Helicopter transmissions are the heart and soul of the rotary aircraft. The design of this critical flight system includes a variety of precisely machined components produced from a highly scrutinized selection of materials. All of these components are protected by a commonly known conversion coating called Black Oxide.
Black oxide coatings are chemical conversion coatings that are the result of a chemical reaction between the iron in a ferrous alloy and the hot alkaline salt bath solution. This reaction produces a magnetite (Fe3O4) on the surface of the substrate. The magnetite is black in color and provides limited corrosion resistance. The Black Oxide coating can be combined with an after finish of oil, wax or lacquer to increase the corrosion protection by a factor of 10X.
Black oxide coatings are used extensively in helicopter transmission assemblies for the protection of individual components. A significant advantage provided by these coatings in this particular application is that the critical “as machined” dimensions of the transmission components are unchanged by the application of the black oxide finish. Since no coating build up occurs, the design and machining operations are greatly simplified and dangers associated with tolerance stacking are eliminated.
Because the black oxide coatings are conversion coatings the danger of chipping, peeling or flaking inherent in other plated coatings is non-existent. This presents a huge advantage when you are designing an assembly of intricate gears and drive mechanisms. You can imagine the potential for disaster if metal flakes and chips mingle with lubricating fluids in the drive assembly. Gear teeth don’t do well in a situation like that. Not to mention potential damage to pumps and filters. While plated coatings have their distinct advantages, conversion coatings, as you can see, are better suited for these types of applications.
Black Oxide coatings are also not affected by temperature changes. Thermal expansion does not pose a problem for these coatings like it could for barrier coatings like chrome or nickel. While the components are immersed in a stream of lubricant during the operation of the transmission, the mechanics of the system do generate heat. The heated lubricant does not, however, adversely affect the black oxide coating.
Black Oxide also provides limited anti-galling protection. Most helicopter transmission components are manufactured from either mild steels like 4140 or stainless steel like 17-4 PH. Galling is a potential problem for all types of stainless steel threaded assemblies including fasteners and inserts. While Black Oxide alone won’t prevent galling, the combination of black oxide and oils or transmission fluids raises the anti-galling capability significantly.
Coating Technologies Inc. provides Black Oxide coatings meeting the following specifications:
Mil-DTL-13924, AMS2485, HP4-50, BPS4084 and M690330.