The present invention relates to coating processes and compositions, and more particularly to processes and coating compositions for suppressing combustion of metals, and to the products of such processes.
The relatively high strength, low density and high melting point of titanium has led to increasing use of titanium and titanium alloys in aircraft engine components such as fan and compressor vanes and blades, and in other applications. However, titanium can be made to ignite and react with air in an exothermic self-sustaining rapid oxidation process referred to as combustion. Titanium has a high heat of combustion, low thermal conductivity and a spontaneous ignition temperature below its melting point. These properties favor ignition and subsequent propagation of titanium or titanium alloy combustion rather than melting, thus producing additional rapid local temperature increases and rapid propagation of the resultant combustion once ignition occurs. During operation of aircraft jet engines, titanium blade ignition is often caused by rub. An abradable seal of rubber, felt metal or nickel-graphite, is present on the engine shroud adjacent to the compressor blades to prevent gas backflow and consequent loss of pressure. Without such a seal, efficiency of the compressor would be severely impaired. Accordingly, seal-blade separation is necessarily small. During engine operation, the blades heat up, expand with heat, and even creep, so that some blades thereby rub against the seal. Severe rubbing and abrasion may occur when an engine part fails, putting the engine out of balance, or when a failed blade imbeds itself in the seal. This abrasion between seal and blade(s) can cause ignition of the titanium. The high-velocity air stream in jet engines aids further to the combustion of any titanium or titanium alloy component that ignites and causes burning particles and molten metal to be sloughed off therefrom. These particles can be entrained in the engine air stream and impinge on downstream engine components to ignite them and thereby spread combustion in the engine. Some single-layer metallic and organic coatings have been used for reducing combustion of titanium substrates. However, an organic coating, particularly its binder, will generally vaporize or burn off, before its protection is needed, in a 600.degree. F.-900.degree. F. working environment such as found in operating jet turbine aircraft engines. Also, many metals, particularly those capable of forming an intermetallic with the substrate metal, can if coated on the substrate diffuse into and significantly degrade engineering or mechanical properties, such as fatigue endurance, of the substrate, particularly in a relatively hot gas turbine engine environment.