friction reduction and frictional wear reducing systems


Low friction and wear reducing systems

Well designed material and coating combinations can deliver a major
improvement in friction, tribo-oxidation, fretting and wear control.

Friction and wear of metals, ceramics,
semi-ceramic, composites, plastics and seals

Possible improvements by using optimized (thus incompatible) tribological pairs:

Prevailing friction coefficient:

  identical incompatible Lubricated
Metal on metal 0.82 0.36 0.07
(Non)metal on nonmetal 0.53 0.12 0.08

The ideal material or ideal material combination for low wear and friction purpose does simply not exist. However, during our laborious tribological wear and friction research program a staggering amount of possible optimizations became available. In many application Lunac 2+ coatings are able to reduce the friction and wear in especially water lubricated axle-seal systems due to the highly incompatible nature. We did not only test Lunac versions, but many other material combinations as well. Because of the almost endless amount of possible combinations a considerable
amount of optimizations with other materials were found as well. This offers us an extra possibility to act as an advisor with new ideas. The following diagrams display a part of the wear and friction data of seals and bearings acquired from our tribo-tester. The WMV tribo-tests offer information about the influences of couple variations as well as speed, kind of movement, temperature, pressure, moisture and lubrication. Conclusively, the results proved that seal and bearing systems in new applications should always be tested first, because exceptions can often show up.


Dry and wet friction

tribo wear'Water lubricated' and dry friction systems follow essentially different tribo pathways. This figure represents two PA 12 pins that were tested on our tribo tester (equal conditions, but different medium). The left pin displays the effect of the prevailing tribo-oxidation in the dry PA 12 / stainless steel 1.4404 system. The right pin displays the effect of adhesive wear in the 'water lubricated' PA 12 / stainless steel 1.4404 system. The wear rate of the right 'water lubricated' system turned out to be a 7-fold higher. Conclusively; tribo-oxidation is not always unwanted. In the 'water lubricated' system the stainless steel must be replaced or plated with an anti-galling coating or the plastic must be replaced by a plastic with lower covalent bonding tendency such as UHMWPE (within an acceptable load range).

Water lubricated friction:

Tested on the WMV pin ring tribo tester according to the GLP norm.

Materials pin: Polyurethane, viton®, ecopur®, x-ecopur®, HT-HNBR, Fluoroloy 45, P6000, GUR 4120 PE, Duralion PE
Materials ring: Flame sprayed Cr2O3, Lunac 2+, 1.8550 nitrided steel, 18-8 stainless steel
Date: September 2007

Diagram 1: Click to enlarge (click again to reduce)


seal wear and burning

This picture displays the effects of a (coated) propeller shafts on 3 similar seals (HT-hnbr is a high temperature nbr version). Clearly visible is the low thermal conductivity effect of a full ceramic (chromium oxide) coating. A closer inspection revealed micro cracks in the black contact zone after bending. The high thermal conductivity of Lunac 2+ lowers the interface temperature. No leakage inducing micro cracks are developed on the Lunac 2+ coated shaft. Consequently, Lunac 2+ can spare seals.

Dry friction:

Tested on the WMV pin ring tribotester, according to the GLP norm.

Tested materials: Poly urethane, viton®, ecopur®, x-ecopur®, POM, PEEK, Ertalyte Tx®, PTFE, C-PTFE, Fluoroloy gold®, Fluoroloy 45®, HT-HNBR, GUR 4120 PE, epoxy-fenol.
Materials ring: Flame sprayed Cr2O3, Lunac 2+, 1.8550 nitrided steel, 18-8 stainless steel
Date: November 2007

seal wear

Example of unexpected detrimental tribological behavior of Cr2O3 on HT-HNBR under dry conditions. The rubber wear is minor, but the wear of the much harder Cr2O3 is severe and green flakes were deposited on the HT-hnbr rubber. In this case hard metal(lic) surfaces are in favor.

PEEK is a favourable material in metal-plastic bearing systems. Hardly known is that serious galling can show up despite the low c.o.f. and initial low wear rate in this system. In our tests this galling issue always showed up within 2 hours! Conclusively, PEEK is able to tear stainless steel particles out of the Steel surface! Hard (semi) ceramic coatings can effectively stop this PEEK tribo issue.

PEEK wear
Teared out stainless steel particles arrested in PEEK surface.

Click to enlarge (click again to minimise)


Diagram 2

Lunac 2+ can both reduce and stabilize the coefficient of friction

Lunac 2+ - hardened steel:

One of the most important systems to compare is the hardened steel-hardened steel
(with slightly different hardness) pair versus hardened steel-Lunac 2+ pair.

Pay attention to :

- very low starting Lunac 2+ - hardened steel c.o.f.
- moderate 0.40 final Lunac 2+ - hardened steel c.o.f.
- 6.25 fold lower wear rate of the steel surface, although this steel surface
is runnning against a harder (Hv 1150/2100) semi-ceramic surface.
- higher 0.55 final hardened steel - hardened steel c.o.f.

Click to enlarge

wear steel on steel

wear steel on ceramic

Diagram 3 and 4
Pressure: 422 MPa
Velocity: 12.4 mm/s
Blue: coefficient of friction
Orange: wear rate

Lunac 2+ - Bronze:

nickel bronze friction

bronze friction

Diagram 5 and 6
Blue: coefficient of friction
Yellow: wear

During the first running-in, the coefficient of friction of the Lunac 2+ / Nickelbronze tribo system raised to over 0.5, just as the hardened 100Cr6 steel-Nickelbronze system did. However, after a restart the Lunac 2+-Nickelbronze system shows a stable coefficient of friction unlike the hardened 100Cr6 steel-Nickelbronze system.

Lunac 2+ - Polyurethane:

In a similar frequently tested system with: wear ring Lunac 2+ / NBR seal Shore A 70, the wear rate of the Lunac 2+ coating compared to a steel wear ring, hardened by chromium diffusion enrichment has been raised by 307 x.

Conclusion: In most tested systems Lunac 2+ coatings lower and stabilise the c.o.f. as well as the wear of both surfaces.
Only resonance at high pressure loads were sometimes able to hammer and slowly fracture the Lunac 2+ surface.

Nowadays, WMV laboratory is still almost continuously testing new material pairs and expanding its experience. Feel free to contact us for a technical exchange of ideas without any obligation.