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h-BN functionally graded materials
Materials Research Bulletin 37 (2002) 1269±1277
Fabrication and characterization of machinable Si3N4/h-BN functionally graded materials Wang Ruigang*, Pan Wei, Chen Jian, Jiang Mengning, Fang Minghao State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China (Refereed) Received 26 November 2001; accepted 11 April 2002
Abstract A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si3N4 ceramics. Silicon nitride/hexagonal boron nitride (h-BN) functionally graded materials (FGMs) were fabricated by hot pressing at 17508C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si3N4/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. Such design can improve the machinability of composite, and at the same time can make the mechanical properties of Si3N4 ceramic not to be sacri®ced too much. The texture of h-BN and b-Si3N4 was observed during hot pressing sintering. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Nitride; B. Multilayer; C. Ceramic; D. Microstructure
1. Introduction Machining is emerging as an inevitable requirement for ¯exible use of advanced ceramics, especially for structural ceramics. However, extremely high hardness of ceramics makes conventional machining very dif®cult or even impossible. In the past years, a lot of researches have been focused on the improvement of ceramic machinability [1±4]. Generally, two methods were used in improving the *
Corresponding author. Tel.: 86-10-6277-2859; fax: 86-10-6277-1160. E-mail address: [email protected] (W. Ruigang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 7 7 3 - 0
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Fig. 1. Schematic illustration and sintered specimens of machinable Si3N4/h-BN FGMs.
machinability of ceramic materials. One method is to introduce a weak interface phase or layered structure material in matrix to facilitate crack de¯ection and propagation during machining. This method is named compound machinable ceramics, such as mica-containing glass-ceramic [5]. The other is structure-design method, which is to optimize machinability of ceramics by adjusting the distribution of phase, porosity and three-dimension macrostructure and microstructure, such as porous ceramic, graded machinable ceramics [6]. In this work, hexagonal boron nitride (h-BN), which has excellent machinability due to a plate-like structure similar to that of graphite, was combined with silicon nitride to improve machinability of composite. A machinable Si3N4/h-BN FGM was designed and fabricated. At the same time, Si3N4/h-BN ceramic composites with such composition and structure design possess a variety of interesting properties such as thermal shock resistance, low friction coef®cient, and erosion resistance to molten metals. Fig. 1 illustrates the design concept and sintered specimens of the machinable Si3N4/ h-BN FGM. The sandwich-like microstructure of FGM and the distribution of h-BN are represented. The middle layer of the Si3N4/h-BN FGM is pure Si3N4; the h-BN content is graded increased from center to two sides. As the result of the increased h-BN content with soft and layered structure, composites can be machined using cemented carbide drills. Present research aims to design and fabricate a functionally graded machinable Si3N4/h-BN composite, which could be partly machined using the cemented carbide tools. While the other part is hard to machine with ordinary tools, the advantages such as the strength, hardness, and other mechanical properties of Si3N4 could remain. 2. Experimental 2.1. Materials preparation and sintering In this experiment, starting powders (Si3N4, 0.71±1.2 mm, Founder Co., China; hBN, 0.53 mm, GY Powder Industries, China) were used as raw materials. According
