Effects of TiC content and melt phase on microstructure and mechanical properties of ternary TiB2-based ceramic cutting tool materials

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Abstract

Ternary TiB2–WC–TiC ceramic composites were fabricated by hot-pressed sintering at 1650 °C. Sintering additives such as WC, TiC, Mo, Ni and Co were used to create a liquid phase and promote densification. The effect of TiC content, (Mo,Ni) and Co on microstructure and mechanical properties of the ternary TiB2–WC–TiC ceramic composites was presented. Flaw structures such as pores and microcracks reduced as the TiC content increased. Wettability of (Mo,Ni) and Co to TiB2 had a significant influence on the microstructure. The microstructure of these composites showed a typical core/rim structure. The core was TiB2 and the rim was mainly composed of TiC. The relative density, microhardness, flexural strength and fracture toughness of these composites increased as the TiC content increased. Results indicated that ternary TiB2–WC–30 wt.% TiC–(Mo,Ni) ceramic composite provided the optimal combination of dense microstructure and mechanical properties, including the relative density of (99.3±0.3)%, the microhardness of 24.8±0.3 GPa, the flexural strength of 946.9±24.9 MPa, and the fracture toughness of 7.4±0.2 MPa·m1/2. The high fracture toughness of ternary TiB2–WC–30 wt.% TiC–(Mo,Ni) ceramic composite was due to an intensive coupled mechanism of the near-full density, the typical core/rim structure, crack deflection, crack bridging, crack branching and pull-out by a large number of fine WC grains.

Introduction

As one of the most promising high-temperature ceramic materials, TiB2 has attracted much attention for its unique combination of properties including exceptionally high hardness, high melting point and strength retention at elevated temperatures, good thermal and electrical conductivities and high wear resistance [1], [2], [3], [4]. This unique combination of properties renders TiB2-based materials suitable for a wide range of technological applications, such as armour materials, wear components and conductive coatings, and so on. However, like most other kinds of ceramics, TiB2 has a tendency toward low flexural strength and low fracture toughness, which remains an obstacle to its more widespread utilisation. TiB2-based ceramic composites can be reinforced with various oxides, carbides, melts, nitrides and borides such as Al2O3, SiC, TiC, WC, Ni, Mo, TiN, MoSi2, ZrB2 and B4C [5], [6], [7], [8], [9], [10], [11], [12]. Among reinforcements, WC, TiC and melts such Ni, Mo have attracted much attention due to their superior properties. WC and TiC have high hardness, good corrosion resistance and good chemical stability [13], [14]. TiB2-based ceramic composites are being produced by different methods, such as the vacuum hot-pressed sintering technique [15], the combustion synthesis technique [16], the spark plasma sintering technique [17] and the pressureless sintering technique [18]. Among these methods, the vacuum hot-pressed sintering technique is considered to be easily adaptable and economically viable due to its low processing cost and high productivity. Also, the vacuum hot-pressed sintering technique is an attractive processing method since it offers a wide selection of materials and processing conditions. In our previous work, TiB2–TiC–WC composites have been fabricated by the vacuum hot-pressed sintering technique and the raw materials of these composites were composed of TiB2–25 wt.% TiC composite powder, WC powder and Ni powder [19]. In this paper, ternary TiB2–WC–TiC composites will be fabricated with TiB2, WC, TiC and melts such as Mo, Ni and Co by the vacuum hot-pressed sintering technique. The characteristics of the composites are addressed according to their microstructure and mechanical properties.

Section snippets

Experimental procedures

Commercially available TiB2 powder, whose average particle size and purity were 1.5 μm and 99.32%, respectively, was used as the raw material. The average particle size and purity of WC were 0.6 μm and 99%, respectively. The average particle size and purity of TiC were 500 nm and 99%, respectively. Ni, Mo and Co powders with an average particle size of 2.3 μm and a purity of 99% were added as sintering additives. The compositions of materials are shown in Table 1.

The powders were mixed and milled

Microstructure

Fig. 1 shows XRD patterns of ternary TiB2–WC–TiC composites. The major crystal phases in the composites are TiB2, TiC and WC except for minor by-products. By-products such as W2C, Ni4B3 and MoNi4 are present in the ternary TiB2–WC–TiC–(Mo,Ni) composite in Fig. 1(a). Formation of W2C and Ni4B3 arises from the dissolution of TiB2 and WC in the liquid nickel at the sintering temperature of 1650 °C which is higher than the melting point of Ni(Tm=1453 °C). Ni4B3 is commonly formed in the sintering

Conclusions

Ternary TiB2–WC–TiC ceramic composites were fabricated by hot-pressed sintering at 1650 °C. Sintering additives such as WC, TiC, Mo, Ni and Co were used to create a liquid phase and promote densification. The effect of TiC content, (Mo,Ni) and Co on microstructure and mechanical properties of the ternary TiB2–WC–TiC ceramic composites was presented. Flaw structures such as pores and microcracks reduced as the TiC content increased. Wettability of (Mo,Ni) and Co to TiB2 had a significant

Acknowledgements

This project is supported by the National Natural Science Foundation of China (51005136 and 51175305), Key Special Project of Numerical Control Machine Tool (2012ZX04003-051), Youth Foundation of Taiyuan University of Technology (No. 2013Z059) and Qualified Personnel Foundation of Taiyuan University of Technology (QPFT) (No. tyut-rc201232a).

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