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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Observations in scanning electron microscope showed well developed microstructure (Fig. 1) in all experimental materials. The carbonitride ceramic grains were uniformly localized, and had bimodal size distribution: i.e. there were two population of grains with average sizes of approximately 2 µm and 15 µm, respectively. They were well bonded and enclosed with the binder. There was some residual porosity, in all cases smaller than 1 vol. %. The pores were connected to occasional clusters of the ceramic grains and their typical size was around 2 µm.

a)

b)

c)

8 µm

Fig. 1 Microstructure of the experimental materials - scanning electron microscopy: a) 0% C, b) 1.8 % C, c) 2.2 % C.

Fig. 2a shows comparison of wear rates of all experimental materials at all experimental temperatures. It can be seen that at room temperature the wear rates are the lowest. At medium temperature (200 °C) wear rates increased and at 400 °C wear rate reached the highest values. Wear rate values for WC-Co ball as counterpart were about one order of magnitude higher than those for the steel ball counterpart. Materials with higher carbon additions show higher values of wear rates so it seems that at the chosen conditions the effect of the higher hardness is more beneficial than that of the improved fracture toughness.

1,0

0.0 5.0x10 -7 1.0x10 -6 1.5x10 -6 2.0x10 -6 2.5x10 -6 3.0x10 -6 3.5x10 -6 4.0x10 -6 4.5x10 -6 5.0x10 -6 Wear rate [mm 3 /N.m]

0 wt.% C WC-Co ball 0 wt.% C Steel ball 1 wt.% C WC-Co ball 1 wt.% C Steel ball 2 wt.% C WC-Co ball 2 wt.% C Steel ball

a)

0,9

0,8

0,7

0,6

0,5

0,4

0 wt.% C WC-Co ball 0 wt.% C Steel ball 1 wt.% C WC-Co ball 1 wt.% C Steel ball 2 wt.% C WC-Co ball 2 wt.% C Steel ball

0,3

0,2 Friction coefficient [-]

b)

0,1

0 50 100 150 200 250 300 350 400 450 500 0,0

0 50 100 150 200 250 300 350 400 450

Temperature [°C]

Temperature [°C]

Fig. 2 a) Wear rates of all experimental materials at all temperatures. b) Friction coefficients at all temperatures for each material.

Friction coefficients results are summarized in Fig. 2b). At room temperature friction coefficient vary between values of 0.45 – 0.75. At medium temperate (200 °C) friction coefficient vary in the wide range of values (0.4 – 0.9). At the temperature of 400 °C friction coefficient vary in the smallest range of values (0.6 – 0.75) and is slightly higher in average in comparison with friction coefficient at 25 °C. The addition of carbon did not lower friction coefficient values, which confirmed the absence of any remaining free graphite which could act as solid lubricant. This is also consistent with our observation of relatively high roughness of the wear tracks. Figs. 3 shows behavior of coefficient of friction curve during whole tests at 200 °C for both WC-Co and steel (100Cr6) counterpart. Wear testing with WC-Co counterpart show dependence between friction coefficient and carbon content. Friction coefficient significantly drops with increasing of graphite volume. There was found no clear dependence between friction coefficient and carbon content if using steel ball as counterpart.