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to the different proportion of Si3N4 and h-BN with 8 wt%Y2O3±2 wt%Al2O3 (Y2O3± Al2O3, >99.5%, GRINM, China) as sintering aids, the mixture were carefully weighed, mixed and milled in a roller mill with alcohol for 48 h, then dried at 808C and sieved through a 100 mesh sieve. The fabrication process of the machinable Si3N4/h-BN FGM is a powder-layering route. The powders were put into a steel die layer-by-layer and cold-pressed into cylinder compact (1 50 mm 10 mm) at 100 MPa. The composition gradient is symmetrical. From the middle layer to side layer, the volume of h-BN is 0, 5, 10, 15, 20, 25 vol%. The thickness of each layer was decided by the equation xp (1) f x t where f(x) is volume fraction of h-BN at the position x, p the gradient distribution exponent and t is the thickness of the gradient region. In this experiment, the thickness of the gradient region is about 1 mm. Green compacts were sintered at 17508C for 2 h under pressure of 25 MPa in a nitrogen atmosphere. The total sintered thickness of all graded composite was 6 mm. 2.2. Characterization of properties and microstructure XRD was carried out at room temperature using an X-ray diffractometer (Model Automated D/Max-rb, Japan). The Vickers indentations were made on polished samples at each layer with a load of 5 kg held for 15 s. Microstructures of fracture surface of Si3N4/h-BN FGM with different h-BN content layer were observed under scanning electron microscopy, using Hitachi SEM-450. Element distribution (including Si, N and B) in the pro®le of graded Si3N4/h-BN composites was characterized by line-scanning using scanning electron microscope (JSE-6301F). The machinability of the specimen was tested using cemented carbide drills. The drilling tests were done using a standard drill press operating at 2500 rpm with a drop of water placed on the drill tip at the beginning of each run. Cross-sectional microstructures of the graded materials were characterized. The cross-sections were polished to a 0.5 mm ®nish using routine ceramographic techniques, and were goldcoated before observing in an optical microscope (Olympus SZ12 DF PLFL, Japan). 3. Results and discussion 3.1. Characterization of Si3N4/h-BN FGM Fig. 1(a) and (b) showed cross-sectional microstructures of the symmetric eleven graded materials with gradient exponent P 1 and P 2, respectively. The layer boundaries are faintly visible in these micrographs. Note the uniformity of the layers in all cases, and the uniformity of the microstructures within individual layers. Most importantly, note the lack of delamination and or severe defects at the interlayer
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Fig. 2. Element cross-sectional pro®le of Si3N4/h-BN functionally graded materials with gradient exponent P 2 from middle layer to side layer.
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boundaries. The higher magni®cation microstructures of pure Si3N4 (middle layer) showed good densi®cation. More h-BN, higher porosity. Fig. 2 showed the result of element line scanning of FGM from middle layer (pure Si3N4) to end layer (25 vol% Si3N4/h-BN). That con®rmed the distribution of h-BN in the Si3N4/h-BN FGM. 3.2. Texture of h-BN and b-Si3N4 The application of a force perpendicular to the top surface during hot-pressing resulted in the orientation growth of h-BN and b-Si3N4. From the XRD result, there exist h-BN and b-Si3N4 textures in this composite parallel and perpendicular to the hot-pressing direction. Fig. 3 illustrates the X-ray diffraction patterns of 25 vol% h-BN layer of the Si3N4/h-BN FGM. Only h-BN and b-Si3N4 phase was founded in the composites. An examination of the diffraction patterns for the hot-pressed sample on the side and top faces reveals that the relative intensity for the (0 0 2) peak for h-BN and ratio of (1 0 1)/(2 1 0) for b-Si3N4 are different on this two surfaces with signi®cantly greater intensity on the top surface than on the side surface. Such texture can also be observed in the microstructure photographs. 3.3. Vickers's hardness of Si3N4/h-BN FGM The effect of h-BN content on Vickers's hardness of each layer of Si3N4/h-BN FGM was investigated. The Vickers hardness of Si3N4/h-BN layer decreased with increasing h-BN volume fraction from middle layer to side layer. The hardness of Si3N4/h-BN with 25% h-BN volume content is as high as H V 5:67 GPa, which matches the request of machining. The hardness is close to the machinable mica glass-ceramic [5]
Fig. 3. XRD patterns taken on the surfaces parallel and perpendicular to the hot-pressing direction for sintered Si3N4/h-BN FGMs at 25 vol% h-BN layer.
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Fig. 4. Effects of the h-BN content on the hardness of Si3N4/h-BN FGM.
