8 Ceramics, Glasses, and Composites

Material categories and structures

Ceramics and glasses are inorganic, nonmetallic materials, while composites are deliberately made by combining distinct constituents. These categories can overlap: a ceramic-matrix composite is both ceramic-based and composite. Their structures help explain their different properties and uses.

Ceramic structure and behavior

Ceramics are generally crystalline solids made from metallic and nonmetallic elements, often as oxides, carbides, or nitrides. Their atoms are held together by strong ionic and/or covalent bonds. Many engineering ceramics are polycrystalline, meaning they consist of small crystals called grains, separated by grain boundaries.

These strong bonds help ceramics resist high temperatures, wear, and chemical attack. They are commonly hard, stiff, and strong in compression, but usually have little ability to deform plastically. A crack or flaw can concentrate stress and grow suddenly, so many ceramics are brittle and vulnerable to impact or thermal shock. Strength in service can depend strongly on flaws and microstructure.

Glass structure and processing

Glass is also inorganic and nonmetallic, but it is generally amorphous: its atoms lack the repeating, long-range order found in crystals. Common silica-based glasses can be transparent, chemically resistant, and electrically insulating. Like many ceramics, glass is hard but brittle, and surface flaws can initiate cracks under tension.

Glass is shaped while softened by heat, for example by blowing, drawing, or forming. Controlled cooling and later strengthening treatments can affect its performance. A glass-ceramic is made by forming glass and then inducing controlled crystallization, resulting in crystalline regions within a residual glassy phase. Its properties depend on its composition and microstructure.

How composites are structured

A composite combines two or more distinct materials that remain identifiable as different phases. Most composites have a matrix that surrounds and binds a reinforcement such as fibers or particles. The matrix transfers load and protects the reinforcement, while the reinforcement contributes properties such as stiffness or strength.

Composite performance depends on the materials selected, their arrangement, and the quality of the interface between them. Its properties are not simply the average of its ingredients. Poor bonding, voids, or damage can weaken the matrix–reinforcement interface and cause separation or failure.

Tailoring composite properties

In glass-fiber-reinforced polymer, strong glass fibers are embedded in a polymer matrix. When fibers are aligned in one direction, the composite can be especially strong and stiff along that direction, while its properties across the fibers may be lower. This direction-dependence is called anisotropy. A particle-reinforced composite may have more similar properties in different directions, depending on how its particles are distributed.

Some composites use ceramic fibers in a ceramic matrix. These ceramic-matrix composites are designed to retain useful performance at high temperatures while reducing the catastrophic brittleness of a monolithic ceramic. Fiber bridging and controlled cracking can make a crack harder to propagate. The fiber–matrix interface must be designed carefully: an overly strong bond can cause fibers to break rather than help slow the crack.

Typical properties and applications

Typical characteristics and applications include:

• Crystalline ceramics: Their grains form one or more crystals. Common advantages include hardness, stiffness, wear and chemical resistance, and high-temperature capability. Limitations include brittle fracture and sensitivity to impact and thermal shock. Examples include cutting tools, electrical insulators, and catalytic-converter components.

• Glass: Its structure is amorphous and noncrystalline. It can offer transparency, chemical durability, and electrical insulation, and it can be formed when softened. It is brittle, and flaws can initiate cracks. Examples include windows, containers, and optical fibers.

• Composites: Their structure consists of a matrix and reinforcement, such as fibers or particles. Their properties can be tailored; fiber composites can combine low mass with high stiffness and strength in selected directions. Direction-dependent behavior, interface damage, and manufacturing complexity are limitations. Examples include aircraft structures, sporting equipment, and reinforced concrete.

These properties are typical, not universal. Composition, defects, geometry, and processing affect the performance of a particular product. Ceramic and glass applications also include catalytic converters and optical fibers, respectively; composites are selected when combining materials can provide a useful balance of properties. Fiber composites may be difficult to inspect, repair, recycle, or manufacture consistently.

Choosing a material

Selection begins with the conditions a part must withstand. A transparent, weather-resistant window may favor glass, while a hot, wear-prone cutting edge may favor a hard ceramic. A lightweight structure needing high stiffness in particular directions may be suitable for a fiber composite.

Designers must consider failure risk, manufacturing method, cost, inspection, and repair, not just a material’s strongest property. The appropriate choice depends on the required properties and service conditions.