2.1

Heat-pressing

Whether using leucite-based first generation or lithium disilicate-based second generation heat-pressed ceramics, the heat-pressing process is likely to lead to the creation of porosity within the ceramic restoration, particularly if the pressing temperature exceeds manufacturer’s recommended temperature. The amount of porosity after heat-pressing of leucite based ceramics is between 8 and 10%, while it is about 3% for lithium disilicate based heat-pressed ceramics . Control of pore formation in heat-pressed ceramics requires the use of an ash-free wax that will burnout completely as well as a strict conformity to the recommended heat-pressing schedule, particularly pressing temperature and time.

Due to the fact that lithium disilicate crystals exhibit an elongated prismatic shape, partial alignment along the pressing direction is possible in heat-pressed lithium disilicate ceramics. It was shown that crystal alignment led to a higher fracture toughness value in the direction perpendicular to pressing compared to the direction parallel to pressing . Conversely, heat-pressing of leucite-reinforced ceramics is sought to lead to an homogeneous crystalline phase dispersion throughout the material.

Heat-pressed ceramics are also susceptible to contamination as they react with phosphate-bonded investment at high temperature during the heat-pressing cycle. As a result, an air abrasion procedure is necessary to remove loose and attached investment particles. These steps may introduce surface defects and abrasive particle impingement if not conducted according to manufacturer’s recommendations. Surface defects in heat-pressed ceramics may also originate from sectioning of the sprue and/or occlusal adjustments.

2.2

Slip-casting

The first step in the slip-casting process is to deposit and shape the slip onto the die. This step is technically delicate and microcracks may develop if drying occurs too rapidly, leading to uneven drying and a so-called “onion skin effect”. The glass infiltration step may be associated with the formation of pores. These are due to inhomogeneous glass infiltration from an inadequate amount of glass powder or from dust produced by grinding and obstructing capillaries in the porous coping. Additionally, if the viscosity of the slip is too high, pores are likely to be formed. The total porosity reaches 5% in slip-cast alumina and 9% in slip-cast zirconia toughened alumina . Finally, control of the firing temperature at the glass infiltration step is critical. Microstructural heterogeneities that can potentially cause failure could develop if the firing temperature is too high during the glass infiltration stage.

2.3

Hard machining

Briefly, three main types of ceramic materials are commonly available for hard machining dental restorations: feldspar-based, leucite-based and lithium disilicate-based . Recent work has shown that flaws such as pores (up to 20–50 μm in diameter), pore clusters and inclusions may be present in feldspar-based ceramics prior to machining. These large pores were identified as failure origins. In addition, it was shown that the lower strength end of the Weibull distribution graph corresponded to grinding damage that was not completely eliminated by polishing .

Leucite-based and lithium disilicate-based ceramics are also likely to contain some degree of porosity, together with flaws and mechanical residual stresses due to the machining process. It is worth noting that lithium disilicate-based ceramics are machined in what is called the “blue stage”, prior to full crystallization, while the crystalline phase present is lithium metasilicate (Li ₂ SiO ₃ ), which is more machinable than lithium disilicate. The restorations are then subjected to a heat treatment to ensure final crystallization into lithium disilicate (Li ₂ Si ₂ O ₅ ). This heat treatment is likely to reduce the extent of mechanical residual stresses from machining and ensures the development of a house of cards microstructure composed of interlocked lithium disilicate crystals. However, machining damage is not eliminated by this heat treatment. Fig. 1 A shows machining grooves and associated macrocracking on the proximal surface of a machined lithium disilicate restoration. Machining grooves are also present on the occlusal surface ( Fig. 1 B) and evidence of crystal pullout can be seen at higher magnification ( Fig. 1 C). Although restorations produced by hard machining undergo a final glazing or veneering step, it is likely that large flaws produced at the machining stage could become fracture origins at try-in or after some time in service.

(A) Machining grooves and associated macrocracking on the proximal surface of a lithium disilicate restoration, (B) machining grooves on the occlusal surface of a lithium disilicate restoration and (C) higher magnification showing evidence of grain pullout.

