Basic requirements for dental alloys
High-quality prosthetic material is always a strict balance between biological safety and technical reliability. All requirements are divided into two main groups:
- General medical properties. The metal must not have a toxic effect on the tissues and fluids of the oral cavity and must be hypoallergenic.
- Technological properties. These determine the quality of the dental technician's work and the lifespan of the prosthesis. The alloy must possess high corrosion resistance, strength, and hardness. During casting, low shrinkage, high fluidity, and a low melting point are valued. In addition, the material must be amenable to soldering, welding, and electrolytic polishing.
Key physical and mechanical characteristics
To ensure that the prosthetic appliance can withstand masticatory pressure, the following parameters are taken into account:
- Strength — the ability to resist the action of external forces without failure.
- Elasticity — the ability to completely restore the original shape after the load is removed (elastic deformation).
- Plasticity — the ability to retain a modified shape after the load is removed without the appearance of cracks and fractures.
- Hardness — the resistance of a material to the penetration of a harder object into it.
If the load exceeds the elastic limit, plastic (residual) deformation occurs. It is irreversible and leads to negative consequences: a change in the metal's structure, a decrease in its density, and an increase in electrical resistance.
Alloy groups for metal-ceramics
In global dental practice, more than a hundred variants of compositions are used to manufacture frameworks for ceramics or glass-ceramics. Globally, they are classified as follows:
- Noble alloys: based on gold, palladium, and silver. They are distinguished by benchmark chemical inertness and excellent casting properties. The main drawback is that they are inferior to non-noble analogs in strength and resistance to deformation.
- Base-metal alloys: based on cobalt, chromium, nickel, and iron. They are cheaper and have better mechanical characteristics. However, working with them is more difficult: their melting point is approximately 500 °C higher, they conduct heat less well, have lower fluidity, and greater chemical reactivity.
- Semi-noble alloys: intermediate compositions with a low gold content.
- Titanium-based alloys: stand out in a separate group due to their unique lightness and exceptional biocompatibility.
Influence of alloying components
The properties of the final product directly depend on which elements are added to the base. For example, in classic stainless steel and popular dental alloys, elements function as follows:
| Element | Effect on alloy properties |
|---|---|
| Chromium | Provides corrosion resistance. |
| Nickel | Imparts plasticity, enhances toughness and malleability. |
| Palladium | Increases heat resistance, reduces segregation (inhomogeneity), and forms a protective PdO film. |
| Molybdenum | Creates a fine-grained structure, increasing overall strength. |
| Manganese | Lowers the melting point and removes sulfur compounds. |
Casting and aesthetics of prostheses
Casting is a key method for obtaining shaped parts by pouring liquid metal into pre-prepared molds (based on wax patterns). The main enemies of precise casting are shrinkage (volume reduction upon cooling) and segregation (uneven distribution of chemical elements).
To make the prosthesis look aesthetic, various methods are used. A budget option is the application of protective and decorative coatings (titanium nitride compounds) in a vacuum. They are bioinert and wear-resistant, but visually cannot imitate a real tooth. To achieve ideal aesthetics, metal-ceramics are used: a strong metal framework is joined with porcelain mass by firing in a vacuum furnace.
Briefly: answers to topical questions
What requirements are imposed on alloys specifically for the manufacture of metal-ceramic prostheses?
The following requirements are imposed on alloys for the manufacture of metal-ceramic prosthetic frameworks:
- Softening temperature — must be higher than the firing temperature of porcelain.
- Bonding — the ability to bond with porcelain is necessary.
- Thermal compatibility — good thermal compatibility with the ceramic coating is necessary; for high adhesion strength, the expansion coefficients of the metal and ceramics must be as close as possible. The coefficient of linear thermal expansion for Ni-Cr and Co-Cr alloys must be (13.0–14.5)×10⁻⁶ °C.
- Physical and mechanical properties — satisfactory strength and casting properties, durability, and stability of properties are required.
- Chemical properties — corrosion resistance and absence of toxicity are necessary.
What types of casting shrinkage of dental alloys exist?
Available sources do not provide an exact classification of the types of casting shrinkage of alloys, but indicate the forms in which it manifests.
- Linear shrinkage — reduction in the linear dimensions of a body.
- Volumetric shrinkage — reduction in the volume of a body.
These changes occur during the cooling, solidification, and storage of the metal. The magnitude of shrinkage directly depends on the chemical composition of the alloy, its degree of heating, and the chosen cooling method.
To easily remember the roles of additives in steel alloys, use the mnemonic rule: Chromium - Preserves (against corrosion), Nickel - Tenderness (imparts plasticity and malleability), Molybdenum - Might (provides strength).
Class and exam questions
Why is the coefficient of thermal expansion so critical for metal-ceramics?
If the metal and ceramics expand differently upon heating and contract upon cooling, stress will arise between them, and the brittle porcelain will simply crack. The coefficients must be as close as possible.
Why is palladium added to dental alloys?
Palladium is significantly cheaper and lighter than gold, while making the alloy heat-resistant and chemically stable. In the aggressive environment of the oral cavity, it is protected by a durable oxide film.
What is the danger of plastic deformation for a prosthesis?
It irreversibly changes the shape of the product, disrupting the marginal fit. In addition, the internal structure of the metal changes: its density drops and electrical resistance increases, which can provoke galvanism in the oral cavity.
What else to study on this topic
- Stages of dental prosthesis casting using lost-wax techniques
- Vacuum deposition technology for titanium nitride coatings
- Dental polymeric materials and their classification
- Application of plasticizers, stabilizers, and cross-linking agents in dentistry
- Polymerization features: thermoplastics and thermosets
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