Dental Ceramics_ Fracture Mechanics and Engineering Design PDF

Chemistry and Microstructure 2 The effort to make sense of the broad assortment orous, seen that hybrid ceramics here is naming of ceramic-based materials available for dental...

Chemistry and Microstructure 2 The effort to make sense of the broad assortment orous, seen that hybrid ceramics here is naming of ceramic-based materials available for dental scaffolds made out of glass or zirconia/alumina applications can be frustrating, admittedly. This particles by partial sintering, which are later infil- is more so for young professionals that have trated by a low-viscosity polymer or a molten missed the train of development, which got glass, respectively. The class of silicate ceramics momentum in 1980s with the translation of tech- is also hybrid in terms of microstructure, once nical processing techniques to the dental field they are composed by a SiO2-rich glass fraction (e.g., hot pressing, CAD-CAM), which evolved and one or more crystals types. Crystals can be rapidly by means of different material fabrication crystallized from the glass following a nucleation techniques, and is today at full speed, with new and crystallization process (glass-ceramic pro- ceramics invading the market in an ever-­ cess) or be added separately (particle reinforce- increasing pace. In that sense, it is unhelpful for ment). Glass-free systems are contained in the the clinician and laboratory technician to cling to class of oxide ceramics (though non-metallic product names instead of referring to materials glasses are also oxides)—which could well be by their chemical composition and/or microstruc- termed “polycrystalline ceramics” or “non-­ ture. Not to lose sight of product brands is none- silicate ceramics”—in dentistry represented by theless advisable, once materials of the same type alumina (aluminum oxide, Al2O3), zirconia (zir- can slide widely across the spectrum of mechani- conium dioxide, ZrO2), and composites thereof. cal performance, depending on oxide chemistry In Fig. 2.1, the subclasses are also distinguished and fabrication parameters employed by the whether by the specific fabrication technique manufacturer. Meaning that not every lithium (partial sintering, full sintering, or glass-ceramic disilicate product, for instance, is equivalent to process) or by the processing technique of the some lithium disilicate that one might be expected commercialized product (powder layering, CAD- to be referring to. Subtle changes in oxide ratios, CAM, or injection molding). particle size of the raw particulates, firing param- In this chapter, we will be waiving on histori- eters, and so on, can lead to substantially differ- cal fairness to attend mainly to the most relevant ent materials, including phase fractions, crystal systems in modern prosthodontics, such as lith- type and shape, structural homogeneity, and ium (di)silicates and zirconias, with brief incur- internal stresses. sions in ceramic systems that find increasingly A simple way to classify dental ceramic-based fewer applications, such as aluminosilicates materials is illustrated in Fig. 2.1, into three main (feldspar- and leucite-based) and hybrid materi- categories: hybrid ceramics, silicate ceramics, als (glass-infiltrated polycrystalline scaffolds and and oxide ceramics. This terminology is not rig- polymer-infiltrated glass scaffolds). © The Author(s), under exclusive license to Springer Nature Switzerland AG 2022 3 U. Lohbauer, R. Belli, Dental Ceramics, https://doi.org/10.1007/978-3-030-94687-6_2 4 2 Chemistry and Microstructure Fabrication technique Hybrid Ceramics Silicate Ceramics Oxide Ceramics Partial sintering Sintering Glass-ceramic process Glass-infiltrated Feldspar-reinforced Polycrystalline polycrystalline scaffolds glasses Alumina Processing technique Powder layering Polymer-infiltrated Leucite Polycrystalline glass scaffolds glass-ceramics Zirconia CAD-CAM Injection molding Lithium (Di)Silicate glass-ceramics Polycrystalline Alumina/Zirconia Fluorapatite glass-ceramics Fig. 2.1 A simplistic classification of dental ceramics based on chemistry and microstructure, fabrication, and processing techniques 2.1 Aluminosilicates uct for powder layering (veneering) or mixed to organic binders to be compacted, extruded to In dentistry, high-content Al3+ glasses are mainly block shape, and sintered in the factory for use as of the tectosilicate family, having as alkali metal CAD-CAM material. Feldspar reinforced blocks ions (M+) sodium Na+ or potassium K+ in an have a crystallinity of 20–40 vol.