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A Material Difference


Developers of energy-efficient, next-generation spark-ignition and compression-ignition diesel engines have long sought a lower-cost alternative to full-scale testing in order to screen the frictional characteristics and durability of candidate piston ring and cylinder bore materials. In evaluating new materials, it isnt only the lubricants performance, but also the composition, finish and properties of the solid surfaces that concern designers.

Prompted by stricter emissions requirements, rising fuel prices, and the trend toward lower-sulfur and less-lubricative fuels, new diesel engine designs are emerging. To meet more severe operating demands, traditional materials may need to be replaced by advanced materials like ceramics and composites, or by novel coatings and surface treatments. Fabricating full-sized parts from such experimental materials and running them in engine test cells is very costly – especially when multiple tests are required to establish confidence in the repeatability of the new materials performance.

Several years ago, Sidney Diamond, a heavy vehicle technologies program manager at the U.S. Department of Energys Office of FreedomCAR and Vehicle Technologies, became aware of the need for a better, more cost-effective bench test for new diesel engine piston ring and liner materials. He asked Oak Ridge National Laboratorys tribology staff to develop one. The main question we faced as we took up his challenge was: How much does the laboratory test have to look like a full-sized engine test in order to provide useful, yet cost-effective screening of candidate materials?

Having been involved with simulative testing in years past, Oak Ridge had compiled a survey of various bench-scale tests for ring and liner materials. Not surprisingly, we found a diversity of approaches and levels of sophistication. A new look at the problem revealed that additional work had taken place, but no method had really emerged in the United States as an industry standard.

Getting Started

A large number of variables are known to affect friction and wear. In addition to the composition and condition of bearing materials, a partial list of factors includes: contact geometry; speed and type of relative motion; roughness, waviness, and lay of the mating surfaces; contact pressure changes during operation; temperature; the presence or absence of wear particles; the manner of running-in; lubrication regime (hydrodynamic, boundary, mixed film, etc.); and of course, the lubricant chemistry.

To make the problem even more challenging, a number of additional variables lie nestled within these general categories. Surface roughness characterization alone, for example, can be represented by more than two dozen standard parameters, and it is not straightforward to know which of them best correlates with friction and wear performance in a given engine.

Past DOE-sponsored work by Malcolm Naylor at Cummins Engine Co. showed us that the wear rates of the same set of piston ring coatings can rank differently in order of merit, depending on whether fresh or used oil was applied in the tests. The material ranking wasnt reversed or changed in a systematic way – it was simply different. Therefore, one of our primary challenges in developing a new bench test was to find a means to induce materials to behave in the laboratory as they would in engine-conditioned oil – and that includes forming appropriate boundary films and wear by a similar combination of degradation processes.

Calling in Experts

We decided to bring this challenge to ASTM Committee G-2 on Wear and Erosion. Shortly thereafter, a new task group was established under subcommittee G02.40 to develop a friction and wear standard for ring and liner materials, and a group of industry experts was recruited to help us. Task group members included representatives from General Motors, Caterpillar, Cummins, Bendix Commercial Vehicle Systems, Penn State University, the National Institute of Standards and Technology, Phoenix Tribology Ltd., Falex Corp. and Mid-Michigan Test Laboratories. Working with me at Oak Ridge was John Truhan, a research professor with the University of Tennessee, Center for Materials Processing, with past experience at Cummins Inc. Truhan complemented our materials background with experience in filtration and lubricant characterization. In addition, Jun Qu of Oak Ridges tribology staff developed new and more accurate wear-measurement methods for test specimens having compound curvatures.

Voluntary ASTM standards must be capable of being performed by as many interested users as possible. Therefore, rather than attempting to design an entirely new testing apparatus that had no prior record of performance and that potential users might be reluctant to build, we selected instead the Plint Model TE-77 machine as our platform. This type of friction and wear tester was already in widespread use. The devices upper sample holder was modified to incorporate a section from a Caterpillar C-15 diesel engine production piston (see Figure 1, page 26). For the purposes of test development, we used a gray cast-iron coupon for the mating face. A convenient method to simulate the surface roughness and lay of a production, plateau-honed cylinder surface on a flat test coupon was devised. The details of that procedure were published by ASME in 2001. However, the procedure also allows actual cylinder-liner segments to be used if desired.

Truhan assembled a collection of engine-conditioned diesel oils from Jim Wells of Southwest Research Institute, and had these samples commercially analyzed for composition and soot content. They were used in our development efforts to ensure that the friction test standard would accurately and sensitively reveal the sometimes subtle effects of lubricant condition. In addition, a method was designed for running-in the fresh ring and liner samples before testing. Application of this running-in procedure was the key to obtaining repeatable friction data and reducing variability in the data.

