PREPARATION AND PROPERTIES OF Cr 2 N-Al 2 O 3 NANOCOMPOSITES

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1 PREPARATION AND PROPERTIES OF Cr 2 N-Al 2 O 3 NANOCOMPOSITES Lian Gao 1 and Yaogang Li 2 1 State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai People s Republic of China 2 Institute of Materials Science and Engineering, Donghua University, Shanghai ABSTRACT Nanosized Cr 2 N reinforced Al 2 O 3 composites were prepared by hot pressing of nano CrN-Al 2 O 3 powders. Dense sintered bodies were obtained by hot pressing at 1400 o C-1550 o C and 30 MPa for 60 min. X-ray diffraction (XRD) analysis indicated that the CrN converted to Cr 2 N during the sintering process. Microstructure studies found that the Cr 2 N particles were uniformly dispersed in the Al 2 O 3 matrix. The bending strength of the Cr 2 N-Al 2 O 3 nanocomposite with the addition of vol.% Cr 2 N hot pressed at 1400 o C reached 848 MPa, and the fracture toughness of samples was over 4 MPam 1/2. 1. INTRODUCTION Alumina (Al 2 O 3 ) with its unique combination of great hardness, corrosion resistance, thermal stability, and economic advantages is the material that acquired the best acceptance in industry [1]. Alumina ceramics are essential structural materials, but lower mechanical properties limit their applications [2]. In recently years there has been increased interest in the area of ceramicceramic composites. A primary goal for this research has been to develop a new generation of ceramics having higher strength, better toughness, and greater reliability [3]. Al 2 O 3 -based composites could find applications as wear-resistant materials and cutting tools if appropriate second phase are added [4]. Since Niihara reported that nanocomposites reinforced with sub-micrometer or nanometer second phase show excellent mechanical properties [5], Al 2 O 3 -based nanocomposites have been widely studied. For alumina ceramics, reinforcements with SiC, TiC, TiN, or metals are usually used. Al 2 O 3 -SiC composites hold great promise for applications as structural components and as wear-resistant elements [6,7]. Niihara [5] reported that the strength of the nanocompsite with addition of 5 vol.% of SiC particles increases from 320 to 1050 MPa, and its fracture toughness increases from 3.2 to 4.7 MPa m 1/2. Al 2 O 3 -TiC composites with high wear resistance, high strength and fracture toughness, and good electrical conductivity have been the focus of significant attention primarily because of their potential application in cutting tools and tape-head materials [8]. Various methods have been used to prepare Al 2 O 3 -TiC composites [8,9]. The properties of titanium nitride (TiN) particles reinforced alumina composites have been widely studied, because the addition of an electroconductive second phase to the matrix was reported not only to improve some mechanical properties but also to lower drastically the electrical resistivity [10,11]. Li et al [11] reported that the resistivity of Al 2 O 3 -TiN nanocomposites reaches Ω cm at 25 vol.% TiN additions. The addition of 20 vol.% TiN could increase the bending strength of hot-pressed Al 2 O 3 from 370 MPa to 725 MPa and the fracture toughness from 3.40 to 5.27 MPa m 1/2, respectively. Chromium nitride (CrN) has outstanding physical and mechanical properties and has been identified as a better coating material than currently used titanium nitride (TiN) for wear applications [12]. On the other hand, little attention has been paid to the fabrication and mechanical properties of CrN ceramics. Recently, CrN/ZrO 2 (2Y) composites, chromium nitride 1

