Effect of fluorine and nitrogen content on the properties of Ca-Mg-Si-Al-O- (N)-(F) glasses

X-ray amorphous glasses of composition (in equivalent percent) 15Ca:15Mg: 55Si:15Al:(100-x-y)O: xN:yF with x = 0, 10, 15 and y= 0, 1, 3, 5, were prepared by melting and casting. The effects of oxygen substitution by fluorine and/or nitrogen on the physical, mechanical, thermal and optical properties of the glasses have been investigated. Molar volume, fractional glass compactness, microhardness, Young’s Modulus, glass-transition temperature, dilatometric-softening point and refractive index increased linearly with nitrogen substitution for oxygen, whereas molar volume and thermal expansion coefficient decreased linearly with nitrogen increase. In contrast, all properties except glass-transition temperature and dilatometric-softening point, are virtually unaffected by fluorine substitution for oxygen. Significant and linear, decreases in thermal properties occurred with increasing fluorine substitution level. All the data collected and its analysis clearly showed that the substitution effects of fluorine for oxygen on the studied properties of the glasses of the system with general formula Ca-Mg-Si-Al-O-(N)-(F) are totally independent and additive with respect to the substitution effects of nitrogen for oxygen on glass properties.


I. Introduction
Oxynitride glasses are special types of silicate or aluminosilicate glasses in which the oxygen atoms of the glass network are partially replaced by nitrogen atoms. Numerous studies have focused on the formation and properties of oxynitride glasses in different systems [1][2][3][4][5][6][7][8][9][10] showing that increases in glass transition temperature, hardness, elastic modulus, or viscosity, with regard to the corresponding oxide glasses, are due to a higher crosslink density provided by nitrogen within the glass network. In the case of fluorine-containing glasses, monovalent fluorine acts as a network terminator, thus reducing the connectivity of the glass network and causing a marked reduction in Tg and viscosity as the fluorine content of the glasses is increased 11,12 .
These fluorine-containing glasses are used for a wide variety of purposes, among them bioglasses and bioglass ceramics, where fluoride release stimulates hydroxyapatite formation 13 which bonds to human bone due to similar phase structure [14][15][16] . Fluorine is also introduced into ionomer glasses which are used for glass polyalkenoate dental cements, where fluorine atoms are added to lower the refractive index of the glass as well as to enable fluoride ion release from the set cement 17 to prevent secondary caries 18 . Fluorine ions in human saliva and plasma also play an important role in development of hard tissues in the body 19 .
There is very little information available on oxyfluoronitride glasses and their properties. Vaughn and Risbud 20 incorporated nitrogen into glasses in the Zr-Ba-Al-Y-O-F system with the intention of increasing their thermal stability and improving their mechanical properties. What they actually observed was an increase in the glass transition temperature and crystallisation temperatures, as well as hardness. Later, Fletcher and Risbud 21 used nitrogen to increase the stability and chemical durability of some fluorophosphate glasses in the M-Al-P-O-F-N system (where M = Ba, Na). However, it was not until 20 years later that the first systematic studies were carried out in order to determine the combined effect of adding nitrogen and fluorine to aluminosilicate glasses 22 .
The authors of this study explored the glass-forming region in the Ca-Si-Al-O-N-F system and compared it with that obtained previously in the Ca-Si-Al-O-N system 23 . The addition of fluorine, even in low quantities (1 eq%), was shown to expand the glass-forming region, facilitating the dissolution of the modifier cations and nitrogen into the melt. The authors concluded that fluorine affects nitrogen dissolution into the melt, lowering the melting temperature and preventing the formation of crystalline phases. This study opened up the possibility of obtaining glasses with even higher nitrogen contents by using fluorine as a melting agent, harnessing the wellknown beneficial effects of nitrogen incorporation on the their physical and mechanical properties.
In a previous study 24 , a systematic investigation was carried out of the effects of oxygen replacement by fluorine and nitrogen simultaneously on the physical (density, molar volume and compactness), thermal (glass-transition temperature, dilatometric-softening temperature and thermal expansion coefficient) and mechanical (Young's modulus and microhardness) properties of calcium-and calcium-yttrium-modified aluminosilicate glasses with a constant cation composition. In the previous work, an increase in fluorine substitution for oxygen was observed to produce an increase of thermal expansion coefficient, a decrease of glass-transition temperature and dilatometric-softening point and virtually no effect on molar volume, fractional glass compactness, Young's modulus and microhardness. Furthermore, increase in nitrogen substitution for oxygen was observed to produce an increase of glass fractional compactness, glass-transition temperature, dilatometric-softening point, Young's modulus and microhardness and a decrease of glass molar volume and thermal expansion coefficient. The results clearly showed that the effects of substitution of fluorine for oxygen and nitrogen for oxygen were independent and additive and that the effects on the property were very similar for the two different modifier compositions investigated.
This paper reports on an analogous study of calcium-magnesium aluminosilicate oxyfluoronitride glasses and how the modification of the fluorine and/or nitrogen substitution for oxygen affects their physical (density, molar volume and compactness), mechanical (microhardness, Young's modulus, shear modulus, compressive modulus and Poisson's rate), thermal (glass-transition temperature, dilatometric-softening point and thermal expansion coefficient) and optical (refraction index) properties.

