Mechanical and Tribological Performance of Graphite/Silicon Nitride Composites: A Comparison between Pressureless and Sp
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SILICON nitride (Si3N4) is one of the most promising high-performance non-oxide ceramics suitable for wear parts applications in both dry and lubricated states even under harsh service condition.[1–5] However, sintering of Si3N4 is a difficult task due to its high Si-N covalent bonding and extremely low self-diffusion coefficient.[6] In the past, several research works have been done to effectively sinter Si3N4 ceramics in the presence of sintering additives, e.g., magnesium oxide (MgO), aluminum oxide (Al2O3), aluminum nitride (AlN), silica (SiO2,) and rare-earth oxides through pressureless (PLS), hot-press (HP), and hot-isostatic press (HIP) sintering.[7–14] Beside these, the state-of-the-art sintering technique, i.e., spark plasma sintering (SPS), has also been utilized for rapid consolidation of Si3N4 in the SUBRATA KR. MAITI, M.Tech. Scholar, and MD. FAROOQ WANI, Professor and Head, are with the Department of Mechanical Engineering, National Institute of Technology-Srinagar, Srinagar, J&K 190006, India. SOUMYA SARKAR, Senior Technical Officer (2), and PROBAL KUMAR DAS, Chief Scientist and Head, are with the Non-oxide Ceramics and Composites Division, CSIR-Central Glass & Ceramic Research Institute, Kolkata 700032, India. Contact e-mail: [email protected] Manuscript submitted July 2, 2015. Article published online October 13, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
presence of a transient liquid phase.[10,15–19] On the other hand, attempts have also been made to further improve the mechanical and tribological properties of Si3N4 through various second phase addition.[20–23] Reinforcements with carbon black (CB), graphite particles (GPs), carbon nanotubes (CNTs), and graphene are not exceptions in this regard.[24–30] Limitations of utilizing CNT or graphene as reinforcement are their cost, poor dispersion in matrix phase due to strong inter-tubular van der Waal attractive forces, resistance to matrix densification, and structural instability at desired sintering temperature and pressure when consolidated using PLS, HIP, or SPS techniques.[25,26,28] On contrary, CB and GP are free from such limitations and can easily be dispersed in the matrix phase and are structurally stable up to sufficiently high temperature and pressure in inert atmosphere.[10,25,31] The primary objective of this paper was to compare the sintering behavior of GP/ Si3N4 composites having filler loading in the range of 1.5 to 10 wt pct using PLS and SPS techniques in inert atmosphere. Physical property evaluation, microstructure and X-ray diffraction (XRD) phase analyses, and mechanical and tribological property measurements of the composites were carried out using standard procedures. The properties were compared with those of pure Si3N4 prepared under similar conditions to assess the extent of property changes through GP reinforcement. VOLUME 46A, DECEMBER 2015—5719
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to 1173 K (900 C) and thereafter, normal furnace cooling was used.
EXPERIMENTAL DETAILS
A. Raw Materials, Batch Preparation, and Sintering Si3N4 powder (M/s UBE Chemic
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