CERAMICS INTERNATIONAL ELSEVIER Ceramics International 32(2006)549-554 www.elsevier.com/locate/ceramint Mechanical and tribological properties of hot-pressed h-BN/Si3 N4 ceramic composites Daqing Wei, Qingchang Meng, Dechang Jia Istitute of Technology Harbin 150 Received 25 November 2004; received in revised form 23 February 2005; accepted 18 April 2005 Available online 31 August 2005 Abstract h-BN/Si N4 ceramic composites were fabricated by hot-pressing using sub-micrometer sized a-Si3N4, h-BN powders and Y2O3-Al2O3 sintering additives. The microstructural analysis indicated the growth of B-SigNa grains to be hindered by h-BN particles and the diameter and aspect ratio of elongated B-Si3N4 grains to decrease with increasing h-BN content. As expected, this increase of h-BN content resulted in slight decrease in the bending strength and fracture toughness and a sharp decrease in the elastic modulus and vickers hardness of h-BN/Si3 N4 ceramic composites. In addition, the friction coefficient of the h-BN/Si3 N4 ceramic composites decreased with increasing h-BN content. A spalling mode of failure has been observed on the worn surface of monolithic Si3 N4 and 10 vol %h-BN/Si3 N4 ceramic composite C 2005 Elsevier Ltd and Techna Group S.r. l. All rights Keywords: B. Composites; C Mechanical properties; C. Friction; D. Si3N4 1. Introduction For structural and tribological applications, the mechan- ical properties of the self-lubricating ceramic composite also Si3N4 ceramic is promising for structural and tribological should be taken into account besides its tribological applications because it has low density, a low coefficient of properties. Generally speaking, the strength of thermal expansion, high strength and toughness and high lubricating ceramic composite with a great quantity of a esistance to wear and thermal shock [1-7. However, the lubricating phase is not high due to the low strength of a high hardness and elastic modulus of Si3N4 ceramic in lubricating phase. In addition, the previous investigations hybrid steel/Si3N4 bearings result in high contact stress have indicated that the mechanic properties have between bearing ball and groove, which reduces load significant effect on the tribological properties of ceramic capacity and shortens the rolling contact fatigue life of material [2, 11], for instance, high fracture toughness can hybrid bearings [2]. In addition, in some cases, ceramic enhance the wear property of ceramic material [11] bearings are used under vacuum and exposed to a condition It is well known that h-bn possesses a number of of elevated temperature, therefore liquid lubricants are interesting properties such as lubrication action, low hardness unsuitable for these environments. In order to improve the and low friction coefficient. However, it has less reported that tribological properties of ceramic materials, a self-lubricat- the effect of adding a small quantity of h-BN on the ing phase such as graphite was introduced in ceramic microstructure and tribological and mechanical properties of matrixes[8, 9]. However, graphite has significant effects on Si3N4 ceramic. In this experiment, h-BN was introduced into the mechanical properties of ceramic materials such as the Si3N4 ceramic. The aim of adding a small quantity of h-BN is strength and toughness [10] to decrease appropriately the hardness and elastic modulus of Si3N4 ceramic, consequently, to decrease the high contact stress between bearing ball and groove. It is also expected to maintain the high flexural strength and fracture toughness of laqingwei@hit.edu.cn(D.Wei) Si3NA ceramic matrix. In addition, the effects of h-BN on the 0272-8842/$30.00 2005 Elsevier Ltd and Techna Group S.r.L. All rights reserved doi:10.1016 j ceramist.2005.04010
Mechanical and tribological properties of hot-pressed h-BN/Si3N4 ceramic composites Daqing Wei *, Qingchang Meng, Dechang Jia Institute for Advanced Ceramics, Harbin Institute of Technology, Harbin 150001, PR China Received 25 November 2004; received in revised form 23 February 2005; accepted 18 April 2005 Available online 31 August 2005 Abstract h-BN/Si3N4 ceramic composites were fabricated by hot-pressing using sub-micrometer sized a-Si3N4, h-BN powders and Y2O3–Al2O3 sintering additives. The microstructural analysis indicated the growth of b-Si3N4 grains to be hindered by h-BN particles and the diameter and aspect ratio of elongated b-Si3N4 grains to decrease with increasing h-BN content. As expected, this increase of h-BN content resulted in a slight decrease in the bending strength and fracture toughness and a sharp decrease in the elastic modulus and Vickers hardness of h-BN/Si3N4 ceramic composites. In addition, the friction coefficient of the h-BN/Si3N4 ceramic composites decreased with increasing h-BN content. A spalling mode of failure has been observed on the worn surface of monolithic Si3N4 and 10 vol.