2.4

Soft machining followed by sintering

Machining in the fully sintered stage is impractical and time-consuming with hard ceramics that exhibit stress-induced phase transformation such as partially stabilized zirconia. A more elegant approach is to machine partially sintered blocks, softer and easier to machine, and later complete the sintering process . This is the preferred fabrication technique for zirconia-based dental restorations, using zirconia stabilized with 3 mol.% yttria (3Y-TZP). Since its introduction in dentistry more than a decade ago, zirconia has generated considerable interest and led to an unprecedented number of publications for such a relatively short time in service. This is partly due to the fact that the exceptional fracture toughness of zirconia finds its origin in a reversible stress-induced phase transformation, accompanied by a 4–5% volume increase and generating compressive stresses in the vicinity of the propagating crack, thereby slowing down crack propagation . The clinical implications of this fully reversible crystallographic transformation will be assessed in a latter part of this review.

Fabrication defects and machining grooves are present after machining partially sintered zirconia. As shown in Fig. 2 , a crack emanating from a corner of a Vickers indentation performed in the partially sintered stage ( Fig. 2 A) grows significantly wider after a full sintering cycle at 1500 °C for 2 h ( Fig. 2 B). The sensitivity of 3Y-TZP to the presence of sharp flaws has been unequivocally pointed out in fatigue testing . Careful handling of the machined restorations prior to full sintering appears therefore imperative.

(A) Optical micrograph showing a crack emanating from the corner of a Vickers indentation in a partially sintered 3Y-TZP specimen and (B) same indentation after sintering at 1500 °C for 2 h.

Although much smaller than in other dental ceramics, pores (0.3–0.5 μm in diameter) are also present in partially stabilized zirconia ceramics for dental applications. Depending on the heating rate and firing temperature, porosity gradients may develop in zirconia parts and have been shown, alone or in combination with low temperature degradation, to be particularly detrimental to the clinical performance of zirconia femoral heads used in total hip replacement . As shown in Fig. 3 , pore clusters can also be found in prefabricated dental abutments, and should be regarded as highly undesirable as later illustrated in this review.

Scanning electron micrograph showing porosity in a 3Y-TZP dental abutment.

It is well established that the average grain size of 3Y-TZP ceramics increases with sintering temperature . According to Scott’s phase diagram , a greater amount of cubic phase is also present at higher sintering temperatures. As demonstrated by Chevalier et al. , large cubic grains are associated with a higher concentration of stabilizer, while the surrounding tetragonal grains are yttrium-depleted, and therefore less stable . This situation may trigger low temperature degradation. Segregation of yttrium at grain boundaries and in the cubic grains was also reported by Matsui et al. in 3Y-TZP after sintering at 1500 °C for 2 h , a sintering schedule commonly used for dental zirconia. An example of large cubic grain found in the microstructure of a 3Y-TZP dental abutment is displayed in Fig. 4 . Although large grains are an indirect consequence of the fabrication process, they clearly represent an anomaly in the otherwise homogeneous microstructure of 3Y-TZP. Microstructural heterogeneities such as large cubic grains are an indication of a change in the relative proportions of tetragonal and cubic phase and therefore in the amount of tetragonal phase available for transformation toughening .

Scanning electron micrograph showing an example of large cubic grain in the microstructure of a 3Y-TZP dental abutment.

Finally, it is evident that residual stresses of mechanical or thermal origin play a key role in the mechanical performance of partially stabilized zirconia. Mechanical residual stresses are produced during grinding or air abrasion while the development of thermal residual stresses originates from the veneering process. We will consider the effect of residual stresses in a latter section of the present review.

As mentioned earlier, independent of the fabrication method, all-ceramic systems for dental restorations are all highly sensitive to three major factors that can contribute to failure: (1) the presence of porosity, (2) the presence of residual mechanical and thermal stresses and (3) subcritical crack growth. We will now examine each of these factors as they relate to dental ceramics.

