Diamond Like Carbon (DLC) on bucket type cam followers 6

[red4re]

I am rebuilding a double overhead V-6 engine and have noticed wear on the bucket cam followers and would like to know if anyone has had a good result with DLC coatings applied to the cams and or followers?

 

[CoryPad]

Yes, DLC provides high wear resistance for bucket tappets.

 

[TVP]

This technology is beginning to be used in mass produced engines. Nissan and Caterpillar are both actively working with DLC coatings. There are a number of SAE Technical Papers documenting some of the recent research, implementation, etc. The annual conference of the Society of Vacuum Coaters is another source of information on automotive applications.

http://www.svc.org/TC/TC08/CFP08Tribological.html

http://www.sae.org/servlets/SiteSea...=simplesearch.jsp&qt=DLC+coating&ofType=PAPER

 

[tbuelna]

red4re,

If designed, manufactured and maintained properly, a cam and bucket follower interface would not benefit from an (expensive) DLC surface treatment. The reason is that when the parts are properly designed and manufactured, the contact between the bucket and cam lobe should always be hydrodynamic (ie. the parts separated by a thin dynamic oil film), except during start-up and stopping. As long as the cam lobe and bucket surfaces have adequate compressive fatigue strength for the magnitude and number of load cycles it must endure, based on the hydrodynamic oil film pressures in the local contact zone, then the parts should have adequate life.

Adding a very thin, but very hard, DLC surface won't really help fatigue life either. The classic failure mode for this type of structure (ie. a thin hard case over a softer core under high localized contact stresses), is case spalling due to sub-surface initiated shear fractures. What this means is that it does no good to have a very hard, high strength case unless you have a core material with very high shear strength to back it up.

DlC's are great due to their hardness and low inherent friction properties. And they have their place when utilized properly. They are most beneficial for boundary-type contact conditions, where oils or greases cannot be used, or where a loss of lube may occur. But an engine valvetrain bucket follower and cam lobe would not likely have these operating conditions. So save your money.

Regards,
Terry

 

[CoryPad]

Terry,

You mentioned one of the prime reasons to consider DLC - low friction. To increase engine efficiency, low viscosity lubricants are used. This means the metallic surfaces that move will be more likely to wear, so a wear-resistant coating improves part durability. Also, the low friction reduces powertrain losses for reduced fuel consumption.

 

[BrianPetersen]

Cam / lifter wear on flat-tappet lifters has been associated with the reduction of ZDDP antiwear additive in new-formulation engine oils. The hot-rodders have been using ZDDP supplements for some time because of this ... and it's not only classic-car engines that are affected by this.

 

[ivymike]

Terry, as I mentioned in another thread, I believe you're mistaken w/regard to the lubrication regime at the cam-follower interface. It is my understanding that it's not uncommon for the cam-follower interface to operate in the mixed lubrication regime under load (opening, for example), with periodic excursions to boundary lubrication (In each regime there is metal-to-metal contact present, where a wear-resistant friction-reducing coating would be of benefit).

http://www.obitet.gazi.edu.tr/makale/makale/internalcombustionengines/055.pdf

However, although it has been convincingly demonstrated that lubrication of a ‘hydrodynamic’ nature does have a role to play, the modern cam and follower has traditionally been associated with the boundary lubrication regime where the role of chemical actions in thin surface films is vital.

Automobile engine tribology—design considerations for efficiency and durability , C. M. Taylor
A cam and follower contact is a form of concentrated contact (notionally elastohydrodynamic) which is traditionally reflected as operating in the boundary lubrication regime (see Fig. 4). That is, the viscosity of the lubricant is seen to be unimportant in the lubrication process, but the establishment of thin surface films of molecular proportions due to physical and chemical actions linked to the additive package, are seen to be crucial....The satisfactory lubrication of the cam and follower contact in IC engines has proved to be the most difficult of all the tribological components. To this day, extensive problems persist for many manufacturers, both in the mass and higher quality markets.

see also this diagram:

http://www.sciencedirect.com/cache/MiamiImageURL/B6V5B-3W382JW-1R-7/0?wchp=dGLzVlz-zSkWb

 

[RossABQ]

Can this type of coating be used to "restore" a used part? Do they have adequate control of the coating as-deposited or is finishing required?

