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Monday, May 15, 2006 - 3:30 PM
SEAA061.4

Copper-Nickel-Indium, a Classic Basis for Coating Development

R. K. Betts, Cincinnati Thermal Spray, Inc., Cincinnati, OH

 

The enigma of fretting as a wear phenomenon with consequences of fatigue-life reduction  in machines of many kinds, is well recognized. Incursion of jet engine propulsion into aircraft piston engine realms further raised imperatives for performance and reliability of both military and commercial aircraft. Jets became Fan engines, propelling by-pass air, improving primary thrust efficiency. Engines enlarged, and Ti alloys, with fortuitous strength-weight ratios, were introduced. However, compromised by wear, Ti alloy surfaces may be sensitized to fatigue crack initiation, thus requiring effective coating protection to assure reliability. Thermal spraying has been utilized for this from the earliest days of jet engine service. Review traces origins, testing and adaptation of unique Cu-Ni-In thermally-sprayed wear-resisting coating for aircraft turbine engines. Wear effects upon Ti-6Al-4V alloy and coatings are compiled from patent, contract and unpublished research literature, including dedicated machines built for simulative sliding and fretting modes. Specimens were first subjected to sliding wear at increments up to 10,000 cycles of 0.006 inch stroke, under 50,000 psi contact stress, with friction history continuously monitored. Materials were evaluated by relating friction to surface disruptions such as striations, galling, and debris. In-situ fretting was generated under high-cycle fatigue conditions. Fatigue tests quantified integrity reduction due to wear, providing baselines to specifically qualify wear protection afforded by coatings.

Cu-Ni-In, augmented by MoS2-based dry film lubrication, emerged as the most effective  system. Applied to both surfaces, assuring no disruption of Ti surface integrity, it is shown durable for wear cycles up to prescribed limits, with friction coefficients remaining less than 0.06. Fretting fatigue life run-out exceeded 106 cycles. Specialized plasma spray  facilitizing is described for production coating of engine components. Attributes of Cu-Ni-In and other materials from contracted research are characterized, providing a basis for ongoing studies toward improved protection methods.


Summary: Paper traces the origin, testing and adaptation of the unique alloy Cu-Ni-In as a thermally-sprayed fretting wear resisting coating for world-wide aircraft turbine engine applications. Review of patent, contractural report and documented literature provides a foundation for ongoing work on newer, improved functional replacements. Copper and Nickel are widely used pure, together and separately as bases or constituents of many useful, well known alloys. But spiced with the indigo-blue spectral characterized rare metal Indium, the trimet Cu-Ni-In, though ubiquitous in aircraft engines, serves relatively unknown. Unusual, for it helped launch Ti alloys safely in commercial and military engines, coincident with late 1960s advent of the large turbo-fan high bypass designs powering C5A, 747, L1011 and DC-10 aircraft. Cu-Ni-In use for wear protection began in earlier aft-mounted fan bypass engines, compound designs with the last stage LP turbine blade lengthened into a fan-configured airfoil, called a "flade". Patented in 1958, the trimet originated as a brazing alloy featuring low grain boundary diffusion. Studies of thermally sprayed wear protection materials for flade mid-span seal abutments found Indium oxides, formed in-situ, provided dry lubrication augmenting the Cu-Ni base into a more durable coating. From this limited backgriund, extensive fretting fatigue research was performed on Ti alloy substrates for the large fan engines. Cu-Ni-In emerged, synergic with low-friction MoS2 dry surface lubricant, as the effective wear protection coating for mating fan blade and disk dovetail, or root bearing surfaces. Adapted into production using a unique plasma spray system for fan disk slots, its applications spread from fans to compressors, to smaller engines for helicopters, and to other life-cycle limited components. Cu-Ni-In use grew with the rapidly expanding competitive aircraft turbine engine markets of the 1970s and beyond, wherein its reliability still pervades.