TECHNOLOGY
Norton men instrumental in developing Rokide coatings. Left to right: Edwin Lowe, W. M. Wheildon, and N. N. Ault. Coatings resist temperatures in excess of 3000° F.
Found: Flarne-Resisîanî Coatings 3000
Pure metallic oxides resist temperatures in excess of F. and erosion of supersonic velocities
W O R C E S T E R , M A S S . - H i g h l y refractory crystalline oxides which protect metal parts from extreme temperatures were developed b y t h e Norton Co. Deposited as a fine spray on the surface of metals, such as alloys, aluminum, and steels, t h e oxides offer protection from erosion a n d provide thermal insulation. Industrial scientists, in the past, have concentrated efforts t o find high temperature, refractory, a n d wear-resistant coatings for metals. Most work was devoted to ceramic coatings similar to porcelain enamels used on stoves, and refrigerators. Experiments with these type of coatings were confined, for the most part, to temperature ranges of from 1400° to 2200° F. However, research programs in jet and rocket propulsion required protection for metal parts in temperature ranges in excess of 3000° F . At these high temperatures enamel-type coatings w e r e not adequate. Aluminum O x i d e . Norton development work commenced several years ago w i t h the idea of finding a m e a n s to deposit a high-melting oxide, such as aluminum oxide, directly on a metal surface. A reasonable degree of adherence and impermeability were prime requirements. After much mechanical difficulty in application, it conceived that a sintered 2608
rod could be fed through a metalizing gun at a controlled rate using oxyacetylene flame to melt the oxide. T h e molten metal could then b e projected onto a metal surface by a jet of air. T h e coating applied in this m a n n e r was extremely hard, had good wear, and chemical resistance qualities. Once applied, the oxide coating would not chip or flake. Rod P r e p a r a t i o n . Fine powdered alumina is mixed with water and an organic material, such as cornstarch. T h e plastic material formed is then extruded to form rods 24 in. long and V 8 in. in diameter. After drying and baking in a kiln, rods are ready for use. Obviously, this method of preparation is not restricted to aluminum oxide. Some of the oxide coatings prepared include zirconium oxide, titanium oxide, magnesium aluminate, iron oxide, zirconium silicate, and chromium oxide. Of all the materials w h i c h can b e used as coatings, alumina, zirconia, and zircon h a v e received most attention from Norton. It will produce them commercially as Rokide A, Z, and ZS, respectively. C o a t i n g P r o p e r t i e s . T h e r m a l expr ision, thermal conductivities, and densities are lower for the oxides than for steel. Melting points and hardness values are higher. T h e s e proper-
ties indicate that such coatings are a n improvement over b a r e steel for resisting t h e erosion and corrosion of combustion gases. T h e low thermal conductivities reduce t h e temperature of the metal backing by insulating it. Melting points and hardness are higher than steel. Hence, the coating is not worn away or melted by rapidly moving hot gases. Oxide coatings also have certain limitations. Ceramic materials are brittle. Their compressive strength is about 10 times greater than their tensile strength. T h u s coatings will withstand more stress in compression than in tension. This was borne out in thermal cycling tests of coatings o n concave a n d convex surfaces, Coatings on concave surfaces, such as t h e inside diameter of a tube, will withstand m a n y more cycles of heating and cooling than will coatings on t h e outside of a t u b e or cylinder. Another limitation of the coatings is their slight porosity. A p p l i c a t i o n . Coatings can h e applied to practically a n y materials, b u t it is most commonly applied to metals. Application is b y m e a n s of the previously mentioned metalizing-type spray gun which was designed for this purpose by Norton Co. Thickness of t h e coating is usually between 0.OO5 and 0.100 inch. O n e rod will cover 15 square inches, 0.010 inch thick in 6 minutes. Products that h a v e been coated thus far include rocket a n d guided missile components. Coatings have also been placed on ramjets, gas turbines, and miscellaneous b u r n e r parts. Silicon C a r b i d e . Also announced by Norton was a silicon carbide coating on graphite. This product is known as Rokide C. T h e coating increases t h e surface hardness of graphite to that of silicon carbide ( w h i c h stands next to diamond a n d boron carbide in hardness). Oxidation resistance is increased b y 300 to 4 0 0 % . Rokide C coating is applied b y reacting the outer graphite surface with vapors from boiling silica sand t o form silicon carbide. Therefore, the graphite surface itself is transformed to a strongly adherent silicon carbide coating. Coating thickness is usually b e tween 0.002 and 0.010 inch. Graphite-coated products will b e useful in combustion chambers of rocket motors. F u r t h e r use is expected in nozzle exhaust cones and ramjet exhaust nozzles. Rokide coatings a n d the methods of application are both protected b y patents. Norton will make these processes available, u n d e r license, on a nonexclusive basis to all manufacturers.
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