February 2011 / Volume 63 / Issue 2|
By Susan Woods, Contributing Editor
Courtesy of United Grinding Strategies for grinding HVOF-coated aerospace parts.
Courtesy of United Grinding
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
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
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.”
While most people think a resin-bond diamond wheel should be applied, in many applications vitrified-bond wheels work better. Although these wheels are more expensive, they generally last longer than resin-bond wheels and condition more easily. Also, vitrified-bond diamond wheels can be diamond-dressed on the machine during the grinding process. Resin-bond wheels are typically trued first and then dressed in a secondary manual stick dressing operation.
“Resin-bond diamond wheels traditionally have been used,” said Rutkiewicz. “However, resin- and hybrid-bond technologies have a drawback in that online truing and dressing can be difficult because of the frequency of truing due to form or finish loss and, more importantly, the need to dress or condition (stick-dress) the face after truing. This is because the resin- and hybrid-bond technology has little porosity and, when trued, the abrasive no longer has exposed cutting edges.”
The newest technology employs vitrified-bond diamond wheels. Vitrified wheels are trued and dressed simultaneously with a rotary diamond disk. “With a properly designed truing and dressing diamond rotary-disk dresser and by using the correct parameters, in many applications vitrified wheels may yield the lowest total process cost,” said Rutkiewicz.
For dressing, the grinding machine must be set up for rotary dressing, not with a stationary tool. Stationary diamonds do not have the strength and hardness to efficiently dress vitrified diamond wheels. “A rotary dressing tool is used when dressing a vitrified-bond diamond wheel because there are many more diamonds cutting and it acts like a cutting saw,” said Beat Maurer, president of Complete Grinding Solutions, Springboro, Ohio, which grinds HVOF-coated aerospace parts.
The dressing system must be equipped with a high-frequency drive so it is adjustable in speed and direction to make the wheel cut properly and achieve desired surface finish requirements, according to Maurer. “You can adjust parameters accordingly with the dresser because it is a rotary dresser that can go unidirectional or counterdirectional,” he said. “Unidirectional makes the wheel cut better and counterdirectional provides a better surface finish.”
Maurer added that more sophisticated machines use an acoustic sensor to automatically detect contact between the wheel and the dressing disc so a precisely defined, minimal amount can be dressed.
Other factors to consider are proper application of cutting fluid and selection of feeds and speeds. “For the most part, companies are using emulsion over straight oil due to the cost savings. In general, emulsion is a better coolant but straight oil is a better lubricant,” said Maurer. Diamond wheels are typically run at about 5,000 sfm, according to Maurer.
The coating must be ground without burning the base part, but that is less of an issue with HVOF coatings than chrome. “These coatings are more heat resistant than hard chrome,” Devereaux said. “If you are grinding hard chrome, you definitely have to be careful not to burn the steel underneath it. But we have not seen that with HVOF coatings.”Superfinishing to the Rescue?
When ordinary grinding does not produce an acceptable surface finish on HVOF-coated aerospace parts, manufacturers turn to superfinishing. “Superfinishing, or microfinishing, is a material-removal application that produces a very smooth, highly uniform surface finish, characterized by a high bearing area, while maintaining or improving part geometry,” said Saint-Gobain’s Rutkiewicz. The superfinished surface is less than 8µin., typically around 4µin. .
The AMS 2449 specification does not cover superfinishing, but an Aerospace Material Specification for HVOF-applied, tungsten-carbide coatings is expected to be published this year, according to Devereaux.
Courtesy of Clint Forrest, ES-3 Landing Gear Systems
Superfinishing may not always be required, depending on the application. However, tungsten-carbide coatings have hard, sharp peaks that emerge from the surface, according to Devereaux. “The grinding process (with coarser grit sizes) inherently leaves some of these tungsten-carbide peaks exposed after grinding,” he said. “If not removed or smoothed by the superfinishing process, these particles will damage a mating seal in a hydraulic system, such as landing gear or flight control actuators, which often results in a hydraulic oil leak and premature removal of aircraft components.”
Because of their coarser grit sizes, diamond wheels cannot be used to perform superfinishing. Therefore, diamond belts, stones or paste are used.
While superfinishing of hard chrome is not needed, the benefits of HVOF coatings over hard chrome still are apparent. They provide better wear and corrosion resistance, in addition to environmental benefits. CTE
About the Author: Susan Woods is a contributing editor for CTE. Contact her by e-mail at firstname.lastname@example.org.
Courtesy of MesoCoat
Can microcomposite coatings replace HVOF?
Abrasive Technology Inc.
Complete Grinding Solutions
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Saint-Gobain Abrasives Inc.
United Grinding Technologies Inc.
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