Metals & Alloys in Dentistry - Dr Alex Cresswell-Boyes (2024-25) PDF

Material Science of METALS & ALLOYS Dr Alex Cresswell-Boyes [email protected] Learning Objectives  Explain fundamental metallurgical principles, including crystal structure, phases, and alloying, as they relate to dental materials.  Relate these principles to the mechanical...

Material Science of METALS & ALLOYS Dr Alex Cresswell-Boyes [email protected] Learning Objectives  Explain fundamental metallurgical principles, including crystal structure, phases, and alloying, as they relate to dental materials.  Relate these principles to the mechanical and physical properties of dental metals and alloys.  Classify dental alloys based on composition, structure, and application.  Differentiate between noble and base metals and understand the advantages and limitations of each category. Outline  Introduction.  Metallurgic principles.  Mechanical & physical properties.  Classifications.  Commonly used metals & alloys.  Considerations. Introduction to Metals & Alloys  Metals: Elements that readily lose electrons to form positive ions and have metallic bonds.  Alloys: Mixtures of two or more metals, or metals with non- metals, designed to have enhanced properties. Photo: © Ruitai Mould 2024  Metals and alloys are essential materials in restorative and Why are pure metals rarely used alone in dental applications? prosthetic dentistry. Pure metals often lack the necessary strength, hardness, or corrosion resistance required for dental applications; alloys enhance these properties to meet clinical demands. Characteristics of Metals  Physical Properties  Lustre: Shiny appearance due to the reflection of light.  Conductivity: High thermal and electrical conductivity.  Malleability and Ductility: Can be shaped and stretched without breaking.  Chemical Properties  Reactivity: Tendency to lose electrons and form cations.  Corrosion: Susceptibility to Photo: © Brainard 2024 oxidation and degradation over time. Metallic Bonding  Delocalised Electrons: Valence electrons are not bound to any specific atom.  "Sea of Electrons": Allows atoms to slide over each other, contributing to malleability.  Impact on Properties Photo: © Chemistry Student 2024  Conductivity: Free electrons enable electrical and thermal How does metallic bonding contribute to conduction. the adaptability of dental appliances during  Ductility and Malleability: Atoms fitting? Metallic bonding allows for the deformation of metal can move without breaking bonds. components without fracturing, enabling adjustments to dental appliances for a better fit. Introduction to Alloys  Definition and Purpose  Alloys: Combination of metals to achieve desired properties not found in pure metals.  Enhancement of Properties: Strength, hardness, corrosion resistance.  Types of Alloys  Substitutional Alloys: Atoms of similar size replace each other. Photo: © Brunning 2015  Interstitial Alloys: Smaller atoms fit What is the significance of atomic size in substitutional into the spaces between larger versus interstitial alloys regarding their mechanical metal atoms. properties? In substitutional alloys, similar-sized atoms maintain ductility, while interstitial alloys with smaller atoms increase hardness and strength by distorting the lattice and hindering dislocation movement. Why use alloys in dentistry?  Limitations of Pure Metals  Softness: Pure metals like gold are too soft for certain applications.  Corrosion Susceptibility: Pure metals may corrode easily.  Benefits of Dental Alloys  Improved Mechanical Properties: Increased strength and durability.  Tailored Physical Properties: Adjusted melting points, thermal expansion.  Enhanced Biocompatibility: Reduced allergenicity and improved tissue response. Photo: © Brunning 2015 Common Metals in Dentistry  Gold (Au)  Properties: Excellent malleability, corrosion resistance.  Use: High noble alloys for restorations.  Silver (Ag)  Properties: High thermal conductivity, ductility.  Use: Component in some dental alloys.  Titanium (Ti) Photo: © Smart 2024  Properties: High strength-to-weight ratio, biocompatibility.  Use: Dental implants, orthodontic wires. Common Dental Alloys  Amalgams  Composition: Alloy of mercury with silver, tin, and copper.  Use: Dental fillings (less common today).  Gold Alloys  Composition: Gold with copper, silver, palladium.  Use: Crowns, bridges, inlays.  Base Metal Alloys  Composition: Nickel-chromium, cobalt- Photo: © Ezza Dental Care 2022 chromium alloys.  Use: Partial denture frameworks, orthodontic wires. Fundamental Metallurgical Principles  Crystal Structure  The ordered arrangement of atoms in a metal.  Affects mechanical properties like ductility and strength.  Phases  Different structural forms of a material, each with distinct physical properties.  Predicts how alloys behave under different conditions. Photo: © Iowa State University 2024  Crystal Structures in Metals Photo: © GGS DT 2024 Body-Centred Cubic (BCC) Face-Centred Cubic (FCC) Hexagonal Close-Packed (HCP)  Structure: Atoms at each corner  Structure: Atoms at each  Structure: Hexagonally and one in the cube’s centre. corner and one at the arranged atoms.  Example: Chromium. centre of each face.  Example: Titanium.  Properties: High strength, lower  Example: Gold.  Properties: Good strength, ductility.  Properties: High ductility moderate ductility. and malleability. Phases and Phase Diagrams  Single-Phase Alloys: Uniform structure; consistent properties.  Multi-Phase Alloys: Mixture of phases; can enhance strength through mechanisms like precipitation hardening.  Phase Diagrams  Visual representation of phases at different temperatures and compositions.  Key Points:  Eutectic Point: Lowest melting point of an alloy system. Photo: © Warren 1997  Solidus and Liquidus Lines: Boundaries between solid and liquid phases. Phases and Phase Diagrams Photo: © van Noort & Barbour 2023 Alloying in Dentistry  Purpose of Alloying  Enhance Mechanical Properties: Strength, hardness.  Improve Corrosion Resistance: Essential for longevity in the oral environment.  Adjust Melting Temperatures: For casting and fabrication processes.  Common Alloying Elements  Palladium (Pd): Increases hardness and strength.  Platinum (Pt): Raises melting point.  Silver (Ag): Improves ductility.  Copper (Cu): Adds strength but can reduce corrosion resistance. Photo: © Deloro 2024  Zinc (Zn): Acts as a deoxidiser. Relationship Between Metallurgy and Properties  Crystal Structure Impact  FCC Metals: More ductile (e.g., gold), easier to shape.  BCC Metals: Stronger but less ductile (e.g., chromium).  Phase Influence  Single-Phase Alloys: Generally, more ductile.  Multi-Phase Alloys: Can be stronger due to phase interactions.  Alloying Effects  Solid Solution Strengthening: Solute atoms hinder dislocation movement. Photo: © van Noort & Barbour 2023  Precipitation Hardening: Fine particles impede dislocations, increasing strength. Classification of Dental Alloys  By Composition  High Noble Alloys: ≥60% noble metals, ≥40% gold.  Noble Alloys: ≥25% noble metals.  Base Metal Alloys: