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Journal of Prosthetic Dentistry

Influence of yttria content and surface treatment on the strength of translucent zirconia materials

Open AccessPublished:August 24, 2021DOI:https://doi.org/10.1016/j.prosdent.2021.07.001

      Abstract

      Statement of problem

      Newly developed translucent zirconia materials have been used for anterior monolithic complete coverage restorations. Surface treatments can improve adhesion, as well as decrease or increase the strength of ceramics. However, information on the influence of surface treatments on the strength of translucent zirconias is sparse.

      Purpose

      The purpose of this in vitro study was to measure and characterize the effects of different surface treatments, including airborne-particle abrasion, on the strength of different translucent 4 mol% and 5 mol% yttria-stabilized zirconia materials.

      Materials and methods

      Disks (N=160) made from 4 types of translucent yttria-stabilized zirconia materials were surface-treated in 4 ways: Control groups were hand-polished with 2000-grit silicon carbide abrasive paper; as-machined; glass bead airborne-particle abraded; and alumina airborne-particle abraded. The biaxial flexural strength was measured by using a piston-on-3-ball test in a universal testing machine. The simple main effects of material type and surface treatment and their interaction on biaxial flexural strength were evaluated with 2-way ANOVA (α=.05). A priori, 1-way ANOVA and the Tukey multiple comparisons tests were used within material and treatment types (α=.05). Surface morphology was assessed by using scanning electron microscopy. Translucency, absolute transmittance, was measured by using a spectrophotometer.

      Results

      Two-way ANOVA revealed that the effects of zirconia type, surface treatment, and their interaction all significantly affected biaxial flexural strength (P<.001). One-way ANOVA revealed that the 4Y material was stronger than all 5Y materials, regardless of surface treatment; all 5Y materials were ranked from strongest to weakest as polished; as-machined, or glass bead abraded; and alumina abraded. The 4Y material was stronger when alumina abraded than when glass bead abraded. Scanning electron microscopy showed that as-polished surfaces were smoother than all others; as-machined and glass bead abraded surfaces displayed little difference; alumina abraded was the roughest; and differences among materials were not discerned. The 1-way ANOVA and multiple comparisons testing showed that the 4Y material had less absolute transmittance, approximately 5% less, than all the 5Y materials.

