ENGINEERING MATERIALS IENGINEERING MATERIALS ICH-5 HARDNESS & HARDNESS TESTINGCH-5 HARDNESS & HARDNESS TESTINGVickers hardness testThe Vickers hardness test was developed in the early 1920s as an alternative method to measure the hardness of materials. Vickers tests has two versions: macro Vickers (load over 1kg and micro Vickers (load less than 1kgThe Vickers macrotest is often easier to use than other hardness tests since the re!uired calculations are independent of the si"e of the indenter# and the indenter can $e used for all materials irrespective of hardness. %owever# Vickers macrohardness test is not as widely used as the &rinell or 'ockwell test.( diamond# in the form of a right pyram)d w)th a s!uare $ase and an angle of 1*+$etweenoppos)te faces )s forced )nto the mater)al under a load ,. The two diagonals# dand d # of the indentat)onleft)n the surface of the mater)al after remov)ng the load are measured# and the)r ar)thhmet)cmean d calculated.VHN V%-.1./011,2d3 ,.load #kg d.mean of diagonal impression#mm 4.face angle of diamond.1*+

d.(d 5d22 6iagonals of the indentation produced are appro7imately seven times larger than the depth of indentation# especially with high hardness metals it provides $etter accuracy than that could $e o$tained with the &rinell or 'ockwell. This is $ecause of the fact that# $y virtue of its indentation# higher measurementaccuracy can $e o$tained even if the indentation depth is small. 8o this makes the Vickers test especially suita$le for hardnessmeasurements in thin layers and very hard alloys.Vickers hardness num$er is nearly independent of the test loads for homogeneous materials due to the fact that the ratio $etween the diagonals of the impression remains constant under different test loads a$ove 0kg. (t lower loads it may $e load dependent.8tandart designation of Vickers hardness is usually given with load and duration values: eg 100V%-2*0220means a 100Vickers hardness value measured under a load of *0kg for a duration of 20secs (similar to the &rinellVickers hardness tests and &rinell test are similar to each other $oth in principle and also the hardness values.&oth calculate the hardness as the load2area of impression9hile &rinell use a $all as indenter Vickers use a diamond with angle of 1*+degrees resem$ling a $all as seen in fig.0.9Therefore numerical values of %& and V%- of the same test piece are so close to each other up to%&100(dvantages of Vickers %ardness Test :ethod1.8oft# as well as hard metals may $e tested.2.Tests may $e conducted in low;load and micro range as well*.The vickers hardness ndentation hardeness was closely related to the tensile strength of most metalsCikewise# the resistance of a material to wear is closely related to its peformance in a scratch test as a$ove.6uo to this relation the scratch test is of the technological interest. This property of resistance to wear is measured $y means of various scratch hardness tests.Types of 5cratch TestThe :D%?sscale catogori$es the materials on the basis of relative hardness.&ach ne material is used to scratch the old materials and placed in a proper placeon the scale.')())* *scratch(3)+ +, *-).. , + etc.'/)( ne material N is used as the diamond to scratch materals starting ith the hardest material in the scale, . in this case-ote : #his test is not ob0ective test since the hardness of the to material can be so close that 0ust by scratching them and observing the surface they can not be classified or scaled.&ier$aum 8cratch %ardness is defined as the ratio of load to the s!uare of measured width of the scratch.6#er,Sc,H4- 7 89 # (kg7""9) :arten?s 8cratch %ardness is the value of the load (in kg re!uired to produce a scratch of w.0.01mm.'elative 8cratch %ardness -um$er of materials can $e determined $y comparing the scratches produced on the specimen materials against a reference scratch. (nother non;o$Eective testCHOICE O6 TESTIN= METHODThe points or the considerations that play a role in the choice of the suita$le hardness testing method are:;The scope of the test (whether it is done to do a direct hardness measurement or to do an indirect strength or wear resistance measurement of the material;The degree of accuracy re!uired (Vickers# =noop or 'ockwell;The rapidity with which the tests must $e conducted (time.;The specimen or part si"e (macro test or micro test or 8chleroscope etc.8hore 8cleroscope is the fastest $y all standartd means# to $e followed $y the 'ockwell tester.The &rinell or Vickers method of testing are comperatively slower even if these tester are e!uipped with micro;proEectors.The structure of the material effect the selection of the si"e of the indenter(hence type of the test,or %eteregeneous structures # with large inclusions of individual structural components (e.g .gray iron or nonferrous hardness tests should $e done with a large si"e indenterF $ecause the large si"e of indentation averages out local heterogeneties.9hether it $e a com$ination of the large indenter low load or small indenter large load.%omogeneous structure :re!uires to $e done at smallerareas with smaller indenters since test done at this point will $e sufficiently representative of the $ulk of the metal.,errous :aterials :Gractically all ferrous meterials of suita$le thickness can $e tested $y the standart &rinell# 'ockwell# Vickers and 8hore schleroscope method. %owever# the &rinell test gives $etter results with coarse grained materials $ecause of the averaging provided $y large10 mm $all.,or hardened and tempered stell:'ockwell and Vickers testing produce more accurate results.9hen materialis thin 'ockwell ( scale or superification testing can $e used.Base %ardened 8teels: the penetrator should not produce an impression which distru$s this layer.The 'ockwell tester is more advantageous in this case.The scale to $e used depends upon the thickness of the case.Cikewise# Vickers tester can also $e used effectively.Variation of hardness across the depth can only $e achieved $y microhardness test.8teel ,orging:&rinell test with 10mm $all and *000kg load gives $etter results.( good relationship e7ists $etween &%- tensile streght of the forgings. Bast >ron: Bast iron is generaly tested $y the standard &rinell test $ecause of the nonun)formsty of the stracture of B>.Glastics: There are three important (8T: standarded procedures for testing plastic.16H'D:ITI' 2&('BDC>:G'I88D'*'DB=9ICC TI8T('#C#:#I#= The relationship $etween their hardness and strength is not so determinate as with metals.Ilastomers: 'u$$erand ru$$er like materials are tested on durometer. Very %ard :aterials are tested with diamond indenter or penetrators and very high contact pressure are re!uired to produce on impression of measura$le si"e.The hardness of ceramics is is usually determined according to =noop scale.'ockwell test is relativelyinstensive to surface finish.&rinell test can also $e said not to $e highly influenced$y the surface scratches and roughness as long as the diameter of the impression could $e defined accurately.The Vickers and microhardness tester re!uire reasona$ly $etter surface finish.( Jeneral 'uleis that ligher the test load the higherthe degree of surface finish re!uired.