(3 GPa) and layered ternary compounds Ti3SiC2 [7] (4±5 GPa). Easy cleavage of basal plane of h-BN platelets causes hardness to decrease with h-BN addition as shown in Fig. 4. Along with the addition of h-BN, a steep decrease in the hardness of Si3N4/h-BN composites occurs due to the forming the weak interface, leading to good machinability. As a weak phase, the addition of h-BN will bene®t the crack de¯ection and reduce the hardness of composite. An important reason is that the Si3N4/h-BN is hard to densi®cate due to the h-BN addition, which brings many pores in the microstructure. 3.4. Machinability of Si3N4/h-BN composites and FGM A variety of methods have been utilized to improve the machinability of Si3N4 ceramics, including porous ceramics and microstructure design (controlling the ratio of a-Si3N4/b-Si3N4 etc.). In order to combine the good machinability of h-BN and the excellent mechanical properties of Si3N4 at both room temperatures and high temperatures, a graded Si3N4/h-BN ceramic composite was developed in this work. In general, ease in machining is inversely proportional to the hardness. It can be seen that the hardness of each Si3N4/h-BN composite layer decreases gradually with the increasing h-BN content. The hardness of Si3N4/h-BN with 25% h-BN volume content is as high as H V 5:67 GPa, which matches the request of machining. Fig. 5 showed the whole optical images of machinable 25 vol% Si3N4/h-BN and Si3N4/hBN FGM using a cemented carbide drill. According to the hardness change and experiment results, the layers of 20 and 25 vol% h-BN addition can be machined using cemented carbide drills. Moreover, the material depth, which can be easily machined, can be designed by adjusting the thickness of 25 vol% h-BN layer according to the requirement of the engineering application. The materials removal
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Fig. 5. Hole drilled with cemented carbide drill in machinable 25 vol% h-BN/Si3N4 composite and Si3N4/h-BN FGM.
mechanism of Si3N4/h-BN composite during drilling seems to rely on the cleavage of layered boron nitride crystals. During drilling, the cleavage of h-BN crystals localizes fracture of the composite on a microscopic scale and allows powdered chips to form easily, giving rise to good machinability. The intercrystal porosity can be attributed to another reason for improvement of machinability because of preventing growth of cracks associated with machining. Moreover, it appears that the more porous ceramic, the weaker and more machinable it becomes. 3.5. Microstructure evolution of Si3N4/h-BN FGM Fig. 6 showed the each layer microstructures of sintered Si3N4/h-BN FGM. SEM micrographs of fracture surface parallel to the pressing direction con®rm the preferred orientation of h-BN plates in the hot-pressed h-BN composites. A similar preferred orientation of h-BN grains in hot-pressed BN/Al2O3 [8], SiC/BN [9], BN/oxide ceramic [10], and BN/B2O3 has previously been observed. A scanning electron micrograph of the resulting microstructure clearly shows the presence of acicular b-Si3N4 grains with high aspect ratio at the monolithic Si3N4. Each microstructure of composite shows the layered crystal structure and the preferential orientation of the ¯aked h-BN grains with the planar planes perpendicular to the hot pressing direction, which may be due to the rotation of the h-BN ¯akes during the viscous ¯ow of the glass phase under hot pressing. h-BN has a layered structure similar to graphite, with a strong bonding within each layer and a weak bonding between the layers. When a crack tip meets the h-BN grain, it will propagate either along the interface between the Si3N4 and h-BN grains or along the interlayer within the h-BN grains. Whereas, the crack de¯ection and propagation across h-BN ¯ake are dif®cult because of the strong bonding within each layer of the h-BN grains. The fracture surface of the composite shows large degrees of crack de¯ection, pull-out, and cleavage cracking of the h-BN grains. All of which explained the improvement of machinability of Si3N4/h-BN FGM.
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Fig. 6. SEM micrographs of Si3N4/h-BN FGM at different layer with graded h-BN content, showing: (a) Si3N4 layer; (b) 5 vol% h-BN/Si3N4 layer; (c) 10 vol% h-BN/Si3N4 layer; (d) 15 vol% h-BN/Si3N4 layer; (e) 20 vol% h-BN/Si3N4 layer; (f) 25 vol% h-BN/Si3N4 layer. The HP direction corresponds to the hot-pressing direction.
