How to choose between diamond and cubic boron nitride CBN
Apr 03, 2025
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In the field of mechanical processing, diamond (PCD) and cubic boron nitride (CBN) are known as the "twin giants of superhard materials". They become powerful tools for difficult machining tasks due to their excellent hardness and wear resistance. However, the performance characteristics of the two are significantly different, and choosing the wrong material may lead to a surge in costs and a sharp drop in efficiency. This article will provide in-depth analysis from dimensions such as hardness, thermal stability, and applicable scenarios to help you make the optimal choice.
who is better?
Hardness and thermal stability
| abrasives |
Diamond
|
CBN
|
| products |
 |
 |
| Hardness |
Mohs hardness level 10, the hardest material known in nature, with unparalleled wear resistance. |
The Mohs hardness is 9.5, slightly lower than diamond, but still the second hardest artificial material. |
| Thermal stability |
At high temperatures (above approximately 700 ℃), it is prone to chemical reactions with metals such as iron, cobalt, and nickel, leading to the failure of "graphitization" (the transformation of diamond into graphite structure). |
It can maintain stable performance at high temperatures of 1200 ℃ and has extremely strong antioxidant properties. |
| Chemical inertness |
inability to process iron-based metals (such as steel and cast iron), sudden drop in performance in high-temperature environments. |
It hardly reacts with iron-based metals and is the "natural choice" for processing ferrous metals. |
Conclusion:
- Diamond is the 'hardness king', but CBN is the 'high-temperature war god'!
- Diamond is afraid of both iron and fire, CBN is specifically designed for iron-based metals.
Material adaptability
Iron based metal battlefield: materials such as quenched steel (HRC58-65), high-speed steel, cast iron, etc. can be considered as the "forbidden zone" for diamonds. When the processing temperature exceeds 400 ℃, carbon in diamond will undergo diffusion reaction with iron: C+Fe → Fe3C (cementite), causing abrasive particles to be "eaten". The B and N atoms of CBN hardly undergo chemical reactions with iron-based metals, and can maintain sharp cutting even at high temperatures generated by dry grinding (up to 800-1000 ℃).
In the field of hard and brittle materials, ceramics, glass, hard alloys (such as YG20), gemstones, and other materials require extreme wear resistance. The ultra-high hardness and isotropic structure of diamond can perfectly match the brittle removal mechanism of these materials. Although CBN can also process these materials, its wear rate is 3-5 times faster than diamond
How to choose?
Types of processed materials
Choose diamond:
- Non ferrous/hard brittle materials: ceramics, glass, hard alloys, stone, copper aluminum alloys, etc.
- Advantages: High hardness brings an ultra long lifespan, and the processed surface has excellent smoothness.
- Taboo: Strictly prohibited for use on iron-based metals such as steel and cast iron!
Choose CBN:
- Iron based high hardness metals: quenched steel, high-speed steel, cast iron, titanium alloys, high-temperature alloys, etc.
- Advantages: It does not soften at high temperatures, and its chemical stability avoids the phenomenon of "sticking to the knife".
- Taboo: Processing non-ferrous materials is inefficient and cost-effective.
Typical case:
- A certain hard alloy cutting tool factory mistakenly used CBN to process hard alloys, resulting in a 50% decrease in efficiency; After switching back to diamond, the production capacity doubled.
- A certain automobile gear factory originally used diamond to grind quenched steel gears, with a monthly loss of 150000 yuan; After switching to CBN, the tool life is increased by three times and the annual cost is saved by 500000 yuan.
Processing temperature and environment
- High temperature dry grinding: CBN is a must-have!
- Diamond rapidly graphitizes under high temperature during dry grinding, causing the cutting edge to crack.
- Wet grinding or low-temperature processing: prioritize diamond.
- Adequate coolant can suppress the high-temperature failure of diamond and leverage its hardness advantage.
Practical Application Scenario Guide
Scenario 1: Machining of Automotive Engine Cylinder Blocks (Cast Iron)
Wrong choice: Diamond cutting tools - rapidly fail at high temperatures, resulting in increased costs.
Correct solution: CBN blade - resistant to high temperature and wear, with a 40% increase in processing efficiency.
Scenario 2: Grinding of ceramic backplate for mobile phones
Wrong selection: CBN grinding wheel - insufficient grinding force, surface roughness does not meet the standard.
Correct solution: Diamond grinding wheel - high hardness achieves nanometer level smoothness.
Remember the principle:
- Diamond: the "lifelong companion" of non-ferrous materials, but it "dies" upon seeing iron.
- CBN： The terminator of iron-based metals, but the non-ferrous field is not adapted to the local conditions.
Conclusion
In today's increasingly competitive manufacturing industry, choosing the right tool materials can directly determine a company's profit margin. Diamond and CBN have their own advantages, and there is no eternal "king". Only by accurately matching processing needs can we make a wise choice!
Although theoretically, diamond cannot be used to process black metals. But in practical applications, composite abrasives, which are mixed abrasives of diamond and CBN, are often used. It usually has a good effect. For example, in the production of CBN grinding wheels for grinding railway tracks, diamond abrasives and silicon carbide abrasives are added, which makes it easier to remove chips and improves the service life during rail track grinding.
In the manufacturing of saw blades for cutting reinforced concrete, diamond is usually used as the abrasive. In practical applications, diamond circular saw blades with diamond as the main abrasive have good results in cutting steel bars. If CBN is used as the abrasive at this time, not only will the cost increase significantly, but the service life will also decrease significantly.
In summary, the experience and practice of Huice Tools over the years have proven that the use of diamond and CBN abrasives cannot be solely based on theoretical knowledge from textbooks. For example, in the processing of many black metals, diamond is used as the abrasive for cutting, a mixed abrasive of diamond and CBN can be used for rough grinding, and a single CBN abrasive is used for fine grinding and polishing.
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