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.2005.04010Mechanical 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-lubricat￾ing 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 mechan￾ical properties of the self-lubricating ceramic composite also should be taken into account besides its tribological properties. Generally speaking, the strength of the self￾lubricating 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