Study on the local damage mechanisms in WC-Co hard metals during scratch testing
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Study on the local damage mechanisms in WC-Co hard metals during scratch testing Siphilisiwe Ndlovu, Karsten Durst, Heinz Werner Hoeppel, and Mathias Goeken Department of Materials Science and Engineering, University of Erlangen-Nuernberg, Erlangen, 91058, Germany ABSTRACT The effect of the cobalt content and WC grain size on the deformation behaviour of WC-Co hard metals was investigated by studying materials with a varying WC grain size and cobalt content. The WC grain size ranged from 0.25 to 2.65 µm and the binder content ranged from 6 to 15 wt%. Single and multiple scratch tests were conducted using a Nanoindenter with a Berkovich diamond tip and the load ranged from 5 to 500 mN with a tip sliding velocity of 10 µm/s. Several damage mechanisms were observed and these show a combination of ductile and brittle wear. The bulk properties i.e. composite properties of the hard metal determine the wear in the 6 wt% Co samples. On the other hand the 15 wt% Co samples exhibited a localised response to the wear i.e. the wear is determined by the individual phases in the hard metal. INTRODUCTION WC-Co hard metals are well established powder metallurgy products. The unique composite structure of hard WC grains in a tough cobalt matrix results in excellent wear resistance. Recent developments have led to the production of nanostructured WC-Co hard metals which consist of nanoscale tungsten carbide grains in a cobalt matrix. These nanostructured hard metals are reported to have enhanced wear properties as a result of their increased hardness and the increased constraint of the WC grains in the binder phase which results from the reduced binder mean free path [1]. Nanoindentation is a well established experimental technique to investigate the localised mechanical properties of a system and a Nanoindenter can also be used for scratch testing of materials [2]. The local wear behaviour of WC-Co hard metals will thus be investigated using scratch testing. The mechanical and wear properties of WC-Co hard metals on the local scale will be investigated with emphasis on the WC grain size and the cobalt content. EXPERIMENT Materials Seven commercial hard metal grades with varying cobalt content ranging from 6 to 15 % were tested in this study. The grades are classified as ultra fine (UFG), medium (MG) and coarse grained (CG) depending on the size of the WC grains. The characterisation data is given in Table 1.
Table I: Composition and properties of the investigated WC-Co cemented carbides Sample
Average WC grain size (µm) 2.65 0.25 1.21 0.60 0.48 0.66 0.25
J15 NY15 T06M T06MF T06MG T06F T06SMG
Description
Binder wt%
Vickers hardness Hv
Indentation hardness (GPa)
Young’s Modulus (GPa)
CG UFG CG MG UFG MG UFG
15 15 6 6.5 6 6 6
1017 1486 1380 1575 1760 1710 1940
12.7 19.7 18.4 22.1 21.8 22.0 25.5
555 561 530 638 476 615 623
Instrumented scratch testing Nanoscratch tests on the hard metals were performed using a Nano Indenter XP with a load controlled head. The maximum load capacity for the standard system is
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