| Objective: To observe surface structure of metal-ceramic crowns with different metal thickness; To observe changes of its surface after loading; To observe porcelain's fracture sections by scanning electron microscope (SEM) and through destruction tests on metal-ceramic crowns, effects of the different of thickness of metal on metal-ceramic crown microstructure and strength are studied, and compared with the metal-ceramic with thicker porcelain, which provide experimented basis for technical manufacture of metal-ceramic crown.Method: 1. Models preparation: a metal die of simulating crown core is precision machine.4 group of metal-ceramic crown were made , they are: group A with metal's thickness 0.3mm, porcelain's thickness 1.5mm; group B, with metal's thickness 1.5mm, porcelain's thickness 2.1mm; group C, with metal's thickness 2.1mm, porcelain's thickness 1.5mm; group D with metal's thickness 0.3mm, porcelain's thickness 3.3mm. 2.Observe the surface feature of test specimens through SEM . 3. 300N pressure was loaded on the occlusal surface of each model for 5 minutes. 4. Analyze the surface microstructure changeafter being loaded. 5.Destruction test on each model is performed to test fracture force values by universal testingmachine.6 .Statistical analysis of those values are performed. 7.Each model's structure in fracture section is observe with SEM.Result:1.metal-ceramic crown surface microstructure: in group A ,surface is even ,a few pores are shown,small crystals evenly scattered, no clearly crack are found ; in group B ,there are more pores than that of group A ,they are shallow ,a few cracks are shown, some crystals evenly scattered; in group C ,many pores scatters unevenly , longer cracks and bigger crystals are found which was round by craks; in group D similar with group C ,the crack of 10μm length is seen . 2 . The surface microstructure after loading : in group A , few small shallow cracks of 0.5~2.0μm occurred; in group B ,the number of pores decrease ,the cracks of 2~30μm are more and larger than that of group A;in group C ,more cracks are shown, there is crack of 110μm length and 10μm width; In group D, many cracks occurred such as group C, a crack of 100μm length was found. 3. Even values of strength obtained from destruction test are as follows(unit: kg):group A, 520.8, group B, 491.5, group C,142.7, group D,128.7. Analysis of variance (ANOVA) results indicate that there are significant of destruction test values on models with difference of metal thickness.(P<0.01),multiple comparison result indicate that significant difference in destruction testvalues exist between group A and C,A and D,B and C, B and D; no difference between any other tow groups(P>0.05).4. The fracture surface features: group A, a few pores and cracks found; there is few clear cracks and pores in metal- ceramic interface; Group B, glassy structures are uniform, some pores are shown; few saw-toothed cracks scatter,there are a few pores and cracks in metal-ceramic interface; in group C, many saw-toothed cracks are shown, glassy structures are even, edge of crack are irregular, the structures of metal- ceramic interface are loosen ,a lot of pores and cracks are shown; in group D , The fracture surface features are similar with group of B, there are long saw-toothed cracks around the pores. Conclusion: 1.The thickness of metal has effects on metal-ceramic crown microstructure and strength. When the thickness of metal is 2.1mm, the strength of metal-ceramic decrease significantly. 2.Increasing the thickness of metal and porcelain in proper rang has no significant effect on the strength of metal-ceramic crown. 3. In clinic, we can decrease the thickness of porcelain though increase the thickness of metal in proper rang.
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