Availableonlineatwww.sciencedirect.com ° ScienceDirect CERAMICS INTERNATIONAL ELSEVIER Ceramics International 35(2009)747-753 www.elsevier.com/locate/ceramint Pore geometry of 3D-C/Sic composites by mercury Intrusion porosimetry Wei li.Zhao Hui Chen State Key Laboratory of Advanced Ceramic Fibers Composites, College rospace and Materials Engineering National University of Defense Technology, Changsha 410073, PR China Received 27 August 2007: received in form 15 November 2007; accepted 8 February 2008 Available online 4 June 2008 Abstract The 3D-CSiC composites fabricated via precursor infiltration and pyrolysis(PIP)are porous inside due to their specific processing. To evaluate the porosity of 3D-CSiC, a novel procedure of mercury intrusion porosimetry (MIP)was adopted to extract information from the hysteresis and entrapment. This method is able to eliminate the temporarily retained Hg at atmospheric pressure from the real entrapment due to topological reasons From the interpretation of the MIP primary and secondary intrusion-extrusion data, accompanied by scanning electron microscopy(SEM) analysis and bubble point measurement, the pore geometry of 3D-CiSiC is supposed to be a 3D network originating from the architecture of braided carbon fabrics. This network is composed of hundreds of micron-sized large chambers between bundles, micro-cracks below 0. 1 um and medium-sized channels about 20-4 um that bridge the former two and provide passages for fiuids permeating the material. C 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Porosity; CMCs; 3-Dimensional reinforcement; Analytical model; Mercury intrusion 1. Introduction two classic methods to characterize the porosity, specific surface area, pore size distribution(PSD), and the surface Precursor infiltration and pyrolysis(PlP) is one of the most roughness/surface fractal dimensions etc. [5,6]. Examples of important fabrication processes for 3-dimensional carbon the characterization of FRC by MIP are also reported [7, 8]. The fabric reinforced SiC (3D-Cr/SiC) composites. However, due interpretation of MIP results is based on the Washburn equation to the pyrolysis gas escape and incomplete precursor [9], under the assumption that the pores are bundles of infiltration, there are inevitably some voids and cracks in the capillaries with various sizes which are equally accessible to 3D-C/SiC even after successive infiltration-pyrolysis cycles. the exterior mercury reservoir. The deviation of real cases from Thus, the 3D-CSic composites are not totally dense but the above simplified model results in the intrusion-extrusion actually porous. This specific microstructure has critical hysteresis and mercury entrapment. Several explanations have infuence on C/SiC mechanical and thermal properties [1, 2], been offered to account for these phenomena, the most accepted thus its evaluation is attractive and valuable undoubtedly. For one is the so-called"ink-bottle"effect [10] i.e., the pores fibers reinforced composites(FRC), scanning electron micro- chambers are surrounded by smaller pore throats and access to scopy(SEM) is prevalently used to characterize the inner the outside via them. Due to the shielding of throats on morphology [3, 4], however, it is difficult to evaluate the chambers, intrusion hysteresis occurs and the calculated PSD porosities of the composites by SEM only, for its 2- will be biased towards smaller than actual status, thus is also dimensional, limited sight-fields called the pore throat size distribution(PTSD). with the MIP For porous media, especially solids, mercury intrusion becoming popular in many fields, this throat-chamber model porosimetry (MIP)and isothermal N2 sorption(INS)are the faces increasing challenges and some new concepts have been raised to describe the complicated experimental results, e.g he hysteresis of MIP is separated to"structural hysteresis"and Corresponding author. Tel: +86 731 4576397; fax: +86 731 4573162 the"parametric hysteresis"[ll], according to their sources. onductor1979@yahoo.com.cn(w.Li The former is caused by the connectivity of pore network and 2-8842/34.00@ 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved 0.1016 JceramIst.2008.0201lPore geometry of 3D-Cf/SiC composites by mercury intrusion porosimetry Wei Li *, Zhao Hui Chen State Key Laboratory of Advanced Ceramic Fibers & Composites, College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China Received 27 August 2007; received in revised form 15 November 2007; accepted 8 February 2008 Available online 4 June 2008 Abstract The 3D-Cf/SiC composites fabricated via precursor infiltration and pyrolysis (PIP) are porous inside due to their specific processing. To evaluate the porosity of 3D-Cf/SiC, a novel procedure of mercury intrusion porosimetry (MIP) was adopted to extract information from the hysteresis and entrapment. This method is able to eliminate the temporarily retained Hg at atmospheric pressure from the real entrapment due to topological reasons. From the interpretation of the MIP primary and secondary intrusion–extrusion data, accompanied by scanning electron microscopy (SEM) analysis and bubble point measurement, the pore geometry of 3D-Cf/SiC is supposed to be a 3D network originating from the architecture of braided carbon fabrics. This network is composed of hundreds of micron-sized large chambers between bundles, micro-cracks below 0.1 mm and medium-sized channels about 20–4 mm that bridge the former two and provide passages for fluids permeating the material. # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Porosity; CMCs; 3-Dimensional reinforcement; Analytical model; Mercury intrusion 1. Introduction Precursor infiltration and pyrolysis (PIP) is one of the most important fabrication processes for 3-dimensional carbon fabric reinforced SiC (3D-Cf/SiC) composites. However, due to the pyrolysis gas escape and incomplete precursor infiltration, there are inevitably some voids and cracks in the 3D-Cf/SiC even after successive infiltration–pyrolysis cycles. Thus, the 3D-Cf/SiC composites are not totally dense but actually porous. This specific microstructure has critical influence on Cf/SiC mechanical and thermal properties [1,2], thus its evaluation is attractive and valuable undoubtedly. For fibers reinforced composites (FRC), scanning electron micro￾scopy (SEM) is prevalently used to characterize the inner morphology [3,4], however, it is difficult to evaluate the porosities of the composites by SEM only, for its 2- dimensional, limited sight-fields. For porous media, especially solids, mercury intrusion porosimetry (MIP) and isothermal N2 sorption (INS) are the two classic methods to characterize the porosity, specific surface area, pore size distribution (PSD), and the surface roughness/surface fractal dimensions etc. [5,6]. Examples of the characterization of FRC by MIP are also reported [7,8]. The interpretation of MIP results is based on the Washburn equation [9], under the assumption that the pores are bundles of capillaries with various sizes which are equally accessible to the exterior mercury reservoir. The deviation of real cases from the above simplified model results in the intrusion–extrusion hysteresis and mercury entrapment. Several explanations have been offered to account for these phenomena, the most accepted one is the so-called ‘‘ink-bottle’’ effect [10] i.e., the pores’ chambers are surrounded by smaller pore throats and access to the outside via them. Due to the shielding of throats on chambers, intrusion hysteresis occurs and the calculated PSD will be biased towards smaller than actual status, thus is also called the pore throat size distribution (PTSD). With the MIP becoming popular in many fields, this throat-chamber model faces increasing challenges and some new concepts have been raised to describe the complicated experimental results, e.g., the hysteresis of MIP is separated to ‘‘structural hysteresis’’ and the ‘‘parametric hysteresis’’ [11], according to their sources. The former is caused by the connectivity of pore network and www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 35 (2009) 747–753 * Corresponding author. Tel.: +86 731 4576397; fax: +86 731 4573165. E-mail address: superconductor1979@yahoo.com.cn (W. Li). 0272-8842/$34.00 # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2008.02.011