D. Wei et al. /Ceramics International 32(2006)549-554 Table 1 The compositions of raw materials(vol %b) Si3N4+Y2O3+Al,O3 %92 02468 Fig. 1. Effect of h-BN content on the relative density of h-BNSi3N ceramIc composite friction coefficient and wear behaviour of Si3 N4 matrix were investigated. Fig. I shows the relative density change of h- also investigated preliminarily. BN/Si3 N4 ceramic composites with the different addition of h-Bn content. The h-BN/Si3 N4 ceramic composites are densified completely as the addition of h-BN content is less 2. Experimental than or equal to 8 vol % However, the relative density of h- BN/Si3N4 ceramic composite with 10% h-BN volume Six kinds of compositions were designed, as shown in content reduces slightly as shown in Fig. 1. Due to the low- Table 1; the mass ratio of Si3N4, Y203 and Al2O3 was chemically active nature of h-BN in h-BN/Si3 N4 ceramic 94: 4.5: 1. 5. Powders of a-Si3N4(85% Si3N4 phase, Shang- composite, therefore h-Bn reduces the sinter ability of hai Institute of Material Research, China), h-BN(99.5%, SN10, leading to the decrease in relative density of SN1O. Beijing Chemical Factory, China), Y2O3(99.9%, Shanghai Nevertheless, the h-BN/Si3 N4 ceramic composites with a Yuelong Chemical Factory, China) and Al,O3(99.5%, small quantity of h-BN maintain high relative density Beijing Chemical Factory, China) were mixed and ball- Fig 2 shows SEM micrographs of the microstructure of milled in alcohol for 24h. After the slurry was dried, the h-BNSi3N4 ceramic composites. The microstructure of SNO powders were hot-pressed in flowing N2 at 1800C for I h in primarily consists of coarse elongated B-Si3N4 grains with a h-BN-coated graphite die. To minimize the effect of high aspect ratios, equiaxed B-Si3N4 grains and intergral- relative density on the mechanical and tribological proper- nular phase as shown in Fig 2a. However, when 10 vol %h ties, a high hot-pressure of 40 MPa was applied to fabricate bN is introduced into the Si3N4 matrix, a number of short h-BN/Si3 N4 ceramic composites of a high relative density. and rod-like B-Si3N4 grains are observed as shown in Fig. 21 The relative density was calculated on the basis of th The formation of elongated B-Si3N4 grain can b heoretical and bulk density measured by the Archimedes's explained by the anisotropic grain growth. Due to the method. Microstructure of h-BN/Si3 N4 ceramic composites lower boundary energy in the c-direction than in a-direction was characterized using a Hitachi S-570 scanning electron of hexagonal crystal, the energetically more favorable microscope(SEM). Bending strength was determined by three nucleation takes place on the surface of the basal plane [12] point bend testing(test bars 4 mm x 3 mm x 36 mm), with a That results in a higher growth rate in the c-direction and the bending span of 30 mm and across-head speed 0.5 mm/min formation of elongated grain. In addition, as can be seen at room temperature on a universal testing machine(Instron- from Fig. 2, the diameter and aspect ratio of elongated B- 5569) Fracture toughness was determined by the single-edge- Si3N4 grains in composites decrease with increasing h-BN notched beam(SENB)method (test bars 2 mm x 4 mm x content. That is due to the growth of B-Si3 N4 grains to be 20 mm)on a universal testing machine (Instron-5569). Elastic hindered by h-Bn particles. In the grain growth stage of modulus was measured using a universal testing machine microstructural evolution, the growth of the B-Si3N4 nucleus Instron-5569). The Vickers hardness was measured using a is limited by h-BN particles as the B-Si3N4 nucleus contacts hardness testing machine(HBV-30A) with a load of 30kn h-bn particles in the growth direction of the B-si3N4 held for 15 s. The friction coefficient was measured using nucleus, thereby B-Si3 N4 nuclei are unable to develop friction and wear testing machine(CJSl1lA)with a load of sufficiently in time. However, nucleation takes place 4N. The polished and worn surfaces of specimens were continually. This suggests that the rate of nucleation characterized using a hitachi S-570 SEM increase relatively, resulting in microstructure refining. Therefore, the diameter and aspect ratio of B-Si3N4 grains decrease with increasing h-bN content. 3. Results and discussion 3. 2. Effect of h-BN on the mechanical properties 3. 1. Effect of h-BN on the relative density and microstructure 3.2.1. Flexural strength Fig 3 shows the relation of the flexural strength of h-BN/ The effect of h-BN content on the relative density and Si3 n4 ceramic composites versus h-BN content. The bendin microstructure of h-BN/Si3 N4 ceramic composites were strength of h-BN/Si3 N4 ceramic composites decreases withfriction coefficient and wear behaviour of Si3N4 matrix were also investigated preliminarily. 