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Monday, 21 September 2015

VICKER'S HARDNESS TEST

This is similar to Brinell Hardness test, but in this method the drawback of the flattening of the steel ball in testing harder materials is eliminated. It uses a similar relationship and most of the errors and limitations of Brinell Hardness test are eliminated. In this method of hardness testing a diamond square based pyramid indentor with 136° angle between opposite faces is used. The load varies from 5 kg to 120 kg in increments of 5 kg. Similar to Brinell and Rockwell hardness measuring methods, this method also uses the indentation produced by the indentor (Diamond pyramid). The indentor gives geometrically similar impressions under different loads.
A piston and a dash pot of oil is used for controlling the rate and duration of loading. The specimen is placed on the anvil, which is then raised to indentor. Load is applied and then removed. The value of Vicker's Hardness can be obtained by the following relation:

VPN=DPN=
             
                

Where VPN = Vicker's Pyramid Number and
             DPN = Diamond Pyramid Number

Let P is the load applied,
       d is the average length of two diagonals, in mm, and 
       q is the angle between opposite faces of diamond pyramid (136°). Then


              This method is used for the determination of hardness of very thin and very hard materials. This method also facilitates the ease of measurement of a diagonal of the indentation area ( Fig ), as compared to circular dimensions, which are difficult to measure. This method is rapid, accurate and suitable for metals as thin as 0.15 mm. The indentor is capable of giving geometrically similar impression with different loads. Obviously, the hardness number is independent of the load applied. Some typical values of VPN are:

Material
VPN Value (Kg/mm2)


Diamond
8400
Steel
210
Aluminium
22
Lead
12
Tungsten carbide
2100


             The values of Brinell and Vickers hardness are practically the same up to 300. We must note that Vicker's test can be carried out accurately on polished surfaces but does not give accurate results when used for rough surfaces.


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