| Objective: Zirconia all-ceramic restorations are widely used in the field of oral restorations.However,chipping is one of the major issues that could be observed on zirconia surface after coating by veneer ceramics.Therefore,improving coating property and bond strength between zirconia and veneering ceramics is the key to increase the success rate of zirconia all-ceramic restorations.Several methods have been reported to improve the bonding strength between zirconia and veneering ceramic materials,including: sandblasting,grinding,acid etching,fine brush coating.However,chipping still is the main issue.The intimate contact of melted veneering materials on zirconia is quite mandatory.Therefore,this study is the first experimental work that focused on different laser methods to form melted veneer ceramic as coted layer on zirconia,and improve the bond strength of different veneering ceramics to zirconia core.The study was aimed to investigate certain aspects such as 1.Developing appropriate laser methods to coat zirconia by veneering ceramics and detect the effectiveness of the different approaches over the other methods.2.Detect interfacial improvements through ‘double surface' treatment and selecting the best veneer ceramic material that could coat zirconia following laser method.3.Evaluate details of coating and micromechanical properties of zirconia core through conventional sintering and laser method.Methods: The following materials were used either as a core or veneering ceramics based on study design and groups: Zenostar?T zirconia used as a core.The IPS e.max Ceram,VITA VM?9,and Noritake Cerabien ZR were used as veneering ceramics.Coating of zirconia core was carried out by veneering ceramic material through air-brush gun spraying technique,then subjected to treatment methods.Characterizations: SEM,EDS,XRD,surface roughness,contact angle measurement,shear bond strength test in unaged and aged conditions(specimens were immersed in aqueous medium for 6 months at 37 oC for each study design approach),and fracture modes were also investigated.1.As a starting point,it was necessary for the study to set the required parameters as a standard protocol to be followed for subsequent laser irradiations.Therefore,30 zenostar?T zirconia cubic-shape specimens were milled though CAD/CAM machine.Then,specimens were divided into three study groups;25 W,50 W,and 75 W(n = 10 each group),then the best parameter was determined and surface of specimens were evaluated through SEM and XRD.First approach(part one,method): zirconia discs(n = 110)were prepared and divided into 5 study groups(n = 22,each group):(1)milling without receiving surface treatment ‘control group'(Group M),(2)Sandblasting(Group S),(3)single laser irradiation(group LX1),(4)two laser irradiations(group LX2),and(5)three laser irradiations(group LX3).For more than one laser irradiation,specimens were 90o rotated then started new laser irradiation to maximize high energy surface and provide better surface area for veneer materials on zirconia.IPS e.max Ceram was selected to use for coating zirconia in the first approach.Specimens group 3,4,and 5 were coated by veneering ceramic through spraying process,then subjected to laser irradiation.Second approach(part one,method): The similar approach as first approach was followed at this stage.However,the number of specimens and groups were as following;n = 132 total zirconia cubes,n = 22 each group,and 6 study groups.The veneer ceramic material that used in this approach was Noritake Cerabien ZR.Groups were divided into:(1)milling without receiving surface treatment ‘control group'(Group M),(2)grinding group(G),(3)sandblasting group(S),(4)three laser irradiations(L3),(5)four laser irradiations(group L4),and(6)five laser irradiations(group L5).Specimens in the laser groups were coated by veneering ceramic through spraying technique,then laser irradiated.2.Zenostar?T ‘zirconia' cubic-shape specimens(n = 136)were prepared and divided into 4 study groups(n = 34,each group): Different materials and techniques were employed,and direct comparison between study groups was performed.The veneer materials used in this approach were;Noritake Cerabien ZR,and VITA VM?9.The first treatment was either grinding under wet condition or sandblasting,then followed by laser irradiation(second treatment)according to the corresponding methods of surface treatment and materials used.The groups included;(1)grinding followed by laser irradiation using Noritake Cerabien ZR ‘GLN',(2)sandblasting followed by laser irradiation using Noritake Cerabien ZR ‘SLN',(3) grinding followed by laser irradiation using VITA VM?9 ‘GLV',and(4)sandblasting followed by laser irradiation using VITA VM?9 ‘SLV'.Further,Noritake Cerabien ZR,and IPS e.max Ceram were also used as a second approach to compare material's affinity to respond to laser coating.Zenostar?T cubic-shape specimens(n = 96)were prepared and divided into 4 study groups(n = 24,each group):(1)sandblast followed by laser irradiation using IPS e.max Ceram ‘group A',(2)grinding followed by laser irradiation using IPS e.max Ceram ‘group B',(3)sandblast followed by laser irradiation using Noritake Cerabien ZR ‘group C',and(4)grinding followed by laser irradiation using Noritake Cerabien ZR ‘Group D'.3.Zenostar?T ‘zirconia'(n = 80)specimens were prepared using CAD/CAM machine and divided into two study groups;conventional sintering group(n = 40)and laser irradiation group(n = 40).The zirconia was coated by IPS e.max Ceram through spraying technique,then treated through mentioned techniques,then coating microstructure and mechanical properties were studied.Results: 1.Investigated specimens for setting parameters showed that the suitable laser output power was 25 W to coat zirconia surface by laser method.The XRD analysis did not show phase transformation after specimens were subjected to different output powers.First approach: The XRD showed that phase transformation from tetragonal to monoclinic tm occurred after sandblasting,laser method just showed tetragonal phase.Specimens in groups LX2 ‘two laser irradiations' and