Effect of Atmosphere

2.7.3.4. Effect of Atmosphere

In order to understand the sintering process of silicon carbide under various atmospheres, the general sintering behaviour should be made clear. It has been shown by SEM, TEM and electron diffrac- tion experimental data [3] that the sintering mechanism of nano particles of SiC, through doping of boron carbide and carbon, is a solid-state process. This might have occurred by the creation of vacancies formed by the dopants and by an increase of the bulk diffusion by many orders of magnitude.

This solid-state sintering mechanism was also evident from an extensive analysis of fracture toughness data as a function of temperature [19, 20]. The sintering process does not appear to be either

a surface phenomena, as proposed by Prochazka [23] , or a liquid phase sintering, as proposed by Lange and Gupta [26]. The chemical analysis of both β-silicon carbide and 6H α-silicon carbide single crys- tals by three different laboratories has shown the crystals to be silicon rich. The ratio of silicon to carbon of both β-silicon carbide and 6H α-silicon carbide was reported to be 1.049 and 1.032 respectively [56, 57], as said earlier. The excess silicon must result from the carbon vacancies. A high concentration of carbon vacancies is precluded by the following reasons :

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1. The low diffusion coefficient,

2. A high activation energy, and

3. A large pre-exponential term This is a conclusion which is supported by a high value of the calculated Schottky energy by

Vechten [51]. It was found that the self-diffusion rate of each element is enhanced by the presence of the other element, due to the creation of vacancies. Thus, the carbon atmosphere would equalize the ratio of Si to C to unity or less than unity in silicon carbide, thereby creating the silicon vacancies and thus increasing the bulk diffusion of silicon. A further enhancement of silicon diffusion coefficient occurs due to carbon rich atmosphere. In ‘argon atmosphere’, the carbon diffusion coefficient is two orders of magnitude larger than that of silicon.

However, in ‘carbon rich atmosphere’, the carbon diffusion coefficient is less than the diffusion coefficient of silicon, and also the enhancement of diffusion coefficient of silicon occurs, as reported by Rijswijk and Shanefield [42]. Here, carbon also prevents the formation of B 4 C structure, and also decreases the vacancies of carbon. Thus, it lowers the diffusion coefficient of carbon as reported in carbon-rich environment.

By this process, the addition of boron increases the bulk diffusion of silicon and carbon by many orders of magnitude, and they eventually become equal to grain boundary diffusion coefficient of both silicon and carbon vacancies. For the densification to occur, the mass transport of silicon and carbon should be more or less equal. Without the addition of boron carbide and carbon, the mass transport of silicon and carbon is not equal, and it is also low, and the densification does not occur [3]. The addition of boron carbide increases the diffusion coefficient and the carbon creates a partial carbon atmosphere, which make both the diffusion coefficients equal. Thus, the addition of dopants makes the mass trans- port of silicon and carbon equal in order to make the densification to occur [3].

It is already noted from the above description on the creation of vacancies and consequent diffu- sion for sintering of nano particles of SiC in terms of the vacancy formation and consequent diffusion should be helpful in order to understand the overall densification behaviour under different atmos- pheres, as detailed below.

The evolution of the density for the α-silicon carbide samples, sintered under three different atmospheres like vacuum, argon and nitrogen, is shown as a function of sintering temperature in Figure

2.30. It is seen that the sintered density is higher for vacuum atmosphere compared to those of the other two atmospheres at all temperatures. It is also seen that the maximum densification occurs in vacuum, while the minimum densification is observed in nitrogen atmosphere.

The densification behaviour of α-silicon carbide in vacuum and argon atmosphere is quite simi- lar, but they are distinctly different than that in nitrogen atmosphere, although the densification level in case of argon is lower than that in vacuum. The sintered density initially increases linearly up to 2050°C for all the samples and then almost saturates towards higher temperatures, or rather the sintering rate is considerably slowed down. This shows that the optimum sintering temperature of nano-crystalline α- silicon carbide doped with boron carbide and carbon is 2050°C.

Moreover, it is noteworthy that the initial rate of increase of density is somewhat higher for vacuum atmosphere compared to those of the other two atmospheres. This can be ascribed to higher diffusion of carbon and silicon under vacuum atmosphere. The number of vacancies of carbon and silicon are high in vacuum, compared to that in argon atmosphere. The creation of a higher vacancy

NANO MATERIALS

leads to a higher diffusion as a function of temperature, which is the main driving force for densification. Therefore, the higher sintered density of α-silicon carbide has been obtained in vacuum than in argon atmosphere as mentioned earlier, as shown in Figure 2.30.

2.7 IN VACUUM IN ARGON IN NITROGEN

TEMPERATURE (°C)

Figure 2.30: Sintered density of α-SiC in three different atmospheres. The microstructures of α-silicon carbide, doped with 0.5 wt% boron carbide and 1 wt% carbon

sintered at 2050°C in three different atmospheres are shown in Figures 2.31, 2.32 and 2.33 respec- tively, which show the different levels of densification.

