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Vickers Hardness Test Method

The Vickers hardness test was developed by George E. Sandland and Robert L. Smith at Vickers Ltd. in the year 1921, as an alternative to the Brinell method to determine the hardness of materials. This Vickers hardness test is usually easier to use compared to other hardness tests. The reason is the required calculations are not dependent on the size of the indenter. Similarly, the indenter can be used for all the materials irrespective of the hardness. The basic principle of the Vickers hardness test is the same as that of all other hardness tests. With the help of this hardness test, it is possible to observe the material’s ability to resist plastic deformation from a standard source. The Vickers hardness test can be used for all the available metals and it has got one of the widest scales among all hardness tests. The unit of hardness can be measured as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). This hardness number can be converted into the units of pascals. However, it should not be confused with pressure, which is expressed in the same units. The hardness number is calculated by the load over the indentation surface area and not the area normal to the force.

About Vickers Hardness Testers. What You Should Know.

What is Vickers Hardness Testing?

The Vickers hardness testing method is also known as a microhardness test, commonly employed for examining thin sections, small components, or work involving case depth.

The Vickers hardness testing method operates using an optical measurement system, adhering to ASTM E-384 standards. This method involves applying a range of light loads through a diamond indenter to create an indentation. The indentation is accurately measured and converted into a hardness value. While applicable to a broad spectrum of materials, specific conditions must be met when preparing test samples. These samples need to be meticulously polished to facilitate easy measurement of the impression size. A square base pyramid-shaped diamond is consistently used for testing on the Vickers scale. Typically, the loads applied are very light, ranging from 10g to 1kgf, although 'Macro' Vickers loads can exceed 30 kg.

Microhardness methods are employed to inspect ceramics, metals, composites, and virtually any material type. Due to the small size of the Vickers test indentation, it finds utility in diverse applications, including measuring individual microstructures, testing very thin materials like foils, assessing the surface of a single part or multiple small parts or areas, and determining the depth of case hardening by creating a series of indentations to define a hardness alteration profile.

For microhardness testing, sectioning is often necessary to provide a small enough specimen for the tester. Additionally, sample preparation is crucial to achieving a smooth test sample surface, ensuring a consistent indentation shape, proper measurement, and confirming that the sample is positioned perpendicular to the indenter. Prepared samples are frequently mounted on a plastic medium for easy preparation and testing. Larger indentations are preferred to enhance measurement resolution, as decreasing the indentation size magnifies errors. However, it's important to note that this testing procedure is susceptible to operator influence on the results.

The Vickers hardness testing method entails indenting the test material with a diamond indenter in the form of a right pyramid with a square base at an angle of 136 degrees between the opposite faces relative to a load of 1 to 100 kgf. The complete load is typically applied for approximately 10 to 15 seconds. The two diagonals of the indentation left on the material surface after load removal are precisely measured using a microscope, and their average is calculated. Additionally, the area of the sloping surface of the indentation is determined. The Vickers hardness is then obtained by dividing the load in kgf by the indentation area in square mm.

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F= Load in kgf

d = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

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By determining the mean diagonal of the indentation, the Vickers hardness can be calculated using the aforementioned formula; however, it is more convenient to utilize conversion tables. The Vickers hardness should be reported as 800 HV/10, signifying a Vickers hardness of 800 obtained using a 10 kgf force. Different load settings yield similar hardness numbers on homogeneous materials, which is typically more consistent than the arbitrary scale changes seen in other available hardness testing methods.

The advantages of the Vickers hardness test include exceptionally accurate readings, as a single type of indenter can be applied to all metals and surface treatments. Despite being systematically adaptable and highly accurate for testing materials across a wide hardness spectrum under varying loads, the Vickers machine is a floor-standing unit and is considered more expensive compared to Brinell or Rockwell machines.

Classification of Vickers Hardness Testers:

Vickers Hardness testers are categorized into eight types:

  • Microhardness Tester
  • Vickers hardness tester with a small load
  • Digital microhardness Tester
  • Digital Vickers hardness tester
  • Video Screen microhardness Tester
  • Digital touch screen microhardness Tester
  • Automatic rotating Tavis Hardness Tester
  • Automatic Vickers Hardness Tester

Working Procedure of Vickers Hardness Testing:

The Vickers hardness test is an optical method, where the size of the indentation (i.e., the diagonals) left by the indenter is measured. In contrast, the standardized Rockwell testing method measures the depth of indentation caused by the indenter. The larger the indent left at a well-defined test force on the specimen's surface, the softer the test material.

To determine Vickers hardness (HV) according to ISO 6507 standards, a pyramid-shaped indenter with an interfacial angle of 136 degrees is pressed into the test specimen with a well-defined test load, typically starting from 1 gf.

Calculation of Vickers Hardness (HV):

The Vickers hardness (HV) is calculated by dividing the applied test force (F in newtons N) by the surface area of the residual indent on the specimen. The formula is as follows:

HV= F / 1/2 * (d​1 + d2)

To determine the surface area of the residual pyramidal indentation, the mean of the two diagonals (d₁ and d₂ in mm) is calculated. This is because the base area of Vickers indents is often not precisely squared.

The recommended range of Vickers hardness values can be found in ISO 6507 standards. Depending on the test force and the specimen material used, Vickers hardness values typically range between 1 and 3,000 HV.