Figure 2, page 26, exemplifies the effects of test load on the average friction coefficient in different kinds of standard test oils obtained from Southwest Research Institute; test temperature was 100 degrees C (212 F). A segment from a new chrome-plated ring designed for a Caterpillar C-15 diesel engine was used for the moving specimen and gray cast-iron was used as the stationary specimen in each case. The highest friction coefficient was for the M11 high-soot test oil (ASTMD6838-02), at a load of 20 Newtons. Data in the figure confirm that the friction coefficient is affected by both oil condition and the applied load.

Interestingly, the unused 15W-40 diesel oil had higher friction than three of the engine-tested oils and lower friction than two of the others. This demonstrated that the engine-conditioning process does not necessarily degrade the frictional characteristics of a lubricant but may improve it for some period of time before replacement is needed.

ASTM publishes standard test methods, practices, guidelines, and specifications. A standard test method generally specifies exact testing parameters. Since there are many kinds of engine designs, we did not want to restrict the test to a single load, speed or temperature. Rather we opted to develop a less-rigid standard practice that would provide procedural guidance yet allow the user to adjust the variables to reflect the behavior of different kinds of engines.

After months of testing and retesting, the final procedures evolved. It became evident from those studies that separate methods were needed for friction testing and for wear testing. We focused first on completing the friction standard, and it was approved in late fall 2004. Designated ASTM G 181-04, its title is Standard Practice for Conducting Friction Tests of Piston Ring and Cylinder Liner Materials Under Lubricated Conditions, and it appears in the Annual Book of Standards, Volume 03.02.

At this writing, the companion standard on wear is still undergoing the ASTM balloting process. Some results for the proposed wear test method are shown in Figure 3, below. They indicate that the relative wear of ring and liner specimens can be affected by oil conditioning. Also, the wear rates of the liner materials are considerably larger than those for the harder chrome-plated rings, and the ring and liner wear rates do not rank in the same order within this set of test oils.

Applying the Results

One of the first uses to which we put the new friction standard and its sister wear procedure, involved another Department of Energy project at Oak Ridge National Laboratory, on the durability of diesel engine components. Titanium alloys offer excellent corrosion resistance and elevated temperature strength, but are not particularly good bearing surfaces. If appropriate surface treatments could be found and the material cost could be reduced, titanium might be considered for a future generation of high-performance engines.

Results of initial tests of chrome-plated production piston rings against bare titanium alloy Ti-6Al-4V (as a cylinder liner material) were discouraging, but a thermal oxidation treatment being investigated by Jun Qu produced dramatic improvements. In fact, the wear rate of the titanium in diesel engine oil was reduced by more than 40,000 times over non-treated titanium, and the friction coefficient was comparable to that for cast iron (Fig. 4).

Surface chemical analysis indicated that the thermal oxidation-treated surfaces react more favorably with traditional anti-wear additives than do non-treated titanium. That finding has some exciting implications for lightweight, corrosion-resistant titanium bearings as well. Therefore, the new ASTM standard has become a useful tool for our companion research projects on advanced materials for engines.

For a deeper look at the research see: The effect of lubricating oil condition on the friction and wear of piston ring and liner materials in a reciprocating bench test, Wear, Vol. 259 (2005), pages 1048-1055.

This research was sponsored by the U.S. Department of Energy, Office of FreedomCAR and Vehicle Technologies, under contract DE-AC05-00OR22725 with UT-Battelle LLC, Oak Ridge, Tenn.

The new friction standard and its companion wear procedure have been added to the suite of tribology research capabilities that are available to U.S. industry and universities at Oak Ridge National Laboratory, through the Department of Energy-sponsored High Temperature Materials Laboratory User Program. More than fifteen specialized tribology instruments – some of them one-of-a-kind – are supported by a state-of-the-art materials characterization laboratory, to enable friction and wear measurements to be linked with a wide range of complementary analytical tools like electron microscopy, surface chemical analysis, residual stress measurements, nanoindentation, and infrared imaging.

U.S. industry and universities can tap into Oak Ridges world-class materials characterization facilities, including those for friction and wear testing, by participating in the Energy Departments High Temperature Materials Laboratory User Program. Short-term, collaborative research can be done on a non-proprietary basis (no charge to users, but a publication is required), or on a proprietary basis (no publication is required, but costs must be paid by the user). More information about the HTML User Program, the application process, and the equipment is available at

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