%, resulting in Al/M ratio of 1, which compensate the deficit in fracture toughness in the order of 1.2 MPa√m electrical charge around the tetrahedral AlO4 [2, 3], with hints of a probable modest R-curve units, resulting in a highly polymerized glass net- behavior. work containing very few non-bridging oxygens In the glass-ceramic processing, potash alumi-. They have been used for the production of nosilicate glasses crystallize leucite (KAlSi2O6), dental powder veneering and machinable block historically employed to adjust the coefficient of “porcelains” via the synthetic route of glass-­ thermal expansion (CTE) of the veneering mate- ceramic process or by milling natural feldspar rial to be compatible with metallic infrastructures rocks. The latter technique has been widely , today also applicable as veneering onto poly- employed to produce the pioneering block crystalline ceramics. Bulk crystallization of leu- ceramic materials for CAD-CAM processing, cite is induced by nucleation agents such as namely the Vitablocks® from Vita Zahnfabrik. nano-sized leucite [6, 7] or Na-Ca titanate seeds The basic fabrication steps of this class are illus- , to form single or bundle of crystals, typically trated in Fig. 2.2, composed of the selection of in crystal fractions between 10 and 30 vol.% in high-purity feldspar natural rocks (usually albite commercial products. The residual stresses and NaAlSi3O8 and/or nepheline (Na, K)AlSiO4), cracking around crystals that occur due to high their milling into a fine powder, its melting and CTE mismatches between crystal and residual water quenching into a frit that is again milled for glass, coupled with the cubic to tetragonal phase remelting and homogenization. To the powder transformation of leucite taking place during resulting from the milling of the second frit, very cooling , seem to act as toughening mecha- fine milled particles of the original feldspar rock nism in this system. However, although increas- are added as reinforcing particles. Some products ing leucite fraction leads to a linear increase in result from the mixture of two or more glass frits toughness up to about 1.3 MPa√m , crystal- of different compositions and thermal properties. linities over ~30 vol.% become deleterious to its Those powders can be employed as a final prod- mechanical performance [11, 12]. 2.2 Lithium-Based Glass-Ceramics 5 Fig. 2.2 Example of a fabrication route of feldspar-reinforced aluminosilicate glass Although aluminosilicates based on feldspar and leucite have been important in the early development of all-ceramic systems, some fac- Lithium Disilicate (IPS e.max® CAD) tors have been responsible for the decline in their 100 use in recent years, most importantly: (1) the increase in popularity of monolithic approaches inlays Survival from fracture [%] that forgo the veneering step; (2) the advent of 99 other highly esthetic veneering materials, such as onlays fluorapatite-based; (3) the development of alter- native techniques to veneering, such as the glass 98 fusion or luting of ceramic overlays; (4) the con- solidation of machinable lithium disilicate as competitor material having higher mechanical properties, among others. A testament to item (4) 97 is illustrated in Fig. 2.3, where a higher fracture rate of leucite inlays and onlays has been reported, inlays when compared to restorations made out of a Leucite onlays 96 (IPS Empress® CAD) lithium disilicate glass-ceramic. 2.2 Lithium-Based 95 0 1 2 3 4 Glass-Ceramics Time [years] The importance that glass-ceramics based on the Fig. 2.3 Clinical fracture rates of inlay and onlay restora- SiO2–Li2O system have on the landscape of cur- tions produced by the same machining center, comparing the performance of leucite and lithium disilicate materi- rent materials for prosthetic dentistry is difficult als. From Ref. to exaggerate. Looking at the profusion of newly 6 2 Chemistry and Microstructure commercialized glass-ceramic products, with the restoratives is deemed to shrink. By the start of vast majority falling under the enclave of lithium the 2020s, the assortment of products appears to (di)silicates, one cannot escape to notice the consolidate lithium (di)silicates and dental zirco- silent operation of a natural selection type-of-­ nias as major players in the class of ceramics due process taking place in that domain over the last to extended spectrum of use. Overlapping clini- decades. This favors certain compositions to cal indications, such as partial restorations (i.e., carry along their “traits” to next generations, inlay and onlays), are up for grabs, with indirect while “unfit” materials are progressively dropped composites competing for space. For short- to from the market. A similar phenomenon set the medium-range indications, such as crowns and fate of the InCeram® product line, broadly used 3-unit bridges anterior bridges, formerly con- in the 1990s and early 2000s as all-ceramic fined to the purview of veneered infrastructures framework materials, discontinued by the com- and lithium disilicates, are now claimed by pany Vita Zahnfabrik in 2015 after being over- monolithic translucent zirconias. performed by the newcomer class of dental What is oblivious to the most ordinary practi- zirconias. tioners is the fierce legal fight for a billions worth Lithium disilicate, in particular, became a market that is in play, and lithium-based glass-­ brand in itself, and its popularity is driving other ceramics take center stage. Companies not satis- glass-ceramic compositions to a rapid extinction. fied as bystanders of the commercial success of In today’s dentistry, dominant traits are mainly the lithium disilicate products from Ivoclar-­ mechanical and optical properties, with the fabri- Vivadent AG, and eager to cash in in the lithium cation route helping to tip the balance. For hype, sought to launch products that could cir- instance, the pioneering hot pressed injectable cumvent patent protected compositions and fab- lithium disilicates (originally IPS Empress® 2, rication processes. That task becomes nearly now IPS e.max® Press, both from Ivoclar-­ impractical, considering the broad spectrum of Vivadent AG), despite having superior mechani- oxide composition that falls under the umbrella cal performance and comparable esthetics , of a single patent, and how compositions overlap are losing territory to machinable analogs due to among different patents. Readers are referred to a ease in processing, cost-effectiveness, and the list of the most important patents involving den- chairside element. Also, psychological aspects tal lithium-based glass-ceramics, recently sum- imprinted in consumer behavior have their share marized by Huang et al.. This has resulted in in securing the survival or demise of entire prod- numerous patent infringement litigations, some uct classes. For example, novel lithium silicates of which have found resolution, with others are surfing the same wave lithium disilicates extending over many years. While some compa- gained by reputation, thereby conquering consid- nies choose the way of court battles, others yield erable market share by inertia, despite inferior to license agreements not to suffer from commer- mechanical properties. Conscious of that, compa- cialization restrictions that come into effect dur- nies have resorted to efficient strategies to push ing legal disputes. for lithium silicates, for instance by coupling spe- cific products to their proprietary CAD/CAM systems, or shaping the branding to profit from 2.2.1 Compositional Variations the prestige of some of its composition (i.e., by using the slogan “zirconia-reinforced”). Lithium-based glass-ceramics are not inventions Unlike in biology, where natural selection of the dental industry. Early dental products assures biodiversity, the evolution of dental mate- appearing in the end 1990s stem from the tailor- rials is sorted out by the invisible hand of the ing of lithium disilicate compositions, such as the market, where economic factors set the pace. The photoetchable lithium disilicate glass Fotoceram®, consequence is being that the diversity within the the first synthetic glass-ceramic material ever class of ceramic materials available as dental produced, discovered accidentally by Stanley 2.2 Lithium-Based Glass-Ceramics 7 D. Stookey in 1953, when working at Corning and Li2Si2O5, we have been using the term lith- Inc. A similar product fabricated by Schott AG is ium (di)silicate, incognizant of more appropriate named Foturan®. A brief history of the early denominations. The vast majority of dental com- developments of glass-ceramic materials can be positions also present some small (1000 MPa—a value that as discussed in Sect. 3.1. As in any monolithic is of course dependent on the size of the tested ceramic material, the high strength achieved in specimen (See Sect. 3.1). In Table 2.4, the zirconia components—a material property which strength of dental zirconias stabilized with is much easier to be digested in terms of service 3–5.4 mol% Y2O3, the