II. Experimental (1) Design of glass composition
For the formulation of the studied glasses, a preliminary study was carried out in order to find a suitable Ca:Mg:Al:Si cation ratio which would allow a systematic study of the progressive substitution of oxygen by fluorine for different nitrogen contents. The initial chosen compositions are in the Mg-Si-Al-O-N system because its glass formation region is shorter than in the corresponding Ca-Si-Al-O-N system 25  The value of the partial Mg substitution by Ca (15Ca:15Mg:55Si:15Al in eq%) was selected in order to be able to obtain similar fluorine and nitrogen substitution levels than the ones reached in previous work 24 . The nitrogen composition was modified between 0 and 15 eq% and the fluorine content between 0 and 5 eq%. It was impossible to prepare glasses with higher nitrogen contents due to devitrification which occurred when these high nitrogen content compositions were formulated with also high contents of fluorine (5 eq%).
The compositions of the twelve glasses in the Ca-Mg-Si-Al-O-(N)-(F) system used in this study are given in Table I. temperature for 1 h, to relieve cooling stresses, and then slow furnace-cooled to ambient temperature.
Specimens of each glass were cleaned and dried and then weighed dry and immersed in water to enable glassdensity determination by the Archimedes principle. Glass compactness (C) was calculated according to the expression: where xi is the fraction of ionic species "i", νi the volume of ionic species and mi the ionic mass of the species; N is Avogadro´s number and ρ the glass density. The value of νi was calculated using the ionic radii given by Shannon 27 and the expression: The molar volumes (MV) of the glasses were calculated according to the expression: Specimens of each glass, 10 mm x 3 mm x 3 mm in size, were cut from the cast bars and placed in a dilatometer, Netzsch Dil 402-C. The specimens were then heated under flowing nitrogen, at a rate of 5°C/min, to above the dilatometric-softening point (TDS). The inflection point of the expansion curve was taken as the glass-transition temperature (Tg,dil) while the maximum was taken as the TDS. The thermal expansion coefficient (300-600) was calculated between 300 and 600ºC using the following equation: where l0 is the original length, l is the change in length of the specimen and T is the temperature change. From Dilatometry curves for each glass were examined to ensure that there was no decrease in the Δl/l0-temperature slope before the rapid increase in gradient above Tg,dil. Any such decrease in slope would have reflected incomplete glass annealing and, consequently, any glass showing such behaviour should be re-annealed for an additional hour.
Differential thermal analysis was carried out using a Stanton-Redcroft STA 1640 instrument. 50 mg of the glass powder were heated at 10°C/min under 0.1 MPa nitrogen atmosphere. The glass transition temperature (Tg,DTA) is represented by the point of inflection of the endothermic peak.
Specimens of the glass were mounted in a cold setting resin, polished to a 1 μm finish and then subjected to microhardness testing (Leco microhardness tester) using a Vickers indenter with a 300 g load for 10 s. Young´s modulus (E) was measured using the ultrasonic pulse-echo-overlap technique and the expression: where Vl and Vt are the longitudinal and transverse ultrasonic wave velocities, respectively. Samples of 15 mm x 10 mm x 3 mm in size with parallel surfaces were used for the measurements.
Shear modulus (G), compressive modulus (B) and Poisson's ratio (υ) were determined using the following expressions: Finally, refractive index was measured using an Ellipsometer (J. A. Woollam and Co. Inc.). Values taken were for 500 nm (visible spectrum) using a glass slide polished to 1 μm.

III. Results and discussion
As expected, all glasses were X-ray amorphous and fully dense, that is, they contained no internal bubbles or pores. However, some of them were not completely homogeneous, displaying different colour shades with some exhibiting a grey-blue hue. Only the glasses with higher nitrogen content (15 eq%) are not transparent to light (1.5 mm thickness).

(1) Effects of fluorine and nitrogen on physical properties
The physical properties (density, molar volume and fractional compactness) of the Ca-Mg-Si-Al-O-(N)-(F) glasses are also given in Table I and, the last two, are represented in Figures 1 and 2, respectively, against fluorine and nitrogen contents (discrete values). As can be observed, oxygen substitution by fluorine has no significant effect on molar volume or fractional compactness, even though this last property seems to increase very slightly. This is in contrast to the introduction of nitrogen into the glass network, which results in a linear increase of fractional compactness, and the resulting reduction, also linear, of the molar volume.
In silicate glasses, the network comprises SiO4N tetrahedra bridged by bi-coordinate O-T linkages whereas, in oxynitride glasses, it is known from the literature 1,4,9,28 that the network comprises SiO(4-x)Nx tetrahedra, with average x values controlled by N:Si ratios, which are bridged either by bi-coordinate O-T linkages or tricoordinate N-T linkages. Thus nitrogen increases the crosslinking of the glass network. AlO4 tetrahedra are present and are mainly bridged by bi-coordinate O-T linkages. Each eq.% nitrogen substitution introduces the same number of additional crosslinks into the glass network. The extent of network crosslinking will increase linearly with eq.% N which will have the result of contracting the glass network, thus increasing glass compactness and decreasing molar volume in a linear manner.

