Ceramic Processing:

Processing of ceramic materials describes the way in which ceramic objects (e.g., glass windows, turbocharger rotor blades, optical fibers, capacitors) are produced.

Processing begins with the raw materials needed to produce the finished components, and includes many individual steps that differ significantly depending on the type of ceramic material, crystalline versus glass.

Processing of Crystalline Ceramics Glass Processing

Raw Material Selection Raw Material Selection

Preparation Melting

Consolidation Pouring

Sintering Annealing

Raw material selection involves obtaining and preparing the right materials for the final product. Traditional ceramics use various forms of clay. Glass makers start with primarily silica. Advanced ceramics use several different raw materials depending on the applications (i.e., properties needed).

Material Uses

Al2O3 (aluminum oxide) Spark-plug insulating bodies, substrates for microelectronic packaging

MgO (magnesium oxide) electrical insulators, refractory brick

SiO2 (Silicon dioxide) cookware, optical fibers

ZrO2 (zirconium oxide) cubic zirconia, oxygen sensors

SiC (silicon carbide) kiln parts, heating elements, abrasives

Si3N4 (silicon nitride) turbocharger rotors, piston valves

For crystalline ceramics, the characteristics of the raw materials (powders) such as their particle size and purity are very important as they affect the structure (e.g., grain size) and properties (e.g., strength) of the final component. Since strength increases with decreasing grain size, most starting powders are milled (or ground) to produce a fine powder (diameter < 1 µm). Since dry powders are difficult to shape, processing additives like water, polymers, etc. are added to improve their plasticity.

Consolidation involves forming the ceramic mixture into the specified shape. There are many techniques available for this step:

Figure 11: Ceramic processing aides.

Sintering is the final step in the process. Sintering at high temperatures (800˚ to 1800˚ C) causes densification that gives the ceramic product its strength and other properties. During this process, the individual ceramic particles coalesce to form a continuous solid network and pores are eliminated. Typically, the mictrostructure of the sintered product contains dense grains, where an individual grain is composed of many starting particles.

Figure 12: Microstructure of raw, formed, and sintered ceramic products

Glass processing is different from crystalline processing. One of the considerations that must be examined is the solidifying behavior of glass. Glasses are most commonly made by rapidly quenching a melt. This means that the elements making up the glass material are unable to move into positions that allow them to form the crystalline regularity. The result is that the glass structure is disordered or amorphous.

One of the most notable characteristics of glasses is the way they change between solid and liquid states. Unlike crystals, which transform abruptly at a precise temperature (i.e., their melting point) glasses undergo a gradual transition. Between the melting temperature (Tm) of a substance and the so-called glass transition temperature (Tg), the substance is considered a supercooled liquid. When glass is worked between Tg and Tm, one can achieve virtually any shape. The glass blowing technique is a fascinating demonstration of the incredible ability to deform a glass.

Figure 13: Specific Volume vs. Temperature graph for a typical ceramic material

Glass processing does not require an optimum size particle (although smaller pieces melt faster). The selections of glass raw materials and chemical additives (which, for example, can alter the color of the glass) are heated up (700˚ - 1600˚ C), melted and finally poured onto or into a quick-cool form or plate. There are four different forming techniques used to fabricate glass.

Technique Application

Pressing Table ware

Blowing Jars

Drawing Windows

Fiber forming Fiber optics

During the glass formation, there may be stresses that have been introduced by rapid cooling or special treatments that the glass needs (such as layering or strengthening). Additional heat treatment is needed to “heal” the glass. Annealing, in which the glass is heated to the annealing point (a temperature just below the softening point where the viscosity is approximately 108 Poise) and then slowly cooled to room temperature, is one such process. Tempering is also a follow-up heat treatment in glass processing in which the glass is reheated and cooled in oil or a jet of air so that the internal and external parts have different properties. The tempering reduces the tendency of glass to fail. Tempered glass can then be used in conditions prone to stresses like car windows.