The power piston and cylinder are made of titanium 6A1-4V (Ti6A1-4V) alloy. This material was chosen because of its light weight and high strength. Unfortunately, titanium and its alloys have poor tribological properties, especially in regard to their tendency to scuff and gall when in sliding contact with themselves or many other alloys. In the current design, the piston is a free piston that spins in the cylinder and functions as a gas bearing. The piston/cylinder clearance is very small, on the order of 10 )*m. Under ideal steady state conditions, a hydrodynamically pressurized gas film prevents contact between the piston and cylinder. However, contact occurs during starts and stops and occasional high speed rubs may occur. This necessitates the use of a back-up selflubricating coating to prevent scuffing, galling and possibly catastrophic seizure of the piston and cylinder in the event of a scuff. This is especially important because of the severe galling tendency of uncoated titanium. PM212 sliding against PS212coated Ti64 gave low fric- PS212coated Ti64. Wide wear tracks, and evidence of severe tion and wear at 25 and 125OC in a dry nitrogen adhesive wear are evident on the uncoated Ti64 disks.In atmosphere.contrast, wear areas are barely distinguishable on the PS212. The plasma-sprayed PS212 coatings, with a thin, intercoatings.The combination of a thin bond coat of nickel- mediate bond coat of nichrome, adhered strongly to the chromium with a top coat of PS212 is an adherent coating Ti64 substrates.
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