4. Conclusion Poor machinability and high machining cost prohibit the potential application of Si3N4 ceramics. Combining the excellent thermal and mechanical properties of Si3N4 with good machinability of h-BN, a machinable Si3N4/h-BN FGM was designed and fabricated, and the properties of those FGMs were ®rst investigated. It was shown that
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hardness of Si3N4/h-BN composite layer decreased with the increase of h-BN content, which is advantageous to the machinability of Si3N4/h-BN composites and FGM. At the same time, it is well known that the remarkable properties of Si3N4/h-BN are high corrosion resistance to molten metal, high thermal shock resistance and excellent machinability, while retaining relatively high strength. Thus, Si3N4/h-BN FGM has a variety of applications in engineering ®eld. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
D.G. Grossman, J. Am. Ceram. Soc. 55 (1972) 446. M.W. Barsoum, T. El-Raghy, J. Am. Ceram. Soc. 79 (1996) 1953. N.P. Padture, C.J. Evans, H.H.K. Xu et al., J. Am. Ceram. Soc. 78 (1995) 215. K. Chihiro, Y. Akira, J. Ceram. Soc. Jpn. 106 (1998) 1135. H.H.K. Xu, S. Jahanmir, J. Am. Ceram. Soc. 78 (1995) 497. W. Pan, R.G. Wang et al., Rare Metal Mater. Eng. 29 (2000) 84. T. El-Raghy, S. Chakraborty, M.W. Barsoum, J. Eur. Ceram. Soc. 20 (2000) 2619. D. Goeuriot-Launay, G. Brayet, F. Thevenot, J. Mater. Sci. Lett. 5 (1986) 940. R. Ruh, L.D. Bentsen, D.P.H. Hasselman, J. Am. Ceram. Soc. 67 (1984) C83. R.W. Trice, J.W. Halloran, J. Am. Ceram. Soc. 82 (1999) 2563.
Fabrication and characterization of machinable Si3N4/h-BN functionally graded materials Wang Ruigang*, Pan Wei, Chen Jian, Jiang Mengning, Fang Minghao State Key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China (Refereed) Received 26 November 2001; accepted 11 April 2002
Abstract A new design method of machinable ceramic composites was proposed, which applies the graded-structure concept to the design of machinable Si3N4 ceramics. Silicon nitride/hexagonal boron nitride (h-BN) functionally graded materials (FGMs) were fabricated by hot pressing at 17508C for 2 h, varying the alignment of the amount of hexagonal BN using powder layering method. The improved machinability of Si3N4/h-BN composite can be attributed to addition of layered structure hexagonal BN. Hexagonal BN possesses excellent cleavage planes perpendicular to the c-axis. Ease of machining depends on degree of crystal interlocking; hence volume content of h-BN crystals and their aspect ratio affect machinability. Such design can improve the machinability of composite, and at the same time can make the mechanical properties of Si3N4 ceramic not to be sacri®ced too much. The texture of h-BN and b-Si3N4 was observed during hot pressing sintering. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Nitride; B. Multilayer; C. Ceramic; D. Microstructure
1. Introduction Machining is emerging as an inevitable requirement for ¯exible use of advanced ceramics, especially for structural ceramics. However, extremely high hardness of ceramics makes conventional machining very dif®cult or even impossible. In the past years, a lot of researches have been focused on the improvement of ceramic machinability [1±4]. Generally, two methods were used in improving the *
Corresponding author. Tel.: 86-10-6277-2859; fax: 86-10-6277-1160. E-mail address: [email protected] (W. Ruigang).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 7 7 3 - 0
1270
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Fig. 1. Schematic illustration and sintered specimens of machinable Si3N4/h-BN FGMs.
