New technology worth its weight in chrome

  • Published
  • By Jeanne Grimes
  • Oklahoma City Air Logistics Center Public Affairs
TINKER AIR FORCE BASE, Okla. (AFPN) --  A powdered blend of tungsten carbide and cobalt used to coat jet engine parts, and new thermal-spray technology used to apply it are being tested by people at the Oklahoma City Air Logistics Center here.

The powdered blend is emerging as the military’s best bet for replacing cancer-causing chrome, officials said.

“Chrome repair has been around for 60 years,” said Johnny Tsiao, an aerospace engineer in the Propulsion Directorate. “It works great. It’s cheap.”

But chrome is a carcinogen and an Environmental Protection Agency-banned substance.

Air Force, Army and Navy officials organized the Propulsion Environmental Working Group to find an environmentally friendly alternative that would at least match chrome in durability.

It turns out that the sprayed-on powdered-blend coating actually fares far better than chrome, Mr. Tsiao said.

“(It) is a superior coating,” he said.

Endurance tests show the coating lasts as long as chrome.

“We are confident it will last twice as long,” Mr. Tsiao said. “We would like to test it to see if it can (last) three times as long.”

The group selected a TF33 engine as a test subject, and engineers identified seven engine parts for application and endurance tests. It also designed and equipped specialized booths to apply the coating here.

“They cost about $1 million per booth, and we got two of them,” Mr. Tsiao said.

A third booth is under construction, said Jeff Marnix, plasma shop team leader. Unlike other plasma spray booths, these have 12-foot ceilings. That is because a shaft selected as one of the seven test parts measures 5 feet long, and engineers wanted to be able to spray it upright.

The process will work with any number of powders, Mr. Marnix said. Contracted engineers selected tungsten carbide and cobalt.

A seven-axis robot applies the spray at a velocity of Mach 2.5. Because the high velocity generates so much heat, an infrared pyrometer monitors the temperature. The temperature of the part must be kept below 400-degrees Fahrenheit, and cooling jets blow air on the part. Other equipment in the booth includes a powder feeder and dust collector.

The high velocity also generates noise levels to rival a jet engine, so the booths are soundproofed.

“The gun produces 130 decibels. It’s not an environment where you’d want to have an operator, even with ear protection,” Mr. Marnix said.

Between five and 10 pounds of powder is needed to coat one part with 17-percent cobalt and 83-percent tungsten carbide.

With upfront costs for the booths, the new process’ cost-savings are not immediately apparent, Mr. Tsiao said. But over time, he said he believes the Air Force will recoup the investment.

The Air Force’s payback will not come in dollars, but in mission capability, he said.

There are increasing restrictions on the use of chrome. With a more durable coating, engine parts will not have to be recoated as often.

“The [engine division] folks will tell you chrome restrictions are so tight it’s economically unfeasible to continue using it and meet EPA requirements,” Mr. Tsiao said.

Still, the chemical is not likely to disappear from here anytime soon, he said.

The one downside to the new process is that it is “a line-of-sight” application only, meaning some engine parts that are out of reach will continue to require chrome; at least until the group can tackle that problem using new technologies, Mr. Tsiao said.