Metallurgy Quiz: Precious Metals

Metals and Alloys

• Precious metals are high in the electromotive series, resistant to corrosion, and chemically stable. Examples include gold (Au), platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir), and osmium (Os). Silver (Ag) is considered noble but not precious.
• Precious metals resist oxidation, tarnish, and corrosion during heating, casting, and soldering.
• The carat/karat and fineness express the gold content of an alloy. Carat is a part of the total weight (24K is pure gold). Finess is the number of pure gold parts per 1000 parts of alloy (1000 is pure gold).
• ADA (American Dental Association) in 1977 adopted percentage expressions.

Methods of Processing

• Cold working includes deep drawing (metal disc into a cup), stamping (metal cup into a dental crown)
• Heat working comprises melting and casting, melting and soldering and melting and welding.
• Thermal treatments improve the properties of cold-worked alloys.
• Galvanoplating and pulverization create metallic casts and models.

Classification

• Classifications by chemical composition arrange elements in descending order by percentage (e.g., Au-Ag-Pt, with Au 78%, Ag 12%, Pt 10%).
• Noble alloys have a high gold content.
• Non-noble alloys have reactive metals like Ni-Cr, Co-Cr, titanium, and iron (stainless steels).
• ADA (1981, revised 1984) established high noble (HN), noble (N), and predominantly base (PB) alloy categories based on noble metal percentages (>60%, ≥25%,<25%, respectively) and gold content (≥40%).

Crystal Structure

• Metal cooling in a liquid state creates crystals.
• Atoms in a crystal have specific arrangements in a three-dimensional lattice.
• Cubic (simple), face-centered cubic (FCC), and body-centered cubic (BCC) are common atomic arrangements.
• Crystallization nuclei form as the metal cools.
• Crystals grow from the nuclei as dendrites (branched network structures).
• Dendritic crystals contact with each other but retain irregular shapes rather than perfect shapes.
• Homogeneous nucleation forms crystals from uniform material, leading to smaller, more uniform grains.
• Heterogeneous nucleation uses impurities as crystal formation sites resulting in more numerous, smaller grains).
• Grain boundaries between crystals are less ordered and act as barriers to dislocation movement but favour fracture propagation. Greater concentration of grain boundaries comes with smaller grain size.
• Reducing grain size increases yield strength but decreases ductility, and enhances fracture toughness. Conversely, larger grains reduce yield strength, increase ductility, and decrease fracture toughness.
• Fine grain structure is useful for partial denture frameworks owing to their strength and resistance to deformation.

Crystal Structure of Alloys

• An alloy is a mixture of two or more metals.
• During cooling, metal atoms can randomly occupy positions in crystals.
• Solid solutions are formed if metals are soluble with each other. Irregular solid solution means random atom positions and regular solid solution indicates specific atom positions in the lattice. Interstitial solid solution has metal atoms in interstitial spaces of the crystal lattice.
• Insoluble solid solutions form separate crystals of each metal in an alloy with different zones (phases). These phases can lead to corrosion.
• Partially soluble alloys have some mutual solubility of metal atoms into their respective crystals. These alloys have identical lattice structures, similar atomic radii, and the same valence electrons.

Intermetallic Compounds

• Atoms of two metals have a strong affinity, forming intermetallic compounds, high bonding in them, low crystal imperfection, leading to reduced dislocation possibilities.
• These compounds result in brittle alloys.

Practical Applications

• Impurities influence alloy properties which influence grain size and shape
• Fine grain size alloys are harder to deform, but partial dentures frequently use these alloys due to hardness and strength.
• Diverse aspects of microstructure are influenced by processing methods, impurities, components, and factors like homogeneity.

