Properties of Ceramics

Functional ceramics in the sense of components of electronic or

electric devices such as capacitors, piezo ceramics, chip carders, insulating

housings, spark plugs, ceramic ball valves, ceramic pumps, etc., are prepared by thin film techniques or extrusion

processes, sometimes followed by glazing, yielding suitable surface roughness

and sufficient accuracy in final dimensions. Grinding and polishing operations

are usually not requested as an additional finishing step. Therefore, this

class of ceramics will not be treated further in the following paragraphs.

Structural ceramics, however, which have to sustain external loads and to fit

into a mechanically active construction consisting of a large variety of

different materials, e.g. an engine, must strictly meet the desired final

dimensions and surface qualities to guarantee the requested properties in service with sufficient reliability and life time.

Since hardness, stiffness (Young's modulus), toughness, and strength

are the most important mechanical properties of structural ceramics determining

the wear resistance, the goal of this article is to introduce one to the

fundamentals of material-inherent properties as well as of wear mechanisms

and reinforcing strategies which have been applied to technical ceramics.

This is of a particular importance because grinding and polishing (i. e., mechanical

material removal during shaping) of ceramics which have been

especially optimized to resist material removal (i.e., wear in service) is

accordingly difficult. These conflicting properties, case in machining and

simultaneous resistance in service, are surprisingly not yet regarded by the

material developers nor by the manufacturing engineers.

Additionally, a basic understanding will be developed to enable the

reader to choose suitable material combinations for appropriate applications

and to understand the difficulties in manufacturing but also the risks and

origins of failure during service live. Besides parts of structural ceramics,

grinding grits or small cutting tools suffer basically from the same problems

and can therefore be strengthened by the same methods. Another goal of this

article is, however, to show the chances and the limits of a future materials

development.

WEAR MECHANISMS OF CERAMICS MATERIALS

Because of their partially covalent and partially ionic chemical

bonding, ceramics are extremely hard and corrosion resistant and therefore

excellent wear resistant materials at both room temperature and high

temperatures. One important limiting factor is, however, their inherent brittleness.

High stiffness, high hardness, and consequently the brittleness, are

based upon the little deformability of the crystal lattice in contrast to metals

and polymers. At low temperatures, strain energy in the vicinity of a crack tip

cannot be released by dislocation movement or creep. In comparison to

metals, the activation energy for the movement of dislocations is so high that

the ultimate fracture strength is by far exce~ed. As the crystal structures of

ceramic possesses lower symmetries compared to metals, even an increase of temperature closest to a melting point does not result in the activation of more

than two or three dislocation slip systems. Therefore, the plastic deformability

remains poor which means that the brittleness and also the high

hardness persists to high temperatures. Talking in terms of stress-strain

relationships, the linear elastic range of the stress-strain curve is terminated

by immediate catastrophic fracture releasing the entire stored elastic strain

energy. This is in particular the case if the stored elastic strain

energy exceeds the work of fracture required for the formation of a new crack

surface or if at a tip of a preexisting crack or microstructure inhomogeneity

tensile stresses are accumulated in the order of the theoretical strength of the

material.