MATERIALS 兴 HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 475(2008)217-223 www.elseviercom/locate/msea and silicon nitride ceramic matrix composites prepard ide Microstructure and properties of particle reinforced silicon c by chemical vapor infiltration Yongsheng liu Laifei Cheng, Litong Zhang, Yunfeng hua, Wenbin Yan National Key Laboratory of Thermostricture Composite Materials, Northwestern Polytechnical University. Xi'an Shaanxi 710072, People's Republic of China Received 28 November 2006; received in revised form 10 April 2007: accepted 10 April 2007 Introduction: Particle reinforced silicon carbide andsilicon nitride ceramic matrix composites were fabricated using designed particle agglomeration and chemical vaporinfiltration(CVi)technique. Scanning electron microscopy(SEM)and Tra electron microscopy (TEM)were employed to observe the microstructures of the preforms and as-infiltrated composites. In the preform, the inter-agglomeration and intra-agglomeration pores had an approximate size of 500-800 um and 5-10 um, respectively. After infiltrated, a large amount of silicon carbide and silicon nitride matrix yere infiltrated in the preform, the sizes of inter-agglomeration and intra -agglomeration pores were 200-400 um and 2-4 um, respectively. The Sic and Si3N4 whiskers were observed in the residual intra-agglomeration pores. The flexural strength of SiC(p/SiC composites changed with first-step and second-step pressure. The maximum of the strength was 284 MPa, and the ratio of the retained strength was 95.4% at 1600C. The fracture toughness of SiC(p)/SiC composites was around 7 Mpa m". The Si3 Na(p/Sia N4 composite attained an acceptable strength, 113.4 MPa and low dielectric constant, about 4.2-4.3 @2007 Elsevier B v. All rights reserved Keywords: Particle agglomeration; Ceramic matrix composites; Chemical vapor infiltration; Silicon carbide; Silicon nitride 1. Introduction particular applications depends on their high temperature per- formance, which is intimately linked to the amount and nature Sic and Si3N4 are two of the most important thermo- of the sintering additives, on the microstructure characteristics, structural ceramic materials [1]. In addition, Si3N4 is considered and eventually, on the second phase distribution and content to be a suitable candidate for high temperature radomes because [10-14 of the following properties:(1)a high mechanical strength; (2) The conventional methods to fabricate particle reinforced Sic a good thermal shock resistance; (3)excellent resistance to rain or Si3N4 composites are sintering methods such as pressureless and sand erosion; (4)an acceptable dielectric constant; (5)a low sintering, hot pressing, and hot isostatic pressing, employing dielectric losses[2]. However, the brittleness of these ceramics sintering additives. The residue of sintering additives existed mits their applications. Therefore, these ceramics have been as continuous intergranular glassy phase, which will soften at reinforced by particle [3], whisker [4] and fiber [5]. Particle elevated temperature. Furthermore, CTE mismatch of matrix, reinforced ceramic matrix posites have received great con- intergranular phase and the reinforcements resulted in the for- cern due to their improved fracture toughness and thermal shock mation of microcracks in the composites, which had a notably resistance and isotropic properties for a variety of high temper- negative effect on high temperature mechanical and chemical ture, high stress and severe erosion applications in aerospace, properties [15-19 hot engine, and energy conversion devices [6-9]. The suitabil- SiC and Si3 N4 matrix composites without sintering additives ity of the isotropic SiC and Si3N4 matrix composites to these can be fabricated by using hot isostatic pressing, which needs ultra-high temperature and pressure and ultra-fine and purity powders. Besides the high costs, it is difficult to fabricate SiC or Corresponding author. Tel +8629 8848 6068 823: fax:+8629 88494620. Si3N4 matrix composites by employing HIP process due to the E-mailaddressliuys99067@163.com(y.Liu). echnical difficulties [13, 20]. Chemical vapor infiltration(CVi)Materials Science and Engineering A 475 (2008) 217–223 Microstructure and properties of particle reinforced silicon carbide and silicon nitride ceramic matrix composites prepared by chemical vapor infiltration Yongsheng Liu ∗, Laifei Cheng, Litong Zhang, Yunfeng Hua, Wenbin Yang National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, People’s Republic of China Received 28 November 2006; received in revised form 10 April 2007; accepted 10 April 2007 Abstract Introduction: Particle reinforced silicon carbide and silicon nitride ceramic matrix composites were fabricated using designed particle agglomeration and chemical vapor infiltration (CVI) technique. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) were employed to observe the microstructures of the preforms and as-infiltrated composites. In the preform, the inter-agglomeration and intra-agglomeration pores had an approximate size of 500–800 m and 5–10 m, respectively. After infiltrated, a large amount of silicon carbide and silicon nitride matrix were infiltrated in the preform, the sizes of inter-agglomeration and intra-agglomeration pores were 200–400 m and 2–4m, respectively. The SiC and Si3N4 whiskers were observed in the residual intra-agglomeration pores. The flexural strength of SiC(p)/SiC composites changed with first-step pressure and second-step pressure. The maximum of the strength was 284 MPa, and the ratio of the retained strength was 95.4% at 1600 ◦C. The fracture toughness of SiC(p)/SiC composites was around 7 Mpa m1/2. The Si3N4(p)/Si3N4 composite attained an acceptable strength, 113.4 MPa and low dielectric constant, about 4.2–4.3. © 2007 Elsevier B.V. All rights reserved. Keywords: Particle agglomeration; Ceramic matrix composites; Chemical vapor infiltration; Silicon carbide; Silicon nitride 1. Introduction SiC and Si3N4 are two of the most important thermo￾structural ceramic materials[1]. In addition, Si3N4 is considered to be a suitable candidate for high temperature radomes because of the following properties: (1) a high mechanical strength; (2) a good thermal shock resistance; (3) excellent resistance to rain and sand erosion; (4) an acceptable dielectric constant; (5) a low dielectric losses [2]. However, the brittleness of these ceramics limits their applications. Therefore, these ceramics have been reinforced by particle [3], whisker [4] and fiber [5]. Particle reinforced ceramic matrix composites have received great con￾cern due to their improved fracture toughness and thermal shock resistance and isotropic properties for a variety of high temper￾ature, high stress and severe erosion applications in aerospace, hot engine, and energy conversion devices [6–9]. The suitabil￾ity of the isotropic SiC and Si3N4 matrix composites to these ∗ Corresponding author. Tel.: +86 29 8848 6068 823; fax: +86 29 8849 4620. E-mail address: liuys99067@163.com (Y. Liu). particular applications depends on their high temperature per￾formance, which is intimately linked to the amount and nature of the sintering additives, on the microstructure characteristics, and eventually, on the second phase distribution and content [10–14]. The conventional methods to fabricate particle reinforced SiC or Si3N4 composites are sintering methods such as pressureless sintering, hot pressing, and hot isostatic pressing, employing sintering additives. The residue of sintering additives existed as continuous intergranular glassy phase, which will soften at elevated temperature. Furthermore, CTE mismatch of matrix, intergranular phase and the reinforcements resulted in the for￾mation of microcracks in the composites, which had a notably negative effect on high temperature mechanical and chemical properties [15–19]. SiC and Si3N4 matrix composites without sintering additives can be fabricated by using hot isostatic pressing, which needs ultra-high temperature and pressure and ultra-fine and purity powders. Besides the high costs, it is difficult to fabricate SiC or Si3N4 matrix composites by employing HIP process due to the technical difficulties [13,20]. Chemical vapor infiltration (CVI) 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.04.031