Hardness and refractive index of Ca–Si–O–N glasses Ali Sharafat, Jekabs Grins, Saeid Esmaeilzadeh * Department of Physical, Inorganic and Structural Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden article info Article history: Received 8 August 2008 Received in revised form 23 October 2008 Available online 7 January 2009 PACS: 60.0 70.0 Keywords: Oxynitride glasses Hardness Nitrogen-containing glass abstract Vickers hardness and refractive index was determined for Ca–Si–O–N glasses with 14.6–58 e/o N and 19– 42 e/o Ca. By applying slow cooling rates, transparent glasses were obtained for compositions near Ca 9.94 Si 10 O 17.73 N 8.14 , while the majority of the glasses were opaque due to small inclusions of elemental Si and/or Ca-silicide. Determined glass densities varied between 2.80 and 3.25 g/cm 3 . Hardness was found to vary from 7.3 to 10.1 GPa at a load of 500 g and, respectively increase and decrease linearly with N and Ca content. The refractive index was found to increase linearly with N content from 1.62 to 1.95 and showed no significant dependence on Ca content. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Oxynitride glasses have continued to attract interest since the late 1970’s, at which time it was realized that incorporation of nitrogen into silicate glasses causes profound alterations of mechanical properties as strength, toughness, elastic modulus, and hardness [1–5]. Moreover, the introduction of nitrogen results in higher glass transition temperatures and lower thermal expan- sion coefficients [6]. Largely due to the feasibility to produce refractory silicon nitride based ceramics containing oxynitride glasses as grain-boundary phases, a number of investigations have been carried out on oxynitride glass formation and properties [7– 9]. Oxynitride glasses have since been prepared in systems includ- ing Y–Si–Al–O–N [10–14], Mg–Si–O–N [15,16], La–Si–O–N [17–19], Na–Ca–Si–O–N [20], Na–B–Si–O–N [21], Ca–Si–Al–O–N [22–24], Mg–Si–Al–O–N [8,24], Li–B–O–N [25], Na–B–O–N [25] and K–B–O–N [25]. Nitrogen-containing silicate glasses are usually less transparent than their counterpart oxide glasses and are often gray or black and translucent only in thin sections, due to inclusions of other phases. The opacity of oxynitride glasses has been investigated [14,17,26,27], however, with no substantial progress attained to- wards obtaining fully transparent glasses with high contents of nitrogen. The most common reason for the diminished transpar- ency is allegedly precipitation of silicon [13,14,27] which is as- sumed to result from the decomposition of Si 3 N 4 [28] at temperatures above ca. 1650 °C according to the reaction, Si 3 N 4ðsÞ þ SiO 2ðsÞ ! 2SiO ðgÞ þ 2Si ð1Þ þ 2N 2ðgÞ Two methods have been suggested to avoid the decomposition of Si 3 N 4 ; preparation at high nitrogen pressures (1.6 MPa) [14] and replacement of Si 3 N 4 by Al–N as starting source of nitrogen [14,26]. Despite evidence provided by thermodynamic calculations and sintering experiments on Si 3 N 4 based materials [29], which indicate that higher nitrogen pressures should suppress the decom- position of Si 3 N 4 , the experimental observations by Messier and Deguire [14] shows that the silicon content in Y–Si–Al–O–N glasses is not much reduced by applying high nitrogen pressure. The trans- parency of the glasses has been found to increase by selecting com- positions which can be melted at lower temperatures, e.g. 1450 °C, at which a decomposition of Si 3 N 4 is not expected to occur. Observed property changes for oxynitride glasses with increas- ing N content is consistent with a structural model in which nitro- gen substitutes for oxygen and, due to differences in bonding and anion coordination, produces a more tightly linked glass network. An increase of the nitrogen content increases the microhardness, as evidenced by the studies of the Na–Ca–Si–O–N [20] and Ca–Si–Al– O–N [22,23] systems. The microhardness also varies substantially with the type of modifier; e.g. Lu- and Yb-containing glasses exhi- bit higher hardness values than corresponding La-containing glasses [30]. It is well known that the refractive index n of glasses changes as a rule continuously with composition [31]. Oxynitride glass sys- tems that have been investigated with regard to refractive index 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.11.019 * Corresponding author. Tel.: +46 8 161258; fax: +46 8 152187. E-mail address: zaida@inorg.su.se (S. Esmaeilzadeh). Journal of Non-Crystalline Solids 355 (2009) 301–304 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol