Influence on microstructure of dental zirconia ceramics prepared by two-step sintering / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To investigate the microstructure of dental zirconia ceramics prepared by two-step sintering.</p><p><b>METHODS</b>Nanostructured zirconia powder was dry compacted, cold isostatic pressed, and pre-sintered. The pre-sintered discs were cut processed into samples. Conventional sintering, single-step sintering, and two-step sintering were carried out, and density and grain size of the samples were measured. Afterward, T1 and/or T2 of two-step sintering ranges were measured. Effects on microstructure of different routes, which consisted of two-step sintering and conventional sintering were discussed. The influence of T1 and/or T2 on density and grain size were analyzed as well.</p><p><b>RESULTS</b>The range of T1 was between 1450 degrees C and 1550 degrees C, and the range of T2 was between 1250 degrees C and 1350 degrees C. Compared with conventional sintering, finer microstructure of higher density and smaller grain could be obtained by two-step sintering. Grain growth was dependent on T1, whereas density was not much related with T1. However, density was dependent on T2, and grain size was minimally influenced.</p><p><b>CONCLUSION</b>Two-step sintering could ensure a sintering body with high density and small grain, which is good for optimizing the microstructure of dental zirconia ceramics.</p>

Study on corrosion resistance of three non-noble porcelain alloys / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To study the electrochemical corrosion behavior of Co-Cr, Ni-Cr and Ni-Cr-Be based porcelain alloys in NaCl solution.</p><p><b>METHODS</b>Five samples of each alloy were made respectively, electric polarization curve of each alloy was obtained using potentiodynamic polarization technique. Self-corrosion potential (E(corr)), self-corrosion current density (I(corr), passive region and transpassivation potential were tested. Microstructure and constituent was examined using scanning electron microscopy and energy dispersive spectroscopy.</p><p><b>RESULTS</b>Co-Cr alloy possessed the most desirable corrosion resistance because of its integrated, homogeneous and compact passive film. The poor compactness of Ni-Cr alloy's passive film decreased its corrosion resistance. Ni-Cr-Be alloy exhibited the worst corrosion resistance due to the Cr and Mo depleted Ni-Be eutectic phases in the alloy.</p><p><b>CONCLUSION</b>Taking biological security into consideration, it is necessary to avoid the application of porcelain alloys with Be element. Co-Cr alloy with better biocompatibility possesses much broader prospect in the field of dental restoration.</p>

The effect of solid phase transformation on the metal-ceramic compatibility of Co-Cr alloy / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To study the effect of solid phase transformation on the metal-ceramic compatibility of Co-Cr alloy during firing programs.</p><p><b>METHODS</b>9 foils of Co-Cr and Ni-Cr alloy with the dimension of 25 mmx3 mmx0.5 mm were casted using lost wax technology respectively. Among them, 6 specimens were subjected to metal-ceramic bonding strength test (three point bending method), the ceramic layer of 3 specimens were removed for X-ray diffraction (XRD) analysis. One cylindrical specimen of each alloy was casted for the test of coefficient of thermal expansion, cooling curves were recorded.</p><p><b>RESULTS</b>The metal-ceramic bonding strength of Ni-Cr, Co-Cr alloy was (49.1 +/- 3.40), (40.9 +/- 2.02) MPa respectively. There was significant difference between the two groups' bonding strength (P = 0.00). The coefficient of thermal expansion in the 20-500 degrees C interval of Ni-Cr and Co-Cr alloy was 13.9 x 10(-6), 13.8 x 10(-6) x K(-1) respectively. XRD analysis indicated that the microstructure of Ni-Cr alloy was austenite. While Co-Cr alloy was constituted by fcc phase, hcp phase and sigma phase.</p><p><b>CONCLUSION</b>During the cooling procedure, the transformation of fcc phase to hcp phase and segregation of needle-like sigma phase intensify the linear contraction speed of Co-Cr alloy, which decreases the metal-ceramic adaptability.</p>