Chemistry And Microstructure Of Dental Ceramics PDF

Chemistry and Microstructure of Dental Ceramics Prof. Muawia Qudeimt Dr. Nada Moustafa Chemistry and Microstructure of Dental Ceramics Introduction Importance of Microstructure in Ceramics Ceramic Types Composition and Stoichiometry Introduction The rapid evolution of dental ceramics s...

Chemistry and Microstructure of Dental Ceramics Prof. Muawia Qudeimt Dr. Nada Moustafa Chemistry and Microstructure of Dental Ceramics Introduction Importance of Microstructure in Ceramics Ceramic Types Composition and Stoichiometry Introduction The rapid evolution of dental ceramics since the 1980s, driven by techniques like CAD-CAM, has led to an overwhelming variety of materials. Clinicians and technicians should focus on ceramic materials' chemical composition and microstructure rather than just brand names, as performance varies widely even within the same material type. Emax or Lithium disilicate? Importance of Microstructure in Ceramics Differences in oxide chemistry, fabrication methods, and particle size can impact mechanical properties like strength, toughness, and wear resistance, even for similar products like lithium disilicate. (ie: not every lithium disilicate is the same) Importance of Microstructure in Ceramics Factors affecting material Which will lead to Which will lead to: performance: differences in the material: Wide spectrum of Oxide chemistry Phase fractions mechanical Fabrication parameters Crystal type and shape performance Oxide ratios Structural homogenity Particle size Internal stresse Firing parameters Ceramic Types Dental ceramics are classified into three main categories: Hybrid ceramics: consist of glass or zirconia/alumina particles partially sintered and later infiltrated with a polymer or molten glass. Silicate ceramics: a mix of SiO2-rich glass and crystals, which can be crystallized or added separately for reinforcement. Oxide ceramics: glass-free, polycrystalline materials like alumina and zirconia. Ceramic Types Fabrication Technique: Fabrication involves creating objects from raw or semi-finished materials. It typically includes methods like: Partial Sintering: Heating materials below their melting point to bond particles together. Full Sintering: Heating materials to a point where they fully fuse together. Glass-Ceramic Process: Combining glass and ceramic materials to create a composite. Processing Technique: Processing, on the other hand, involves transforming or refining materials into their final form. This can include: Powder Layering: Adding layers of powder material to build up a product. CAD-CAM (Computer-Aided Design and Manufacturing): Using computer software to design and manufacture products. Injection Molding: Injecting molten material into a mold to create a product. In summary, fabrication techniques are about creating parts from raw materials, while processing techniques are about refining and shaping those parts into final products. Ceramic Types We will briefly cover older ceramic systems that are used less frequently today, such as aluminosilicates (feldspar and leucite-based) and hybrid materials (glass- infiltrated polycrystalline scaffolds and polymer-infiltrated glass scaffolds). Our primary focus will be on the most relevant materials in modern prosthodontics, including lithium (di)silicates and zirconia. Aluminosilicates Typically, tectosilicates, containing alkali metal ions like sodium (Na+) or potassium (K+) with an aluminum ions (Al³⁺) to metal ions (M⁺). These glasses have a highly polymerized network with very few non- bridging oxygens, which contributes to their strength. Feldspar-based ceramics are used for dental veneering and machinable blocks, made through glass-ceramic processes or by milling natural feldspar rocks. Early CAD-CAM materials like Vitablocks® from Vita Zahnfabrik were made using feldspar, a technique that’s widely employed in dental ceramics. The fabrication process involves milling feldspar rocks (e.g., albite or nepheline), melting, and quenching to form frits, which are then milled and mixed with reinforcing particles. Aluminosilicates Tectosilicate/Framework Silicates: 3D framework of shared tetrahedra Each tetrahedro shares all four O atoms aith neighboring terahedra (SiO2) Silicate Classification: Cont…. Feldspar blocks typically have 20-40% crystallinity, contributing to a fracture toughness of around 1.2 MPa√m. Leucite crystallization (KAlSi2O6) in glass-ceramics is used to adjust the thermal expansion to match metallic frameworks and polycrystalline ceramics. Leucite crystals (10-30% by volume) are formed using nucleation agents and help toughen the material by causing residual stresses and cracking. Excess leucite (over 30% volume) can harm the material’s mechanical properties, reducing its toughness. Aluminosilicates based on feldspar and leucite have seen a decline in use due to the rise of monolithic ceramics, alternative veneering techniques, and stronger materials like lithium disilicate.  