Investigation on carbonizing behaviors of nanometer-sized Cr 2 O 3 particles dispersed on alumina particles by metalorga
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Wen-Cheng J. Wei Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China (Received 30 October 2004; accepted 20 April 2005)
Nanoscaled Cr2O3 powder with an average particle size of 20–40 nm, coated on alumina particles, has been produced by means of chemical vapor deposition (CVD) in a fluidized chamber, using the pyrolysis of Cr(CO)6 precursor. Amorphous and crystalline Cr2O3 particles were obtained when the temperatures of the pyrolysis were 300 and 400 °C, respectively. To prepare nanoscaled Cr3C2 powder from the nanometer-sized Cr2O3, carbonizing behavior of the Cr2O3 particles was investigated. It was found that, when amorphous Cr2O3 powders were carbonized in graphite furnace at 1150 °C for 2 h in vacuum (10−3 Torr), the powder was transformed into Cr3C2, while the crystalline Cr2O3 was transformed into a mixture of Cr7C3 and Cr3C2. The examinations by x-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy confirmed the transformation of the nano-sized Cr3C2 powders. The results of thermogravimetry and differential thermal analysis indicated that the transformation temperature was ∼1089 °C for amorphous Cr2O3 and ∼1128 °C for crystalline Cr2O3.
I. INTRODUCTION
Alumina has been broadly used in structural ceramic applications such as wear, cutting tools, balls, catalysts, spark plugs, etc. because of its excellent mechanical properties, good chemical stability, and high-temperature characteristics.1,2 Its intrinsic brittleness and relatively poor reliability, however, made the improvement of the toughening of alumina ceramics to be an important and challenging area. The incorporation of secondary phases (e.g., particulates, fibers, or platelets) has been proven to be an easy, safe, and economical toughening technique for alumina ceramics.3–5 Among those approaches, incorporating chromium carbide (Cr3C2) particles into Al2O3 matrix has been successfully achieved for toughening purposes.6–9 Other materials such as SiC reinforced particles on Al2O3 composites mostly improve the mechanical properties, but the toughness enhancement (3.5 ∼ 5.8 MPa.m1/2) is not significant.10–12 Therefore, to improve the toughness behavior, Cr2O3 dispersed in the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0268 2154
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 8, Aug 2005 Downloaded: 14 Mar 2015
alumina matrix (4.8–7.5 MPa m1/2) was carbonized. The advantages of adding nanometer inclusions have been reported by Niihara to achieve several modifications,10 such as the reduction of grain size of matrix grains, strengthening, and toughening. When traditional mixing techniques are used for preparing particle-reinforced composites, nanoscale reinforcing particles are difficult to uniformly disperse on the micro-scale matrix particles. This problem is ascribed to the fact that nano-sized particles easily agglomerate due to the interaction between the particles.13 The agglome
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