[Reinforcement of cores on porcelain jacket crowns. Influence of alumina powder properties on the cores].

Various alumina particles were investigated for use as core reinforcement to strengthen aluminous-porcelain jacket crowns. The alumina particles were either spherical or nonspherical and were between 5.3 microns and 60 microns in size. Initially comparisons were made among such properties as particle-size distribution, apparent density, specific surface area, and flow. Then each alumina powder was mixed with feldspar at rates of from 10wt% to 70wt% at increments of 10wt%. Formed into columns, the powder was fired at 1350 degrees C for 5 minutes. The resulting feldspar-alumina composites were compared for shrinkage then cut and prepared into disc specimens. The disc-rupture test was employed to measure composite strengths. A scanning electron microscope was employed to observe alumina particles and composite cross sections. Results 1. Spherical alumina (average particle size 5.4 microns), spherical alumina (average particle size 28.9 microns), and nonspherical alumina (average particle size 58.0 microns) were higher in apparent density and flow rate but lower in specific surface area. 2. Feldspar-alumina composites shrank approximately 30%; shrinkage of 15-20% occurred only in composites involving spherical alumina (average particle size 5.4 microns). 3. Composites of feldspar and spherical alumina were observed to be stronger than those of feldspar and nonspherical alumina. Composites containing 50-60wt% (especially when the alumina particle size was 5.4 microns) were the sturdiest. 4. Increasing the amount of alumina proportionally increased composite strength. In composites containing nonspherical alumina, volume amount and strength were not always proportional. 5. Microscopic examination of composite cross sections showed uniform dispersion of spherical alumina but no dispersion of nonspherical alumina. In conclusion, spherical alumina (especially when average particle size is 5.4 microns) demonstrated the best powder properties as a reinforcing core material and therefore is most suitable for reinforcing the core of the porcelain jacket crown.