《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_alumina-BN.pdf_大学文库

1402 Y. Li et al. /Materials Research bulletin 37(2002)1401-1409 engineering ceramic components have complex shapes and hence, require machining by diamond tools. They cannot be machined using conventional metal tools such as cemented carbide, high-speed steel, etc. One of the significant disadvantages of ceramics is the high machining costs, which limits their application fields. It has been shown that rendering ceramics machinable usually caused significant decrease in fracture strength [1, 2] In recent years, attempts have been made to develop machinable ceramics with high fracture strength and high fracture toughness. Uno et al. [3 showed mica-based nanocomposite glass-ceramics, consisting of fine tetragonal zirconia particles, 20- 50 nm in size, embedded in plate-like mica crystals, which exhibited excellent mechanical properties with bending strength of 500 MPa, fracture toughness of 3.2 MPa mand good machinability. Baroum and El-Raghy [4]reported that Ti3SiC not only could be drilled and taped using high-speed steel tools, but also had a fracture strength up to 600 MPa. Its crystal structure was comprised of planar Si layers linked gether by TiC octahedra, and the microstructure was an analogous texture with plate shaped graphite, leading grains to be easily cleaved. Suganuma et al. [5]had fabricated machinable SiC/C ceramic composites, which still retained high strength over 200 MPa at 1500"C Niihara and co-workers [6, 7] reported Si3N4/BN nanocomposites with dispersed nano-sized h-BN. They claimed excellent machinability and high fracture strength(1100 MPa in max), which were attributed to the weak grain boundaries and nano-sized h-BN dispersions. These results imply that the super fine dispersoids with laminar structure and a low modulus could be expected to play a crucial role in improving the machinability of ceramics and its mechanical properties Since there has been no information about Al2O3-based machinable ceramics with both high strength and high fracture toughness, to our knowledge, we report in this paper the fabrication processes and mechanical properties of Al2O3/BN nanocom posite ceramics, and also discuss the machinable mechanism. 2. Experimental procedure In order to have homogeneous dispersion of h-BN into Al2O3 ceramics, a chemical process to precipitate the bn precursor on a-Al2O3 powders were adopted [8, 9]. In nis study, boric acid and an excess of urea were selected as the bn source to coat the a-AL2O3 particle surface. Fig. I shows schematically the experimental procedure. The Bn content was adjusted to be 10, 20 and 30 vol %o a-Al2O3 powders were ball milled with boric acid and urea in a plastic bottle with ethanol as medium using alumina balls. The slurry was dried at 60-65'C. Before drying, the electric-blower was used to evaporate most of the ethanol with mechanical stirring. The dried mixtures were kept at 1000C for 6 h in flowing hydrogen gas. The products were ball milled with the sintering aids (10 wt. %0 Si3N4, 0.5 um in grain size), and then after drying, the composite powders were hot-pressed at 1600-1800C in nitrogen gas. For compar ison, Al2O3 with 10 wt. Si3N4 was also hot-pressed at 1600-1800"C and pure Al2O3 powders at 1500"C in nitrogen gas

Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 1405 react with urea on the surface of a-Al2O3grains. It shows the disappearance of peaks of boric acid and urea, and the appearance of peaks arising from t-BN (20= 26.20 after reduction, which agrees with the XRd data of the mixtures comprising only boric acid and urea under the same condition. t-BN grain size was about 30 calculated using the Scherrer equation [10] according to the typical t-BN peak (20= 26.2%)in Fig. 2b. Additionally, Al-O-B compounds as semifinished product mainly containing Al18B4033, appeared during the reduction procedure h-Bn (from the peak of 1002) reflection, 20= 26.8%)appeared, taking the place of t-Bn during the hot-pressing process. Most of the Al18B4O33 disappeared in this step, as evidenced by the much lower intensity(I/lo) of the XRD peaks, compared to that before hot-pressing. Notably, Sialon phases(containing varied a-Sialon with different components) appeared in the hot-pressing step, and no Si3N4 peaks remained; while no Sialon phases appeared in Al2O3/10 wt %o Si3 N4 mic ites.However, Si3N4 peaks disappeared and glass phase peaks appeared arising from Sio2. It is indicated that the semifinished products, Alg B4O33, turned into Al2O3 and BN, and si3 N4 reacted with the matrix Al2O3 in the hot-pressing step. The estimated equations are expressed as following Al18B4O3+N2+6C→9A2O3+4BN+6CO↑ (1) xAlO3+ySi3 N4 - zSialon In order to obtain fully dense bodies at lower temperatures, a small quantity of Si3n4 was added into the composites as sintering aids. The dispersion of nano-sized BN made sintering temperatures rise due to its inhibition to Al2O3 boundary migration. Furthermore, if Si3 N4 is not added into the sintering bodies, macro-cracks ill emerge when BN content is more than 20 vol %, which leads to poor strength This was mainly caused by a difference in the coefficient of thermal expansion between a-Al2O3 and h-BN, in our point of view. The coefficient of thermal expansion of a-Al2O3 is 8 x 10-/C, but that of h-BN is-2.9 x 10/C in a-axis direction and 40.5 x 10-b/oC in c-axis direction, respectively. The tensile stress that originated at the interfaces between the matrix and the dispersoid accumulated with the increase of bn volume fraction and, finally, resulted in destructive cracking. Microcracks were also observed in Si3//BN nanocomposites [11]. The formation of Sialon phases in the composite ceramics contributed to reduce thermal expansion due to its lower coefficient of thermal expansion. < Fig 3a shows a representative TEM picture for the reduced powders. It is observed hat the al-Al2O3 particles are surrounded with a disordered layer of t-BN, which was identified using XRD and EDS Fig 3b shows the corresponding SEM micrograph of polished and etched surface of hot-pressed body. The surface is parallel to the hot- pressing direction Matrix grains are regular, about 1 um in size. Coarse grains larger than 2 um were not found. BN grains 100-200 nm in diameter can be seen dispersed within the matrix grains as well as club-shaped bn grains 400-600 nm in length, 100-200 nm in width at the grain boundaries. No evident directional alignment for bN grains was observed in the materials for the surface morphology perpendicular to the hot-pressing direction being similar to that of the parallel one

Y. Li et al. /Materials Research bulletin 37(2002)1401-1409 (b) t-BN a-Al2O3 Fig. 3. Micrographs from: (a) TEM of Al2O3/BN nanocomposite powders; and(b) SEM of the polished surface from Al,O/30 vol. BN nanocomposite ceramics. 3.2. Mechanical properties Variations in fracture strength and fracture toughness with BN volume fraction for AL2O3/BN nanocomposite ceramics are plotted in Fig. 4. The fracture strength of composite ceramics in this work decreased with an increase of BN content. Even so, the materials exhibited a high strength of 478 MPa with BN content up to 20 vol %, which was approximately 85%0 strength of Al2O3 monolithic ceramics. While the hardness was only 60% of that(8 10 GPa) for the composite and 13.52 GPa for Al2O3 monolith Considerably low strength and toughness for Al2O,Si3N4 microcomposite ceramics were caused by a large amount of Sio2 glass in the material after hot-pressing Based on linear fracture mechanics concept of brittle ceramics, fracture strength of ceramics is controlled by fracture toughness and the size of critical flaws introduced by processing defects, such as large pores, agglomerates of addition phases and abnormally grown grains. Soft h-Bn particles with layer-structures can relax stress and absorb energy at the crack tip through microcracking or crack-particle interac tion,and then prevent the main crack from extending which is propitious to improve fracture toughness. BN-toughened oxide composites have been studied systemically [12, 13]. Thus, as shown in Fig 4, the fracture toughness of Al2O3/BN nanocomposite ceramics increases with BN content and a maximum toughness of 5.5 MPa m-is obtained. Although, the addition of bn no doubt introduces faws and influences strength of the materials to some degree, a remarkable decrease in fracture strength has not been observed in this work because the fracture toughness was improved due to fine h-Bn dispersions 3. 3. Mach The Al2O3/BN nanocomposite ceramics containing BN, no less than 20 vol % in this work, exhibited excellent machinability, which is very important and has practical applications for engineering ceramics. Machinability improved with the increase of bn volume fractions or the decrease of vickers hardness

《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_alumina-BN

《复合材料 Composites》课程教学资源（学习资料）第五章陶瓷基复合材料_alumina-BN