Microstructure and mechanical properties of functionally gradient cemented carbides fabricated by microwave heating nitriding sintering

Microstructure and mechanical properties of functionally gradient cemented carbides fabricated by microwave heating nitriding sintering

Int. Journal of Refractory Metals and Hard Materials 58 (2016) 137–142 Contents lists available at ScienceDirect Int. Journal of Refractory Metals a...

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Int. Journal of Refractory Metals and Hard Materials 58 (2016) 137–142

Contents lists available at ScienceDirect

Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM

Microstructure and mechanical properties of functionally gradient cemented carbides fabricated by microwave heating nitriding sintering Siwen Tang a,⁎, Deshun Liu a, Pengnan Li b, Lingli Jiang a, Wenhui Liu b, Yuqiang Chen b, Qiulin Niu b a b

Hunan Provincial Key Laboratory of Health Maintenance for Mechanical Equipment, Hunan University of Science and Technology, Xiangtan 411201, China Hunan Provincial Key Laboratory of High Efficiency and Precision Machining of Difficult-to-Cut Material, Hunan University of Science and Technology, Xiangtan 411201, China

a r t i c l e

i n f o

Article history: Received 1 December 2015 Received in revised form 21 March 2016 Accepted 26 April 2016 Available online 27 April 2016 Keywords: Functionally graded material Cemented carbides Microstructure Mechanical properties Microwave sintering Nitriding

a b s t r a c t A new method is presented for the fast preparation of functionally graded cemented carbide materials by microwave heating nitriding sintering. The influence of composition and sintering temperature on the mechanical properties, microstructure, and phase composition of the materials was studied. Results showed that functionally graded cemented carbides with the desired mechanical properties can be obtained rapidly by microwave heating nitriding sintering. A gradient layer with a Ti(C, N)-enriched surface layer, and underneath a Co-enriched layer formed on the top of the hard alloy substrate. The nitriding process had little effect on the microstructure of the matrix. A lower surface roughness, and the similar layer thickness as seen in conventional heating nitriding was obtained by microwave heating nitriding sintering in a short period of time. The thickness of the gradient layer increased with increasing temperature. The high Ti content in the raw material was beneficial to the formation of the gradient layer; however, the Co content had little effect on the gradient layer thickness when it increased from 6% to 10%. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Cemented carbide is an advanced composite material composed of: WC, Co, TiC, TaC, NbC, Mo2C, etc., prepared, typically, by powder metallurgy, including mixing, drying, pressing, and sintering. It has good overall mechanical properties, is widely used in tools, moulds, drilling equipment, and cutting tools, but under some onerous working conditions, such as high-speed machining, the low wear resistance limits its application. A functionally graded, wear-resisting layer formed on the surface of cemented carbide to form functionally graded cemented carbide (FGCC) can effectively improve the cutting performance of a cutting tool [1–6]. At present, the functionally graded layer is mainly prepared through an in situ diffusion nitriding sintering process. The sintering process is primarily conducted in a traditional resistance furnace, in which the heat flow goes from the surface to the center. The binder melts at the surface during the nitriding process blocked the channel for outgassing, leading to a cessation of the densification process of the materials [7], thereby significantly reducing the performance thereof. To improve its mechanical properties, functionally graded cemented carbides need to be processed by vacuum sintering [3,8–10] or hot isostatic pressing process [3] to increase the density of material before or after nitriding sintering: the process is time-, and energy-consuming.

⁎ Corresponding author. E-mail address: [email protected] (S. Tang).

http://dx.doi.org/10.1016/j.ijrmhm.2016.04.013 0263-4368/© 2016 Elsevier Ltd. All rights reserved.

