Tungsten carbide platelet-containing cemented carbide with yttrium containing dispersed phase

ZHANG Li(张 立)¹, CHEN Shu(陈 述)², WANG Yuan-jie(王元杰)¹,

YU Xian-wang(余贤旺)¹, XIONG Xiang-jun(熊湘君)¹

1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;

2. Changsha Mining and Metallurgy Research Institute, Changsha 410012, China

Received 8 February 2007; accepted 22 August 2007

Abstract: A fine and platelet tungsten carbide patterned structure with fine yttrium containing dispersed phase was observed in liquid phase sintered WC-20%Co-1%Y₂O₃ cemented carbide with ultrafine tungsten carbide and nano yttrium oxide as starting materials. By comparing the microstructures of the alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it is shown that hexagonal and truncated trigonal plate-like WC grains are formed through the mechanism of dissolution-precipitation (recrystallization) at the stage of liquid phase sintering. Yttrium in the addition form of oxide exhibits good ability in inhibiting the discontinuous or inhomogeneous WC grain growth in the alloy at the stage of solid phase sintering.

Key words: cemented carbide; platelet tungsten carbide grain; grain growth inhibition; yttrium; sintering

1 Introduction

The concept of whisker and platelet reinforcement is well known in the field of advanced ceramics with improved toughness[1]. It has been demonstrated that this idea can be applied successfully as well to WC-based cemented carbide. Trigonal or truncated trigonal WC platelets were shown to form in-situ by reaction sintering processes or recrystallization[2-5]. Compared with the conventional platelet-free cemented carbide, optimized platelet-containing cemented carbide showed superior hardness and toughness combinations, improved wear and impact resistance, improved high-temperature creep resistance[2-3,5]. In this work, hexagonal plate-like WC grains were formed in situ through the mechanism of dissolution-precipitation (recrystallization) at the stage of liquid phase sintering of WC based cemented carbide, and inhomogeneous grain growth which is a very vexing problem in cemented carbide with ultrafine or nano tungsten carbide as starting material during the early-stage densification was suppressed by the addition of yttrium oxide.

2 Experimental

2.1 Starting materials

Tungsten carbide powder with FSSS (Fisher subsieve sizer) of 0.51 μm and total carbon content of 6.09% (mass fraction), and cobalt powder with FSSS of 0.93 μm and Y₂O₃ powder, 20 nm in single particle determined by BET specific surface area, were used as the starting materials of WC-20%Co-1%Y₂O₃ (mass fraction) cemented carbide.

2.2 Alloy preparation

As it is reported before[6-7], high efficiency of grinding and dispersion during ball-milling can be achieved via the surfactant effect of multicomponent organic medium and the particle-stabilization effects of surfactant and macromolecule. Based on the principle mentioned above, a new cemented carbide slurry preparation technique was developed. Tungsten carbide powder, cobalt powder, Y₂O₃ powder and paraffin wax based macromolecule compound were wet milled in multicomponent organic medium for 72 h with mass ratio of milling media (cemented carbide ball) to powder of 6?1. The liquid medium in the colloidal slurry was evaporated under vacuum.

Hot-press at the temperature of 1 260 ℃ for 60 min which is below the eutectic temperature was carried out by uniaxial hot press under vacuum. Liquid phase sintering was carried out at the temperature of 1 470 ℃ for 90 min in a conventional vacuum sintering furnace.

2.3 Microstructure observation and analysis

Polished sample was first observed by Olympus PMG3 optical microscope(OM) to check if there exists graphite phase, then was etched in sequence by: 1) the mixture of 10% KOH (mass fraction) and 10% K₃Fe(CN)₆ with equal volume for 5-10 s to detect whether η phase exists; 2) the mixture of 20% KOH and 20% K₃Fe(CN)₆ with equal volume for 3-4 min; 3) the saturated solution of FeCl₃ in concentrated HCl for 20-30 s to give good contrast between WC grains and the binder.

For stereographic observation of WC grains, the WC grains in the sintered compacts were extracted by removing the cobalt matrix with boiling hydrochloric solution. Cobalt residue was removed by dilute hydrogen peroxide solution and the extracted WC grains were rinsed sequentially with distilled water and acetone.

JEOL JSM-6360 LV scanning electron microscope (SEM) was used in the observation of the microstructure of the extracted WC grains. EDAX Genesis 2000 energy dispersive X-ray spectroscopy(EDXS) was used in the qualitative analysis of the dispersed phase in the alloy.

3 Results and discussion

3.1 Microstructure and dispersed phase analysis

The microstructures of WC-20%Co-1%Y₂O₃ cemented carbide prepared by hot-press and by conventional liquid phase sintering are given in Fig.1 and Fig.2, respectively. Neither graphite phase nor η phase was observed in the alloy. It can be observed from Fig.1 that WC grains in the alloy by hot-press below the eutectic temperature are fine and homogenously distributed. Abnormal grain growth which is a very vexing problem in cemented carbide with ultrafine or nano tungsten carbide as starting material[8-10] was not observed. Accordingly, Y₂O₃ exhibits good ability in inhibiting both the continuous and discontinuous WC grain growth at the solid phase sintering stage. Nevertheless, a fine and platelet tungsten carbide patterned structure was observed in the liquid phase sintered sample. The maximum length of the platelet WC grains is as large as 10 μm, shown in Fig.2. Obviously, the ability of Y₂O₃ to inhibit the continuous grain growth can still remain to a considerable extent; however, it becomes ineffective in controlling the preferential grain growth at the liquid phase sintering stage.

