The standard “EN ISO 6507-1 Metallic materials - Vickers hardness test - Part 1: Test method”, which has its basis in a European standard and was later published by the International Organization for Standardization (ISO), describes the Vickers hardness test method for three different test force ranges for metallic materials, including hard metals and other cemented carbides.
In the table titled “Test force ranges” given in the standard, the test force ranges are defined as follows:
The Vickers hardness test is specified in this standard for indentation diagonal lengths between 0,020 mm and 1,400 mm. Using this method to determine Vickers hardness from smaller indentations is beyond the scope of this standard because the results are subject to large uncertainties due to the limitations of optical measurement and imperfections in the tip geometry. A periodic verification method is specified for routine checking of the testing machine in service by the user. Specific international standards exist for specific materials or products.
A diamond indenter, in the form of a right pyramid with a square base and a specified angle between opposite faces at the apex, is forced into the surface of a test piece and then the diagonal length of the indentation remaining on the surface after the test force is removed is measured.
The Vickers hardness is proportional to the test force divided by the area of the inclined surface of the indenter, assuming that this surface is a right pyramid with a square base and the same angle at its apex as the indenter. The apex of a right pyramid is aligned with the centre of the base. As applicable, this standard adopts the hardness test parameters defined by the International Committee for Weights and Measures and related organisations.
The testing machine used in these tests shall be capable of applying a predetermined force or forces within the required range of test forces in accordance with ISO 6507-2. The indenter shall be a square-based, steeply pyramidal diamond as specified in ISO 6507-2.
The diagonal measuring system shall meet the requirements of ISO 6507-2. Magnifications shall be provided so that the diagonal can be magnified by more than 25 percent but not less than 75 percent of the maximum possible optical field of view. Many objective lenses are non-linear towards the edge of the field of view. A diagonal measuring system using a camera for measurement can use the entire field of view of the camera, provided that the optical system is designed to take into account the field of view limitations. The resolution required by the diagonal measuring system depends on the size of the smallest indentation to be measured and shall be in accordance with the table entitled “Resolution of the measuring system”. In determining the resolution of the measuring system, the resolution of the microscope optics, the digital resolution of the measuring scale and the step size of any stage movement, if any, shall be taken into account.
Unless otherwise specified in the product standards, the test shall be carried out on a smooth and even surface, completely free from oxide scale, foreign matter and especially lubricants. The surface finish shall permit the diagonal length of the indentation to be determined accurately. For hard metal samples, the thickness of the layer removed from the surface shall be at least 0,2 mm.
Surface preparation must be carried out in a manner that will prevent surface damage or changes in surface hardness due to excessive heating or cold working. Since the depth of the Vickers microhardness indentations is small, it is important to take special precautions during preparation. It is recommended to use a polishing or electropolishing process appropriate to the material to be measured.
The thickness of the test piece or the layer under test shall be at least 1,5 times the diagonal length of the indentation as defined in the standard annex (Annex A). No deformation shall be visible on the back of the test piece after the test. The thickness of the hard metal test piece shall be at least 1 mm. The depth of the indentation is approximately 1/7 of the diagonal length.
For tests on curved surfaces the corrections given in the tables should be applied.
For a test piece with a small cross-section or irregular shape, either a special support must be used or it must be mounted in a manner similar to a metallographic micro-section made of suitable material, so that it is adequately supported and does not move during force application.
The test is normally carried out at an ambient temperature of 10 degrees to 35 degrees. If the test is carried out at a temperature outside this range, this will be stated in the test report. Tests carried out under controlled conditions are carried out at a temperature of 23 degrees plus/minus 5 degrees.
The test forces shown in the table given in the standard are typical. Other test forces greater than 980,7 N, but not less than 0,009 807 N, may be used. Test forces should be selected to result in indentations with diagonals greater than 0,020 mm. The preferred test force for hard metals is 294,2 N.
Periodic verification as defined in the annex to the standard (Annex C) should be performed one week before use for each test force used, but is recommended on the day of use. Periodic verification is recommended each time the test force is changed. Periodic verification should be performed each time the indenter is changed.
The test piece should be placed on a rigid support. The support surfaces should be clean and free from scale, oil, dirt and similar foreign matter. It is important that the test piece lies firmly on the support so that no displacement can occur during the test which could affect the test result. For example, for extremely cold worked anisotropic materials, there may be a difference between the lengths of the two diagonals of the indenter. Therefore, whenever possible, the indentation should be made so that the diagonals are oriented in the plane at an angle of approximately 45 degrees to the cold working direction. The product specification may specify limits for differences between the lengths of the two diagonals.
The diagonal measuring system microscope should be focused so that the sample surface and the desired test location can be observed. Some testing machines do not require the microscope to be focused on the sample surface.
The indenter shall be brought into contact with the test surface and the test force shall be applied in a direction perpendicular to the surface without shock, vibration or overload until the applied force reaches the specified value.
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