Introduction to materials science

The science of dental materials has a relatively short history, spanning approximately 300 years. The emergence of dental materials science as a formal discipline dates back to 1728, when Pierre Fauchard (1678–1761) published a book on materials and methods of their applications in dentistry, although there is archeological evidence on the practical use of materials in the oral cavity even before the Common Era. In the early years of dentistry, dental prostheses were quite rare, demanding exceptional skills for processing precious and semiprecious alloys (gold, silver) or manual processing of animal bones, shells, tusks. Dental prostheses were made approximately, «by eye» with repeated fittings in the mouth. It was not until 1721 that Matthaus Gottfried Purmann, the city doctor of Breslau, proposed the innovative idea of taking impressions of the jaws as a preliminary step in the production of artificial teeth. Various materials were used to fill tooth cavities, from cotton fabrics and molten lead to gold foil. The introduction of amalgam marked a turning point, initiating scientific research in the field of restorative dentistry.

Modern dental materials science delves into a broad spectrum of topics concerning the production of materials and their clinical application. For example, in prosthodontics, a variety of processes are used, including modeling, dental impressions, pressing, casting of metal alloy denture parts, polymerization of plastics, brazing, ceramic and plastic coatings. All this requires the dentist not only to know the technology of denture production and the correct use of equipment, but also to understand how specific technological processes affect the properties and quality of the material. Deviations from technology when working with a material can lead to a decrease in its strength and the appearance of unfavorable properties that can affect the oral organs and the body as a whole.

Dental materials science is the science that studies the relationship between the composition, structure, properties, manufacturing and use of materials for dentistry, as well as the patterns of change in the properties of materials under the influence of various factors. All materials used in dentistry can be categorized based on their chemical composition into the following groups: inorganic salts, ceramics, metals and alloys, polymers. The polymer-based materials include composites, elastomeric materials and plastics. The classification of materials is presented in Fig. I.1.

Materials used in dentistry have physical, chemical, mechanical, technological and biological properties.

Physical properties are based on the laws of mechanics, acoustics, optics, thermodynamics, electricity, magnetism, radiation, atomic structure, and nuclear phenomena. Phenomena of light related to its perception (color, saturation, and brightness) are based on the laws of optics. Thermal conductivity, heat capacity and thermal expansion are based on the laws of thermodynamics.

Chemical properties are based on the ways in which substances interact, combine, and change, as determined by their external orbital electrons. External electrons are responsible for bonding of atoms in molecules and for the electrical, thermal, optical, and magnetic properties of solids. The properties of some materials are presented in Fig. I.2.

It is not always possible to strictly distinguish the physical, chemical, and mechanical properties of materials, so complex concepts are often employed to characterize different materials: physicomechanical and physicochemical properties.

Physiomechanical properties

Physicomechanical properties include density, thermal conductivity, electrical conductivity, heat capacity, coefficient of thermal expansion, melting point, boiling point, crystallization/recrystallization temperature, phase transformation, surface tension, rheological and optical properties, color, adhesive properties, strength, hardness, viscosity, elasticity, plasticity, brittleness, stretching, compression, bending, torsion, and others under functional loads.

Density is defined as the mass of a substance per unit volume, typically measured in grams per cubic centimeter (g/cm³), it depends on temperature, aggregate state of the substance and external pressure. Thus, for instance, as the pressure increases, the distance between the molecules of a substance decreases, therefore, the density becomes higher. Conversely, elevated temperatures generally cause an expansion of the distances between molecules, resulting in a decrease in density.

Examples of density of substances are presented in Table I.1.

Thermal conductivity (K) is the ability of a material to transfer thermal energy from hotter areas to cooler ones through the chaotic movement of particles (atoms, molecules, electrons, etc.). It is measured by the amount of heat per second that passes through a material sample that is 1 cm thick and has a cross-sectional area of 1 cm² with a temperature difference of 1 °С (or 1 K) at the ends of the sample. The unit of thermal conductivity in the international system is watt per Kelvin degree (W×m⁻¹×Κ⁻¹). Generally, thermal conductivity increases as follows: polymers → ceramics → metals, although there may be exceptions (see Table I.1).