HOME

TheInfoList



OR:

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at Vickers Ltd as an alternative to the Brinell method to measure the
hardness In materials science, hardness (antonym: softness) is a measure of the resistance to plastic deformation, such as an indentation (over an area) or a scratch (linear), induced mechanically either by Pressing (metalworking), pressing or abrasion ...
of materials. The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe a material's ability to resist plastic deformation from a standard source. The Vickers test can be used for all
metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
, which uses the same units. The hardness number is determined by the load over the surface area of the indentation and not the area normal to the force, and is therefore not pressure.


Implementation

It was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size; the impression should have well-defined points of measurement; and the indenter should have high resistance to self-deformation. A
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...
in the form of a square-based pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression was of the ball diameter. As two tangents to the circle at the ends of a chord 3''d''/8 long intersect at 136°, it was decided to use this as the included angle between plane faces of the indenter tip. This gives an angle from each face normal to the horizontal plane normal of 22° on each side. The angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load. Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured. The HV number is then determined by the ratio ''F''/''A'', where ''F'' is the force applied to the diamond in kilograms-force and ''A'' is the surface area of the resulting indentation in square millimeters. : A = \frac, which can be approximated by evaluating the sine term to give, : A \approx \frac, where ''d'' is the average length of the diagonal left by the indenter in millimeters. Hence, : \mathrm = \frac \approx \frac \quad textrm^2/math>, where ''F'' is in kgf and ''d'' is in millimeters. The corresponding unit of HV is then the kilogram-force per square millimeter (kgf/mm2) or HV number. In the above equation, ''F'' could be in N and ''d'' in mm, giving HV in the SI unit of MPa. To calculate Vickers hardness number (VHN) using SI units one needs to convert the force applied from
kilogram-force The kilogram-force (kgf or kgF), or kilopond (kp, from ), is a non-standard Gravitational metric system, gravitational metric unit of force. It is not accepted for use with the International System of Units (SI) and is deprecated for most uses. T ...
to N then
kilogram-force The kilogram-force (kgf or kgF), or kilopond (kp, from ), is a non-standard Gravitational metric system, gravitational metric unit of force. It is not accepted for use with the International System of Units (SI) and is deprecated for most uses. T ...
is divided by 9.806 65 (
standard gravity The standard acceleration of gravity or standard acceleration of free fall, often called simply standard gravity and denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is a constant ...
). This leads to the following equation: : \mathrm \approx \frac \quad textrm^2 where ''F'' is in N and ''d'' is in millimeters. A common error is that the above formula to calculate the HV number does not result in a number with the unit newton per square millimeter (N/mm2), but results directly in the Vickers hardness number (usually given without units), which is in fact one
kilogram-force The kilogram-force (kgf or kgF), or kilopond (kp, from ), is a non-standard Gravitational metric system, gravitational metric unit of force. It is not accepted for use with the International System of Units (SI) and is deprecated for most uses. T ...
per square millimeter (1 kgf/mm2). Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or if duration of force differs from 10 s to 15 s, e.g. 440HV30/20, where: * 440 is the hardness number, * HV names the hardness scale (Vickers), * 30 indicates the load used in kgf. * 20 indicates the loading time if it differs from 10 s to 15 s


Precautions

When doing the hardness tests, the minimum distance between indentations and the distance from the indentation to the edge of the specimen must be taken into account to avoid interaction between the work-hardened regions and effects of the edge. These minimum distances are different for ISO 6507-1 and ASTM E384 standards. Vickers values are generally independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf. However, lower load indents often display a dependence of hardness on indent depth known as the indentation size effect (ISE). Small indent sizes will also have microstructure-dependent hardness values. For thin samples indentation depth can be an issue due to substrate effects. As a rule of thumb the sample thickness should be kept greater than 2.5 times the indent diameter. Alternatively indent depth, t, can be calculated according to: :t = \frac \approx \frac,


Conversion to SI units

To convert the Vickers hardness number to SI units the hardness number in kilograms-force per square millimeter (kgf/mm2) has to be multiplied with the
standard gravity The standard acceleration of gravity or standard acceleration of free fall, often called simply standard gravity and denoted by or , is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is a constant ...
, g_0, to get the hardness in MPa (N/mm2) and furthermore divided by 1000 to get the hardness in GPa. \text = \fracHV = \fracHV Vickers hardness can also be converted to an SI hardness based on the projected area of the indent rather than the surface area. The projected area, A_, is defined as the following for a Vickers indenter geometry: A_ = \frac = \frac This hardness is sometimes referred to as the mean contact area or Meyer hardness, and ideally can be directly compared with other hardness tests also defined using projected area. Care must be used when comparing other hardness tests due to various size scale factors which can impact the measured hardness. \text = \frac\fracHV \approx \frac


Estimating tensile strength

If HV is first expressed in N/mm2 (MPa), or otherwise by converting from kgf/mm2, then the
tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
(in MPa) of the material can be approximated as ≈ HV/ , where is a constant determined by yield strength, Poisson's ratio, work-hardening exponent and geometrical factors usually ranging between 2 and 4. In other words, if HV is expressed in N/mm2 (i.e. in MPa) then the
tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
(in MPa) ≈ HV/3. This empirical law depends variably on the work-hardening behavior of the material.


See also

* Indentation hardness * Leeb Rebound Hardness Test *
Hardness comparison A variety of hardness-testing methods are available, including the Vickers, Brinell, Rockwell, Meyer and Leeb tests. Although it is impossible in many cases to give an exact conversion, it is possible to give an approximate material-specific co ...
* Knoop hardness test * Meyer hardness test *
Mohs scale The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Ger ...
* Rockwell hardness test * Vickers toughness test of ceramics *
Superhard material A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high Covalent bond, bond covalency. As a res ...


References


Further reading

* {{cite book , author = Meyers and Chawla , year = 1999 , chapter = Section 3.8 , title = Mechanical Behavior of Materials , publisher = Prentice Hall, Inc *
ASTM ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...
E92: Standard Test Methods for Vickers Hardness and Knoop Hardness of Metallic Materials * ASTM E384: Standard Test Method for Microindentation Hardness of Materials *
ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...
6507-1: Metallic materials – Vickers hardness test – Part 1: Test method * ISO 6507-2: Metallic materials – Vickers hardness test – Part 2: Verification and calibration of testing machines * ISO 6507-3: Metallic materials – Vickers hardness test – Part 3: Calibration of reference blocks * ISO 6507-4: Metallic materials – Vickers hardness test – Part 4: Tables of hardness values * ISO 18265: Metallic materials – Conversion of hardness Values


External links


Video on the Vickers hardness test



Conversion table
– Vickers, Brinell, and Rockwell scales Hardness tests de:Härte#Härteprüfung nach Vickers (HV)