Effects of Mo and VC on the Microstructure and Properties of Nano-Cemented Carbides

In this study, four unique groups of nano-cemented carbides with 8 wt.% Co and trace amount of Mo and VC have been successfully synthesized. The effect of Mo and VC has been investigated comprehensively on both the microstructure and mechanical behavior of nano-cemented carbide. The results show that Mo significantly increases hardness but decreases the fracture toughnesses and VC slightly increases hardness and strongly increases fracture toughness of nano-cemented carbide. It is found that the optimum Mo and VC contents are both 0.5 wt.%. The average WC grain size is about 370 nm. The hardness is about 2350 HV30 and the fracture toughness is about 11.2 MPam.


Introduction
Cemented carbide has been used widely in the applications of cutting, machining, mining, rock drilling, forming tools, dies and wear resistant parts [1][2][3][4][5] due to its desirable combination of mechanical, physical, and chemical properties.Cemented carbides are processed from carbide and metal powders per liquid phase sintering technology in which the carbide grain growth may have the critical effect on the final properties [2,4] .The hardness, strength and toughness of cemented carbides are all closely related to the WC grain size [6] .In recent years, nano-cemented carbides with the carbide grain size 100-500 nm have attracted more and more research and commercial interest due to their extremely high hardness and wear resistance.
In order to synthesize the nano-cemented carbide, the carbide of transition metalsin Group VB (V, Nb, Ta) and VIB ( Cr , Mo ,W) groups of the periodic Tab. has been used as the grain growth inhibitor during liquid sintering process [7][8][9][10] .In addition, in order to overcome the extensive coarsening of WC grain when nanometer sized WC-Co starting powder mixtures are sintered by the standard liquid phase sintering [11][12][13] special sintering methods such as microwave sintering [11] , hot isostatic pressing (HIP) [14] , and spark plasma sintering (SPS) [15,16] have been investigated to lower sintering temperature or to shorten sintering time.These special sintering methods have the problems of high cost and being unable to make complicated shaped parts.So far, the most successful way to control carbide grain size is the addition of cubic carbides such as VC, TaC, Cr 3 C 2 or NbC into the starting powder mixtures [17][18][19][20][21] .Among these cubic carbides, vanadium carbide has been proven the most effective grain growth inhibitor in WC-based cemented carbides due to the facts of forming a solid solution with WC and/or dissolving preferentially into metallic binder phase (Co) [6,[20][21][22][23][24][25][26] .Mo 2 C and Mo have rarely been used as the carbide grain growth inhibitor although both Mo 2 C and Mo can improve the wettability between hard phase (Ti(C,N), (W,Ti)C) and metallic binder phase (Co, Ni) [27][28][29] .
The combination effect of both VC and Mo on the WC grain growth behavior and the mechanical behavior of nano-cemented carbide, which has rarely been investigated before, has become the main research focus in this study.We have designed four unique groups of cemented carbides: (a) pure cemented carbide (WC and Co), (b) cemented carbide with the additive of Mo; (c) cemented carbide with the combinative additives of Mo and VC; and (d) cemented carbide with the additive of VC.The effects of Mo/VC addition on the microstructure, density, hardness and fracture toughness were investigated in this study.

Experimental procedures 2.1. Materials preparation
Tab.I shows the nominal composition of the 4 groups of WC-8Co alloys with or without VC/Mo additions.The raw material powders used in the present study include WC powder (purity 99.9 wt.%, average oxygen content of 0.05 wt.%, mean particle size of 0.8 μm), Co (purity 99.9 wt.%, mean particle size of 0.8 μm), Mo (purity 99.9 wt.%, mean particle size of 2 μm) and VC (purity 99.9 wt.%, mean particle size of 1 μm).Their morphologies are shown in Fig. 1.Mo and VC powders were added to WC-8Co powders before milling.Then 1.0 wt.% paraffin wax was added as the pressing aid, mechanical milling was carried out for 72 h in 4-planetary ball milling system in carbon tetrachloride solution with 8 mm carbide milling ball.The mass ratio of milling ball to powder was 10:1 and the milling speed was 400 r/min.After milling the powder mixture slurry was dried in a vacuum oven at 70 o C for 24 h.The mean particle size of the WC-Co powder was measured for the powder after milling.The powder mixture was granulated and pressed into cylindrical green parts with 25 mm diameter and 12 mm height under a pressure of 200 MPa.The green parts were dewaxed and sintered with sinter-hip process.The sintering temperature is 1410 o C. The holding time at 1410 o C was (30, 60, 90, 120) min and the pressure in the dwell time was 5 MPa in argon atmosphere to avoid the prominent evaporation of cobalt during the liquid phase sintering.After sintering, the alloys were cooled down to room temperature in the vacuum sintering furnace.

