Fibre pull out
Long
When a crack propagates through a material reinforced with long fibres there are many processes that happen. For the crack to propagate transversely to an applied stress parallel to the length of the fibres, each fibre will have to break; this usually occurs somewhere along the fibre where there is a relatively large flaw because the strength of the fibre is greater than the strength of the material it is reinforcing. This means that for the crack to propagate, the surfaces between the crack and where the fibre has broken will need to to debond to enable the fibre to be extracted. Both of these processes absorb energy, helping to toughen the composite.
An issue with long fibre composites is that they are difficult to produce. For fibres to achieve high toughening they must be packed tightly. However, this means powder routes for a ceramic matrix are not the best choice because the powder is unlikely to fill the space between fibres very efficiently. This is likely to create a very uneven composite matrix. One way of overcoming this problem is to produce the matrix as a glass which is then devitrified, such as in Figure 8 below. Here, the matrix has crystallised as a magnesium aluminosilicate.
A second potential problem with such processing routes is that the high sintering temperatures needed are likely to cause large stresses at the matrix-fibre interfaces when cooled because of different thermal expansion coefficients of the fibre and the matrix. This can potentially lead to failure along the matrix-fibre interfaces immediately upon cooling.
Other ways to produce long fibre ceramic composites are to use liquid or gaseous precursor processing routes. In these routes, the resultant ceramic is more likely to deposit on the fibres near the surface of the composite, rather than penetrate into the interior of the composite. This means that it can often take several infiltration steps to produce sufficient ceramic in the middle of the composite. This repeated infiltration is expensive and time consuming, making these processes impractical in industry for anything other than niche applications where the cost is not a major factor. A second consideration for these processing routes is that because the temperatures involved in the breakdown of the precursors to form the ceramics are relatively modest, such composites are unlikely to be able to be used at temperatures > 1000 °C because of thermal mismatch considerations. One notable exception is that of SiC fibres in a SiC matrix: although the matrix is not fully dense, this does not have thermal mismatch problems.
Long fibre polymer matrix composites are more easily processed. For example, carbon fibre-reinforced epoxy resin matrix composites typically require curing temperatures less than 160 °C. Such composites are used in a wide variety of sports, automotive and aerospace applications where light, stiff and tough materials are required for ambient temperature use.
Short
With short fibres, the amount of toughening is limited by the length of the fibres, rather than the strength of the fibres, as in long fibre composites. The main advantage of short fibres is that there is not much alteration required during processing. For example, in alumina reinforced with SiC, the short fibres ('whiskers') are mixed in before hot pressing. Alumina reinforced with SiC whiskers is often used for cutting tools. Such tools have good wear resistance because SiC is hard and sufficiently tough. As well as this advantage, cutting tools tend to get hot during use. This helps to oxidise the SiC into SiO2. This is good because SiO2 is also hard, which is also useful for the cutting ability of the tool.
However, as the material is still quite brittle, there is the possibility that small particles could break off the cutting tools in service. If these particles are the SiC whiskers, this has the potential to be dangerous because such small fibres tend to be toxic. For example, asbestos is a naturally occurring fibrous silicate mineral which causes asbestosis if inhaled over a prolonged period, leading eventually to death. Therefore, suitable ventilation precautions must be used in both the processing and use of such composite materials.
Elongated grains
Some toughening can be provided to a ceramic if the microstructure can be sufficiently modified in situ. This requires making grains with elongated structures that promote crack deflection and fibre-like pull out. These grains can also act to bridge gaps ('cracks'), similar to the behaviour of fibres.
An example of a material that undergoes such toughening is silicon nitride, Si3N4. In silicon nitride the crystals are anisotropic so there are anisotropic surface energies that lead to preferred growth directions. A powder compact can use a liquid phase to help create the desired microstructure. \( \alpha \)-Si3N4 is mixed with \( \beta \)-Si3N4 (5 wt%) and either Al2O3 or Y2O3 (5 wt%). During consolidation at high temperature (typically around 1700 °C) by hot pressing, the oxides melt and form a liquid phase which acts as a sintering aid. The \( \alpha \) crystals dissolve in preference to the \( \beta \) crystals and reprecipitate out from the liquid as the \( \beta \) phase. The newly precipitated \( \beta \) phase is highly anisotropic so that \( \beta \)-rods form, with the original \( \beta \) crystals being used to help nucleate these new rods. After suitable time at this high temperature, the ceramic is cooled quickly enough so that the oxide-rich liquid does not have time to crystallise and instead forms a glass. With careful choice of starting materials in the initial powder compact, the final microstructure can be formed of \( \beta \)-Si3N4 rods with nanometre thick residual grain boundary glassy phases and only small amounts of residual equiaxed grains of \( \alpha \)-Si3N4.
The glass at the boundaries has a lower fracture resistance than the \( \beta \)-Si3N4, and so it is able to deflect growing cracks around these elongated grains. This process acts to toughen the material by crack deflection and fibre pull out, similar to the behaviour of fibres in a composite material used to provide toughness.
Figure 8: Micrograph 548 from the DoITPoMS library: SiC fibre-reinforced magnesium aluminosilicate where fibres are bridging the crack
For more information on types of fibres used in composites, click here.