The IPD method comprises placing a cathode formed of a coating material into a vacuum environment within a vacuum chamber, providing a workpiece having a surface within the vacuum environment, supplying a current to the cathode to form a cathodic arc upon a cathode surface resulting in erosion or evaporation of coating material from the cathode surface, and depositing the coating material from the cathode upon the workpiece surface. The surface of the workpiece may be referred to more generally as a substrate surface. The workpiece may be a turbine blade part or component in need of coating or repair, but the method and apparatus can be used to deposit coating materials upon any substrate surface that can be subject to an IPD method.
A cathodic arc is formed upon a surface of a cathode when a cathode and an anode are placed in a vacuum environment, and a sufficient voltage is applied to the cathode to create a sufficient potential between the cathode and the anode to form a discharge on a surface of the cathode. This discharge creates a highly ionized metal plasma from the erosion of the cathode. The arc is formed for a short period of time, extinguishes and reforms rapidly, giving the appearance that the arc is moving continuously over the surface of the cathode.
The erosion of the cathode forms a cloud of coating material containing ions, charged particles, vapors and neutral droplets in the vacuum environment. This coating material is then available for deposition within the vacuum environment. The coating material may deposit upon cool surfaces in the vacuum environment by condensation, and also upon an anode surface by electrical attraction as well as condensation.
A workpiece may be placed in the vacuum environment to form a coating upon a surface of the workpiece. The workpiece may be manipulated within the vacuum environment to produce a uniform coating over the entire surface of the workpiece, or the workpiece may be masked in order to produce a coating on a selected surface. The workpiece may be held at ground potential or a voltage bias may be applied to the workpiece to affect material deposition by increasing the attraction of ions and charged particles to the surface.
The anode may be a conductive surface of the vacuum chamber or a separate structure within the vacuum chamber. The function of the anode is to sink electrons from the cathode to sustain the electrical discharge.
In the past, cathodes have been formed of solid cylindrical stock having a flat end-face. These cathodes are typically cut to short lengths of about 4.4 cm with an outside diameter of about 6.3 cm. The plasma arc is formed at the flat end-face of the cathode open to the vacuum environment.
Multiple cathodes are often positioned in a vacuum chamber around the workpiece to increase the amount of coating material available for coating upon the workpiece surface. These conventional cathodes operate on current of approximately 40 A to approximately 1200 A.
A plasma deposition system has been disclosed by S. A. Mouboiadjian, E. N. Kablov and Ya. A. Pomeloov in their paper “Equipment, technology and protective coatings produced by ion bombardment deposition.” This system discloses a cylindrical cathode that has an arc formed on a circumferential surface. However, the structure requires a ring-shaped cooled anode and electromagnetic coil fixed coaxially around the cathode to effect arcing.
Currently, the cost of an IPD process is dominated by factors pertaining to the cathode. A cathode that allows for more coating material to be available to the coating process will have a significant impact on the process cost by allowing the process to be operated for greater periods of time without having to replace the cathodes. Additionally, by not requiring the cathode to be replaced as often, a more uniform and improved coating material may be deposited.
Other features and advantages of the present invention will be apparent from the following more detailed description of a preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate by way of example, the principles of the invention.