%h-BN/Si3N4 ceramic composite. # 2005 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Composites; C. Mechanical properties; C. Friction; D. Si3N4 1. Introduction Si3N4 ceramic is promising for structural and tribological applications because it has low density, a low coefficient of thermal expansion, high strength and toughness and high resistance to wear and thermal shock [1–7]. However, the high hardness and elastic modulus of Si3N4 ceramic in hybrid steel/Si3N4 bearings result in high contact stress between bearing ball and groove, which reduces load capacity and shortens the rolling contact fatigue life of hybrid bearings [2]. In addition, in some cases, ceramic bearings are used under vacuum and exposed to a condition of elevated temperature, therefore liquid lubricants are unsuitable for these environments. In order to improve the tribological properties of ceramic materials, a self-lubricating phase such as graphite was introduced in ceramic matrixes [8,9]. However, graphite has significant effects on the mechanical properties of ceramic materials such as the strength and toughness [10]. For structural and tribological applications, the mechanical properties of the self-lubricating ceramic composite also should be taken into account besides its tribological properties. Generally speaking, the strength of the selflubricating ceramic composite with a great quantity of a lubricating phase is not high due to the low strength of a lubricating phase. In addition, the previous investigations have indicated that the mechanic properties have a significant effect on the tribological properties of ceramic material [2,11], for instance, high fracture toughness can enhance the wear property of ceramic material [11]. It is well known that h-BN possesses a number of interesting properties such as lubrication action, low hardness and low friction coefficient. However, it has less reported that the effect of adding a small quantity of h-BN on the microstructure and tribological and mechanical properties of Si3N4 ceramic. In this experiment, h-BN was introduced into Si3N4 ceramic. The aim of adding a small quantity of h-BN is to decrease appropriately the hardness and elastic modulus of Si3N4 ceramic, consequently, to decrease the high contact stress between bearing ball and groove. It is also expected to maintain the high flexural strength and fracture toughness of Si3N4 ceramic matrix. In addition, the effects of h-BN on the www.elsevier.com/locate/ceramint Ceramics International 32 (2006) 549–554 * Corresponding author. Tel.: +86 451 8640 2040 8530; fax: +86 451 8641 4291. E-mail address: daqingwei@hit.edu.cn (D. Wei). 0272-8842/$30.00 # 2005 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2005.04.010
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 with
friction 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, Shanghai 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 ballmilled 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 properties, 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-edgenotched 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 hBN/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 hBN/Si3N4 ceramic composite with 10% h-BN volume content reduces slightly as shown in Fig. 1. Due to the lowchemically 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 intergranular phase as shown in Fig. 2a. However, when 10 vol.%hBN 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 bSi3N4 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
