Compact nitridation, silicon powders

The reaetion-bonded materials (RBSN) produced by the nitridation of compacts of silicon powder (see Table 2) are not dense, whereas the other kinds of materials in the table are dense. 

Sintering involving a chemical reaction is typical for the manufacture of some carbides and nitrides. For example, dense SiC is obtained by firing carbon compacts in silicon vapours. A similar example is provided by the manufacture of densified Si3N4 w here the nitride is formed in the powdered metal compact by reaction with gaseous nitrogen the product fills up the pores and thus gives low porosity. 

Silicon powder is compacted to a density of 60% of the theoretical value and heated in N2 to form reaction bonded silicon nitride (RBSN). If the reaction goes to completion and there is no change in the external dimensions of the compact, determine the porosity of the RBSN. 

In addition to the above-mentioned benefits, the SRBSN process has the potential to fabricate silicon nitrides with enhanced mechanical and thermal properties. A typical example of the morphological change before and after the nitridation of a Si powder compact is shown in Figure 16.14 where, clearly, the particle size of the nitrided compact is much finer than that of the starting Si powder. This phenomenon, combined with the higher density achieved in nitrided compacts, makes the RB process very attractive for fabricating high-performance silicon nitride ceramics. 

Not unlike the case of superplastic ceramics, ductility and strength relations are influenced by strain rate. The conditions of the experiment must be above the DBT to observe plastic flow, which is different for various ceramics. An illustration of the effect of strain rate and temperature on the strain (ductility) at some stress level can be seen in monolithic Si-C-N. Silicon-nitride-based ceramics are quite promising candidates for mechanical applications at elevated temperatures. Specimens were prepared by hot isostatic pressure (henceforth HIP) of pyrolyzed powder compact at 1500 °C and 950 MPa, without any sintering additives. These compression tests were conducted at temperatures from 1400 to 1700 °C in a nitrogen atmosphere with a servo-hydraulic-type testing machine at constant crosshead speed in an induction heating furnace. In Fig. 2.5, stress-strain curves... 

From these data it is evident that the behavior of these powders during compaction seems to be roughly similar however, the influence of pressure on relative density is more evident in the case of TiN. This can be e q)lained by more covalent and req>ectively more brittle nature of silicon nitride. It has be pointed out that there is significant influence of chemical bond on conq>ressibility for UFPs. It is also important to indicate that there is no possibility to obtain near dense conq>acts fi om UFPs in the conditions of cold compaction even at the conq>action pressures up to 8-9 GPa. 

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