Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions

doi:10.1016/j.jmst.2019.07.016

Abstract

Abstract:

The grain-size dependence of wear resistance of WC-Co cemented carbides (with mean WC grain sizes of 2.2 μm, 1.6 μm, 0.8 μm and 0.4 μm, respectively) was investigated under different tribological conditions. The results showed that the grain size had opposite effects on wear resistance of the cemented carbides in dry sliding wear and microabrasion tests. In the former condition, with decrease of WC grain size hence the increase of hardness, plastic deformation, fracture, fragmentation and oxidation were all mitigated, leading to a drastic decrease in the wear rate. In the latter condition, pull-out of WC grains after Co removal dominated the wear, so that the hardness of cemented carbide was not a core factor. As a result, the wear resistance of the cemented carbide generally showed a decreasing trend with decrease of the grain size, except for a slight increase in the ultrafine-grained cemented carbide. Single-pass scratching of the cemented carbides under various loads indicated the same failure mechanism as that in the sliding wear tests. Furthermore, the reasons for severe surface oxidation of the coarse-grained cemented carbides were disclosed.

Key words: Cemented carbide, Grain size, Fracture, Surface oxidation, Grain pull-out, Wear resistance

Haibin Wang, Mark Gee, Qingfan Qiu, Hannah Zhang, Xuemei Liu, Hongbo Nie, Xiaoyan Song, Zuoren Nie. Grain size effect on wear resistance of WC-Co cemented carbides under different tribological conditions[J]. J. Mater. Sci. Technol., 2019, 35(11): 2435-2446.

Figures/Tables 15

Table 1 Preparation details of the WC-Co powders.

Powder	Composition (wt%)	Particle size of raw powders (μm)				Milling time (h)
Powder	Composition (wt%)	WC	Co	VC	Cr₃C₂	Milling time (h)
S1	WC-8Co	3	10	-	-
S2	WC-8Co	3	30	-	-
S3	WC-8Co- 0.5VC-0.5Cr₃C₂	3	30	0.5	0.5
S4	WC-8Co- 0.5VC-0.5Cr₃C₂	0.8	30	0.5	0.5

Table 2 Experimental details of the wear tests.

Parameter	Microabrasion	Dry sliding wear
Normal load (N)	0.2	70
Wear counterpart	Stainless steel ball, Φ25	Si₃N₄ ball, Φ5
Wear time (min)	30	30
Wear distance (m)	220	150
Abrasive flow rate	SiC (4 μm) slurry with a concentration of 0.2 g/ml and flow rate of 44 ml/h	—

Fig. 1. Microstructures of the cemented carbides prepared with WC-Co powders of different characteristics: (a) S1; (b) S2; (c) S3; (d) S4.

Fig. 2. WC particle size distribution of the prepared cemented carbides.

Table 3 Properties of the prepared cemented carbides.

Specimen No.	Relative density (%)	Amount of magnetic Co (wt%)	Vickers hardness (HV₃₀)	Fracture toughness (MPa m^1/2)
S1	99.5	7.87	1489 ± 10	—
S2	99.8	7.68	1549 ± 9	—
S3	97.7	7.34	1715 ± 25	11.3 ± 0.7
S4	98.5	7.41	2154 ± 27	11.3 ± 0.3

Fig. 3. Wear rate of the cemented carbides with different WC grain sizes.

Fig. 4. Typical optical images of craters caused on the sintered samples with (a) 2.2 μm, (b) 1.6 μm, (c) 0.8 μm and (d) 0.4 μm after microabrasion tests, and (e) the cross-sectional profiles of the craters.

Fig. 5. Friction coefficient of the sintered WC-Co cemented carbides during sliding wear tests.

Fig. 6. 3D surface topographies obtained from vertical scanning white-light interference microscopy and corresponding cross-sectional profiles of sliding wear scars of the sintered WC-Co cemented carbides with (a) 2.2 μm, (b) 1.6 μm, (c) 0.8 μm and (d) 0.4 μm.

Fig. 7. Variation of scratch width (a) and calculated compressive stress (b) with the increase of applied load on the sintered WC-Co cemented carbides.

Fig. 8. Microstructures of the sintered WC-Co cemented carbides after microabrasion test under the load of 0.2 N: (a, b) 2.2 μm; (c, d) 1.6 μm; (e, f) 0.8 μm; (g, h) 0.4 μm.

Fig. 9. Typical wear characteristics and elemental analyses of the coarse-grained cemented carbide after sliding wear test under the load of 70 N: (a) low-magnification image; (b) mixed WC and Co fragments; (c) powdery oxides containing W and Co; (d) rod-like W oxides; (e) fracture of WC grain. EDS maps show the distributions of O, Co and W in (a). The area marked with circle in (a) shows irregular fracture and fragmentation of WC grains.

Fig. 10. Typical wear characteristics EDS maps of the submicron-grained cemented carbide after sliding wear test under the load of 70 N: (a) low-magnification image; (b) irregular fracture of WC; (c) oxide scales. The EDS maps show the distributions of O, Co and W in (a).

Fig. 11. Worn microstructures of the ultrafine-grained cemented carbide after sliding wear test at 70 N: (a) low-magnification image; (b) high-magnification image; (c) EDS spectrum obtained by scanning the entire imaged area of (a); (d) EDS spectrum obtained at the point marked with “+” in (b).

Fig. 12. Typical microstructures of scratches on the coarse-grained cemented carbides with loads of (a) 10 N, (c) 40 N, (e) 50 N and (g) 70 N and ultrafine-grained with loads of (b) 10 N, (d) 40 N, (f) 50 N and (h) 70 N.

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