Grains are the key difference
A diamond film grown by vapor deposition is not one crystal. It is a packed layer of many small crystals, called grains, that nucleate on the surface and grow until they meet. What distinguishes the types of film is mainly how big those grains are.
Microcrystalline films have grains on the scale of micrometers, large enough to see as faceted crystals under an electron microscope. Nanocrystalline films have grains on the scale of tens of nanometers, far too small to see as individual facets without high magnification. There are also intermediate and ultrananocrystalline forms, with the boundaries between the categories defined loosely, so the names are best taken as a spectrum.
How grain size is controlled
Grain size comes from the growth recipe. Several variables matter, and none has a universal setting.
- A high density of starting nucleation sites leads to many small crystals that meet before they can grow large.
- A higher fraction of carbon-bearing gas in the mix, or the addition of other gases such as argon in some systems, tends to cause frequent renucleation, which keeps grains small.
- Lower hydrogen content reduces the etching of non-diamond carbon, so fine-grained films typically contain more of it at the grain boundaries.
- Substrate temperature and pressure change growth rates and the shapes of crystals.
Both hot filament and microwave plasma systems can produce either type, although each tends to have a recipe range in which it works best.
Surface smoothness
Smoothness is the most visible contrast. Large crystals grow with exposed facets and the film becomes rougher as it thickens, because larger crystals overgrow smaller neighbors. A rough surface raises friction against the workpiece, may let chips stick and can leave a texture on the finished part.
Nanocrystalline films are far smoother as grown. That makes them attractive for finishing tools, for sliding surfaces and for micro-tools where the edge must stay sharp. A smooth film also reduces the extra edge radius contributed by large crystal tips, so it works better when a thin, sharp coating is needed, a point connected to the balance described in articles on coating thickness.
Microcrystalline films can be polished after growth, but diamond is hard to polish and the step adds cost, particularly on complicated tool shapes.
Toughness and cracking
Grain size also changes how a film cracks. Larger grains mean fewer grain boundaries per unit length, and diamond itself is brittle, with cleavage planes along which fracture can run. A crack in a coarse film can run along a significant length of crystal.
In finer-grained films, cracks meet many boundaries and tend to be deflected, which often gives better resistance to chipping. The same boundaries also carry non-diamond carbon, which is softer than diamond, so hardness and thermal conductivity are generally somewhat lower in nanocrystalline films. The result is a trade: more toughness and smoothness against slightly reduced hardness and heat conduction. Different films vary a great deal, so a general statement should not be read as a rule for any particular product.
Hardness and heat
Microcrystalline films come closest to the properties of bulk polycrystalline diamond, including high thermal conductivity and the hardest surfaces. That helps in demanding abrasive cutting where the highest wear resistance is needed and a rougher finish is acceptable.
Nanocrystalline films typically give up some of that peak performance. In many practical applications the difference is outweighed by the benefits of smoothness and reduced chipping, which is part of why they have become common on small tools.
Stress and adhesion
Grain structure influences residual stress, because the grain boundaries and the way crystals fill the film affect how stress is distributed. Fine-grained films are often reported to develop different stress levels than coarse ones, and the details depend on the process. What can be said safely is that stress, thickness and adhesion are linked in both types, so the choice of grain size does not remove the need for good surface preparation.
Choosing between them
Typical guidelines, with plenty of exceptions:
| Consideration | Microcrystalline | Nanocrystalline |
|---|---|---|
| As-grown surface | Rough, faceted | Smooth |
| Edge sharpness | Reduced by large crystals | Better retained |
| Resistance to chipping | Lower | Often better |
| Peak hardness and heat conduction | Highest | Somewhat lower |
| Typical use | Roughing, heavy abrasion | Finishing, small tools, sliding faces |
Most real tool coatings are designed for one end of the job. A shop machining graphite electrodes with a large cutter and heavy removal may favor a thick microcrystalline film. A shop making small features in composites or in a precision mold may prefer a thin nanocrystalline one. Some designs use multilayers, alternating the two types to combine toughness with wear life.
What is not the same
It is easy to confuse nanocrystalline diamond with diamond-like carbon. The latter is an amorphous carbon coating without a true diamond crystal structure, so it differs in hardness, thermal behavior and how it is deposited. Nanocrystalline diamond is still crystalline diamond, only in very small grains.
Takeaway
Grain size trades smoothness and toughness against ultimate hardness and thermal performance. Microcrystalline films favor heavy abrasion, nanocrystalline films favor sharp, smooth and chip-resistant tools, and the best choice follows the job rather than a general ranking.