Vacuum plasma spraying (VPS) has been demonstrated to be an effective technique for the fabrication of refractory-metal components of solar-thermal engines. Heretofore, such components have been fabricated by specialized techniques that include electrical-discharge machining, shear spinning, sintering under pressure, chemical vapor deposition, and electrochemical machining. Though effective, these specialized techniques are time-consuming and costly. On the other hand, VPS makes it possible to fabricate components with complex shapes, simply and at relatively low cost.
VPS is a thermal-spray process conducted in a low-pressure, inert-gas atmosphere. A hot plasma is generated by making an inert gas flow through a direct-current arc. The plasma flows through a nozzle to a downstream vacuum chamber, where the pressure is maintained at about 100 torr (about 13 kPa) and the deposition substrate is located. A powder of the material to be deposited is injected into the plasma, causing the material to be heated and accelerated toward the substrate. The process is continued until the deposit reaches the desired thickness.
In the present application of VPS, the substrates used to form solar-thermal-engine components are graphite mandrels. Each mandrel acts as a male mold to form the inside of the deposit to the required size, shape, and surface finish. Inasmuch as graphite is soft and easy to machine, complex shapes and precise surface textures can be achieved without difficulty. For example, helical flow channels for transfer of heat to a working fluid can be formed integrally into a component of a generally cylindrical absorber cavity (see figure), and textures necessary for maximizing absorption of solar radiation can be imparted to the inner surface of the cavity by the outer surface of the mandrel used to make the innermost component.
After the refractory metal has been deposited to the desired thickness, the mandrel is removed by blasting with acrylic or other polymeric beads, which are hard enough for eroding the graphite but soft enough not to damage the refractory-metal deposit. Removal of the graphite in this manner is rapid and simple and complies with environmental laws.
Because the interior of a component formed by VPS is inherently of net size, shape, and surface finish imparted by the mandrel, no additional machining or surface conditioning of the interior is necessary. The exterior dimensions can be controlled by controlling the deposition rate and time, so that the exterior of the VPS-formed component is near net size and shape. Thus, little or no subsequent machining is necessary; this is especially advantageous for fabricating components of tungsten, rhenium, and molybdenum, which are difficult to machine by conventional techniques. Even in cases in which tolerances are tighter than are achievable by VPS, the amount of subsequent machining and grinding needed is less than in older methods and is confined to exterior surfaces, which are more accessible than interior surfaces are. While VPS shares these advantages with other vacuum-vapor-deposition processes (chemical vapor deposition and physical vapor deposition), VPS deposits materials at rates orders of magnitude greater than those of the other processes.
This work was done by Frank R. Zimmerman, D. Andy Hissam, and Harold P. Gerrish of Marshall Space Flight Center and William Davis of Boeing North American.
Inquiries concerning rights for the commercial use of this invention should be addressed to
the Patent Counsel
Marshall Space Flight Center; (205) 544-0021
Refer to MFS-31242.