Two step sintering of a novel calcium magnesium silicate bioceramic: Sintering parameters and mechanical characterization Ali Nadernezhad a , Fathollah Moztarzadeh a,∗ , Masoud Hafezi b,∗∗ , Hadi Barzegar-Bafrooei c a Biomaterials Group, Faculty of Biomedical Engineering (Center of Excellence), Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran b Biomaterials Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center, P.O. Box 31787-316, Karaj, Iran c Ceramic Division, Materials and Energy Research Center, P.O. Box 31787-316, Karaj, Iran Received 31 October 2013; received in revised form 2 May 2014; accepted 8 May 2014 Available online 28 May 2014 Abstract Two-step sintering (TSS) was applied to control the grain growth during sintering of a novel calcium magnesium silicate (Ca 3 MgSi 2 O 8 – Merwinite) bioceramic. Sol–gel derived nanopowders with the mean particle size of about 90 nm were sintered under different TSS regimes to investigate the effect of sintering parameters on densification behavior and grain growth suppression. Results showed that sintering of merwinite nanopowder under optimum TSS condition (T 1 = 1300 ◦ C, T 2 = 1250 ◦ C) yielded fully dense bodies with finest microstructure. Merwinite compacts held at T 2 = 1250 ◦ C for 20 h had the average grain size of 633 nm while the relative density of about 98% was achieved. Mechanical testing was performed to investigate the effect of grain growth suppression on the hardness and fracture toughness. Comparison of mechanical data for samples sintered under two sintering regimes, including TSS and normal sintering (NS), showed TSS process resulted in significant enhancement of fracture toughness from 1.77 to 2.68 MPa m 1/2 . © 2014 Elsevier Ltd. All rights reserved. Keywords: Two-step sintering (TSS); Merwinite; Mechanical properties; Sinterability 1. Introduction During the past two decades, many researchers have shown interest on development of novel processes and techniques to produce nanostructured synthetic ceramics. These efforts have mainly focused on using novel synthesis methods as well as improved processing techniques. Sintering, as an important stage in ceramics processing, has also undergone significant modifications and some novel sintering routs were introduced. Since vast microstructural changes occur during sintering, the main goal of these novel techniques was to control the microstructural features of ceramics like grain size and density. ∗ Corresponding author. Tel.: +98 21 64542393; fax: +98 21 64542393. ∗∗ Corresponding author. Tel.: +98 26 36280040 9; fax: +98 26 36201888. E-mail addresses: a.nadernezhad@aut.ac.ir (A. Nadernezhad), moztarzadeh@aut.ac.ir (F. Moztarzadeh), mhafezi@merc.ac.ir, masoud.hafezi@gmail.com (M. Hafezi), hadi.merc@gmail.com (H. Barzegar-Bafrooei). Using of sintering additives was reported as a successful method to control grain growth during sintering. 1–4 However, there are some limitations and disadvantages for using of a second phase particles due to the possible adverse effects on physical and chemical properties. Many researchers reported successful uti- lization of pressure and extremely high rates of heating during sintering like hot pressing (HP), 5 spark plasma sintering (SPS) 6 and microwave sintering (MWS). 7 The main aim of these non- conventional sintering methods was shortening the heating time to hinder the grain growth. However, the application of pressure and also high rates of heating would be costly and needs special equipment. A new approach which has been introduced by Chen and Wang 8 is to suppress the grain growth during final stage of sin- tering, by taking advantage of the difference between required energy for migration of grain boundaries and the activation energy required for diffusion of mater through grain boundaries. The simple nature of this promising method, which is called two- step sintering (TSS), made it attractive for many researchers