D. Wei et al /Ceramics Intemational 32(2006)549-554 55 5 um Fig. 2. SEM micrographs of the etched surface of (a)SNO, (b)SN2, (c) SN4,(d) SN6,(e)SN& and(f)SN10 increasing h-BN content as shown in Fig 3, which is mainly shows the relation of the fracture toughness of h-BN/Si3 N4 due to the low strength of h-BN and weak interface between h- ceramic composites versus h-BN content. As can be seen BN and Si3N4 grains from this figure, the monolithic Si3N4 has high fracture Nevertheless, the bending strength of h-BN/Si3 Na ceramic toughness. However, the fracture toughness of h-BN/Si3 N4 opposites decreases slightly with increasing h-BN content, ceramic composites deceases slightly with increasing h-BN which is due to the reduction of B-Si3N4 grain size. Generally conter of Si3N4 ceramic can be enhanced by Many investigators have demonstrated that the fracture reducing the grain size of B-Si3N4 [13]. According to the toughness of B-Si3N4 ceramic is strongly dependent upon microstructural analysis, the diameter and aspect ratio of B- grain morphology [14-16]. The improved fracture tough Si3N4 grains in composites decrease with increasing h-Bn ness of B-si3N4 ceramic is the result of a reinforcin content. Therefore, the strength has a tendency to be improved phenomenon from the whisker-like B-Si3N4 grains, similar with the reduction of B-Si3N4 grain size. This result reveals to the behavior observed in whisker-reinforced ceramics. It that Siz Na ceramic still has a high bending strength as a small has been demonstrated that the fracture toughness of Si3N4 quantity of h-BN is incorporated into SiNa ceramic matrix. ceramic can be enhanced by increasing the diameter and igh strength ofh-BN/Si3N4 ceramic composites is necessary aspect ratio of elongated B-Si3 N4 grains [1, 17-20). In for its structural application addition, other factors can also affect the fracture toughness such as volume fraction of the bridging grains and the compositions and properties of the boundary phase [21] The effect ofh-BN content on the fracture toughness of h- As mentioned above, the fracture toughness of B-si3N4 BN/Si3N4 ceramic composites was investigated. Fig. 3 ceramic is strongly dependent upon B-Si3 N4 grain morphol ogy. That the monolithic Si3 N4 has high fracture toughness is mainly attributed to large numbers of coarse elongated B- Fracture toughness3日 Si3N4 grains with high aspect ratios, yielding various toughening mechanisms such as grain bridging, crack deflection and pull-out of the elongated B-Si3 N4 grains. As shown in Fig. 4a, many coarse voids due to the pull-out of m0+ the elongated B-Si3N4 grains were observed on the fractured surface of monolithic Si3 N4. However, with increasing h-BN content, the diameter and aspect ratio of elongated B-si3N4 grains decrease, which weakens the toughening mechanisms Fig. 3. Effect of h-BN content on the flexural strength and fracture toughness of h-BN/Si3N4 ceramic composites decreases toughness of h-BN/ N4 composites. with increasing h-BN content
increasing h-BN content as shown in Fig. 3, which is mainly due to the low strength of h-BN and weak interface between hBN and Si3N4 grains. Nevertheless, the bending strength of h-BN/Si3N4 ceramic composites decreases slightly with increasing h-BN content, which is due to the reduction of b-Si3N4 grain size. Generally speaking, the strength of Si3N4 ceramic can be enhanced by reducing the grain size of b-Si3N4 [13]. According to the microstructural analysis, the diameter and aspect ratio of bSi3N4 grains in composites decrease with increasing h-BN content. Therefore, the strength has a tendency to be improved with the reduction of b-Si3N4 grain size. This result reveals that Si3N4 ceramic still has a high bending strength as a small quantity of h-BN is incorporated into Si3N4 ceramic matrix. High strength of h-BN/Si3N4 ceramic composites is necessary for its structural application. 3.2.2. Fracture toughness The effect of h-BN content on the fracture toughness of hBN/Si3N4 ceramic composites was investigated. Fig. 3 shows the relation of the fracture toughness of h-BN/Si3N4 ceramic composites versus h-BN content. As can be seen from this figure, the monolithic Si3N4 has high fracture toughness. However, the fracture toughness of h-BN/Si3N4 ceramic composites deceases slightly with increasing h-BN content. Many investigators have demonstrated that the fracture toughness of b-Si3N4 ceramic is strongly dependent upon grain morphology [14–16]. The improved fracture toughness of b-Si3N4 ceramic is the result of a reinforcing phenomenon from the whisker-like b-Si3N4 grains, similar to the behavior observed in whisker-reinforced ceramics. It has been demonstrated that the fracture toughness of Si3N4 ceramic can be enhanced by increasing the diameter and aspect ratio of elongated b-Si3N4 grains [1,17–20]. In addition, other factors can also affect the fracture toughness such as volume