Self-Lubricating Coatings (26167)
Multilayer tool coatings that generate, at elevated service temperatures, an oxidized lubricous material that reduces surface friction and the need for additional lubricant or cooling.
ADVANTAGES: Stable machine tool coatings suitable for high temperature operation with reduced or no lubricant.
SUMMARY: Hard coatings for cutting tools have been in use for thirty years and evolved from simple nitride and oxide coatings (TiN and Al2O3) to more complex, high-performance alloy nitrides such as TiAlN employed today. Interest in green manufacturing has spurred development of coatings for dry machining. Tools that can effectively tolerate or lower operating temperatures would reduce or eliminate lubricant usage and their associated enviromental impact. Reduction of the friction level in machining processes would also favorably lower power requirements. Incorporation of elements that readily oxidize to form lubricious oxides (e.g. V) has been explored. However ready diffusion to the surface in the host material generates destabilized structure in the bulk. Use of low melting metals such as silver also creates undesireable surface and structural changes. Thus the unmet need for coatings that remains strong, generate adequate amounts of lubricious oxide on the contact surface at elevated operating temperatures, without degrading the underlying tool substrate.
The present invention provides a composite tool coating with CrN/MoN layers which are sufficiently immiscible to maintain their individual layer structure and strength/hardness at elevated service temperatures. Futhermore the MoN layer undergoes oxidation to the friction-reducing lubricious oxide MoO3 during machining.
A reactive sputtering process was developed including Cr /Mo cathode power settings and nitrogen pressure conditiions that establish CrNx and MoNx compositions and thickness. Optimal deposition of CrN/ MoN afforded nanolayer films stable to 1000°C with hardness in excess of 25 GPa, suitable for tool coating. Sliding friction tests of a CrN/Mo2N multilayer coating exhibited an intial coefficient of friction increase from 0.4 (25°C) to ~1.0 (300°C), then steady decrease to ~0.55 at 600°C. A pattern similar to that observed in the VNx systems. Formation of the lubricious MoO3 was confirmed. This combination of conditions and materials shows significant potential as an in-situ activated lubricating machining system.
Michael Graham, Laurence Marks, Robin Koshy
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