Figure 1.1.
Market forecast for high-performance ceramics.
(Courtesy, Hoechst 1988)
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Not in all cases, however, have ceramics been able to meet the sometimes extraordinarily high demands of the applying industry. The progress in understanding the particular influence of the manufacturing procedures to the microstructure and mechanical properties was slower than expected. The market did not develop as projected due to the lack of reliability of the ceramic parts and due to problems in its acceptance by construction engineers. Furthermore, the request for high quality products led to high-cost raw materials and products which had some time to compete with metals or even with polymers. Thus, some strategic investments by big companies came too early and turned out risky, especially in Europe, but the competition with Japan and the United States, as the two most important providers of advanced ceramics, was severe. Imports from Japan where part development and production was strongly supported and funded by the government, were sometimes preferred to imports from the European providers.
Today (1994-96), the worldwide economic problems govern the entire market. The exponential increase in market demand for high-tech ceramics is stopped, even in Japan (Figure 1.2). New machining techniques for shaping sintered parts to final dimensions, however, have significantly lowered the costs of structural parts. On the other hand, more accurate analyses of the mechanical properties being really requested for ceramics in automotive engines show a clearly lower level of performance being necessary than aimed at before, hence the dramatically decreasing costs in raw materials, processing, and final machining. Together with new fields of application (e.g. tools for semiconductor fabrication, Figure 1.2) these facts bring about new prospects for high-tech ceramics in the near future, because they are still what they have been designed for: key materials of a modern technology.
Figure 1.2.
Fine ceramic market development in japan as it is.
(Courtesy, Hitachi Metals Inc., 1996)
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One field of application that developed with an increasing intensity, as was predicted, is related to the excellent wear behavior of ceramics: the application as cutting tools and grinding grits. In the last decades, ceramic grinding and cutting tools initiated a strong impact to the manufacturing technology of metals. New turning and milling machines were developed; these required high hardness and toughness materials that were capable to work at very high feed rates, speed, and therefore at high temperatures yielding smooth surfaces free of damage. Numerically controlled manufacturing techniques, the strong increase in process reliability, and quality reproducibility were made possible by especially developed alumina and silicon nitride ceramics. The most important step towards high performance ceramics was the basic understanding of fracture initiating mechanisms and strategies to minimize the material-inherent brittleness.
Functional ceramics in the sense of components of electronic or electric devices such as capacitors, piezo ceramics, chip carriers, insulating housings, spark plugs, 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, ease 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.
1.2. 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 exceeded. 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 (Figure 1.3). 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.