 

[thundair]

IMO the cam followers put the lubricant in shear.
I always thought the cam should be in a gear oil instead of a hydrodynamic insert oil.

I don't know anything but the people that do.

 

[TVP]

RossABQ,

DLC coatings are applied by vacuum processes (PVD or PACVD) with a thickness of ~ 2-5 micrometers on surfaces that have already been highly finished (combinations of grinding, polishing, etc.), usually with a roughness of Ra ~ 0.1 to 0.2 micrometers. Maximum roughness should be Rz ~ 0.4 micrometers, otherwise the coating essentially acts as a microscopic grinding wheel and degrades the mating surface during operation. Anyway, DLC coatings do not require subsequent finishing when the substrate surface has been properly finished. "Restoring" a previously worn surface is not really practical, because the surface is too rough (> Ra ~ 0.4).

 

[tbuelna]

ivymike,

You may be correct in your statement about the contact between cam and follower being mixed regime conditions for part of a valve lift event, for some engines. Whether the sliding contact is boundary, mixed, elastohydrodynamic, or full hydrodynamic conditions depends upon many variables, and each regime is generally defined by the dynamic oil film thickness versus surface asperity heights of the two mating surfaces (ie. usually referred to as the Lambda ratio). I don't know why an engineer would intentionally design a non-oscillatory, sliding contact that did not always operate under hydrodynamic conditions, since it is normally easy enough to accomplish. But maybe it has to do with manufacturing costs or packaging constraints. Who knows?

Highly viscous oils and smooth parts with low surface asperity heights will be able to maintain hydrodynamic contact under higher loads than parts with a rougher surface using less viscous oils. But smooth surfaces cost more to produce, and engines like low viscosity oils to maximize efficiency.

Parts having full hydrodynamic contact should have zero wear as long as there are no corrosive elements present in the oil, there are no debris particles present in the oil that are larger than the contact zone hydrodynamic film thickness, the part surfaces have adequate surface compressive fatigue strength and sub-surface shear fatigue strength for the local dynamic fluid film pressures produced, and the oil film contact temps never exceed the flash temp capability of the oil which would lead to film breakdown and scuffing.

The reason some oils use EP additives like ZDTP, is to provide an extra margin of safety in case mixed or boundary contact occurs. These EP additives work by creating a surface oxide layer on the parts that prevents local diffusion bonding between the mating surface asperity tips when they locally contact (ever try to weld parts that had dirty or oxidized surfaces?). As these asperity tips bond together and then snap apart during their brief moment of contact, over the course of millions of load cycles, they slowly transfer material from one part surface to another and this results in the classic surface pitting and smearing that you see in failed rotary bearing surfaces. EP additives like ZDTP work well in gear oils, but they have properties that are detrimental when used in engine oils.

As I noted in my previous post, a DLC coating would only help under boundary contact conditions, where a low Mu value is beneficial. But unless both parts (bearing surface and journal surface) had the DLC, then the softer surface without the DLC would simply be slowly sheared away.

Interesting discussion.
Regards,
Terry

 

[ivymike]

I can't recall ever having seen a cam which did not wear. Have you?

 

[EdDanzer]

A cam and flat surface cam follower have line contact at all times. This creates an area that will exceed the film strength of oil. Once that happens dry metal is running against dry metal causing wear. The contact areas are probably deforming some so super hard coating may flake off. Using another type of coating or directing an oil jet at the interface may provide a better solution.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[tbuelna]

EdDanzer,

Most of the "flat" surface cam followers I have seen are not truly flat, and thus do not have true line contact. The lifter face is normally ground with a slight crown on its surface, which yields an elliptical contact zone. The slight crown profile prevents edge loading that may occur due to slight manufacturing misalignments between the cam bore and the lifter bore. The same is true for cylindrical roller followers. They are also ground with a slight crown profile.

The center of the elliptical lifter/cam lobe contact area is also designed to be slightly offset from the lifter axis, with flat tappets. This slight offset causes the lifter to slowly rotate, helping to equalize any wear.

Regards,
Terry

 

[drwebb]

"EP additives like ZDTP work well in gear oils, but they have properties that are detrimental when used in engine oils."

