6. Vickers Hardness Test

Lab Session: 04 To determine the hardness of a given material using Vickers Hardness Tester

Vickers Hardness Test: Vickers Hardness Test was formally known as the Diamond Pyramid Hardness (DPH) test. Vickers test has two distinct force ranges, micro (10gf to 1000gf) and macro (1kgf to 100kgf), to cover all testing requirements. The indenter is the same for  both ranges therefore Vickers hardness values are a re continuous over the total range of hardness for metals (typically HV100 to HV1000). With the exception of test forces below 200g, Vickers values are generally considered test force independent. In other words, if the material tested is uniform, the Vickers values will be the same if tested using a 500g force or a 50kg force. Below 200g, caution must be used when trying to compare co mpare results. It is the standard method for measuring the hardness of metals, particularly those with extremely hard surfaces: the surface is subjected to a standard pressure for a standard length of time by means of a pyramid-shaped diamond. The diagonal of the resulting indention is measured under a microscope. The indenter employed in the Vickers test is a square-based pyramid whose opposite sides meet at the apex at an angle of 136º. The diamond is pressed into the surface of the material at loads ranging up to approximately 120 kilograms-force, and the size of the impression (usually no more than 0.5 mm) is measured with the aid of a calibrated microscope. The Vickers number (HV) is calculated using the following formula:

Where: F being the applied load (measured in kilograms-force) and d^2 the area of the indentation (measured in square millimeters). The applied load is usually specified when HV is cited.

Standards: Vickers test methods are defined in the following standards:   

ASTM E384 - micro force ranges - 10kg to 1kg ASTM E92 - macro force ranges - 1kg to 100kg ISO 6507- 1,2,3 - micro and macro ranges

Principle of Vickers Hardness Test: All Vickers ranges use a 136° pyramidal diamond indenter that forms a square indent. The indenter is pressed into the sample by an accurately controlled test force. The force is maintained for a specific dwell time, normally 10  –  15 seconds. After the dwell time is complete, the indenter is removed leaving an indent in the sample that appears square shaped on the surface. The size of the indent is determined optically by measuring the two diagonals of the square indent. The Vickers hardness number is a function of the test force divided by the surface area of the indent. The average of the two diagonals is used in the following formula to calculate the Vickers hardness. HV = Constant x test force / indent diagonal squared 

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The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 100 kgf. The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated. The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation. Where;

F= Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2in mm HV = Vickers hardness

The constant is a function of the indenter geometry and the units of force and diagonal. The Vickers number, which normally ranges from HV 100 to HV1000 for metals, will increase as the sample gets harder. Tables are available to make the calculation simple, while all digital test instruments do it automatically. A typical Vickers hardness is specified as follows: 356HV0.5: Where 356 is the calculated hardness and 0.5 is the test force in kg.

Applications: Because of the wide test force range, the Vickers test can be used on almost any metallic material. The part size is only limited by the testing instrument's capacity.

Merits: 

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We can get extremely accurate readings One scale covers the entire hardness range A wide range of test forces to suit every application  Nondestructive, sample can normally be reused Very precise for testing the softest and hardest of materials Just one type of indenter is used for all types o f metals and surface treatments

Demerits: 

Under varying loads, the Vickers machine is a floor standing unit that is more expensive than the Brinell or Rockwell machines.

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The main drawback of the Vickers test is the need to optically measure the indent size. This requires that the test point be highly finished to be able to see the indent well enough to make an accurate measurement

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Slow testing can take 30 seconds not counting the sample preparation time

Lab Working: We put the sample under testing in Vickers hardness tester, having the data below

Load

4.903 N

HU

0.5 grams

Time (sec) 10

Lens

40X

d1

60.32

d2

59.6

Mean

Hardness

(d1+ d2)/2

(HV)

59.96

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