Clinical fracture rates of inlay and onlay restorations produced by the same machining center, comparing the performance of leucite and lithium disilicate materials. Lithium-Based Glass-Ceramics Glass-ceramics based on the SiO2–Li2O system, especially lithium (di)silicates, are vital in prosthetic dentistry, reflecting a natural selection process favoring superior compositions in the market. The InCeram® product line, once popular, was discontinued due to competition from newer dental zirconias, illustrating the shift in material preferences. Lithium disilicate has become a prominent brand, overshadowing other glass-ceramics and driving their decline, with mechanical and optical properties influencing market dynamics. Machinable analogs of lithium disilicate are gaining popularity due to ease of processing and cost- effectiveness, even as hot-pressed variants like IPS e.max® Press maintain strong performance. Intense legal battles over patents related to lithium-based glass-ceramics are ongoing, with companies either engaging in litigation or opting for licensing agreements to navigate commercialization challenges. Compositional Variations of Lithium-Based Glass-Ceramics Historical Background: Lithium-based glass-ceramics originated from tailoring lithium disilicate compositions, with Fotoceram® being the first synthetic glass-ceramic developed in the 1990s. Key Definitions: Lithium Disilicate: refers specifically to the stoichiometric composition of 2SiO2·Li2O (Li2Si2O5 crystals). This material is known for its excellent mechanical properties and esthetics in dental restorations. Lithium Silicate: involves a different stoichiometric composition of SiO2·Li2O, leading to lithium metasilicate (Li2SiO3) crystals, which have different properties and applications. Composition and Stoichiometry Dental glass-ceramics are multicomponent, incorporating oxides like aluminum oxide (Al2O3), potassium oxide (K2O), and cerium oxide (CeO2) to help lower melting temperatures and viscosity, making the materials easier to process and to enhance chemical resistance and accelerates crystallization kinetics, critical for achieving desired mechanical properties. Stoichiometric ratios (SiO2/Li2O) can vary, with recent products showing ratios below 2 or around 3.  SiO2–Li2O binary system, with superposed dental multicomponent compositions Chemical composition (in mol%) and crystallinity of some dental lithium-based glass-ceramics Crystallization Process The crystallization process for 2SiO2·Li2O typically yields lithium disilicate crystals while retaining some residual glass. This process can take several hours to achieve near-complete crystallization. In contrast, SiO2·Li2O crystallizes primarily into lithium metasilicate. The incorporation of nucleation agents, particularly phosphorus pentoxide (P2O5), plays a crucial role in improving crystallization rates and properties. For example, the use of P2O5 has been shown to enhance the crystallization kinetics of lithium silicates, making them more effective in clinical applications. Zirconium Dioxide Addition Recently, zirconium dioxide (ZrO2) has been integrated into some dental glass-ceramics in amounts up to 10 wt.% (approximately 4.5 mol%). This addition is intended to enhance the strength of the glass-ceramic: ZrO2 contributes as a network former rather than a glass modifier, which can enhance the glass network's stability. It has been found to increase the viscosity of the melt and shift the glass transition temperature to higher values, which is beneficial for processing. ZrO2 influences the nucleation and growth of crystalline phases, as seen in materials like Suprinity® PC, which show altered crystallization behavior due to ZrO2 presence. Complex Interactions The interaction between various oxides and the evolving glass network adds complexity to the crystallization behavior of multicomponent glass-ceramics. For example: The presence of ZrO2 alters the elastic properties of the glass due to longer bond lengths, complicating generalizations about performance. The network modifications can lead to varied crystallization temperatures and behaviors in the final product, which can differ significantly from predictions based solely on stoichiometric compositions. Chemistry and Microstructure of Dental Ceramics- II Prof. Muawia Qudeimat Dr. Aliah Alnafisi