Effect of Li2O on Structure and Properties of Glass-Ceramic Bonds

SiO 2 -B 2 O 3 -Al 2 O 3 -CaO vitried bonds are widely used in the diamond abrasive tools preparation. The effect of Li 2 O on structure and properties of the glass-ceramic bonds was investigated. The structure of the glass-ceramic bonds was analyzed by XRD and FTIR. The bending strength, the sintering properties of different glass-ceramic bonds and the thermal expansion coecient were tested at meantime. The sintering and interfacial bonding state between L-4 glass-ceramic bonds and diamond grains were observed by SEM. The results showed that with the increasing of Li 2 O, the sintering temperature of the glass-ceramic bonds was impactful reducing. When the content of Li 2 O was 4 wt%, the sintering temperature corresponding to the optimal bending strength was 630 °C, which had a certain reduction compared with other studies. The main crystal phase precipitated in the glass-ceramic bonds was Li x Al x Si 3−x O 6 . With the increase of Li 2 O, the number of crystals in the glass-ceramic bonds gradually increased. The highest bending strength could attain about 136 MPa. Meanwhile, the bending strength performed regular change as the rising of temperature accompanied by the linear shrinkage rate. The L-4 glass-ceramic bonds and diamond grains were found to have very good wetting property and the interfacial bonding strength was insured enhance. The average bending strength of the composite sinter of the glass-ceramic bonds and diamond was up to 87.8 MPa.


Introduction
Diamond is the hardest material in nature, which is often used to cut other materials. Diamond grinding wheels maintain mechanical properties through the combination of binding agents. Vitri ed bonds diamond grinding wheels with excellent properties are widely used for grinding and machining materials [1][2][3], such as high strength, high elastic modulus, low fracture toughness, self-dressing, shape-retaining ability, high accuracy and low cost. These make high demands of diamond abrasive tools from high e ciency grinding to high precision grinding nowadays [4,5]. Vitri ed bonds diamond grinding wheels are normally composed of three materials: diamond abrasive, ller and vitri ed bonds. To some extent, the properties and qualities of diamond grinding wheels are determined by vitri ed bonds. For diamond grinding wheels, oxidation may occur on surface during sintering. Therefore, two measures are often used. One is to use N 2 atmosphere to protect sintering, the other is to sinter at low temperature. At present, Chinese manufacturing enterprises are studying low temperature sintering mode without N 2 atmosphere protection. The sintering temperature of diamond grinding wheels mainly depends on the service temperature and content of binding agents. The researchers reduced the sintering temperature of diamond grinding wheels by reducing the sintering temperature of binding agents. SiO 2 -B 2 O 3 -Al 2 O 3 -CaO vitri ed bonds were often used as the basic system of diamond grinding wheels which had the properties of high bending strength and low softening temperature.
Zhao [6] reported that increasing Bi 2 O 3 content decreased the thermal expansion coe cient of vitri ed bonds, decreased the sintering temperature and improved their mechanical properties to some extent. Liu [7] suggested that Zr 4+ could enter the glass network as mending nets and agglomeration composition, the addition of ZrO 2 increased the bending strength, crystallinity, sintering temperature of vitri ed bonds and considerably decreased the wetting angle with diamond. The effect of other additives on the properties of vitri ed bonds also had been investigated, such as titanium oxide [8], alkali oxides [9], alkali earth oxides [10]. However, only a few studies were reported on decreasing the sintering temperature and improving mechanical properties. Glass-ceramic bonds had demonstrated de nite performance superiority compared with other vitri ed bonds system for the presence of microcrystal precipitated by controlled nucleation and crystallization of glass [11,12]. The addition of Li 2 O could reduce the melting temperature and increase mechanical strength of glass-ceramic bonds. Aiming at this target, there were two primary goals of this research. One was to increase the strength of glass-ceramic bonds and reduce the sintering temperature of glass-ceramic bonds. The evolution of structure and properties of glassceramic bonds was studied by changing the content of Li 2 O and regulating the sintering system. The other was to ascertain the bonding state of diamond and glass-ceramic bonds for a better understanding of the microstructure of glass-ceramic bonds diamond grinding wheels.

Preparation of glass-ceramic bonds
The chemical composition of glass-ceramic bonds was shown in Table 1. The parent glass was prepared by melting method, accurate weighing and mixing, and melting in a platinic crucible at 1450 o C for 2 h.
Then the glass-frit was rapidly quenched in deionized water, dried, ball milled, and sieved to obtain the parent glass powders. The particle size range of powders was 2-5 µm. The mixed powders were pressed into the rectangular mould (45 mm × 6 mm × 6 mm) with an appropriate amount of 5% PVA solution under a pressure of 25 kN. Finally, the compacted specimens were sintered under a certain heat treatment and cooled naturally in the mu e furnace in air atmosphere to prepare the glass-ceramic bonds. The XRD pattern of parent glass with different Li 2 O content was shown in Fig. 1. It could be seen from the gure that there was no sharp diffraction peak in the pattern which was the typicality of amorphous state. The presence of a broad halo in between 15° and 35° indicated that parent glass was amorphous state and did not crystallize during melting at high temperature. The parent glass was mainly glass phase, which was conducive to the appearance of liquid phase during sintering and coating of diamond particle.
The FTIR spectra of parent glass was shown in Fig. 2. Five distinct absorption bands were observed in the FTIR spectra. It could be seen from gure that with the increase of Li 2 O content, the main absorption bands of parent glass did not change, but the absorption vibration peak was affected.  4 ] existed in a dense form, that was, there was a boron-rich phase, and the appearance of boron-rich phase would lead to another silicon-rich phase.

Thermal analysis of parent glass
The DSC curve of parent glass with different Li 2 O content was shown in Fig. 3. It could be seen from DSC curve that the obvious crystallization exothermic peak appeared from 500 o C. With the increase of Li 2 O content, the peak temperature of crystallization exothermic peak gradually decreased from 536 o C to 526 o C. It indicated that with the increase of Li 2 O content, the crystallization temperature shifted to low temperature, that was, the addition of Li 2 O reduced the crystallization temperature. With the increase of Li 2 O content, the area of crystallization exothermic peak gradually increased, that was, the number of crystals increased, indicating that the Li 2 O promoted the precipitation of crystals. In addition, after the crystallization exothermic peak, the DSC curve showed a clear downward trend. That was because with the increase of temperature, the glass phase of parent glass gradually melted and the amount of liquid phase increased. The glass phase melting process was an endothermic process, so the DSC curve declined. It could be known from thermal analysis results that the onset crystallization temperature of parent glass was around 500 o C. In order to achieve the best sintering, it was necessary to ensure appropriate amount of liquid phase to achieve the compact sintering of glass-ceramic bonds. The sintering temperature was set in the range of 590 o C-720 o C, and a sintering temperature was taken every 10 o C. By adjusting the sintering temperature of each sample to control sintering and crystallization, the glass-ceramic bonds sintered body at different sintering temperatures was prepared.

XRD analysis of glass-ceramic bonds
The XRD pattern of glass-ceramic bonds sintered body with different Li 2 O content at 660 o C was shown in Fig. 4. According to the analysis of Jade 5.0 software, at the same sintering temperature, the crystal phases precipitated in the sintered body were all SiO 2 crystal, Al 2 SiO 5 crystal and Li x Al x Si 3−x O 6 crystal.
With the increase of Li 2 O content, the intensity of diffraction peak of each crystal increased, indicating that the number of crystals increased and the crystals grew. When the content of Li 2 O exceeded 4%, the diffraction peak of crystal phase in the XRD pattern was particularly sharp and strong. That might cause the crystal grains to be too large, which would lead to the uneven structure of glass-ceramic bonds and the reduction of mechanical strength. The too many precipitations of crystal would also weaken the network structure and mechanical strength of glass phase, which affected the overall strength of glassceramic bonds [19].
In order to investigate the effect of sintering temperature on crystallization of glass-ceramic bonds sintered body, the XRD test was taken on the sintered body of L-4 glass-ceramic bonds at different sintering temperatures. The XRD pattern of L-4 glass-ceramic bonds sintered body at different sintering temperatures was shown in Fig. 5. According to the analysis of Jade 5.0 software, at different sintering temperatures, the glass-ceramic bonds precipitated the same crystals, which were all SiO 2 crystal, Al 2 SiO 5 crystal and Li x Al x Si 3−x O 6 crystal. It could be known from Fig. 5 that with the increase of sintering temperature, the intensity of diffraction peak of crystal phase gradually increased and then remained unchanged. It indicated that with the increase of temperature, the type of crystal phase precipitated by glass-ceramic bonds was unchanged and the content of crystals increased and then remained unchanged. When the sintering temperature exceeded 630 o C, with the further increase of sintering temperature, the intensity of diffraction peak of crystal was unenhanced, indicating that in a certain composition, only increasing the sintering temperature within a certain temperature range could promote the precipitation of crystal.

micro-morphology analysis of glass-ceramic bonds
The micro-morphology of glass-ceramic bonds with different Li 2 O content at 660 o C sintering temperature was shown in Fig. 6. The distribution of glass phase and crystal phase in the glass-ceramic bonds could be seen from micro-morphology and the encapsulation effect of glass-ceramic bonds on diamond particle could be roughly predicted.
It could be known from Fig. 6 that at this sintering temperature, the sintered body of each component was sintered densely, without a large number of obvious pores, but there were many thin pores. Li 2 O could play a role of uxing, that was, it had a function of lowering the melting point and promoting the melting of glass phase. Therefore, at the same temperature, with the increase of Li 2 O content, the content of liquid phase in the sintered body increased, and the uidity was improved, which was conducive to promoting densi cation of sintering and reducing the porosity. An appropriate amount and evenly distributed micro-pores would be conducive to heat removal and chip removal of diamond grinding wheels during grinding process. At the same time, with the increase of Li 2 O content, it could be seen from the gure that obvious regular crystals appeared, the number of crystals increased, and the grain size became larger. That was because Li 2 O could promote the precipitation of crystal and the growth of crystal, and the appearance of an appropriate number of microcrystals would be conducive to improving the strength of glass-ceramic bonds.