Fig.1 SEM microstructure of WC-20%Co-1%Y₂O₃ cemented carbide by hot-press below eutectic temperature (not etched by saturated solution of FeCl₃ in concentrated HCl)

Fig.2 OM microstructure of WC-20%Co-1%Y₂O₃ cemented carbide by liquid phase sintering (additionally etched by saturated solution of FeCl₃ in concentrated HCl)

Liquid phase sintered sample which was not etched by saturated solution of FeCl₃ in concentrated HCl was observed by OM and SEM. The microstructure is shown in Fig.3. Along with WC and the binder phase, a fine dispersed phase (the dark one) with an average grain size less than 1 μm is observed. This dispersed phase cannot be identified if the binder is etched by saturated solution of FeCl₃ in concentrated HCl as shown in Fig.2, and has not been detected in the solid phase sintered sample. EDX spectra of the dispersed phase in three different positions indicated by the arrows in Fig.3(b) are given in Fig.4. Y, O, W, Co and C are detected in all the positions. Based on EDXS analysis results and the fact that although the addition amount of Y₂O₃ in the alloy is only 1%, the dispersed phase is in considerable amount, it can be deduced that yttrium has a very strong ability to combine the elements in the alloy to form a new phase during the liquid phase sintering process. Considering the fact that the subjacent and adjacent phases, i.e. WC and cobalt based solid solution binder may play an effect on the EDX analysis result of the third phase, it is hard to determine the exact composition of the third phase in this way.

Fig.3 Microstructures of WC-20%Co-1%Y₂O₃ cemented carbide by liquid phase sintering: (a) OM; (b) SEM

Fig.4 EDX spectra of dispersed phase in three different positions shown in Fig.3(b): (a) Position 1; (b) Position 2;  (c) Position 3

3.2 Morphology of WC crystal

The stereographic morphologies of WC grains extracted by removing the cobalt matrix with boiling hydrochloric solution are shown in Fig.5.

Fig.5 SEM morphologies of WC crystal in WC-20%Co-1%Y₂O₃ cemented carbide by liquid phase sintering at different magnification

Tungsten carbide has a hexagonal crystal structure with an aspect ratio (c/a), a ratio of c-axis length against a-axis length of 0.976 (a=0.290 6 nm and c=0.2 837 nm). Fig.6 shows the models of the three-dimensional morphologies of well crystallized WC grains in WC based cemented carbide, i.e. conventional trigonal prism (a), plate-like trigonal prism with an aspect ratio of much less than 0.976 (b) and truncated trigonal prism (c), as well as the plate-like hexagonal prism (d). From Fig.5, it can be observed that along with the conventional truncated trigonal and irregular WC grains, there exist large hexagonal plate-like WC grains, indicated by the arrows. It is reported[11-12] that hexagonal plate-like WC grains can be formed by a shock wave technique used in the synthesis of tungsten carbide powder through a vapor—liquid—solid(VLS) growth mechanism. However, by comparing the microstructures of the  alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it can be concluded that the hexagonal plate-like WC grains in the alloy can be formed through the mechanism of dissolution-precipitation at the stage of liquid phase sintering. The recombination of Y, O and C/W/Co atoms results in the forming of the complicated yttrium-containing fine dispersed phase, which can lead to the change of the interface energy between the WC solid phase and the liquid binder phase. It can be deduced that the formation of the well crystallized hexagonal plate-like WC grains is related with the change of the interface energy, which plays a critical role in determining the WC grain growth shape[13-15] as well as enhanced dissolution-precipitation process caused by the high sintering temperature.

Fig.6 Models of three-dimensional morphologies of well crystallized WC grains in WC based cemented carbide: (a) Conventional trigonal; (b) Plate-like trigonal; (c) Truncated trigonal; (d) Plate-like hexagonal

It is shown[16] that when the Co/WC interfaces are coherent, the morphology of WC grains in WC-Co cemented carbide is hexagonal. As it is known that the strength of coherent interface is much higher than that of the incoherent interface, it can be deduced that optimized hexagonal platelet-containing cemented carbide must show high crack propagation resistance ability and thus high fracture toughness.

Platelet tungsten carbide is characterized by its higher surface area ratio of (0001) plane to (1100) plane than conventional tungsten carbide. Because the hardness of (0001) plane is approximately twice as large as that of (1100) plane, platelet-containing cemented carbide shows higher hardness than the conventional cemented carbide when the grain size and cobalt content are comparative.

4 Conclusions

1) Yttrium in the addition form of oxide exhibits good ability in inhibiting the WC grain growth in WC based cemented carbide with ultrafine starting materials at the stage of solid phase sintering. At the stage of liquid phase sintering, the ability of yttrium containing compound to inhibit the continuous grain growth can still remain; however, it becomes ineffective in controlling the preferential grain growth.

2) Yttrium exhibits strong ability to combine the elements from the alloy during the liquid phase sintering processes, forming a complicated yttrium containing fine dispersed phase.

3) Well crystallized hexagonal plate-like WC grains can be formed in-situ by dissolution-precipitation (recrystallization).

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Foundation item: Project(50574104) supported by the National Natural Science Foundation of China

Corresponding author: ZHANG Li; Tel: +86-731-8876424; E-mail: zhangli@mail.csu.edu.cn

(Edited by YUAN Sai-qian)