Microwave heating imparts rapid, selective, volumetric heating [11]. Compared with traditional resistance heating, microwave heating can reduce time and energy consumption, improve the material performance, is more environmental friendly, etc. [12]. More importantly, the heat flow direction of microwave heating is inside-out [13], this provides a potential means of preparing functionally graded cemented carbide material by combined sintering and nitriding from billet by microwave heating. However, the ability to absorb microwave energy of most materials at low temperature is low [12], so most of the microwave heating processes use SiC as an auxiliary heating medium to form a hybrid heating process [14–16], but this can cause a loss of energy and damage of the heat insulation material. Willert-Porada and Rödiger, et al. [16] reported the fabrication of hardmetal by microwave sintering. The result shows that microwave sintering can obtain a finer microstructure than traditional sintering. They also [17,18] report a method of fabricating functionally graded cemented carbides by microwave heating reactive sintering, which deals with sintering and nitriding simultaneously; but they used microwave hybrid heating sintering by using SiC for auxiliary heating. According to Luo, et al. [19], the pore length (Lp) of commercial pure Ti fabricated from TiH2 by pure microwave sintering is half that fabricated from hydride–dehydride (HDH) Ti by microwave hybrid heating sintering, and the former has a higher density and tensile strength which show the acceleration of pure microwave sintering on material shrinkage behaviour. Here, a new method for the preparing of functionally gradient cemented carbides tool materials by microwave heating nitriding sintering is presented. It includes two basic processes as follows: firstly,

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the rarefied nitrogen was used to form a plasma under microwave irradiation at low temperature. The radiant heat from the plasma was used to replace the heat produced by the SiC, which is currently commonly used in microwave heating as an auxiliary heating body which can accelerate the heating process. Secondly, pure microwave heating was achieved by adjusting the nitrogen pressure to suppress the plasma at higher temperatures; the densification and nitriding processes were conducted simultaneously on this material, therefore a high-performance functionally graded cemented carbide was formed. The combination of these two steps can avoid the material surface overheating due to the skin effect produced by microwave plasma that Willert-Porada and Rödiger reported [20], and obtain excellent mechanical properties of the final sintered body. The heating mechanism governing microwave heating nitriding sintering methods at different temperatures is shown in Fig. 1. At low temperature, the temperature inside the material is constant, the material undergoes no obvious shrinkage, and its porosity distribution is uniform. At high temperatures, pure microwave heating is achieved, the opposite temperature gradient is established, i.e. the internal temperature of the material exceeds that of the exterior, internal material shrinks before the external material, and porosity migrates along the yellow arrow (Fig. 1). When the surface temperature reached the sintering temperature, the pores moved further along the yellow arrow and out of the material: the material was thus densified. Based on the method, this research prepared functional gradient cemented carbide cutting tool materials, investigated the effect of sintering temperature and alloy composition on the mechanical properties, formation and phase composition of the gradient layers. 2. Experimental methods

Table 1 Nominal composition of samples (wt.%). Samples

WC

TiC

Co

FGCC-T5 FGCC-T15

Balance Balance

5 15

10 6

respectively. The surface roughness was measured by a Surf-gauge (Mahr, German). After sintering, the samples were cut, embedded in resin, and polished. The microstructures of the polished specimens were observed in cross-section by electro-probe micro-analyser (EPMA, JEOL Corp., Japan) in back-scattered-electron (BSE) mode. The phase compositions were evaluated by an X-ray diffractometer (XRD, Bruker Corp., Germany). 3. Results 3.1. Strength and hardness Fig. 3 shows the mechanical properties of the sintered FGCC samples. With increasing sintering temperature, the strength and hardness of FGCC-T5 and FGCC-T15 tended to decrease after an initial increase. The strength and hardness of FGCC-T5 were maximised, when sintered at 1400 °C for 15 min, at 2010.2 MPa and HV1580, respectively; however, the strength and hardness of FGCC-T15 were maximised, when sintered at 1430 °C for 15 min, at 1560 MPa and HV1835, respectively. Differences in composition result in differences in sintering temperature and mechanical properties. In addition, a matrix with more pores at low temperatures, upon being sintered, underwent grain coarsening and the volatilisation of its binding phase at high temperature, which caused the aforementioned mechanical properties changes.

Cemented carbides samples from an industrial-grade mixture of WC, Co, and TiC were prepared by standard powder metallurgy methods. The nominal composition of the cemented carbides used here is given in Table 1. The green compacts were sintered in a vacuum sintering furnace heated to 400 °C for 1 h to remove the wax. The experimental apparatus is that used elsewhere [21]. The experiment was conducted in a microwave oven with an operating frequency of 2.45 GHz. Fig. 2 shows the typical temperature curve for preparation of FGCC, changed by controlling the input power, so giving different heating rates. To realise simultaneous sintering and nitriding, after a brief cryogenic vacuum degassing process, nitrogen, at a pressure of 0.02 MPa and a purity of 99.9% was saturated at about 300 °C. And the pressure was increased to 0.08 MPa upon reaching the required sintering temperature. The required sintering temperature ranged from 1380 °C to 1460 °C, and the soaking time was 15 min. The strength and surface hardness were evaluated by three-point bending test and a Vickers hardness tester,

