Hardness testing

1. Hardness Test
Hardness is ability of materials to resist localized plastic deformation cause by scratching or indentation.
Scratch hardness, when testing coatings, scratch hardness refers to the force necessary to cut through the film to the substrate
Indentation hardness measures the resistance of a sample to material deformation due to a constant compression
load from a sharp object; they are primarily used in engineering and metallurgy fields.
Indenter may be rounded or pointed made from very hard materials such as, hardened steel, tungsten carbide, or
diamond.
Figure 1. Scratching hardness

2. Common types of hardness tests include:
1- Brinell Hardness
2- Rockwell hardness
3- Vickers
4- Knoop
1- Brinell Hardness
In Brinell tests, a hard, spherical indenter is forced into the surface of the sample to be tested. The diameter of the hardened steel (or
tungsten carbide) indenter is 10.00 mm. Standard loads range between 500 and 3000 kgf during a test, the load is maintained constant
for a specified time (between 10 and 30 s). Harder materials require greater applied loads. The Brinell hardness number, HB, is a
function of both the magnitude of the load and the diameter of the resulting indentation. This diameter is measured with a special low-
power microscope. The measured diameter is then converted to the appropriate HB number using a chart
Where , BHN unit is Kgf /mm2 and diameter in ( mm ), F = Kgf
Figure 2. schematic of Brinell Hardness

3. Rockwell
The Rockwell tests constitute the most common method used to measure hardness because they are so simple to
perform and require no special skills. Several different scales may be utilized from possible combinations of
various indenters and different loads, which permit the testing of virtually all metal alloys (as well as some
polymers). Indenters include hardened steel balls having diameters of (1.588, 3.175, 6.350, and 12.70 mm), and a
conical diamond ( indenter, which is used for the hardest materials. With this system, a hardness number is
determined by the difference in depth of penetration resulting from the application of an initial minor load
followed by a larger major load; Call this difference, in millimeters, h. On the basis of the magnitude of both major
and minor loads, there are two types of tests: Rockwell and superficial Rockwell. For Rockwell, the minor load is
10 kg, whereas major loads are 60, 100, and 150 kgf.. For superficial tests, 3 kgf is the minor load; 15, 30, and 45
kgf are the possible major load values. Superficial tests are frequently performed on thin specimens.

4. if the test uses a conical diamond:
If the test uses a ball:

5. Regular Rockwell Scales
Scale
symbol
Indenter
Type
Dia mm
Minor
load
Newton
(KgF)
Major
load
Newton
(KgF)
Type applocations
A
Sphero-
conical
diamond
98.07
(10)
588.4
(60)
Cemented carbides, thin steel, and shallow case hardened steel.
B
Ball
1.588mm
98.07
(10)
980.7
(100)
Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
C
Sphero-
conical
diamond
98.07
(10)
1471
(150)
Steel, hard cast irons, pearlitic malleable iron, titanium, deep
case hardened steel, and other materials harder than 100 on the
Rockwell B scale
D
Sphero-
conical
diamond
98.07
(10)
980.7
(100)
Thin steel and medium case hardened steel, and pearlitic
malleable iron.
Also, There are E,F, etc scales. The most common scales are B and C
Superficial Rockwell scales are used for thin sheet and very soft materials.

6. Knoop and Vickers Microindentation Hardness
Both Brinell's and Rockwell tests are used in macroscales. Two other hardness-testing techniques are Knoop and Vickers
(sometimes also called diamond pyramid). For each test a very small diamond indenter having pyramidal geometry is forced into
the surface of the specimen. Applied loads are much smaller than for Rockwell and Brinell, ranging between 1 gf and 1000gf. In
the Vickers test, the force is applied smoothly, without impact, and held in contact for 10 to 15 seconds. The force must be
controlled precisely. After removing the force, both diagonals are measured and the average is used to calculate the HV. Careful
specimen surface preparation (grinding and polishing) may be necessary to ensure awell-defined indentation that may be
accurately measured. The Knoop and Vickers hardness numbers are designated by HK and HV, respectively, and hardness scales
for both techniques are approximately equivalent. Knoop and Vickers are referred to as microindentation-testing methods on the
basis of indenter size. Both are well suited for measuring the hardness of small, selected specimen regions. furthermore, The
Knoop indenter is similar to the Vickers indenter except that the diamond pyramid has unequal length edges, resulting in an
impression that has one diagonal with a length approximately seven times the shorter diagonal, thus Knoop is used for testing
very hard materials such as ceramics.

7. HV = F/A , where A is resulting area of indentation
A= d2 / 2 sin (136/2) A= d2 / 1.854
So, HV= 1.854 F / d2 , F in KgF and d = d1 +d2 / 2 in (mm)
HV = KgF/ mm2
HV can be converted to SI units
HV= 1.854 x (9.8) F / d2 = 18.16 F/ d2 ( MPa)
Figure (4) Vickers Hardness test

8. Types of Hardness tests

9. Correlation Between Hardness and Tensile Strength
Both tensile strength and hardness are indicators of a metal’s resistance to plastic deformation. Consequently, they are roughly
proportional, as shown in Figure (5),for tensile strength as a function of the HB (Brinell Hardness) for cast iron, steel, and
brass. The same proportionality relationship does not hold for all metals, as Figure (5) indicates. As a rule of thumb for most
steels, the HB and the tensile strength are related according to:
TS (MPa) = 3.45 x HB 5

10. Nanoidentation: It is used for thin film materials. In nanoindentation small loads and tip sizes are used, so the indentation
area may only be a few square micrometres or even nanometres. Instead, an indenter with a geometry known to high
precision (usually a Berkovich tip about 20 nm radius , which has a three-sided pyramid geometry) is employed. During the
course of the instrumented indentation process, a record of the depth of penetration is made, and then the area of the indent is
determined using the known geometry of the indentation tip. While indenting, various parameters such as load and depth of
penetration can be measured. A record of these values can be plotted on a graph to create a load-displacement curve (such as
the one shown in Figure 6).
hc

11. These curves can be used to extract mechanical properties of the material such as :
1. Hardness H = Pmax / Ac
(area of contact)Ac = π( tan(θ) hc)2 = 24.5 hc
2
hc = depth of contact
θ = 70.320 for Berkovich tip
2- Young's modulus: The slope of the curve dP/dh upon unloading is indicative of the stiffness of the
contact.
3- fracture toughness (ability of materials containing crack to resist fracture KIC)
4- elastic and plastic energy