DLC coatings

Thursday, February 09, 2012

Tags :  con-rods

con-rodsThere has been mention of coatings and other surface treatments for con rods in previous RET Monitor articles, and in the articles on con rods in Race Engine Technology magazine (see issues 40 and 58).

At the recent Autosport Engineering show in Birmingham, England, I discussed the matter of rod coatings with a supplier, who showed me a couple of con rods he had to hand. The supplier explained to me that the con rods were coated with diamond-like carbon (DLC) with the aim of reducing friction and wear of the thrust faces of the big end. Other coatings have been used over the years for the same purpose and with some success. Metallic molybdenum has been used for a long time, although this has been supplanted to a large extent by the very hard, thin and lower-friction engineering coating processes such as DLC and chromium nitride (CrN).

DLC has an advantage over CrN and the metallic coatings in that it has a very low coefficient of friction. While this should have little advantage once conditions for full hydrodynamic lubrication are satisfied, lower friction and improved wear characteristics are useful at start-up and when lubrication is in the mixed regime.

In addition to the application of DLC to big-end thrust faces, it could also be usefully applied on the thrust faces of con rods at the small end. In this application, where the motion between sliding surfaces is at relatively low velocity and where the motion is intermittent, such coatings might prove to be very useful.

There is a further use for low-friction coatings for the manufacture of con rods with interfered small-end bushes, especially in highly stressed applications that use titanium for the con rod. Titanium is prized for its combination of strength and low density, and has found common use in racing as well as an increasing acceptance as a material for production car and motorcycle con rods.


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Where a bush is pressed into the small end of a titanium con rod, there is a chance that the bush may 'pick up' in the bore of the rod, with the consequence that the bore of the rod is damaged. Depending on the location of the damage, this may be of little significance, but there is always the chance that the damage is in a critical location which could cause the rod to fail. For such applications a DLC coating applied to the bore of the small end of the rod can be used to reduce the chances of such damage occurring. If we can prevent the damage happening, we don't have a 'sore spot' from which a fatigue crack could emanate.

In addition, the use of coatings in the bore of a con rod could allow the rod to run 'bushless'. This has been discussed a number of times, but the main application of this design philosophy has been with steel rods. If the same could be achieved with a titanium rod then we could reduce the size of the small end while reducing the risk of bore damage by not having to fit bushes.

Fig. 1 - DLC-coated rods spotted at a trade show recently

Written by Wayne Ward

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Comments

I can't really see where DLC coating of conrods would be beneficial.  With the conrod small end side faces, there is not normally any contact here.  The piston is bore guided and the conrod is located by the big end faces.  A DLC on the big end faces would not help much either, since the thrust forces are very low and there is lots of oil flow at the surface.

The DLC process itself can also present problems with conrods.  High performance conrods are typically shot-peened to improve fatigue life.  The relatively high temperatures (400degF or more) used in the DLC process can produce a stress relief of the conrod, negating the benefits of shot peening.

If smearing of the small end bushing during a press-fit installation is a concern, thermal fitting of the bushing would be a better option.