      Conclusions

      Zirconia material type and surface treatment influenced the strength of translucent zirconia materials; a 4 mol% zirconia material was stronger than 5 mol% zirconia materials for all surface treatments tested; airborne-particle abrasion using alumina had a slight strengthening effect on a 4 mol% zirconia but had a weakening effect on 5 mol% materials; airborne-particle abrasion by using alumina produced the roughest surfaces on all materials; and the 4 mol% material was slightly less translucent than the 5 mol% materials.
      Clinical Implications
      A zirconia containing 4 mol% yttria was substantially stronger and only slightly less translucent than 5 mol% materials. Unlike the 5 mol% materials, the 4 mol% material was not weakened by alumina airborne-particle abrasion.
      Zirconia first gained attention as an engineering ceramic in the 1970s and began to be used in prosthodontics in the late 1990s.
      • Garvie R.C.
      • Hannink R.H.
      • Pascoe R.T.
      Ceramic steel?.
      • Garvie R.C.
      • Nicholson P.S.
      Phase analysis in zirconia systems.
      • McLaren E.A.
      • White S.N.
      Glass-infiltrated zirconia/alumina-based ceramic for crowns and fixed partial dentures.
      Zirconia exhibits polymorphism, existing in cubic, tetragonal, and monoclinic forms. A martensitic phase transformation from tetragonal to monoclinic (t→m) zirconia forms is accompanied by a 4% increase in volume. However, a small amount of controlled expansion can decrease crack propagation, thereby making a material tougher through a process called transformation toughening.
      • McMeeking R.M.
      • Evans A.G.
      The mechanics of transformation toughening in brittle materials.
      ,
      • Heuer A.H.
      • Lange F.F.
      • Swain M.V.
      • Evans A.G.
      Transformation toughening: an overview.
      The addition of stabilizing oxides, typically yttria (Y2O3), can create tough multiphase partially or fully stabilized zirconia materials.
      • Garvie R.C.
      • Hannink R.H.
      • Pascoe R.T.
      Ceramic steel?.
      ,
      • Garvie R.C.
      • Nicholson P.S.
      Phase analysis in zirconia systems.
      Zirconias that are capable of undergoing transformation toughening do not exhibit a proportional relationship between toughness and strength.
      • Heuer A.H.
      • Lange F.F.
      • Swain M.V.
      • Evans A.G.
      Transformation toughening: an overview.
      ,
      • Kelly J.R.
      • Denry I.
      Stabilized zirconia as a structural ceramic: an overview.
      A wide variety of zirconia formulations are possible, each having their own distinctive physical properties.
      • Kelly J.R.
      • Denry I.
      Stabilized zirconia as a structural ceramic: an overview.
      ,
      • Zhang Y.
      • Lawn B.R.
      Novel zirconia materials in dentistry.
      The first zirconias used in dentistry typically contained approximately 3 mol% yttria (3Y).
      • Kelly J.R.
      • Denry I.
      Stabilized zirconia as a structural ceramic: an overview.
      ,
      • Kwon S.J.
      • Lawson N.C.
      • McLaren E.A.
      • Nejat A.H.
      • Burgess J.O.
      Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
      Monolithic zirconia became a material of choice for posterior crowns.
      • Makhija S.K.
      • Lawson N.C.
      • Gilbert G.H.
      • Litaker M.S.
      • McClelland J.A.
      • Louis D.R.
      • et al.
      Dentist material selection for single-unit crowns: findings from the National Dental Practice-Based Research Network.
      However, these 3Y materials lacked the translucency needed for monolithic anterior restorations.
      • Zhang Y.
      • Lawn B.R.
      Novel zirconia materials in dentistry.
      ,
      • Kwon S.J.
      • Lawson N.C.
      • McLaren E.A.
      • Nejat A.H.
      • Burgess J.O.
      Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
      ,
      • Zhang Y.
      Making yttria-stabilized tetragonal zirconia translucent.
      ,
      • McLaren E.A.
      • Lawson N.
      • Choi J.
      • Kang J.
      • Trujillo C.
      New high-translucent Cubic-phase-containing zirconia: clinical and laboratory considerations and the effect of air abrasion on Strength.
      In order to improve translucency, yttria content was increased to approximately 5 mol% (5Y), stabilizing the materials at approximately 50% cubic and 50% tetragonal phases.
      • Zhang Y.
      • Lawn B.R.
      Novel zirconia materials in dentistry.
      ,
      • Kwon S.J.
      • Lawson N.C.
      • McLaren E.A.
      • Nejat A.H.
      • Burgess J.O.
      Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
      ,
      • Zhang Y.
      Making yttria-stabilized tetragonal zirconia translucent.
      ,
      • Inokoshi M.
      • Shimizu H.
      • Nozaki K.
      • Takagaki T.
      • Yoshihara K.
      • Nagaoka N.
      • et al.
      Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
      These 5Y materials had translucencies comparable with those of lithium disilicate glass-ceramics but were substantially weaker than the 3Y materials.
      • Kwon S.J.
      • Lawson N.C.
      • McLaren E.A.
      • Nejat A.H.
      • Burgess J.O.
      Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
      More recently, approximately 4 mol% (4Y) materials have been introduced, reducing the cubic phase to about 30%.
      • Zhang Y.
      Making yttria-stabilized tetragonal zirconia translucent.
      ,
      • Inokoshi M.
      • Shimizu H.
      • Nozaki K.
      • Takagaki T.
      • Yoshihara K.
      • Nagaoka N.
      • et al.
      Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
      In order to improve the adhesion of resin cements, the intaglio surfaces of zirconia restorations can be airborne-particle abraded to increase wettability and micromechanical interlocking.
      • Mattiello R.D.L.
      • Coelho T.M.K.
      • Insaurralde E.
      • Coelho A.A.K.
      • Terra G.P.
      • Kasuya A.V.B.
      • et al.
      A review of surface treatment methods to improve the adhesive cementation of zirconia-based ceramics.
      • Michida S.M.
      • Kimpara E.T.
      • dos Santos C.
      • Souza R.O.
      • Bottino M.A.
      • Ozcan M.
      Effect of air-abrasion regimens and fine diamond bur grinding on flexural strength, Weibull modulus and phase transformation of zirconium dioxide.
      • Ozcan M.
      • Bernasconi M.
      Adhesion to zirconia used for dental restorations: a systematic review and meta-analysis.
      • Su N.
      • Yue L.
      • Liao Y.
      • Liu W.
      • Zhang H.
      • Li X.
      • et al.
      The effect of various sandblasting conditions on surface changes of dental zirconia and shear bond strength between zirconia core and indirect composite resin.
      Airborne-particle abrasion using alumina at high pressure is generally considered to produce the roughest surfaces, those most conducive to adhesion.
      • Kwon S.J.
      • Lawson N.C.
      • McLaren E.A.
      • Nejat A.H.
      • Burgess J.O.
      Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
      ,
      • Inokoshi M.
      • Shimizu H.
      • Nozaki K.
      • Takagaki T.
      • Yoshihara K.
      • Nagaoka N.
      • et al.
      Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
      ,
      • Sawada T.
      • Schille C.
      • Zoldfoldi J.
      • Schweizer E.
      • Geis-Gerstorfer J.
      • Spintzyk S.
      Influence of a surface conditioner to pre-sintered zirconia on the biaxial flexural strength and phase transformation.
      However, airborne-particle abrasion has been assumed to generally weaken brittle materials by creating new microcracks; however, conflicting data have been published.
      • McLaren E.A.
      • Lawson N.
      • Choi J.
      • Kang J.
      • Trujillo C.
      New high-translucent Cubic-phase-containing zirconia: clinical and laboratory considerations and the effect of air abrasion on Strength.
      ,
      • Michida S.M.
      • Kimpara E.T.
      • dos Santos C.
      • Souza R.O.
      • Bottino M.A.
      • Ozcan M.
      Effect of air-abrasion regimens and fine diamond bur grinding on flexural strength, Weibull modulus and phase transformation of zirconium dioxide.
      ,
      • Sawada T.
      • Schille C.
      • Zoldfoldi J.
      • Schweizer E.
      • Geis-Gerstorfer J.
      • Spintzyk S.
      Influence of a surface conditioner to pre-sintered zirconia on the biaxial flexural strength and phase transformation.
      • Passos S.P.
      • Linke B.
      • Major P.W.
      • Nychka J.A.
      the effect of air-abrasion and heat treatment on the fracture behavior of Y-TZP.
      • Alao A.R.
      • Stoll R.
      • Song X.F.
      • Miyazaki T.
      • Hotta Y.
      • Shibata Y.
      • et al.
      Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes.
      • Sulaiman T.A.
      • Abdulmajeed A.
      • Shahramian K.
      • Lassila L.
      Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia.
      • Lawson N.C.
      • Jurado C.A.
      • Huang C.
      • Morris G.P.
      • Burgess J.O.
      • Liu P.
      • et al.
      Effect of surface treatment and cement on fracture load of traditional zirconia (3Y), translucent zirconia (5Y), and lithium disilicate crowns.
      Recently, airborne-particle abrasion has been reported to improve the strength of 3Y zirconia materials, likely attributable to the induction of compressive residual stresses leading to phase transformation from tetragonal to monoclinic forms.
      • Sawada T.
      • Schille C.
      • Zoldfoldi J.
      • Schweizer E.
      • Geis-Gerstorfer J.
      • Spintzyk S.
      Influence of a surface conditioner to pre-sintered zirconia on the biaxial flexural strength and phase transformation.
      ,
      • Passos S.P.
      • Linke B.
      • Major P.W.
      • Nychka J.A.
      the effect of air-abrasion and heat treatment on the fracture behavior of Y-TZP.
      ,
      • Sato H.
      • Yamada K.
      • Pezzotti G.
      • Nawa M.
      • Ban S.
      Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment.
      • Ozcan M.
      • Melo R.M.
      • Souza R.O.
      • Machado J.P.B.
      • Valandro L.-F.
      • Botttino M.A.
      Effect of air-particle abrasion protocols on the biaxial flexural strength, surface characteristics and phase transformation of zirconia after cyclic loading.
      • Song J.Y.
      • Park S.W.
      • Lee K.
      • Yun K.F.
      • Lim H.P.
      Fracture strength and microstructure of Y-TZP zirconia after different surface treatments.
      The effects of surface treatment, airborne-particle abrasion, on translucent 4Y and 5Y zirconia materials remain largely unknown.
      • Inokoshi M.
      • Shimizu H.
      • Nozaki K.
      • Takagaki T.
      • Yoshihara K.
      • Nagaoka N.
      • et al.
      Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
      ,
      • Lawson N.C.
      • Jurado C.A.
      • Huang C.
      • Morris G.P.
      • Burgess J.O.
      • Liu P.
      • et al.
      Effect of surface treatment and cement on fracture load of traditional zirconia (3Y), translucent zirconia (5Y), and lithium disilicate crowns.
      The purpose of this in vitro study was to measure and characterize the effects of different surface treatments, including airborne-particle abrasion, on the strength of different translucent 4Y and 5Y zirconia materials. The null hypothesis was that the effects of surface treatment and type of zirconia material would not influence biaxial flexural strength.