same commercial products load—cannot, therefore, be single-handedly due as in Table 2.3, is given for two specimen sizes to its high fracture toughness, but by decreasing having effective surfaces and volumes analogous the size of defects in the sintered material. It to a small restoration (single crown) and a larger becomes therefore of fundamental importance to construct such as a 4-unit posterior bridge. As identify the type and source of defects that are expected, the larger the size, the lower the typical for a specific material, a task that usually strength. The reliability represented by the leads one back to evaluate all the processing Weibull modulus m varies in a wide range from 5 steps during production so to act on eliminating up to 18, even within the same manufacturer. The or reducing the size of defects that become criti- increase in Y2O3 content can be seen to reduce the cal at different relevant volumes in service. In strength substantially, thus narrowing clinical machinable dental zirconias, it has been repeat- applicability for translucent zirconias. In that edly demonstrated that the nature of critical respect, the decrease in strength is solely due to defects can be traced back to the compaction of the lower KIc of the more stabilized YSZs, due to the powder [101, 102], resulting in fine elon- the fact that an equivalent defect size distribution gated voids after sintering. Figure 2.16 shows seems to be present in most materials. That one such defect in the partially sintered white indicates that most manufacturers are using feed- body and it surviving the final sintering. Their stocks that stem from a small number of powder presence in the white body indicates that they providers, or that granulate size distributions are a b Fig. 2.16 (a) Compaction defect seen on the partly sin- pacted at their junctions (from Ref. ). Reprinted with tered white body of a dental zirconia resulting from the permission from Elsevier. These defects are not healed dur- inability of powder granulates to be appropriately com- ing sintering, resulting in voids as the one observed in (b) 2.3 Zirconium Dioxide 27 a 3000 b 3000 Lava™ Plus Lava™ Esthetic Fully-sintered Flexural Strength [MPa] Fully-sintered 300 300 White-body White-body 30 30 0,1 1 10 100 1000 0,1 1 10 100 1000 2 2 S eff [mm ] S eff [mm ] c d 10 10 Lava™ Plus White-body 10 8 g [m-4 ] 10 6 Fully-sintered 10 4 10 50 100 a c [ m] Fig. 2.17 Flexural strength vs. effective surfaces (Seff) for specimen, with its critical crack size ac shifted to lower the fully sintered and white bodies of a 3Y-TZP in (a) and values. (d) shows a typical compaction defect (star) with a for a 5Y-TZP in (b) for different specimen sizes, showing similar shape as in Fig. 2.16 being the critical defect trig- the deviation of the Weibull prediction for the largest gering fracture in fully sintered specimens. From Ref. specimen size. (c) shows lower values for the relative fre-. Reprinted with permission from Wiley quency flaw size density function g(a) for the largest very similar across different powder therefore exist. This was suggested by Ref. manufacturers. as the cause for all materials in Table 2.4 in fail- A consequence of the strength distribution ing to show a behavior befitting the size effect on being governed by compaction defects is that the strength, a requirement for materials to be con- size distribution of critical defect sizes must be sidered Weibull materials. This is shown exem- related to the size distribution of granulates plarily for a 3Y-TZP and a 5Y-TZP in Fig. 2.17, within the powders, and an upper threshold must where the red and blue predictions, based respec- 28 2 Chemistry and Microstructure tively on the strength of a small and the largest ratio zirconia:veneer were identified as contribut- specimens, do not align was per a typical Weibull ing to the severity in the built-up of residual behavior. The largest specimen shows a shift stresses in the veneer layer [110–114]. toward higher strengths, suggesting that size dis- Laboratorial testing later confirmed those vari- tribution of compaction defects reached a pla- ables as playing decisive roles in determining the teau, with no other second defect distribution susceptibility of the veneer layer to fracture [115, competing at this size scale. This also reveals that 116]. Figure 2.18a shows, for example, increased the applicability of zirconias for large volume lifetimes obtained for veneered-zirconia crowns constructs is benefited by this behavior, but the that were cooled