Fluorosilicone mica glass ceramics have good aesthetic properties, machinability and mechanical properties, and have a wide application prospect as a machinable ceramic for dental restoration CAD/CAM systems. The conventional process for preparing the material includes a melting method and a sintering method ra, wherein the sintering method is more suitable for the individualization requirements of the oral cavity repair because it can prepare articles having complicated shapes and various colors, and has great application potential. However, compared with the melting method, the sintering method is technically difficult, has many influencing factors, and has a complicated sintering mechanism. Therefore, many key technical problems are not well solved. In the sintering process of fluorosilicone mica glass ceramics, in addition to the sintering temperature, the holding time and heating rate are also important process factors that directly affect the compactness and crystal size and distribution of the sintered body. However, the influence of different holding time of the glass ceramics in the densification sintering on the sintering behavior of glass ceramics with different particle sizes has not been reported. Therefore, this paper will compare the shrinkage and microscopic morphology of two kinds of particle size fluorosilicone mica glass powders under the same sintering temperature, the same heating rate and different heat preservation time, and discuss the particle size, holding time, etc. The influence of factors on the sintering behavior, in order to formulate more reasonable sintering process parameters, so as to improve the comprehensive performance of the material, and provide theoretical support for the establishment of a reasonable sintering process parameter system.
1 Materials and Methods 1.1 Materials B2O3 and P25 were used as fluxing agents. Specific ingredients (mass fraction 2wt%, other ingredients 1.2 method The above ingredients are melted by 1500 ft for 4h, quenched, ball milled in agate tank for 24h, passed through 100 mesh sieve, spare, named P0. P0 ball milled again for 36h, standby, It was named Pm. The particle size distribution of P0 and Pm was measured using a SA-CP3 type centrifugal particle size analyzer (SHIMADZU, apan).
Two kinds of granular glass powder packaging boxes were formed by cold isostatic pressing at a pressure of 200 MPa for 0.5 h. The blank size was 1.5 mm x 5 mm x 5 mm strips, and the two square end faces of the blank were parallel.
The formed P0 and Pm blanks were placed in a controlled high temperature electric furnace and burned without deformation.
The trajectory of the 0.02 mm precision caliper was used to clamp the two parallel end faces of the sample to read the line length of the blank before and after sintering. Each set of 5 test pieces, each sample was measured 3 times, and the average value was taken. The line shrinkage is calculated by the formula 0): where U is the line length before the green body is sintered; U is the line length after the body is sintered; a is the line shrinkage.
2 Results 2.1 Particle size distribution curve Scanning electron micrograph and particle size distribution curve showed that there were significant differences in particle size distribution between P0 and Pm. Particle size analyzer from the scanning electron micrographs of PO and Pm powders P0 and Pm particle size distribution curve Pm holding time (min gives the average particle size, P0 is 72.8xm, Pm is 4.5xm. 2.2 Sintering shrinkage can be clearly It can be seen that the variation of the shrinkage rate of the two-size sintered bodies varies greatly with the increase of the holding time.
During the initial period of isothermal sintering, the shrinkage rate changes significantly, and after reaching a certain period of time, the shrinkage rate is basically saturated, and the curve shows a clear platform. P0 is almost close to the maximum shrinkage rate from about 240 minutes, and Pm starts to appear from about 60 minutes. In addition, the overall shrinkage of Pm is significantly higher than that of P0. 2.3 The microscopic morphology is a scanning electron micrograph of P0 sintered body at different holding times of 1000 feet. The figure shows that the internal particle size of P0 is quite different. The size and distribution of irregular pores formed by the accumulation of large and small particles are very uneven. The phenomenon of particle boundary fusion is also uneven. Small particles and large particles fuse very early, and large. There are large irregular pores between the particles (the tens of microns to hundreds of microns are always difficult to eliminate the shrinkage curve of P0 and Pm sintering at 1000 different holding times, until the maximum shrinkage is reached, the large particles There are still many irregular pores between them, and it is difficult to see the occurrence and increase of circular regular pores. In addition, the crystallization phenomenon appears later, and the microstructure of the TO sintered body at different holding times of crystal lO'C Scanning electron micrograph of Pm sintered body at different holding time of 1000 ft. The figure shows that the contact between Pm particles is tight, the irregular pores caused by powder accumulation are evenly distributed, and the pore size is on the order of micrometer. The phenomenon of glass particle fusion occurs very quickly, and the number and size of irregularly distributed irregular pores are significantly reduced, replaced by the appearance of circular pores and their number and size. After 60 min, the number and size of circular pores did not change significantly, and the irregular pores disappeared completely. The crystallization phenomenon appeared earlier in the Pm constant sintering process, and the crystal content and size were higher than P0.丨(XO'IM': The micro-觇 shape of the MKPm sintered body at M insulation is discussed. The sintering process as a powder is discussed. In most cases, the purpose is to obtain the maximum density of the sintered body. It is called densification. The degree of densification of sintering will directly affect the mechanical and optical properties of the sintered body. Sintering shrinkage is a key characterization parameter for sintering density, while the microscopic morphology is a visual representation of the density of sintering. The most powerful evidence for the cause of its appearance. Therefore, this paper uses two judgments to compare the advantages and disadvantages of sintering performance under different factors.
