Grinding, Processing HVOF Coated Materials Requires Super Abrasive-Ready Machine

Grinding new, exotic materials occurs more frequently these days, so United Grinding, Miamisburg, OH, has decided to approach these changes with a turnkey strategy. To help understand the process and the planning needed to achieve success in grinding theses materials, United Grinding presented the "Grinding Ceramic and Carbide Coatings" webcast, delivered by Hans Ueltschi, national sales manager.

According to Ueltschi, "In the last few years, United Grinding has seen an increase in the grinding of coated materials such as carbide and ceramic, and other materials such as Triballoy, where the coatings have been applied via the High Velocity Oxy Fuel (HVOF) process." Increased use of these coated materials is due to industry demand – especially within aerospace, which requires tougher, more repairable surfaces with more wear-resistance, better corrosion resistance, and a higher-performance bond coating.

Typical applications for HVOF within the aerospace industry are for grinding bearing journals, landing gear components, actuators, flap tracks, etc. There are a number of other industries that utilize more and more of this particular process, such as power generation, automotive, transportation/heavy equipment, printing and paper, and petrochemical, just to name a few.

What is HVOF?

The coatings mentioned above are applied via a HVOF spray process using either a gas fuel HVOF gun or a liquid fuel HVOF gun. The HVOF process provides high density, high bond strength, optimum hardness, improved toughness, and higher coating thickness.

Coatings are applied via a HVOF spray process using either a gas fuel or liquid fuel HVOF gun.

HVOF offers high density with approximately a 2% porosity to as low as 0.5% porosity. Typical carbide coating sprayed with HVOF exhibits bond strength in excess of 12,000psi. The HVOF process is what improves the bond strength. "Coating materials sprayed with HVOF have significantly higher bond strength than the same materials applied using other atmospheric thermal spray processes such as air plasma spray," says Ueltschi.

Optimum hardness of, for example, 12% tungsten carbide/cobalt coating will have a typical micro hardness of 1,100 to 1,350 on the diamond pyramid hardness scale (DPH300). Depending on chemistry or other factors, the short dwell time and lower temperature of HVOF can produce wear-resistant coatings with excellent impact resistance, improving its toughness.

HVOF can also achieve a thicker coating than plasma-, combustion-, or wire-coating of the same material. These higher thickness limits are attributed to the stress-relieving affect produced by the high velocity particles impacting on the previous layers of coating.

For example, the HVOF process has been used to create tungsten carbide coatings with a thickness greater than 6.4mm (¼").

"Compressive residual stress, and in some cases, very low tensile stresses enhance the fatigue life of a coated component, reducing the susceptibility of cracking and permit greater coating thickness limits," Ueltschi states. "HVOF coatings can exhibit superior resistance to sliding, adhesive wear, fretting, erosion, or cavitation depending on material and chosen parameters." The high density and exception metallurgical properties of HVOF coatings provide enhanced resistance to the effects of corrosion including hot corrosion, oxidation, and the effect of corrosive media such as acidic and alkaline gases and liquids.

"The area that United Grinding is most involved in is with fine surface finishes," Ueltschi says. "HVOF coatings are sometimes smooth enough to be used in an "as sprayed" condition. But the coatings can also be machined, ground, lap-honed, and super-finished to produce very fine surface finishes and to very precise tolerances."

Right machine, right options

Machines built for grinding carbide or other high-hardness materials require special attention with regard to configuring options and accessories to achieve the required tasks. "United Grinding offers experience and application enhancements for success in this process," Ueltschi says. "Items required for successful grinding of these materials are a rigid machine base, linear scales, super abrasive grinding wheels, variable speed wheel drives, variable speed rotary dressing tools, and dressing monitoring capability."

Successful grinding of carbides or other high-hardness materials require special attention to the configurations, options, and accessories of the machine.

