The Vickers hardness test is ideal for testing of all metals and is therefore the method with the widest range of application. The hardness test method according to Vickers is described in standards ISO 6507 (Metallic materials – Vickers hardness test – Part 1: Test method) and ASTM E384 (Standard Test Method for Microindentation Hardness of Materials to Vickers and Knoop).
The indenter used in the Vickers test method is a diamond pyramid with a square base, whose opposite sides meet at the apex at an angle of α = 136°. It is applied to the specimen with a test force (to standard starting at 10 g) and held according to the holding time.
The lengths of both diagonals of the residual test indentation are optically measured. The Vickers hardness is then calculated from the average of the diagonals and the test force applied.
In the Vickers hardness test, an optical method, the size of indentation (the diagonals) left by the indenter is measured. In contrast, the depth of indentation caused by the indenter is measured in the depth measurement methods (only Rockwell is standardized).
The larger the indent left by the indenter at a defined test force in the surface of a workpiece (specimen), the softer the tested material.
In order to determine the Vickers hardness (HV) according to ISO 6507, the pyramid-shaped indenter (with interfacial angle of 136°) is pressed into a specimen (workpiece) with a defined test load from 1 gf.
- The Vickers hardness (HV) results from the quotient of the applied test force (F in newtons N) and the surface area of the residual indent on the specimen (see formula below). To calculate the surface area of the residual pyramidal indentation, the average of the two diagonals d1 and d2 (in mm) is used, because the base area of Vickers indents is frequently not exactly square.
- The recommended Vickers hardness range can be found in the standard (ISO 6507). Depending on the test force and specimen material used, the Vickers hardness value lies between 1 and 3,000 HV.
- Ideally, the test force is increased from 0 to its final value within 7 seconds (minimization of measurement uncertainty). The maximum permissible interval for application time from the standard is 2 to 8 seconds (nominal time duration 7 s).
- Generally, the dwell time for the test force is 10 to 15 seconds (nominal time duration 14 s). If the dwell time is any longer, the duration in seconds must also be specified in the hardness value, e.g.: 610 HV 10/30 (dwell time of 30 s).
- The test forces used in the macro range in the Vickers method are mostly substantially smaller than those used in the Brinell method. The preferred choice for the macro range is 49, 98, 196, 294, 490 or 980 N, with 294 N being used most frequently for testing in practice.
The minimum distances between test points (indentations) and to the specimen edge, which are used for the Vickers test method are defined in standard ISO 6507. The reason for these minimum distances is to avoid distortion of the hardness testing results, which could result from the deformation of the material structure.
The minimum values that must be adhered to according to the standards can be found in the diagram to the left.
The Vickers method offers the following advantages:
- The Vickers method can be used with any and all materials and test specimens, from soft to hard, as the procedure covers the entire hardness range.
- There is only one type of indenter, which can be used for all Vickers methods.
- Non-destructive testing is possible, so the test specimen can be used for other purposes.
The Vickers method has the following disadvantages:
- The surface quality of the specimen must be good, because the indent is measured optically. This means that the test location must be prepared (ground and polished), otherwise precise evaluation is difficult.
- The process is rather slow (compared with the Rockwell method). The test cycle takes somewhere between 30 and 60 seconds, not including the time taken to prepare the specimen.
- Due to the need to conduct optical indent evaluation, Vickers hardness testers must be equipped with an optical system, which makes them more expensive to purchase than Rockwell testers.
- In general, it can be said that the Vickers method is becoming the most common testing method in practice due to its diverse applications, even though specimens need to be prepared and evaluation of the hardness value is laborious.