Hardness Testing Guide: Vickers, Rockwell & Knoop

Hardness testing is a key way to assess material strength, wear resistance and heat-treatment quality. This guide covers the principle, calculation and range of Vickers, Knoop and Rockwell tests, and the relationship between specimen thickness, test force and indentation size, to raise accuracy and material-quality judgement.

1. What Hardness Testing Is

Hardness is a material's resistance to deformation or indentation, widely used for metal strength judgement, heat-treatment quality inspection, surface-hardened-layer analysis, and weld/microstructure evaluation.

2. Common Test Types

2a. How Vickers and Knoop Are Actually Calculated

These two definitions are what the table above is built on. F is in newtons and d is in millimetres; the constants come from the gravity conversion (1/9.80665 = 0.102), so every line below can be recomputed.

Vickers HV — a diamond square pyramid with a 136° face angle is pressed in; after the indenter is withdrawn you measure the diagonal in both directions and average it to get d, then divide the test force by the contact area of the indent:

HV = 0.1891 × F / d²

Check: 0.102 × 2 × sin(136°/2) = 0.18915. Vickers covers the widest range of test forces, including the micro range below 9.807 N, which is why thin material and surface-treated layers usually end up on Vickers.

Knoop HK — the indenter is a rhombic diamond pyramid whose included angles are 172°30′ and 130°. You measure the longer diagonal d and divide by the projected area of the indent:

HK = 0.1451 × F / d²

Check: 0.102 / 0.07028 = 1.45134. At the same test force a Knoop indent is shallower than a Vickers one, which is why coatings, films and small areas go to Knoop; a micro hardness tester measures it simply by swapping the Vickers indenter for a Knoop one.

2b. Rockwell and Superficial Rockwell: the Difference Is the Preliminary Force

The indenter is a diamond cone with a 120° included angle and a 0.2 mm tip radius, or a steel or carbide ball. A preliminary force is applied, then the major force, then the load returns to the preliminary force, and the hardness comes from the difference in indentation depth between the two.

A 98.07 N (10 kgf) preliminary force is Rockwell; 29.42 N (3 kgf) is superficial Rockwell — that number is the difference between them, not the scale letter. Each fixed combination of indenter, test force and formula has its own symbol, and those are the scales in the two tables in the next section.

3. Selection Principle

Choose by material and shape: ultra-hard → Vickers or Knoop; steel → Rockwell (HRC); thin sheet/coating → superficial Rockwell or Knoop; plastic and rubber → dedicated hardness tests.

3a. Material, Shape and Purpose Mapped onto the Tests

Work backwards from what you are trying to judge. A blank row means none of the five tests covered here fits, and you need another method such as Brinell, Shore or Leeb.

● suitable  ▲ partly suitable  blank: use another method

3b. The Rockwell and Superficial Rockwell Scales

Rockwell scales, indenters and typical materials (per JIS)

Superficial Rockwell scales, indenters and typical materials (per JIS)

4. Thickness Relationship (Important)

Vickers: specimen thickness should exceed 1.5× the indent diagonal; indent depth is about 1/7 of the diagonal. If too thin, results distort and are influenced by the substrate. Rockwell: each scale has its own minimum thickness — HRC, HRB etc. must meet it; superficial Rockwell suits thinner material.

The number after a Vickers symbol is the test force in kgf — 185HV1 means 185 measured with 1 kgf (9.807 N). The usual pairs are:

Every row is the symbol number in kgf multiplied by 9.80665 - you can recompute any line yourself.

5. Sources of Error

Mainly indenter geometry error, test-force error, indent-reading error and specimen surface condition. A flatter, cleaner surface gives more reliable readings. For material properties see the Metal Materials Guide.

6. Applications

Common uses: carburised-layer depth, quench-depth determination, weld-zone hardness analysis and high-temperature material performance evaluation.

This article is part of Precision Measurement Complete Guide: Ask What You Are Measuring First, Then Pick the Instrument; that guide shows how the whole topic fits together.