The basic idea
Most hardness tests work the same way. A small, hard tip is pressed into the surface with a known force. The size of the permanent mark, or the depth the tip reaches, becomes a hardness number. Harder surfaces leave smaller marks. This approach is known as indentation testing, and nearly all methods used on engineering materials are variations of it.
The measurement is not a pure material constant. It combines the surface’s resistance to yielding, its elastic stiffness, its tendency to crack, and the geometry of the tip. That is why the method and conditions always need to be stated alongside a number.
Common methods
Mohs scratch scale. Minerals are ranked from 1 to 10 by which can scratch which, with diamond at 10. It is simple and useful for field identification, but it is only a ranking. The gaps between steps are not equal, and diamond is far harder than the step below it suggests. Mohs numbers are best treated as a rough hardness scale for identification, not as measurements.
Brinell and Rockwell. These are used mainly on metals. Brinell presses a ball into the surface and measures the diameter of the dent. Rockwell measures the depth of penetration under a preload and a main load, and has several scales for different materials. A common Rockwell scale for hardened steel and cemented carbide uses a diamond cone tip. These tests are quick and widely used in workshops, but they are not suited to the hardest ceramics or superhard crystals.
Vickers. A square-based diamond pyramid is pressed in, and the lengths of the diagonals of the resulting square mark are measured under a microscope. The number comes from the load divided by the surface area of the mark. Vickers works across a wide range of materials, from soft metals to ceramics, and it is the usual reference for hard materials. Values are often reported in gigapascals for superhard work.
Knoop. The Knoop tip is a long, narrow diamond pyramid that leaves an elongated mark. Only the long diagonal is measured. Because the tip is shallow, Knoop is helpful for brittle materials and thin layers, since it penetrates less deeply and reduces the risk of cracking.
Nanoindentation. Here the tip is pressed in by extremely small loads, and the instrument records force against depth continuously. From the curve, it is possible to calculate both hardness and elastic modulus without viewing the mark. It is the standard approach for thin films and coatings, where deep indents would sample the substrate, not the coating.
Why the load changes the answer
Results for the same material can differ for several reasons:
- Indentation size effect. At low loads, measured hardness often appears higher than at larger loads. Surface effects, elastic recovery and the way dislocations behave all contribute.
- Cracking. In brittle materials, a high load produces cracks at the corners of the mark. Hardness is then mixed up with toughness. In fact, the length of such cracks is sometimes used as a rough estimate of fracture toughness, though that approach is debated.
- Elastic recovery. After the tip is removed, the mark may shrink slightly, especially in very hard materials, and this changes the measured size.
- Crystal orientation. Single crystals are not equally hard in every direction. Diamond measurements, for instance, differ depending on which face and direction are tested.
- Microstructure. In a sintered or composite material, a mark may land on a hard grain, a soft binder or across both. Large loads average over many grains; small loads may not.
Testing diamond and superhard materials
Diamond poses a special problem: the tip of the indenter is itself usually diamond. Measuring very hard samples can damage or distort the tip, and results are less certain. Superhard materials are also tiny in many cases, such as small crystals or thin films, so sample preparation, polishing and flatness have a large effect on the numbers.
For these reasons, reported hardness values for the hardest materials vary between laboratories. A sensible reader treats a figure as a range, and looks for the load and method before comparing two materials. When one source quotes a Vickers number at a low load and another at a high load, a direct comparison can mislead.
Choosing a method
| Material or sample | Typical approach |
|---|---|
| Hardened steel | Rockwell or Vickers |
| Cemented carbide | Rockwell A or Vickers |
| Bulk ceramics | Vickers or Knoop |
| Thin coatings | Nanoindentation |
| Small superhard crystals | Vickers or Knoop at several loads |
Beyond hardness
Hardness is one input to materials testing, and not a complete description of performance. Real wear depends on the counter-material, temperature, lubrication and contact conditions, which is why wear testing is often a better predictor. Looking at the damage under an electron microscope can also show whether a mark deformed plastically or cracked.
A high hardness number remains useful as a first screening tool. It becomes misleading only when it is treated as the single deciding property, because tough, heat-resistant or chemically stable materials may be better for a given job.