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Project ID: G1RD-CT-2002-00712
Finanziato nell'ambito di: FP5-GROWTH
Paese: United Kingdom

Low friction and wear resistant PVD coatings

Low Friction and Wear PVD Coatings

IPN deposited further examples of the optimised W-Ti-N coatings (i.e. V=-70V with the relative pressures of the deposition gases, pN2/pAr, in the range 1/3-1/5, resulting in a chemical composition WxTiyNz=(35-40) at.% (x+y+z=100%)) for evaluation in WP 8.
Development of PVD Coatings - TCL

During year 3, in addition to depositing the standard MoST and Graphit-iC coatings for evaluation in WP 7, TCL carried out some limited additional development of the coatings in an effort to extend the available range of operating characteristics.

MoST/TiB2.

One MoS2 target was substituted with a TiB2 target. The investigation focused mainly on a multilayer approach. The TiB2 sputtering power was varied, relative to the MoS2 target power, to produce multi-layers with different thickness, typically 20-30nm for each individual layer. The TiB2 magnetron current was set at 1.0, 1.4, and 1.8 amps. Glancing angle XRD was used to evaluate the resulting structures. An SEM investigation showed no columnar features in the cross section of the multilayer coating. The back-scattered image in the SEM enabled the multi-layers to be seen (for thicker multi-layers).

The thickness of the coating was varied in the range 1'Ým to 2'Ým. It was possible to reduce the wear rate further to 10-18 m3/Nmm. There was no increase in hardness. Adhesion, as evidenced in the Rockwell test, was worse if layer thickness was increased. At much higher thickness (e.g. 3'Ým) adhesion scratch test results also degraded, although wear rates, as measured by pin on disc were still very good. Try to control the sputtering of TiB2 by a more gradual change V can maintain high wear resistance and low friction.

The advantages of the combined coatings, compared to the standard MoST, were that thicker coatings could be used, with a higher temperature capability and reduced humidity sensitivity (e.g. very good results were still obtained when testing at 45% RH).

MoST/ Carbon coatings

One MoS2 target was substituted by a carbon (i.e. graphite) target. Both MoST with carbon additions as a composite coating, and MoST/C multi-layers were investigated. The objective was to minimize the resulting friction coefficient (and therefore the investigation did not focus on pin on disc performance at high applied load). Because both carbon and MoS2 were sputtered the Ti target was unavoidably ¡§poisoned¡¨ and would normally have required cleaning between coating runs. To save time the intermediate cleaning run was been omitted for the preliminary series of experiments.

With C addition to the MoST there was no obvious decrease in friction coefficient. With the multilayer approach, an attempt was made to increase the coatings wear resistance, reducing the MoS2 coating content gradually. However, overall there was no significant improvement compared to the standard MoST coating.

MoST in combination with hard coatings

The self-lubricating coatings are often used in combination with a hard underlayer. The effect of different hard coatings in combination with MoST was evaluated: MoST, TiN+MoST (4x increase in wear life), CrTiAlN+MoST (5x wear life), Graphit-iC+MoST (14.5x wear life)

It was clear that, in combination with MoST, a Graphit-iC underlayer is more than just a mechanical support ¡V the whole coating can work as a lubricant. For CrTiAlN+MoST, when the top layer was worn through the friction increases dramatically.

For Graphit-iC, tested at 40N & 80N load it was possible to detect an increase in friction when the G-iC layer was exposed. When testing a pure Gi-C the starting values of the friction was higher ¡V ie the Graphit-iC exhibited a ¡§running in¡¨ effect, but a MoST top-coat changed this behaviour significantly.

In summary, the low friction coatings have been shown to be potentially beneficial in their intended applications.

Informazioni correlate

Reported by

Teer Coatings Ltd.
West Stone House, Berry Hill Industrial Est.
WR9 9AS Droitwich, Worcestershire
United Kingdom