Figure 2.31. TEM photo of α-SiC sintered in vacuum.

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Figure 2.32: TEM photo of α-SiC sintered in argon.

Figure 2.33: TEM photo of α-SiC sintered in nitrogen.

The intermediate behaviour of densification in argon atmosphere can be ascribed to that during later stage of sintering, the argon gas could have remained entrapped in the disconnected pores, which prevent full densification, as observed by a lower level of densification in argon atmosphere compared to that in vacuum. From the SEM micrograph of the sample sintered under argon atmosphere, some evidence of such disconnected pores was observed. The disconnected pores do play a role in sintering, but here the entrapment of argon gas in these pores may prevent mass transport into these pores, thereby causing a lower level of densification, compared to that in vacuum.

NANO MATERIALS

In case of the densification in nitrogen atmosphere, the Schottky defects, i.e., the vacancy for- mation of carbon and silicon will be lower compared to the atmosphere of vacuum or argon. Moreover, in boron carbide doped α-silicon carbide samples, nitrogen may form BN with boron as observed by

Venkateswaran and Kim [47], as mentioned earlier, preventing the formation of SiB 4 and B 4 C, which actually increases the diffusion coefficients of silicon carbide. There is no experimental proof in this case, since small amounts of BN formed on the surface of the SiC grains is difficult to detect by the usual technique of SEM. But under the thermodynamic situation discussed above in the section - 2.7.3.3, it is most likely that BN forms under nitrogen atmosphere. Hence, the possible formation of boron nitride, which is not helpful in increasing the diffusion as an aid to sintering, might be responsible for the lower level of densification for sintering under nitrogen atmosphere.

It is quite plausible to mention here that in the case of nitrogen atmosphere, the onset of densification has been observed to be retarded by 150°C in the present investigation, which was also observed by Mirzah et al [38]. Therefore, the slower densification can be considered to be due to the decrease of diffusion coefficient, as opposed to the situation under vacuum. The level of densification also becomes lower in case of nitrogen atmosphere compared to vacuum or argon atmosphere, where the densification has been increased by the addition of dopants like boron carbide and carbon to speed up the required vacancy formation with a consequent increase in diffusion, as stated above through a diffusion model proposed earlier to explain the sintering behaviour of α-silicon carbide [3]. The impor- tance of the diffusion mechanism through vacancy formation, as detailed above, may be considered relevant for explaining the maximum sintering under vacuum, and for the lowest level of densification in case of nitrogen atmosphere.

In summary, it can be stated that the diffusion model proposed here predicts the formation of SiB 4 and B 4 C, which create vacancies and thereby increase the diffusion coefficients. This situation prevails in case of sintering of α-silicon carbide in vacuum and a maximum densification has been observed. The scenario is quite similar, but to a lesser extent in argon atmosphere. In nitrogen atmos-

phere, the possible formation of BN with boron prevents the formation of SiB 4 and B 4 C, thereby de- creasing the formation of vacancies and consequently their diffusion coefficients. Moreover, the forma- tion of the Schottky defects in nitrogen atmosphere is also lower compared to that in vacuum and argon atmosphere, thereby a further decrease in diffusion coefficient occurs. Therefore, the onset of densification of nano-crystalline silicon carbide doped with boron carbide (0.5 wt%) and carbon (1 wt%) retarded by 150°C is due, presumably, to the formation of BN and to the creation of lower amount of defect sites as compared to that occurred in vacuum and argon atmospheres.

Finally, it should be mentioned from the recent studies that silicon carbide can be sintered up to high densities by means of ‘liquid phase sintering’ under low gas pressure with a combination of ‘sintering aids’ such as AlN-Y 2 O 3 , AlN-Yb 2 O 3 , and AlN-La 2 O 3 . As-sintered materials of SiC exhibited fine- grained homogeneous microstructure, and this has the advantage of sintering SiC at comparatively low temperature with improved mechanical properties [58 - 60].

The densification behavior of materials with Y 2 O 3 and La 2 O 3 additions has been shown to be opposite. While the former materials densify better in Ar, the latter show better sinterability in N 2 . Moreover, the densification of La 2 O 3 containing materials has a complex nature most probably due to a complex phase formation sequence on heating, which has to be confirmed yet. The Yb 2 O 3 containing materials exhibited sintering behavior more similar to the behavior of Y 2 O 3 containing ones [58].

The transformation-controlled ‘grain growth’ during post-sintering heat-treatment of the devel- oped materials was established. A high degree or even a complete β-SiC to α-SiC transformation was achieved by such techniques in reasonably short time intervals. The kinetics of the phase transformation

113 has been shown to depend on the composition of the sintering additives, the use of less refractory rare

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earth oxides accelerating the process. Thus, the possibility of in-situ platelet reinforced SiC ceramics was shown, which might be the reason of higher flexural strength and better fracture toughness [58]. However, an optimization process of several factors involving these different ‘sintering additives’ vis-à- vis the processing techniques has to be undertaken to get better information for SiC for high performace applications.