Polycrystalline ceramics are manufactured by sintering, and they are also generally referred to as sintered ceramics.
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Among chairside digital prosthetic materials, polycrystalline ceramics are generally stronger mechanically than glass ceramics. They are a popular class of materials used in chairside digital prosthetics today. In polycrystalline ceramics, no glass phase is present and all crystals are arranged in a dense conventional matrix. This arrangement limits the extension of cracks and provides them with superior mechanical properties [35]. Additionally, the absence of a glass matrix allows the ceramics to possess resistance to surface etching with hydrofluoric acid [36]. Polycrystalline ceramics, which usually have low transparency due to the lack of a glass phase, are mostly used for the manufacture of crowns and bridges, which are then fitted with veneering porcelain to improve the esthetics [37]. Currently, polycrystalline ceramics that are frequently employed in dental restorative materials include alumina ceramics and zirconia ceramics.
In recent years, due to the advent of zirconia ceramics and their superior physical and chemical properties, alumina ceramics have been gradually replaced by zirconia ceramics in the field of restorative dentistry. However, compared to zirconia ceramics, alumina ceramics still have advantageous characteristics, in that they are very stable at high temperatures and in physiological fluids. In conclusion, alumina ceramics may still have a promising future in the field of prosthodontics.
Normally, the microstructure of alumina ceramics consists of isometric particles, with a polycrystalline structure consisting of ionic or covalent bonds. Therefore, the fracture toughness of alumina ceramics is low. Under the action of external forces, the stress will cause fine cracks on the surface of the ceramic, and the rapid expansion of cracks makes alumina ceramics undergo brittle fracture. Toughening research is a central topic in the study of alumina ceramic materials. Currently, there are a few main methods used to improve the fracture toughness of alumina ceramics: particle dispersion toughening, fiber and whisker toughening, zirconia phase change toughening, composite toughening, and self-toughening. Today, alumina ceramics are often toughened by zirconia phase change toughening, and ceramics toughened by this method are called zirconia-toughened alumina ceramics (ZTA). The process involves adding Y-TZP (yttrium-oxide-stabilized zirconium oxide) to the alumina matrix and distributing it evenly throughout the alumina matrix to achieve a good toughening effect. A study of the wear and corrosion phenomena on the toughened alumina ceramics showed that the wear and corrosion resistance of ZTA was superior to that of yttrium-oxide-stabilized zirconium oxide (YSZ) [ 51 ]. The clinical application of ZTA has also been investigated, with Larsson et al. [ 52 ] conducting a five-year follow-up study of implantable ZTA (using the In-Ceram Zirconia, VITA Zahnfabrik [InZ] repair system), which showed considerable clinical advantages. The In-Ceram Zirconia repair system is illustrated in Figure 8 .
The first completely intensive dental polycrystalline ceramic was Procera&#; AllCeram (Nobel Biocare, Göteborg, Sweden), introduced in . Procera&#; AllCeram has a transparency between Empress&#; 1 and Empress&#; 2 and has been tested to have a consistent marginal fit of 60-80 μm, which is within the clinically acceptable range [ 45 ]. In a six-year evaluation of the clinical use of Procera&#; AllCeram single crowns, the accumulated survival and success rates over six years were 95.2% and 90.9%, respectively, which is a great advantage for clinical use [ 46 ]. Figure 7 shows the patient's recovered portion of the broken Procera&#; AllCeram premolar crown and the SEM image of the Procera&#; AllCeram. In-Ceram AL appeared, made by VITA Zahnfabrik, also a representative example of an alumina ceramic. It has a higher mean fracture load [941.8 (±221.66) N] (p > 0.05) compared to other ceramic materials like IPS-Empress II and Top-Ceram (a more detailed comparison of some of the properties of the three materials is shown in Table 3 ) [ 47 ]. In-Ceram alumina is a suitable material for anterior and posterior crowns, as well as for anterior single-retainer RBFPD. Certain other specific data for the two alumina ceramic products are shown in Table 4 .
