# The Effect of the Chemical Composition on Mechanical Properties of CMAS Diopside Glass Ceramics
## 1 Introduction  
Glass ceramics (GCs) are relatively new materials created by the partial crystallization of glass at a carefully controlled temperature [1; 2]. Therefore, GCs are inorganic and non-metallic materials with both glassy and crystalline phases. Because of the presence of crystalline phases in the residual and continuous glassy phases, GCs have a variety of unusual mechanical, electrical, optical and magnetic properties, which make GCs widely used in industrial areas such as architecture, building, optics, electronics and domestic [1; 3; 4; 5]. Among these features, the superior mechanical properties are significant since the crystalline phase in GCs can contribute strength and the glassy phase can offer ductility [6; 7]. Furthermore, through the control of the chemical composition, crystal size and thermal treatment, the mechanical properties can also be modified significantly [8; 9; 10; 11]. There are various types of GCs. One common classification method is based on the composition and crystalline structure, such as for silicate GCs, phosphate GCs and borate GCs. CaO-MgO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) (CMAS) glass is an important system to produce GCs owing to easy access to their natural raw materials [12; 13]. As the CMAS system is a quaternary system with a complicated phase diagram, there are vast different crystalline structures that can be nucleated by adjusting the temperature and composition. Some common and important crystalline phases nucleated in the CMAS system are dipside (CaO-MgO-2 SiO\({}_{2}\)), northite (CaO-Al\({}_{2}\)O\({}_{3}\)-2 SiO\({}_{2}\)) and cordierite (MgO-2 Al\({}_{2}\)O\({}_{3}\)-5 SiO\({}_{2}\)) and most of these crystalline phases have chemical stability [14]. Among these crystalline phases, the dipside phase can make the CMAS GCs have superior mechanical properties compared to others, which makes the diopside GCs have a wide range of applications [3; 15; 16; 17; 18]. Moreover, the CaO-MgO-SiO\({}_{2}\) glass systems (CMS) are shown to have high biocompatibility andbioactivity and stimulate new bone growth [15; 16; 17; 19]. So, the diopside GCs have become an ideal candidate in the biomedical domain. The mechanical properties of diopside GCs in the CMAS system can be controlled by many factors, such as the temperature, chemical composition, nucleating agents, crystal size, etc. [3; 15; 17]. Most previous works have focused on the effect of the sintering temperature and the addition of various nucleation agents, such as ZrO\({}_{2}\), B\({}_{2}\)O\({}_{3}\) and P\({}_{2}\)O\({}_{5}\)[3; 20; 21]. To make the diopside GCs work well in biomedical applications, it is necessary to systematically evaluate the strength and fracture toughness, so that they can fit as closely as possible with the corresponding biological tissues, such as cortical bone, trabecular bone and dentine [22; 23]. The nucleation agent, sintering temperature and crystal size can significantly influence the production and morphologies of GCs, while the composition of the CMAS glass phase can also affect the mechanical properties, such as the strength and fracture toughness. However, it is by no means a trivial task since the composition effect with a wide range of variable space using an experimental approach is usually time-consuming and expensive. Moreover, the in situ capture of the crack propagation events in GCs is extremely challenging using traditional SEM and TEM approaches and the important atomic details may also be neglected. As an alternative approach to experiments, atomistic simulation can offer some unique capabilities for observing the atomic structures of glass, crystals and their interface, providing insights into the complex deformation mechanism involved in the strength and fracture properties [24]. However, the atomistic simulations on GCs are extremely limited in the existing literature.