3.1. Effect of Li2O on glass-ceramic bonds structures
3.1.1. Structural analysis of parent glass
The XRD pattern of parent glass with different Li2O content was shown in Fig. 1. It could be seen from the figure 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 figure that with the increase of Li2O content, the main absorption bands of parent glass did not change, but the absorption vibration peak was affected.
The parent glass exhibited IR band at around 461 cm− 1 derived from bending vibration of O-Si-O in [SiO4] [13]. The absorption peak around 790 cm− 1 was symmetric stretching vibration of Si-O [14]. With the increase of Li2O content, these two absorption peaks gradually weakened. That was because Li2O played a role of breaking network structure, which could provide a large amount of free oxygen. The free oxygen would destroy the Si-O-Si structure, break the glass network structure, and change bridging oxygen to non-bridging oxygen, resulting in a reduction in the amount of [SiO4]. Therefore, the Si-O-Si bending vibration peak and the symmetric stretching vibration peak of Si-O were weakened. The spectral band at 1027 cm− 1 became narrower and split into three spectrum bands, 1027 cm− 1, 1151 cm− 1, and 1211 cm− 1. Wherein, the absorption peak around 1027 cm− 1 was the result of combined effect of the asymmetric stretching vibration of Si-O-Si and the asymmetric stretching vibration of [BO4] [15, 16]. The characteristic bond of glass-ceramics appeared at 1151 cm− 1, and 1211 cm− 1 was assigned to the asymmetric vibration of Si-O-Si [17]. With the increase of Li2O content, the asymmetric stretching vibration of Si-O-Si tended to be obvious. The absorption peak around 1415 cm− 1 was the asymmetric stretching vibration of [BO3] [18], which gradually increased with the increase of Li2O content. That was because in the glass system, [BO4] and [BO3] connected directly and the strength of B-O bond in [BO3] was stronger than that in [BO4]. The free oxygen introduced by Li2O would reduce the amount of [BO4] and increase the amount of [BO3], which would cause the enhancement of the asymmetric stretching vibration of [BO3] in the network structure, that was, the absorption peak at 1415 cm− 1 gradually enhanced. It could be known from pattern that only the synergic absorption peak of the asymmetric stretching vibration of Si-O-Si and the asymmetric stretching vibration of [BO4] appeared in the glass-ceramic bonds, but there was no the synergic absorption peak of [BO3] and Si-O-Si bond. It indicated that in the glass network structure, the [SiO4] was only connected to [BO4], but not connected to [BO3] and the [BO3] was only connected to [BO3], indicating that the [BO3] and the [BO4] 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.
3.1.2. Thermal analysis of parent glass
The DSC curve of parent glass with different Li2O content was shown in Fig. 3. It could be seen from DSC curve that the obvious crystallization exothermic peak appeared from 500 oC. With the increase of Li2O content, the peak temperature of crystallization exothermic peak gradually decreased from 536 oC to 526 oC. It indicated that with the increase of Li2O content, the crystallization temperature shifted to low temperature, that was, the addition of Li2O reduced the crystallization temperature. With the increase of Li2O content, the area of crystallization exothermic peak gradually increased, that was, the number of crystals increased, indicating that the Li2O 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 oC. 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 oC-720 oC, and a sintering temperature was taken every 10 oC. 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.
3.1.3. XRD analysis of glass-ceramic bonds
The XRD pattern of glass-ceramic bonds sintered body with different Li2O content at 660 oC 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 SiO2 crystal, Al2SiO5 crystal and LixAlxSi3−xO6 crystal. With the increase of Li2O 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 Li2O 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 glass-ceramic 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 SiO2 crystal, Al2SiO5 crystal and LixAlxSi3−xO6 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 oC, 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.
3.1.4. micro-morphology analysis of glass-ceramic bonds
The micro-morphology of glass-ceramic bonds with different Li2O content at 660 oC 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. Li2O could play a role of fluxing, 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 Li2O content, the content of liquid phase in the sintered body increased, and the fluidity was improved, which was conducive to promoting densification 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 Li2O content, it could be seen from the figure that obvious regular crystals appeared, the number of crystals increased, and the grain size became larger. That was because Li2O 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.
3.2. Effect of Li2O on glass-ceramic bonds properties
3.2.1 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 specific 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 Li2O content, the optimal sintering temperature of glass-ceramic bonds gradually decreased.
When the content of Li2O increased to a certain extent, the maximum bending strength decreased instead. That was because in borosilicate glass, B atom could exist in the form of [BO3] in a chain or layer structure, which connection strength was low. B atom could also exist in the form of [BO4] tetrahedron, which had high connection strength. The two forms normally coexist in the glass network, but the relative content could be changed. The bonding interaction of Li2O was weak. With the addition of Li2O, a large amount of free oxygen was brought into glass network, and some [BO3] combined with free oxygen and changed to [BO4]. In the glass-ceramic bonds system, [BO4], [AlO4] and [SiO4] constituted the main body of the three-dimensional network structure, and Li+ could fill the network interspace as a network modifier. [BO4] had a higher network connection strength and a smaller volume than [SiO4]. Therefore, the network structure formed by more [BO4] combining with [AlO4] and [SiO4] was denser and had better mechanical properties. However, when the content of Li2O 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 significant fluxing effect, but it also reduced the bending strength of the glass-ceramic bonds. When the content of Li2O was 4 wt% and the sintering temperature was 630 °C, the glass-ceramic bonds exhibited the highest bending strength, which could reach 136 MPa.
3.2.2 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 glass-ceramic 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 first and then decreased. With the increase of Li2O content, the maximum shrinkage rate increased, but when the content of Li2O 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 reflected the changing law of the strength.
3.2.3. 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 oC was 136 MPa. It indicated that appropriate content of Li2O 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 oC. It could be seen from pattern that the diamond particles were buried in glass-ceramic 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.