Titanium dovetail joints between the turbine blades and disk are often susceptible to fretting and fatigue. While lubricious Cu-Ni-In coatings have been used to delay fretting, complex mechanisms appear to transfer titanium to the softer coating at the temperatures typical for the fan. At the higher compressor temperatures, copper segregation leaves the coating significantly weakened. Given the need for improved coating performance, a new coating method known as High Velocity Particle Consolidation (HVPC) warrants review. HVPC is a promising lower-temperature spray method that creates a coating through a process related to friction welding by exposing a substrate to a high-velocity jet of solid particles. The unique feature of HVPC is that the particles are accelerated by a supersonic gas jet at temperatures well below the melting temperature of most materials. Hence, the problems of oxidation, crystallization, and residual stresses can be avoided. To test coatings and simulate the dovetail, an accelerated tribotesting procedure that employs a combination of gross-slip displacements ranging from 25-125ƒÝm was developed. This methodology was used to assess hard coatings including molybdenum and cobalt for up to 3x106 fretting cycles. Results indicate that the mixed range of gross-slip displacements can initiate limited titanium transfer, but not the segregation experienced by the compressor. The study also found that the fretting resistance and likelihood of Ti adhesion of the HVPC coatings is virtually equivalent to that of Plasma Spray. Finally, the study also found that both molybdenum and cobalt coatings show promise for reducing fretting damage and eventual fatigue.
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