Fig. 4 shows the typical cross-sectional view of the microstructure of FGCC-T5 sintered at 1400 °C for 15 min. A grey surface layer, about 10 μm thick was formed at the homogeneous bright white and grey matrix in EPMA/BSE mode (Fig. 4(a)). Combining elemental analyses by EPMA, it may be seen that a graded layer, rich in titanium and nitrogen, had formed on the surface of the matrix, in which the tungsten content was lower than that of the matrix (Fig. 4(b), (c), and (e)). A Co-enriched layer was located at the bottom of the graded layer adjacent to the matrix (Fig. 4(d)). From Fig. 4(f), the carbon content of the graded layer was greater than that of the substrate. Fig. 5 shows the typical cross-sectional microstructure of FGCC-T15 sintered at 1430 °C for 15 min. The cross-sectional morphology of FGCC-T15 was similar to that of FGCC-T5, but the thickness of its surface

Fig. 1. Schematic diagram of heating mechanism underpinning the microwave heating sintering nitriding method at different temperatures.

Fig. 2. Typical temperature curve.

3.2. Gradient structure and phase composition

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It was worth noting that there was coarse-grained WC formed in the FGCC-T5 matrix at the contact area of the graded layer and matrix (see arrow, Fig. 4(a)); however, there was no coarse WC found in the FGCC-T15. This may have been as a result of the higher TiN content in the FGCC-T15, the formation of TiN having played an important role in refining grain size [4]. Fig. 7 shows the gradient layer thickness of FGCC-T15 at different temperatures: with increasing temperature, the thickness of the gradient layers increased. The thickness of a graded layer sintered at 1460 °C for 15 min was about 40 μm, which was twice that of the layer sintered at 1430 °C. From the cross-sectional micrograph (Fig. 8), it may be seen that there are some grey islands of W-enriched matter (WC) distributed within the grey Ti-enriched gradient layer surface (see white arrow, Fig. 8), there was also a Co-enriched layer, with a low W content, located at the bottom of the gradient layer adjacent to the matrix (see red lined area, Fig. 8). Fig. 3. Effect of sintering temperature on the mechanical properties of FGCC.

layer reached 20 μm (Fig. 5(b) and (e)), and also the Co-enriched layer in FGCC-T15 was more obvious (Fig. 5(d)). Similarly, the carbon content in the graded layer was higher than that in the matrix (Fig. 5(f)). Beneath the gradient layer, the microstructure was not affected by nitriding. The XRD diffraction of the alloy surface and cross-section shows (Fig. 6) that FGCC-T5 and FGCC-T15 surfaces and cross-sections were composed of a three-phase alloy of Ti(C,N), WC, and Co, with no other new phase formation. The main diffraction peak of the cross section matched that of WC, and that of the surface indicated the presence of Ti(C,N): this showed obvious nitriding at the surface of the two materials. Compared with FGCC-T5, the Ti(C,N) diffraction peak intensity on the surface of FGCC-T15 was higher, the WC diffraction peak intensity was lower, and this suggested that the nitriding effect of FGCC-T15 was higher than that of FGCC-T5. Due to the influence of the solid solution, a partial diffraction peak arose at an offset compared with the standard diffraction peak, especially in Co, the diffraction peak position of Co having largely left-shifted compared to the standard sample.

3.3. Surface roughness Nitriding at liquid phase temperature readily causes an increase in surface roughness, as shown in Table 2, the Ra value of microwave heating sintering FGCC material was between 0.9 and 1.42 μm. The increase in both temperature, and TiC content of the raw material caused the surface roughness Ra increase. 4. Discussion 4.1. Effect of microwave heating nitriding sintering on mechanical properties and formation of graded layer The underpinning mechanism of microwave sintering was bulk heating and a hot spot effect; the resulting shortening of the heat transfer distance, and partial liquid phase for the promotion of particle rearrangement are also useful [22]. Additionally, the inverse temperature gradient helped to densify the material during liquid sintering. These were the reasons why microwave heating nitriding sintering shortened sintering time, and improved the mechanical properties of the final materials.

Fig. 4. Typical cross-sectional microstructure of FGCC-T5 (EPMA, 1400 °C, 15 min).

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Fig. 5. Typical cross-sectional microstructure of FGCC-T15 (EPMA, 1430 °C, 15 min).

