1. The Science Behind the Hardness

The “superhard” nature of both CBN and diamond stems from their atomic structure. Both feature a covalent bond angle of 109°28′, which creates an extremely rigid lattice.

• Atomic Structure:​ CBN atoms have a spacing of 1.57 Å, slightly larger than diamond’s 1.5 Å.

• Hardness Comparison:​ This structural similarity results in exceptional hardness. CBN boasts a micro-hardness of 71.54 GPa, significantly higher than Alumina (17.64–27.44 GPa) and Silicon Carbide (25.48–35.28 GPa).

• Anisotropy:​ It is crucial to note that CBN hardness is anisotropic. Knoop hardness tests show:

  • [100] Direction: ~4,600 kg/mm² (vs. Diamond’s 10,000 kg/mm²)

  • [110] Direction: ~3,200 kg/mm² (vs. Diamond’s 7,000 kg/mm²)

Despite being slightly softer than diamond, CBN’s true value lies in its thermal and chemical properties.

2. Thermal Superiority: Stability and Conductivity

Unlike diamond, CBN excels where heat is involved.

• Unmatched Thermal Stability:​ CBN can withstand temperatures exceeding 1,200°C, far superior to diamond which oxidizes and graphitizes above 720°C. In a vacuum, CBN remains stable up to 1,550°C before transforming into Hexagonal BN (HBN).

• Oxidation Resistance:​ The boron atoms on the CBN crystal surface saturate valences, eliminating “dangling bonds.” This prevents oxidation at high temperatures, forming a stable oxide layer instead of disintegrating like diamond.

• High Thermal Conductivity:​ With a conductivity of 79.54 W/m·K​ (compared to Diamond’s 146.5 W/m·K but much higher than carbide’s ~25 W/m·K), CBN efficiently dissipates heat from the cutting zone, reducing diffusion wear.

3. Chemical Inertness: The Ferrous Metal Advantage

This is where CBN truly separates itself from diamond.

• Inert to Iron:​ CBN exhibits extreme chemical inertness toward iron, steel, and nickel-based alloys. It does not react or form carbides.

• Diamond’s Weakness:​ In contrast, diamond readily reacts with iron at high temperatures, causing severe “galling” (build-up of metal on the abrasive) and rendering it ineffective for steel machining. CBN’s inertness makes it the onlychoice for grinding hardened steels and superalloys.

4. Industrial Applications: Grinding vs. Cutting

Due to its properties, CBN is manufactured into two primary tool types:

A. CBN Grinding Wheels (The Most Common Use)

Small-grained CBN bonded into wheels solves the “difficult-to-grind” problem for tough materials.

• Performance:​ Offers a metal removal rate 10 times higher​ than diamond wheels when grinding ferrous metals.

• Materials:​ Ideal for high-temperature alloys, chilled cast iron, and hardened steel.

• Efficiency:​ Enables high-speed, high-efficiency grinding, drastically reducing auxiliary time for wheel dressing and machine adjustments.