LX3 ‘three laser irradiations' showed the highest surface roughness values;1.18±0.23 ?m ‘LX2' and 1.21±0.22 ?m ‘LX3';P < 0.05.Group LX3 demonstrated highest bond values(32.08±2.45 MPa unaged condition,31.43±2.07 MPa aged condition).The top surface of specimens showed grooves in milled specimens,while tiny roughness was detected in certain areas treated by sandblasting.Laser irradiated specimens showed micro-holes within coated veneer ceramic layer and irregularities on the zirconia core.Laser group showed higher cohesive fracture mode than other groups.EDS detected silicon and aluminum as prominent elements,indicating for adequate chemical bonding.Second approach: The XRD showed phase transformation from tetragonal to monoclinic t-m after grinding and sandblasting.Laser-irradiated specimens did not show monoclinic phase.After comparing all groups for microstructures and mechanical properties,it was observed that three laser irradiations yielded improved properties than groups: 1.10±0.23 ?m surface roughness,37.30±1.33o contact angle,31.70±2.90 MPa unaged condition,and 29.80±2.97 MPa aged condition(P < 0.05).Following the mentioned method,the interface showed better interlocking property.However,four and five laser irradiations led to tiny cracks within the core material.Groups: milling,grinding,and sandblasting,could not show adequate adhesion between zirconia-veneer materials similar to 3 laser irradiations.Cohesive fracture mode was higher in 3 laser irradiated specimens.Different shapes of zirconia core either discs or cubes did not present significant impact on the bond strength values.2.Grinding under wet condition as a first surface treatment followed by laser irradiation as a second surface treatment using Noritake Cerabien ZR;GLN provided satisfactory roughness values in both treatment approaches(1.05±0.24 ?m grinding alone,1.30±0.21 ?m after laser irradiation)compared to sandblasting and other groups(P < 0.05).Line grooves and various tiny roughness could be observed on the surface of ground and sandblasted specimens,respectively.The micro-holes with irregularities could be differentiated on the coated surface by veneer ceramics through laser method.The GLN group showed the highest bond strength values(30.97±3.12 MPa unaged condition,29.09±4.17 MPa aged condition),while group SLV ‘sandblasting followed by laser irradiation using VITA VM?9 recorded the lowest values(23.96±3.60 MPa unaged condition,22.95±3.68 MPa aged condition;P < 0.05).Sandblasting showed phase transformation from t-m.Analyzed collected spectra of Noritake Cerabien ZR on zirconia core after shear test showed cohesive fracture mode and high silicon element.Indicating for sufficient adhesion and micro-mechanical interlocking with adequate chemical bonding at interface of GLN group.This study detected that(Noritake Cerabien ZR)was better than both(IPS e.max Ceram)and(VITA VM?9).Grinding followed by laser irradiation using Noritake Cerabien ZR presented highest roughness values in both treatments approaches(1.11±0.22 ?m grinding alone,1.31±0.22 ?m;P < 0.05 after laser irradiation)compared to other groups.After first surface treatment,ground specimens showed obvious deeper grooves than sandblasted specimens which showed flake-like appearance and tiny roughened surface.After coating the core by veneer ceramics and applied laser irradiation ‘second treatment',the surfaces showed condensed coat layer with certain tiny micro-holes within the coat layer.The treated specimens showed tortuous-like appearance after grinding followed by laser irradiation,increased interfacial bonding.In the second approach,group D ‘grinding followed by laser irradiation using Noritake' showed the highest bond strength values(31.19±3.18 MPa unaged condition,29.79±4.03 MPa aged condition),while group A ‘sandblasting followed by laser method using IPS e.max' recorded the lowest values(24.00±3.50 MPa unaged condition,22.15±3.75 MPa aged condition;P < 0.05).The relation between roughness and bond strength was proportional,it led to optimize interfacial bond strength.Sandblasting showed phase transformation for these trials.However,grinding under wet condition using low rotary speed showed tetragonal phase.Grinding followed by laser method used Noritake.3.XRD did not show phase transformation in either sintering or laser method.The highest surface roughness Ra,contact angle q,and shear bond strength SBS values were recorded though laser process(1.087±0.143 ?m Ra,24.39±1.09o contact angle,33.2±4.184 MPa unaged condition,31.7±3.56 MPa aged condition;P < 0.05).The top surface of specimens treated by sintering displayed less irregularity than laser method.The coated veneer ceramic layer through sintering ranged from 114.2-128.8 ?m,while a more homogenous thinner layer was achieved by laser coating 72.2-99.4 ?m.The coated layer followed laser method was more condense,uniform,presented less defect and pores,and showed adequate adhesion than sintering.The Adhesive and cohesive fracture modes were recorded through sintering and laser method,respectively.Conclusion: 1.Laser method can be used successfully for coating veneering ceramics on zirconia,the recommended laser output power is 25 W.Two and three laser irradiations provided low contact angle leading to intimate contact at interface and increasing the bond strength.The bond strength in unaged and aged conditions were better in laser irradiated specimens than other specimens.The laser method in the current study permitted melting of veneer ceramics on zirconia without phase transformation.The microstructure,adhesion,and mechanical properties were optimized.2.The following recommended double surface treatment and material could increase success rate of fixed dental prostheses in load bearing areas(grinding under wet condition followed by laser irradiation using Noritake Cerabien ZR).3.Laser method permitted melting and coating of a uniform thin layer of veneering ceramic on zirconia with presenting better micro-structure and micro-mechanical properties than conventional sintering. |
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