Study On The Color And Aging Properties Of Dental Y-TZP Ceramics

Yttria-stabilized tetragonal zirconia polycrystal(Y-TZP)ceramics have been reported to possess exceptionally high flexural strength and fracture toughness due to it's so called martensite transformation under stress,in addition to it's excellent biocompatibility and low thermal conductivity,it became an ideal material for dental all-ceramic restorations,compared with other dental ceramics, it has very high strength and toughness.Normally,dental zirconia is white to ivory color,also relatively translucent,which makes it easier to mimic the natural teeth color,However,white framework are not desirable in every case,applying opaquer as metal ceramic will lose some translucence.If the zirconia structures were shaded to adapt the brightness and chroma of the tooth to be restored, harmony color match with neighboring teeth would be arrived simpler,tooth preparation would conserve more substance.Basically,two methods were used to shade dental zirconia,the first was that presintered porous zirconia is infiltrated with an aqueous solution of metal oxides before the sintering process,the second was that zirconia was shaded during the production of the powder before sintering. The former is easier,but dipping in coloring liquid to provide internal color characterization may cause inhomogeneous color,also coloring liquid can not be used among different systems due to the different materials making process, sintering procedure etc.,while the latter is more promising because the color would be homogeneous and accurate.As a core ceramic material,Y-TZP offers the advantage of relatively translucent,to improve the esthetic appearance,the milled Y-TZP frameworks are veneered with dental porcelain forming bilayers, To what extent of the shading destination of the core ceramics Should be reached is the key for reproduction of natural teeth with dental porcelain.The influencing factors may including the translucence,lightness and chroma of the core ceramics, also the translucence or opacity of the veneering porcelain.Stabilized tetragonal zirconia transform to the monoclinic phase at relatively low temperatures spontaneously especially if water or humid environment present,this phenomenon is called low-temperature degradation(LTD),which is responsible for the formation of microcracks and macrocracks,cause loss of strength,which may accelerate stress corrosion in oral condition.In this study,zirconium oxide was shaded by mixing metal oxides with 3Y-TZP powder,the purpose is to investigate the effects of different colorants and it's composites on the color and mechanical properties of dental zirconia,to develop a series of intrinsic colored dental zirconia.To study the aging properties of commercial available and self-made dental zirconia.Results show as follows:1.Color shaded by single metal oxides:The lightness of zirconia by adding cerium oxides was about 85,a~* decreased slightly,the range was between -5～-4,b~* value increased,the color shift to more yellow-green color,the chroma was increased while little change in lightness.When erbium oxide was added,the lightness slightly decreased but remain in high level above 80, a~* value increased apparently,b~* value lowered,more erbium oxide makes the color shifting to red-blue,chroma was lower.The chroma of the ceramics became more intensity and the lightness decreased with the increasing of the content of ferrum oxides,the color change to red-yellow direction,lightness value range was 75.27-83.85.When bismuth oxides was added,the lightness value range was between 83.31～85.24,a~* value was 1.5～2.15,b~* value was 7.37～9.19,the color was light saffron yellow.2.Color shaded by mixed metal oxides:Combined use of cerium oxide and erbium oxide could shift the color from yellow-green to yellow-red,while its ability of reducing L~* value was still weak,changing from 83.99 to 86.99 scope,a~* value changed little with the increasing amount of cerium oxide,but increased rapidly with erbium oxide amount,b~* value increased with cerium oxide amount while slightly decreased with erbium oxide amount.Compared with the chromaticity of adding ferrum oxides or bismuth oxides alone, combined use of these two colorants made the lightness decreased,a~* value increased,particular at the range 0.06wt%～0.12wt%of ferrum oxides,b~* value increased a little,the color became darker and intensity.3.Within the range of this study,the bending strength was decreased with adding ferrum oxides,while it was not going down at 0.15wt%level,overall bending strength is above 988MPa;the bending strength was decreased with adding cerium oxides,it was not going down at 4wt%level,overall bending strength is above 965MPa,the influence of these two colorants was not significant and acceptable in clinical use.After adding erbium oxides alone,the bending strength decreased significantly,the lowest value was 426MPa,large amount of it will bring detrimental effects to mechanical properties.4.After shading by mixing metal oxides with 3Y-TZP powder,a series of intrinsic colored dental zirconia was developed with 5 graded lightness,the color space was L~*:67.76～77.78;a~*:-2.19～3.80;b~*:12.13～25.01,which was similar to Vita In-Ceram YZ coloring liquid shade guide,and accorded with the color space of Chinese people's nature teeth,it may be suitable for color matching with dental veneering porcelain.5.Three-point bending strength were not significantly differences between monolithic specimens of white and colored materials or between bilayered specimens,while the strength of monolithic specimens were significantly higher than their bilayered counterpart.The force-displacement curves of the the bilayered specimens show one peak.In all bilayered specimens,the bulk fracture initiated at the tensile surface of the core material and was consistently accompanied with porcelain crush and delamination at the interface.6.Tetragonal-to-monoclinic phase transformation was detected for specimen of TZ-3YS,Vita In-Ceram YZ,Ivoclar,and Kavo zirconia ceramics except for Cercon Smart ceramics after aging,and the relative content of monoclinic phase was increasing with the prolonged aging time,the most affected material is TZ-3YS,then Kavo,Vita and Ivoclar was similar.Aging had no significant negative effects on flexural strength on TZ-3YS,average bending strength was over 1100MPa;The nucleation and growth of monoclinic phase was detected by AFM in surface of Cercon Smart zirconia on which no monoclinic phase was detected by XRD.