MECHANICAL PROPERTIES

4.4. MECHANICAL PROPERTIES

Many details about the mechanical properties are given in the previous sub-sections along with some details of different experimental techniques, including some descriptions of the Weibull’s statisti- cal theory, which are all relevant for the present study in the interpretation of the data of fracture strength,

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Weibull modulus and fracture toughness, i.e. criticl stress intensity factor. Here, first of all, a comparison will be made between the mechanical data of both α- and β-silicon carbides in terms of their flexural strength, and then the mechanical data of α-silicon carbide will be presented in order to be able to correlate them with their microstructural behaviour.

4.4.1. Comparison of Mechanical Data of α- and βββββ-SiC α α αα The flexural strength of both α- and β-silicon carbides, which are doped with boron carbide +

carbon and aluminium nitride + carbon, has been studied. For boron carbide doping, the flexural strength of α- and β-silicon carbides are found to be 390 and 330 MPa respectively. Such a behaviour of the flexural strength of two types of silicon carbide could be interpreted by the sintered density and the polytype formation, as described in various sections of the chapter 2. The grains of these silicon car- bides doped with boron carbide are mostly eleongated in shape. The sintered density of α-silicon car- bide is 3.18 g/cc, which corresponds to 99% of theoretical density, whereas the density of β-silicon carbide is 3.10 g/cc that corresponds to 96.5% of the theoretical density. Therefore, the higher density of α-silicon carbide compared to that of β-silicon carbide could be one reason for a higher flexural strength of the former material. But there could be other reasons related to material properties.

As β-silicon carbide contains 3C-polytype, which is a thermodynamically unstable phase, the growth of the crystals in a particular direction has been very fast compared to that of α-silicon carbide according to the energy considerations. The very fast growth of the elongated crystals during the sintering of β-silicon carbide is expected to ‘entrap the pores’, and thus it might have prevented the densification to be completed. Therefore, as mentioned in the section 2.7.3, two types of porosities have been identi- fied : (a) some within the grains, and (b) others between the grain-boundaries. The length of the crystal of α-silicon carbide is 5.7 µm, whereas that of β-silicon carbide is 12 µm. This should explain why the flexural strength of α-silicon carbide is higher than that of β-silicon carbide, which are both sintered with the same dopant of the same concentration.

As shown later from the microstructure, the fracture mode in α-silicon carbide is found to be inter-granular, which is also similar in β-silicon carbide. This type of fracture presumably originates from the surface, and propagates through the crystals through inter-granular porosities. The flexural strength of both these materials remained constant from room temperature up to 1400°C. This invari- ance of strength against temperature can be attributed to the fact that there is neither any liquid phase nor any glassy phase in the grain boundaries, as shown later for α-silicon carbide.

Therefore, the lower value of strength for β-silicon carbide can be attributed to the larger grain size (d), which is related to the flexural strength by Petch equation [24] as :

y = σ x + kd

where, σ x is the base strength for the single crystal and k is a constant. Thus, the flexural strength of β- silicon carbide is found to be lower due to the presence of such larger grains, i.e. 12 µm.

4.4.2. Flexural Strength of α αα α α-SiC There are many factors which affect the strength of ceramic materials such as porosity, grain

size, grains shape and surface conditions etc. Figure 4.2 shows a SEM micrograph of a polished and etched surface of the dense α-silicon carbide. The grains had mostly tabular shapes The average grain

159 size was found to be 5700 nm. The Figure 4.3 shows the flexural strength as a function of temperature.

MECHANICAL PROPERTIES

No variation of strength data with temperatures was found up to 1400°C. The average flexural strength was found to be 390 MPa. The Figures 4.4 and 4.5 show the flexural strength as Weibull modulus (m) at room temperature and at 1400°C. The number of specimens was 30 in each case. The Weibull modu- lus was 11.3 at room temperature and 13.3 at 1400°C. The Weibull modulus is found to increase at higher temperature presumably due to healing of surface cracks, which were induced during the cutting of the samples by high speed diamond blades, i.e. the curing of the surface damages at higher tempera- ture.

The Weibull modulus for β-silicon carbide (not shown here) was found to be 10.9 and 12.9 at room temperature and 1400°C respectively. The low value of ‘m’ for β-silicon carbide could be due to the presence of pores of different sizes and also due to a relatively wider distribution of flexural strength at room temperature. The increase of Weibull modulus by about 20% for both these materials from room temperature to 1400°C might be due to the ‘healing of surface cracks’ and possibly due to the ‘release of residual stresses’.

Figure 4.2: SEM photo of etched and polished surface of a dense α-silicon carbide (dopant : 0.5 wt% boron carbide + 1 wt% carbon)

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