Materials Research Bulletin Materials Research Bulletin 37(2002)1401-1409 Machinable Al2O3/bn composite ceramics with strong mechanical properties Yongli Li, Guanjun Qiao, Zhihao Jin State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University Xi'an 710049. PR China ( Refereed) Received 22 August 2001: accepted I May 2002 Abstract Al2O /BN composite ceramics with nano-sized BN dispersions ranging from 0 to 30 vol. were successfully fabricated by hot-pressing a-Al2O3 powders with turbostratic BN(t-BN coating, which was prepared through chemical processes using boric acid and urea. SEM observations revealed that the nano-sized hexagonal bn(h-BN) particulates were homoge- neously dispersed within Al2O3 grains as well as at grain boundaries. Vickers hardness of materials decreased with an increase in BN content. The fracture toughness was improved but the fracture strength had a small decrease, in comparison to Al2O3 monolithic ceramics. The nanocomposite ceramics with bn content more than 20 vol. exhibited excellent machin- ability, which could be drilled using conventional hard metal alloy drills. Drilling rates and normal forces demonstrate the ease of machining of these materials. The preliminary informa- tion on the relationship between microstructures and properties are provided. The mechanism of naterial removal is also discussed. C 2002 Elsevier Science Ltd. All rights reserved Keywords: A Ceramics; B Chemical synthesis; C Electron microscopy; D Mechanical properties; D Machinability 1. Introduction eramic materials like alumina have been widely used as engineering materials in automotive parts and cutting tools because of their high strength, high hardness high wear resistance, and abundant reserves in the earths crust. However, most Corresponding author E-mailaddress:waterle@hotmail.com(Y.Li) 0025-5408/02/S- see front matter C 2002 Elsevier Science Ltd. All rights reserved PI:S0025-5408(02)00786-9
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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
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Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 H3BO3+ CO(NH2)2 +a-SiaN(10wt Riddling(#300) Fig. 1. Flow chart of fabrication processes of the Al2O/BN composites The crystalline phases of the powder mixtures and hot-pressed bodies were identified by X-ray diffraction(XRD) analysis. The microstructure was observed by transmission electron microscope (TEM, JEM-200CX, Japan Electron Co. ) and scanning electron microscope (SEM, AMRAY-1000B, USA), respectively. The microchemical analysis was done using the energy dispersion spectroscopy(EDs) analyzer attached to the sem Bulk density was measured by the Archimedes immersion technique in distilled water. The Vickers hardness was measured by the vickers indentations method fracture strength was determined by a three-point bending test using a rectangular bar (mm x 4 mm x 25 mm) with the bend span of 20 mm and a crosshead speed of 0.5 mm/min. The tensile surface of specimens was parallel to the hot-pressing direction. The fracture toughness was measured using single edge notched beam (SENB) with three-point bending. The beam dimensions were 2 mm x 4 mm x 20 mm and the notch width and depth were 200 um and 1.5 mm, respectively. Each test for both or and Kic was done eight times in order to eliminate large occasional errors. Machinability was evaluated using hard metal alloy drill bits of two diameters under the varied conditions 3. Results and discussions 3.1. Phase compositions and microstructures Fig. 2 illustrates the transformation of crystalline phases in the processes of preparing the Al2O,BN composite ceramics through the chemical route using boric acid and urea as BN source. It can be observed in this figure that the starting powders before reduction in H, consisted of a-Al2O3, boric acid and urea boric acid does not
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Y. Li et al. /Materials Research bulletin 37(2002)1401-1409 (a) mixed powders (b)reduced powders (c)hot-pfessed body Fig. 2. XRD patterns of the Al2O3/30 vol. BN (a)mixed powders; (b)reduced powders and (c)hot- pressed body(●)aAl2O3;(口)tBN;(■)hBN:(◇) boric acid;(△)urea;(o) Sialon;(V) Al18B4O33
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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
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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
-*/* 2 2 4 % & % ( % % & L/ </ 6 % % 5 & & % / 2( 3 & % <;@ B ) (/0 3 , @?0 % / Q & , E)0 % #@/*) B' % & * /? B % / 8& , & % 1 < & , % 1 % - / & % % % % & , % & . % M & & , & % % && & , / 1% - , - 3 & , 5 5 5- - & ( 5 % 3 & ( % / - & 3& ( & & , , I* * J/ 6 L/ < % % & 3 % ?/? B * &/ && % & & M & M % & 5 & % ( & 5 % ( & & H - & / -*-* 6 ) (/0 5 3 & 3 , ( , & % / , ( & % ( % & % 45 & / L/ / %D #' 62 % & + & #' 12 % & % % ) (/0 / *<)E )* * + ! , - ./ /0 1
Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 ■- Fracture Strentgh 口- Fracture Toughness BN Content(vol‰%) Fig. 4. Effects of h-BN content on fracture strength and fracture toughness of Al2O3/BN composites. Two macrographs of a series of typical drill holes in the Al,O3/30 vol. %0 BN anocomposite ceramics are shown in Fig. 5. The holes are cleanly drilled with no evidence of cracking or chipping. The machinability was also tested with the sample bonded onto a piece of graphite liner, and the holes drilled without lubrication. Load force on the drill bit ranged from 1 to 12N and a fixed drill revolutions 700 rpm. Drilling depths were approximately 2 mm. Each test was done three times under the same conditions. A new drill bit was used at the beginning of each set to avoid systematic effects due to wearing of the drill. Drilling rates show a nonlinear relation with the load in the experimental range, as shown in Fig. 6. When the load on the drill bit gets to 10 N, we obtain the satisfying drill rate. The surface roughness(R,) of Fig. 5.(a) Top surface and(b) section views of the holes drilled in Al2O,/BN composite ceramics sing hard metal alloy drill bits with different diameter 4.1 and 3.0 mm, respectively
6 % % , & ) (/0 L/ ?/ 6 , & & (& % 5 / 6 , & & & % & & & / & % & & % * * & H3 & & ( ;)) / 7 & 3 , / 2 & & &/ & & % (& , %% & % &/ 7 & 3 L/ E/ Q & & *) %, & / 6 % #!.' % L/ </ 2%% % - % & % % / L/ ?/ #' 6 % & #' ( % & & & , & &%% & </* & /) ( ,/ )* * + ! , - ./ /0 1 *<);
Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 EEEE品∝a 02▲ Drilling Load(N) ig. 6. Measurements of drilling rates as a function of normal force for Al,O3/BN ite ceramics drilled section was 3+0.2 um, measured by a light-section microscope under the conditions that satisfied a fixed load of 10 N on the drill bit. Significant cracks were not observed on the surface The convenience of machining is presumably attributed to the cleavage of the ano-sized h-bn particles homogeneously dispersed in the matrix, which may confine the machining damage to a very small region under the tip of the tools through microcracking to absorb and disperse the damage force during the machining operations. Linking of the microcracks results in the ease of machining. Separate phases could not be distinguished on the machined surface by scanning electron microscopy, and the ratio of Al2O3 and bn was consistent with Al2O3/30 vol %o BI by EDs analysis. However, further confirmation of the microstructure within this surface layer needs a higher resolution of transmission electron microscopy(TEM) 4. Conclusions (1) The Al2O3/BN nanocomposite ceramics, including the nano-sized Bn up to 30 vol %, were fabricated by hot-pressing the al-Al2O3 powders covered with t-BN. The SEM observations revealed the h-BN particles were homogenously dispersed within the a-Al2O3 grains and at the grain boundaries 2) There was no significant decrease in the fracture strength of these nano- omposite ceramics with the increase of h-BN content. The enhancement of the fracture toughness was chiefly attributed to the soft layer-structure and
& & ) & , - & & H & H3 & & % *) & / 1H 5 ( & % / 6 ( % , & ( % -. & - , & & 3 , H & ( , & % 5 & & & % & / 5 % 5 % / 1 & & & & % , , & % & ) (/0 , 271 ,/ : ( % H % % , & % , #62'/ #*' 6 & -. & ) (/0 % & , - - & ( & -/ 6 12 ( ( & - , & & - & & / # ' 6 H & % % - % - / 6 % % M, & % , - & L/ E/ % & % % % % / *<)@ )* * + ! , - ./ /0 1
Y. Li et al. /Materials Research Bulletin 37(2002)1401-1409 1409 large aspect ratio of h-bn grains and the fine microstructure owing to inhibition of matrix Al2O3 grain growth by nano-sized h-Bn dispersions ()The observed excellent machinability derived from the cleavage of the h-BN crystals beneath the cutting tools, and the weak interfaces between Al2O matrix and the h-Bn were caused by thermal expansion mismatch. The machinability of the materials maybe also connected with the decrease of Vickers hardness Acknowledgments The authors wish to thank the National Natural Science Foundation of China(No 50072017) for financial support and the State Key Laboratory for Mechanical Behavior of Materials for their technical support. References [1] G Launay, G. Brayet, F. Thevenot, J Mater. Sci. Lett. 5(1986)940 [2] Y.L. Li, G. Qiao, Z Jin, J Inorg. Mater. 16(2)(2001)207, (in Chinese) [3] T. Uno, T Kasuga, S Nakayama, A.J. Ikushima, J. Am. Ceram Soc. 76(2)(1993)539. [4] M.W. Baroum, T El-Raghy, J. Am. Ceram Soc. 79(7)(1996)1953 [5]K Suganuma, G. Sasaki, T. Fujuta, M. Okumura, K. Niihara, J. Mater. Sci. 28(1993)1175 6 T Kusunose, K Niihara, Chem Ind 51(8)(1998)1221, (in Japanese) [7] T. Kusunose, Y.H. Choa, T. Sekino, K Niihara, Ceram. Trans.: Innovative Proc /Synth. Ceram, Glasses, Composites Il(1999)443 [8] R.T. Paine, C K Narula, Chem. Rev. 90(1)(1990)73 [9] Z.H. Jin, J.Q. Gao, GJ. Qiao, Engineering Ceramic Materials, Xi'an Jiaotong University Press, Xian, 2000, 198-202 pp (in Chinese). [11 T. Kusunose, Newly Developed Silicon Nitride Based Nanocomposites with Multiple Functionality, Doctor Thesis, Osaka University, 1999, p. 55(in Japanese). [12] W.S. Coblenz, D. Lewis Ill, J. Am. Ceram Soc. 71(12)(1988)1080. [13] D. Lewis, R. P Ingel, WJ. McDonough, R w. Rice, Ceram. Eng. Sci. Proc. 2(7-8)(1981)719
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