High-Velocity Grind: General Industry Coverage

Courtesy of United Grinding

An HVOF-coated aerospace part being ground on a Studer S-33 machine.

Strategies for grinding HVOF-coated aerospace parts.

Chrome may be flashy, but its days in the sun are numbered—including as a coating for aerospace parts.

Engineered hard chrome has been applied to aerospace parts to increase wear resistance and repair or rebuild worn sections. However, the use of hard chrome in aerospace and other applications is being phased out because of environmental and health concerns with hexavalent chrome emissions from the chrome plating process. As a result, high-velocity oxygen-fuel thermal spray coatings are replacing hard chrome on many aerospace parts. HVOF coatings can be used for newly manufactured parts and for repairing worn parts.

With worn parts, machinists typically remove a certain amount of material, eliminating the wear that has developed in the part. They rebuild that surface with HVOF coatings, according to Glen Rosier, applications engineering/business development, Abrasive Technology Inc., Lewis Center, Ohio, a superabrasives manufacturer.

While HVOF coatings were initially targeted as an alternative to hard chrome for health and safety reasons, they have proven to provide better wear and corrosion resistance than hard chrome.

“The environmental reasons are important, but are not as big a driver as everyone thinks,” said Keith O. Legg, a senior analyst at Rowan Technology Group, Libertyville, Ill., a marketing and analysis firm specializing in advanced technologies, materials and coatings. “[Manufacturers] use HVOF coatings because they work better and last longer. With chrome, there are striations after a few years. The chrome becomes worn and damaged. With HVOF coatings, that is not the case.”

A Different Take

The HVOF process is unlike plating, which can coat the entire part. HVOF coatings are deposited in a thermal-spray process where a powdered material is injected into a high-pressure, hot gas stream. The powder is sprayed with a gun onto the part surface and forms a dense, well-adhered coating. HVOF coatings are typically about 0.003 ” to 0.005 ” thick on original parts, and 0.015 ” thick on rebuilt parts because manufacturers are building up the worn area.

The resulting HVOF coating has a hardness of 1,200 to 1,500 HV compared to 800 to 1,000 HV for hard chrome. This makes it highly wear-resistant, but more difficult to grind and otherwise finish.

Courtesy of United Grinding

A tungsten-carbide HVOF-coated turbine shaft before (top) and after grinding.

The HVOF coating process can deposit a range of different alloys and cermets. A cermet is a composite material composed of ceramic and metallic materials. The most common alloy coating materials for aerospace applications are tungsten carbide cobalt (WC-Co) and tungsten carbide cobalt chrome (WC-CoCr).

“You typically apply tungsten carbide on moving parts that have surfaces in constant contact,” said Larry Marchand, aerospace account manager, United Grinding Technologies Inc., Miamisburg, Ohio, a grinding machine builder. “You want something extremely hard and that will withstand the constant frictional forces.”

The most common aerospace applications for tungsten-carbide HVOF coatings are landing gear parts, flight control and hydraulic actuators, landing gear and hydraulic system pins, flap and slat tracks and turbine engine shafts. “[The list includes] almost any component subject to wear by rubbing or abrasion, which differs from aircraft to aircraft,” said Legg.

The coated aerospace parts are typically made from HSS, although some titanium parts are HVOF-coated. The use of titanium in aerospace applications is growing because it weighs half as much as steel in similar applications. “And for aircraft, weight is dollars,” said Jon Devereaux, materials and processes engineer, NASA–Kennedy Space Center in Florida. (All opinions expressed by Devereaux in this article are his own and not necessarily those of NASA.)

One limitation with the HVOF coating process, however, is coating deep IDs. With chrome plating, any ID or OD can be coated because a part is immersed in a bath. But the HVOF spray does not reach into deep holes.

“We can coat inner diameters, but it depends on the depth, size and location of these diameters,” said Roger Maragh, process engineer for Hitemco, Old Bethpage, N.Y., which provides coating and grinding services. “After all, this is a line-of-sight process.

The parts themselves are typically from 6 ” to 30 ” diameter in size, although one company noted it has coated landing gear pins as small as 1.5 ” in length with a diameter of less than 0.5 “.

Grinding Approach

Because HVOF coatings are denser and harder than hard chrome, they require a different approach to grind the material to achieve the required finishes and geometries. “The coating has a thick, bumpy surface, so it has to be ground back to the correct geometry or with the proper finish, as far as smoothness and texture,” Marchand said.

A process specification is available—Aerospace Material Specification 2449—for grinding tungsten-carbide HVOF thermal spray coatings applied to HSS for applications requiring wear, heat and corrosion resistance or dimensional restoration. However, usage is not limited to those applications.

NASA’s Devereaux, who sponsored the specification, said, “documenting all aspects of the grinding procedures in some sort of written grinding process control sheet prior to the start of the grinding operation is vital.”

Often, larger and more rigid grinding machines equipped with high-frequency drives are required. Also, minimizing vibration is especially important when grinding HVOF coatings. “Machines with hydrostatic guide ways dampen vibration and offer much smoother grinding with these very hard materials,” Marchand said. “The machine moves on a film of oil and that layer of oil breaks the vibration energy.”

Courtesy of Abrasive Technology

A diamond grinding wheel.

While hard chrome is usually ground with an aluminum-oxide or silicon-carbide grinding wheel, the hardness of HVOF coatings requires that they be ground with a diamond wheel. As a result, the grinding machine requires higher static and dynamic stiffness.

“The diamond wheel itself requires a higher threshold force to make it work,” said Brian Rutkiewicz, manager of applications engineering, Saint-Gobain Abrasives Inc., Worcester, Mass., a manufacturer of high-performance materials, including abrasives. “A wheel that has aluminum-oxide or silicon-carbide abrasives doesn’t need as much force to make it cut.”

Wheel Care

“Because diamond is as hard as it is, diamond wheels can grind substantially higher levels of the coating compared to silicon-carbide and aluminum-oxide wheels,” said Abrasive Technology’s Rosier. “It maintains size and keeps the conformity of the part better.”

The key to grinding HVOF coatings is following correct procedures for diamond wheels, including proper selection of the grinding wheel, grit size, grade and bond type, and proper mounting, balancing, truing and dressing of the wheel prior to grinding, according to NASA’s Devereaux.

Courtesy of CGS

Complete Grinding Solutions uses an acoustic sensor to automatically detect contact between the wheel and the dressing disc.

The first step is properly truing the wheel. “You have to have that wheel running true before you start grinding parts,” said Devereaux. “If you don’t properly true and dress it before you start, you will have problems. You can’t just take a diamond wheel (from the manufacturer) and put it on your grinding machine.”