Effects of nitrogen on phase formation, microstructure and mechanical properties of Y–Ca–Si–Al–O–N oxynitride glass–ceramics Zhiwei Luo, Anxian Lu ⁎, Xiaolin Hu, Weizhen Liu School of Materials Science and Engineering, Central South University, Changsha 410083, PR China abstract article info Article history: Received 9 January 2013 Received in revised form 25 February 2013 Available online 3 April 2013 Keywords: Oxynitride glass; Glass–ceramics; YAG; SEM; Mechanical properties The effect of nitrogen substitution on the crystallisation of an oxynitride glass in the Y–Ca–Si–Al–O–N system has been studied. The appropriate heat treatment temperatures were selected according to the information provided by the differential scanning calorimeter (DSC) measurement. There is a significant increase in T g and T c with increasing nitrogen content. X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis demonstrated that, for the oxide glass and oxynitride glasses containing 6 equiv.% and 12 equiv.% nitrogen, crystallisation results in the formation of irregular lath-shaped Ca 4 Y 6 O(SiO 4 ) 6 and stick-shaped anorthite. As the nitrogen content increases to 18 or 24 equiv.%, irregular plate-like yttrium-aluminium garnet (YAG) is identified as the main crystalline phase. As the nitrogen content increases to 30 equiv.%, microscopic needle-like crystals of Al 6 O 3 N 4 become the only crystallised phase. The best composition, owing to the mechanical properties (e.g. flexural strength of 162 MPa and Vickers hardness of 8.5 GPa), was found to correspond to a nitrogen content of 24 equiv.%. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Silicon oxynitride glasses were first discovered as grain boundary phases in silicon nitride based ceramics [1]. Silicon nitride ceramics are usually densified with the aid of various sintering additives (such as MgO, Y 2 O 3 , etc.). These additives in combination with silicon dioxide, which originates from the furnace atmosphere, or from the dissolution of silicon nitride particles in the melt, cause the formation of a liquid phase that transforms into a glass upon cooling [2]. Since it is very diffi- cult to investigate experimentally the properties of grain boundary phases, research on them in bulk form was initiated [3]. Oxynitride silicate glasses are a branch of high performance glasses, obtained by incorporating nitrogen atoms into silicate or alumino-silicate glasses [4,5]. The unique properties of silicon oxynitride and SiAlON glasses have led to a search for areas of potential application [6–8]. The improvement in physical and mechanical properties when nitrogen is incorporated into silicate or alumino-silicate glasses is also realised in glass–ceramics. As with other silicate or alumino-silicate glasses, oxynitride glasses may be heat treated at the appropriate temperatures to crystallise as glass–ceramics and many studies of crystallisation of these types of glasses have been reported [9–14]. The crystallization improves the mechanical and thermal properties. The specific crystalline phases formed upon heat treatment, and the extent of their formation, determine the properties of the material [15–18]. In general, nitrogen increases the stability of oxynitride glasses. The conventional process to produce a glass–ceramic involves two steps: a lower temperature heat treatment of glasses, generally just above the glass transition temperature, to induce nucleation, followed by heating to a second higher temperature, the so-called crystallisation tempera- ture, to allow growth of the pre-formed nuclei. The crystalline phases formed depend on both the composition of the parent glass and the heat-treatment process but in many cases, oxide phases form first leav- ing the residual glass more N-rich [19]. The present work investigates the influence of the nitrogen addi- tion on phase formation, microstructure and mechanical properties of Y–Ca–Si–Al–O–N glass. XRD and SEM were used to study the crys- tallization behaviour of the glass–ceramics. An understanding of the roles played by nitrogen atom in this system is clearly vital to a coher- ent approach to improved material performance. 2. Experimental procedure 2.1. Preparation of materials A base Y–Ca–Al–Si–O oxide glass was prepared with a cation com- position (in equiv.%) of 12Y:12Ca:61Si:15Al and 100 equiv.% oxygen. 6, 12, 18, 24 and 30 equiv.% N was substituted for oxygen in order to evaluate the effects of nitrogen on crystallization of oxide glass or oxynitride glasses. Y 2 O 3 (99.99%), α-Si 3 N 4 (99.8%), SiO 2 (99.9%), CaCO 3 (99.9%), and Al 2 O 3 (99.9%) were used as raw material powders to prepare Y–Ca–Si–Al–O–N glasses. Dried powders were weighed and performed by mechanical agitation (using an attritor mill), mixed in isopropyl alcohol for 24 h, and then dried again. The mixture was Journal of Non-Crystalline Solids 368 (2013) 79–85 ⁎ Corresponding author. Tel.: +86 731 88830351; fax: +86 731 88877057. E-mail addresses: axlu@csu.edu.cn, loaswell@163.com (A. Lu). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.03.005 Contents lists available at SciVerse ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol