| Due to better mechanical properties and aesthetic effects,porcelain-fused-to-metal(PFM)restorations have become the main way to repair teeth and dentition defects.Co-Cr alloy has excellent mechanical properties,corrosion resistance and biocompatibility,which has been widely used in the production of dental restorations.With its personalized design and rapid processing of complex shapes,selective laser melting(SLM)technique is particularly suitable for the production of Co-Cr PFM restorations.SLM technique can greatly improve the production efficiency and product quality stability,and reduce the production cost.At the same time,the SLM Co-Cr prosthesis manufactured according to the patient’s tooth data has a high matching degree with the patient,which shortens the patient’s adaptation period and reduces the incidence of complications.Therefore,SLM technique has gradually replaced precision casting and CNC milling as the main manufacturing method for Co-Cr PFM restorations.At present,the following scientific problems in the study of microstructure and performance of SLM Co-Cr porcelain restorations need to be solved urgently : 1)The influence mechanism of the microstructure of SLM Co-Cr alloy on the mechanical properties and metal-ceramic bonding properties is still unclear.Compared with casting Co-Cr alloys,systematically studying the microstructure of SLM Co-Cr alloys and exploring the effect mechanism of its microstructure on mechanical properties and metal-ceramic bond properties is the basis for accurately assessing the service life and the risk of porcelain cracking of SLM Co-Cr PFM restorations.2)There is a lack of research on the evolution of the microstructure,mechanical properties,and metal-ceramic bond properties of SLM Co-Cr alloy during annealing.Annealing can not only eliminate the residual stress formed during the SLM forming process,but also improve the microstructure and performance.Establishing the mechanism of annealing on mechanical properties and metal-ceramic bond properties can provide a theoretical basis for the design of annealing process.3)There is a lack of research on the evolution mechanism of the microstructure of SLM Co-Cr alloy during the porcelain firing.4)The metal-ceramic bond properties of Co-Cr alloy is low.In view of the above scientific problems,this paper studies the following contents: 1)Study the formation mechanism of the microstructure of As-SLM Co-Cr alloy;2)Study the microstructure of SLM Co-Cr alloy,and explore the influence mechanism of SLM Co-Cr alloy microstructure on mechanical properties and metal-ceramic bond properties;3)Study the microstructure evolution of SLM Co-Cr alloy during the annealing and porcelain firing,and explore the influence mechanism of annealing temperature on the mechanical properties and metal-ceramic bond properties of SLM Co-Cr alloy;4)Combining the principle of metallology and the metal-ceramic bonding mechanism,a method to improve the metal-ceramic bond performance of SLM Co-Cr porcelain parts is proposed and verified.The influence mechanism of heat treatment after porcelain firing on the metal-ceramic bond properties of SLM Co-Cr restorations is studied.This paper has obtained the following results:1)The microstructure characteristics of As-SLM Co-Cr alloy were related to the solidification rate,temperature gradient,the wetting and spreading behavior of the molten metal on the solid metal.The content of γ phase with fcc structure of As-SLM Co-Cr alloy was more than 90%,and the extremely rapid solidification rate resulted in a high nucleation rate and fine grains.There were a large number of oriented columnar crystals in the molten pool and grains,and the growth direction of the columnar crystals was generally the fastest heat dissipation direction.The columnar grains may grew across the molten pool boundaries.On the x-y plane,a large number of strip-shaped cladding channels were shown,while in the z-axis direction,there was an obvious "fish scale" structure.During the SLM forming process,the wetting,spreading behavior and rapid solidification of the metal melt on the substrate and the solidified metal surface resulted in the unique microscopic morphology of the As-SLM Co-Cr alloy.2)The mechanical properties of SLM Co-Cr alloy were significantly better than casting alloys,and ε martensite was unfavorable to the mechanical properties.Co-Cr alloy was dominated by isothermal martensitic transformation(γ→ε).Needle-shaped ε martensite easily teared the matrix,increased the brittleness of the material,and was detrimental to the mechanical properties.Compared with the casting alloy,the tensile strength,yield strength and elongation after fracture of SLM Co-Cr alloy were all higher than 50%.The micro-Vickers hardness was also significantly higher than that of cast alloys.Refinement strengthening and second phase strengthening were the main reasons for the excellent mechanical properties of SLM Co-Cr alloy.3)The γ→ε phase transformation at the metal-ceramic bonding interface could significantly improved the metal-ceramic bond performance of Co-Cr porcelain parts.The ε phase had a lower coefficient of thermal expansion(CTE)and had better thermal matching with ceramics;The γ→ε phase transformation at the interface produced more defects such as vacancies,which was conducive to the diffusion of atoms at the interface and increased the chemical bonding force,and the atom diffusion also created conditions for the γ→ε phase transformation at the interface.4)Porcelain firing treatment had little effect on the grains and second phase precipitation of SLM Co-Cr alloy,but significantly affected the γ→ε phase transformation.Due to the short processing time,the thermal cycle of the porcelain firing had basically no effect on the grains and second phase precipitation.However,higher firing temperature and faster cooling rate affected the γ→ε phase transformation.During the pre-oxidation treatment,due to the higher firing temperature(980°C)and the faster cooling rate,the content of γ phase increased sharply;however,in the subsequent firing procedure,the thermal matching between the ceramic and the metal matrix should be taken into consideration.The temperature and cooling rate were both significantly reduced,and isothermal martensitic transformation occurred during the cooling process.With the increase of the porcelain step,? martensite gradually increased,and the γ phase gradually decreased.5)After porcelain applications,heat treatment could promote the diffusion of atoms at the metal-ceramic bond interface,improve the chemical bonding force,promote the occurrence of the γ→ε phase transformation at the interface,and improve the metal-ceramic bonding force.The effect of heat treatment on the metal-ceramic bond properties of SLM Co-Cr PFM restorations: 1)it could promote the diffusion of atoms at the metal-ceramic bonding interface,and improve the chemical bonding force.2)By causing a change in the chemical composition at the interface,conditions were created for the γ→ε phase transformation,and the γ→ε phase transformation produced a higher concentration of defects such as vacancies at the interface,further promoting atom diffusion.3)The ε phase had a lower CTE value and had a better thermal matching with the ceramic layer.The heat treatment could produce more ε phase at the bonding interface,reducing the possibility of microcracks formation.4)The heat treatment could release the residual stress at the metal-ceramic interface,which had a great effect on delaying crack propagation at the interface.The heat treatment at750℃ could significantly improve the metal-ceramic bond strength without affecting the microstructure of the metal matrix. |
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