2.1

Heat-pressing

Whether using leucite-based first generation or lithium disilicate-based second generation heat-pressed ceramics, the heat-pressing process is likely to lead to the creation of porosity within the ceramic restoration, particularly if the pressing temperature exceeds manufacturer’s recommended temperature. The amount of porosity after heat-pressing of leucite based ceramics is between 8 and 10%, while it is about 3% for lithium disilicate based heat-pressed ceramics . Control of pore formation in heat-pressed ceramics requires the use of an ash-free wax that will burnout completely as well as a strict conformity to the recommended heat-pressing schedule, particularly pressing temperature and time.

Due to the fact that lithium disilicate crystals exhibit an elongated prismatic shape, partial alignment along the pressing direction is possible in heat-pressed lithium disilicate ceramics. It was shown that crystal alignment led to a higher fracture toughness value in the direction perpendicular to pressing compared to the direction parallel to pressing . Conversely, heat-pressing of leucite-reinforced ceramics is sought to lead to an homogeneous crystalline phase dispersion throughout the material.

Heat-pressed ceramics are also susceptible to contamination as they react with phosphate-bonded investment at high temperature during the heat-pressing cycle. As a result, an air abrasion procedure is necessary to remove loose and attached investment particles. These steps may introduce surface defects and abrasive particle impingement if not conducted according to manufacturer’s recommendations. Surface defects in heat-pressed ceramics may also originate from sectioning of the sprue and/or occlusal adjustments.

2.2

Slip-casting

The first step in the slip-casting process is to deposit and shape the slip onto the die. This step is technically delicate and microcracks may develop if drying occurs too rapidly, leading to uneven drying and a so-called “onion skin effect”. The glass infiltration step may be associated with the formation of pores. These are due to inhomogeneous glass infiltration from an inadequate amount of glass powder or from dust produced by grinding and obstructing capillaries in the porous coping. Additionally, if the viscosity of the slip is too high, pores are likely to be formed. The total porosity reaches 5% in slip-cast alumina and 9% in slip-cast zirconia toughened alumina . Finally, control of the firing temperature at the glass infiltration step is critical. Microstructural heterogeneities that can potentially cause failure could develop if the firing temperature is too high during the glass infiltration stage.

2.3

Hard machining

Briefly, three main types of ceramic materials are commonly available for hard machining dental restorations: feldspar-based, leucite-based and lithium disilicate-based . Recent work has shown that flaws such as pores (up to 20–50 μm in diameter), pore clusters and inclusions may be present in feldspar-based ceramics prior to machining. These large pores were identified as failure origins. In addition, it was shown that the lower strength end of the Weibull distribution graph corresponded to grinding damage that was not completely eliminated by polishing .

Leucite-based and lithium disilicate-based ceramics are also likely to contain some degree of porosity, together with flaws and mechanical residual stresses due to the machining process. It is worth noting that lithium disilicate-based ceramics are machined in what is called the “blue stage”, prior to full crystallization, while the crystalline phase present is lithium metasilicate (Li ₂ SiO ₃ ), which is more machinable than lithium disilicate. The restorations are then subjected to a heat treatment to ensure final crystallization into lithium disilicate (Li ₂ Si ₂ O ₅ ). This heat treatment is likely to reduce the extent of mechanical residual stresses from machining and ensures the development of a house of cards microstructure composed of interlocked lithium disilicate crystals. However, machining damage is not eliminated by this heat treatment. Fig. 1 A shows machining grooves and associated macrocracking on the proximal surface of a machined lithium disilicate restoration. Machining grooves are also present on the occlusal surface ( Fig. 1 B) and evidence of crystal pullout can be seen at higher magnification ( Fig. 1 C). Although restorations produced by hard machining undergo a final glazing or veneering step, it is likely that large flaws produced at the machining stage could become fracture origins at try-in or after some time in service.

(A) Machining grooves and associated macrocracking on the proximal surface of a lithium disilicate restoration, (B) machining grooves on the occlusal surface of a lithium disilicate restoration and (C) higher magnification showing evidence of grain pullout.