Mechanical Testing
The bulk density of the sintered alloys was measured using FA2104J density balance (Satorius, 0.0001 mg sensitive quantity, China) by Archimedes method.The average WC grain size was determined based on the SEM micrographs.Image-pro Plus software was used to measure the linear intercept length of at least 100 WC grains for each material grade.The microstructures of the cemented carbides were evaluated with scanning electron microscope (SEM, NOVA NANOSEM 430), and the phases were characterized by the X-ray diffraction (XRD, Bruker D8 Advance) method.
The hardness was measured with Vickers's hardness tester under a constant load of 30 kg.The fracture toughness (K IC ) of the alloys was determined by measuring the crack length from the four corners of the indentation generated by Vickers's indentation load of 30 kg, and the Palmqvist indentation toughness was calculated as follows [30] : K IC =0.15(HV30/∑L ) 1/2 (1) Where ∑L is the sum of crack lengths (mm).Crack length measurement was carried out on an optical microscope.

Microstructure of composite powders after ball milling
The SEM micrograph, grain size distribution and XRD spectrum of the Alloy 3 composite powder after ball milling are shown respectively in Fig. 2

3.2Microstructure and phases of sintered alloys
Fig. 3 shows SEM images of the fractured surface of ultrafine WC-8Co cemented carbides for the four groups after sintering.As it can be seen from Fig. 3, the additives of Mo or VC are very effective for the reduction of grains size.The particle size distribution of these four groups of carbides is shown in Fig. 4. Fig. 4 shows that for base cemented carbide (Alloy 1, no addition of any carbide grain growth inhibitor) has the average carbide grain size of 750 nm.With the 1 wt.% addition of Mo (Alloy 2), the average carbide grain size was reduced to 420 nm.With the 1 wt.% addition of VC (Alloy 4), the average carbide grain size was 470 nm, which is similar to Alloy 2. The co-addition of both Mo and VC has more significant effect on carbide size reduction.The Alloy 3 (0.5 wt.% Mo and 0.5 wt.% VC) has the carbide size of 370 nm, 55% lower than the base cemented carbide.Both Fig. 3 and 4 show that Mo and VC are the effective WC grain growth inhibitors in WC-8Co cemented carbides and the combination of both components can be more effective.The decrease in growth rate is attributed to the lowering of interfacial free energies (between carbide and metal binder) due to the segregation of the Mo and VC to the interfaces of the WC grains [33,34] .Therefore, Addition of Mo or VC can change the interface energy and slow down the dissolution-reprecipitation step.
The XRD patterns of all the Alloys after sintering at 1410℃ for 60 min are presented in Fig. 5.The results show that WC-8Co cemented carbide contains WC, Co and Co 6 W 6 C phases.The Co 6 W 6 C phase can affect the mechanical properties of cemented carbides.Meanwhile, the volume fraction of Co 6 W 6 C phase in the Alloys 2 and 4 is lower than that in the Alloy 1.Since frequent oxygen concentrate on the surface of nanocrystalline powder, Co 6 W 6 C phases are generated because of carbon shortage during sintering process.Adding Mo or VC may decrease the oxygen concentration, and therefore decrease the volume fraction of Co 6 W 6 C phases [33] .