slowly inside the oven, com- potential for smaller constructs is not entirely pared with the protocol of bench cooling. exhausted, provided that improvements in pow- Evidences also negated any fault of the quality of der technology and compaction processes can be the adhesion between layers [115, 118]; it was a guided toward reducing junction gaps between chipping problem, not a delamination problem. granulates. Clinical recommendations from the scientific Sandblasting with alumina particles, a com- community advised using veneering materials mon procedure meant to increase the bonding with matching CTEs, performing slow-cooling potential of cements onto the intaglio zirconia protocols, and modelling so-called “anatomical surface, tends to increase the strength of 3Y-TZP copings” that allowed for a veneer layer having a by inducing surface t → m transformation more homogeneous thickness (see Fig. 2.18b). with consequent generation of compressive That reputational crisis helped in boosting the stresses on the outer layer. If this layer is thick popularity of the monolithic use of zirconia, thus enough in the subsurface, compressive stresses avoiding the veneer layer altogether. may engulf existing surface defects and act as a shielding term similar to Eq. (2.8). 2.3.6 Low-Temperature Degradation 2.3.5 Veneered-Zirconia Bilayers Perhaps more famous than zirconia itself, is the Before any attempt to make zirconia more trans- term Low-Temperature Degradation (LTD), lucent for use as monolithic material, zirconia stemming from a spontaneous t → m transforma- has been extensively used mostly as a thin infra- tion in the absence of any triggering mechanical structure to be veneered with highly esthetic stress, at temperatures as low as body tempera- glass-rich porcelains. Some years in, the clinical ture, sufficing the presence of moisture. Some application of veneered-zirconia took a blow in use the alternative terminology Hydrothermal the form of reported high rates of chipping of the Aging for the same phenomenon. The exact veneer material in clinical service [106–108]. underlying mechanism being nevertheless unre- That inconveniency triggered a wave of scientific solved, some consensus is enjoyed by the theory interest targeting the causes of that increased sus- that oxygen anions dissociated from water mole- ceptibility to fracture. Theoretical models and cules destabilize the tetragonal symmetry by experimental observations gave insights on an occupying the oxygen vacancies formerly cre- expected thermal incompatibility issue ensuing ated by the stabilizing oxide (in this case Y2O3) from the low thermal diffusivity of zirconia,. This process begins at the surface with which generates a high thermal gradient within grain uplifting and roughening and progresses to the veneer layer. Factors such as the magni- the subsurface following a diffusion-controlled tude of the mismatch in linear coefficient of ther- nucleation-and-growth process , with the mal expansion between both materials, the stress induced by the volume increase setting off cooling rate employed through the glass transi- the transformation of neighboring grains. As the tion temperature of the veneer, and the thickness transformed zone evolves inward into the bulk 2.3 Zirconium Dioxide 29 Nf [cycles] a b 1x105 1x107 2 99.9 99 95 90 80 Failure Probability, -PF [%] 0 63.2 50 In(In(1/(1-PF))) Fast-cooled 20 -2 Slow-cooled 10 5 -4 2 1 0.5 -6 5 10 15 20 In Nf Fig. 2.18 (a) Weibull plot showing the lifetime of (b) Conventional zirconia coping having a homogeneous veneered-zirconia crowns having a high CTE mismatch thickness (left-hand side) and an “anatomically-shaped” and cooled down fast (bench cooling) or slow (under Tg coping (right-hand side) inside the oven) loaded cyclically in a chewing simulator. material, microcracking, grain pull-out, and ulti- bending strength have been reported [122, 123], mately surface pitting mark the repercussions to opposite results can also be found [124–126]. the structural integrity. Due to signs of LTD being The effects of LTD on the strength are related to present throughout the surface and subsurface of the layer of compressive (strengthening) stresses recovered 3Y-TZPs belonging to a batch of femo- generated by the transformation, the compensat- ral heads that had fractured clinically at an abnor- ing tensile (weakening) stress zone neighboring mally rapid rate in the early 2000s, LTD came the transformed zone , and how this change into the spotlight as a potential vulnerability in stress