2.4

Soft machining followed by sintering

Machining in the fully sintered stage is impractical and time-consuming with hard ceramics that exhibit stress-induced phase transformation such as partially stabilized zirconia. A more elegant approach is to machine partially sintered blocks, softer and easier to machine, and later complete the sintering process . This is the preferred fabrication technique for zirconia-based dental restorations, using zirconia stabilized with 3 mol.% yttria (3Y-TZP). Since its introduction in dentistry more than a decade ago, zirconia has generated considerable interest and led to an unprecedented number of publications for such a relatively short time in service. This is partly due to the fact that the exceptional fracture toughness of zirconia finds its origin in a reversible stress-induced phase transformation, accompanied by a 4–5% volume increase and generating compressive stresses in the vicinity of the propagating crack, thereby slowing down crack propagation . The clinical implications of this fully reversible crystallographic transformation will be assessed in a latter part of this review.

Fabrication defects and machining grooves are present after machining partially sintered zirconia. As shown in Fig. 2 , a crack emanating from a corner of a Vickers indentation performed in the partially sintered stage ( Fig. 2 A) grows significantly wider after a full sintering cycle at 1500 °C for 2 h ( Fig. 2 B). The sensitivity of 3Y-TZP to the presence of sharp flaws has been unequivocally pointed out in fatigue testing . Careful handling of the machined restorations prior to full sintering appears therefore imperative.

(A) Optical micrograph showing a crack emanating from the corner of a Vickers indentation in a partially sintered 3Y-TZP specimen and (B) same indentation after sintering at 1500 °C for 2 h.

Although much smaller than in other dental ceramics, pores (0.3–0.5 μm in diameter) are also present in partially stabilized zirconia ceramics for dental applications. Depending on the heating rate and firing temperature, porosity gradients may develop in zirconia parts and have been shown, alone or in combination with low temperature degradation, to be particularly detrimental to the clinical performance of zirconia femoral heads used in total hip replacement . As shown in Fig. 3 , pore clusters can also be found in prefabricated dental abutments, and should be regarded as highly undesirable as later illustrated in this review.

Scanning electron micrograph showing porosity in a 3Y-TZP dental abutment.

It is well established that the average grain size of 3Y-TZP ceramics increases with sintering temperature . According to Scott’s phase diagram , a greater amount of cubic phase is also present at higher sintering temperatures. As demonstrated by Chevalier et al. , large cubic grains are associated with a higher concentration of stabilizer, while the surrounding tetragonal grains are yttrium-depleted, and therefore less stable . This situation may trigger low temperature degradation. Segregation of yttrium at grain boundaries and in the cubic grains was also reported by Matsui et al. in 3Y-TZP after sintering at 1500 °C for 2 h , a sintering schedule commonly used for dental zirconia. An example of large cubic grain found in the microstructure of a 3Y-TZP dental abutment is displayed in Fig. 4 . Although large grains are an indirect consequence of the fabrication process, they clearly represent an anomaly in the otherwise homogeneous microstructure of 3Y-TZP. Microstructural heterogeneities such as large cubic grains are an indication of a change in the relative proportions of tetragonal and cubic phase and therefore in the amount of tetragonal phase available for transformation toughening .

Scanning electron micrograph showing an example of large cubic grain in the microstructure of a 3Y-TZP dental abutment.

Finally, it is evident that residual stresses of mechanical or thermal origin play a key role in the mechanical performance of partially stabilized zirconia. Mechanical residual stresses are produced during grinding or air abrasion while the development of thermal residual stresses originates from the veneering process. We will consider the effect of residual stresses in a latter section of the present review.

As mentioned earlier, independent of the fabrication method, all-ceramic systems for dental restorations are all highly sensitive to three major factors that can contribute to failure: (1) the presence of porosity, (2) the presence of residual mechanical and thermal stresses and (3) subcritical crack growth. We will now examine each of these factors as they relate to dental ceramics.

