Topic

Ceramics and glasses

In other topics we deal fairly comprehensively with polymers and metals, but these aren’t the only materials that are used. Particularly in components, you will come across both ceramics, a group of materials with a wide range of properties, including some with magnetic properties and others that are best described as ‘glasses’.

Some ceramics basics

The word ‘ceramic’ is derived from the Greek for potters’ clay, though the term is now used much more generally, to apply to a wide range of inorganic materials that are generally non-metallic and in most cases have been treated at high temperature at some stage during manufacture.

Ceramics can be classified into four main groups:

The structures of ceramics fall into two main groups:

Generic manufacturing methods

Many engineering ceramics are made from powders, by cold pressing the powder to produce a ‘compact’ which is strong enough to be handled. This compact is then sintered at a temperature high enough to cause fusion of the particle boundaries. The temperatures involved depend on the nature of the ceramic material and whether any ‘glass formers’ are included in the powder. With alumina for example, where materials are typically quoted at 96% or 99% purity, the balance consists of glass-formers, which help give the fired part a smooth surface finish, and reduce its porosity.

Until recently, most ceramic materials consisted of crystalline particles cemented together by glass, but sintering at sufficiently higher temperatures can produce wholly crystalline structures which maintain their strength at elevated temperatures.

Other ways of shaping ceramic products, such as the sheets of material used to make resistors and capacitors, include modifications of processes traditionally used with clay:

Whether extruded, or cast as a sheet, the unfired ceramic has relatively little strength, and needs handling with care. It is, however, possible to carry out operations such as printing precious metal inks, which can be co-fired with the ceramic, and form part of the eventual structure. Any forming or cutting is best carried out while the ceramic is in the ‘green’ (or unfired) state, as the task is easy – you can cut a sheet of green ceramic with a razor blade; once fired, the ceramic will blunt the razor!

Ceramic properties

Ceramics are hard and reasonably strong: Table 1 gives some typical values, from which it can be seen that, weight for weight, alumina is stronger than stainless steel. However, ceramics are more rigid (higher Young’s modulus). More crucially, they have almost no ductility, because of the directional nature of the covalent bonds. Without ductility, stress concentrations are prevented from being relieved by plastic flow, so ceramics tend to fracture readily.

Table 1: Mechanical properties of selected electronic materials (ceramics are shown in red)
material melting point (°C) density
(g/cm)
CTE
(10–6/°C)
Young’s modulus
(GPa)
tensile strength
(MPa)
alumina 2050 3.99 5.8 380 620
aluminium 660 2.70 23.5 69 50–195
aluminium nitride 2400 3.25 5.3 350 270
beryllia 2530 3.01 8.4–9.0 311 172–275
copper 1083 8.96 17.0 180 see footnote 1
nickel 1453 8.9 13.3 199 660
304 stainless steel (annealed)
  8.0 17.2 193 >525
1 The tensile strength of copper depends on its treatment history: typical values are 125 MPa for cast copper (50% elongation), 220 MPa for annealed wrought copper (56% elongation), and 386 MPa for cold-drawn copper (6% elongation).

Internal imperfections such as porosity reduce both strength and ductility. Because most engineering ceramics are compacted from powders, some porosity is inevitable, so most ceramics are very brittle.

Ceramics also tend to suffer from the presence of micro-cracks, which act as stress raisers, and tensile stresses must generally be kept low if sudden failure is to be avoided. Also, because the number of faults will vary from specimen to specimen, in ceramics there can be a much bigger scatter of measured strengths than with metals.

Creep only takes place in crystalline ceramics at relatively high temperatures. However, non-crystalline glasses have low softening temperatures, and considerable creep occurs at moderate temperatures.

Most ceramics are non-conductors of electricity, and have many uses because of this. They are particularly useful because they are reasonable conductors of heat. Table 2 shows alumina to be a very much better conductor of heat than FR-4 laminate, and comparable in performance to leadframe materials and solder. Other ceramics have substantially better characteristics, comparable to metals, but their specification and use is outside the scope of this module.

Table 2: Thermal characteristics of selected electronic
materials (ceramics are shown in red)
material CTE
(10–6/°C)
thermal cond.
(W/(m.K))
diamond 1.7 2300
copper 16.5–17.3 398
beryllia 8.0 275
aluminium nitride2 4.0–4.5 250
aluminium 22.3 237
silicon 2.8–3.2 150
solder (95/5) 28 36
alumina 6.7–7.0 21
kovar 5.9 17
304 stainless steel 16.3 16
FR-4 at T<Tg X, Y 15.8 Z 80–90 1.7
FR-4 at T>Tg X, Y 20 Z 400
polyimides 45 8
2 Aluminium nitride (AlN) is a stable covalent compound with low thermal expansion that makes it useful in electronics as a heat sink material. Unfortunately, the material cannot be sintered in air and exhibits considerable variations in thermal performance due to the presence of oxide in the structure. However, it does not present the same toxic hazards as beryllia.