      Material and methods

      Four different 4Y and 5Y zirconia materials were included in this study and are listed in Table 1. Disk-shaped specimens were milled from presintered blanks of each material and sintered (N=160). All the specimens were preshaded (Vita shade A2) and were not additionally colored. The specimens were hand dry-polished with 2000-grit silicon carbide paper.
      Table 1Zirconia materials, trade names, manufacturers, yttria content, and sintering parameters
      • Zhang Y.
      • Lawn B.R.
      Novel zirconia materials in dentistry.
      ,
      • Inokoshi M.
      • Shimizu H.
      • Nozaki K.
      • Takagaki T.
      • Yoshihara K.
      • Nagaoka N.
      • et al.
      Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
      MaterialManufacturerY2O3 mol% (Cubic Phase %)Sintering Temperature °CSintering Time, h:minHold Time at High Temperature
      ArgenZ Anterior ZirconiaArgen Corp5 mol% (>50% cubic)14508:102:00
      Lava Esthetic Fluorescent Full-Contour Zirconia3M Co5 mol% (>50% cubic)15005:032:00
      Katana Zirconia STMLKuraray Noritake Dental Inc5 mol% (>50% cubic)15506:452:00
      IPS e.max ZirCAD MTIvoclar Vivadent AG4 mol% (>25% cubic)14758:442:00
      All specimens were sintered (Zircom; KDF Co, Ltd) according to the manufacturers’ instructions with a 10 °C/min temperature rise and a hold time/dwell time of 2 hours at the high temperature (Tables 1 and 2). The sintering furnace was calibrated to ±4 °C by using process temperature control rings (PTCR; Orton Ceramic Foundation). The final dimensions of the specimens were Ø14 ±2×1.2 ±0.2 mm, measured with a digital micrometer (227-211; Mitutoyo Corp.).
      Table 2Mean ±standard deviation biaxial flexure strength (MPa) with 95% confidence limits by surface treatment (ST) and material (M)
      Horizontal and vertical lines link similar groups (P >.05).
      Four different surface treatments were included. A control group was polished with 2000-grit silicon carbide paper under water. A second group was as received from the milling machine. A third group was airborne-particle abraded with 50-μm alumina particles at a distance of 1.27 cm under 0.2 MPa for 20 seconds.
      • Su N.
      • Yue L.
      • Liao Y.
      • Liu W.
      • Zhang H.
      • Li X.
      • et al.
      The effect of various sandblasting conditions on surface changes of dental zirconia and shear bond strength between zirconia core and indirect composite resin.
      ,
      • Alao A.R.
      • Stoll R.
      • Song X.F.
      • Miyazaki T.
      • Hotta Y.
      • Shibata Y.
      • et al.
      Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes.
      ,
      • White S.N.
      • Miklus V.G.
      • McLaren E.A.
      • Lang L.A.
      • Caputo A.A.
      Flexural strength of a layered zirconia and porcelain dental all-ceramic system.
      • Tan J.P.
      • Sederstrom D.
      • Polansky J.R.
      • McLaren E.A.
      • White S.N.
      The use of slow heating and slow cooling regimens to strengthen porcelain fused to zirconia.
      • White S.N.
      • Green C.C.
      • McMeeking R.M.
      A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces.
      A fourth group was airborne-particle abraded with 50-μm glass beads at a distance of 1.27 cm and 0.25 MPa for 20 seconds. Each of the 16 material-surface treatment groups contained 10 specimens. Specimens were assigned to surface treatment groups by using a random numbers table.
      Biaxial flexural strength was measured by using a universal testing machine (1123; Instron Corp.) at a crosshead speed of 1 mm/min according to the International Organization for Standardization (ISO) 6872:2015.
      International Organization for Standardization
      ISO 6872:2015. Dental ceramic.
      The specimens were placed so that the treated surfaces were in tension. The biaxial flexure strength was calculated from the following equation:
      σ=−0.2387 P (X−Y)/b2, where X=(1+ν) ln(r2/r3)2+[(1−ν)/2](r2/r3)2 and Y=(1+ν) [1+ln(r1/r3)2]+(1−ν)(r1/r3)2, where ν is Poisson ratio, r1 is the radius of the support circle in millimeters, r2 is the radius of the loaded area in millimeters, r3 is the radius of the specimen in millimeters, and b is the specimen thickness at the fracture origin in millimeters.
      The means and standard deviations of the material-surface treatment group were calculated and plotted. To elucidate the influence of the simple main effects of material type and surface treatment, as well as their interaction, on biaxial flexural strength, a 2-way ANOVA was performed (α=.05).
      A priori, 1-way ANOVA and Tukey multiple pairwise comparison testing were used to determine which of the 4 zirconia types differed from one another for each of the 4 surface treatments (α=1.05). Likewise, 1-way ANOVA and Tukey multiple pairwise comparison testing were used to determine which of the 4 surface treatment types differed from one another for each of the four zirconia types (α=.05). Although ANOVA is robust, it assumes a normal distribution; for this reason, box and whisker plots of all 16 subgroups were reviewed for indications of nonnormal distributions. All distributions were reasonably symmetrical around their means and medians.
      Surface morphology was assessed by using scanning electron microscopy in a secondary electron and backscatter electron imaging mode at 20 kV (Quanta FEG 650; FEI) for gold sputter-coated specimens of each material at magnifications ranging from ×100 to ×5 000.
      Absolute transmission was measured by using a spectrophotometer (Color-i7; X-Rite) for 4 polished specimens of each material.
      • Spink L.S.
      • Rungruanganut P.
      • Megremis S.
      • Kelly J.R.
      Comparison of an absolute and surrogate measure of relative translucency in dental ceramics.
      Disk specimens, 1.0 ±0.05 mm in thickness, were placed inside a closed chamber; all the light that passed through the specimen—full spectrum from UV through visible to infrared—came from the spectrophotometer. Means and standard deviations were calculated. One-way ANOVA and Tukey multiple pairwise comparison tests were used to determine which of the 4 materials differed from one another (α=.05).