According to Chen et al. [5], carbides and carbonitrides can react with nitrogen. The reaction can be written as: bTiCN þ N2 ⇔bTiNN þ C

ð1Þ

Here, the angle brackets represent a part of the solid solution, not a separate phase. Reaction 1 prompted N atoms to replace some of the C atoms in TiC to form Ti(C,N) due to the strong thermodynamic coupling between Ti and N, and also the unfavourable thermodynamic conditions between W and N [4,23]. At the same time, elemental Ti, close to the surface,

migrated to the surface under the action of the external nitrogen potential. These two processes caused Ti(C,N)-enrichment on the surface of the FGCC matrix. According to Reaction 1, the nitriding reaction resulted in a higher content of elemental C in the surface than that in the substrate (Figs. 4(f) and 5(f)). The difference in carbon content between surface and matrix caused “liquid phase migration” when the particle size and other factors are the same [24], i.e. Co migrated from areas of high carbon content to those of low carbon content due to the difference in interfacial energy. Liquid phase migration resulted in a Co-enriched layer being formed at the bottom of the surface Ti(C,N) layer. The

Fig. 6. XRD diffraction spectrum of FGCC.

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Table 2 Ra values of the samples prepared by microwave sintering. Sample

Temperature

Ra

Temperature

Ra

FGCC-T5 FGCC-T15

1400 1400

0.9 1.21

1430 1430

1.01 1.42

FGCC-T5. Therefore, it was concluded that the Co content of the two components provided enough lubrication and diffusion channels for N and C, and the Ti content played a key role in the thickness of the gradient layer. This implied that, in the case of a certain Co content, the formation of the graded layer was related to the number of Ti elements in the alloy, and the effect of Co content on the gradient layer was insignificant. Exactly which Co content was the critical value in this TiC, will be discussed in future research. Fig. 7. Effect of temperature on the thickness of the graded layer.

existence of a Co-enriched layer is of benefit in that it hinders crack initiation and propagation; furthermore, it improves the cutting performance of such tools [2,25]. During microwave heating nitriding sintering, the nitrogen flux into the sintering furnace before the density of material increased meant that the nitrogen ran into the pores of the billets, reacted with particulate material under microwave irradiation, and thus accelerated the reaction. The increased sintering temperature caused an increase in the diffusion coefficient increases, and also accelerated the diffusion of the material, therefore the thickness of gradient layers increased accordingly. On the other hand, the increased sintering temperature led to an increase in the driving force needed for nitriding: i.e., under the same nitrogen pressure, the temperature increase would decrease the driving force behind the nitriding, decreasing the efficiency of the nitrogen. Here, however, nitriding and densification were simultaneous: nitriding is an accumulative process; the increased temperature was advantageous in that it caused the gradient layer thickness to increase. 4.2. Effect of composition on formation of the gradient layer According to Janisch et al. [7], pure TiC is hard to nitride, however, TiC-31%Co alloy obviously nitrided under the same conditions. The promoting effect of Co on TiC was derived from three aspects: the first was that Co played the role of a lubricant, which helped particle rearrangement, accelerated densification, and shortened the diffusion pathway; the second was that Co provided the dissolution and diffusion path for N and C atoms; the third was that liquid phase Co can dissolve more carbon, which was conducive to reaction (1) to the right, as shown. The experimental results showed that FGCC-T15, with a low Co content and high Ti content, formed thicker gradient layers than

4.3. Effect of microwave heating nitriding sintering on phase formation X-ray diffraction analysis of the surface, and EPMA analysis of the cross-section showed that a wear-resistant Ti(C,N) layer was formed in the surface of the FGCC material, and beneath it a Co-enriched layer was formed. Bao et al. [15] reported that an eta phase (W3Co3C) was formed in the surface of WC-8Co cemented carbide when sintered by microwave hybrid heating sintering under a pure nitrogen atmosphere. They believe that this is a result of decarburization, which is the reaction among N2, carbon in the sintered materials, and Al2O3 in thermal insulation materials, under microwave irradiation. However, in this experiment, due to the presence of TiC, sintering decarburization generated an eta-phase reaction with TiC and N2, as shown in Eqs. (2) and (3), generated Ti(C,N), WC, and Co. Therefore, the surface did not show the signs of the presence of an eta phase. 1 1 3 3 TiC þ xCo3 W3 C þ xN2 →TiðC1−x Nx Þ þ xWC þ xCo 2 2 2 2