Mechanical Properties of Glass-Ceramics Introduction Importance of Microstructure in Ceramics Ceramic Types Composition and Stoichiometry Lithium-Based Glass-Ceramics Interrelation of Microstructure and Mechanical Properties Microstructure's Role: The mechanical properties of glass-ceramics, such as strength and toughness, are intrinsically linked to their microstructure, encompassing the arrangement and characteristics of both glassy and crystalline phases. Complex Interactions: In glass-ceramics, the interaction between the glass matrix and crystalline phases creates a unique mechanical behavior that cannot be fully understood without considering these microstructural elements. Microstructural Parameters Particulate Properties: o Young’s Modulus: A measure of stiffness; higher values indicate stiffer materials, which can enhance overall strength. o Fracture Toughness: A critical property for assessing resistance to crack propagation; higher values correlate with improved performance in load-bearing applications. Geometrical Attributes: o Particle Size: Smaller particles can create a more homogenous microstructure, enhancing mechanical properties. o Aspect Ratio: Elongated particles or crystals can provide interlocking structures that enhance toughness through crack deflection mechanisms. Thermal Compatibility Issues- Phase Interactions The presence of two distinct phases (glass and crystal) can lead to thermal incompatibility: § Thermal Expansion Coefficients (TECs): Differences in TECs can result in thermal stresses during cooling. For example, Li2SiO3 with a TEC of 15.4 × 10⁻⁶ K⁻¹ creates significant tensile stresses in adjacent phases. § Stress-Induced Cracking: Such thermal stresses can lead to microcracking during the cooling process, particularly problematic for dental applications where aesthetic and mechanical integrity are critical. Cracking Behavior Localized vs. Distributed Cracking: o Low Crystallization: At low crystallization levels, cracks are often confined to regions near single crystals, minimizing structural damage. o High Crystallization: Higher levels of crystallization increase the risk of overlapping tensile zones, leading to microcracking throughout the material. This phenomenon is especially evident in high Li2SiO3 content ceramics. Persistence of Cracks: Cracks induced during the pre-crystallization phase can persist through subsequent crystallization, affecting overall material performance and reliability. Product Variability Suprinity® PC and Celtra® Duo: These products exhibit low Weibull modulus, indicating significant variability in strength due to thermal incompatibility and the presence of cracks. Obsidian®: Demonstrates improved mechanical properties post-crystallization, attributed to phase transformation and increased thermal stability. Crystallization and Fracture Toughness Degree of Crystallization: o A well-documented linear relationship exists between the degree of crystallization and fracture toughness, with higher crystallized volume fractions leading to better toughness performance. o For instance, a stoichiometric 2SiO2·Li2O glass shows improved toughness correlating with crystallization from 0% to 100%. Comparison of Crystal Phases Li2SiO3 vs. Li2Si2O5: Materials primarily containing Li2Si2O5 demonstrate minimal thermal mismatch effects, leading to compressive residual stresses in crystals that do not significantly harm structural integrity. In contrast, those with high Li2SiO3 content can experience serious mechanical failures due to significant thermal incompatibilities. Toughening Mechanisms- Mechanisms at Work 01 02 03 Crack Deflection: The Crack Branching: Energy Crack Bridging: Crystalline presence of crystalline dissipation occurs as cracks structures can span cracks, phases causes cracks to branch off, contributing to preventing further growth change direction, absorbing increased toughness. and enhancing load-bearing energy and reducing crack capacity. propagation. Toughening Mechanisms- R-Curve Behavior This phenomenon indicates that materials with elongated crystals can exhibit increasing toughness with crack extension, vital for applications under cyclic loading conditions. Pressable Materials- Enhanced Fracture Resistance Local crystal orientation in pressable materials like lithium disilicates can significantly enhance fracture resistance. Aligning crystal bundles perpendicular to the crack propagation direction can result in toughness increases of up to 25%, improving performance under stress. Machinability Considerations- Machinable Two-Step Process This process involves machining followed by crystallization, which compromises toughness: Pre-crystallized