Tough problems? Tougher ceramics!

DN Staff

December 2, 1996

11 Min Read
Tough problems? Tougher ceramics!

Silicon nitride cam roller adds miles to engine

Think your commute is bad? Longhaul truck drivers can cover upwards of 250,000 miles a year. And with their livelihood on the line, engine reliability is of utmost importance. Yet other parties--such as the EPA--have their own priorities. Clean air, for instance.

For 1994, regulators tightened control over diesel exhaust emissions, presenting engineers at Detroit Diesel Corp. (DDC) with a design dilemma. The solution: small cylinders of silicon nitride.

One route to improving diesel emissions is through increased fuel-injection pressures. On DDC's Series 50 and 60 engines, six electrically activated injectors--one for each cylinder--generate the pressure. The pumps are driven by extra lobes on the camshaft acting against rocker arms fitted with rollers. Over the years, injection pressures have risen until, to meet the new emission requirements, they've reached a heady 28,000 psi.

"With those kinds of pressures there would be a very low oil film, and that would drastically shorten the life of the roller," says Jon DePentu, DDC senior project engineer. The existing parts consisted of steel followers and bronze bushings running on steel pins. Each must last the more than 1 million miles between overhauls. "We knew that conventional technology wasn't going to get us there this time," DePentu adds.

So he turned to ceramics. After evaluating ceramic materials from several suppliers, DePentu chose to make the cam roller out of silicon nitride from Kyocera. Silicon nitride exceeds other technical ceramics in thermal shock resistance. It also maintains strength at elevated temperatures, making it perfect for turbocharger rotors, glow plugs, and other engine components.

At first, engineers embarked on an ambitious plan to offset the ceramic part's cost increase by modifying the engine oil system to reduce flow to the new rollers. "Ceramics run quite well at the low oil films resulting from the increased injection pressures," says DePentu.

But ultimately he chose the simpler route and opted to create a drop-in replacement part for the steel rollers. Though functionally identical to their predecessors, the ceramic parts are more brittle. To address this, workers on the manufacturing line were schooled to be a bit more careful during engine assembly.

Extensive field testing followed the redesign, with well over a million miles logged on some engines. "We'd tear them down, and the ceramic cam followers showed so little wear that you could have put them back in and run them through the test again," DePentu says. "It's incredible stuff."

Telescope's success reflected in CVD Silicon Carbide

Atop Mauna Kea in Hawaii and Cerro Pachon in Chile, scientists are busy constructing a pair of 8m telescopes designed to exploit some of the best viewing conditions on Earth. Called Gemini, these twin instruments depend on secondary mirrors produced from a revolutionary bulk silicon carbide developed by Morton Advanced Materials.

Said to be 99.9995% pure, the ceramic is formed by chemical vapor deposition (CVD), the pyrolytic decomposition of methyltrichlorosilane gas in a vacuum chamber. "We build solid material atom by atom over the top of graphite mandrels attached to the walls of the chamber," says Dr. Michael Pickering, technical manager at Morton. "It's similar to crystal growth." Not a speedy process, it takes about 100 hours to deposit a thickness of 1/8 to 1/4 inch--but the result is worth the wait.

Unlike CVD methods used to form ceramic or metal coatings, Morton's bulk CVD process yields shapes limited, so far, by the size of the reaction chamber. Plates can be made as large as one inch thick and 60 inches in diameter. The company hopes to bring online chambers that can produce depositions of several hundred square feet. Morton is the world's only supplier of bulk SiC produced by CVD.

Besides its purity, the material's advantages include: attainment of maximum theoretical density, zero porosity, superior thermal conductivity, and extreme polishability--on the order of 2 angstroms. "It can be polished as smooth as any material known to man," says Pickering.

These qualities led to the material's initial development for lightweight mirrors through an SDIO Air Force contract. Other applications: mirrors for high powered lasers, molds for glass forming, and semiconductor wafer carriers--a market that currently accounts for about 75% of the material's sales.

For Gemini, Morton is constructing one-meter-diameter blanks. They will be formed in two depositions: one to mold the mirror surface, the second to form the intricate back structure and integral mounting points. The resulting mirrors will weigh just 50 kg. This low mass is a critical advantage, since the components will be mounted high up in the telescope and "chopped" at 5-10 Hz. Chopping involves tilting the mirror slightly on and off the object being viewed to allow subtraction of background radiation for improved contrast in the thermal infrared.

Silicon carbide's high thermal conductivity quickly evens thermal gradients to reduce distortion. And the material's extreme hardness and chemical inertness allow for the reflective coating to be chemically stripped and recoated as needed over the life of the telescope. "They will probably never have to repolish the mirror," Pickering predicts.

Spectroscope relies onultrafine-grain alumina

In 1998, the Space Shuttle will lob into high elliptical orbit a 39-foot-long, 10,000-lb observatory called the Advanced X-ray Astrophysics Facility, or AXAF. Designed to study high-energy events, such as supernovae, black holes, quasars, and stellar coronae, AXAF will demonstrate the highest angular resolution and greatest sensitivity of any x-ray telescope ever put into space.

"It's going to be used to study the oldest events in the universe by looking at the most distant objects," says John Polizotti, mechanical engineer at the Smithsonian Astrophysical Observatory (SAO). Third in a series of orbiting x-ray observatories, AXAF will be five times more sensitive at detecting faint objects than its predecessor ROSAT. At its heart lie several extremely precise instruments, one of which, the high-resolution camera spectroscope (HRC-S), depends on an ultra-fine grain, 99.98%-pure alumina supplied by Astro Met.