machinability of ceramic materials. One method is to introduce a weak interface phase or layered structure material in matrix to facilitate crack de¯ection and propagation during machining. This method is named compound machinable ceramics, such as mica-containing glass-ceramic [5]. The other is structure-design method, which is to optimize machinability of ceramics by adjusting the distribution of phase, porosity and three-dimension macrostructure and microstructure, such as porous ceramic, graded machinable ceramics [6]. In this work, hexagonal boron nitride (h-BN), which has excellent machinability due to a plate-like structure similar to that of graphite, was combined with silicon nitride to improve machinability of composite. A machinable Si3N4/h-BN FGM was designed and fabricated. At the same time, Si3N4/h-BN ceramic composites with such composition and structure design possess a variety of interesting properties such as thermal shock resistance, low friction coef®cient, and erosion resistance to molten metals. Fig. 1 illustrates the design concept and sintered specimens of the machinable Si3N4/ h-BN FGM. The sandwich-like microstructure of FGM and the distribution of h-BN are represented. The middle layer of the Si3N4/h-BN FGM is pure Si3N4; the h-BN content is graded increased from center to two sides. As the result of the increased h-BN content with soft and layered structure, composites can be machined using cemented carbide drills. Present research aims to design and fabricate a functionally graded machinable Si3N4/h-BN composite, which could be partly machined using the cemented carbide tools. While the other part is hard to machine with ordinary tools, the advantages such as the strength, hardness, and other mechanical properties of Si3N4 could remain. 2. Experimental 2.1. Materials preparation and sintering In this experiment, starting powders (Si3N4, 0.71±1.2 mm, Founder Co., China; hBN, 0.53 mm, GY Powder Industries, China) were used as raw materials. According
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to the different proportion of Si3N4 and h-BN with 8 wt%Y2O3±2 wt%Al2O3 (Y2O3± Al2O3, >99.5%, GRINM, China) as sintering aids, the mixture were carefully weighed, mixed and milled in a roller mill with alcohol for 48 h, then dried at 808C and sieved through a 100 mesh sieve. The fabrication process of the machinable Si3N4/h-BN FGM is a powder-layering route. The powders were put into a steel die layer-by-layer and cold-pressed into cylinder compact (1 50 mm 10 mm) at 100 MPa. The composition gradient is symmetrical. From the middle layer to side layer, the volume of h-BN is 0, 5, 10, 15, 20, 25 vol%. The thickness of each layer was decided by the equation xp (1) f x t where f(x) is volume fraction of h-BN at the position x, p the gradient distribution exponent and t is the thickness of the gradient region. In this experiment, the thickness of the gradient region is about 1 mm. Green compacts were sintered at 17508C for 2 h under pressure of 25 MPa in a nitrogen atmosphere. The total sintered thickness of all graded composite was 6 mm. 2.2. Characterization of properties and microstructure XRD was carried out at room temperature using an X-ray diffractometer (Model Automated D/Max-rb, Japan). The Vickers indentations were made on polished samples at each layer with a load of 5 kg held for 15 s. Microstructures of fracture surface of Si3N4/h-BN FGM with different h-BN content layer were observed under scanning electron microscopy, using Hitachi SEM-450. Element distribution (including Si, N and B) in the pro®le of graded Si3N4/h-BN composites was characterized by line-scanning using scanning electron microscope (JSE-6301F). The machinability of the specimen was tested using cemented carbide drills. The drilling tests were done using a standard drill press operating at 2500 rpm with a drop of water placed on the drill tip at the beginning of each run. Cross-sectional microstructures of the graded materials were characterized. The cross-sections were polished to a 0.5 mm ®nish using routine ceramographic techniques, and were goldcoated before observing in an optical microscope (Olympus SZ12 DF PLFL, Japan). 3. Results and discussion 3.1. Characterization of Si3N4/h-BN FGM Fig. 1(a) and (b) showed cross-sectional microstructures of the symmetric eleven graded materials with gradient exponent P 1 and P 2, respectively. The layer boundaries are faintly visible in these micrographs. Note the uniformity of the layers in all cases, and the uniformity of the microstructures within individual layers. Most importantly, note the lack of delamination and or severe defects at the interlayer
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Fig. 2. Element cross-sectional pro®le of Si3N4/h-BN functionally graded materials with gradient exponent P 2 from middle layer to side layer.
