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The Vickers (HV) test was developed in England is 1925 and was formally
known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has
two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg),
to cover all testing requirements. The indenter is the same for both
ranges therefore Vickers hardness values are 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 results.
Standards
Vickers test methods are defined in the following standards:
- ASTM E384 – micro force ranges – 10g to 1kg
- ASTM E92 – macro force ranges - 1kg to 100kg
- ISO 6507-1,2,3 – micro and macro ranges
Vickers Test Method
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
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.
Strengths
- One scale covers the entire hardness range.
- A wide range of test forces to suit every application.
- Nondestructive, sample can normally be used.
Weaknesses
- 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.
- Slow. Testing can take 30 seconds not counting the sample preparation time.
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