Minimising Friction and Wear on Diamond-Like Carbon Surfaces


Diamond-like carbon (DLC) exists in many different forms of varying hydrogen content and chemical bondings.1


The aim of applying


Diamond-like carbon (DLC) coatings to some engine components is


ultimately to help reduce CO2 emissions, improve vehicle fuel economy and extend equipment durability.


The materials of construction within the engine and the lubricant must be complimentary to ensure the performance of the vehicle is maintained throughout the drain interval period and throughout the lifetime of the vehicle with both esters and friction modifiers2 helping to enhance the properties of the lubricant.


Croda’s polymeric friction modifiers (PFMs) have demonstrated unique frictional characteristics on steel-steel contacts and they also exhibit unique performance in DLC - steel contacts, helping to reduce friction and minimise wear.


It is very important to consider both friction and wear when conducting experiments as low friction does not necessarily equate to low wear. Often it can be observed that low friction is actually the direct result of high wear, so additives which have the ability to reduce both friction and wear are highly desirable.


, friction and wear characteristics of two types of DLC coated discs, (in contact with a steel ball) have been evaluated under pure sliding conditions using the original oil and oil top-treated with friction modifiers. Steel disc–steel ball contact results are also described below. See Table 1 for a short description of the steel and DLC coated discs used in this work.


In order to exemplify the effect of organic friction modifiers and PFMs on DLC coatings, Croda has tested a selection of friction modifiers top-treated into a commercially available 5W30 ACEA C3 European engine oil. As hydrogen content can have a big effect on the DLC material3


Table 1. Surface contacts used in MTM testing.


To establish friction and wear, two stribeck curves are generated; the first is recorded immediately on virgin surfaces and a second after a period of two hours rubbing. The two hour rubbing profile is used to enable tribo-film formation to take place. The conditions used to determine the stribeck curves and the rubbing profile are shown in Table 2. In this short article, only the friction curves after 2 hours rubbing will be presented alongside wear profiling for the reference oil and the top-treated oils. However, a full copy of the recent paper presented at the TAE 2016 Conference and a recording of the presentation given can be found in the links at the end of this article.


Figure 1. MTM graph - friction of EU Oil with and without 0.5% FM in steel disc– steel ball pure sliding contact at 120 minutes rubbing.


The addition of GMO, PFM1 and PFM2 to the reference oil had a positive impact in reducing the depth of the disc wear track (see Figure 2).


Figure 2. Disc track after 120 minutes in steel disc-steel ball contact.


Table 2. MTM pure sliding test conditions.


Following completion of the second Stribeck curve, wear profiles for both balls and discs were studied using 3D image profilometry.


Steel Disc–Steel Ball In Figure 1 it can be observed that the 5W30 reference oil and the reference oil containing 0.5% glycerol mono-oleate (GMO) have similar friction profiles, that is, under these specific sliding conditions GMO does not provide any friction reduction benefits. In comparison, at medium and low speeds, both PFM 1 and PFM 2 provide approximately 5% reduction in friction.


3D images of the ball wear scars are shown in Figure 3 and the ball total displaced wear volumes are shown in Table 3. Addition of both PFM 1 and PFM 2 to the reference oil resulted in a significant reduction in wear. PFM1 gave a volume scar reduction of 10% and PFM2 gave an exceptional 31% wear reduction, compared to an increase in wear for GMO (+ 22%).


Figure 3. Ball wear scar after 120 minutes in steel disc–steel ball contact.


14


LUBE MAGAZINE NO.134 AUGUST 2016


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