fraction of the bridging grains and the compositions and properties of the boundary phase [21]. As mentioned above, the fracture toughness of b-Si3N4 ceramic is strongly dependent upon b-Si3N4 grain morphology. That the monolithic Si3N4 has high fracture toughness is mainly attributed to large numbers of coarse elongated bSi3N4 grains with high aspect ratios, yielding various toughening mechanisms such as grain bridging, crack deflection and pull-out of the elongated b-Si3N4 grains. As shown in Fig. 4a, many coarse voids due to the pull-out of the elongated b-Si3N4 grains were observed on the fractured surface of monolithic Si3N4. However, with increasing h-BN content, the diameter and aspect ratio of elongated b-Si3N4 grains decrease, which weakens the toughening mechanisms of elongated b-Si3N4 grains. Therefore, the fracture toughness of h-BN/Si3N4 ceramic composites decreases with increasing h-BN content. D. Wei et al. / Ceramics International 32 (2006) 549–554 551 Fig. 2. SEM micrographs of the etched surface of (a) SN0, (b) SN2, (c) SN4, (d) SN6, (e) SN8 and (f) SN10. Fig. 3. Effect of h-BN content on the flexural strength and fracture toughness of h-BN/Si3N4 composites
D. Wei et al. /Ceramics International 32(2006)549-554 -vickers hardness 12a Fig. 5. Effect of h-bN content on the elastic modulus and vickers hardness Fig 4. SEM micrographs of the fractured surface of (a)SNO and(b)SNI0. of h-BNSi,N4 ceramic composites. In contrast to the fractured surface of SNO, even though versus h-BN content. The elastic moduli of SNO and no large number of coarse B-Si3N4 grains of SN1O was SN10 are 293.5 and 236.6 GPa, respectively. As can be pulled out as shown in Fig 4b, no significant reduction in seen from this figure, the elastic modulus of h-BN/Si3N4 fracture toughness was observed. This is due to a crack ceramic composites decreases sharply with increasing h- deflection originated from the effect of h-BN particles on BN content. That is caused by the low elastic modulus of crack propagation. When the crack tip contacts the h-BN h-BN and the weak interface between h-BN and B-si3N4 particle, cracks will propagate within the h-BN particles or grains along the interface between h-BN and Si3 N4 grains due to The Vickers hardness of h-BNSi3N4 ceramic compo- he weak interface between h-Bn and Si3 N4 grains. Other sites decreases sharply with increasing h-BN content as nvestigators have shown that adding a small quantity of h- shown in Fig. 5. The Vickers hardness of SNO and SN1O bn to Si3 N4 can increase the fracture toughness of Si3N4 are 14.5 and 10.9 GPa, respectively. The sharp decrease ceramic matrix [22]. However, in this investigation the B- in Vickers hardness of h-BN/Si3 N4 ceramic composites Si3N4 grain size is a key factor in determining the fracture can be explained by the low hardness of h-BN and the toughness. Therefore, the fracture toughness of h-BN/Si3 N4 weak interface between h-BN and Si3 N4 grains. The h-BN ceramic composites decreases due to the reduction in the has a laminated crystal structure similar to graphite diameter and aspect ratio of elongated B-Si3N4 grains. and the bond strength between layers is minimal due to Nevertheless, the extent of reduction in fracture toughness of the molecular linkage. Therefore, h-bn has lower h-BN/ N4 ceramic composites is not significant owning to hardness and is easily broken along interlayers. As a he effect of h-BN particles on crack propagation. weak phase, the addition of h-BN leads to the reduction in the Vickers hardness of h-BN/Si3 N4 ceramic composites 3. 23. Elastic modulus and vickers hardness In addition the reduction in vickers hardness is also ig. 5 shows the relation of the elastic modulus and related to the weak interface between h-Bn and Si3n4 Vickers hardness of h-BN/Si3N4 ceramic composites graIns. A的 Time(min) (e) (f)0.9 W时部 Time(min) Fig. 6. The friction coefficients of (a)SNO. (b) SN2, (c)SN4. (d) SN6, (e)SN8 and(f) SN10
