Brittle Ceramics

Metallic Versus Ceramic/Brittle Materials Recovery... 

So why aren t today s engines made of ceramics The short answer is that, unlike metals, ceramics cannot bend and deform to absorb impacts. Intense research is currently under way to solve the problem of ceramic brittleness, with some success. Improved resistance to fracturing, for example, can be attained by careful quality control of starting materials and processing. As we shall see in the next section, brittleness can also be combated by compositing ceramics with other materials. 

Some substrates must be heated for deposition. For example, large ceramic (brittle) substrates must be evenly heated to avoid large temperature gradients, which could result in fracture, across their surface. Many coating materials and phases also require a warm or hot substrate to deposit. This characteristic is related to the surface diffusion and thermodynamics of the materials. A material s dependence on a heated substrate may be only for the deposition itself to achieve a dense, continuous film or for the deposition of a particular phase or morphology. For example, many materials require a hotter substrate to form a crystalline film as opposed to an amorphous film. 

In ceramics, brittle fracture is controlled by the extension of small flaws which are dispersed in a material or component s surface, and which behave like cracks. Flaws can arise not only from the production process, but also from handling and service. Some examples of critical flaws are shown in Figures 12.1 and 12.7. 

For glass or ceramics, brittleness and eventual consequences of thermal shock treatment... 

A study of Vickers hardness of polycrystalline ceramics revealed that cracking may cause critical transition points in the Vickers ISE trends. The transition point was associated with extensive cracking in and around the indentation and a shift in the energy balance during indentation. Different ratios of the indentation work are expended on volumetric deformation and surface fracture processes above and below the transition point. The transition point was very distinct for brittle materials such as silicon carbide. The Vickers hardness transition point was related to a new index of ceramic brittleness defined as ... 

The identification between ceramic and terra cotta gathers together the basic concepts that we will continue to encounter throughout this book powdery mineral raw materials [RIN 96], the shaping which is made possible by the plasticity of wet clay, the heat treatments which start by drying (reversible dehydration) and continue with firing (irreversible dehydration and permanent physicochemical modifications). We have not yet mentioned in the description a major characteristic that conditions the preparation techniques as well as the uses of ceramics brittleness. The flower pot is hard (it can scratch a metal sheet) but is vulnerable to impact. This brittleness is a hydra with many heads, as it implies ... 

V. D. Frnchette, Failure Analysis of Brittle Materials, Advances in Ceramics, Vol. 28, The American Ceramic Society, Inc., Westervike, Ohio, 1990. 

HTS materials, because of their ceramic nature, are quite brittle. This has introduced problems relative to the winding of superconducting magnets. One solution is to first wind the magnet with the powder-in-tube wire before the ceramic powder has been bonded and then heat treat the desired configuration to form the final product. Another solution is to form the superconductor into such fine fila-... 

Limiting the use of brittle metals such as ceramic or porcelain. Although, avoiding the use of such materials in some cases may not be practical, as in the case of lightning arresters, bushings and insulators used in an HT switchgear, instrument and power transformers and reactors. 

While the structure/property behavior of numerous shock-recovered metals and alloys has received considerable attention in the literature to date, the response of ceramics, cermets, and other brittle solids (including geological materials) to shock loading remains poorly understood [9], The majority of shock-recovery studies on brittle materials have concentrated on examining... 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

These requirements severely limit our choice of creep-resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aero-engines because they are far too brittle (they are under evaluation for use in land-based turbines, where the risks and consequences of sudden failure are less severe - see below). Cermets offer no great advantage because their metallic matrices soften at much too low a temperature. The materials which best fill present needs are the nickel-based super-alloys. 

As with metals, the number of different ceramics is vast. But there is no need to remember them all the generic ceramics listed below (and which you should remember) embody the important features others can be understood in terms of these. Although their properties differ widely, they all have one feature in common they are intrinsically brittle, and it is this that dictates the way in which they can be used. 

Ceramics, without exception, are hard, brittle solids. When designing with metals, failure by plastic collapse and by fatigue are the primary considerations. For ceramics, plastic collapse and fatigue are seldom problems it is brittle failure, caused by direct loading or by thermal stresses, that is the overriding consideration. 

Fig. 18.1. If small samples are cut from a large block of o brittle ceramic, they will show o dispersion of strengths because of the dispersion of flaw sizes. The average strength of the small samples is greater than that of the large sample.

A more complicated, and more effective, mechanism operates in partially stabilised zirconia (PSZ), which has general application to other ceramics. Consider the analogy of a chocolate bar. Chocolate is a brittle solid and because of this it is notch-sensitive notches are moulded into chocolate to help you break it in a fair, controlled way. Some chocolate bars have raisins and nuts in them, and they are less brittle a crack, when it... 

Ceramics cannot be bolted or riveted the contact stresses would cause brittle failure. Instead, ceramic components are bonded to other ceramic or metal parts by techniques which avoid or minimise stress concentrations. 

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