A computer-based approach for developing functionally graded and layered coatings with detonation spray coating process

Introduction Some commonly used coating processes such as High Velocity Oxy-Fuel Gun1-2, Plasma Spray3-5, Detonation Spray Coating6-8, Laser surface alloying and cladding9 are widely used in various industries for enhancing the life of critical components. These processes modify the tribological properties or corrosion or oxidation resistance at elevated temperatures10. Such improvements in coating properties are all the more beneficial when the composition is graded or layered across the thickness. In this way, coatings can be tailored to suit specific applications. By properly designing the microstructure, abrupt failures of components can be avoided. In conventional DSC processing, a dedicated powder feeding device is employed to carry powder into the high temperature zone, generated by a detonation. Examples of such feeders are mechanical-disc type of feeding, screw feeding and fluidized bed feeding. In our original DSC system, the fluidized bed type is used to inject a small amount of powder into the detonation stream so that it gathers sufficient heat and acceleration before bombarding the substrate. This process is repeated enough number of times until the deposited particles form homogeneous, well-adhered and moderately smooth surfaces with thicknesses ranging from 40 microns to 1 mm. However, such a powder feeder system has limitations as only one type of powder can be coated. To overcome this problem, we have used multiple powder feeders, where the quantity of each powder can be dynamically controlled during a process run. This is achieved by electronically controlling the time of opening of the solenoid valves that inject each powder. Thus the quantity and composition of the injected powder mix can be varied, either sequentially or simultaneously as a function of time, so that gradient coatings or layered coatings can be formed. The specific focus of this paper will be on the development of a reliable and repeatable control mechanism to achieve these objectives.