state affects the natural defect popula- staining the great expectations for zirconia as a tion of the material. For example, critical defects mechanically stable biomedical ceramic. can be completely or only partly engulfed in this Reverberations were felt across medical and transformation layer, depending on their size dis- engineering disciplines, reaching also dentistry, tribution. Possibly, a different flaw population with the research community eager to find out to underneath of the transformation zone can which degree were dental zirconias susceptible to become activated, thus starting to dominate the any loss in expected performance. That episode fracture initiation behavior. Also, with the of mass fracture of orthopedic 3Y-TZP hip evolution of transformation toward the bulk, implants was found to be due to newly intro- inter- and intragranular cracks start to develop duced—deficient—fabrication steps leading to within the transformed layer, in an orientation high porosity batches. Subsequent machin- plane parallel to that of the surface. That particu- ing of the sintered pieces had induced surface lar orientation is less dangerous under bending residual stresses, which is believed to have conditions than if orthogonal to the surface, but if ignited a process (LTD) that usually takes a long they coalesce, the transformed layer thickness time in the body to become significant. can become itself the critical defect size. In other Some mechanical testing have shown contra- loading orientations composed of shear compo- dictory results; while negative effects of LTD on nents, such as when contact wear is involved, 30 2 Chemistry and Microstructure a b Fig. 2.19 (a) Surface of a 3Y-TZP material severely 1650 °C after 0 or 50 h of accelerated aging in an auto- affected by LTD, with black spots being grain pull-outs. clave at 134 °C. The pattern of transformation is already Focus-ion beam trenches are seen prepared on the surface visible at the surface layer at 0 h, and extends 25 μm deep for the evaluation of the subsurface material. (b) View after 50 h, with severe cracks that develop in a plane paral- from the prepared trenches of a 3Y-TZP sintered at lel to the surface those cracks can quickly become detrimental. In on the framework of a clinical setting. To date, Fig. 2.19, the subsurface transformation zone is there are still no clinical trials singling out LTD exposed by Focus-Ion Beam milling on a 3Y-TZP as either main or contributing cause to clinical sintered at 1650 °C showing already one-grain-­ failure in dental prosthetic constructs. thick surface transformation due to larger grain Laboratorial experiments seem to negate such a sizes, and after 50 h of artificial aging, where pessimistic view of LTD as an agent that limits extensive subsurface cracking is seen. Such the lifetime of 3Y-TZP, providing even indication effects are obviously dependent on how LTD is of an increase in fatigue resistance [126, let on to evolve. 138–140]. Many factors seem to affect the susceptibility to, and evolution rate of LTD, especially those compositions containing large amounts of the 2.4 Hybrid Ceramics transformable t-phase; translucent zirconias are much less affected. Regarding 3Y-TZP, grain size The subset of hybrid ceramics holds an important (controlled mainly by sintering temperature) place in the historical development of all-ceramic [128–130], amount of alloying oxides (Y2O3, systems for dentistry, allowing in the 1990s rein- CeO2, Al2O3) [131–134], and initial amount of forced infrastructures for bridges to be con- cubic phase are most important factors. In structed without metallic components for the first dental implants, the most relevant are those con- time. The first hybrid ceramics consisted of three cerning surface modification. Post-sintering variants according to the composition of the sandblasting and roughening the surface, for polycrystalline scaffold: Al2O3, Al2O3 + Ce-ZrO2, example, have been shown to induce compressive and MgAl2O4, respectively, InCeram® Alumina, stresses beneficial to the strength of the piece, InCeram® Zirconia, and InCeram® Spinell. The increasing its resistance to LTD [135, 136], while latter was slightly more translucent and used annealing promotes the reverse effect. exclusively for the anterior region. The slip (dis- Although LTD is referred to as a malignant persion of powder in water) was applied using the process that is expected to lead to mechanical slip-casting technique directly on top of the plas- degradation [126, 137], especially