One lattice versus many
A single crystal diamond is one continuous lattice: every atom is positioned according to one orientation throughout the piece. Polycrystalline diamond is an assembly of many small crystals, called grains, each with its own orientation, joined at grain boundaries. The grains themselves are still diamond. What differs is how they are arranged, and that arrangement changes many practical properties.
There are two broad ways polycrystalline diamond is made. One is sintering diamond particles under very high pressure and temperature, usually with a metal binder, to produce compacts. The other is growing a polycrystalline layer by vapor deposition, which gives free-standing plates or coatings with no binder. They behave differently, so “polycrystalline” is not a single material.
Strength and fracture behavior
A single crystal has preferred directions along which it splits, the cleavage planes. A crack can run a long way along one plane. In polycrystalline diamond, the grains point in random directions, so a crack heading along an easy plane in one grain meets a harder orientation in the next and must turn or stop. The result is generally better resistance to chipping and fracture, which is why polycrystalline forms are favored for tools that take shocks.
The cost is that grain boundaries are not as perfect as the lattice inside the grains. In binder-containing compacts, the binder phase can limit high-temperature performance and chemical stability, and differences in expansion between binder and diamond can create stress on heating. Grain size also matters: finer grains tend toward more uniform, tougher material, while coarser grains can approach the properties of single crystals but may have more variable wear.
Wear behavior
A single crystal is hard in a direction-dependent way. Tools made from a single crystal, such as those used for ultra-precision diamond turning, can produce extremely sharp, smooth edges because there are no grain boundaries to leave microscopic steps. That is valuable for optical surfaces and fine finishes, but the crystal must be oriented carefully and protected from impact.
Polycrystalline diamond wears more uniformly in all directions, with no weak orientation. That makes it well suited to rough and semi-finishing work, to rock-drilling cutters such as polycrystalline diamond cutters, and to turning inserts used on aluminum and abrasive composites. The edge is generally less perfect than a single-crystal edge, which is acceptable in most industrial tasks.
Thermal and optical quality
For heat conduction, the highest figures come from high-purity single crystals and the best vapor-grown plates. Grain boundaries scatter the lattice vibrations that carry heat, so polycrystalline material is typically somewhat lower, especially with small grains. Even so, good polycrystalline diamond conducts heat far better than most metals, and it is used for heat spreaders because it can be made in larger sizes.
For optics, single crystals can be extremely clear, while polycrystalline plates may scatter light at grain boundaries and from included non-diamond carbon. Optical-grade polycrystalline plates exist and are used for large windows, but they are made carefully to keep scatter low. Size works in polycrystalline diamond’s favor: large single crystals are hard to grow, while vapor deposition can cover wide areas.
Size, shape and cost
- Size. Large, flawless single crystals are difficult and slow to produce, so sizes are limited. Polycrystalline compacts and plates can be made larger or in useful shapes.
- Shape. Sintered compacts can be formed into tool-ready blanks, often bonded to a carbide substrate. Single crystals typically must be cut, ground and polished from the grown stone.
- Cost. Cost depends on the grade, size, process and quality, and varies widely. As a general rule, high-quality large single crystals are costly to produce, while standard polycrystalline blanks are produced in volume for ordinary tools. Exact prices change and should be checked with suppliers rather than assumed.
Typical uses side by side
| Need | Often suited to |
|---|---|
| Ultra-sharp edge, optical finish | Single crystal |
| Heavy-duty cutting or drilling | Sintered polycrystalline |
| Large heat spreader or window | Vapor-grown polycrystalline |
| Electronic or quantum devices | Single crystal, high purity |
| Grinding grit | Single-crystal particles, selected by toughness |
How to choose
The choice is not a matter of one form being better. It depends on whether the job rewards a perfect edge and clarity or toughness, size and economy. The same trade-off appears at the nanometer scale in thin films, where nanocrystalline diamond differs from coarser-grained types. In all cases, performance depends on the specific grade, so data on the actual product is more dependable than general rules.