Similar to a prism's effect on visible light, the spectrometer works with the HRC-S to separate and analyze wavelengths of x-rays to reveal the material composition and chemical makeup of the radiant source. The spectroscopic detector consists of three major assemblies: a UV/ION shield, a pair of micro-channel plates (MCP), and a cross grid charge detector (CGCD) made from the Astro Met ceramic.

An x-ray photon entering the camera strikes a photocathode layer in front of the first MCP and is converted into an electron. This electron enters the MCP and is turned into a cascade of electrons that jumps a 5-mil gap to the second MCP, where the amplification process is repeated. "Ultimately, one photon is turned into a cloud of more than 1 million electrons," Polizotti explains. The cloud then strikes the CGCD.

Usually such detectors consist of two separated layers of finely spaced gold wires wrapped in orthogonal directions around an insulating substrate, such as alumina. But in the spectroscopic detector, the CGCD presents a special problem.

It comprises a long and thin ceramic "dog bone" measuring about 400mm x 33mm that has slight facets machined on its top face. Wire can't be wound along their length because it would vary in height above the surface due to the facets. So engineers decided to deposit an array of 7-mil-wide gold traces just 0.7 mils apart on the substrate. This gave them fits, since the original alumina material they'd specified had too coarse a grain and caused shorts or breaks in the traces.

The solution: Astro Met's AMALOX 87. "They said they could give us a 1- to 3-micron grain size as opposed to 17 microns with the less pure material," says Polizotti. Further tests led him to HIP (hot isostatic press) the dog bones at Industrial Materials Technology, Andover, MA, to close any remaining voids. He also relaxed the machining requirements on Insaco (Quakertown, PA) by specifying a 15-microinch finish that was less apt to create grain pullouts.

"The final dog bones look flawless," says Polizotti. "And the only way we could get there was to start with this super-fine- grain material."

Ceramic bearing lengthens pump life

By turning to zirconia, engineers at Reda, a division of Camco International, reduced bearing wear in the company's electric submersible pumps by a factor of one hundred. The redesigned pumps, called ARZ for Abrasion Resistant Zirconia, employ a patent-pending compliant mount for the ceramic bearing that withstands most rough oil-field handling. With pump replacement in areas like the North Sea costing upwards of half a million dollars, the savings to REDA's customers can be substantial.

One would envision that pumps used in oil wells lead lubricious lives. Yet crude oil is sometimes more crude than oil, containing enough abrasive sand to quickly destroy even the toughest metal pump bearings. "Wells sometimes produce oil from unconsolidated sand formations," says Steve Kennedy, manager of pump engineering. "They actually pull sand in with the oil and water."

Over the years, engineers dealt with the abrasion problem in a variety of ways, using fluted rubber journal bearings, hard plating, alloy sleeves, ceramic coatings, and ceramic inlays. In the harshest conditions, pumps sometimes lasted just a few weeks to a couple of months, explains Kennedy. "With zirconia, we can get six months to a year or more under the same conditions," he says.

Reda procures its bearings from Coors Ceramics. The company has an exclusive agreement with Coors for use of the material in downhole electric submersible pumps, and worked closely with Coors engineers to develop the bearings. "We did a lot of testing of other zirconias and found that they were inferior," says Kennedy.

Reda also offers pumps with silicon carbide bearings for the most abrasive and corrosive fluids. But zirconia's toughness makes it seven times more resistant to shock than SiC, Kennedy notes.

Breaking, in fact, is the only possible caveat. With Reda's compliant mounting system, however, Kennedy says he's only seen a couple pumps ever come back with the zirconia broken from rough handling. The bearings have proved such a success that the marketing department told engineers, "You're finished with this; now go find something else to do."

Ceramic foam filters out competition

Cruising down the freeway in your new '02 sedan, you and the environment might be the beneficiary of a catalytic converter based on ceramic foam. At least that's the hope of engineers at Ultramet.

Currently under development, the company's UltraCat products target the demands of strict turn-of-the-century ULEV emission requirements. It's just one of dozens of uses of novel refractory ceramic foams-- generically called Ultrafoam--slowly finding applications in everything from medicine to aerospace.

Ultrafoam and UltraCat begin as vitreous carbon mesh, a vast collection of open interconnected cells shaped roughly like pentagonal dodecahedrons. The sizes of the cell openings or pores vary, depending on the application, from 150 to 2,200 microns. Complete cells range from 500 to 6,500 microns.

Ultramet sells the carbon foam as a product. But more interesting are foams coated via chemical vapor deposition with any of a dozen different non-oxide ceramics. Silicon-carbide foam, for instance, weighs 0.32 g/cc, one tenth as much as pure SiC. It also possess a remarkable void fraction of 90%. That's where catalytic converters come in.

"For ULEV, this material has a great advantage," says Ed Stankiewicz, manager of new product development. "It has low mass and high mass transfer, so it heats up rapidly. And its high temperature capability enables it to be used closer to the engine." A high void fraction means lower backpressure and substrates just one-third to one-half the volume of honeycomb substrates.

The cell shape creates turbulence that better mixes the flow and exposes more exhaust to the catalyst. This gives a quicker reaction and potentially a smaller converter. "Heat transfer is generally five to 10 times greater than current converters," adds Stankiewicz.

The challenge is showing the big auto companies that the material is reliable and that Ultramet can produce it in quantity.

Other applications include: tantalum-coated foam (Hedrocel(TM)) for artificial bone, stiff substrates for space mirrors, core material for structural sandwich panels, lightweight thermal protection systems, and filters for harsh conditions. Says Stankiewicz, "if you need to filter hot nitric acid at 900C, I'm your man."

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