Alumina has other uses within electronic packaging, because it is not permeable to gas, provided that it is correctly sintered. Ceramics therefore are the basis of many advanced packages. Their use in this way uses the fact that layers of ceramic can be co-fired to form a robust joint, making package assembly possible, and the structure allows enough penetration by glass to make it possible to metallise ceramic using glass threads containing precious metals. It is materials of this sort, referred to as ‘thick film inks’ which are used both in package manufacture and in making chip resistors.

Magnetic ceramics

There is a wide range of magnetic ceramic materials, of which true ferrites are only one group of structures exhibiting ferromagnetic behaviour. Despite this, the term ‘ferrite’ is generally used for the whole range of such materials.

Ceramic magnets, and the ferrites used in making inductors, are magnetic because of a different mechanism, which is distinguished by the slightly different spelling of ‘ferrimagnetism’. However, whilst the mechanism at an atomic level is different, the hysteresis behaviour, and the presence and movement of domains, is exactly the same as with ferromagnetic materials.

The most commercially important ceramic magnets are associated with the spinel (MgAl2O4) crystal structure, an extremely complex crystal containing 56 ions. Whilst spinel itself isn’t magnetic, some compounds containing transition metal ions crystallise in this structure or the closely-related inverse spinel structure.

From the user point of view ferrites are hard ceramic materials, which are generally pressed, because they are too hard to form in other ways. They are also extremely brittle, and this must be borne in mind during assembly operations.

Some glass basics

Glasses can be classified as amorphous ceramics. They may have a wide range of compositions, but have the property in common that during manufacture they are cooled quickly enough to prevent crystallisation from taking place, so that the glass (amorphous) state is retained at ambient temperatures. This happens because the chemical ‘unit’ in glasses is very large, being formed of large silicate networks, bonded internally covalently, and held together by ionic bonds provided by metallic ions within the structure. Movement of these ‘units’ is difficult even when they are thermally activated, so it is easy to cool the material past its normal melting point without crystallisation taking place. As shown in Figure 9, the material cooled slowly will crystallise at the normal melting point.

Figure 9: The glass transition temperature for a glass

Figure 9: The glass transition temperature for a glass

Typically this will not happen, and more rapid cooling results in the formation of a supercooled liquid. Even in this state, some contraction takes place, until temperature reaches a point where a sudden change occurs because there is no further possibility of molecular readjustment. This temperature is referred to as the ‘glass transition temperature’: as with polymers this is not a well-defined temperature, and depends on the cooling rate.

Ordinary commercial glass is produced from various inorganic oxides, of which silicon in the form of sand is generally the most important constituent. Common ‘soda-lime’ glass also contains lime (from limestone) and soda ash (crude sodium carbonate). This mixture is then heated to 1590°C, when the acidic silica reacts with the basic lime and soda to form the mixed silicates that we know as glass.

Because there are no crystallographic planes to make slip possible, glass cannot deform when stress is applied, and the viscous flow is too little and far too slow. This makes glass brittle at room temperature. However, they are elastic up to the point of fracture. Even though most of the load may be in compression, failure in glass always results from a tensile component under stress.

Because glass is a poor conductor of heat, there is often a considerable temperature gradient between the inside of a piece and its cooler outside as the part cools. Given the difference in CTE with temperature, the tendency is for the core to contract more than the surface, leading to the formation of internal stresses. Failure to anneal glass, to remove the stresses, leaves it weak and brittle. Annealing3 takes the glass up to the glass transition temperature, and slowly down to ambient, so that a small amount of viscous flow can take place leading to stress relaxation within the glass.

3 An alternative to annealing is the ‘tempering’ process, which tries to reduce the formation of surface cracks by putting the surface in a state of compression. Glass is heated to near its glass transition temperature,and then the surface cooled rapidly by air jets. Because the outside surface cools, contracts and hardens more quickly than the inside, the outer surface is left in a state of compression, whilst the interior material is in tension. Because the surface layers are in compression, the internal forces will balance any moderate tensile forces to which the glass may be subjected. Such material has considerable internal strain, and the strain patterns become visible when materials such as ‘Perspex’ are viewed by polarised light.

 

Ordinary glass will not cope with sudden temperature changes, but a range of temperature resistant glasses have been developed where as much as 20% of glass formers such as boron oxide (B2O3) are added to the silica. These produce a material with a higher viscosity, resistance to chemical attack and a very low coefficient of expansion, which makes materials such as ‘Pyrex’ suitable for kitchen, laboratory and industrial uses.

Whilst creep takes place in crystalline ceramics only at relatively high temperatures, non-crystalline glasses have low softening temperatures and considerable creep occurs at moderate temperatures.

Glass-ceramics

In modern glass-ceramics, controlled devitrification is employed to initiate large numbers of nuclei for crystallisation in glasses of suitable composition to produce polycrystalline materials. After shaping, the material is given a heat treatment in two stages. First, a low temperature treatment to promote the formation of large number of crystal nuclei within the glass; second a higher temperature treatment at which crystal growth rate is maximised. This continues until the crystalline content is high (perhaps 90%), so that only a little glass remains. The mechanical properties of such glass-ceramics are intermediate between those of a glass and a true ceramic such as alumina. Not only used in cooking wear (for example, Pyrex), glass ceramics with different materials are used for missile nose cones and heat exchangers, being resistant to thermal shock and transparent to radar signals.

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