Dental Materials and Bonding

• Metals utilized in dentistry require high corrosion resistance and biological inertness.
• Metals offer strength and ductility.

Metallic Bonding

• Metals bond through metallic bonding.

Ceramic Bonding

• Ceramics bond through ionic/covalent bonds between metal and non-metal elements.

Polymer Bonding

• Polymers bond through covalent bonds in long, organic chains.

Semiconductor Bonding

• Semiconductors bond through covalent bonds.

Composite Bonding

• Composites bond through a mixture of two or more materials such as resin-based composites, which include both organic and inorganic components.
• Dental amalgam bonds through a metal alloy and metal alloy mixture.

Lost Wax Casting Technique

• Reproduces a wax pattern in metal, accounting for wax shrinkage, metal casting shrinkage, investment setting expansion, and thermal expansion.

Subtractive Processes

• CAD/CAM is an example of a subtractive process used to produce prostheses.

Phases

• A phase in material science is a structurally homogeneous part of a system with clear physical boundaries.

Cooling Curve of Pure Metals

• Transformation from liquid to solid occurs at a well-defined, discrete temperature.

Crystallization

• Crystallization releases heat, known as the latent heat of fusion.

Unit Cell

• The smallest atomic unit in a crystal, repeated in three directions.

Crystalline Phase

• The local atomic arrangement is repeated at regular intervals in three dimensions, exhibiting long-range order.

Crystal Lattice Systems

• There are seven 3D crystal lattice systems.

Unit Cell Types

• There are different types of unit cells.

Packing Factor

• Calculated as (volume of atoms inside the cube) / (volume of the cube).
• Simple cubic lattice: 0.52
• Body-centered cubic lattice: 0.68
• Hexagonal close-packed lattice: 0.74
• Face-centered cubic lattice: 0.74

Energy States

• Metals seek the lowest energy state to achieve the best packing factor.

Cooling Curve Alloys

• Pure metals have a constant temperature during crystallization at the melting point (Tm).
• Alloys crystallize over a temperature range, starting at T1 and completing at T2.

Phase Diagrams

• Constructed from cooling curves, plotting temperature (Y-axis) against composition of elements (X-axis).
• Indicate composition and types of phases at a given temperature and at equilibrium.
• Define conditions for liquid only, liquid + solid, and solid only phases.

Types of Alloys

• Solid solutions (substitutional and interstitial)
• Intermetallic compounds
• Eutectic mixtures

Solid Solutions

• Metals must be completely miscible, with atomic radii within 15% to fit into the lattice.
• Enhance properties with ductility.

Substitutional Solid Solution

• Solute atoms directly substitute for solvent atoms at normal lattice sites.
• Example: gold/copper alloy at any concentration.

Substitutional Solid Solution Requirements

• Atoms must have similar valency (electronic structure).
• Atoms must have similar crystal structure (e.g., FCC).
• Atom sizes must be within 15% of each other.

Interstitial Solid Solution

• Solute atoms occupy spaces between solvent atoms, distorting the solvent lattice.
• Example: steel with carbon atoms.

Interstitial Solid Solution Requirements

• Solute atoms must be much smaller than solvent atoms (less than 60% in diameter).

Inter-Metallic Compounds

• Formed when two or more metals react at a stoichiometric ratio.
• Example in amalgam: Ag3Sn & Cu6Sn5 phases.

Physical Properties of Inter-Metallic Compounds

• Higher melting point
• Usually more brittle
• Clear, distinct phases

Atomic Diffusion

• Atomic diffusion occurs in the solid state through the exchange of atom lattice positions.
• It is a function of time and temperature.
• Surface welding can occur between metals.
• It’s a known problem in transistor and integrated circuit electronics.

Eutectic Alloy Structure

• Some metals are completely insoluble in the solid phase, resulting in a mixture of pure component grains.

Deformation of Grains

• Results from distortion of lattice structure and slip.
• Slip involves the movement of vacancies and imperfections within the crystal structure, extending to the edge of the structure.

Wrought Structures

• All dental instruments are wrought structures.
• Plastic deformation of metal increases yield strength and hardness.

Creating A Wrought Structure

• Exceed the elastic limit and deform the crystal structure.

Cold Working/Strain Hardening

• Mechanically deforming a metal at relatively low temperature (e.g., machining).
• Deformation in the grains leaves residual stress from the distortion of the lattice structure.

Lattice Impurities and Imperfections

• Reduce the force needed to break an object.

Slip and Plastic Deformation

• (Incomplete information)

Annealing

• Heating a wrought structure enhances atomic diffusion, allowing grain relaxation and growth.
• It’s a diffusion-controlled process, dependent on temperature and time.

Recrystallization and Residual Stress

• More extensively wrought structures with more residual stress in the grains begin recrystallization at a lower temperature.

Endodontic Files Structure

• Highly wrought structures.
• Heat treated for a hard surface that provides wear resistance.
• Inner core is not hardened to allow for flexibility.
• High flexibility

Endodontic Files

• After three turns in a root canal, endodontic files must be discarded to avoid breakage due to increasing brittleness.