Materials science is an interdisciplinary field that studies the relationships between the internal structures, properties, processing methods, and performance of solid materials. At its core, it aims to identify the constituent materials that form a structure and understand their properties. This field not only seeks to better comprehend existing materials but also strives to design new materials through various processes for human use. At the heart of materials science lies the accurate establishment of the structure–property relationship of a material. Once this relationship is understood, the performance of a material in a specific application can be predicted and improved.

Materials science acts as a bridge between basic sciences such as physics and chemistry and engineering applications. The structure of materials is examined on multiple scales, starting from the atomic level and extending to larger scales such as crystals, phases, and grains. These examinations help explain the causes of mechanical, thermal, electrical, optical, and magnetic properties of materials. A raw material without a defined shape is transformed into a new and functional material through a shaping or processing stage known as "processing." Therefore, materials science is built upon the structure–property–processing–performance framework.

History

The origins of materials science date back to the earliest periods of human history. The production of ceramics is considered one of the oldest applications in the field. However, its emergence as a modern scientific discipline is closely linked to the development of metallurgy and materials engineering. Significant advances were made in the 19th century as the thermodynamic properties of materials became better understood. During this period, studies on the behavior of metals and alloys laid the groundwork for materials science. Until the 1960s, courses in this field were largely taught under the name of metallurgy. After this point, with the increasing importance of non-metallic materials such as polymers and ceramics, the scope of the field expanded and evolved into a more inclusive discipline called “materials science and engineering.” This transformation enabled the field to adopt a holistic approach encompassing all material classes, not just metals.

Structure of Materials

The properties of a material largely depend on how its atoms are arranged. Materials science examines this arrangement across different scales.

Atomic Structure and Interatomic Bonds

At the core of everything lies the structure of the atom. The way electrons are distributed in orbitals around the atomic nucleus—known as electron configuration—determines how an atom interacts with other atoms. Valence electrons, which are the electrons in the outermost shell, play a key role in the formation of interatomic bonds. When two atoms come close to one another, the attractive and repulsive forces between them reach a balance, and a stable bond forms at this equilibrium distance. These bonds define the fundamental properties of a material. The main types of bonds include metallic, ionic, and covalent bonds (primary bonds), as well as Van der Waals bonds (secondary bonds)^【1】. The type of bond directly affects how atoms are arranged in the solid state.

Crystalline and Amorphous Structures

Based on the atomic arrangement in solid materials, two primary structures can be distinguished: crystalline and amorphous.

• Crystalline Structure: Metals and many ceramic materials exhibit a crystalline structure in which atoms are arranged in a regular and repeating three-dimensional pattern. The smallest repeating unit of this structure is called the "unit cell." This regular atomic arrangement gives materials specific characteristics, such as anisotropy—properties that vary depending on direction. In crystalline materials, directions and planes serve as important references for understanding mechanical behavior.
• Amorphous Structure: In materials like glass (a type of ceramic) and many polymers, atoms do not form a regular structure. These materials lack long-range atomic order and are referred to as "disordered" or "amorphous." This structural irregularity causes such materials to exhibit isotropic properties, meaning their characteristics are independent of direction.

Classes of Materials

Materials are generally categorized into three main classes based on their atomic bonding and structure: metals, ceramics, and polymers.

Metals

Metals consist of one or more metallic elements held together by metallic bonds. In this bonding structure, valence electrons are free to move, which gives metals high electrical and thermal conductivity. They typically possess a crystalline structure. This structure allows atomic planes to slide over one another, making metals ductile (easily shaped) and tough. In addition to pure metals, alloys—combinations of two or more elements—also belong to this class. Alloys often exhibit superior mechanical properties compared to pure metals. They can exist in the form of solid solutions or intermediate compounds such as intermetallics, interstitial compounds, or electron compounds.

Ceramics

Ceramics are compounds typically formed between metallic and non-metallic elements. Their atoms are bonded through ionic and/or covalent bonds. Due to these strong bonds, ceramics are hard, have high compressive strength, and exhibit excellent resistance to high temperatures and chemical attack. However, they are also brittle. Ceramics can be either crystalline (e.g., alumina) or amorphous (e.g., glass). They are widely used, ranging from traditional ceramics (clay, porcelain) to advanced ceramics used in applications such as engine components and electronic devices.

Polymers

Polymers are composed of long chains formed by repeating units of small molecules called "monomers," connected by covalent bonds. They are typically organic compounds with carbon atoms forming the backbone of the molecular structure. Organic molecules such as alkanes and alkenes form the building blocks of polymers. Due to the weak secondary bonds (Van der Waals forces) between polymer chains, polymers generally have low density, are flexible, and possess low mechanical strength. However, by modifying the structure and length of the molecular chains, a wide range of material properties can be achieved.

Crystal Structure Defects

Although a crystal structure is theoretically considered perfect, in reality, it contains various types of defects. These structural imperfections significantly affect the properties of a material, especially its mechanical strength and electrical conductivity. Defects are classified based on their size:

• Point Defects: These are atomic-scale defects that affect a single lattice point. Examples include vacancies (missing atoms), interstitial atoms (atoms squeezed between regular lattice positions), and substitutional atoms (foreign atoms replacing host atoms).
• Line Defects (Dislocations): These are one-dimensional defects occurring along a line in the crystal lattice. The two main types are edge dislocations (where an extra half-plane of atoms ends within the crystal) and screw dislocations (where atomic planes shift around a helical axis). The motion of dislocations is the primary mechanism behind plastic deformation (permanent shape change) in metals.
• Planar Defects: These are two-dimensional surface defects. The most common are grain boundaries, which separate regions (grains) with different crystal orientations. Grain boundaries impede dislocation motion, thereby strengthening the material. Other types of planar defects include twin boundaries, stacking faults, and phase boundaries that separate regions of different phases.