Properties of sintered alloys
The relative bulk densities of the sintered alloys are shown in Fig. 6.With the increase of holding time at 1410 o C, the density increases for all the alloys.Except Alloy 2, all other alloys reached above the 99.5% relative density after 120 minutes of holding time.Fig. 6 shows that the introduction of Mo to the WC-8Co cemented carbides (Alloy 2) decreases the relative bulk density.It may be due to the higher viscosity of liquid phase containing Mo and the lower capability to fill the porosities [29,34,35] .Therefore, alloy 2 does not achieve the maximum density even at 120 minutes holding time.In contrast, alloys 1, 2 and 4 have easily reached high density at 120 minutes holding time.The hardness values of the alloys with Mo or VC addition in all cases are higher than those of the base alloy due to the finer WC grain size.Among all the samples, Alloy 3 has the highest hardness of 2350 HV 30 sintered at 1410°C for 90 min, 28% higher than that of alloy 1.As reported that the industrial cemented carbide alloys in particular WC-8%Co possess hardness of 1400-1800 HV 30 and fracture toughness of 9-11MPa-m -1/2 [36] .Fig. 7 also shows that all the carbides reach the maximum hardness value at 90 minutes holding time and after 90 minutes holding time, the hardness begins to drop.With the increase of sintering holding time at 1410 o C, there are two phenomena occurred, which contribute to this hardness drop after peak value.The first is the decrease of porosity, which improves the hardness of cemented carbide.The second is the WC grain growth, which reduces the hardness value.The combination of these two phenomenon leads to the maximum hardness of all the alloys at 90 minutes holding time.The fracture toughness of the alloys versus holding time is shown in Fig. 8.Although Alloy 3 show remarkable enhancement in hardness (~2350 HV 30 ),Alloy 3 doesn't reduce K 1C (~11.2MPa-m -1/2 ) compared to Alloy 1 or the reported data of WC-8%Co composite (~9-11 MPa-m -1/2 )) [2,11,36] .Compared with base alloy (Alloy 1), all the carbide with Mo and/or VC addition has lower toughness due to the significantly finer WC grain size.Fan et al. [37] found that the transgranular fracture strength is decreases with increasing WC grain size.The finer WC grain size results in the shorter mean free path of Co phase, which directly contribute to the lower toughness [37,38] .Alloy 2 has the lowest toughness compared with all other alloys mainly due to the significantly lower relative density as shown in Fig. 6.

Conclusions
1) Both Mo and VC addition decreases WC grain size and the volume fraction of Co 6 W 6 C phase of ultrafine WC-8Co alloys after vacuum sintering at 1410 o C for 60 min.
2) The increase of the holding time enhances the relative bulk density of the samples.The addition of Mo decreases the density of the sample whereas VC increase the density of the sample.
3) The hardness values of the alloys with Mo or VC addition are higher than those of the base alloy.The alloy 3 has the highest hardness of 2350 HV 30 sintered at 1410°C for 90 min, which is 28% higher than that of the base alloy.
4) The addition of Mo or VC has a negative effect on fracture toughness of WC-8Co cemented carbides.The alloy 1 has the highest fracture toughness of 13.2MNm -3/2 sintered at 1410°C for 120 min.

Fig. 6 .
Fig. 6.Variation of density of sintered alloys at 1410°C sintering temperature for different holding time.

Fig. 7 .
Fig. 7. Hardness of sintered alloys at different sintering temperature for different time.

Fig. 7
Fig. 7 shows the Vickers hardness of the samples sintering soaking time at 1410 o C.The hardness values of the alloys with Mo or VC addition in all cases are higher than those of the base alloy due to the finer WC grain size.Among all the samples, Alloy 3 has the highest hardness of 2350 HV 30 sintered at 1410°C for 90 min, 28% higher than that of alloy 1.As reported that the industrial cemented carbide alloys in particular WC-8%Co possess hardness of 1400-1800 HV 30 and fracture toughness of 9-11MPa-m -1/2[36] .Fig.7also shows that all the carbides reach the maximum hardness value at 90 minutes holding time and after 90 minutes holding time, the hardness begins to drop.With the increase of sintering holding time at