In contrast to the fractured surface of SN0, even though no large number of coarse b-Si3N4 grains of SN10 was pulled out as shown in Fig. 4b, no significant reduction in fracture toughness was observed. This is due to a crack deflection originated from the effect of h-BN particles on crack propagation. When the crack tip contacts the h-BN particle, cracks will propagate within the h-BN particles or along the interface between h-BN and Si3N4 grains due to the weak interface between h-BN and Si3N4 grains. Other investigators have shown that adding a small quantity of hBN to Si3N4 can increase the fracture toughness of Si3N4 ceramic matrix [22]. However, in this investigation the bSi3N4 grain size is a key factor in determining the fracture toughness. Therefore, the fracture toughness of h-BN/Si3N4 ceramic composites decreases due to the reduction in the diameter and aspect ratio of elongated b-Si3N4 grains. Nevertheless, the extent of reduction in fracture toughness of h-BN/Si3N4 ceramic composites is not significant owning to the effect of h-BN particles on crack propagation. 3.2.3. Elastic modulus and Vickers hardness Fig. 5 shows the relation of the elastic modulus and Vickers hardness of h-BN/Si3N4 ceramic composites versus h-BN content. The elastic moduli of SN0 and SN10 are 293.5 and 236.6 GPa, respectively. As can be seen from this figure, the elastic modulus of h-BN/Si3N4 ceramic composites decreases sharply with increasing hBN content. That is caused by the low elastic modulus of h-BN and the weak interface between h-BN and b-Si3N4 grains. The Vickers hardness of h-BN/Si3N4 ceramic composites decreases sharply with increasing h-BN content as shown in Fig. 5. The Vickers hardness of SN0 and SN10 are 14.5 and 10.9 GPa, respectively. The sharp decrease in Vickers hardness of h-BN/Si3N4 ceramic composites can be explained by the low hardness of h-BN and the weak interface between h-BN and Si3N4 grains. The h-BN has a laminated crystal structure similar to graphite and the bond strength between layers is minimal due to the molecular linkage. Therefore, h-BN has lower hardness and is easily broken along interlayers. As a weak phase, the addition of h-BN leads to the reduction in the Vickers hardness of h-BN/Si3N4 ceramic composites. In addition, the reduction in Vickers hardness is also related to the weak interface between h-BN and Si3N4 grains. 552 D. Wei et al. / Ceramics International 32 (2006) 549–554 Fig. 4. SEM micrographs of the fractured surface of (a) SN0 and (b) SN10. Fig. 5. Effect of h-BN content on the elastic modulus and Vickers hardness of h-BN/Si3N4 ceramic composites. Fig. 6. The friction coefficients of (a) SN0, (b) SN2, (c) SN4, (d) SN6, (e) SN8 and (f) SN10
D. Wei et al /Ceramics Intemational 32(2006)549-554 55 Fine flake 100μr 20 Fig. 7. SEM micrographs of the worn surfaces of monolithic Si3 N4 and 10 vol. ch-BNSi3 N4 ceramic composite:(a),(c) SNO and(b),(d)SN10 3.3. Effect of h-BN on the tribological properties However, the characteristic of spalling morphology on worn surface changes from fine flakes to large flakes by 3.3.. Friction coefficient incorporating 10 vol %h-BN into the Si3N4 ceramic matrix Fig. 6 shows the relation of the friction coefficient of h- as shown in Fig. 7a and b. In addition, no coarse cracks BN/Si3 N4 ceramic composites versus h-BN content. In all observed on the worn surface of SNo but on that of SN10 as cases the friction coefficient rose quite quickly at the start of shown in Fig. 7c and d the test before reducing to a level which remained relatively le formation of coarse cracks on the worn surface of sn constant for the remainder of the test. It can be seen that the can be explained by the surface pores and weak interface friction coefficient of h-BN/Si3 N, ceramic composites has a between h-BN and B-Si3N4 grains. Fig. 8 shows the SEM tendency to decrease with increasing h-BN content. micrograp ohs of the polished surfaces of SNO and SN10. No h-bn has a lower friction coefficient and lubrication evident pores are observed on the polished surface of SNO as action,thereby the friction coefficient of h-BN/Si3N4 shown in Fig. &a. However, as can be seen from Fig 8b, pores ceramic composites decreases with increasing h-bn are produced by incorporating 10 vol %h-Bn into Si3 n4 content. In addition, the reductions in diameter and aspe ceramic matrix, which is caused by the reduction in its sinter ratio of elongated B-Si3 N4 grains also result in the decrease ability. The pores of SN10 lead to the stress concentration, of friction coefficient. Investigations of ZrO2, Al2O3 and thereby microcracks are easily produced from thesites of pores ZrO2-Al2O3 ceramics system have shown that the frictional existed by applying stress. In addition, the weak interface also ceramic is more smooth and the friction coefficient is lower than that of coarse-grains ceramic [23-25]. Therefore, the reduced B-Si3N4 grain size is also the factor to decrease the friction coefficient of h-BN/ Si3N4 ceramic composites(see Fig. 6) 3 3.2. Wear behaviour The wear behaviour of sno and nio were investigated preliminarily. Fig. 7 shows the SEM micrographs of the worn surfaces of SNO and SN10 A surface spalling mode of failure is observed on the worn surfaces of sno and sn10 Fig 8. SEM micrographs of the polished surfaces of (a) SNO and(b)SN10
