PSI - Issue 5

Pavol Hvizdoš et al. / Procedia Structural Integrity 5 (2017) 1385–1392 Pavol Hvizdoš et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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3. Experimental methods

Hardness values were obtained by Vickers method using 98.1 N load and 10 s holding time. Indentation toughness values were obtained by measuring of indentation cracks and calculated according to Anstis proposal, (Anstis et al. (1981)): = 0.016 ( ) 1 2 ( 3/2 ) (1) where E – Youngs modulus, H – hardness in GPa, P – load, c – crack length. Tribological behavior of the experimental materials was studied under dry sliding in air atmosphere. Surface of each sample was prepared by polishing to surface roughness Ra < 0.1 µm. Wear testing was carried out on the THT Tribometer (CSM Instruments) in dry conditions using ball-on-disc technique. Tribological partners used for wear testing were highly polished WC-Co and steel (100Cr6) balls with 6 mm diameter. The applied load was 5 N, sliding speed 0.1 m/s and total sliding distance was 300 m. The testing temperature were 25 °C, 200 °C, 400 °C. The tangential forces during the test were measured and friction coefficients calculated. The worn surfaces were subsequently observed and the wear regimes, damage type and micromechanisms were identified. The material losses (volume of the wear tracks) due to wear were measured by a high precision confocal microscope PLu neox 3D Optical Profiler, by SENSOFAR, and then specific wear rates (W) were calculated in terms of the volume loss (V) per distance (L) and applied load (F p ) according to the standard ISO 20808 (2016): = . (2) The microstructure and the wear tracks created at the surfaces of the investigated materials were observed using scanning electron microscopy (JEOL JSM 7000). Porosity and average pore size was observed and evaluated using image analysis of micrographs of polished ceramographic sections. At least 1 mm 2 of surface was documented and then treated using ImageJ 1.47e software. Three materials with nominal composition Ti 0,95 Ta 0,05 C 0.5 N 0.5 -20% Co were prepared by high energy milling with various amounts of graphite addition - 0 % C, 1.8 % C, and 2.2 % C. Their phase composition was analyzed by XRD and SEM in detail by Chicardi et al. (2015). Table 1 summarizes phase composition identified by XRD, hardness and indentation toughness of each experimental material. It was confirmed that with increasing amount of added graphite the solid solution in the intergarnular phase contained decreasing amount of titanium and tantalum, and for 2.2 % graphite addition metallic cobalt dominated the binder. Its influence was clearly seen in slight decrease of hardness from 11.65 HV10 for the reference sample down to 9.34 HV10 for the sample with 2.2 % C. On the other hand, the toughness markedly improved from 3.89 MPa.m 1/2 up to 9.23 MPa.m 1/2 , which was certainly the desired outcome. 4. Results and discussion

Table 1 Phase composition, hardness and indentation toughness of the experimental materials wt.% C Binder phase Ceramic phase HV10 (GPa) K IC (MPa.m 1/2 ) 0 Ti x Ta 1-x Co 2 Ti 0.95 Ta 0.05 C 0.5 N 0.5 11.65 ± 0.17 3.89 ± 0.17 1.8 Ti x Ta 1-x Co 3 Ti 0.95 Ta 0.05 C 0.5 N 0.5 10.76 ± 0.14 8.44 ± 0.83 2.2 α -Co Ti 0.95 Ta 0.05 C 0.5 N 0.5 9.34 ± 0.25 9.23 ± 1.28

Figs. 1 – 3 summarize results of the microstructure studies and tribological testing in terms of coefficient of friction (COF) and wear rate (W) of the experimental materials as functions of temperature.