Metal-Ceramic Alloys Quiz

Metals and Alloys

• Precious metals are high in the normal voltage series, more inert, and thus more resistant to corrosion. This "noble quality" makes them chemically more stable. Examples include Au, Pt, Pd, Ru, Ir, Os. Silver (Ag) is considered noble but not precious.

• Precious metals resist oxidation, tarnishing, and corrosion during heating, casting, and soldering processes.

Terminology

• Precious metals are defined by their intrinsic value. Noble metals are precious metals, but not necessarily the opposite. Non-noble metals are reactive metals that combine easily with their surroundings.

• The carat/karat and fineness express the gold content in an alloy. The carat is a relative measure of gold content (24k = pure gold). Fineness is the amount of pure gold per thousand parts of the alloy (1000 = pure gold).

Methods of Processing

• Cold working involves shaping a metal without heating, e.g., deep drawing, stamping.

• Heat working involves heating a metal for shaping, e.g., melting, casting, soldering, welding.

• Thermal treatments improve alloy properties after cold working.

• Galvanoplating and pulverization are methods to create metallic casts or models.

Classification

• Alloys are classified by chemical composition. Noble alloys include high gold content, and low gold content, with Ag-Pd and Pd as bases for some. Non-noble alloys are based on Ni-Cr, Co-Cr, titanium, and iron (stainless steel).

• ADA (1981, 1984) classification distinguishes high noble, noble, and predominantly base alloys, determined by the percentage of noble metals in the alloy.

• ADA specification #5 for high noble alloys defines Types I-IV based on gold content, and their suitability for inlays, onlays, crowns, and partial dentures. This classification also considers casting temperature.

Crystal Structure

• Metal solidification forms crystals during cooling from the liquid state.

• Atoms within a crystal have a specific arrangement. This repetitive structure forms a three-dimensional crystal lattice (cubic, face-centered cubic, body-centered cubic).

• Crystallization occurs with the formation of nuclei (centers). Developing crystals form grains. Crystal growth is dendritic, creating a branching network emanating from the nucleation centers.

• The crystals contact each other but don't necessarily have a perfect geometric form (like a cube or sphere).

• Homogeneous nucleation relates to uniform crystal growth with a rapid cooling rate leading to higher nucleation centers and smaller grains. Heterogeneous nucleation is affected by impurities that act as agents for crystallization, which also results in smaller grains.

• Grain boundaries form restrictions to dislocation movement, but can promote fracture propagation. Smaller grain sizes lead to higher grain boundary area. Large grains have fewer boundaries and encourage fracture.

Practical Applications

• Fine grain properties are preferred in situations where enhanced strength (higher Young's modulus) and fracture toughness are needed. Fine grain (small and numerous grains) alloys are frequently used in partial denture frameworks.

• Larger grains (few grains) exhibit high ductility and low fracture toughness

Crystal Structure of Alloys

• Alloys are mixtures of 2 or more metals.

• During cooling, metals' atoms randomly occupy lattice positions, creating a solid solution.

Solid Solution

• Metals dissolve into each other.

• Solid solutions can be irregular (atoms randomly disposed) or regular (specific positions within the lattice) or interstitial (atoms of one metal are positioned within the interstices of the other metal).

Insoluble Solid Solution

• Metals form separate crystals.

• Alloys with two or more phases are prone to electrolytic corrosion (more when the metals' potentials differ).

Partially Soluble Solid Solution

• The solubility of the metals depends on their structures and atomic radius and valency. Similar characteristics increase solubility

Intermetallic Compounds

• Atoms with strong affinity form stable compounds

• These compounds have fewer crystal defects, exhibiting decreased ductility. They are typically brittle.

Practical Applications of Impurities

• Useful impurities can improve grain size for specific applications (alloys with fine grains).

• Insoluble impurities create defects. Oxides and air inclusions create defects that can accelerate corrosion and promote fractures.

### Deformation

• Elastic deformation is reversible and proportional to applied force.

• Plastic deformation is permanent. The removal of force does not bring a return to the original shape

• Hardening through mechanical and heat treatments influences crystal lattice structure, altering strengths and ductilities.

Casting

• Melting range varies across alloy types, including factors like solidus and liquidus temperatures. Narrow ranges signify homogeneous crystallization.

• The casting coefficient of thermal contraction impacts restoration sizing. High values mean risks of undersized restorations.

Adhesion

• Adhesion impacts metal-ceramic and metal-acrylic applications. Also relevant to cement in metal-ceramic scenarios.

Noble Dental Alloys

• Properties of noble alloys include high corrosion strength and biocompatibility.

Noble Alloys; Indications

• Types of noble alloys (I-IV) are indicated for specific restorations (inlays, onlays, crowns, and partial dentures), and dependent upon hardness and ductility requirements.

The Role of Elements in Alloy Composition

• Elements in alloy compositions affect stiffness, tarnish resistance, casting porosity, and hardness

Properties

• Homogenization treatments, hardening treatments, and alloys' composition are considered crucial.

Medium and Low-Gold Alloys

• These alloys have a lower gold content than high-noble alloys. Pd and Cu elements are typically added to counteract tarnishing tendencies or for improvements in homogenization treatment outcomes. Higher variations in properties are typical across this class.

Noble Alloys based on Ag-Pd

• These alloys are similar to high-noble alloys, and are prone to porosity issues related to the oxygen affinity of Ag and Pd. The ratio of Ag to Pd helps to lessen corrosion risk.

Noble Alloys based on Pd-Ag

• These alloys have high yield strength, improved corrosion resistance, and are reasonably inexpensive. Some disadvantages include potential colour change and susceptibility to porosity issues.

Non-Noble Alloys

• These alloys lack Au, Ag, Pt, and Pd. General properties include reduced biocompatibility, significant contraction, high casting temperatures, and diminished ductility.

Alloys based on Co-Cr

• Co and Cr compositions create a solid solution with high corrosion resistance. Mo, Ti additions impact ductility and reduce allergic reactions.

Alloys based on Ni-Cr

• Ni and Cr form an alloy used for many dental restorations that feature good corrosion resistance and hardness. Other additions affect ductility and susceptibility to allergic responses.

Titanium and Titanium Alloys

• Titanium's purity is graded according to oxygen and iron content.

• Common titanium alloys in dentistry include Ti-6Al-4V, a notable alloy because of its strength, and fatigue resistance.

Casting

• High thermal shrinkage is characteristic when casting titanium. Casting techniques (e.g., vacuum or inert gas atmosphere) can address these concerns in particular cases.

Alloys for Metal-Ceramic Technique

• Alloys used in metal/ceramic restorations should have a strong bond with ceramics, minimizing issues with ceramic discoloration or fracturing risks.

MCA Composition

• Specific elements and percentage compositions are needed for accurate alloy production.

MCA Properties

• The modulus of elasticity, hardness, and density of different MCA types vary.

Used Criteria to Select Alloys

• Clinical indications (e.g., crowns, bridges, PFM restorations) guide alloy selection

Specific properties including hardness, accuracy, modulus of elasticity, Easy soldering, and biocompatibility are used when choosing suitable alloys for a given use.

Test your knowledge about Metal-Ceramic Alloys (MCA) with this quiz. Explore their thermal compatibility, corrosion resistance elements, and biological conditions. Answer questions regarding their composition and mechanical properties.