Cubic boron nitride (CBN) is a superabrasive traditionally used to grind steels and nickel alloys. Though not as hard as diamond (4,500kg/mm2 vs. 9,000 kg/mm2), CBN is not chemically reactive with ferrite-based and nickel alloys, allowing it to outperform diamond-based grinding wheels in life and material removal rates on these materials1. A 1987 study by Hitchiner and Wilks showed that when grinding nickel, the chemical wear of a diamond single-point turning tool exceeded abrasive mechanical wear by 105x, highlighting the importance of using CBN in these applications2. Using CBN requires fewer wheel changes and machine downtime, offers low and controlled wear rates, and provides excellent thermal stability. As CBN superabrasive wheels wear with use, the wear rate and wear mechanisms largely determine wheel performance.

Structure

To understand the wear mechanisms in single-layer CBN wheels, it is first important to understand their construction. Single-layer wheels typically consist of a steel core that has been machined or ground to a specific size and tolerance. Next, wheels are masked to keep only the abrasive regions exposed and the CBN grains are then tacked in place. The wheels are submerged in an electrolytic solution, and a current is passed through the wheel and solution, which draws the Ni-based bond onto the wheel as a cathodic reaction occurs2. Manufacturers can control the thickness of the bond layer by varying the amount of time the wheel is plated.

Plating thickness plays a significant role in CBN grain wear mechanisms during grinding. Thick coatings tend to hold grains at the expense of material removal rate (due to lower grit protrusion from the top surface of the bond) and higher grinding temperatures (due to less room for coolant to enter and grinding swarf to escape). Thinner coatings can increase grit protrusion, at the risk of grain pullout, macro-fracture, and lower wheel life.

Wear in a single CBN grain from new to light to substantial, with micro-fracturing. Grain micro-fracture is favorable for consistent, predictable grinding.

Wear mechanisms

Hitchiner2, Malkin3, Upadhyaya4, and Ding5 all describe four possible modes of wear in grinding with electroplated products.

Attritious wear of the grains (grain flattening) – Grain wear is minimal and is concentrated at small scales around the cutting surfaces. Grinding forces are often insufficient to cause grain fracture, so grain dulling occurs. This can result in an increase in the power draw and is often associated with more plowing and rubbing between the grains and work material, leading to a higher likelihood of thermal damage. To combat attritious grain wear, an increase in grinding forces (more aggressive grinding parameters) should be used to promote grain fracture. In dressable grinding products (vitrified or resin bonded wheels), dressing parameters and frequency can help combat attritious grain wear as well.

Grain micro-fracture (microcrystalline splintering) – Grinding forces are sufficient to cause small scale grain fracture, resulting in continuous generation of sharp cutting points. This is often accompanied by steady-state grinding power. Grain micro-fracture is favorable for consistent, predictable grinding.

Grain macro-fracture or large-scale cleaving (partial grain break-off) – When forces are substantially high, grains may cleave or break off in large fragments. This typically results in low wheel life and is an indication that the grinding parameters are too aggressive.

Grain/bond interface wear or bond wear (causing total grain breakoff) – Though not common, it can happen if the grain exposure level (percentage of the grain that is above the plating bond material) is high (>50%) and the forces are high. In grain pullout, the grains are ripped out of the bond, resulting in low wheel life. This was highlighted by Ghosh and Chattopadhyay6 who note the relationship between plating thickness and tendency for grit pullout.

A visual summary of the four common wear modes is included in Figure 1 (right).

Performance characteristics

Assuming a grinding process has been developed that results predominantly in micro-fracture of CBN grains, common and predictable wheel performance trends are often observed with single-layer wheels. As described by Hitchiner2 and Upadhyaya3, 4, wheel wear and grinding power of new plated wheels tend to increase quickly during break-in, where only the tips of the highest grains are cutting the material, leading to a lower active grit density and rougher surface.

However, this break-in period is often short, replaced by more stable grinding where rates of change for wheel wear, power, and surface finish can decrease by up to 10x after the initial break-in rates. Wheel failure tends to occur once the grains have worn, causing grinding power to increase and surface roughness to drop, often resulting in workpiece burn. Failure can also be caused by grain and bond stripping off the steel core during more aggressive applications. In both failure modes, predicting the end of life is difficult, and is best estimated using empirical life data from previous wheels.

Figure 1: Four common wear modes in single layer superabrasive grinding wheels.

Troubleshooting guide

The final performance is only partially a function of the wheel. Other influential factors include coolant system filtration, setup, and nozzles, machine stiffness and forces, and the workpiece fixture. The final stress-state of the abrasive grain is a result of thermal and mechanical wear, which are driven by machine parameters, coolant, and lubricity. Once the entire system has been evaluated, focus may be placed on designing a grind cycle that promotes micro-fracture and self-sharpening wheels.

If the wear modes are observed when using single layer products, it is recommended to check troubleshooting topics to improve the wheel performance and part quality in the application.

Norton | Saint-Gobain

www.nortonabrasives.com

About the author: Norton | Saint-Gobain Applications Engineer Andrew Biro can be reached at andrew.biro@saint-gobain.com or 508.471.6769.

References
  1. Norton | Saint-Gobain. Superabrasive Wheels for Industrial Applications. 2016.
  2. Hitchiner, Mike, et al., Handbook of Machining with Grinding Wheels. Boca Raton: Taylor & Francis Group - CRC Press, 2007. pp. 93-98, 103-107,185-193.
  3. Upadhyaya, R and Malkin, S. “Thermal Aspects of Grinding with Electroplated CBN Wheels.” Transactions of the ASME. 2004, Vol. 126.
  4. Upadhyaya, R and J, Fiecoat. “Factors Affecting Grinding Performance with Electroplated CBN Wheels.” CIRP Annual Manufacturing Technology. 2007, Vol. 56.
  5. Ding, Wenfeng, et al., et al. “Review on Monolayer CBN Suberabrasive Wheels for Grinding Metallic Materials.” Chinese Journal of Aeronautics. Article in Press, 2017.
  6. Ghosh, A and A, Chattopadhyay. “On Grit Failure of an Indigenously Developed Single Layer Brazed CBN Wheel.” Industrial Diamond Review. 2007, Vol. 67.