Effects Of Molding And Sintering Process On Dental Recycled Zirconia Ceramics

Objective:① To explore the methods to purify the dental recycled zirconia powder,and the optimal route of the recycled zirconia powder molding.②To determine the optimal pre-sintering parameters of recycled zirconia green body.③To evaluate the physical and mechanical properties of the fully-sintered recycled zirconia.Methods:① Commercial zirconia blocks（Lava TM Plus,3M,USA）were obtained and pulverized into powders,sieved by 120 and 300 mesh screen to obtain the large（particle diameter<125 μm）and small（particle diameter≤50 μm）particles of recycled zirconia powders.According to the purification treatment,the powders were divided into three groups,including control group（non-treatment）,heat treatment group（calcining at 900℃ for 30 min）and acid treatment group（pickling with 0.5 mol/L nitric acid for 5 min）.The microstructure of the powder was observed by SEM（scanning electron microscope）,and the purified effects were determined by EDX（energy dispersive X-ray spectroscopy）.The purified recycled zirconia powder was then formed into blocks by bidirectional dry-pressing method under the pressure of 50,60,70,and 80 MPa with a duration of 30,60,and 90 s,respectively.The optimal molding parameters of recycled zirconia powder were determined by measuring the green body.The recycled zirconia green body was further processed by cold isostatic pressing at a pressure of 300MPa for 10 min.The dimension of the green body was measured,and the morphology were observed.②According to the heating rate and pre-sintering temperature,the large particles recycled zirconia green bodies were divided into 9 groups with different heating rates and pre-sintering temperatures.The heating rate was set as 2℃/min,5℃/min,and 8℃/min,respectively.The pre-sintering temperature was set as 950℃,1000℃,and 1050℃,respectively.The relative density,open porosity,machinability,and surface roughness of the recycled zirconia pre-sintered body were evaluated by comparing with commercial zirconia pre-sintered body.Three groups of large particles recycled zirconia pre-sintered body with the best performance were selected based on their microstructure,and their pre-sintering parameters were applied to the small particles recycled zirconia green body.The relative density,open porosity,machinability,surface roughness,and microstructure of the small particles recycled zirconia pre-sintered body were measured and compared with those of the large particles recycled and commercial zirconia pre-sintered body.③Three groups of large particles recycled zirconia pre-sintered bodies,three groups of small particles recycled zirconia pre-sintered bodies and commercial zirconia pre-sintered body in experiment 2 were fully sintered according to the manufacturer’ s instructions.The relative density,open porosity,surface roughness and flexure strength of fully sintered zirconia were further compared.The microstructure of fracture surfaces was analyzed by SEM,and the crystalline structure and Y element content were analyzed by XRD（X-ray diffractometer）and EDX,respectively.Results:①The shape and particle size of the recycled zirconia powders were irregular and different.The powders contained O,Zr,Y,Al,and C elements,while the C element significantly decreased after the heat treatment.The recycled zirconia powder can be formed to a green body under the condition of 70 MPa bidirectional pressing pressure and 60 s holding time.Moreover,the recycled zirconia green body with smooth and flawless surface was obtained after cold isostatic pressing at 300 MPa for 10 min.②At the same heating rate,with the increase of pre-sintering temperature,the relative density and machinability of recycled zirconia pre-sintered body increased,and the open porosity decreased.The results showed that the relative density of the recycled zirconia pre-sintered at 1050℃ was significantly higher than that of the recycled zirconia pre-sintered at 950℃ and 1000℃（all P<0.05）,and its open porosity was significantly lower than that of the recycled zirconia pre-sintered at 950℃ and 1000℃（all P<0.05）.There was no significant difference in the relative density and open porosity between the recycled zirconia pre-sintered at 1050℃ and commercial pre-sintered zirconia.At the same pre-sintering temperature,the relative density and open porosity of the pre-sintered zirconia sintered at different heating rates had no significant difference.Under the same pre-sintering parameters,the relative density of small particles recycled zirconia pre-sintered body was significantly higher than that of large particles recycled zirconia pre-sintered body（all P<0.05）,and its open porosity was significantly lower than that of larger particles recycled zirconia pre-sintered body（all P<0.05）.There was no significant difference in surface roughness among the groups.③The relative density,linear shrinkage and flexural strength of the recycled zirconia were significantly lower than those of the commercial zirconia（P<0.05）,and the porosity of the recycled zirconia was significantly higher than that of the commercial zirconia（P<0.05）.The flexural strength of the small particles recycled zirconia was significantly higher than that of the large particles recycled zirconia（P<0.05）.The phase of the recycled zirconia was t-ZrO₂（tetragonal zirconia）,and the content of Y element has no significant difference with commercial zirconia.There was no significant difference in the surface roughness among the groups.Conclusion:①The content of organic impurities in recycled zirconia powder decreased after heat treatment.Under the condition of bidirectional molding pressure 70 MPa and holding time 60 s,the recycled zirconia green body can be formed.The recycled zirconia green body with smooth and flawless surface can be obtained after cold isostatic pressing at 300 MPa for 10 min.②The relative density,open porosity and machinability of the recycled zirconia pre-sintered at 1050℃ were similar to those of commercial zirconia.Different heating rates had minor effect on the relative density and machinability of the recycled zirconia.Under the same pre-sintering parameters,the small particles recycled zirconia was easier to densify than the large particles recycled zirconia.③Compared with the commercial zirconia,the relative density and flexural strength of recycled zirconia were lower.The flexural strength of small particles recycled zirconia was significantly higher than that of large particles recycled zirconia.The flexural strength of small particles recycled zirconia was 72%of that of commercial zirconia,while the flexural strength of large particles recycled zirconia was only 45%of that of commercial zirconia.According to ISO 6872:2015,the flexural strength of large particles,small particles recycled zirconia and commercial zirconia corresponded to the strength requirements of 3-unit anterior or posterior single crown,3-unit posterior and 4 or more units of posterior restorations respectively.The recycled zirconia only existence in the form of t-ZrO₂,which was consistent with the commercial zirconia.