3.3. Effect of h-BN on the tribological properties 3.3.1. Friction coefficient Fig. 6 shows the relation of the friction coefficient of hBN/Si3N4 ceramic composites versus h-BN content. In all cases the friction coefficient rose quite quickly at the start of the test before reducing to a level which remained relatively constant for the remainder of the test. It can be seen that the friction coefficient of h-BN/Si3N4 ceramic composites has a tendency to decrease with increasing h-BN content. h-BN has a lower friction coefficient and lubrication action, thereby the friction coefficient of h-BN/Si3N4 ceramic composites decreases with increasing h-BN content. In addition, the reductions in diameter and aspect ratio of elongated b-Si3N4 grains also result in the decrease of friction coefficient. Investigations of ZrO2, Al2O3 and ZrO2–Al2O3 ceramics system have shown that the frictional surface of fine-grains ceramic is more smooth and the friction coefficient is lower than that of coarse-grains ceramic [23–25]. Therefore, the reduced b-Si3N4 grain size is also the factor to decrease the friction coefficient of h-BN/ Si3N4 ceramic composites (see Fig. 6). 3.3.2. Wear behaviour The wear behaviour of SN0 and SN10 were investigated preliminarily. Fig. 7 shows the SEM micrographs of the worn surfaces of SN0 and SN10. A surface spalling mode of failure is observed on the worn surfaces of SN0 and SN10. However, the characteristic of spalling morphology on worn surface changes from fine flakes to large flakes by incorporating 10 vol.%h-BN into the Si3N4 ceramic matrix as shown in Fig. 7a and b. In addition, no coarse cracks are observed on the worn surface of SN0 but on that of SN10 as shown in Fig. 7c and d. Theformation ofcoarsecracks ontheworn surfaceofSN10 can be explained by the surface pores and weak interface between h-BN and b-Si3N4 grains. Fig. 8 shows the SEM micrographs of the polished surfaces of SN0 and SN10. No evident pores are observed on the polished surface of SN0 as shown in Fig. 8a. However, as can be seen from Fig. 8b, pores are produced by incorporating 10 vol.%h-BN into Si3N4 ceramic matrix, which is caused by the reduction in its sinter ability. The pores of SN10 lead to the stress concentration, therebymicrocracksareeasilyproducedfromthesitesofpores existed by applying stress. In addition, the weak interface also D. Wei et al. / Ceramics International 32 (2006) 549–554 553 Fig. 7. SEM micrographs of the worn surfaces of monolithic Si3N4 and 10 vol.%h-BN/Si3N4 ceramic composite: (a), (c) SN0 and (b), (d) SN10. Fig. 8. SEM micrographs of the polished surfaces of (a) SN0 and (b) SN10
D. Wei et al. /Ceramics international 32(2006)549-554 results in the formation of microcracks. Eventually, the [61 R.S. Bhattacharya, A.K. Rai, High energy (Me v)ion beam modifica- accumulation and propagation of microcracks result in th Mos2 coatings on ceramics, ASME/STLE Tribology formation of coarse cracks on the worn surface of sn1o ference Preprints 1992 Preprint No. 92-TC-6A [7 F. Brenscheidt, S Oswald, A Muchlich, Wear mechanisms in titanium Consequently, the propagation of coarse cracks leads to anted silicon nitride ceramics, Nucl. Instrum. Methods Phys. Res. the formation of large flakes on the worn surface of SN10. In B129(1997483-486 addition, as can be seen from Fig. &d, the surface of each 8] PJ. Blau, B. Dumont, D N. Braski, Reciprocating friction and wear coarse flake is smooth which is due to the lubrication action behavior of a ceramic-matrix graphite composite for possible use in and low hardness of h-BN diesel engine valve guides, Wear 225-229(1999)1338-1349 [91 A Gangopadhyay, S. Jahanmir, Friction and wear characteristics of In order to understand thoroughly the effect of h-BN silicon nitride-graphite and alumina-graphite composites, TriboL. e tribological properties of h-BN/Si3 N4 ceramic co Trans.34(2)(1991)257-265 sites, further works need to be conducted [10 S. Jahanmir, Firction and Wear of Ceramics, Marcel Dekker Inc, New York,1994,pp.163-198. [11] A.G. Evans, P B. Marshall, Wear