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This improvement of the physical properties with nitrogen content is consistent with other earlier studies of oxynitride glasses found in the literature 6,7,9,23,[28][29][30][31][32] . However, if the oxyfluoronitride glasses results are compared with those obtained for oxyfluoride glasses, a significant decrease in molar volume with the nitrogen content is observed, which confirms the cross-linking role of the nitrogen in the glass network, even in the presence of a strong modifying ion such as fluorine.
In view of the above results, and taking into account the linearity of the composition-properties relationship, a linear least-square fit of the experimental data has been performed. The results of the fitting are shown in Table   IV  The results given in Table IV Table I Table II.

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As can be observed, oxygen replacement by fluorine has a negligible effect on the mechanical properties, while oxygen replacement by nitrogen results in a significant linear increase of these properties, without any data deviation from the general tendency. Also the present results are consistent with other data on oxynitride glasses from the literature 6, 7, 23, 28, 30, 31, 33-35 . A linear least-square fit of the experimental data allows an empirical formula to be obtained which relates each of the mechanical properties with the content of nitrogen or fluorine (see Table IV). In this way, for example, Young's modulus can be expressed as: GPa, and the experimental value, as can be seen in Table II, is 116.1 GPa. The good correlation between the experimental data and the calculated values is further evidence that the effects of nitrogen and fluorine substitution on glass mechanical properties are independent and additive rather than synergistic.
The changes in properties with nitrogen substitution for oxygen, as shown by the gradients in Table IV, as with the physical properties discussed above, are very similar to those for the Ca-Si-Al-O-(N)-(F) system previously reported 24 . However, for the changes in properties with fluorine substitution, the present calculated gradients are significantly lower than determined previously.

(3) Effects of fluorine and nitrogen on thermal properties
The thermal properties of the glasses are shown in Table III:  The obtained data confirms the trends observed in previous studies of similar oxynitride glass systems 24 . The bridging oxygen substitution by non-bridging terminating fluorine reduces the network connectivity, explaining the reductions in Tg,DTA and TDS. On the other hand, as nitrogen substitutes for oxygen, it becomes chemically bonded to silicon in the glass network and produces a more tightly packed and highly linked structure, which explains the increase of these thermal properties.
The changes in TgDTA and TDS with fluorine and nitrogen substitution are clearly linear. However, for the thermal expansion coefficient, there is no clear relationship, although it seems that the general trend is that 300-600 increases with fluorine substitution and decreases with nitrogen content as also observed previously 4 .
As with the physical and mechanical properties, Table IV

(4) Effects of fluorine and nitrogen on refractive index
Values for refractive index as a function of fluorine and nitrogen content of the glasses are given in Table III and Refractive index increases linearly with nitrogen, and shows a slight decrease with fluorine, these effects being consistent with data in the literature 11,25,40 and with the polarizability of oxygen, nitrogen and fluorine ions.
The refractive index of glasses depends both on density and electronic structure of each of its ions, as is well established by the Lorenz-Lorenz relation: where Rm is the molar refractivity, M is the molecular weight, ρ is the density and n is the refractive index.
Equation (12) can be rearranged as follows: Page 11/29 So, the refractive index varies with molar refractivity, which in turn depends on polarizability of the ions in the glass, its density and molecular weight, which depend on chemical composition. Since the cation composition of the glasses is constant, the value of refractive index will only depend on the fluorine and nitrogen contents of the glass. According to Coon and Goyle 41 , the ionic refraction (which is proportional to ion polarizability) is 66.6 for nitrogen, but only 13.3 for oxygen, which explains the increase in refractive index when oxygen is substituted by nitrogen. Moreover, nitrogen substitution for oxygen in the glass network increases density, thus allowing an increase in this optical property. In contrast, the polarizability of the oxygen ion is three times higher than that of the fluorine ion 42 , which explains the decrease in refractive index with fluorine substitution, since both density and molecular mass remain almost constant.

V. Conclusions
In the present study, the effects of oxygen substitution by fluorine and/or nitrogen on various physical,  and oxygen replacement by nitrogen (for fixed fluorine content) on physical, mechanical, thermal and optical properties of these oxyfluoronitride glasses are independent and additive, rather than synergistic.         The discrete points represent the experimental data and the lines correspond to the best fit of the data.