      Results

      A plot of all 16 groups showed a wide 2-fold range of mean ±standard deviation biaxial strengths from 464 ±43 MPa for an alumina airborne-particle-abraded 5Y zirconia to 975 ±89 MPa for an alumina-abraded 4Y zirconia (Table 2; Fig. 1).
      Figure thumbnail gr1
      Figure 1Plot of mean biaxial strength (MPa) by surface treatment and material. Error bars indicate standard deviation.
      The 2-way ANOVA showed that the simple main effects of zirconia type and surface treatment, as well as their interaction, all influenced biaxial strength (P<.001) (Table 3). Although the interaction between zirconia type and surface treatment was significant, that is, zirconia materials responded unequally to surface treatments, this effect was much less influential than either of the simple main effects.
      Table 3Two-way ANOVA for effects of zirconia material type, surface treatment, and their interaction
      Degrees of FreedomSum of SquaresF ValueP
      Zirconia material (M)32 648 023283<.001
      Surface treatment (T)3853 15291<.001
      Interaction

      M×T
      9450 69016<.001
      Error144448 602
      Corrected total1594 400 468
      The four 1-way ANOVAs for the 4 zirconia materials by surface treatment all found significant differences among surface treatments (Table 4). Multiple comparison tests showed that for all 5Y zirconia materials, the polished control groups were the strongest, the as-machined and glass-bead-abraded groups were tied in an intermediate position, and alumina-abraded group was the weakest (P<.05) (Table 2). For the 4Y zirconia material, multiple comparison testing found only one difference among surface treatments, that glass-bead abrasion produced a lower strength than alumina abrasion (P<.05) (Table 2).
      Table 4One-way ANOVAs for zirconia materials by surface treatment and surface treatments by material
      Degrees of FreedomMean Square ModelMean Square ErrorF ValueP
      e.max ZirCAD by surface treatmentModel: 3

      Error: 36
      19 33150794.02
      Katana STML by surface treatmentModel: 3

      Error: 36
      126 125368234<.001
      Lava Esthetic by surface treatmentModel: 3

      Error: 36
      128 3371212106<.001
      ArgenZ Anterior by surface treatmentModel: 3

      Error: 36
      160 821248765<.001
      Polished control by zirconia materialModel: 3

      Error: 36
      60 658505012<.001
      As-machined by zirconia materialModel: 3

      Error: 36
      604 27391 51679<.001
      Glass beads by zirconia materialModel: 3