mechanism in ceramics, in: Proceed- gs of the International Conference on fundamentals of friction and 4. Conclusions Wear of Materials, ASME, Pittsburgh, 1980, pp. 439-452. [12]L K. Rueh, T.T. Ying, Kinetics of B-Si3N4 grain growth in Si3N4 Hot-pressed h-BN/Si3N4 ceramic composites with a ramics sintered under high nitrogen pressure, J. Am. Ceram Soc. 76 small quantity of h-BN can maintain high relative density The growth of B-Si3N4 grains is hindered by h-bN [13] PJ. Becher, Silicon nitride ceramics scientific and technology particles and the diameter and aspect ratio of B-Si3N4 grains [14) L. Chien-Wei, L. Siu-Ching, Jeffrey Goldacker, Relation between mposites decrease with increasing h-BN content. strength, microstructure, and grain-bridging characteristics in in-situ The aim of decreasing the hardness and elastic modulus inforced silicon nitride J Am Ceram Soc. 78(2)(1995)449-459 of Si3 N4 ceramic appropriately and maintaining its high 15 flexural strength and fracture toughness by introducing a pressure sintering of a-silicon nitride, J. Am. Ceram. Soc. 75(1) (1992)103-108 mall quantity of h-BN into Si3N4 ceramic was achieved [16] F.C. Peillon, F. Thevenot, Microstrucral designing of silicon nitride The friction coefficient of h-BN/Si3N4 ceramic compo- related to toughness, J. Eur. Ceram Soc. 22(2002)271-278. sites decreases with increasing h-bn content due to the [171 F.P. Becher, Y.E. Sun, P K Plucknett, Microstructural design of silicon lubrication action of h-BN and the reduced B-Si3N4 grains tride with improved fracture toughness I, effect of grain shape and size,J.Am. Ceram.Soc.81(11)(1998)2821-2830 size. A spalling mode of failure has been observed on the [18] N. Hirosaki, Y. Akimune, Effect of grain growth of B-Si3N4 on surface of sno and SN10 ength, weibull modulus, and fracture toughness, J. Am. Ceram. c.76(7)(1993)1892-1894 [19] P.J. Becher, Silicon nitride ceramics scientific and technology References dvances, Mater. Res. Soc. 8(1993)147-158. 201 A.J. Pyzik, A.R. Prunier, B. Pyzik, Microstructural engineering of [1] G. Shuqi, H. Naoto, Y. Yashinobu, Hot-pressed silicon ceramics with silicon nitride ceramics. in: Int. Conf. Silicon nitride 93. Stuttgart lerman. 1993 Lu,O3 additives: elastic moduli and fracture toughness, J. Eur. Ceram. Soc.23(2003)537-545 [21]R V. Weeren, C D. Stephen, The effect of grain boundary phase [2] Y. Sang-Young, A. Takashi, Y. Eiichi, The microstructure and creep characteristics on the crack deflection behavior in a silicon nitride deformation of hot-pressed Si N4 with different amounts of sintering aterial, Scripta Mater. 34(1996)1567-1573 dditives, J Mater Res. 11(1)(1996)120-126. [22] D.R. Petrak, J D. Lee, Silicon nitride/boron nitride composite with [3 H L Ekkehard, V.S. Michael. Fracture toughness and thermal shock enhanced fracture toughness. US Patent 5,324, 694(28 June 1994) [23] K.H. Zum Gahr, w Bundschuh, B Zimmerlin, Effect of grain size on behavior of silicon nitride-boron nitride ceramics.J. Am. Ceram. Soc friction and sliding wear of oxide ceramics, Wear 162-164(1993) 75(192)67-70 [4] J. Yiping, Current study on hybrid bearing and ceramic ball, Wear 2 [24A.K Mukhopadhyay. Y.M. Mai, Grain size effect on abrasive wear (2002)33-35 (in Chinese) [5] Y Shimura, Y. Mizutani, Wear of ceramics at high temperatures and its mechanisms in alumina ceramics, Wear 162-164(1993)314-321 [25] B. Wallis, Influence of the microstructure of ceramic materials on their ment by metallic coating. in: Wear of Materials, ASME (1991)405-410. wear behavior in mechanical seals, Lub. Eng. 50(1994)789-799
results in the formation of microcracks. Eventually, the accumulation and propagation of microcracks result in the formation of coarse cracks on the worn surface of SN10. Consequently, the propagation of coarse cracks leads to the formation of large flakes on the worn surface of SN10. In addition, as can be seen from Fig. 8d, the surface of each coarse flake is smooth, which is due to the lubrication action and low hardness of h-BN. In order to understand thoroughly the effect of h-BN on the tribological properties of h-BN/Si3N4 ceramic composites, further works need to be conducted. 