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Application of Test Force:

Ideally, the test force should be rapidly increased from 0 to its final value within 7 seconds, a practice known as minimizing measurement uncertainty. The permissible interval for the application time, deviating from the standard, ranges between 2 to 8 seconds, with a nominal duration of 7 seconds. The dwell time for the test force is typically 10 to 15 seconds, with the recommended nominal duration being around 14 seconds. If the dwell time extends beyond this, the additional duration in seconds must be specified in the hardness value, such as 610 HV 10/30 (indicating a dwell time of 30 seconds). Test forces in the macro range of the Vickers method are notably lower than those used in the Brinell method, with common choices being 49, 98, 196, 294, 490, or 980 N. However, 294 N is the most frequently used force for testing at the macro level.

Minimum Distance of Test Points:

In the Vickers hardness test method, the indents should be strategically positioned to ensure sufficient clearance from the specimen's edge and between individual indents. The minimum observed values, as per standards, are detailed below.

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Specimen Requirements for Vickers Hardness Testing:

When utilizing the Vickers hardness testing method, careful preparation of the specimen surface is mandatory. The surface quality requirements are more stringent compared to the Rockwell method, where a clean and impurity-free surface is typically sufficient.

The specimen to be tested should meet the following criteria:

  1. The specimen should be well-polished for microhardness testing or precision-ground for macro hardness testing.
  2. It is advisable to securely clamp the specimen to prevent any movement during the testing process. A practical tip is to fix the embedded specimen in a specimen holder after measuring with a suitable test anvil.
  3. Special attention should be given to avoiding any vibration or shock during the test. Using an anti-vibration table is recommended to ensure that external factors do not affect the test results.

Available Methods and Applications for Vickers Hardness Testing:

The Vickers process can be subdivided into various methods based on the magnitude of the test force, as per ISO standards. These include HV 0.01, HV 1, HV 10, etc. Additionally, the Vickers hardness testing method encompasses three hardness testing load ranges:

  1. Micro Hardness Range: This is suitable for testing small and thin specimens, often used for microstructures, coatings, and thin layers.

  2. Low-Load Range: This range is commonly employed for softer materials or thin sections where the application of a high load may damage the specimen.

  3. Conventional (Macro) Hardness Range: Used for standard hardness testing of larger and more robust materials. This range involves higher test forces suitable for a broader range of materials.

The selection of the appropriate method and load range depends on the specific characteristics of the material being tested and the testing requirements.

The applications of Vickers methods depending on the respective load range are listed below:

Load range

Method

Indenter

Test force F

Applications

 

Micro hardness range

HV 0.01

Diamond pyramid 136°

0.098 N

Structural constituents, very thin layers

Range 1 HV – 3000 HV

HV 0.025

0.245 N

HV 0.05

0.490 N

HV 0.1

0.980 N

Low force hardness range

HV 0.2

Diamond pyramid 136°

1.961 N

Thin layers CHD/SHD/NHD series, thin sheet metals, small specimens

HV 0.3

2.942 N

HV 0.5

4.903 N

HV 1

9.807 N

HV 2

19.61 N

HV 3

29.42 N

Macro range (Conventional hardness range)

HV 5

Diamond pyramid 136°

49.03 N

Normal specimens

HV 10

98.07 N

HV 20

196.1 N

HV 30

294.2 N

HV 50

490.3 N

HV 100

980.7 N

*recommended hardness range according to ISO 6507-4

 

Factors in the Selection of Test Force and Test Method:

  1. Specimen Size:

    • Consider the lowest clearances between test points (indent spacing) and specimen edge (edge clearance) as defined in standards for the Vickers method.
  2. Specimen Thickness:

    • Ensure the specimen thickness is at least 1.5 times the indentation diagonals. The minimum thickness should be between 0.085 and 6.5 mm.
  3. Surface Quality:

    • The specimen surface should be polished, especially for very low-test loads. A good surface is crucial for accurate measurements, as even small irregularities can impact results.
  4. Test Specimen Preparation:

    • Measure the test specimen with the highest possible test force, minimizing potential influencing factors that could alter the final result.

Reading and Representing Vickers Hardness Value:

A Vickers hardness value includes:

  • Numeric hardness value (1 to 3000)
  • Letters 'HV' (Hardness according to Vickers)
  • Applied test load in kgf
  • Dwell time of the test load, considered only if not between 10 to 15 seconds (rare in practice, according to ISO 6507 standards).

Advantages of the Vickers Method:

  1. Applicability to All Materials:

    • Suitable for materials across the complete hardness range, from soft to hard.
  2. Single Type of Indenter:

    • Utilizes a single type of indenter for all Vickers methods.
  3. Non-Destructive Testing:

    • Test specimens can be used for other purposes as it is a non-destructive testing method.

Disadvantages of the Vickers Method:

  1. Surface Quality Requirement:

    • Precise specimen preparation is essential, as the method relies on optical indent measurement.
  2. Speed of Testing:

    • Slower compared to the Rockwell method, with a test cycle taking between 30 to 60 seconds (excluding specimen preparation time).
  3. Cost and Equipment:

    • Requires an optical system for indent evaluation, making Vickers hardness testers more expensive compared to Rockwell testers.

Despite its time-consuming nature and specimen preparation challenges, the Vickers method is widely used due to its versatility and varied applications.

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