It is critical that the machine base is very rigid, as far as its static stiffness, and it must dampen any vibration, because of the type of grinding wheels used. The machine must also be thermally-stable in order for the process to remain stable throughout the day. Linear scales are typically used for such applications, as opposed to rotary scales. Linear scales have proven to be necessary for maintaining the stability of the process, in part because the required super-abrasive wheels are not as easily dressable as conventional wheels.

"Next is the grinding wheel itself. Like grinding carbide, ceramics, or other hardened materials, when grinding such HVOF coatings, it is best to use either diamond or CBN wheels," Ueltschi says. "In the case of carbide coatings, diamond wheels are typically used, or where there is a choice, it is between vitrified-bond diamond wheels or resin-bond diamond wheels."

Ueltschi explains that one of the most important considerations when using super abrasive wheels is the spindle drive. The speed must change while grinding, and for this purpose, United Grinding uses variable-speed grinding drive motors. As opposed to a system that changes speed by changing a pulley, the variable wheel speed has to be programmable by the CNC control so that the grinding wheel speed can be different than the dressing wheel speed.

Variable rotary dressing tools and dressing monitoring capabilities help complete the success of grinding carbides of other high-hardness materials.

When using super abrasive wheels, either diamond or CBN, United Grinding typically recommends using a more automated dressing system – variable speed rotary dressing tools. This allows the user to be more automated, which gives them a more stable overall process. Compensating for wheel and dresser wear is more controllable with vitrified than resin bonded wheels. Electrically-driven rotary disc dressers are typically recommended. The dresser should offer adjustable speeds up to 16,000rpm and the ability to reverse the rotation so that the contact between the dressing disc and the wheel can be uni-directional or counter-directional. It is important for the machine to be flexible, to allow set up of a process which will get different surface finishes or different pressures on the wheel or workpiece. "Again, this is accomplished using a variety of different parameters in the dressing process, including different directions," explains Ueltschi. "Using an air or hydraulic dresser is less desirable because in particular, with the air dresser, the dressing wheel spindle speed can not be controlled and potentially the grinding wheel speed takes over and starts moving the dressing wheel in a faster rotational speed."

The last item is a monitoring capability that plots and displays the dressing process for evaluation and trouble-shooting. "Here, we typically recommend a high-quality acoustic monitoring system, also known as an in-sensor system, that picks up the acoustic signal when the grinding wheel touches the dressing spindle," Ueltschi says. "This works together with our software, Touch Dressing, which allows the grinding wheel to be set up each time it goes for a dress, as opposed to a conventional wheel where the wheel is only registered and set up one time during the setup phase."

Ueltschi goes on to explain that in this touch dressing process, the grinding wheel touches the dressing spindle in the X or Z position each and every time it goes through a new dress. This allows tight control of both grinding wheel and dressing disc wear. He also notes that dressing diamond wheels with diamond discs results in particular wear factors, which can accurately be anticipated or predicted and corrections can be programmed accordingly.

Grinding wheels, linear scales, and variable speed wheel drives are just some of the requirements of a super abrasive-ready machine.

Summarizing the considerations for successfully grinding coated materials such as carbide or Triballoy, the user needs to invest in a super abrasive-ready machine with features ensuring a high degree of stability and the ability to properly dress diamond and/or vitrified CBN wheels.

On the machine or process side, the critical components include a rigid machine base; linear scales; variable speed drives; variable speed rotary dressing tools; touch-dressing software; and dressing monitoring capabilities with the ability to plot-display in order to establish the grinding process, trouble-shoot the process should an issue occur, and get a superior dress on the wheel.

"United Grinding is able to offer turnkey projects for just about any industry," Ueltschi says. "Our applications group has a lot of experience in different areas and in grinding different materials. We work closely with a number of different wheel manufacturers to allow for the optimum wheel to be applied for a particular process. It's important to us to be open and to be able to use the best technology available on the market in order to improve the process stability and to improve the capability and reliability of the systems we provide."