Base Metal Alloys in Dentistry

• Base metal alloys are alternatives to gold alloys, introduced due to the increasing cost of gold in the 1930s
• Base metal alloys are derived from heavy metals in the periodic table

Types of base metal alloys

• Cobalt-chromium alloy

• Cobalt-chromium-nickel alloy

• Nickel-chromium alloy

• Titanium and titanium alloys

• Cobalt-chromium and cobalt-chromium-nickel alloys are used for removable dental prosthesis frameworks

• Nickel-chromium alloys are used in ceramic-metal restorations

Cobalt Chromium Alloys

• Major elements constitute ~90% of the alloy's weight
  • Cobalt: ~35-65% by weight
  • Chromium: ~28-30% by weight
  • Nickel: ~0-30% (can be used interchangeably with Cobalt)
• Minor elements constitute ~10% of the weight
  • Molybdenum: ~3-6% by weight
  • Silicon and Manganese: act as deoxidisers
  • Carbon: ~0.4% by weight

Nickel Chromium Alloys

• Major elements constitute ~90% of the alloy's weight
  • Nickel: ~70-80%
  • Chromium: ~12-20%
• Minor elements constitute ~10% of the alloy's weight
  • Molybdenum: ~3-6% by weight
  • Aluminum: ~2-6% by weight
  • Silicon and Manganese: ~0.02-0.5% by weight
  • Beryllium: ~0.5% by weight

Commercially Pure Titanium and Titanium Alloys

• Titanium offers resistance to electrochemical degradation, light weight, low density, low modulus, and high strength
• Titanium forms a stable oxide layer, which facilitates corrosion resistance and biocompatibility
• Commercially pure titanium is available in four grades, based on oxygen (0.18-0.4%) and iron content (0.2-0.5%)
• Titanium has polymorphic forms: alpha (α) phase and beta (β) phase
  • At room temperature, it exists as α phase (closed packed hexagonal)
  • When heated above 883°C, it transforms into a stronger, more brittle β phase (body-centered cubic)
• Aluminum is an α stabilizer, while copper, palladium, or vanadium are β stabilizers
• Commercially pure titanium (Cp Ti) is used for dental implants, surface coatings, crowns, partial removable dental prostheses, and orthodontic wires
• The most common titanium alloy for dental purposes is Titanium 6 Aluminum 4 Vanadium

Role of Constituent Elements in Base Metal Alloys

• Chromium, cobalt, and nickel make up about 85% of the total weight, but their effect on physical properties is limited
• Minor alloying elements like carbon, molybdenum, tungsten, manganese, nitrogen, tantalum, gallium, and aluminum control the physical properties

Cobalt

• Increases strength, modulus of elasticity, and hardness

Chromium

• Provides resistance to tarnish and corrosion via a passive layer of chromium oxide
• A minimum of 12% chromium is required for the passive layer
• A maximum of 30% chromium is the limit of solubility; additional chromium would produce a brittle sigma phase

Nickel

• Increases strength, modulus of elasticity, and hardness, similar to cobalt but to a lesser degree
• Nickel is responsible for the ductility of the alloy
• Nickel may cause allergic reactions in some patients; nickel-free Co-Cr alloys advised in such cases

Molybdenum

• Acts as a grain refiner, increasing strength

Silicon, Manganese, and Tungsten

• Act as deoxidizers, increasing the fluidity of the molten alloy and improving castability
• Tungsten increases density and improves castability

Carbon

• Combines with other alloying elements to form carbides that solidify last during cooling and appear at grain boundaries
• Fine precipitation of carbides can increase strength and hardness
• A change in carbon content by 0.2% can significantly affect the alloy's properties
• Discontinuous carbide formation at grain boundaries is preferable to continuous formation

Aluminum

• Aluminum and nickel form nickel aluminide (Ni3Al), increasing strength

Beryllium

• Lowers the melting range by 100°C and improves fluidity, improving castability; also increases strength
• Beryllium vapor is carcinogenic and may lead to fibrosis of the lungs; most alloys are now beryllium-free

Titanium

• Provides resistance to tarnish and corrosion due to its passivity
• If the film is scratched, the area repassivates rapidly (within nano-seconds)

Biocompatibility

• Determines the biological response and the ability to resist degradation or corrosion
• It is related to the passive layer of chromium oxides (in chromium-containing alloys) and titanium oxides (in titanium alloys)

Health Hazards

• Nickel allergy: 5-10 times higher incidence in females (5-8% sensitivity)
• Beryllium vapor: Carcinogenic to dental technicians

Physical properties

Resistance to Tarnish and Corrosion

• Excellent due to the passive layer

Color

• Lustrous silvery white when properly finished and polished

Fusion Temperature

• 1300°C - 1500°C for cobalt-chromium and nickel-chromium alloys
• ~1700°C for commercially pure titanium; titanium alloys may have lower melting temperatures