Bending strength analysis
The performance of diamond grinding wheels was determined by bending strength of glass-ceramic bonds to a large extent. Therefore, improving the bending strength of glass-ceramic bonds was conducive to improving the performance of diamond grinding wheels. The bending strength curve of glass-ceramic bonds with different composition at different sintering temperatures was shown in Fig. 7. In the speci c sintering temperature range, the bending strength of glass-ceramic bonds had the same variation trend as sintering temperature. It could be seen from curve that the bending strength increased with the increase of sintering temperature. After reaching the highest strength corresponding to sample, the sintering temperature continued to increase but the bending strength decreased. That was because the content of liquid phase changed with sintering temperature. The optimal sintering temperature of each sample was mainly affected by composition. With the increase of Li 2 O content, the optimal sintering temperature of glass-ceramic bonds gradually decreased. properties. However, when the content of Li 2 O further increased, the excess free oxygen played a major role in breaking network. The free oxygen would form bond with Si atom, that was, the bridge oxygen bond in glass network was broken, and the glass network structure became loose, which had a signi cant uxing effect, but it also reduced the bending strength of the glass-ceramic bonds. When the content of Li 2 O was 4 wt% and the sintering temperature was 630 °C, the glass-ceramic bonds exhibited the highest bending strength, which could reach 136 MPa.

Sintering shrinkage rate analysis
In order to investigate the sintering properties of different glass-ceramic bonds, the sintering shrinkage rate of samples were tested. In the sintering process, the phenomenon that the sample contracted in length or volume was called sintering shrinkage. Determining sintering shrinkage rate played an important role in choosing a reasonable glass-ceramic bonds. The linear shrinkage curve of glassceramic bonds with different component at different temperatures was shown in Fig. 8. The changing law of curve was similar to the binding strength curve of glass-ceramic bonds with different component at different temperatures. That was, within a certain temperature range, the shrinkage rate increased rst and then decreased. With the increase of Li 2 O content, the maximum shrinkage rate increased, but when the content of Li 2 O was too high, the maximum shrinkage rate decreased instead.
The consistency of changing law of linear shrinkage rate and bending strength indicated that the best bending strength meant the densest structure, that was, the greater the shrinkage of the glass-ceramic bonds was, the stronger the structure and the higher the strength was. The reason was also the change of liquid phase content of glass-ceramic bonds. With the increase of temperature, the content of liquid phase increased, the glass-ceramic bonds particles were easy to gather and sintered into entirety, and the gap between particles was conducive to the elimination of pores, so the linear shrinkage rate increased. When the sintering temperature was higher than optimal sintering temperature, the content of liquid phase was too much, the gap between particles was blocked, which was not conducive to the elimination of pores, so the linear shrinkage rate of glass-ceramic bonds decreased and the strength decreased. The above analysis indicated that to a certain extent, the change in linear shrinkage rate re ected the changing law of the strength.

Analysis of wettability and coverability
It could be known from the analysis of bending strength, sintering temperature and thermal expansion data that the maximum bending strength of L-4 glass-ceramic bonds at the sintering temperature of 630 o C was 136 MPa. It indicated that appropriate content of Li 2 O played a great role in improving the comprehensive performance of glass-ceramic bonds. The L-4 glass-ceramic bonds with better comprehensive performance were chosen and sintered with diamond particles.
The SEM pattern of composite sinter of L-4 glass-ceramic bonds and diamond was shown in Fig. 9. Wherein, the mass ratio of glass-ceramic bonds to diamond in composite sinter was 7:3, and the sintering temperature was 630 o C. It could be seen from pattern that the diamond particles were buried in glassceramic bonds, and were tightly wrapped by glass-ceramic bonds, indicating that the wettability and coverability of them were satisfactory. At the same time, it could also be seen that appropriate number of small grains were precipitated in the glass-ceramic bonds, which had a certain effect on improving the strength of composite sinter. The average bending strength of composite sinter was up to 87.8 MPa through the bending strength test, which was enough to meet the strength requirement of abrasive tools.

Conclusions
(1) With the increase of Li 2 O content, the sintering temperature corresponding to the optimal bending strength of glass-ceramic bonds shifted to low temperature. When the content of Li 2 O was 4 wt%, the sintering temperature corresponding to the optimal bending strength was 630 °C.
(2) The main crystal phase precipitated in the glass-ceramic bonds was Li x Al x Si 3−x O 6 , and the secondary crystal phase was SiO 2 and Al 2 SiO 5 . With the increase of Li 2 O, the number of crystals in the glassceramic bonds gradually increased. When the content of Li 2 O was 4 wt%, the bending strength was 136 MPa, and the linear sintering shrinkage rate was 11.17%.
(3) L-4 glass-ceramic bonds had a better wettability and coverability for diamond particles. The average bending strength of the composite sinter of the glass-ceramic bonds and diamond was up to 87.8 MPa. Figure 1 XRD pattern of parent glass with different Li2O content     Bending strength of glass-ceramic bonds at different temperatures Figure 8