Aluminum oxide is currently a widely used restorative material in clinical practice. Choosing the bonding agent is very important for all-porcelain alumina crowns. The bond strength can directly affect the restorative effect of an all-ceramic crown, and the use of different bonding agents will produce different bond strengths [ 44 ]. Currently, commonly used bonding agents for alumina crowns include the Fuji multipurpose glass ionomer and flowable composite resins. In contrast, the bonding of alumina crowns can be made stronger with flowable compound resin bonding agents; these have a higher clinical application value.
As the standard of living around the world improves, more and more people are paying attention to the esthetics of their teeth. Thus, we normally try to ensure attractive esthetics in the restoration of teeth. Because of their similar color and natural luster to real teeth, alumina ceramics are commonly used to make crowns, bridges, and veneers for anterior teeth. The use of alumina all-ceramic crowns can achieve better cosmetic restorative dentistry than zirconia all-ceramic crowns and effectively reduce the likelihood of gingivitis. However, alumina ceramics have a high modulus of elasticity, reaching 380 GPa, which makes them prone to fracture [ 43 ]. There is currently no complete solution to this problem, which is one of the reasons for the gradual replacement of alumina ceramics by zirconia ceramics. The use of zirconia is significantly more effective than the use of aluminum oxide for the restoration of posterior areas of the oral cavity.
Alumina ceramics have a very high hardness and density, and their Mohs hardness reaches nine, which is slightly lower than diamond. In medicine, they are often used to make dental prosthetic materials and artificial joints (e.g., hip joint balls). Studies have shown that the Rockwell hardness of alumina ceramics is approximately HRA80-90, while the bending strength of sintered and hot-pressed products can reach 250 MPa and 500 MPa, respectively [ 42 ].
Regarding the sintering of alumina ceramics, this process is usually performed using ultrafine powders with good sintering activity and adding appropriate amounts of sintering aids to lower the sintering temperature required to obtain ceramics with excellent mechanical properties. The usual sintering aid added to alumina ceramics is MgO, which effectively inhibits excessive grain growth and tends to make the sintering completely dense. For ceramic materials, the sintering temperature can generally be reduced by using both ultrafine powders and sintering aids, and the reduction in the sintering temperature results in a ceramic material with excellent mechanical properties [ 41 ].
The main crystalline phase of alumina ceramics is corundum (AI 2 O 3 ), which has four isomorphs: α-Al 2 O 3 , β-Al 2 O 3 , γ-Al 2 O 3 , and δ-Al 2 O 3 . The α-Al 2 O 3 crystalline form has the best thermal and chemical stability [ 38 ]. This is essentially the only form of alumina that is currently used in the field of dental prosthetics. Depending on the content of alumina in alumina ceramics, they can be classified into two categories: high-purity type and normal type. Ultrapure alumina ceramics contain more than 99.9% alumina and have excellent characteristics such as porosity, high dispersion, insulation, and heat resistance [ 39 ]. Ordinary alumina ceramics are divided into different varieties according to their alumina content, such as 99 porcelains, 95 porcelains, 90 porcelains, and 85 porcelains. In some cases, ceramics with a content of approximately 80% or 75% alumina are also classified as ordinary alumina ceramics. In alumina ceramics, the mechanical strength decreases as the content of alumina decreases. Thermal conductivity also increases with increasing alumina content. The alumina content of alumina ceramics used in dental restorative materials is generally above 50 wt%, where the alumina ceramics have good mechanical strength and their bending strength increases with increasing alumina content [ 40 ].
In the early s, zirconium oxide was introduced into the field of denture processing, and today, it is a popular material in the field of dentistry. In general, zirconia is yellow or gray in color, although high-purity zirconia is white.