Sag Resistance

• Because of high melting temperature, there is less potential for sag during porcelain firing

Density

• Affects castability; high-density alloys accelerate faster into the mold
• Base metal alloys require higher casting force due to their low density
• Cobalt-chromium and nickel-chromium alloys: 7-8 gm/cm3
• Commercially pure titanium and its alloys: 4.5 gm/cm3

Coefficient of Thermal Expansion and Contraction

• Affects investment materials and compatibility with porcelain; should contract slightly more than porcelain
• Cobalt-chromium and nickel-chromium alloys: 12-14 x 10-6/°C
• Commercially pure titanium and its alloys: 8.4 x 10-6/°C

Mechanical properties

Modulus of Elasticity

• Affects stress distribution and restoration thickness
• Cobalt-chromium alloys: 250 x 103 MPa
• Nickel-chromium alloys: 200 x 103 MPa
• Commercially pure titanium: 102 x 103 MPa
• Modulus of elasticity of commercially pure titanium is comparable to gold alloys
• Modulus of elasticity of titanium alloys is comparable to base metal alloys

Yield Strength

• Cobalt-chromium and nickel-chromium alloys: 500-600 MPa
• Commercially pure titanium: 170-480 MPa
• Titanium alloys: 500 MPa

Ultimate Tensile Strength

• Cobalt-chromium and nickel-chromium alloys: 600-800 MPa
• Commercially pure titanium: 240-550 MPa
• Titanium alloys: 800 MPa

Vickers Hardness Number

• 200-350 V.H.N.
• Hardness affects finishing/polishing ease and ability to maintain surface finish
• Mechanical properties of commercially pure titanium are similar to gold alloy type III and IV
• Mechanical properties of titanium alloys are similar to cobalt-chromium and nickel-chromium alloys

Casting

• Technique-sensitive; any variable affects microstructure and properties
• Avoid picking up carbon or changing its distribution during manipulation

Casting of Cobalt Chromium and Nickel Chromium Alloys

• Casting should be done in a well-controlled vacuum under argon to avoid nitrogen incorporation
• Investment: Carbon-free phosphate or silicate investment with vents

Melting

• Oxyacetylene flame
  • Oxygen/acetylene ratio must be adjusted to avoid oxidation or increased carbide precipitation
• Electric melting induction
• Melting should be done in a quartz crucible

Casting Machine

• Centrifugal casting machine provides adequate driving force due to low density

Cooling

• Bench cooling

Finishing and Polishing

• Difficult due to high hardness, but alloys retain polished surface during service
• No pickling, as HCl acid can attack the passive layer

Finishing and Polishing Steps

• Sand blasting: Mechanical smoothening to remove investment
• Electrolytic polishing: Restoration placed in the anode position for a smooth surface

Recasting

• Cannot be recast due to carbide precipitation and uncontrolled properties; gold alloys can be recast easily

Microstructure

• Large grains with cored dendritic structure with intergranular and interdentritic carbides
• A precipitated phase of Ni3Al can be identified in nickel-chromium alloys

Heat treatment:

• Cobalt-chromium alloys cannot be heat-treated to improve mechanical properties
• Nickel-chromium alloys' properties can be altered by heat treatment
• Heating 15 minutes at 1500°C followed by water quenching may be used to increase the workability
• Subsequent heating (15 minutes at 1005°C followed by water quenching) will increase toughness of dental casting

Uses of alloys

• Cobalt-chromium alloys are suited for denture frameworks due to high modulus of elasticity (250 GPa), allowing thinner sections while maintaining rigidity
• Nickel-chromium alloys are best used in crown and bridge work due to increased ductility and workability

Casting of Titanium

• Difficult due to low density, high melting temperature, and high reactivity

Challenges

• Low density requires high casting force; requires advanced casting techniques
• High melting temperature requires special investment material and melting method
• High reactivity with gases at high temperatures requires controlled vacuum or inert atmosphere

Investment

• Phosphate or silicate-bonded investment with stable oxides
• Magnesia-based investments are substituting silica-based investments to reduce titanium interfacial activity

Melting

• Electric melting induction

Alloys are cast by a specially designed casting machine

• Cooling: Bench cooling

Finishing

• Difficult due to high hardness, but alloys retain polished surface during service
• No pickling, as HCl acid can attack the passive layer

Finishing and Polishing Steps:

• Sand blasting: Mechanical smoothening to remove investment
• Electrolytic polishing

Recasting

• Titanium alloys cannot be recast due to casting difficulties

Microstructure

• Coarse plate-like α grains having parallel orientation in β matrix (lamellar microstructure)

Heat Treatment

• Lamellar microstructure can be refined by heating slightly above the β transition temperature (1000ºC – 1500ºC) and aging at temperature (800ºC – 900ºC)
• This improves mechanical properties
• Shaping of titanium is done by machine milling and spark erosion

Uses for Titanium Alloys

• Removable partial denture framework
• Crown and bridge
• Dental implants