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boundaries. The higher magni®cation microstructures of pure Si3N4 (middle layer) showed good densi®cation. More h-BN, higher porosity. Fig. 2 showed the result of element line scanning of FGM from middle layer (pure Si3N4) to end layer (25 vol% Si3N4/h-BN). That con®rmed the distribution of h-BN in the Si3N4/h-BN FGM. 3.2. Texture of h-BN and b-Si3N4 The application of a force perpendicular to the top surface during hot-pressing resulted in the orientation growth of h-BN and b-Si3N4. From the XRD result, there exist h-BN and b-Si3N4 textures in this composite parallel and perpendicular to the hot-pressing direction. Fig. 3 illustrates the X-ray diffraction patterns of 25 vol% h-BN layer of the Si3N4/h-BN FGM. Only h-BN and b-Si3N4 phase was founded in the composites. An examination of the diffraction patterns for the hot-pressed sample on the side and top faces reveals that the relative intensity for the (0 0 2) peak for h-BN and ratio of (1 0 1)/(2 1 0) for b-Si3N4 are different on this two surfaces with signi®cantly greater intensity on the top surface than on the side surface. Such texture can also be observed in the microstructure photographs. 3.3. Vickers's hardness of Si3N4/h-BN FGM The effect of h-BN content on Vickers's hardness of each layer of Si3N4/h-BN FGM was investigated. The Vickers hardness of Si3N4/h-BN layer decreased with increasing h-BN volume fraction from middle layer to side layer. The hardness of Si3N4/h-BN with 25% h-BN volume content is as high as H V 5:67 GPa, which matches the request of machining. The hardness is close to the machinable mica glass-ceramic [5]
Fig. 3. XRD patterns taken on the surfaces parallel and perpendicular to the hot-pressing direction for sintered Si3N4/h-BN FGMs at 25 vol% h-BN layer.
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Fig. 4. Effects of the h-BN content on the hardness of Si3N4/h-BN FGM.
(3 GPa) and layered ternary compounds Ti3SiC2 [7] (4±5 GPa). Easy cleavage of basal plane of h-BN platelets causes hardness to decrease with h-BN addition as shown in Fig. 4. Along with the addition of h-BN, a steep decrease in the hardness of Si3N4/h-BN composites occurs due to the forming the weak interface, leading to good machinability. As a weak phase, the addition of h-BN will bene®t the crack de¯ection and reduce the hardness of composite. An important reason is that the Si3N4/h-BN is hard to densi®cate due to the h-BN addition, which brings many pores in the microstructure. 3.4. Machinability of Si3N4/h-BN composites and FGM A variety of methods have been utilized to improve the machinability of Si3N4 ceramics, including porous ceramics and microstructure design (controlling the ratio of a-Si3N4/b-Si3N4 etc.). In order to combine the good machinability of h-BN and the excellent mechanical properties of Si3N4 at both room temperatures and high temperatures, a graded Si3N4/h-BN ceramic composite was developed in this work. In general, ease in machining is inversely proportional to the hardness. It can be seen that the hardness of each Si3N4/h-BN composite layer decreases gradually with the increasing h-BN content. The hardness of Si3N4/h-BN with 25% h-BN volume content is as high as H V 5:67 GPa, which matches the request of machining. Fig. 5 showed the whole optical images of machinable 25 vol% Si3N4/h-BN and Si3N4/hBN FGM using a cemented carbide drill. According to the hardness change and experiment results, the layers of 20 and 25 vol% h-BN addition can be machined using cemented carbide drills. Moreover, the material depth, which can be easily machined, can be designed by adjusting the thickness of 25 vol% h-BN layer according to the requirement of the engineering application. The materials removal
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Fig. 5. Hole drilled with cemented carbide drill in machinable 25 vol% h-BN/Si3N4 composite and Si3N4/h-BN FGM.