Deformation

• Plastic deformation occurs when the limit of elastic deformation is exceeded, by slip of atoms to new positions. In plastic deformation, atoms of the crystal lattice slip resulting in the structure being permanently altered. The movement of dislocations leads to ductility of a material (where the material can be easily bent without breaking).
• Materials with FCC(face-centered cubic), BCC(body-centered cubic) form ductile alloys. Intermetallic compounds form brittle alloys (due to strict atomic position requirements).
• Hardening happens through cold working (repeated deformation and movement of dislocations.

Release Internal Stress

• Annealing (heating material to a specific temperature and cooling slowly), reduces internal stress within metal, and increases ductility.
• Hardening by heat treatment involves slow cooling of the molten alloy in an oven (at specified temps to improve the strength and stiffness of dental bridges and frameworks).

Thermal Treatment of Homogenization

• Rapid cooling (quenching) creates heterogeneous grains in an alloy.
• Heating at recrystallization temperature homogenizes the composition and reduces susceptibility to corrosion.

Imposed Conditions

• Price, biocompatibility, mechanical properties, handling, and adhesion are important in selecting alloys.

Biocompatibility

• Potential side-effects for patients include oral galvanism, irritation, allergies (especially with Ni), and lichen planus (with Cu).
• Potential side-effects for technicians include carcinogenesis and toxic effects (Be, Cr, Ni, and Al).

Mechanical Properties

• Stiffness is important for dental bridges (post and core requirements), as well as for frameworks. Factors include the modulus of elasticity, which measures stiffness.
• Ductility, the ability to undergo plastic deformation, is important in applications like cast clasps and inlays.
• Hardness is an indicator of resistance to mechanical working.
• Strength is linked to yield strength or resistance to permanent deformation/yielding.

Casting

• Range of melting temperatures (solidus and liquidus) is key for casting.
• Alloy density (high density, easier to cast, less risk of defects) and coefficient of thermal contraction (high values lead to undersized castings) influence success.

Adhesion

• Materials used in metal-ceramic restorations form bonds (specifically, metal-ceramic, metal-acrylic, and metal-composite alloys).
• The application of suitable bonding agents and techniques ensures the structural integrity of the restoration.

Noble Dental Alloys

• High corrosion resistance, biocompatibility are notable properties of noble alloys.
• Type I-IV gold alloys have specified gold percentages for different clinical applications.

Role of Alloy Elements

• Copper (Cu) increases stiffness and over 16% impacts tarnish and corrosion
• Zinc (Zn) reduces alloy porosity and enhances hardness, but reduces ductility in alloys by forming a stable Zn-metal layer.
• Ruthenium or Indium promote fine grain structure.

Properties of Alloys

• Homogenization heat treatments, hardening treatments, with elements like copper above 12% in alloys to improve properties of Type III and IV. Homogeneity in solid solution improves mechanical properties and reduces corrosion.
• Platinum and palladium increase melting range which may hinder homogenization, and needs heat treatment to reduce heterogeneity. High heat treatment reduces thermal expansion coefficient.
• Alloys are selected based on properties and treatment methods (homogenization, hardening) that reduce or remove their defects.

Medium and Low-Gold Alloys

• Palladium (Pd) reduces Ag's tendency to tarnish. Copper (Cu) allows for homogenization heat treatments.
• Gold content below 20% and containing silver (with high quantities of Pd, Ag), yields medium and low gold alloys.
• High variability in properties exists impacting clinical outcomes depending on alloy composition.

Noble Alloys Based on Ag-Pd

• Similar properties as Type III noble alloys, with hardening heat treatments, lower density than gold alloys.
• Ag and Pd have affinity for oxygen during casting which creates porous casting, reducing strength and increasing ductility, decreasing hardness and corrosion strength. Ag-Pd has ratio 3:1.
• Application criteria for inlay, crowns, post and cores, and reduced dental bridges.

Noble Alloys Based on Pd-Ag

• Similar properties as Type IV noble alloys
• High advantages in yield strength and better corrosion performance compared to Ag-Pd alloys. However, also more susceptible to gas absorption.
• Non-toxic and cost-effective, thus applicable to many MCA.