      Error: 36
      178 7491167153<.001
      Alumina by surface treatmentModel: 3

      Error: 36
      592 0733702160<.001
      All four 1-way ANOVAS for surface treatment by zirconia material found significant differences among zirconia materials within each type of surface treatment (Table 4). Multiple comparison tests for all surface treatments ranked the 4Y material as being stronger than the 5Y materials, with no differences being discerned within the 5Y materials (Table 2).
      Scanning electron microscopy showed that the as-polished surfaces were smoother than all others; surfaces as-machined and airborne-particle abraded with glass beads displayed little difference; and those airborne-particle abraded with alumina were the roughest (Fig. 2). Differences among materials were not discerned.
      Figure thumbnail gr2
      Figure 2Representative scanning electron micrographs of 5Y material (Katana STML). As-machined and glass-bead-abraded surfaces displayed little difference. Zirconia materials displayed little difference in response to abrasion. A, Polished control. B, Alumina airborne-particle abraded. C, As machined. D, Glass-bead airborne-particle abraded. Original magnification ×1000.
      The 5Y zirconia materials had similar absolute transmissions of approximately 34%, whereas the 4Y material had a significantly lower transmission of approximately 29% (Fig. 3). The 1-way ANOVA found differences among materials (F ratio 18, P<.001), and multiple comparison testing showed that the 4Y material was less translucent than all the 5Y materials.
      Figure thumbnail gr3
      Figure 3Plot of percentage mean absolute light transmission by material error bars indicate standard deviation. Horizontal line links similar materials (P>.05).

      Discussion

      The null hypothesis was rejected, as the zirconia type was more influential than surface treatment. The trends displayed in Figure 1 were supported by the statistical analyses described previously. The 4Y material was significantly stronger than the 5Y materials for all surface treatments. If a dentist chooses to prioritize strength, the 4Y material offers a substantial advantage over the 5Y materials, with only a small loss in translucency (Figs. 1 and 3).
      For the 4Y material, surface treatment choice had a statistically significant effect; airborne-particle abrasion with alumina produced higher strengths than that with glass beads (Fig. 1). Conversely, for all the 5Y materials, airborne-particle abrasion with alumina produced significantly lower strengths than all other surface treatments. For all the 5Y materials, the control polished surfaces produced the highest strengths; this speaks to the damage produced by machining even when followed by sintering. If a dentist prioritizes strength, the 4Y material should be airborne-particle abraded with alumina, and 5Y materials should be left as machined or abraded with glass beads.
      When choosing zirconia materials and their surface treatments for anterior restorations, dentists are faced with competing priorities: restoration strength, restoration translucency, and bond strength or choice of cementation technique. In restorations where the translucency of a 4Y material is sufficient, it will offer substantial improvements in strength over 5Y zirconia materials. Moreover, the results of this study suggest that a 4Y material may have undergone some transformation toughening when airborne-particle abraded with alumina, as has been reported for 3Y materials.
      • Sawada T.
      • Schille C.
      • Zoldfoldi J.
      • Schweizer E.
      • Geis-Gerstorfer J.
      • Spintzyk S.
      Influence of a surface conditioner to pre-sintered zirconia on the biaxial flexural strength and phase transformation.
      ,
      • Passos S.P.
      • Linke B.
      • Major P.W.
      • Nychka J.A.
      the effect of air-abrasion and heat treatment on the fracture behavior of Y-TZP.
      ,
      • Sato H.
      • Yamada K.
      • Pezzotti G.
      • Nawa M.
      • Ban S.
      Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment.
      • Ozcan M.
      • Melo R.M.
      • Souza R.O.
      • Machado J.P.B.
      • Valandro L.-F.
      • Botttino M.A.
      Effect of air-particle abrasion protocols on the biaxial flexural strength, surface characteristics and phase transformation of zirconia after cyclic loading.
      • Song J.Y.
      • Park S.W.
      • Lee K.
      • Yun K.F.
      • Lim H.P.
      Fracture strength and microstructure of Y-TZP zirconia after different surface treatments.
      In contrast, the weaker 5Y materials behaved similarly to lithium disilicate glass-ceramic materials after airborne-particle abrasion with alumina; they were further weakened.
      • Lawson N.C.
      • Jurado C.A.
      • Huang C.
      • Morris G.P.
      • Burgess J.O.
      • Liu P.
      • et al.
      Effect of surface treatment and cement on fracture load of traditional zirconia (3Y), translucent zirconia (5Y), and lithium disilicate crowns.
      Although the 5Y materials allowed more absolute light transmission than the 4Y material, the difference was small (Fig. 3).
      Airborne-particle abrasion with glass beads did not appear to visibly alter the surface topography of machined surfaces (Fig. 2). Alumina abrasion roughened all surfaces, weakening the 5Y materials but strengthening the 4Y material (Figs. 1 and 2). The effects of surface treatment on bond strength were beyond the scope of this article but are also needed to inform clinical decisions. Nonetheless, it is reasonable to assume that rougher surfaces produce higher bond strengths and that the undesirability of alumina airborne-particle abrading of 5Y materials may preclude some clinical applications.
      The polished control groups produced data consistent with their manufacturers’ data sheets and brochures; however, the 5Y materials will have lower strengths in their clinically used as-machined or airborne-particle-abraded states because intaglio surfaces cannot be polished for clinical usage.

