Hardness is a mechanical property that refers to a material's resistance to plastic deformation. There are three main types of hardness tests: scratch, indentation, and rebound hardness. Indentation hardness tests such as Brinell, Vickers, Rockwell, and Knoop hardness tests are most commonly used for engineering metals. Each test uses a different indenter and applied load over various time periods to make an indentation on the material's surface for measuring hardness.

1. Hardness – A
Mechanical Property
Shahadat Hussain
CSIR-AMPRI, Bhopal

2. Properties of Solid Material
Almost all important properties of solid materials can be grouped
into following six properties:-
1. Mechanical Properties
2. Thermal Properties
3. Electrical Properties
4. Magnetic Properties
5. Optical Properties
6. Chemical or deteriorative Properties
Mechanical Properties: Mechanical properties of a solid material is determined
by the response of the material when force or load is applied on it. Eg:
Stiffness, strength, ductility, toughness, creep, fatigue , hardness etc.

3. The Hardness
Hardness is the poorly defined term
The hardness of a materials refer to its resistance to the
deformation (plastic or permanent deformation)
Why we use hardness?
This is very easy to measure
It indicates something about strength and heat treatment of
the metals

4. Types of Hardness
 Scratch Hardness
 Indentation Hardness
 Rebound or dynamic Hardness
Based on the method of determining the hardness of
materials, hardness can be classified into following three
categories.

5. Scratch Hardness
• These hardness values are generally used in minerology.
• Scratch hardness test determines the hardness of a materials due to
scratches and abrasion due to friction from a sharp object.
• It was first developed by German mineralogists Friedrich Mohs in 1820 and
is still widely applied to rank the physical property of minerals.
• This method is applicable to metals, ceramics, polymers and coated
surfaces.
• Scratch hardness is measured according to Mohs Scale.
• Mohs Scale has 10 division from 1 to 10 and these values are assigned to
10 standard minerals. 10 is for diamond and 1 is for talc.
ASTM G171 – 03 (hardness of materials using diamond stylus)

7. Scratch Hardness
Measurement

8. Dynamic Hardness
In dynamic hardness, the indenter is dropped on the surface of metals and the height of its
rebound is calculated as dynamic hardness.
Higher the rebound, the higher is the hardness of the materials
Diamond pointed
hammer is used.

9. Scales in dynamic hardness test
S.No. Model Height of
Hammer
Weight of Hammer
1 C 251.2 mm 23g
2 D 17.9 mm 36g
ASTM A448
1
4
3
2
Minimum materials thickness
Surface finish of the materials should be
good for reliable measurement
5 Hammer should not be impacted more than once at same spot since there occurs
cold working. The spots should be 0.51 mm apart.

10. Indentation Hardness
Indentation hardness test is significant for engineering metals as
compared to other two.
1. Brinell Hardness Test
2. Vickers Hardness Test
3. Rockwell Hardness Test
4. Knoop Hardness Test

11. Brinell Hardness
• Earliest indentation hardness test developed in 1900.
• Load applied :500-3000kgf
• Time: 10-30 seconds
• Indenter: Steel or tungsten carbide ball of 10 mm diameter
• Average value of two reading of indentation taken at right angles to
one another.
• Brinell Hardness is calculated by dividing load by the surface area of
indentation.
• Combination of load and ball size is chosen according to the types of
materials being tested to minimize any distortion of indenter.
ASTM E10-17 for metallic materials

14. Vickers Hardness
• Load applied: 1-100kgf
• Time: 10-15 seconds
• Indenter: Diamond indenter of
right pyramid shape with square
base with 136° angle between
opposite faces
• Length of two diagonals are
measured
Where F is the force applied and D is the
average length of the diagonal, i.e.,
(d1+d2)/2
ASTM E92-17 (for metallic materials)

17. Rockwell Hardness
Rockwell Hardness test is widely
used in industries due to its
speed of testing, simplicity, small
size of indentation, and ease of
operation.
Two loads are applied: Minor
Load (3-10kgf) and Major Load
(15-150kgf) for superficial
testing and 500-3000kgf for
macrohardness
Time: 10-15 seconds
ASTM E18-17e1 (for metallic materials)
Indenters: Variety of indenters are used in Rockwell test ranging from diamond indenter with
round tip for harder materials to ball shaped indenter for softer materials
There are several scales in Rockwell test most popular are B and C.

18. HR= N-
𝑑
𝑠
Where N and S are scale
factor and d is the depth of
penetration

20. Knoop Hardness
Knoop hardness test is mainly used for
brittle materials such as glass and
ceramics and thin specimen.
Load: 1 gf-2kgf
Time: 10-15 seconds
Indenter: Diamond pyramid-shaped
rhombic indenter (longitudinal edge
angle 172.5°, transverse edge angle
130°)
Where F is the force applied and D is the
average length of the diagonal, i.e.,
(d1+d2)/2

21. ASTM C1326 - 13
Standard Test Method for Knoop
Indentation Hardness of Advanced
Ceramics