Brinell hardness test was invented by J.A. Brinell in 1900 using a steel ball indenter with a 10 mm diameter. The steel ball is pressed on a metal surface to provide an impression as demonstrated in figure 1. This impression should not be distorted and must not be too deep since this might cause too much of plastic deformation, leading to errors of the hardness values. Different levels of material hardness result in impression of various diameters and depths. Therefore different loads are used for hardness testing of different materials as listed in table 1. Hard metals such as steels require a 3,000 kgf load while brass and aluminium involve the loads of 2,000 and 1,000 kgf respectively. For materials with very high hardness, a tungsten carbide ball is utilized to avoid the distortion of the ball.

Figure 1: (a) Brinell indentation (b) measurement of impression diameter

Rockwell hardness test is commonly used among industrial practices because the Rockwell testing machine offers a quick and practical operation and can also minimize errors arising from the operator. The depth of an indentation determines the hardness values. The indenter made of diamond or hardened steel is pressed onto a flatly ground metal surface with a minor load of 10 kgf to position the metal surface as shown in figure 2. The depth of the impression caused by the minor load will be recorded onto the machine before applying a major load level according to a standard as shown in table 2. The difference of the depths when applying the minor and the major loads indicates the hardness value of the material. If the depth difference is small, the deformation resistance of the metal is high, resulting in a high Rockwell hardness value. The hardness value will be displayed on a dial or a screen, having 100 divisions and each division represents a depth of 0.002 mm.

1

pressing of the steel ball on to the metal surface is carried out for 30 second, followed by measuring two values of impression diameters normal to each other using a low magnification macroscope. An average value is used for the calculation according to equation 1 where: P = the applied load, kgD = the diameter of the steel ball, mmd = the diameter of the indentation, mmt =s the depth of impression, mmNote: This BHN values has a unit of kgf.mm2 (1 kgf.mm2 = 9.8 MPa) which cannot becompared to the average mean pressure on the impression.BHN = P/(π/2*D)(D-√D2-d2 ) = P/πDt

The Rockwell hardness units are in RA, RB and RC (or HRA, HRB, HRC), depending on material.s hardness. Table 1 summarizes loads and types of an indenter utilized for each scale. There are two types of indenters used, a Brale indenter (a diamond cone with an included angle of 120o) and a steel ball indenter with its diameter varying from 1.6-3.2 mm. The applied major loads vary from 60, 100 and 150 kgf, also depending on the Rockwell hardness scale utilized. For instance, hardened steel is tested on a Rockwell scale C using a Brale indenter and at a major load of 150 kgf. On the Rockwell scale C, the obtained hardness values range from RC 20 B RC 70.Scale Indenter Minor Load (kgf) Major Load(kgf) A Diamond cone 10 50B 1/16" steel ball 10 90C Diamond cone 10 140D Diamond cone 10 90E 1/8" steel ball 10 90F 1/16" steel ball 10 50G 1/16" steel ball 10 140H 1/8" steel ball 10 50K 1/8" steel ball 10 140L 1/4" steel ball 10 50M 1/4" steel ball 10 90P 1/4" steel ball 10 140R 1/2" steel ball 10 50S 1/2" steel ball 10 90V 1/2" steel ball 10 140

Applied loads and types of indenter used in Rockwell scale A,B and C hardness testing

Metals with lower hardness are tested on a Rockwell scale B using a 1.6 mm diameter steel ball at a 100 kgf major load, providing RB 0 and RB 100 hardness values. Rockwell scale A offers a wider range of hardness values which can be used to test materials ranging from annealed brass to cemented carbide. Due to high accuracy, the Rockwell hardness test is commonly conducted for measuring hardness of heat-treated steels. Furthermore, the smaller indenter (in comparison to that of Brinell hardness test) facilitates hardness measurement in small areas. However, this technique requires good surface preparation since the hardness values obtained is significantly affected by rough and scratched surfaces.There are several considerations for Rockwell hardness test:

a) Require clean and well positioned indenter and anvilb) The test sample should be clean, dry, smooth and oxide-free surfacec) The surface should be flat and perpendicular to the indenterd) Low reading of hardness value might be expected in cylindrical surfacese) Specimen thickness should be 10 times higher than the depth of the indenterf) The spacing between the indentations should be 3 to 5 times of the indentation diameterg) Loading speed should be standardized.

Vickers hardness test requires a diamond pyramid indenter with an included angle of 136o.This technique is also called a diamond pyramid hardness test (DPH) according to the shape of the indenter. To carry on the test, the diamond indenter is pressed on to a prepared metal surface to cause a square-based pyramid indentation as illustrated in figure 3.