Non-Noble Alloys

• Non-noble alloys generally have low contents of gold, silver, platinum, and palladium.
• Biocompatibility is reduced compared to noble materials.
• High thermal shrinkage, high casting temperatures, and difficulty with mechanical working (due to thermal conductivity) are issues.
• These alloys frequently have different characteristics in relation to various applications.

Alloys Based on Co-Cr

• Co comprises 55-65%. Cr is 25-30% and higher percentages of Cr lead to improved biocompatibility and higher corrosion strength.
• The amount of Mo does not exceed 4%.
• Nickel(Ni) is 0-30%: promotes higher ductility, but increases the risk of allergic reactions.

Properties of Alloys based on Ni-Cr

• Intermediate hardness between Au type IV and Co-Cr.
• Lower ductility compared with Au Type IV.
• Reducing the thickness of metal coping improves adhesion with ceramic restoration (for MCA).
• Low density with light casting defects.
• Materials such as phosphate-bonded materials, semi/automatic spin casting are compatible with the given alloys.
• Suitable applications to metal-ceramic restorations, polymeric-to-metal restorations, bridges with more than 3 elements.

Titanium and Titanium Alloys

• Extracted Titanium (Ti) has 4 degrees of purity, differentiated by the oxygen and iron content.
• Titanium and titanium alloys are used in dentistry for strong components, often with components such as Aluminum (Al) and Vanadium (V) to achieve stable alloys.
• A- hex phase (low melting temperature) to B-body-centered cubic (high melting temperature) transition improves mechanical properties (by heating above 882 °C).
• Both a and B phases are important in the overall mechanical properties of titanium alloys.

Casting of Titanium Alloys

• Titanium melting point (1670°C) leads to high shrinkage and reactivity.
• Inert gas atmospheres and investment materials are used in casting.
• Forming a layer of a-case at the surface with high hardness and improved biocompatibility helps reduce the issues of surface reactivity.
• Specialized investment materials and processing strategies address associated problems (e.g., porosity).

Alloys for Metal-Ceramic Technique

• Achieving a bond stronger than the oxides' cohesive forces ensures superior metal-ceramic restorations.
• Noble alloys (high gold, Ag-Pd, Pd alloys) and non-noble alloys (Ni-Cr-Be, Co-Cr) are suitable.
• CpTi and titanium based alloys can be used as well; high melting temperatures in conjunction with ceramic types are important for these applications.

Imposed Conditions

• Physical considerations include the solidus and sintering temperatures of metal-ceramic and ceramic materials, important to prevent defects in applications. Thermal expansion coefficients need to be considered for optimal function.
• Chemical factors in metal-ceramic applications include the strong bond formation between metal and ceramic (a layer of metal oxides), corrosion resistance in the alloy/ceramic structure, and colour stability.
• Mechanical properties involve the high modulus of elasticity and hardness of alloys to resist forces and stresses from masticatory actions.
• Biological considerations include biocompatibility (lack of cytotoxic or irritant effects) in the alloys.

MCA Composition

• Compositions of various alloys (Au-Pd, Au-Pd-Ag, Pd-Cu, Pd-Ag, Ni-Cr, Co-Cr, cpTi, Ti alloys) based on gold (and related compounds) are tabulated. These tables offer a concise compilation of elemental contents for different applications.

MCA Properties

• Properties of different metal-ceramic alloys, including hardness, modulus of elasticity, yield strength, and density, are summarized and tabulated for easy reference. Such information is helpful in selecting the appropriate alloy for the desired applications.

Criteria for Choosing Alloys

• Important criteria (e.g., casting accuracy, biocompatibility, tarnish, mechanical strength, hardness) are compared with less important aspects (e.g., ductility, stiffness) for selecting the best alloy.
• Table with different clinical applications, and important and less important properties for choosing the optimal alloy are also included.