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Microhardness testing

By: Mirza Bilal Saleem

Physical Metallurgy Lab. MME, UET. Lahore

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Physical Metallurgy Lab. MME, UET. Lahore

Introduction

• This involves the forcing of a diamond indenter of specific geometry into the surface of the test material with a load ranging from 1 to 1000gf.

• It is usually conducted on metallurgical microscope or special purpose instruments.

• Although the term “microhardness” is well established and is generally interpreted properly by test users, it is best to use the more correct term, microindentation hardness testing.

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Introduction

• The indenters may be the standard Vickers diamond pyramid having a square base and an apical angle of 136o or

• A Knoop indenter which has a rhombic base with diagonals in the ratio of 7:1 and the angle b/w opposite edges of the pyramid of 172o30’ and 130o.

• Individual constituents in the microstructure may tested and the hardness is given by the ratio of the load applied to the area of indentation.

Introduction

• Many metallurgical problems require the determination of hardness over very small areas.

• The measurement of the hardness gradient at a carburized surface, the measurement of hardness of individual constituents of the microstructure or the checking of the hardness of delicate watch gear my be typical problem.

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Types of Microhardness testing

1. Knoop2. Vickers3. Ultrasonic

Vickers Test

• This test uses a square base diamond pyramid as the indenter.

• The included angle b/w the opposite faces of the pyramid is 136o. This angle was chosen because it approximates the most desirable ratio of indentation diameter to ball diameter in Brinellhardness test.

• Due to the shape of the indenter, this test is frequently called diamond pyramid harness test.

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Vickers Test

• The diamond pyramid hardness number (DPH) or Vickers Hardness number (VHN or VPH) is defined as the load divided by the surface area of the indentation.

• In practice the area is calculated by microscopic measurements of the lengths of the impression.

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Vickers Test

• The macro-Vickers test (ASTM E 92) operates over a range of applied forces from 1 to 120 kgf, although many testers cover a range of only 1 to 50 kgf, which is usually adequate.

• The use of forces below 1 kgf with the Vickers test was first evaluated in 1932 at the National Physical Laboratory in the United Kingdom. Four years later, Lips and Sack constructed the first Vickers tester designed for low applied forces.

Vickers Test

• The standard loads are 1, 2.5, 10, 20, 30, 50 and 100Kg.

• The Vickers hardness scale is unique. It extends beyond the microhardness scale to macrohardness scale.

• Vickers hardness test at 120 Kgf is comparable to Rockwell C-scale.

• The microscope is used to determine the size of indentation. Proper magnification is important for accurate measurement.

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Magnification

Indentation length Magnification

Less than 76 µm 400X

76 µm 125 µm 300-800X

More than 125 µm 200-600X

Advantages of the test

1. This test is generally used in research work, because it provides a continuous scale of hardness for a given load from very soft materials with VHN to extremely hard materials with VHN of 1500.

2. The test can be used to measure the hardness of very thin sheets as well as the heavy sections.

3. Vickers hardness test is a linear scale e.g. VPN 800 is twice hard as compared with VPN 400.

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Advantages of the test

4. Because the impression made by the pyramid indenter is geometrically similar, no matter what their size is, VHN should be independent of loads. This is generally the case except at very light loads.

Limitations

1. It is not used for routine testing since it is slow. 2. It gives a noticeable indentation.3. It requires a careful surface preparation of the

specimen. 4. The edges of indentation are not always clearly

defined and may be rather difficult to see on materials of certain colors.

5. The test allows greater chance for the personal error in the determination of diagonal lengths.

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Advantages of Pyramid Diamond indenter

1. The impression irrespective of size is geometrically similar.

2. The deformation of the diamond is plastically nil owing to its enormous hardness. Diamond is the hardest material known to the present time.

3. The hardness no. on homogeneous material is unaffected by variation in load because the angle of indentation is constant. If the load is doubled the area will also be doubled.

Indenter

• The angle 136o was chosen because it approximates the most desirable ratio the indentation to the ball diameter in the BrinellHardness test.

• A perfect indentation made with a perfect diamond pyramid indenter would be square.

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• Pincushion indentation is produced as a result of sinking of the metal around the flat faces of the pyramid.

• This condition is observed in annealed material and result in an over estimate of the diagonal length.

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• Te barreled shape indentation is found in cold worked material.

• It results from ridging or pilling up of metal around the faces of the indenter.

• The measurement in this case results in smaller observed diagonal length resulting in higher observed hardness.

Expression of Test Results

• ASTM E 384 recommends expressing a mean hardness of 425 in the Vickers test using a 100 gf applied force as 425 HV 100.

• While by ISO rules, it would be expressed as 425 HV0.1(because 100 gf would be expressed as 0.1 kgf).

• ASTM Committee E4 is currently recommending adoption of a slightly different approach: 425 HV 100 gf.

• While it has proven difficult to get people to adopt a unified expression style, it is important that the stated results indicate the mean value, the test used, and the test force as a minimum.

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Fixtures for holding test pieces

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Measurement Error

• Experience has shown that a single operator typically exhibits a ±0.5 µm variation when measuring the same indent over a period of time, while multiple operators exhibit approximately a ±1.0 µm variation over time.

• Larger variations have also been observed.• A ±0.5 µm variation in the measured diagonal has a

greater influence on hardness as the test load decreases, that is, as the diagonal size decreases.

ASTM E 384 recommends that the operator should try to keep indents larger than 20 µm in d.

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Hardness versus Applied Test Force• For the Vickers test, especially in the macro applied force

range, it is commonly stated that the hardness is constant as the load is changed.

• For microindentation tests, the Vickers hardness is not constant over the entire range of test forces.

• For Vickers tests with an applied force of 100 to 1000 gf, the measured hardnesses are usually equivalent within statistical precision.

• The Vickers indent produces a geometrically similar indent shape

• at all loads, and a log-log plot of applied force (load) versus diagonal length should exhibit a constant slope, n, of 2 for the full range of applied force (Kick's Law); however, this usually does not occur at forces under 100 gf.

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Hardness versus Applied Test Force

• These results, using the same set of five specimens with a wide range of hardnesses and tests with both micro and macro-Vickers units, revealed basically the same trend.

• At small indent sizes for both testers, measurements yielded lower hardness (indents being oversized) than they should.

• This can only be due to visual perception problems in sizing small indents at the tester magnifications employed (100× for the macro system and 500× for the micro system).

• No material characteristic can possibly explain this problem.

Knoop indentor

By: Mirza Bilal Saleem

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Introduction• In 1939, Frederick Knoop and his associates at the

former National Bureau of Standards developed an alternate indenter based on a rhombohedral-shaped diamond with the long diagonal approximately seven times as long as the short diagonal.

• The Knoop indenter is used in the same machine as the Vickers indenter, and the test is conducted in exactly the same manner.

• Except that the Knoop hardness (HK) is calculated based on the measurement of the long diagonal only.

• Calculation of the projected area of the indent is made rather than the surface area of the indent.

CP = 0.07028

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Introduction

• The Knoop indenter has a polished rhombohedralshape with an included longitudinal angle of 172° 30′ and an included transverse angle of 130° 0′.

• The long and short diagonals in the approximate ratio of 7:1 results in a state of plane strain.

• The narrowness of the indenter makes it ideal for testing specimens with steep hardness gradients.

• In such specimens, it may be impossible to get valid Vickers indents as the change in hardness may produce a substantial difference in length of the two halves of the indent parallel to the hardness gradient.

Introduction• For the same test force, Knoop indents can be more closely

spaced than Vickers indents, making hardness traverses easier to perform.

• The Knoop indenter is a better choice for hard brittle materials where indentation cracking would be more extensive using the Vickers indenter at the same load.

• The Knoop indent is shallower• (depth is approximately 1/30 the long diagonal) than the

Vickers indent (depth is approximately 1/7 the average diagonal).

• Hence, the Knoop test is better suited for testing thin coatings.

• On the negative side, the Knoop hardness varies with test load and results are more difficult to convert to other test scales.

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Thanks