Reading property tables with care
A property table for diamond looks like a list of facts, but each entry depends on the sample behind it. Natural stones, high-pressure synthetic crystals, films grown from gas and sintered compacts can all be called diamond, yet they differ in impurity level, grain structure and defects. Material property data quoted without the form and test method is therefore only a rough guide. This review explains the main measured properties, how each is determined and what moves the numbers.
Hardness
Diamond is the hardest known bulk material under common definitions, which is why it sits at the top of the Mohs scale. That scale is only ordinal, though: it ranks scratch resistance, and the gaps between steps are not equal. Engineers more often use indentation hardness, where a small pointed indenter is pressed into the surface and the size of the impression is measured. The figures reported for diamond are very large and spread over a considerable range because the result depends on crystal orientation, on the load applied and on whether the sample is a single crystal or polycrystalline. Many of these details are covered in the discussion of the hardness scale and of indentation testing.
Hardness should also be kept apart from toughness. A very hard material resists scratching but can still chip. The relationship between the two is the subject of the comparison of fracture toughness and hardness, and it explains why sintered diamond is often preferred to a single crystal for impact-prone cutting.
Thermal conductivity
High-quality single-crystal diamond conducts heat several times better than copper at room temperature, which makes it unusual among electrical insulators. Heat is carried mainly by lattice vibrations rather than electrons, and the very stiff, light carbon lattice transmits them efficiently. The underlying reasons are explained in the article on why diamond conducts heat so well.
Real materials fall short of the ideal. Impurities, especially nitrogen, scatter vibrations and reduce conductivity. In films, grain boundaries do the same, so fine-grained polycrystalline diamond usually conducts less than a large single crystal. Measurements are often made by laser flash or related methods that determine thermal diffusivity, from which conductivity is calculated using density and heat capacity. Because the method assumes particular sample geometry, results from different laboratories may not match exactly.
Thermal expansion
Diamond expands very little as it heats up, much less than most metals. That is helpful for dimensional stability, but it creates a mismatch when diamond is bonded to a metal or fixed to a substrate, since the two change size at different rates. The consequences for assemblies are discussed under thermal expansion generally, and in composites they drive the internal stress that designers have to manage.
Stiffness, density and strength
Diamond has a very high elastic modulus, meaning it deforms very little under load, and a moderate density compared with metals commonly used for tools. Strength is more subtle. Brittle materials fail from flaws, so measured strength depends on the largest defect present rather than on a fixed intrinsic limit. Tests on small samples therefore often give higher numbers than components of practical size, and results are usually reported with a spread rather than as one value.
Chemical and thermal stability
Diamond is chemically inert toward most acids and bases at ordinary temperatures. In air, however, it begins to oxidize at elevated temperature, and in the absence of oxygen it can convert toward graphite when heated strongly. Where it sits in contact with iron-group metals at high temperature, carbon can dissolve into the metal, a reason diamond is poorly matched to steel cutting. These limits are the subject of the discussion of oxidation limits, and they set the practical operating window for many diamond tools.
Electrical and optical behavior
Pure diamond is an excellent electrical insulator, while boron-doped material becomes semiconducting. Optical transparency is broad but depends on nitrogen and other defects, and the pattern of absorption bands is a quick way to distinguish types. These effects matter less for bulk wear parts and more for windows, sensors and electronics applications.
How the form changes the values
| Form | Hardness | Heat conduction | Typical remarks |
|---|---|---|---|
| Single-crystal | Very high, orientation dependent | Highest | Can cleave along certain planes |
| Fine-grained polycrystalline | Very high | Lower than single crystal | Grain boundaries scatter heat |
| Sintered compact with metal binder | High | Reduced by the binder | Temperature limit from the binder |
| Metal-diamond composite | Depends on content | Between matrix and diamond | Interface quality is decisive |
The table is qualitative on purpose. Exact numbers differ between suppliers, grades and test methods, so treat any single figure with caution.
Measuring and verifying
Several techniques are standard for checking diamond materials. X-ray diffraction identifies the crystal structure, Raman spectroscopy reveals bonding type and the presence of non-diamond carbon, and electron microscopy shows grain size and defects. Wear and failure behavior, which no single number captures, is handled by separate testing methods. A fair comparison between materials uses data measured in a consistent way, ideally on samples that match the form actually going into service.