If this refers to Zinc dialkyldithiophosphates it is not completely accurate. These compounds are classical antiwear additives, generally accepted as forming complex phosphate-based glasses on iron or steel surfaces such as cams and lifters under conditions of moderate load. Some form of AW is necessary for most engines to protect the valvetrain during start ups, and zinc compounds serve this purpose in nearly all current engine oils and most hydraulic fluids. They also function as anti-oxidants, protecting the oil from high temperature chemical degradation. I don't recall how they interact with DLCs, but this has been extensively researched so there is literature on the topic for those interested.

Extreme pressure additives are usually based on sulfur, forming iron sulfides under conditions of high load found in the low speed sliding contacts of hypoid and bevel gearsets in axles. They are different chemicals suited for separate applications. They are not generally suitable for crankcase oils, part of the reason why gear oils have different SAE viscosity grades despite comparable viscosity values (in some cases) to engine oils- to avoid misapplication.

As I have pointed out elsewhere, AW additives function at very low levels- consider hydraulic oils can get by for thousands of hours on ca. 50 ppm- and the majority present in engine oils is included for oil durability (i.e. long drain). In cases of premature engine wear one would expect packing the oil full of more ZDDP to have less real effect than making a parts change or selecting a different ZDDP chemistry better 'tuned' to activate at a more suitable temperature/pressure regime.

 

[ivymike]

I concur with the comments about offset crown.

A google search ("cam dlc coating") shows that there are in fact articles discussing the interaction between DLC coatings and various antiwear additives.

Back to the original post, Nissan have apparently had good luck with DLC at the cam-follower interface:

http://www.huliq.com/32228/nissans-hydrogen-free-dlc-coating-wins-meti-award

The award-winning technology is the hydrogen-free DLC valve lifter that delivers improved fuel-efficiency. Nissan's DLC coating is a low-friction, highly abrasion-resistant non-crystalline carbon film with diamond-like properties, that does not contain hydrogen in its molecule structure. It significantly reduces the friction drag under oil lubricated condition. Nissan began applying the coating to its VQ35HR and VQ25HR valve lifters on the new Skyline and Infiniti G35, reducing friction between the cam and valve lifter by about 40%.

 

[drwebb]

Good example- But note that the benefit is fuel economy and not necessarily durability. While phosphate films from ZDDP prevent wear compared to unadditized oils, they somewhat counterintuitively increase the coefficient of friction. From the Nissan application I infer the DLC coatings don't support (or need) a phosphate AW layer, presuming the friction reduction is realized with service-fill ZDDP-containing engine oils. If this is a general effect then we should see DLCs on more vehicles, as valvetrain friction is a large component of passenger car engine friction.

 

[thundair]

Wouldn't the caveat be like a glaze on a donut if the material is soft underneath it would crack away?
I was playing with an aluminum project where I Teflon impregnated a hard anodize, and although you couldn't knock it down with a fie you could distort the material underneath, until it flaked away.
I would think that even size where growth is involved would be a limit.

I don't know anything but the people that do.

 

[tbuelna]

I'm not a chemist, but my understanding of how most EP oil additives work is (as drwebb noted) by creating an oxide layer on the metal surfaces. This oxide layer prevents the repeated localized mechanical "cold welding", and then subsequent breaking, between surface asperity tips that occurs during mixed or boundary contact conditions.

Since a thin diamond coating would not readily oxidize, it does not seem logical to me that a DLC coated surface itself would benefit much from a typical EP oil additive. Maybe someone with a good understanding of lube oil chemistry can better enlighten me.

Interesting thread though.

Regards,
Terry

 

[CoryPad]

ASM Handbook Vol. 18 Friction, Lubrication and Wear Technology

Both antiwear and EP additives provide protection by a similar mechanism, except that the EP additives typically require higher activation temperatures and loading conditions than the antiwear additives. Simply stated, antiwear additives perform under mild conditions, and EP additives, under severe conditions.

...

Both antiwear and EP additives function by thermal decomposition and by forming products that react with the metal surface to form a solid protective layer. This solid metal film fills the surface asperities, thereby reducing friction and preventing welding and surface wear. The metal film consists of iron halides, sulfides, or phosphates, depending on the antiwear and EP agents used. Friction modifiers differ from antiwear and EP agents in that they form the protective film through physical adsorption instead of chemical reaction.