Diamond or CBN?
Having decided to leave conventional grinding wheels in favor of superabrasives, buyers are confronted with a difficult choice: diamond or CBN abrasive?
Both are extremely hard and offer the potential for greatly improved grinding productivity, but there are differences between the two. It's important to understand these and match the abrasive to the task if peak productivity, and lowest cost-per-piece are to be achieved.
A CHEMISTRY LESSON
Diamond is a crystalline form of carbon. Under heat and pressure carbon atoms link with adjacent atoms to create the hardest known material. This makes it an ideal cutting tool, or would do but for it's rarity and price. However, in the mid 20th century scientists figured out how to manufacture diamond, and today most diamond grit used in superabrasive grinding wheels is man-made.
Unlike diamond, cubic boron nitride (CBN) doesn't exist in nature and is synthesized from boron and nitrogen. When chemically bound together these two elements behave much like carbon in that they can create an immensely strong crystal lattice structure. Of the two, diamond is considerably harder at room temperature, (knoop hardness around 7,500 versus the 4,500 of CBN,) but CBN has better thermal and chemical stability, remaining inert at temperatures up to 1,000 oC, versus the 800 oC at which diamond begins to degrade.
CHEMISTRY DETERMINES APPLICATIONS
As the harder of the two, diamond is preferred for shaping extremely hard workpiece materials such as ceramics, carbides, stone and glass. It is not however suitable for use with steels. This is because carbon and iron have a strong affinity for one another, especially at elevated temperatures. This results in rapid erosion of the diamond grit, quickly destroying the grinding wheel.
CBN is better suited to grinding applications that generate high temperatures, meaning it can be used at higher speeds. And it's unreactive nature makes it the preferred choice for grinding most steels, such as tool steel and HSS.
IMPACT ON WHEEL CONSTRUCTION
The relatively low thermal limit of diamond also affects the type of wheel construction it can be used in. Vitrified wheels are made by firing a clay mix at very high temperatures: if the mix includes a proportion of diamond grit the firing temperature must be kept below 800 oC otherwise the diamond will start to react. For this reason, CBN is more common in vitrified grinding wheels.
TAKEAWAY
Use diamond on the hardest workpiece materials, but don't let it get too hot. Use CBN on ferrous workpiece materials to avoid the chemical reaction that will quickly wear diamond.