2

The Vickers hardness value (VHN) can be calculated from the applied load divided by areas of indentation, at which the latter is derived from the diagonals of the pyramid as expressed in the equation below:

Vickers hardness is widely used in experimental and research areas because the VHN scale practically offers a wide range of hardness values. For instance, the VHN values range from 5 to1,500 can be obtained from measuring materials from dead soft to full hard. This method is therefore more convenient and provides a wider range of the hardness values in comparison to those obtained from Rockwell and Brinell hardness tests. The applied loads vary from 1-120 kg, which depends on the materials being tested. However, Vickers hardness test is in commonly used for company daily checks. This is due to errors which might occur in the measurement of the diagonals and longer time required to finish the test.

Vickers hardness indentations a) perfect indentation, b) pincushion and c) barrel-shaped.

Experimental procedure1. Specimen surfaces of aluminium, brass steel and weld are flattened and ground using sand

papers. Polishing of the metal surface is required when using Rockwell and Vickers hardness tests.

2. Hardness measurement is carried out using Brinell, Rockwell and Vickers hardness testing techniques on the prepared samples.

3. Hardness profile testing is conducted across the weld sample at 10 positions and 1 mm intervals using a Vickers hardness testing machine.

4. Summarize the experimental results on the table provided and exhibit the results graphically. Compare and discuss the obtained results in order to relate hardness properties of the metals to their microstructure.

5. Give conclusions.

3

Hardness:

The hardness of a material measures how tightly the atoms are held together within it. This test is done either by scratching one substance with another and using the Moh's Scale. On the Moh's Scale, diamond is ranked 10 and graphite is ranked between 1 and 2. This suggests that diamond is about 40 times harder than graphite. Buckminsterfullerene crystals are also relatively soft, even though the individual C60 molecules are hard.

The Moh's Scale measures the relative hardness of various substances. It uses ten reference minerals. The hardness of a substance is determined by scratching it against a reference mineral. If it scratches that mineral, then it is of equal hardness or harder than that mineral, otherwise it is softer then that mineral. The picture below shows the minerals that are used on the Moh's Scale.

Diamond is ranked as number 10, the hardest substance on this scale. The softest mineral is talc. Graphite ranks between 1 and 2 on this hardness scale. The relation between the Moh's Scale and absolute hardness measured by other means is shown below:

Other methods use indenters that apply a set amount of force on the surface of the test material. The profile of the resultant dent is then measured and calculated using a variety of other scales.

Brinell Hardness Test

The Brinell Hardness Test was one of the first quantitative tests developed to measure hardness. A tungsten carbide or hardened steel ball, with a diameter of 1.0 cm, is pressed into the material to be tested under a standard load. The load is maintained for 5 to 10 seconds to allow the material to undergo plastic deformation. After the load is removed, the spherical indentation left behind by the ball is measured. The load divided by the surface area of the indentation determines the hardness of the test material. This test is good for measuring the hardness of very hard and thick materials such as metals.

Vicker Hardness Test

The operating principles of the Vicker Hardness Test are similar to that for the Brinell Hardness Test. The Vicker test uses a square-based diamond pyramid tip with a characteristic 136 degree angle as the

4

indenter. Since determining the diagonal of a square impression is easier than determining the diameter of a circular impression, the hardness obtained by the Vicker test may be more accurate than the Brinell test. This test is also good for measuring the hardness of very hard and thick materials such as metals.

Vickers Hardness Test Knoop Test

Knoop Hardness Test

This test is a micro-hardness test, used to determine the hardness of specific locations on an inhomogeneous or very thin sample. An elongated diamond pyramid tip as the indenter. The impression left on the test material has a 130 degrees angle along the width with 172 degrees and 30 minutes angle along the length.

This big difference in hardness between the three forms of carbon is a consequence of each substance's crystal structures. The crystalline structure adopted by each substance is a reflection of how the carbon atoms are bonded together within each substance.

The hardness of a substance is not the same in all directions.

Even a highly symmetric crystal such as diamond have a distinct hardness for each of its crystalline faces. The cubic crystalline faces (100, 010, and 001 orientations) of diamond are much softer than a face perpendicular to its body diagonal (111 orientation). The carbon atoms are packed more densely, so more rigid, in this direction. This is the reason why natural diamonds are found as octahedrons.

Graphite is hard parallel to its basal plane, but is bendable normal to the layers. The bonds between the carbons within the graphite sheets are stronger than those in diamond. The forces that hold the sheets together, on the other hand, is very weak so the sheets can be easily peeled away. This is the reason graphite feels so soft and lubricating.

The bonds within each Buckminsterfullerene molecule are very strong. The forces that hold the molecules together to form its crystals are very weak, so its crystals are mechanically soft. A measure of a material's resistance to denting, scratching, and abrasion, related to the yield stress and tensile strength of the material. It is determined using indentation test, which measure the size of a hole formed by a hard indenter driven into the material, as in the Vickers and Brinell tests. It is sometimes classified using the Mohs test of mineral hardness (devised by German mineralogist Friedrich Mohs in 1812), which rates talc as hardness 1 and diamond as hardness 10.

5