2 (Cr 2 N and CrN) ceramics have been prepared by hot isostatic pressing [13,14]. By our knowledge, no study has been concerned on the chromium nitride reinforced Al 2 O 3 composites. In this paper, chromium nitride reinforced Al 2 O 3 nanocomposites with the addition of vol.% of Cr 2 N were obtained by hot pressing for the first time. For this system, mechanical properties were measured and related to the composition and microstructure. 2. EXPERIMENTAL PROCEDURE 2.1 Sample preparation Four compositions were chosen for this study (Table I). Nanosized CrN powder with particle sizes of nm was synthesized by the direct nitridation of nanosized Cr 2 O 3 powder in NH 3 flow gas at 800 o C for 8 h[15]. Commercial available pure α-al 2 O 3 (~60 nm, >99.9%, HFF25, Wusong Fertilizer Factory of Shanghai, China) was used as received. The α-al 2 O 3 and nano-crn powders were mixed in calculated proportion and homogenized by ball milling with silicon nitride balls using anhydrous ethanol as the liquid medium for 24 h. CrN-Al 2 O 3 nanocomposite powders with 5-20 vol.% CrN were sintered by hot-pressing between 1350 and 1550 o C and a pressure of 30 MPa for 60 min under N 2 atmosphere. 2.2 Characterization The phase composition of the powders and the sintered samples was identified by X-ray diffractometry (XRD; Model D/MAX 2550V, Rigaku Co., Tokyo, Japan) with Cu-Kα (λ =1.5418Å) radiation at 40 KV and 60 ma over the scan range of 10 o to 70 o at the room temperature. The density of the sintered samples was measured using the Archimedes method. The theoretical density value of the samples was calculated according to the rule of mixtures using 3.97 g cm -3 for Al 2 O 3 and g cm -3 for Cr 2 N, assuming that no chemical reactions take place between the matrix material Al 2 O 3 and the second phase chromium nitride. For the mechanical testing, the hot pressed samples were cut and ground into rectangular bar specimens ( mm). The bending strength was measured with Instron-5566 using the three point bending test with a span length of 20 mm and crosshead speed of 0.5 mm/min. Measurements of the Vickers hardness and the fracture toughness were conducted with Akashi(AVK-A) by indentation using a pyramidal indenter and applying a 10 kg load for 10 s. The microstructure of the sintered bodies was observed using field emission scanning electron microscopy (FE-SEM; Model JSM-6700F, JEOL, Tokyo, Japan). 3. RESULTS AND DISCUTION 3.1 XRD identification of CrN-Al 2 O 3 nanocomposite powder and sintered composite Fig. 1 shows the XRD pattern of CrN-Al 2 O 3 nanocomposite powder with 15 vol.% CrN. The diffraction peaks corresponding to the cubic CrN and α-al 2 O 3 can be observed, which indicated that the mixture contains CrN and α-al 2 O 3 phases. Fig. 2 is the XRD pattern of a chromium nitride reinforced Al 2 O 3 sintered body with addition of 15 vol.% CrN hot pressed at 1400 o C. No diffraction peaks corresponding to the cubic CrN was observed, while distinct peaks assigned to Cr 2 N were detected. This result indicated that no chemical reaction occurred between the second phase and the matrix, but the CrN decomposed to Cr 2 N during the sintering process. The asprepared chromium nitride reinforced Al 2 O 3 composites are composed of Cr 2 N and α- Al 2 O 3 phases. The as-sintered sample is a Cr 2 N-Al 2 O 3 nanocomposite. 2

3 Al 2 O 3 CrN Intensity / (a.u.) θ / ( ο ) Fig. 1. X-ray diffraction patterns of the CrN-Al 2 O 3 nanocomposite powder with 15 vol.% CrN Al 2 O 3 Cr 2 N Intensity / (a.u.) θ / ( ο ) Fig. 2. X-ray diffraction patterns of the Cr 2 N-Al 2 O 3 nanocomposite with vol.% addition of Cr 2 N hot pressed at 1400 o C 3.2 Densification of sintered composites Table 1 shows the relative density of the samples sintered at 1400 o C and 30 MPa for 60 min, which demonstrates that nearly fully dense samples could be obtained by hot pressing at 1400 o C for Cr 2 N reinforced Al 2 O 3 composites. This densification temperature is lower than that needed for other composite system. The result indicates that CrN-Al 2 O 3 nanocomposite powders have good sinterability. 3

4 Table 1. Chemical Compositions of Starting Powders and As-Sintered Samples Composition (vol.%) Starting powders Al2O CrN Density Sintered samples Al2O Cr2N Theoretical density (g/cm3) Bulk density (g/cm3) Relative density (%) Morphology observation of polished surface and fracture crosssections Fig. 3 is the backscattered electron image of the polished surface for the composite with the addition of vol.% Cr2N hot pressed at 1450oC. Black grains are Al2O3, while white particles located among Al2O3 grains are Cr2N. It could be seen that Cr2N particles were homogenously distributed in the alumina matrix. Fig. 3. Back-scattered SEM micrograph of polished surface of vol.% Cr2N-Al2O3 nanocomposite Fig. 4 shows the microstructure of the fracture cross-sections of Cr2N-Al2O3 nanocomposites with different Cr2N contents. It can be seen that the grain size of Al2O3 was decreased with the increasing of the Cr2N content. The addition of Cr2N had effectively inhabited the grain growth of Al2O3. The fracture mode of Cr2N-Al2O3 composites with 4.25 vol.% Cr2N was mainly intergranular. The fracture modes of Cr2N-Al2O3 composites with vol.% Cr2N were intergranular and transgranular. With the increasing of the Cr2N content, the grain boundaries in Cr2N-Al2O3 composites were strengthed, inhabiting intergranular crack propagation, which resulted the change of the fracture modes. The transgranular fracture mode is beneficial to the 4

5 improvement of the bending strength. The micrographs also show that the specimens are fully densified. Fig. 4(c) and 4(d) are the backscattered electron images corresponding to the Fig. 4(a) and 4(b). From Fig. 4(c), it can be seen that Cr2N (white particles) with particle size of nanoscale distributed in the alumina matrix (black particles) for the Cr2N-Al2O3 composites with 4.25 vol.% Cr2N. With the increasing of the Cr2N content, the size of the Cr2N particles increased. Both of nanometer and sub-micrometer scale Cr2N particles distributed in the alumina matrix for the Cr2N-Al2O3 composites with vol.% Cr2N. From the microstructures, it can be seen that the Cr2N particles have good adhesive nature with the alumina matrix, which is beneficial to obtain high-density and high-strength composite. (a) (b) (c) (d) Fig. 4. Microstructures of Cr2N-Al2O3 nanocomposite (a) SEM micrograph and (b) backscattered image of the fracture surface of 4.25 vol.% Cr2N-Al2O3 nanocomposite, (c) SEM micrograph and (d) backscattered image of the fracture surface of vol.% Cr2N-Al2O3 nanocomposite 3.4 Mechanical properties The Vickers hardness measurement of the composites indicated that the samples sintered at 1400oC had the highest hardness of 19 GPa. The conversion of CrN to Cr2N is beneficial to increase the hardness of the composites [14]. Fig. 5(a) shows the bending strength of the composites with addition of vol.% Cr2N versus the sintering temperature. The highest 5

