Evaluation Of Novel Surface Coating Techniques On Zirconia And Its Composite For Improved Bonding

Objective:Zirconia,specifically yttria-stabilized tetragonal zirconia polycrystal(Y-TZP),became favored as a core material for all-ceramic restorations because of favorable mechanical strength and toughness.On the contrary,zirconia coatings for single or multi-unit frameworks still require the application of veneer ceramics(VC),to achieve excellent aesthetics.Various surface-conditioning methods of the zirconia substructure were researched to achieve enhanced zirconia/VC bonding.Among these methods,airborne-particle abrasion with Al2O3 particle sizes in the range of 30–125?m,with or without silica coating,liner application,and argon plasma treatment are all used alone or in association.However,airborne-particle abrasion altering the crystal structure of zirconia from tetragonal to monoclinic,liner and argon plasma application were found to reduce the shear bond strength(SBS).Although the Y-TZP substrate displays excellent mechanical properties,veneer chipping(de-bonding)is described in clinical service with a failure rate of 15% after 2–5 years,and 21–23% after 5–10 years of service due to poor bond strength to veneering.Another problem is that ceramic crowns made only of zirconia,monolithic zirconia crowns are not used widely in clinical practice because of the single color and the possibility of wear of the opposing teeth due to the hardness of zirconia.Particularly,in relation to the possibility of wear of the opposing teeth,the selection of an appropriate restorative material is important for preserving the function of normal opposing teeth and the balance of articulation.Therefore,it is of great clinical significance to find effective and specific targets to overcome the chipping of VC through coating its surface and reduction of the hardness of zirconia through making a composite.Therefore the aim of this study was to investigate:1.The effects of various materials and two surface-coating methods(airbrush spraying and fine brush painting)on the SBS of VC to zirconia.2.The impact of mixing varying ratio of the VC(5%,10%,20%,and 30 wt.%)with zirconia on mechanical property of zirconia.3.A new surface coating on the composite zirconia through application of laser sintering in improving VC to composite zirconia bonding.Methods:1.To evaluate two surface-coating techniques(airbrush spraying and fine-brush painting),eighty pre-sintered zirconia cubes were prepared through milling of zirconia discs by computer aided design and computer aided manufacturing(CAD/CAM).The specimens were then randomly assigned into four study groups,E60 S group: E.max Alumina particle size < 60?m airbrush Spray(n = 20);E60P group: E.max Alumina particle size <60?m fine brush Paint(n = 20);N60S group: Noritake Cerabien ZR Alumina particle size < 60?m airbrush Spray(n = 20);N60P group: Noritake Cerabien ZR Alumina particle size < 60?m fine brush Paint(n = 20).The zirconia surface was coated with a mixture of two types of glaze and alumina(< 60?m)by airbrush spraying and fine-brush painting.Surface roughness(Ra),X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and shear bond strength(SBS)measurements(both initial and artificial aged conditions,including six months of water storage)were performed.2.To evaluate the mechanical property of the new composite zirconia,a total of 60 samples were prepared in four different compositions;Group 5 wt.%,Group 10 wt.%,Group 20 wt.%,and Group 30 wt.% veneering ceramic)with control group,after weighting using a digital balance.The composite was dry ball milled in a polystyrene container with 60 g zirconia ball for 1 h at 250 rpm,to ensure homogenous mixing.The30 g of the composite zirconia(partially stabilized zirconia powder with 5%,10%,20%and 30 wt.% IPS e.max Ceram VC)was prepared.The four composition of 30 g of each of the blend in each group and the control group was compacted in the rectangular stainless steel mold with a dimension of(20x50 mm)under pressure of(70.65 MPa)for dry pressing and held for 1 min,using Oil Tight Metal powder press machine.Three-point bending mechanical investigation was applied to determine the flexural strength,XRD and SEM were used for phase analysis and surface morphology.3.To evaluate the effect of SBS through laser sintering of composite zirconia with VC,40 samples were prepared in four different compositions;Group 5 wt.%,Group 10 wt.%,Group 20 wt.%,with control group,after weighting using a digital balance.The prepared specimens were fully sintered,cleaned and dried.One surface of study groups were airbrush sprayed with a VC and a CO2 laser irradiation was used to sinter the coating. XRD was applied to observe any crystallographic change of specimens.Then SEM,EDS,Ra,SBS and fracture modes were investigated.Results were statistically investigated applying analysis of variance(ANOVA)and Turkey test.Results:1.The effect of two surface coating techniques: The Ra results revealed significant differences among all groups(P < 0.001).The N60 P group exhibited higher values of Ra(5.71±0.20?m)and SBS before and after water storage with values of 37.22±4.95 MPa and 34.42±3.97 MPa,respectively.The fine-brush coatings showed significantly higher SBS than that of airbrush coatings,in both initial and artificial conditions.Both coating methods and various coating materials(P < 0.001)produced a significant influence on VC-zirconia SBS.A significant correlation between Ra and SBS(Spearman's rho =0.808;P < 0.001)was found.2.For testing the mechanical strength of the composite zirconia,the results indicated that mean three-point bending test was(556.89±7.72 MPa)for the control group,(280.73±8.47 MPa)for 5 wt.%,(237.76±6.78 MPa)for 10 wt.% and(119.71±8.99 MPa)for 20 wt.% composite zirconia.One–way ANOVA test revealed that significant differences were observed among all groups(P < 0.001).The XRD traces of the blended Zirconia-IPS e.max Ceram VC showed tetragonal yttrium zirconium oxide(PDF#-48-022)crystalline phases.The SEM revealed that 5 wt.%,10 wt.%,and 20 wt.%polished surface had small pores.3.The Ra results of the surface coated specimens by laser technique revealed significant differences among all groups(P < 0.01).The Group 5 wt.% exhibited the highest values of Ra(1.67 ± 0.17?m)and SBS values of 29.10 ± 2.188 MPa.The composite group coatings showed significantly higher SBS than that of control group(P < 0.05).A significant positive correlation between Ra and SBS(Pearson's r = 0.008,P < 0.05)was found.The results of the 5 wt.% and 10 wt.% groups revealed a higher percentage of the mixed and cohesive mode of failure than the other tested groups.The XRD results revealed that tetragonal phase of the laser scanned composite specimens was preserved.Conclusion:1.The most striking observation was that the bond strength values of the coated zirconia core to VCs are significantly affected by Ra,technique,and materials used.The novel coating by fine-brush painting is a promising surface coating and an easy technique for obtaining a rougher surface,which subsequently improves the bond strength to VC.2.One of the most significant findings to emerge from fabricating composite zirconia is that the composite zirconia could reach the ISO standard for full ceramic,which can be used for single unit dental application.The structural integrity of tetragonal phase of zirconia has been preserved with the addition of various ratio of VC.3.The new coating by laser application is another promising surface modification and a fast processing technique for obtaining a rougher surface,which subsequently improves the bond strength of composite zirconia to VC.The application of laser could be used as an application modality for enhancing composite zirconia/ VC adhesion and reduce the chance of delamination of VC in dental application.