3.1

Porosity

Pores have been identified as failure origins in a number of clinically failed all-ceramic dental restorations, particularly when located at the surface of the restorations or at the interface between a veneering ceramic and core . Pores are present in most all-ceramic dental restorations, independently of fabrication technique or ceramic type. However, it should be emphasized that due to their spherical shape, pores in glass ( Fig. 5 A ) were not initially viewed as flaws but rather as stress concentrators, “increasing the stress on nearby flaws” . As shown in Fig. 5 B, pores in glass–ceramics ( Fig. 5 B) are associated with a very different configurations: crystals can form and grow towards the center of the pore, creating sharp interfaces and flaws. The same is true for polycrystalline ceramics with weakened grain boundaries surrounding the pores ( Fig. 5 C). This has led to the suggestion summarized by Rice that “pores in polycrystalline materials would form equatorial cracks approximately one-half grain deep into the matrix and thus become sharp flaws.” The size of this sharp flaw ( C ) is given by the pore radius ( R ) plus ( L ), which corresponds to one half of the grain size ( L ≈ G /2) ( Fig. 5 C) . This has been demonstrated experimentally and it was also shown that stresses due to mismatch strains from thermal contraction anisotropy in the grains, as is the case with 3Y-TZP, are exacerbated in the vicinity of a free surface, such as a pore surface . This may explain the more deleterious nature of pores in glass–ceramics and polycrystalline materials as opposed to blunt spherical pores present in glass or the glassy phase of a low crystallinity ceramic , despite their smaller size.

(A) Scanning electron micrograph showing an example of spherical pore in a feldspathic glassy matrix, (B) scanning electron micrograph of a pore in a glass–ceramic and (C) scanning electron micrograph of a pore in a polycrystalline 3Y-TZP dental ceramic and schematic showing total defect size: C = R + G /2 ( R , pore radius; G , grain size).

3.2

Mechanical residual stresses and machining damage

Computer aided machining of ceramics for the fabrication of single and multi-unit dental restorations has gained tremendously in popularity over the past two decades . Depending on the system and type of ceramic, clinical success rates are overall very good (>90%), particularly for single unit restorations . One concern that remains is that machining by diamond grinding is well established as a major source of failure-inducing flaws in dense ceramics . Fractography may be a helpful technique in determining machining flaw sizes using a classical brittle fracture approach . It should be pointed out, however, that the calculated flaw size corresponds to the final flaw size and not the original flaw size. The calculated flaw size includes any subcritical crack growth and does not take into account the influence of residual surface stresses . Nevertheless, fractographic techniques have helped in establishing that machining flaw sizes do not vary significantly as a function of grain size, and correlate with hardness, as do both machining forces and rates . As indicated earlier, machining damage has been identified as failure origin in feldspar-based dental ceramics, even after a polishing step had been performed . In dental applications, machining damage can also be introduced by air abrasion or grinding during clinical adjustments. If most all-ceramic restorations are veneered with a softer ceramic that is relatively easy to polish, the recent introduction of monolithic all-ceramic restorations as well as abutments raises the question of grinding damage elimination, particularly in the case of zirconia-based restorations, due to the difficulty of polishing zirconia. Fig. 6 shows the cross section of a 3Y-TZP ceramic ground on the top surface, with a bonded interface configuration . Machining grooves can clearly be seen, as well as more extensive in depth damage in the form of transverse cracks.

Scanning electron micrograph of the cross-section of a 3Y-TZP ceramic diamond-ground on the top surface (bonded interface configuration).

