Repairing Chipped Silicide Coatings on Refractory Metal Substrates
- Tuesday, 10 April 2007
Two methods have been demonstrated to be feasible.
The space shuttle orbiter’s reaction control system (RCS) is a series of small thrusters that use hypergolic fuels to orient the orbiter in space. The RCS thrusters are constructed from a special niobium-based alloy — the C-103. This alloy retains excellent mechanical properties from cryogenic temperature all the way up to 2,500 °F (1,370 °C). Despite its excellent, high-temperature properties, C-103 is susceptible to rapid oxidation at elevated temperatures. Were the naked C-103 alloy exposed to the operational thruster environment, it would rapidly oxidize, at least losing all of its structural integrity, or, at worst, rapidly “burning.” Either failure would be catastrophic. To prevent this rapid oxidation during thruster firing, the RCS thrusters are coated with a silicide-based protective coating — the R512a. Over time, this protective coating becomes weathered and begins to develop chips. Launch Commit Criteria limit the diameter and depth of an acceptable pit; otherwise, the thruster must be removed from the orbiter — a very costly, time-consuming procedure. The authors have developed two methods to repair damaged R512a coatings on C-103.
For the first repair technique, metal foundries, semiconductor manufacturers, and many other industries have developed and routinely use coatings that can easily be painted on metal to protect it from corrosion, including oxidation, to temperatures in excess of 2,500 °F (1,370 °C). These coatings are typically a well-chosen oxide in a special organic binder that adheres to metallic surfaces. The organic binder is selected, so that upon exposure to elevated temperature, the ceramic is held in proximity to the substrate and forms somewhat of a chemical bond to the surface. If the binder is freed from the surface, the ceramic deposit remains and maintains an effective oxygen barrier. Commercially available, off-the-shelf ceramic paints may be used to repair chipped R512a and protect the underlying C-103 from subsequent oxidation. The authors have identified several candidates that aid in the protection of C- 103. This first repair technique is considered somewhat temporary. The ideal use for the ceramic paints would be to repair an RCS nozzle when a chip is discovered, say, at the launch pad. It would serve as a protective coating for at least one mission, prevent the rollback of the shuttle, and postpone the replacement of the nozzle until a more opportune time in the ground-processing flow.
The second repair technique is based on using the native coating material of the RCS nozzles. In this case, the chipped area is ground out and a “green” R512a coating is applied to the repair area. After the green coating has dried, it must be heated at extreme elevated temperatures while in vacuum or inert atmosphere to initiate the solid-state reaction between the R512a and the C-103. In the early 80’s, a repair process was developed using a variant of the native coating and a focused quartz lamp to heat the local area. Due to the bulky size of the lamp and focusing assembly, only the areas along the outer periphery of the nozzles could be repaired. The authors have developed a technique using a fiber-coupled, high-powered laser as heat source to successfully fuse the green R512a to C-103. The resulting repaired areas on test coupons are chemically and structurally equivalent to the native coated areas. Since the fiber-coupled laser assembly is quite small and easy to handle, all areas of the nozzle are accessible for repair, including the throat area. Since this repair technique results in a protective barrier that is equivalent to the original coating, it is considered to be a permanent repair. Thermal modeling and calculations have shown that during the fusing process, all other areas of the nozzle remain within specifications, so that the processes are viable in situ on actual thrusters, although not while they are installed on the orbiter.
The two techniques are complementary in the sense that the ceramic paints are easily applied and do not require excessive temperatures. While not as desirable as the permanent repair, they could be applied for moderate protection until the permanent laser-repair technique is available to the repair area. Both repair techniques were originally intended for RCS nozzles,but the process could easily be applied to other geometries of R512a/C-103. Additionally, the two repair techniques may be adapted to other high-temperature coating/substrate systems.
This work was done by Robert Youngquist of Kennedy Space Center and Christopher D. Immer and Francisco Lorenzo-Luaces of ASRC Aerospace. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing & Prototyping category. KSC-12690/29