Dental Ceramics Microstructure and Properties

Introduction

• Dental ceramics have evolved rapidly since the 1980s.
• CAD-CAM technology has led to a wide variety of dental ceramics.
• Clinicians and technicians should focus on ceramic materials' chemical composition and microstructure, not just brand names.

Importance of Microstructure in Ceramics

• Differences in oxide chemistry, fabrication methods, and particle size can impact mechanical properties like strength, toughness, and wear resistance.
• Not all lithium disilicate ceramics are the same.
• Factors affecting material performance include oxide chemistry, fabrication parameters, oxide ratios, particle size, and firing parameters.
• These factors can lead to differences in the material's phase fractions, crystal type and shape, structural homogeneity, and internal stresses, which affect mechanical performance.

Ceramic Types

• Dental ceramics are classified into three main categories: hybrid ceramics, silicate ceramics, and oxide ceramics.
• Hybrid ceramics consist of partially sintered glass or zirconia/alumina particles infiltrated with a polymer or molten glass.
• Silicate ceramics are a mix of SiO2-rich glass and crystals, which can be crystallized or added separately for reinforcement.
• Oxide ceramics are glass-free, polycrystalline materials like alumina and zirconia.

Fabrication and Processing Techniques

• Fabrication involves creating objects from raw materials.
• Fabrication methods include partial sintering, full sintering, and glass-ceramic processing.
• Processing involves transforming or refining materials into their final form.
• Processing methods include powder layering.

Persistence of Cracks

• Cracks induced during pre-crystallization can persist through subsequent crystallization, affecting overall material performance and reliability.
• Suprinity® PC and Celtra® Duo demonstrate low Weibull modulus, indicating significant variability in strength due to thermal incompatibility and the presence of cracks.
• Obsidian® shows improved mechanical properties post-crystallization, attributed to phase transformation and increased thermal stability.

Crystallization and Fracture Toughness

• A linear relationship exists between the degree of crystallization and fracture toughness.
• Higher crystallized volume fractions lead to better toughness performance.
• Stoichiometric 2SiO2·Li2O glass exhibits improved toughness as crystallization increases from 0% to 100%.
• Li2Si2O5 content materials have minimal thermal mismatch effects, leading to compressive residual stresses that don't harm structural integrity.
• High Li2SiO3 content materials are prone to mechanical failures due to significant thermal incompatibilities.

Toughening Mechanisms

• Crack deflection, where crystalline phases cause cracks to change direction, absorbing energy and reducing crack propagation.
• Crack branching, where energy dissipation occurs as cracks branch off, increasing toughness.
• Crack bridging, where crystalline structures span cracks, preventing further growth and enhancing load-bearing capacity.
• Materials with elongated crystals exhibit R-Curve behavior, meaning toughness increases with crack extension.

Pressable Materials

• Local crystal orientation in pressable materials like lithium disilicates can significantly enhance fracture resistance.
• Aligning crystal bundles perpendicular to the crack propagation direction can increase toughness by up to 25%.

Machinability Considerations

• Machinable two-step 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 reduce strength by up to 50% in materials like feldspathic ceramics.
• Polishing can help recover some strength, but requires standardization for effective clinical application.
• Improper polishing can introduce new defects.

Healing Mechanisms

• Crystallization healing occurs when the glassy matrix flows into cracks during high-temperature heating.
• Capillary forces facilitate healing during crystallization.
• Pre-crystallization polishing is crucial to minimize surface defects that can act as fracture initiation sites.

Clinical Implications

• Surface integrity is critical, as poorly managed surfaces can become weak points leading to premature failure.
• High porosity in pressable materials can create critical defects, compromising mechanical strength.

Zirconium Dioxide

• Zirconia ceramics are heralded as a key achievement of CAD-CAM technology in dentistry.
• Zirconia's introduction required new techniques capable of machining pieces from monolithic prefabricated blanks.
• Traditional methods used for shaping materials included casting, injection molding, heat pressing, or slip-casting with glass infiltration.

Challenges with Zirconia

• Hard Machining: Fully sintered zirconia is difficult to machine due to prolonged machining times and significant tool wear.

Phase Diagram and Crystal Polymorphs

• High-temperature behavior: Pure ZrO2 can achieve dense monolithic structures at high temperatures.
• Upon cooling, ZrO2 undergoes significant structural changes, leading to fragmentation due to phase transitions.
• Allotropic Changes: ZrO2 exists in monoclinic (m), tetragonal (t), and cubic (c) allotropes.
• Critical volume change: A significant volume change of approximately 4.5% occurs during the transformation from the t to m-phase at 1170 °C.

Stabilization of Zirconia

• Alloying Agents: Oxides like MgO, CaO, CeO2, and Y2O3 are added to stabilize zirconia and retain the tetragonal or cubic phases under thermal stress.
• Yttria-Stabilized Zirconia (YSZ): Particularly relevant in dentistry is 3 mol% Y2O3, which stabilizes the microstructure predominantly in the tetragonal phase at room temperature.

Atomistic Description of Stabilization

• The stabilization mechanism is believed to involve reducing overcrowding of oxygen anions around Zr4+ ions.
• The reduction in overcrowding leads to increased stability by creating oxygen vacancy sites, particularly with trivalent ions like Y3+.

Phase Diagram Analysis

• 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.

High-Temperature Sintering Effects

• Y3+ ions can segregate at high temperatures, destabilizing the parent phases and complicating the microstructure.

Translucent Zirconia

• Conventional 3Y-TZP zirconia is opaque, limiting its aesthetic applications.
• Advancements have been made to improve light transmittance.
• Generational Classification: Zirconia is 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 to enhance cubic phase content for better light transmittance, due to its lattice symmetry and larger grain sizes.

Trade-offs in Mechanical Properties

• The transition to more translucent zirconia's often results in decreased mechanical strength.
• Increased amounts of less anisotropic phases reduce fracture toughness due to the lower volume of transformable t-phase.

Microstructural Strategies

• Reducing grain size to the nanometric range (≤100 nm) enhances translucency by minimizing light scattering.
• Graded zirconia's with varying Y2O3 concentrations mimic the natural transition of enamel to dentin in color and translucency.

Mechanical Properties

• Zirconia’s excellent mechanical performance, particularly its toughness, has led to its widespread use in dental and orthopedic applications.
• The metastability of the t-phase creates compressive stresses that hinder crack propagation.
• Traditional methods for measuring fracture toughness can overestimate values due to wide notch radii.
• Chevron-notched beams (CNB) are recommended for more accurate assessments.
• Increasing Y2O3 content reduces the amount of transformable t-phase, negatively impacting fracture toughness.

Defect Considerations

• 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.

This quiz explores the evolution of dental ceramics, emphasizing the importance of chemical composition and microstructure. Participants will learn about various types of dental ceramics and the factors affecting their mechanical properties. Understanding these elements is crucial for clinicians and technicians in modern dental practice.