Concurrently, both grinding and sandblasting are likely to trigger the tetragonal to monoclinic ( t – m ) stress-induced transformation of partially stabilized zirconia . The efficacy of these surface treatments in promoting the transformation depends in part on the mean grain size, with larger grains being more prone to transformation . The toughness of the material after transformation varies upon the amount of tetragonal phase retained. One key consequence of the t – m transformation is the development of surface compressive stresses that lead to a significant strength enhancement depending on the thickness of the compressive stress zone . It has been shown that sandblasting is more efficient in inducing the t – m transformation than abrasive grinding. The transformation can be reversed either by heat treatment or simply by the heat generated during grinding . The latter explains the more modest strength increase associated with grinding compared to sandblasting. The strength enhancement due to generation of compressive stresses after sandblasting or grinding of partially stabilized zirconia is lost after heat treatment, leaving behind machining damage and significant flaws that are detrimental to the mechanical properties, including the fatigue behavior . Polishing after grinding also reduces the compressive stress zone and is unlikely to fully eliminate the flaws created by grinding, particularly in a clinical setting. Consequently, polishing after grinding is associated with a decrease in strength . It should be reiterated that the variability of the results obtained when performing surface treatments on Y-TZP stems from the fact that the response of the material is dependent upon intrinsic factors, such as grain size and amount of stabilizer, as well as extrinsic factors such as thermal history influencing the amount of cubic phase in the material .

3.3

Thermal residual stresses and veneering process

All-ceramic dental restorations are usually veneered to achieve adequate esthetics. Clinical studies evaluating the in vivo performance of zirconia-based restorations have shown a significant occurrence of chipping of the veneering ceramic on zirconia frameworks, usually as cohesive failure within the veneering ceramic . Recent systematic reviews confirm that chipping of the veneering ceramic is the most frequent complication with a significantly higher incidence than with metal–ceramic restorations . Following the analysis proposed by Swain , it is clear that several physical factors play a role in the mechanism by which chipping fractures occur, mainly residual stresses arising from (1) the difference (even slight) between the coefficient of thermal contraction of the veneer and that of the core ceramic and (2) tempering stresses due to temperature gradients created during rapid cooling. Thermal stresses become locked in below the glass transition temperature of the veneering ceramic and reach very high values due to the low thermal diffusivity of zirconia compared to other glass–ceramics . Tholey et al. recorded temperature differences up to 140 °C between veneer and core ceramics when fast cooling rates were used, while slow cooling rates appeared effective in limiting the development of residual thermal stresses. Slow cooling rates were also shown to nearly eliminate thermal residual stresses with ceramics heat-pressed onto zirconia substructures . However, according to the analysis given by Swain, another key factor in the development of thermal residual stresses in veneered zirconia is the thickness ratio of core to veneering ceramic . This was recently demonstrated by Mainjot et al., who measured residual stress profiles in veneering ceramic supported by a zirconia substructure . The results showed that framework thickness and cooling rate strongly influenced the residual stress profile. Three effects were proposed as key contributions to the development of residual stresses: (1) a tempering effect due to thermal gradients between veneer and core and proportional to the veneer/core thickness ratio, (2) a coefficient of thermal contraction mismatch effect, inducing compressive stresses near the framework, (3) a stress-induced t – m transformation of zirconia due to the presence of tensile stresses which could trigger the transformation. It was concluded that an optimal core/veneer thickness is difficult to establish owing to the complex geometry of dental frameworks . Nevertheless, it is clear that further microstructural analyses of the interface between veneering ceramic and zirconia substructure are needed to better establish the role of pre-existing eigenstresses due to thermal expansion anisotropy of the t -phase, as well as intergranular stresses on the transformability of Y-TZP .

3.4

Role of slow crack growth and time to failure of all-ceramic restorations

Immediate failure at restoration try-in or within a few hours or days after cementation is likely to originate from a major processing flaw . Failure after a few years is more likely to involve subcritical crack growth (SCCG) and/or cyclic fatigue . It is well established that SCCG in soda lime glass is exacerbated by the presence of water or in a humid environment . The presence of a glassy phase in dental ceramics together with the humidity of the oral environment are therefore key factors influencing SCCG, which is defined as occurring at stresses below the critical value until the crack reaches its critical length leading to fast failure . A recent study investigated the SCCG susceptibility coefficients of various dental ceramics which ranged from 17.2 for lithium disilicate heat-pressed ceramics to around 30 for leucite-reinforced heat pressed ceramics and slip-cast alumina . It was pointed out that the establishment of stress-probability-time diagrams, combining SCCG data and Weibull statistics make it possible to achieve obtain a preliminary assessment of life time.