2. Experimental Six kinds of compositions were designed, as shown in Table 1; the mass ratio of Si3N4, Y2O3 and Al2O3 was 94:4.5:1.5. Powders of a-Si3N4 (85%aSi3N4 phase, Shang￾hai Institute of Material Research, China), h-BN (99.5%, Beijing Chemical Factory, China), Y2O3 (99.9%, Shanghai Yuelong Chemical Factory, China) and Al2O3 (99.5%, Beijing Chemical Factory, China) were mixed and ball￾milled in alcohol for 24h. After the slurry was dried, the powders were hot-pressed in flowing N2 at 1800 8C for 1 h in a h-BN-coated graphite die. To minimize the effect of relative density on the mechanical and tribological proper￾ties, a high hot-pressure of 40 MPa was applied to fabricate h-BN/Si3N4 ceramic composites of a high relative density. The relative density was calculated on the basis of the theoretical and bulk density measured by the Archimedes’s method. Microstructure of h-BN/Si3N4 ceramic composites was characterized using a Hitachi S-570 scanning electron microscope (SEM). Bending strength was determined by three point bend testing (test bars 4 mm
3 mm
36 mm), with a bending span of 30 mm and a cross-head speed of 0.5 mm/min at room temperature on a universal testing machine (Instron- 5569). Fracture toughness was determined by the single-edge￾notched beam (SENB) method (test bars 2 mm
4 mm
20 mm) on a universal testing machine (Instron-5569). Elastic modulus was measured using a universal testing machine (Instron-5569). The Vickers hardness was measured using a hardness testing machine (HBV-30A) with a load of 30 kN held for 15 s. The friction coefficient was measured using friction and wear testing machine (CJS111A) with a load of 4 N. The polished and worn surfaces of specimens were characterized using a Hitachi S-570 SEM. 3. Results and discussion 3.1. Effect of h-BN on the relative density and microstructure The effect of h-BN content on the relative density and microstructure of h-BN/Si3N4 ceramic composites were investigated. Fig. 1 shows the relative density change of h￾BN/Si3N4 ceramic composites with the different addition of h-BN content. The h-BN/Si3N4 ceramic composites are densified completely as the addition of h-BN content is less than or equal to 8 vol.%. However, the relative density of h￾BN/Si3N4 ceramic composite with 10% h-BN volume content reduces slightly as shown in Fig. 1. Due to the low￾chemically active nature of h-BN in h-BN/Si3N4 ceramic composite, therefore h-BN reduces the sinter ability of SN10, leading to the decrease in relative density of SN10. Nevertheless, the h-BN/Si3N4 ceramic composites with a small quantity of h-BN maintain high relative density. Fig. 2 shows SEM micrographs of the microstructure of h-BN/Si3N4 ceramic composites. The microstructure of SN0 primarily consists of coarse elongated b-Si3N4 grains with high aspect ratios, equiaxed b-Si3N4 grains and intergra￾nular phase as shown in Fig. 2a. However, when 10 vol.%h￾BN is introduced into the Si3N4 matrix, a number of short and rod-like b-Si3N4 grains are observed as shown in Fig. 2f. The formation of elongated b-Si3N4 grain can be explained by the anisotropic grain growth. Due to the lower boundary energy in the c-direction than in a-direction of hexagonal crystal, the energetically more favorable nucleation takes place on the surface of the basal plane [12]. That results in a higher growth rate in the c-direction and the formation of elongated grain. In addition, as can be seen from Fig. 2, the diameter and aspect ratio of elongated b￾Si3N4 grains in composites decrease with increasing h-BN content. That is due to the growth of b-Si3N4 grains to be hindered by h-BN particles. In the grain growth stage of microstructural evolution, the growth of the b-Si3N4 nucleus is limited by h-BN particles as the b-Si3N4 nucleus contacts h-BN particles in the growth direction of the b-Si3N4 nucleus, thereby b-Si3N4 nuclei are unable to develop sufficiently in time. However, nucleation takes place continually. This suggests that the rate of nucleation increase relatively, resulting in microstructure refining. Therefore, the diameter and aspect ratio of b-Si3N4 grains decrease with increasing h-BN content. 3.2. Effect of h-BN on the mechanical properties 3.2.1. Flexural strength Fig. 3 shows the relation of the flexural strength of h-BN/ Si3N4 ceramic composites versus h-BN content. The bending strength of h-BN/Si3N4 ceramic composites decreases with 550 D. Wei et al. / Ceramics International 32 (2006) 549–554 Table 1 The compositions of raw materials (vol.%) Specimens Si3N4 + Y2O3 + Al2O3 h-BN SN0 100 0 SN2 98 2 SN4 96 4 SN6 94 6 SN8 92 8 SN10 90 10 Fig. 1. Effect of h-BN content on the relative density of h-BN/Si3N4 ceramic composites