For the glass-ceramics prepared by the sintering method, if the precipitated crystals do not break through the boundaries of the original glass particles, there is no significant influence on the densification. Therefore, it can be considered that in the early stage of isothermal sintering, the main factor of densification is the viscous flow of the glass phase. At this time, the particle size, the tightness of particle packing, and the contact area between the particles are viscous flow to the whole glass and the entire sintered body. Densification is crucial. The tiny powders that are closely packed have a large contact area between the particles, and the contact angle is small, so that there are more particles passing through the material transfer, and the work required for particle fusion is smaller, so at the same temperature. The process of viscous flow will be completed very early, and conversely, the viscous flow will be difficult to complete in a short period of time.
The Pm particles used in the experiment have smaller particle size. Therefore, under the same cold isostatic pressure, the powder is densely packed and the contact area between the particles is large. Therefore, the glass phase flow needs to overcome the work of viscous resistance. It will decrease, so the viscous flow process will proceed more quickly and thoroughly. It can be seen that after 60 min of heat preservation, the viscous flow process of Pm has been completed, and the performance characteristic is "the disappearance of irregular pores", and these pores completely become circular pores due to viscous flow. Thereafter, only the mica crystals grew up. However, P0 is completely different. Because the particles are large, the irregular pores between the particles are large and large, the glass phase flow must be done with a large amount of work, and it takes a long time for the particles to flow to other particle surfaces, so the whole The viscous flow process becomes extremely difficult and long, and over time, the accumulated heat causes the crystals inside the large particles to grow gradually, and the viscosity inside the particles rises, which makes the whole viscous flow more difficult, thus making the sintering The densification process of the body is blocked and delayed.
It is shown that P0 still does not completely complete the viscous flow process during the holding time of the maximum shrinkage rate, and there are still a large number of irregular pores between the particles which are not completely eliminated. For the above reasons, in the early stage of isothermal sintering, the absolute value of shrinkage and the growth rate of Pm are significantly higher than P0, and the time taken to reach the maximum shrinkage rate is also significantly shorter than P0.
In the late stage of isothermal sintering, after the viscous flow densification sintering process is basically completed, the change in the sintered body is mainly characterized by crystal growth. Since the glass phase and the precipitated crystal phase exist in the interior of the glass-ceramic ceramic, and there is a competition relationship between densification and crystal growth, the relationship between the sintering shrinkage rate and the holding time no longer satisfies the linear growth relationship. Therefore, there is a phenomenon: in the early stage of isothermal sintering at 1000oC, the shrinkage rate of P0 and Pm increases linearly at the initial stage, and then the increase rate gradually decreases. After reaching the maximum value, the shrinkage rate tends to increase with the increase of holding time. On a smooth, the curve appears on the platform. The isothermal sintering shrinkage of P0 and Pm as a whole is consistent with this trend, but there are subtle differences between the two. Since the Pm has completed the viscous flow process before reaching the plateau period, the crystal growth has almost no effect on the density, so the curved platform appears to be more straight, and the P0 is far from the viscous flow process when it approaches the plateau. At the end, the densification process is slow due to crystal growth factors, but the crystal growth does not completely prevent the viscous flow from continuing. Therefore, the platform in the P0 curve still has a tendency to rise slowly, but the amplitude is very small.
4 Conclusions The use of small-grain glass powder can achieve the equilibrium state of crystal growth and densification in the shortest holding time, which is conducive to the smooth progress of the densified viscous flow sintering process, and the holding time required to achieve the maximum density is only 1h, which can greatly save energy consumption and improve sintering efficiency, so small-sized powder is more suitable for preparing fluorosilicone mica glass ceramic with higher density and superior performance to meet the clinical needs of oral repair.
(Finish)
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