Chem. Listy 105, s532s534 (2011) Materiál v inžinierskej praxi 2011 s532 HARDNESS AND FRACTURE TOUGHNESS OF CEMENTED CARBIDES ANNAMÁRIA DUSZOVÁ , PETER HORĕÁK, PAVOL HVIZDOŠ, FRANTIŠEK LOFAJ, JÁN DUSZA Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 043 53 Košice, Slovak Republic Keywords: WC-Co, microstructure, indentation hardness Introduction Cemented carbides are widely used as cutting, forming and machining tools in different areas of industry because of their high hardness and strength, good fracture tougness and excellent wear resistance. Their mechanical properties are determined by their microstructure parameters but are also dependent on the testing methods 1,2 . During the last decade, the instrumental indentation at very low loads (nanoindentation) has shown an important development to measure the mechanical properties of thin films and multiphase materials as cemented carbides by pre- cise control and measuring of very small displacements and loads applied by the indenter. The main mechanical character- istics that can be obtained from a nanoindentation technique are hardness, Young’s modulus, adhesion, friction coefficient and crack growth 3 . The aim of the present contribution is to study the influ- ence of the applied load, testing method and microstructure parameters on the hardness and fracture toughness of WC-Co cemented carbides. Experimental procedure The experimental materials, supplied by Pramet Sumperk, have been prepared using standard processing routes. The microstructure parameters were obtained using point counting on a planar test section (volume fraction of Co  f Co ) and boundary intercepts method and test lines on planar sections, measuring the average number of intercepts per unit length of test line with traces of the carbide/cobalt interface, (N L ) WC/Co , and of carbide/carbide grain boundaries, (N L ) WC/WC . From these quantities the average carbide grain size 4 : D WC =2f WC /(2(N L ) WC/WC +(N L ) WC/Co ), (1) the contiguity of the WC phase: C WC = 2(N L ) WC/WC / (2(N L ) WC/WC +(N L ) WC/Co ), (2) and the mean free path in the binder (cobalt) phase L Co = 2f Co /(N L ) WC/Co (3) have been calculated. The CMC TM (Continuous Multi Cycle) method was ap- plied using instrumental Nano Hardness Tester (CSM- Instruments SA) with Berkovich diamond indenter. In each test run, the indenter was driven into the specimen surface under a load gradually increased from 5 mN to 450 mN with 10 stops at predefined loads. At each stops the indent was unloaded to 10 % of the load after being held at that load for 10 s, and then driven again into the specimen surface to a higher value. At least 15 test runs were recorded on each sample. Single cycle tests were also carried out for compari- son. The instrumented indentation hardness has been calcu- lated as H = P max /A c = P max /24.5h c 2 (4) where h c , the contact depth was determined using the Oliver & Pharr analyses 5 . Traditional Vickers hardness measurement was per- formed with the loads from 0.981 N to 298 N and calculated using parameters and formula defined in Fig. 1. The fracture toughness was measured by indentation method. The polished surfaces were indented by Vickers in- denter at loads from 298 to 1177 N and the size of indents/ cracks was measured by optical microscopy, (see Fig. 1). The Palmqvist toughness was calculated as 6 W k = P/T (5) were P is the load in N and T is the total crack length in mm and the fracture toughness as W G = 0.0028 (HV) 0.5 (W K ) 0.5 (6) where HV is the hardness in N mm 2 and Wk in N mm 1 . Results and discussion Characteristic microstructures of the studied WC-Co systems are illustrated in Fig. 2, together with the microstruc- Fig. 1. Schematic illustration of the indent/cracks system 6