"Long-Haul Endurance," Feature Article, Aug. 2002

long-haul endurance

Lasers are being investigated as a way to uncover tiny imperfections in crucial ceramic components of diesel engines.

by John DeGaspari, Associate Editor

Heavy-duty truck engines are designed to operate for a million miles or more. In their search for components that resist corrosion and wear, manufacturers have developed engine parts from ceramics, which have found their way into a number of commercial engine applications over the last 10 years. Under some conditions, the materials hold up better than steel, but they are not immune to weaknesses of their own. The machining of ceramic parts, for example, can leave them with flaws that lead to early failure and defeat their purpose.

Ceramic components typically cost more than the steel components they replace, according to William F. Mandler, Jr., director of sales and marketing for Enceratec Inc. in Columbus, Ind. The company, a joint venture of Cummins Inc. and Toshiba of Japan, has developed several high-volume commercial applications of structural ceramics in diesel engines. Because of their higher cost, ceramic components generally replace only those steel components that do not have the required durability, Mandler said.

However, for these components to perform as expected they have to be machined properly; otherwise, mating surfaces will wear quickly, according to Mandler. Harder and more brittle than steel, ceramics are generally more difficult to machine. Ceramics require more expensive abrasive materials, are more prone to edge chipping, and must be machined more slowly than steel to avoid damage. Cummins is one of the engine manufacturers that is working with researchers at Argonne National Laboratory in Argonne, Ill., to develop better methods of detecting imperfections that can lead to early failure.

Ceramic components have been used in diesel engines for the past decade, according to Mandler. Enceratec, for instance, was formed in 1989 to develop ceramics applications for diesel engines. Cummins introduced its first ceramic engine component in 1989—an injector link of silicon nitride used in the step timing control, or STC, unit injector. Since then, the company has added other ceramic components. Many of them are related to fuel systems, and include bearings for a solenoid valve that controls a throttle shaft, check balls for a fuel injector, a fuel pump roller, and an injection-timing plunger. The company has also tested ceramics in other areas, including the valve and injector actuating system, although these are not yet cost effective, in Mandler's view.

A six-axis robot installed at Argonne National Laboratory manipulates complex ceramic parts undergoing laser-light examination for tiny flaws.

Ceramics typically resist wear better than steel, particularly when the materials are exposed to contaminated fuel or lube oil, or to high contact stresses, Mandler said. Engine operating conditions are becoming more severe because of higher fuel system pressures and changes in the fuel that have reduced lubricity, he said. Fuel injection pressures above 30,000 psi force the fuel through the nozzle openings where it is atomized. The higher pressures increase contact stresses on components throughout the system.

In general, Cummins uses silicon nitride in high-contact stress sliding and rolling wear applications, and zirconia where there is a requirement to closely match the thermal expansion of steel. Zirconia costs less and is easier to machine than silicon nitride.

Machining in general—and especially high-speed material removal machining—can cause damage, such as chipping of sharp edges. Or the damage may be less obvious, such as microcracking of the material or grinding marks that are large enough to be detrimental to strength or, in the case of zirconia, cause undesirable changes in the crystalline structure that can reduce the strength of the material, Mandler said.

Laser Focus

The laser technique being developed at Argonne National Laboratory is intended to inspect the quality of ceramic parts after they are machined. According to Bill Ellingson, a senior mechanical engineer who heads the research at Argonne's Energy Technology Division, advances in machining technology that reduce costs and create faster material removal machining rates make it necessary for manufacturers to know what damage they are inducing in ceramic parts.

This work is being supported by the U.S. Department of Energy's Office of Transportation Technologies through its material development program. Sid Diamond is the DOE program manager at Washington headquarters and Ray Johnson of Oak Ridge National Laboratory in Tennessee is the project manager.

One initiative involves a high-pressure fuel injector that contains ceramic parts, while another involves a silicon nitride diesel engine valve. "In the case of new fuel injectors, which run sometimes higher than 30,000 psi, you can't afford to have these damaged parts," said Ellingson. Damage in the fuel injector can alter fuel swirl patterns, resulting in inferior combustion and increased exhaust emissions, he said.

The inspection technique being developed by Argonne uses a low-power helium-neon laser, similar to those found in grocery checkout counters. Since the ceramic materials are optically translucent at selected wavelengths, the laser light will penetrate the ceramic material. By training the laser onto the part and studying the way the light scatters, it's possible to characterize and locate any damage on the part, according to the lab. Damage can be pinpointed either on or below the surfaces of ceramic parts and relate the scattered light data to changes in the strength of the ceramic material.

Ellingson started investigating the structure of ceramic bearing balls using lasers, with funding from the U.S. Department of Defense. Although ceramic structures of silicon carbide or silicon nitride are dark gray or black to the naked eye, they are optically translucent at selected wavelengths. The translucency depends on the type of material and sintering aids, which promote densification.

The basic physics involved in the technique is a form of reflectometry, with the difference that light, rather than reflecting off the first surface that it encounters, penetrates below the surface, said Ellingson, who compared the technique to medical ultrasound imaging used for fetal or cardiac imaging.

Zirconia timing and metering plungers on Cummins' Celect diesel fuel injector are said to eliminate failure from seizing and reduce plunger/bore wear.

The system developed at Argonne is a hybrid of two types of optical apparatus: a reflectometer to measure the intensity of the backscattered light and an ellipsometer, which uses changes in polarization angle of light to detect various features and materials.

The Argonne system operates on a principle that is similar to compact disc players, explained Ellingson. In CD players, the laser penetrates a protective coating to read the information contained on the CD. In Argonne's device, the scattered laser light is used to read subsurface patterns of the ceramic part.

A pair of optical detectors captures the intensity of the reflected light and indicates if there is damage at or just below the surface. For example, if the laser light passes over a surface where there are no subsurface cracks, the returning signal would be fairly strong. If the laser light passes over an area where there are microcracks below the surface, the light would reflect back more diffusely.

At present, all correlations between laser scatter data and damage features are arrived at empirically for each material being studied, said Ellingson. He is currently developing an analytical model that would allow the process to be automated. The intent is to develop a device that would be capable and cost-effective to scan every part in a plant when this is required by the application.

How long it takes to examine a part depends on its size and shape. For complex parts, Argonne has installed a six-axis robot that can manipulate the part to control the angle of the incident laser light. Ellingson said he is still trying to demonstrate different automation aspects, and estimates that a commercial product is perhaps three years away.

In the case of silicon nitride bearing balls, the backscattered light could ensure that there isn't a small void that could result in spalling, he said. He noted that the top few hundred microns of bearing balls are important because this is where most of the load is distributed. This is the depth below the surface that needs to be looked at. Subsurface voids or variations in the microstructure might lead to early failure, he said.

Now Ellingson has taken his initial work on bearings and applied it to machined ceramic components for heavy-duty diesel engines. The lab is working with Cummins, which has designed a high-pressure fuel injector that uses ceramic parts.

Ellingson sees applications for the laser technique beyond automotive parts. He said that one attractive feature of ceramic bearing balls is that they run with minimal lubricants for long periods compared with their steel counterparts.

Among applications are the use of ceramic bearing balls for shafts of turbojet engines and in precision machining tools. So far, the laser technique has been developed to look for imperfections in silicon nitride, silicon carbide, and zirconia, among other ceramic materials, Ellingson said. Another is alumina, a ceramic used in hip implants.