| Lithium disilicate glass ceramics are widely used in the production of various types of prostheses because of their moderate mechanical behavior,excellent aesthetic properties and mature casting processes.However,since the dental ceramic restorations are worn in the mouth,the wear between the teeth and the natural teeth is inevitable,and the wear behavior is a dynamic and long-term process.Excessive wear may cause collapse and failure of restorations.Therefore,the research on the wear behavior of dental ceramic materials has become a hot topic in the field of oral cavity repair in recent years.Due to the limited origin of healthy human isolated teeth,some scholars used bovine enamel with dental ceramics in vitro to study the wear behavior of natural teeth and restorations,and the study of impact of grinding,polishing and glazing on restorations showed the wear behavior of ceramic restorations is closely related to surface treatment.However,most of the former wear experiments are confined to the comparison of the wear of a certain time node,and the dynamic wear behavior of the porcelain restoration in the mouth has not been systematically studied.The mechanical tribology theory holds that the wear behavior of the material is time-dependent and each stage has its corresponding micro-topographic features.Therefore,it is necessary to study the dynamic wear behavior and its corresponding micro wear mechanism by studying the chewing and abrasion of porcelain restoration and enamel in a comprehensive and scientific way.AIM: To investigate the effects of two kinds of materials,bovine enamel and lithium disilicate glass ceramic,on the dynamic wear behavior and its potential micro-wear mechanism.Grinding and polishing methods were studied for lithium disilicate glass Dynamic wear behavior,so as to provide the corresponding experimental data for the rational design of porcelain restoration,the appropriate surface treatment method,and protecting the natural tooth from excessive wear.Methods: 1.The mechanical properties and microstructure of bovine enamel and human enamel were studied.The hardness and elastic modulus of human enamel and bovine enamel were measured by nano-indentation tester.Scanning electron microscopy was used to observe the microstructure of human enamel and bovine enamel after polishing and acid corrosion.2.The dynamic wear behavior of bovine enamel and lithium disilicate glass ceramics.Lithium disilicate glass ceramics and bovine enamel were respectively matched with lithium disilicate glass ceramics.540,000 wear cycles was tested and 10 nodes were selected in the whole wear cycle to measure the height loss and wear rate of the specimen at each node to evaluate the dynamic wear behavior of the two materials.3.Microscopic mechanism of dynamic wear behavior of bovine enamel and lithium disilicate glass ceramics.According to the different wear stages of the wear curve in experiment 2,the corresponding bovine enamel and lithium disilicate glass ceramic specimens were prepared.After the wear cycles to the corresponding wear stage,the microscopic parameters such as the general morphology,microstructure and surface roughness of the specimens in different wear stages were analyzed by means of profilometer and scanning electron microscopy.4.Effect of grinding and polishing on dynamic wear behavior of lithium disilicate glass ceramic.The grinding and polishing of lithium trisilicate glass ceramic were respectively divided into ground group and polished group.In the 1.02 million wear cycles,16 nodes was selected,the test conditions was same with the experiment 2.5.Effect of grinding and polishing on microscopic mechanism of lithium disilicate glass ceramics.According to the different wear stages divided by the wear curve in experiment 4,the corresponding ground group and polished group of lithium disilicate glass ceramic specimen were prepared,the test conditions was same with the experiment 3.Results: 1.The hardness of bovine enamel(3.33 ± 0.19)GPa and elastic modulus(79.51 ± 3.56)GPa,are close to the hardness of human enamel(3.92 ± 0.20)GPa and elastic modulus(93.45 ± 4.75)GPa.SEM shows that human enamel and bovine enamel are both made of enamel rods.2.The wear behavior of bovine enamel and lithium disilicate glass ceramics showed time-dependent behavior.The wear curve of bovine enamel can be divided into: 0 ~ 220 thousand cycles as "running-in period",220 ~ 380 thousand cycles as "steady wear period",and 380~540 thousand cycles as "severe wear period";lithium disilicate glass ceramic wear curve can be divided into: 0 ~ 60 thousand cycles as "running-in period"," 6 ~ 54 thousand cycles as "steady wear period." The wear amout of bovine enamel was significantly larger than that of lithium disilicate glass after the 540,000 cycles.3.The surface roughness of bovine enamel in the "running-in period","steady wear period","severe wear period" stage showed the tendency of increase,decrease,and then increase again,lithium disilicate glass ceramic surface roughness showed the tendency of increase and then decrease.The microstructures of the two materials also show the evolution laws corresponding to the macroscopic wear curves in different wear stages.4.Ground group and polished group of lithium disilicate glass ceramic wear behavior have shown a time-dependent tendency: ground group wear curve can be divided into: 0 ~ 140 thousand cycles as "running-in period",140 ~1020 thousand cycles as "steady wear period";polished group wear curve can be divided into: 0 ~ 60 thousand cycles as "running-in period",60 to 1020 thousand cycles as "steady wear period." The wear amout of upper and lower specimens of ground group was significantly larger than that of polished group after the 1.02 million cycles.5.The surface roughness of the lithium disilicate glass ceramic specimen in the ground group and the polished group showed a tendency to increase first and then decrease in the "running-in period" and "steady wear period" stages.The microstructures of the two materials also show the evolution laws corresponding to the macroscopic wear curves in different wear stages.Conclusion: 1.The mechanical properties and micro-organization of human enamel are similar to the bovine enamel,combined with large size,wide range of sources,easy processing,bovine enamel is the ideal alternative of human enamel for in vitro wear experiments.2.In the process of wear,the enamel specimen of the bovine enamel has a characteristic wear stage of "running-in period","steady wear period" and "severe wear period".The lithium disilicate glass ceramic exhibits "running-in period "," steady wear period " two characteristic wear stage.The surface roughness and microstructure of the two materials also show the dynamic evolution law corresponding to the macroscopic wear curve.3.Compared with the opposing lithium disilicate glass ceramics,the opposing bovine enamel produced more wear amounts,so clinical attention should be paid to reduce the excessive wear of opposing natural teeth.4.Grinding and polishing mainly affect the "running-in period" during the wear process of the material.Polishing can effectively reduce the surface roughness of lithium disilicate glass ceramics,shorten the " running-in period " and reduce the wear amount. |
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