4. Conclusions Hot-pressed h-BN/Si3N4 ceramic composites with a small quantity of h-BN can maintain high relative density. The growth of b-Si3N4 grains is hindered by h-BN particles and the diameter and aspect ratio of b-Si3N4 grains in composites decrease with increasing h-BN content. The aim of decreasing the hardness and elastic modulus of Si3N4 ceramic appropriately and maintaining its high flexural strength and fracture toughness by introducing a small quantity of h-BN into Si3N4 ceramic was achieved. The friction coefficient of h-BN/Si3N4 ceramic composites decreases with increasing h-BN content due to the lubrication action of h-BN and the reduced b-Si3N4 grains size. A spalling mode of failure has been observed on the surface of SN0 and SN10. References [1] G. Shuqi, H. Naoto, Y. Yashinobu, Hot-pressed silicon ceramics with Lu2O3 additives: elastic moduli and fracture toughness, J. Eur. Ceram. Soc. 23 (2003) 537–545. [2] Y. Sang-Young, A. Takashi, Y. Eiichi, The microstructure and creep deformation of hot-pressed Si3N4 with different amounts of sintering additives, J. Mater. Res. 11 (1) (1996) 120–126. [3] H.L. Ekkehard, V.S. Michael, Fracture toughness and thermal shock behavior of silicon nitride-boron nitride ceramics, J. Am. Ceram. Soc. 75 (1992) 67–70. [4] J. Yiping, Current study on hybrid bearing and ceramic ball, Wear 2 (2002) 33–35 (in Chinese). [5] Y. Shimura, Y. Mizutani, Wear of ceramics at high temperatures and its improvement by metallic coating, in: Wear of Materials, ASME (1991) 405–410. [6] R.S. Bhattacharya, A.K. Rai, High energy (MeV) ion beam modification of sputtered MoS2 coatings on ceramics, ASME/STLE Tribology Conference Preprints 1992 Preprint No. 92-TC-6A-1. [7] F. Brenscheidt, S. Oswald, A. Muchlich, Wear mechanisms in titanium implanted silicon nitride ceramics, Nucl. Instrum. Methods Phys. Res. B 129 (1997) 483–486. [8] P.J. Blau, B. Dumont, D.N. Braski, Reciprocating friction and wear behavior of a ceramic-matrix graphite composite for possible use in diesel engine valve guides, Wear 225–229 (1999) 1338–1349. [9] A. Gangopadhyay, S. Jahanmir, Friction and wear characteristics of silicon nitride-graphite and alumina-graphite composites, Tribol. Trans. 34 (2) (1991) 257–265. [10] S. Jahanmir, Firction and Wear of Ceramics, Marcel Dekker Inc, New York, 1994, pp. 163–198. [11] A.G. Evans, P.B. Marshall, Wear mechanism in ceramics, in: Proceedings of the International Conference on Fundamentals of Friction and Wear of Materials, ASME, Pittsburgh, 1980, pp. 439–452. [12] L.K. Rueh, T.T. Ying, Kinetics of b-Si3N4 grain growth in Si3N4 ceramics sintered under high nitrogen pressure, J. Am. Ceram. Soc. 76 (1993) 91–96. [13] P.J. Becher, Silicon nitride ceramics scientific and technology advances, Mater. Res. Soc. 8 (1993) 147–158. [14] L. Chien-Wei, L. Siu-Ching, Jeffrey Goldacker, Relation between strength, microstructure, and grain-bridging characteristics in in-situ reinforced silicon nitride, J. Am. Ceram. Soc. 78 (2) (1995) 449–459. [15] M. Mamoru, U. Satoshi, Microstructural development during gaspressure sintering of a-silicon nitride, J. Am. Ceram. Soc. 75 (1) (1992) 103–108. [16] F.C. Peillon, F. Thevenot, Microstrucral designing of silicon nitride related to toughness, J. Eur. Ceram. Soc. 22 (2002) 271–278. [17] F.P. Becher, Y.E. Sun, P.K. Plucknett, Microstructural design of silicon nitride with improved fracture toughness I, effect of grain shape and size, J. Am. Ceram. Soc. 81 (11) (1998) 2821–2830. [18] N. Hirosaki, Y. Akimune, Effect of grain growth of b-Si3N4 on strength, weibull modulus, and fracture toughness, J. Am. Ceram. Soc. 76 (7) (1993) 1892–1894. [19] P.J. Becher, Silicon nitride ceramics scientific and technology advances, Mater. Res. Soc. 8 (1993) 147–158. [20] A.J. Pyzik, A.R. Prunier, B. Pyzik, Microstructural engineering of silicon nitride ceramics, in: Int. Conf. Silicon nitride 93, Stuttgart, Germany, 1993. [21] R.V. Weeren, C.D. Stephen, The effect of grain boundary phase characteristics on the crack deflection behavior in a silicon nitride material, Scripta Mater. 34 (1996) 1567–1573. [22] D.R. Petrak, J.D. Lee, Silicon nitride/boron nitride composite with enhanced fracture toughness, US Patent 5,324,694 (28 June 1994). [23] K.H. Zum Gahr, W. Bundschuh, B. Zimmerlin, Effect of grain size on friction and sliding wear of oxide ceramics, Wear 162–164 (1993) 269–279. [24] A.K. Mukhopadhyay, Y.M. Mai, Grain size effect on abrasive wear mechanisms in alumina ceramics, Wear 162–164 (1993) 314–321. [25] B. Wallis, Influence of the microstructure of ceramic materials on their wear behavior in mechanical seals, Lub. Eng. 50 (1994) 789–799. 554 D. Wei et al. / Ceramics International 32 (2006) 549–554