Fabrication and performance of Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic cutting tools

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Abstract

Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic cutting tools with different thickness ratios were produced by hot pressing. The residual stresses inside the layered materials during fabrication were calculated by means of the finite element method. The mechanical properties at the outer layer of the layered materials were measured. The cutting performance of the layered tools was investigated and compared with an unstressed reference tool. Results showed that multilayered structure in AWT + AT layered ceramic materials can induce excess residual stresses during fabrication. These residual stresses are compressive in the AWT outer layer and tensile in the AT internal layer. The fracture toughness at the outer layer of the layered ceramic materials is greatly improved compared with that of the stress-free one. These multilayered tools can minimize the flank wear and edge chipping compared with the stress-free tool. The mechanisms responsible were determined to be the formation of compressive residual stress on the outer layer of the layered tools, which led to an increase in resistance to fracture. Thickness ratios were found to have a profound effect on the residual stresses, the fracture toughness, and the cutting performance of the layered tools.

Introduction

Ceramic cutting tools usually perform better in high speed machining and in the machining of high hardness workpiece materials as compared to high-speed steel and carbide tools. However, the use of Al2O3 ceramic cutting tools, even fully densified, may be limited by their properties, such as their low strength and fracture toughness. Since about 1970, Al2O3 ceramic cutting tools have improved remarkably. These improvements are mainly due to [1], [2]: (1) microstructures have been refined by controlling and improving manufacturing processes; (2) toughening mechanisms have been developed, such as whisker toughening and transformation toughening, thus improving the fracture toughness of ceramic tools; and (3) surfaces have been conditioned by the removal of cracks and irregularities. Considerable improvements have been achieved in tool properties such as flexural strength, fracture toughness, hardness, and wear resistance by incorporating one or more other components into the Al2O3 base material to form ceramic composite tool materials. The reinforcing component is often in the shape of particles or whiskers. Some of these tool materials, such as Al2O3/TiC, Al2O3/TiB2, Al2O3/ZrO2, Al2O3/Ti(CN), Al2O3/(WTi)C, and Al2O3/SiCw, have been used in various machining applications and offer advantages with respect to friction and wear behaviors [3], [4], [5], [6].

In the past decade, layered ceramics that take advantage of several strengthening mechanisms, such as crack deflection [7], independence on the initial flaw [8], introduction of weak interfaces [9], containment of martensitic transformation [10], or existence of porous layers [11], have been studied extensively. Among them, one of the most common mechanisms is the incorporation of residual stresses arising from thermal expansion coefficients mismatch, so that the surface layer is under compression. Several models and theoretical calculations, and experimental measurement methods have been proposed [8], [12], [13], [14], [15], [16] to determine the stress amount and distribution in laminated structures. These compressive stresses can increase the fracture strength, damage resistance, fatigue properties, as well as fracture toughness of layered ceramics [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. The effectiveness of laminated structures in improving the tribological properties has been reported by Tarlazzi et al. [29], [30]. Toschi et al. [31] showed that laminated hybrid structures can improve the sliding wear resistance of alumina. Portu et al. [32] reported that laminated structure with compressive residual stresses within the surface regions was a suitable way to obtain composite materials with superior abrasive wear resistance. Deng et al. [33], [34], [35], [36], [37], [38] suggested that layered structures in ceramic nozzles can improve their erosion wear resistance in abrasive air-jet machining. Nicola [39] produced an alumina/zirconia laminated cutting tool and found that laminated structures are effective in avoiding the microchipping on the flank zone. Silicon carbide whisker and titanium carbide particulate reinforced ceramic matrix composites have been designed as multilayer structures and fabricated into cutting tool inserts by Maurice et al. [40], which demonstrate improvements in strength, toughness, and thermal shock resistance compared to the conventional non-laminated ceramic composites.

Al2O3/(W,Ti)C and Al2O3/TiC ceramics are widely used in industrial applications such as cutting tools and dies [2], [3], [41], [42], they both have high hardness and wear resistance. These two materials have different thermal expansion coefficients; and different shrinkage during sintering. These differences are sufficient to induce residual stresses in the laminated structures made from these two materials. In the present study, Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic tool materials with different thickness ratios among constituent layers were produced by hot pressing in order to induce compressive residual stresses in the outer layer. The residual stresses inside these layered tool materials were calculated by means of the finite element method (FEM). The mechanical properties at the outer layers were measured. The cutting performance of the multilayered tools were investigated and compared with an unstressed reference tool.

Section snippets

Preparation of the Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic materials

Laminated hybrid structures constituted by alternate layers with different compositions can be properly designed to induce a surface compressive residual stress. The basic idea is to couple material layers with different thermal expansion coefficients (CTE) so that residual stresses arise during fabrication. Compressive residual stresses are induced in the layers with lower CTE. The materials selected in present study were Al2O3/(W,Ti)C (labeled as AWT) and Al2O3/TiC (labeled as AT). The reason

Microstructural characterization of the AWT + AT multilayered ceramic materials

The SEM micrographs of the cross-section surface of the AWT + AT multilayered ceramic materials with different thickness ratios (p) and number of layers (N) are shown in Fig. 1, Fig. 2, respectively, where the bright layers correspond to AWT and the dark ones to AT composition. The layered architectures can be clearly seen, the AWT and AT layers are all compact without voids, and are reasonably uniform and the interfaces are straight and well-distinguishable.

Closer examination at higher

Conclusions

Al2O3/(W,Ti)C + Al2O3/TiC multilayered ceramic tool materials with different thickness ratios among constituent layers were produced by hot pressing. The following conclusions were obtained:

  • 1.

    Multilayered structure in AWT + AT layered ceramic materials can induce excess residual stresses during fabrication. These residual stresses are compressive in the AWT outer layer and tensile in the AT internal layer. Thickness ratios were found to have a profound effect on the residual stresses. Increasing

Acknowledgement

This work was supported by the “Taishan Scholar Program of Shandong Province”, the “Outstanding Young Scholar Science Foundation of Shandong Province”, and the “National Basic Research Program of China (2009CB724402)”.

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