mechanism of Si3N4/h-BN composite during drilling seems to rely on the cleavage of layered boron nitride crystals. During drilling, the cleavage of h-BN crystals localizes fracture of the composite on a microscopic scale and allows powdered chips to form easily, giving rise to good machinability. The intercrystal porosity can be attributed to another reason for improvement of machinability because of preventing growth of cracks associated with machining. Moreover, it appears that the more porous ceramic, the weaker and more machinable it becomes. 3.5. Microstructure evolution of Si3N4/h-BN FGM Fig. 6 showed the each layer microstructures of sintered Si3N4/h-BN FGM. SEM micrographs of fracture surface parallel to the pressing direction con®rm the preferred orientation of h-BN plates in the hot-pressed h-BN composites. A similar preferred orientation of h-BN grains in hot-pressed BN/Al2O3 [8], SiC/BN [9], BN/oxide ceramic [10], and BN/B2O3 has previously been observed. A scanning electron micrograph of the resulting microstructure clearly shows the presence of acicular b-Si3N4 grains with high aspect ratio at the monolithic Si3N4. Each microstructure of composite shows the layered crystal structure and the preferential orientation of the ¯aked h-BN grains with the planar planes perpendicular to the hot pressing direction, which may be due to the rotation of the h-BN ¯akes during the viscous ¯ow of the glass phase under hot pressing. h-BN has a layered structure similar to graphite, with a strong bonding within each layer and a weak bonding between the layers. When a crack tip meets the h-BN grain, it will propagate either along the interface between the Si3N4 and h-BN grains or along the interlayer within the h-BN grains. Whereas, the crack de¯ection and propagation across h-BN ¯ake are dif®cult because of the strong bonding within each layer of the h-BN grains. The fracture surface of the composite shows large degrees of crack de¯ection, pull-out, and cleavage cracking of the h-BN grains. All of which explained the improvement of machinability of Si3N4/h-BN FGM.
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Fig. 6. SEM micrographs of Si3N4/h-BN FGM at different layer with graded h-BN content, showing: (a) Si3N4 layer; (b) 5 vol% h-BN/Si3N4 layer; (c) 10 vol% h-BN/Si3N4 layer; (d) 15 vol% h-BN/Si3N4 layer; (e) 20 vol% h-BN/Si3N4 layer; (f) 25 vol% h-BN/Si3N4 layer. The HP direction corresponds to the hot-pressing direction.
4. Conclusion Poor machinability and high machining cost prohibit the potential application of Si3N4 ceramics. Combining the excellent thermal and mechanical properties of Si3N4 with good machinability of h-BN, a machinable Si3N4/h-BN FGM was designed and fabricated, and the properties of those FGMs were ®rst investigated. It was shown that
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hardness of Si3N4/h-BN composite layer decreased with the increase of h-BN content, which is advantageous to the machinability of Si3N4/h-BN composites and FGM. At the same time, it is well known that the remarkable properties of Si3N4/h-BN are high corrosion resistance to molten metal, high thermal shock resistance and excellent machinability, while retaining relatively high strength. Thus, Si3N4/h-BN FGM has a variety of applications in engineering ®eld. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
D.G. Grossman, J. Am. Ceram. Soc. 55 (1972) 446. M.W. Barsoum, T. El-Raghy, J. Am. Ceram. Soc. 79 (1996) 1953. N.P. Padture, C.J. Evans, H.H.K. Xu et al., J. Am. Ceram. Soc. 78 (1995) 215. K. Chihiro, Y. Akira, J. Ceram. Soc. Jpn. 106 (1998) 1135. H.H.K. Xu, S. Jahanmir, J. Am. Ceram. Soc. 78 (1995) 497. W. Pan, R.G. Wang et al., Rare Metal Mater. Eng. 29 (2000) 84. T. El-Raghy, S. Chakraborty, M.W. Barsoum, J. Eur. Ceram. Soc. 20 (2000) 2619. D. Goeuriot-Launay, G. Brayet, F. Thevenot, J. Mater. Sci. Lett. 5 (1986) 940. R. Ruh, L.D. Bentsen, D.P.H. Hasselman, J. Am. Ceram. Soc. 67 (1984) C83. R.W. Trice, J.W. Halloran, J. Am. Ceram. Soc. 82 (1999) 2563.