when wear is ter dye abutment and formed by hand, subse- involved, its clinical relevance is yet to be defined quently undergoing a partial sintering firing. That 2.4 Hybrid Ceramics 31 firing took place at lower temperatures (~1100 °C) ~37 GPa—in the intermediary range between than the sintering temperature necessary to fully conventional resin composites and glassy ceram- sinter those materials, so that the polycrystalline ics, thus aimed for competing in both markets. particles would only partially fuse together and The inorganic phase is constituted of an alumino- an interconnecting porosity would remain. In a silicate glass powder compact containing low second step, a Lanthanum oxide-rich glass slip fractions of crystalline particulates of some tecto- was applied by hand on top of the partially sin- silicate sort and trace amounts of baddeleyite tered polycrystalline scaffold, which infiltrated. The ceramic network structure is formed by the porosity during a second firing, ultimately irregular-shaped particulates having approx. providing cohesion to the piece. Lanthanum 1–10 μm in size, interrupted at the beginning of oxide was used to improve the thermal compati- the stage of neck formation by partial sintering bility between the glass and the polycrystalline with necks of approx. 0.1–2 μm in cross-section, phases and improve the refractive index. These resulting in an open concave pore geometry. The products were later on made available for machin- internal ceramic surface is treated with a silane ing, the so-called dry-pressed version. In coupling agent in order to increase wetting and InCeram® Alumina (~68 vol.% Al2O3, 28 vol.% provide chemical bonding to the subsequent glass, ~4 vol.% porosity), the dry pressed version monomer mixture. That mixture is composed of showed a more equiaxial microstructural shape, urethane dimethacrylate (UDMA) and triethyl- resulting in lower fracture toughness ene glycol dimethacrylate (TEGDMA) with suf- (3.6 MPa√m) than the elongated particles in the ficiently low viscosity for infiltration, with curing slip-cast version (4.4 MPa√m). For taking place under high pressure to reduce InCeram® Zirconia (~34 vol.% Al2O3, ~34 vol.% shrinkage effects. The final material con- ZrO2, 22 vol.% glass, ~10 vol.% porosity) both tains ~75 vol.% ceramic phase and a ~ 25 vol.% versions showed a mixture of elongated and polymer fraction. The microstructure of Enamic® rounded granules, resulting in equivalent is shown in Fig. 2.20. mechanical properties (both ~4.8 MPa√m) Although promised to confer improved ; those values might be a bit overestimated mechanical performance and indicated by the due to the testing method employed using Vickers manufacturer for constructs as thin as 1 mm, the indentations. Compared to polycrystalline ceram- structure of Enamic® has been shown not to nec- ics, both InCeram® Alumina and InCeram® essarily upgrade the performance of the materials Zirconia are more susceptible to grinding dam- age and strength degradation. The high amount of glass and porosity in these systems still consisted of the weakest link , limiting their mechanical performance and application in longer span constructs. Due to the high opacity, laborious processing, and lower mechanical properties, the InCeram® Alumina and InCeram® Zirconia hybrid materials lost substantial ground during the 2000s to their direct competitors, namely polycrystalline zirconia and polycrystal- line alumina, being thus discontinued for com- mercialization by the manufacturer in 2015. The manufacturer of the hybrid InCeram® materials, i.e., Vita Zahnfabrik, employed a simi- Fig. 2.20 SEM image of the microstructure of Enamic®, lar synthesis approach to fabricate a glass–poly- showing the glassy scaffold (lighter phase) and the infil- mer hybrid material for CAD-CAM processing trated polymer (darker phase). From Ref.. Reprinted branded Enamic®, having Young’s modulus of with permission from Elsevier 32 2 Chemistry and Microstructure it tries to distinguish itself from (resin compos- and leucite content in dental porcelains. J Dent. 2005;33:721–9. ites). In terms of wear and contact fatigue , 11. Lee HH, Kon M, Asaoka K. Influence of modi- as well as flexural strength , fracture tough- fication of Na2O in a glass matrix on the strength ness (~1.12 MPa√m) [4, 148, 149], edge strength of leucite-containing porcelains. Dent Mater J. , and marginal strength , Enamic® per- 1997;16:134–43. 12. Kon M, Kawano F, Asaoka K, Matsumoto N. 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