      Conclusions

      Based on the findings of this in vitro study, the following conclusions were drawn:
      • 1.
        Zirconia material type, surface treatment, and the interaction between material and surface treatment all influenced the strength of translucent zirconia materials.
      • 2.
        A 4Y, 4 mol% zirconia material was stronger than several 5Y, 5 mol% zirconia materials for all surface treatments tested.
      • 3.
        Airborne-particle abrasion with alumina had a slight strengthening effect over airborne-particle abrasion with glass beads on a 4Y zirconia.
      • 4.
        Airborne-particle abrasion with alumina had a weakening effect on 5Y zirconia materials.
      • 5.
        Airborne-particle abrasion with alumina produced the roughest surfaces on 4Y and 5Y zirconia materials, whereas airborne-particle abrasion with glass beads did not appear to substantively alter machined surface morphologies.
      • 6.
        The 5Y zirconia materials were slightly more translucent than a 4Y material.

      Acknowledgments

      The authors thank the manufacturers for providing materials, the University of Alabama at Birmingham High Resolution Imaging Facility for SEM support, and Argen Corporation for CAD-CAM support.

      CRediT authorship contribution statement

      Edward A. McLaren: Conceptualization, Investigation, Methodology, Project administration, Resources, Supervision, Writing – original draft. Anvita Maharishi: Conceptualization, Data curation, Investigation, Methodology, Project administration, Writing – original draft. Shane N. White: Conceptualization, Formal analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing – original draft.