materials are generally less tough and more susceptible to machining- induced defects. Damage during machining can lead to a strength reduction of up to 50%, particularly in materials like feldspathic ceramics. Machinability Considerations- Polishing Techniques Although polishing can help recover some strength, it is complex and requires standardization for effective clinical application. Improper polishing can introduce new defects. Healing Mechanisms- Crystallization Healing Crystallization can repair cracks introduced during machining: The glassy matrix flows into cracks due to viscous behavior at high temperatures, aided by capillary forces that facilitate healing. Healing Mechanisms- Importance of Pre- Crystallization Polishing Polishing before the crystallization process is crucial to minimize the presence of surface defects that can act as fracture initiation sites. Clinical Implications Surface Integrity: o Proper attention to surface finish and integrity is critical, as poorly managed surfaces can become weak points leading to premature failure. Porosity Management: High porosity in pressable materials can create critical defects, compromising mechanical strength. Effective control during manufacturing is essential to ensure reliability and longevity in clinical applications. Zirconium Dioxide THE INTRODUCTION OF ZIRCONIA CERAMICS IS TRADITIONAL METHODS: PRIOR TO ZIRCONIA, NEED FOR NEW TECHNIQUES: EACH PROSTHETIC HERALDED AS A KEY ACHIEVEMENT OF CAD-CAM MATERIALS WERE SHAPED USING CASTING (METALLIC CONSTRUCT'S UNIQUE SHAPE NECESSITATED A TECHNOLOGY IN DENTISTRY. ALLOYS), INJECTION MOLDING, HEAT PRESSING (E.G., SUBTRACTIVE MANUFACTURING TECHNIQUE CAPABLE IPS EMPRESS® 2), OR SLIP-CASTING WITH GLASS OF MACHINING PIECES FROM MONOLITHIC INFILTRATION (E.G., INCERAM® PREFABRICATED BLANKS. ALUMINA/ZIRCONIA/SPINELL). Challenges with Zirconia Hard Machining: Fully sintered zirconia presents difficulties such as prolonged machining times and significant tool wear. Innovative Solution: The development of partially sintered zirconia materials enabled “soft-machining,” allowing easier shaping and requiring a subsequent full sintering step to achieve desired density. Zirconia Powders and Partial Sintering- Source and Composition Raw Material: Primary zirconia particles are derived from zircon sand (ZrSiO4) found in coastal sand deposits. Powder Suppliers: Most zirconia powders are preprocessed and supplied by a few global chemical companies, like Tosoh Corp. Zirconia Powders and Partial Sintering- Synthesis of ZrO2 Nano powders Preferred Method: The coprecipitation route is favored for its simplicity and cost-effectiveness, involving: Fusion with Alkali Oxides: Typically, sodium hydroxide (NaOH) to yield sodium zirconate (Na2ZrO3) and sodium silicate (Na2SiO3). Acid Leaching: Hydrochloric acid removes silica and NaCl, resulting in zirconium oxychloride (ZrOCl2). Stabilization: Mixing with yttrium nitrate (Y(NO3)3) at controlled ratios for desired yttria stabilization. Zirconia Powders and Partial Sintering- Production of Partially Sintered Blanks Pressing Technique: Involves a double-stage pressing process (uniaxial followed by isostatic). Granule Formation: Primary zirconia nanoparticles mixed with approximately 3 wt.% sintering additives create spherical granules via spray drying. Slurry Preparation: Water-based slurries enhanced with pH-increasing solutions, dispersants, and binders ensure proper viscosity for effective spraying. Drying Process: The slurry is sprayed into a hot air chamber (~150 °C) to evaporate moisture, forming an outer shell around granules that can influence sintering quality. Zirconia Powders and Partial Sintering- Effects of Processing on Properties Influence of Granule Size: The size distribution of spray-dried powders and pressing parameters significantly impact the mechanical properties of the fully sintered material. Partial Sintering Process: Involves long-duration firing at lower temperatures (900–1000 °C), enabling neck formation without complete densification, resulting in a "white-body" material with roughly half the density and one-tenth the Young's modulus of fully sintered zirconia. Phase Diagram and Crystal Polymorphs- Sintering and Crystal Structure High-Temperature Behavior: Pure ZrO2 can achieve dense monolithic structures at high temperatures, but upon cooling, undergoes significant structural changes, leading to fragmentation due to phase transitions. Phase Diagram and Crystal Polymorphs- Sintering and Crystal Structure Allotropic Changes: ZrO2 can exist in three allotropes—monoclinic (m), tetragonal (t), and cubic (c)—with a critical volume change of approximately 