Today, zirconia ceramics are a popular type of dental restorative ceramic on the market. At atmospheric pressure, pure zirconia has three crystalline forms: monoclinic zirconia (m-ZrO2), tetragonal zirconia (t-ZrO2), and cubic zirconia (c-ZrO2). Figure 9 shows transmission electron microscopy images and high-resolution transmission electron microscopy images of the three crystal forms. The material changes forms at different temperatures. At temperatures less than °C, zirconia is in the monoclinic (m) phase with a density of 5.65&#;g/cc. The tetragonal (t) phase occurs from °C to °C with a density of 6.10&#;g/cc, and the cubic (c) phase occurs above °C with a maximum density of 6.27&#;g/cc [53, 54]. Zirconia is most stable when it is in the monoclinic (m) phase. Under certain conditions, zirconia undergoes the t-m transition. When this transition occurs, the volume of zirconia also changes. This change is known as &#;tensorial expansion,&#; which is a phenomenon that occurs widely in steel. Hence, zirconia is also known as &#;ceramic steel&#; [55]. During the t-m transformation process, zirconia's stability can be enhanced by limiting crack extension through the expansion of the particle volume, which is limited by the surrounding material. This phenomenon is known as &#;stage transformation strengthening.&#; The flexural strength of partially stabilized zirconia is generally in the range of -&#;MPa and a modulus of elasticity of approximately 200-210&#;MPa [56]. Zirconia ceramics have a very high mechanical strength at normal temperatures. As a result of the excellent properties of zirconia ceramics, they are currently used in clinical practice for prefabricated root canal crowns, single crowns, all-ceramic crowns, and all-ceramic crown and bridge restorations.
Transmission electron microscope images: (a) m-ZrO2, (b) t-ZrO2, and (c) c-ZrO2. High-resolution transmission electron microscopy images: (d) m-ZrO2, (e) t-ZrO2, and (f) c-ZrO2 [57].
As a biologically inert ceramic, the biocompatibility of zirconia ceramics is clearly excellent, enabling the emergence of zirconia implant abutments. In , zirconia implant abutments were officially used in clinical practice for the first time. Compared to metallic materials, zirconia implant abutments offer better esthetic results and can restore the color of natural teeth to a greater extent. Additionally, the lower surface free energy and wettability of zirconia abutments can effectively reduce the risk of periodontal inflammation after implantation [58].
Although the mechanical properties of zirconia are quite good, the optical properties are relatively poor. The installation of veneered porcelain is usually required to achieve an adequate esthetic effect, and using micromechanical inlays is the main method of bonding between veneer porcelain and zirconia. Although the use of veneers satisfies the desired esthetic effect, veneers are prone to chipping. Currently, a major cause of clinical failure of zirconia restorations is disintegration of the veneer [59].
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For the treatment of zirconia ceramic surfaces, alumina blasting techniques are often used in clinical settings to treat the ceramic surface. However, for zirconia, the alumina blasting technique does not dramatically improve the bond strength on the zirconia surface. It has been demonstrated that sandblasting causes microcracks on the surface of the zirconia material, reducing the mechanical properties of the zirconia material itself [60]. As a result, the alumina blasting technique may not be the best option for zirconia. Some experiments have shown that silicon nitride blasting may be more effective than alumina blasting for zirconia, and further investigation is required to compare the effects of these two types of blasting [61].
For the processing of zirconia ceramics, zirconia is currently processed by CAD/CAM systems, such as CEREC, PROCERA, and KAVO (Figure 10 illustrates the basic flow of a CAD/CAM system) [62]. These systems first use zirconia powder to form the molded body. This is followed by sintering at a lower temperature to obtain a zirconia presintered body, cutting the presintered body to obtain a presintered zirconia inner crown, and finally sintering at a higher sintering temperature to precisely control its shrinkage to obtain a dense zirconia denture. This method has greatly improved the processing efficiency of zirconia prostheses and has led to the promotion of zirconia as a dental prosthetic material.
Dental CAD/CAM system process diagram [63].
In order to obtain different types of zirconia ceramics, we usually need to add different types of stabilizers, such as CaO, MgO, Y2O3, and CeO2. The most widely used stabilizer is Y2O3. In , researchers first observed that t-ZrO2 could be made stable or substable at room temperature by adding Y2O3. Later, it was experimentally demonstrated that TZP ceramics with a fully tetragonal phase could be formed by stabilizing zirconia with 2&#;mol% to 3&#;mol% Y2O3 [64].
Commercially applied zirconia is mostly found in various products as tetragonal zirconia polycrystals (Y-TZP), which must be polished after grinding to ensure adequate mechanical properties in the Y-TZP ceramics [65]. Y-TZP can be broadly divided into two types of material depending on the process technique: hot isostatic compression (HIP) and cold isostatic compression (CIP). More zirconia powder is required to use HIP than CIP, but the strength of zirconia ceramics made using the HIP process is 20% stronger than those made using the CIP process. Procera uses HIP-type zirconia.
