Effect Of Repeated Firings On The Color And Adhesion Of Zirconia Prepared Using Different Coloring Methods

Objective:This study aimed to investigate the effects of repeated firings(2,3,4,5,6times)on the color and adhesion of zirconia produced with different coloring methods(colored by immersion、pre-colored).It explores the impact of the two coloringmethods on the color stability and the adhesion of zirconia,which provides theoretical basis for scientific research,clinical and technicians’practice.Methods:(1)The zirconia blocks(HT type,pre-shaded zironia block 98X14-color and white zironia block 98X14-SHT,Aidite,China)were cut into 225 bar-shaped zirconia specimens(10 mm×10 mm×1.5 mm)using CAD/CAM method.The specimens were randomly divided into 3 groups:colored by immersion(group IM),pre-colored(group PR)and the un-colored(group UN,control group).One hundred and fifty sampleswere prepared from White zirconia block 98X14-SHT,of which 75 samples wererandomly selected to be soaked in the special zirconia ceramic coloring liquid for 1 min and then dried for 30 min(group IM).The remaining 75 sample were assigned to group UN.Seventy-five samples were cut from the pre-shaded zironia block 98X14-color and assigned to group PR.Then all samples were sintered using a zirconia sintering furnace under the following condition:1530℃sintering for 8 h,thermal insulation for 2 h.Each group was further divided into 5 subgroups,according to the number of repeated firings(n=15):sintered for 2 times,3 times,4 times,5 times,and 6 times,respectively.(2)The color of the specimens was analyzed using a spectrophotometer(EasyshadeAdvance,Vita,Germany).The chromaticity parameter L~*、a~*、b~*were measured and used to calculate the color difference△E(△E=(△L~*)+(△a~*)+(△b~*)).Thesurface roughness of all specimens was processed using a surface roughness tester(Kosaka Laboratory Ltd,Japan).Each sample was tested with three segments and values obtained were averaged.Moreover,three samples of each subgroup wererandomly selected and their surface topography and composition were measured using the SEM and EDS(Nova Nano SEM 230,Japan).(3)The self-adhesive resin cement tubes(Speed CEM,Ivoclar Vivadent,Liechtenstein),with a bonding diameter of 1 mm,were bonded on the specimen surfaces.After 2000thermal cycles(cold cycle 5℃±1℃,30 s;thermal cycle 55℃±1℃,30 s;conversiontime 10 s),a universal testing machine(AG-X,Shimadzu,Japan)was used to measure the micro-shear bond strength of the resin cements to zirconia.A stereomicroscope(MM-400,Nikon,Japan)was used to observe the fracture models which were classified using the following criteria:(1)mixed failure:part of the fracture occurs on the bonding interface,part in the resin adhesive;(2)cohesive failure:fracture occurs only in resin;(3)adhesive failure:fracture occurs between sample and resin adhesives,and the samplewas completely separated from resin adhesive.(4)SPSS 22.0 statistical software was used for the data analysis.The results were analyzed by 2-way analysis of variances(ANOVA),chi-square test withα=0.05.Results:(1)The color of the specimens in group IM measured by Easyshade Advance were:L~*ranged from 84.31 to 84.53,a~*ranged from 3.27 to 3.57,b~*ranged from 29.13to 29.97.Group PR:L~*ranged from 84.53 to 85.77,a~*ranged from 2.97 to 3.51,b~*ranged from 29.39 to 31.73.The zirconia ceramic processed with 2 coloring methods in color L~*,a~*and b~*were statistically significant(P<0.001),and with the increasing times of sintering,their color changes△E<3,which could not be observed by naked eyes.(2)The Ra value of the group UN was 0.63±0.24μm,the Ra value of the group PR was0.50±0.25μm,and the Ra value of the group IM was 0.49±0.11μm.The Ra value of the group UN was significantly higher than the group PR and the group IM(P<0.001).Coloring processing zirconia ceramics reduced its surface roughness.There was no significant difference in the surface roughness Ra value(P=0.510),with theincreasing times of sintering.(3)The crystal morphology of zirconia ceramic were different using different treatments.With the increasing numbers of firing,the crystal of zirconia ceramic becomes bigger,and the micro cracks on the surface of the specimen increase.The crystal of dyed zirconia ceramics is more angular than the unprocessed zirconia ceramics.(4)The metal elements on the surface of zirconia ceramics specimens under different treatments were compared the metal atomic percentage(at%)excluding zirconium ions under different sintering times.The results show the differences were statisticallysignificant(P=0.025).(5)Along with the multiple firings,the micro-shear bond strength of the resin cement to the zirconia increased.The specimens fired for 6 times showed the best adhesionproperty(P=0.001).The micro-shear bond strength of the group IM was significantly higher than other groups(P<0.001).(6)The fracture models observed using a stereomicroscope.Mixed failures the main failure mode.Conclusion:Along with the multiple firings,the micro-shear bond strength of the resin cement to the zirconia significantly increased.The micro-shear bond strength of thezirconia colored by immersion is significantly higher than the pre-shaded and theun-colored.The application of colored by immersion could gain good color of zirconia ceramics and improve the adhesion strength of the resin cements,which could beadopted and promoted clinically.