Control of substrate temperature in thermal spraying and its influence on coating quality

       The control of the substrate temperature has a great influence on the thermal spray coating, and it is also an important parameter of thermal spraying.



To control the temperature of the substrate, the ideal spraying process is to preheat the workpiece to the temperature that will be reached during the spraying process before spraying, and then take cooling measures to the workpiece during the spraying process to keep it at the original temperature. This effectively combines preheating and subsequent cooling.



If the temperature rise of the workpiece is too high during the spraying process, the workpiece will be deformed, and oxides are easily generated on its surface, which is very unfavorable for the bonding between the coating and the substrate and the coating layer. Therefore, the workpiece temperature must be controlled.



Most workpieces need to be preheated before spraying begins. The purpose is to remove moisture and control thermal expansion of the substrate relative to the coating to improve bonding. Regardless of whether the thermal expansion coefficients of the sprayed material and the base material are the same, there are issues of thermal expansion and thermal stress and their control. This is determined by the characteristics of the spraying process itself. For example, when spraying ceramic powder (such as alumina), if the substrate is not preheated, it often occurs that the coating will crack before cooling to room temperature. , can withstand very severe thermal shock. As for the selection of the preheating temperature, if the thermal expansion and thermal stress of the coating and the substrate are considered, it can be determined by the thin-film spraying test. According to the degree and direction of warpage of the sprayed sheet (about 1.5mm thick), the thermal expansion and residual stress are qualitatively studied. For example, when spraying aluminum oxide on a steel substrate, the preheating temperature is often selected based on the standard that the sprayed thin sample does not warp at room temperature. Insufficient preheating results in concave warpage of thin samples, and convex warpage occurs when preheating is excessive.



In the spraying process, in addition to controlling the temperature of the workpiece by adjusting the moving speed of the spray gun, cooling measures, such as auxiliary blowing cooling or water cooling, are often required to maintain a certain temperature of the workpiece. For large workpieces and good natural cooling conditions, generally the entire temperature can be controlled only by a suitable gun moving speed. The spray gun moves quickly on the surface of the workpiece, and sprays multiple times according to the required coating thickness, so that the heat sprayed by the spray gun will be evenly distributed on the entire workpiece, and local hot spots will not be generated. The adverse effects of local hot spots on the workpiece on the coating and substrate are more serious than the uniform temperature rise of the workpiece as a whole. When spraying a particularly large workpiece, it is required to keep the substrate at the required temperature, so it is no longer necessary to cool, but to take appropriate auxiliary heating measures. These should be predetermined during coating design and process specification. Of course, in the spraying of general workpieces, most of them want to use cooling measures. The easiest way is to blow air for cooling. It should be noted that there must be a certain distance from the spray point when blowing, so as not to affect the beam current. Whenever possible, the blow should be directed towards the parts of the workpiece that are not sprayed. During the spraying process, the cleaning requirements of the cooling gas will be stricter when the dust blowing is followed, so as not to contaminate the surface of the workpiece and reduce the bonding and interlayer separation. For the spraying of small workpieces, especially when spraying ceramics, the power used for spraying is large, and the thermal conductivity of these materials is very poor. It is necessary to use forced air cooling or water cooling. When spraying materials that oxidize violently at lower temperatures, such as tungsten, the surface temperature must be kept low to avoid excessive oxidation of the coating. Taking the spraying of tungsten as an example, the tungsten coating just after spraying leaves the protective cover of the plasma flame gas and is exposed to the air. At this time, the surface must be cooled to below 300 ℃ to prevent serious oxidation. When spraying, the cooling intensity can be adjusted by observation to control the temperature. When the substrate and coating are not overheated (surface temperature is less than 300°C), the tungsten coating exhibits metallic luster. If the coating is overheated and exposed to air, it will quickly turn black.