One of the biggest drawbacks of zirconia ceramics is their low-temperature ageing. Kobayashi et al. [66] first suggested that Y-TZP undergo a slow t-m phase transition at 250°C in a moist environment, which is essentially a martensitic phase transition followed by a significant reduction in the mechanical properties. The primary elements impacting zirconia aging are the type and amount of stabilizer added inside the zirconia ceramic, the grain size, and the residual stress.
Many discussions have been made about the mechanism of low-temperature ageing. Currently, one of the widely accepted ageing mechanisms is the point defect reaction ageing mechanism based on oxygen vacancies and water molecules [67]. This ageing mechanism can be represented by the following two chemical equations.
H2Oad+Osurf2&#;&#;2OHsurf&#; (2) 2OHsurf&#;+Vo¨&#;OHo+S0surfX (3)Reaction (2) describes the process of breaking the space structure bonds of zirconia, while reaction (3) describes the process by which hydroxyl groups move and diffuse on the surface of zirconia, thereby forming defects.
It has been found that Y-TZP has excellent ageing resistance when the zirconia grain size is controlled to be 0.3-0.4&#;μm [68], but the ageing resistance can also be increased by adding various oxides or nonoxides. 3Y-TZP has the best ageing resistance when AI2O3 is added to it at 0.25&#;wt% [69]. Alumina-toughened zirconia (ATZ) is a zirconia ceramic composite material formed by adding alumina (α-Al2O3) to zirconia as a matrix. The α-Al2O3 dispersive phase particles contained in the composite can, to a certain extent, hinder the t-m phase transition of 3Y-TZP [70]. The physical and chemical characteristics of ATZ support its application in dental applications, and its resistance to ageing is better than that of 3Y-TZP. However, further research is still required to enhance the hydrothermal stability of ATZ. Kohal et al. [71] evaluated the clinical use of ATZ and found that the survival rate of ATZ implants was 94.3% after five years and that the material had an advantage in terms of bone tissue stability with a bone loss of 0.81&#;mm over five years. Based on these results, ATZ can be recommended for clinical use.
Fully anatomic zirconia is a hot topic of research in recent years. Fully desorbed zirconia refers to a restoration with a fully desorbed morphology that is designed and manufactured directly from zirconia by CAD/CAM technology. This process eliminates the need for veneering porcelain and reduces the possibility of restoration failure. Fully desquamated zirconia is extremely strong mechanically and does not cause excessive wear on natural teeth because it is less abrasive than feldspathic ceramics [72]. As this technology has evolved, fully destructive zirconia, which was earlier used mainly in the posterior region, has gradually been applied for the esthetic restoration of anterior teeth.
Graded zirconia was also created as a new material in response to the chipping of veneered porcelain. Graded zirconia materials are manufactured by infiltrating glass into 3Y-TZP while sintering it. Due to the penetration of the glass, the graded zirconia ceramics have a better esthetic effect. A study of the wear properties of the new graded zirconia material showed that polished graded zirconia has better wear properties compared to zirconia. It can be inferred that it has adequate clinical wear properties and has a smooth wear surface that can be used to reduce wear on the tooth [73].
VITA In-Ceram® YZ (VITA Zahnfabrik) and Lava&#; Frame Zirconia (3M ESPE) are two of the better-known products in the world today. Certain other specific data for the two zirconia ceramic products are shown in Table 4. Both of these materials can be used to make zirconia bridges. Models of the zirconia bridge and the fixed zirconia bridge are shown in Figure 11.
(a, b) Illustrations of a zirconia bridge and a fixed zirconia bridge [74].
It is undeniable that zirconia ceramics offer excellent physicochemical features compared to many other dental restorative materials (some of the important properties of zirconia and alumina products are compared in Table 4). Although low-temperature ageing of zirconia ceramics due to t-m phase changes in humid, low-temperature environments (or oral environments) can occur, numerous researchers have addressed the issue of low-temperature ageing of zirconia ceramics in various ways. As their resistance to ageing increases, zirconia ceramics are bound to become more widely used in dentistry.
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