6 value achieved for the sample sintered at 1400 o C is 848 MPa, much higher than that of monolithic Al 2 O 3 ceramics. Fig. 5(b) showed the fracture toughness versus the sintering temperature. As compared with that of Al 2 O 3 ceramic, the fracture toughness values of the composites were also improved. The addition of Cr 2 N particles improved the microstructure and mechanical properties of the alumina matrix. Bending strength / (MPa) (a) (b) Fracture toughness / (MPam 1/2 ) Sintering Temperature / ( o C) Fig. 5. The bending strength and fracture toughness of the composites with addition of vol.% Cr 2 N versus the sintering temperature 4. CONCLUSIONS Dense Al 2 O 3 -Cr 2 N composites with high bending strength were successfully prepared for the first time. Experimental results indicated that nano Al 2 O 3 -CrN mixture powders prepared by ball milling could be sintered to dense at relatively low temperature by the hot pressing method. CrN phase converted to Cr 2 N during hot pressing process. The bending strength of the prepared composites has been greatly improved and both the fracture toughness and the hardness of the composites increased. References 1. Krell, A. and Klaffke, D., Effects of Grain Size and Humidity on Fretting Wear in Fine-Grained Alumina, Al 2 O 3 /TiC, and Zirconia, J. Am. Ceram. Soc., 79/5 (1996) Li J. G., Gao L. and Guo J. K., Mechanical Properties and Electrical Conductivity of TiN-Al 2 O 3 Nanocomposites, J. Europ. Ceram. Soc., 23 (2003) McCluskey, P. H., Williams, R. K., Graves, R. S. and Tiegs, T. N., Thermal Diffusivity/Conductivity of Alumina-Silicon Carbide Composites, J. Am. Ceram. Soc., 73/2 (1990) Bellosi, A. G., Portu, D. and Guicciardi, S., Preparation and Properties of Electroconductive Al 2 O 3 -Based Composites, J. Europ. Ceram. Soc., 10 (1992) Nihara, K., New Design Concept of Structural Ceramics-Ceramics Nanocomposites, J. Ceram. Soc. Jpn., 99 (1991) Yang, Q. and Troczynski, T., Alumina Sol-Assisted Sintering of SiC-Al 2 O 3 Composites, J. Am. Ceram. Soc., 83/4 (2000) Sternitzke, M., Review: Structural Ceramic Nanocomposites, J. Europ. Ceram. Soc., 17 (1997) Xia, T. D., Munir, Z. A., Tang, Y. L., Zhao, W. J. and Wang T. M., Structure Formation in the Combustion Synthesis of Al 2 O 3 -TiC Composites, J. Am. Ceram. Soc., 83/3 (2000)

7 9. Goldstein, A. and Singurindi, A., Al 2 O 3 -TiC Based Metal Cutting Tools by Microwave Sintering Followed by Hot Isostatic Pressing, J. Am. Ceram. Soc., 83/6 (2000) Rak Z. S. and Czechowski, J., Manufacture and Properties of Al 2 O 3 -TiN Particulate Composites, J. Europ. Ceram. Soc., 18 (1998) Li, J. G., Gao L., and Guo, J. K., Mechanical Properties and Electrical Conductivity of TiN-Al 2 O 3 Nanocomposites, J. Europ. Ceram. Soc., 23 (2003) Lai, Y. D. and Wu, J. K., Structure, Hardness and Adhension Properties of CrN Films Deposited on Nitrided and Nitrocarburized SKD 61 Tool Steel, Surf. Coat. Technol., 88 /1-3 (1996) Takano, Y., Hachiya, M., Yoshinaka, M., Hirota, K. and Yamaguchi, O., Processing and Mechanical Behavior of CrN/ZrO 2 (2Y) Composites, J. Am. Ceram. Soc., 83/2 (2000) Hirota, K., Takano, Y., Yoshinaka M. and Yamaguchi O., Hot Isostatic Pressing of Chromium Nitrides (Cr 2 N and CrN) Prepared by Self-Propagating High-Temperature Synthesis, J. Am. Ceram. Soc., 84/9 (2001) Li Y. G., Gao L., Li, J. G. and Yan, D. S., Synthesis of Nanocrystaline Chromium Nitride Powder by Direct Nitridation of Chromium Oxide, J. Am. Ceram. Soc., 85/5 (2002)