4.5% during the transformation from t to m-phase at 1170 °C. Fig. The three allotropes of ZrO2. (a) Monoclinic (ICSD #26488, Space group P21/c), (b) tetragonal (ISCD #90884, Space group P42/nm.), and (c) cubic (ICSD #89429, Space group Fm-3 m). Phase Diagram and Crystal Polymorphs- Stabilization of Zirconia Yttria-Stabilized Zirconia (YSZ): Alloying Agents: Extensive Most relevant in dentistry, research focused on stabilizing particularly with 3 mol% Y2O3, zirconia by adding oxides like which stabilizes the MgO, CaO, CeO2, and Y2O3 to microstructure predominantly in retain the tetragonal or cubic the tetragonal phase at room phases under thermal stress. temperature. Phase Diagram and Crystal Polymorphs- Atomistic Description of Stabilization Mechanism: The stabilization mechanism is believed to involve reducing overcrowding of oxygen anions around Zr4+ ions, leading to increased stability by creating oxygen vacancy sites, particularly with trivalent ions like Y3+. Phase Diagram and Crystal Polymorphs- Phase Diagram Analysis Yttria Content Effects: Varying Y2O3 concentrations yield different microstructures: 1.5–3 mol% Y2O3: Achieves fully metastable tetragonal grains. >8 mol% Y2O3: Forms fully stabilized zirconia's (FSZ) characterized by cubic phases. Phase Diagram and Crystal Polymorphs- High-Temperature Sintering Effects Y3+ Segregation: High temperatures can cause Y3+ ions to segregate, destabilizing the parent phases and complicating the microstructure. Translucent Zirconia's Challenge of Opacity: Conventional 3Y-TZP zirconia is limited in aesthetic applications due to its opacity, necessitating advancements to improve light transmittance. Generational Classification: Zirconia's are classified into generations based on improvements in translucency: First Generation: 3Y-TZP with ~0.25 wt.% Al2O3. Second Generation: Reduced Al2O3 content with minimal gains in translucency. Third Generation: Increased stabilization to 5 mol% Y2O3, aiming to enhance cubic phase content, which has better light transmittance due to its lattice symmetry and larger grain sizes, reducing light scattering at grain boundaries. Trade-offs in Mechanical Properties- Strength vs. Translucency The transition to more translucent zirconia's often results in decreased mechanical strength, leading to a “trade-off ” between optical and mechanical properties. Increased amounts of less anisotropic phases reduce fracture toughness due to the lower volume of transformable t-phase. Microstructural Strategies Nanometric Grain Sizes: Reducing grain size to the nanometric range (≤100 nm) is a promising approach to enhance translucency by minimizing light scattering. This requires overcoming challenges in powder processing and sintering techniques. Graded Zirconia's: Innovations include graded zirconia's with varying Y2O3 concentrations, mimicking the natural transition of enamel to dentin in color and translucency. Mechanical Properties- Mechanical Performance in Dentistry The widespread use of zirconia in dental and orthopedic applications is attributed to its excellent mechanical performance, particularly its toughness. Toughening Mechanism: Dental zirconia's benefit from the metastability of the t-phase, which can transform into the stable m-phase under localized stress, creating compressive stresses that hinder crack propagation. Mechanical Properties- Fracture Toughness Measurement Testing Challenges: Traditional methods for measuring fracture toughness can overestimate values due to wide notch radii. Chevron-notched beams (CNB) are recommended for more accurate assessments. Trends in Toughness: The relationship between toughness and Y2O3 content has been characterized, revealing that increasing Y2O3 reduces the amount of transformable t-phase, negatively impacting fracture toughness. Mechanical Properties- Defect Considerations Role of Defects: The mechanical strength of zirconia components is influenced by the size and nature of defects, often arising from powder compaction during manufacturing. Smaller defects can improve overall strength. Granulate Size Distribution: The relationship between defect size distribution and granulate size distribution in powders is critical. Reducing compaction defects through improved processing can enhance strength, especially in smaller constructs. Mechanical Properties- Defect Considerations Mechanical Properties- Surface Treatments and Strength Impact of Sandblasting: Sandblasting zirconia surfaces with alumina particles can enhance bonding for cements and improve strength by inducing t-to-m phase transformation, creating compressive stresses that mitigate the effects of surface defects. Summary  Thank You