      References

        • Garvie R.C.
        • Hannink R.H.
        • Pascoe R.T.
        Ceramic steel?.
        Nature. 1975; 258: 703-704
        • Garvie R.C.
        • Nicholson P.S.
        Phase analysis in zirconia systems.
        J Am Ceram Soc. 1972; 55: 303-305
        • McLaren E.A.
        • White S.N.
        Glass-infiltrated zirconia/alumina-based ceramic for crowns and fixed partial dentures.
        Pract Periodontics Aesthet Dent. 1999; 11: 985-994
        • McMeeking R.M.
        • Evans A.G.
        The mechanics of transformation toughening in brittle materials.
        J Amer Ceram Soc. 1982; 65: 242-246
        • Heuer A.H.
        • Lange F.F.
        • Swain M.V.
        • Evans A.G.
        Transformation toughening: an overview.
        J Am Ceram Soc. 1986; 69: i-iv
        • Kelly J.R.
        • Denry I.
        Stabilized zirconia as a structural ceramic: an overview.
        Dent Mater. 2008; 24: 289-298
        • Zhang Y.
        • Lawn B.R.
        Novel zirconia materials in dentistry.
        J Dent Res. 2018; 97: 140-147
        • Kwon S.J.
        • Lawson N.C.
        • McLaren E.A.
        • Nejat A.H.
        • Burgess J.O.
        Comparison of the mechanical properties of translucent zirconia and lithium disilicate.
        J Prosthet Dent. 2018; 120: 132-137
        • Makhija S.K.
        • Lawson N.C.
        • Gilbert G.H.
        • Litaker M.S.
        • McClelland J.A.
        • Louis D.R.
        • et al.
        Dentist material selection for single-unit crowns: findings from the National Dental Practice-Based Research Network.
        J Dent. 2016; 55: 40-47
        • Zhang Y.
        Making yttria-stabilized tetragonal zirconia translucent.
        Dent Mater. 2014; 30: 1195-1203
        • McLaren E.A.
        • Lawson N.
        • Choi J.
        • Kang J.
        • Trujillo C.
        New high-translucent Cubic-phase-containing zirconia: clinical and laboratory considerations and the effect of air abrasion on Strength.
        Compend Contin Educ Dent. 2017; 38: e13-e16
        • Inokoshi M.
        • Shimizu H.
        • Nozaki K.
        • Takagaki T.
        • Yoshihara K.
        • Nagaoka N.
        • et al.
        Crystallographic and morphological analysis of sandblasted highly translucent dental zirconia.
        Dent Mater. 2018; 34: 508-518
        • Mattiello R.D.L.
        • Coelho T.M.K.
        • Insaurralde E.
        • Coelho A.A.K.
        • Terra G.P.
        • Kasuya A.V.B.
        • et al.
        A review of surface treatment methods to improve the adhesive cementation of zirconia-based ceramics.
        ISRN Biomater. 2013; (Article ID 185376)
        • Michida S.M.
        • Kimpara E.T.
        • dos Santos C.
        • Souza R.O.
        • Bottino M.A.
        • Ozcan M.
        Effect of air-abrasion regimens and fine diamond bur grinding on flexural strength, Weibull modulus and phase transformation of zirconium dioxide.
        J Appl Biomater Funct Mater. 2015; 13: 266-e273
        • Ozcan M.
        • Bernasconi M.
        Adhesion to zirconia used for dental restorations: a systematic review and meta-analysis.
        J Adhes Dent. 2015; 17: 7-26
        • Su N.
        • Yue L.
        • Liao Y.
        • Liu W.
        • Zhang H.
        • Li X.
        • et al.
        The effect of various sandblasting conditions on surface changes of dental zirconia and shear bond strength between zirconia core and indirect composite resin.
        J Adv Prosthodont. 2015; 7: 214-223
        • Sawada T.
        • Schille C.
        • Zoldfoldi J.
        • Schweizer E.
        • Geis-Gerstorfer J.
        • Spintzyk S.
        Influence of a surface conditioner to pre-sintered zirconia on the biaxial flexural strength and phase transformation.
        Dent Mater. 2018; 34: 486-493
        • Passos S.P.
        • Linke B.
        • Major P.W.
        • Nychka J.A.
        the effect of air-abrasion and heat treatment on the fracture behavior of Y-TZP.
        Dent Mater. 2015; 31: 1011-1021
        • Alao A.R.
        • Stoll R.
        • Song X.F.
        • Miyazaki T.
        • Hotta Y.
        • Shibata Y.
        • et al.
        Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes.
        J Mech Behav Biomed Mater. 2017; 65: 102-116
        • Sulaiman T.A.
        • Abdulmajeed A.
        • Shahramian K.
        • Lassila L.
        Effect of different treatments on the flexural strength of fully versus partially stabilized monolithic zirconia.
        J Prosthet Dent. 2017; 118: 216-220
        • Lawson N.C.
        • Jurado C.A.
        • Huang C.
        • Morris G.P.
        • Burgess J.O.
        • Liu P.
        • et al.
        Effect of surface treatment and cement on fracture load of traditional zirconia (3Y), translucent zirconia (5Y), and lithium disilicate crowns.
        J Prosthodont. 2019; 28: 659-665
        • Sato H.
        • Yamada K.
        • Pezzotti G.
        • Nawa M.
        • Ban S.
        Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment.
        Dent Mater J. 2008; 27: 408-414
        • Ozcan M.
        • Melo R.M.
        • Souza R.O.
        • Machado J.P.B.
        • Valandro L.-F.
        • Botttino M.A.
        Effect of air-particle abrasion protocols on the biaxial flexural strength, surface characteristics and phase transformation of zirconia after cyclic loading.
        J Mech Behav Biomed Mater. 2013; 20: 19-28
        • Song J.Y.
        • Park S.W.
        • Lee K.
        • Yun K.F.
        • Lim H.P.
        Fracture strength and microstructure of Y-TZP zirconia after different surface treatments.
        J Prosthet Dent. 2013; 110: 274-280
        • White S.N.
        • Miklus V.G.
        • McLaren E.A.
        • Lang L.A.
        • Caputo A.A.
        Flexural strength of a layered zirconia and porcelain dental all-ceramic system.
        J Prosthet Dent. 2005; 94: 125-131
        • Tan J.P.
        • Sederstrom D.
        • Polansky J.R.
        • McLaren E.A.
        • White S.N.
        The use of slow heating and slow cooling regimens to strengthen porcelain fused to zirconia.
        J Prosthet Dent. 2012; 107: 163-169
        • White S.N.
        • Green C.C.
        • McMeeking R.M.
        A simple 3-point flexural method for measuring fracture toughness of the dental porcelain to zirconia bond and other brittle bimaterial interfaces.
        J Prosthodont Res. 2020; 64: 391-396
        • International Organization for Standardization
        ISO 6872:2015. Dental ceramic.
        International Organization for Standardization, Geneva2015
        • Spink L.S.
        • Rungruanganut P.
        • Megremis S.
        • Kelly J.R.
        Comparison of an absolute and surrogate measure of relative translucency in dental ceramics.
        Dent Mater. 2013; 29: 702-707