Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Effect of nano-size oxy-nitride starting precursors on spark plasma sintering of calcium sialons along the alpha/(alpha + beta) phase boundary B.A. Ahmed a,d , A.S. Hakeem c, ⁎ , T. Laoui b,d, ⁎⁎ a Department of Mechanical Engineering, College of Electrical and Mechanical Engineering (CE&ME), National University of Sciences and Technology (NUST), Pakistan b Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah, United Arab Emirates c Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia d Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Saudi Arabia ARTICLE INFO Keywords: Phase boundary Ca-α-Sialon Spark plasma sintering Nano precursors Silicon nitride ABSTRACT Several compositions of calcium stabilized sialon ceramics were synthesized by using nano-size starting powder precursors and spark plasma sintering technique at a relatively low temperature of 1500 °C. The formation of calcium alpha/beta-sialons was investigated for the compositions represented by Ca m/2 Si 12-(m+n) Al m+n O n N 16-n where m and n values were varied from 0.6 to 1.6 and 0.4–1.6, respectively. Phase analysis of the selected compositions helped in developing the phase boundary between alpha and alpha/beta phase regimes. Effect of m and n values on the evolution of the final phase(s), densification and mechanical properties were evaluated. All samples yielded densified ceramics with density values (3.13–3.19 g/cm 3 ) comparable with those reported in the literature for similar compositions and synthesized at temperatures greater than 1700 °C via conventional sin- tering techniques. Vickers hardness value HV 10 of 20 GPa was measured for the Ca 0.5 Si 10.6 Al 1.4 O 0.4 N 15.6 com- position (with m = 1.0 and n = 0.4) synthesized at 1500 °C. An increase in n value (higher oxide content) was observed to facilitate the formation of beta-sialon and AlN polytype phases leading to an increase in fracture toughness but a decrease in Vickers hardness. 1. Introduction Silicon nitride based ceramic materials have engrossed themselves as an ideal choice for applications requiring elevated temperatures for about 60 years [1–6]. The significance of these materials remains subject to their outstanding mechanical and thermal characteristics such as high tensile strength, good oxidation resistance, appreciable resistance to thermal shock, the relatively low coefficient of friction and high wear resistance. Although the development of silicon nitride finds its advent almost a century ago, however, it was only in 1961 that hot pressing technique was employed to achieve fully dense silicon nitride [7]. Ever since then a tremendous amount of work has been carried out on the subject material and has resulted in remarkable advancement this class of inert ceramics [8,9]. Nevertheless, outstanding mechanical properties in these ceramic materials remain subject to achievement of full densification [6].A major hindrance in achieving fully densified silicon nitride ceramics is the retardation in diffusion process caused by the highly covalent nature of bonds. Together with this, practical complexity of synthesis at high temperature as well as, sufficiently long sintering periods leads to the addition of metal oxide additives such as MgO, A1 2 O 3 ,Y 2 O 3 , and Ln 2 O 3 , to help synthesize fully compact materials at much lower tem- peratures [6,10–15]. The outcome of this addition was the development of sialon materials having an additive controlled structure-property relationship [16–18]. Sialons normally appear in the form of alpha and beta phases, having distinct microstructure as well as thermal and mechanical properties. Beta-sialon is achieved by simultaneously replacing silicon and nitrogen atoms with equal amounts of aluminum and oxygen, re- spectively. The general formula representing beta-sialon has been ar- ticulated as Si 6-z Al z O z N 8-z , whereas; alpha-sialon is generally re- cognized by the chemical formula of M x v Si 12-(m+n) Al m+n O n N 16-n , where x is less than 2, x = mv, and m (Al-N), n (Al-O) substitute (m+n) (Si-N) bonds [16,18–20]. The microstructural appearance of alpha-sialon has been observed as equiaxed grains with very high hardness as compared to the elongated morphology of beta grains which display relatively high fracture toughness compared to alpha-sialon [21,22]. Of the various oxide additives employed, greater solubility limit as https://doi.org/10.1016/j.ceramint.2018.11.005 Received 28 July 2018; Received in revised form 27 October 2018; Accepted 1 November 2018 ⁎ Corresponding authors. ⁎⁎ Corresponding author at: Department of Mechanical and Nuclear Engineering, University of Sharjah, Sharjah, United Arab Emirates. E-mail addresses: ashakeem@kfupm.edu.sa (A.S. Hakeem), tlaoui@sharjah.ac.ae (T. Laoui). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Ahmed, B.A, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.11.005