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Because of the unique combination of their attractive properties, porous ceramics are considered as candidate materials for several engineering applications. The production of porous ceramics from polysiloxane precursors offers advantages in terms of simple processing methodology, low processing cost, and easy control over porosity and other properties of the resultant ceramics. Therefore, considerable research has been conducted to produce various Si(O)C-based ceramics from polysiloxane precursors by employing different processing strategies. The complete potential of these materials can only be achieved when properties are tailored for a specific application, whereas the control over these properties is highly dependent on the processing route. This review deals with processing strategies of polysiloxane-derived porous ceramics. The essential features of processing strategies-replica, sacrificial template, direct foaming and reaction techniques-are explained and the available literature reports are thoroughly reviewed with particular regard to the critical issues that affect pore characteristics. A short note on the cross-linking methods of polysiloxanes is also provided. The potential of each processing strategy on porosity and strength of the resultant SiC or SiOC ceramics is outlined.

Si 2 N 2 O secondary phase-free, fully dense nano-Si 3 N 4 ceramics ( ! 97% of theoretical density) were developed by combining carbothermal reduction treatment and spark plasma sintering (SPS), and their tribological properties were investigated by subjecting to self-mated sliding under unlubricated conditions. Commercially available Si 3 N 4 nanopowder was used as a starting material and phenolic resin was used for carbothermal reduction, which was conducted at 14501C for 10 h. Fully densified Si 2 N 2 O phase-free Si 3 N 4 ceramics with a wide range of grain size from 90 nm to 1.5 lm were fabricated by varying SPS temperature from 15501 to 17501C. The microstructure of the developed Si 3 N 4 ceramics was changed from nano equi-axed at 15501C to large elongated bimodal grain morphology at 17001 or 17501C. The frictional behavior was not dependent on the microstructure, but the wear rate was strongly influenced such that it decreased by an order of magnitude (from 9.7 Â 10 À5 to 0.88 Â 10 À5 mm 3 /N . m) with decreasing grain size. The dominant wear mechanism was changed from the delamination of tribochemical layer for the ceramics with nano equi-axed grain microstructure to the fracture and grain pull-out for the ceramics with duplex microstructure.

Porous mullite-bonded silicon carbide (SiC) ceramics were prepared from SiC and aluminum hydroxide [Al(OH) 3 ] powders. The Al(OH) 3 content was varied from 14.5 to 47.3 wt %, and porous SiC ceramics were fabricated via reaction sintering at 1450 1550°C for 2 h. The microstructure showed large SiC grains embedded in a matrix of small and loosely packed mullite/alumina particles. It was demonstrated that the porosity decreased and flexural strength increased with increase in Al(OH) 3 content. The porosity varied from 46 to 54%, and flexural strength varied from 3 to 14 MPa with the variation in Al(OH) 3 content and sintering temperature. Typically, porous mullite-bonded SiC ceramics of 14 MPa strength and 47% porosity were obtained when SiC and 47.3 wt % Al(OH) 3 powders were sintered at 1500°C.

In the present work, four aluminum sources (Al/AlN/Al2O3/Al(OH)3) were used to fabricate mullite-bonded porous SiC ceramics via reaction sintering in air at 1450°C and 1550°C for 1-6 h duration, and the role of aluminum source on microstructure and strength was estimated. The microstructures revealed a variation in necking and adhesion characteristics of SiC/mullite/silica grains. The porosity decreased and strength increased with sintering temperature or time for all specimens, except for the one prepared with Al2O3. Amongst the investigated, SiC ceramics prepared with Al2O3 exhibited the lowest porosity of 17% and highest specific strength of 19 kN.m/kg, while the usage of AlN rendered a combination of the highest porosity of 42% and lowest specific strength of 5 kN.m/kg. An attempt has been made to qualitatively explain the strength variation of prepared ceramics on the basis of contributing oxidation reactions towards mullitization.

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