ð2Þ

1 1 6 6 TiC þ xCo6 W6 C þ xN2 →TiðC1−x Nx Þ þ xWC þ xCo 5 2 5 5

ð3Þ

The above reactions only represent the stoichiometry, not the reaction mechanisms. The Co generated by Eqs. (2) and (3) migrated inwards under the action of liquid phase migration, and the WC generated by Eqs. (2) and (3) precipitated in the surface, forming a WC phase. With increased thickness of the gradient layer, the formation of WC increased under Eqs. (2) and (3), and formed the grey island-like deposits of WC in the surface (Fig. 8). The increase in WC content will decrease the surface hardness of the material, which indicated that the thickness of the gradient layer must be controlled to obtain a higher Ti(C,N) content, meant, furthermore, obtaining high surface hardness and wear-resistance. The cutting performance of functionally gradient cutting tools was assessed by Leaguer et al. [26] who showed that conventional heating and nitriding gave the longest tool life to a gradient tool with a 7 μm thickness graded layer. Future research into the cutting performance, and the relationship between microstructure and the cutting performance, of a functionally graded cemented carbide cutting tool prepared by microwave heating nitriding sintering will be reported later. 4.4. Effect of microwave heating on surface roughness

Fig. 8. Cross-sectional microstructure of FGCC-T15 sintered at 1460 °C for 15 min (EPMA/BSE).

The formation of the surface roughness on the material during sintering is closely related to the rearrangement of the particles in the densification process. Ucakar et al. [3] show that the surface roughness value Ra of a WC-fcc phase-Co is 1.2 to 4.5 μm and 0.6 to 7.7 μm when nitrided at the liquid phase temperature (1500 °C) and solid phase temperature (1300 °C), respectively, it was worth noting that the higher roughness values were obtained after longer nitriding times. Nitriding,

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in this research, was carried out in the liquid phase, and the direction of heat flow was from the inside-out. The direction of particle rearrangement was also inside-out, which was beneficial in that it reduced the surface roughness of the material. In addition, the shorter sintering time with microwave heating, compared to that with traditional heating, lowered the surface roughness. The roughness of FGCC-T5 was lower than that of FGCC-T15, which was consistent with the results of microscopic observation; i.e., the thicker the gradient layer, the greater the migration distance of Ti, W, Co, and other elements during particle rearrangement, so the surface roughness value of FGCC-T15 was higher than that of FGCC-T5. 5. Conclusion This investigation reports a new method for the fast preparation of functionally gradient cemented carbide materials by microwave assisted nitriding sintering. Two functional gradient cemented carbide materials with a Ti(C,N)-enriched surface layer, and an intermediate Co-enriched transition layer, were prepared on WC-TiC-Co substrates. In the process of the nitriding sintering of the WC-TiC-Co alloy, the thickness of the gradient layer is mainly affected by the sintering temperature and Ti content of the original composition, while the Co content is less affected, and the formation mechanism of the rich Co transition layer is a liquid phase transfer process. The strength and hardness of FGCC-T5 sintered at 1400 °C reach 2010.2 MPa and HV1580, and the thickness of gradient layer is about 10 μm; however, the strength and hardness of FGCC-T15 sintered at 1430 °C reach 1560 MPa and HV1835, and the thickness of gradient layer is about 20 μm. FGCC-T5 and FGCC-T15 have lower surface roughnesses, at Ra of 0.9 to 1.01 μm and 1.21 to 1.42 μm, respectively. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (grant nos. 51305134 and 51275168), a Chinese Important National Science & Technology Specific Project (grant no. 2012ZX04003051), and the Natural Science Foundation (grant no. 14JJ5015). References [1] K. Tsuda, A. Ikegaya, K. Isobe, et al., Development of functionally graded sintered hard materials, Powder Metall. 39 (4) (1996) 296–300. [2] T. Nomura, H. Moriguchi, K. Tsuda, et al., Material design method for the functionally graded cemented carbide tool, Int. J. Refract. Met. Hard Mater. 17 (6) (1999) 397–404. [3] V. Ucakar, K. Dreyer, W. Lengauer, Near-surface microstructural modification of (Ti,W)(C,N)/Co hardmetals by nitridation, Int. J. Refract. Met. Hard Mater. 20 (3) (2002) 195–200.

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