceramics, overheating The problem is particularly serious. These spray materials tend to hinder heat transfer to the base structure. The result is that almost all of the heat generated is concentrated in the inner holes being sprayed, so supplemental ventilation must be used.