ADVANCED ceramic materials that can withstand high temperatures (over 1,500 °C) without degradation or oxidation are needed for applications such as structural parts for motor engines, gas turbines, catalytic heat exchangers and combustion systems1,2. Hard, oxidation-resistant ceramic composites and coatings are also in demand for use on aircraft and spacecraft. Silicon nitride (Si3N4) and silicon nitride/carbide (Si3N4 /SiC) composites are good candidates for such high-temperature applications2,3. Commercial Si3N4 parts can be used in oxidizing environments up to 1,200–1,300 °C (ref. 4), but are oxidized at still higher temperatures. Here we describe the synthesis of a covalent ceramic composite which is resistant to oxidation at temperatures up to 1,600 °C. The composite is formed from an amorphous silicon carbonitride, which crystallizes at high temperature into a composite of α-Si3N4 microcrystals and α-SiC nanocrystals. The oxidation resistance stems from the formation of a passivating surface layer of SiO2 a few micrometres thick.

A covalent micro/nano-composite resistant to high-temperature oxidation

The progress in monolithic Si3N4 and SiC as well as in Si3N4/SiC-composites for structural applications is reviewed. The conventional processing including the powder synthesis, densification and microstructural design is discussed. The mechanical properties of the resulting silicon based non-oxide ceramics and their industrial applications as structural components are summarized. As an alternative route to fabricate Si3N4/SiC composites the hybrid processing utilizing the thermal conversion or organosilicon precursors to amorphous and polycrystalline multicomponent materials is described. The hybrid processed ceramics exhibit ultra-high temperature stability with respect to crystallization, oxidation and decomposition.

Progress in silicon-based non-oxide structural ceramics

Phase equilibria and thermal analysis of Si-C-N ceramics

In the present work a Si 3 N 4 composite containing 24 wt% SiC particulates was prepared by sintering a polysilazane-derived amorphous SiNC powder. Al 2 O 3 and Y 2 O 3 were added as sintering additives. The amorphous powder was pressureless sintered to 97% relative density at 1750°C in nitrogen atmosphere. α-Si 3 N 4 and β-SiC were generated in situ due to the crystallization of the amorphous powder during the heating in the densification process. Subsequently, α-Si 3 N 4 is transformed to the β-form by the liquid phase sintering mechanism. A fine-grained microstructure (d 50 = 0·2 μ m) with the SiC particulates homogeneously distributed in the Si 3 N 4 -matrix was found in the densified samples. Room temperature fracture strength ( σ B ) and fracture toughness (K Ic ) were similar to those measured in conventionally processed composites. However, the fracture strength was characterized by a relatively low standard deviation indicative of the better reproducibility of the properties due to improved homogeneity in the developed microstructure.

Sintering of amorphous polymer-derived Si, N and C containing composite powders

Polysilazanes have been described as excellent polymeric precursors to amorphous silicon carbonitride (SiCN) glasses. In this work, the synthesis, thermal cross-linking and pyrolysis of a polycyclic silazane network precursor for a SiCN glass, were studied. This polymeric precursor was prepared by free radical polymerization of 1,3,5-trimethyl-1′,3′,5′-trivinylcyclotrisilazane, using dicumyl peroxide as radical initiator. The polymer-to-ceramic conversion was investigated by 29 Si and 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) and infrared spectroscopies, X-ray diffraction and by simultaneous thermogravimetric and differential thermal analyses. Based on the results the polymeric precursor can be characterized as a polycyclic silazane network, containing aliphatic carbon segments produced by the vinyl polymerization. 29 Si and 13 C NMR data of this polymeric precursor heated at different temperatures indicated the occurrence of side-reactions in addition to the vinyl polymerization. The SiCN glass was obtained as an amorphous solid up to 1400 ° C . Above this temperature, the crystallization process of the ceramic product was promoted by rearrangement of the different Si sites in the amorphous network. Heat treatment at 1600 ° C , under N2 atmosphere, produced a ceramic containing 85% silicon carbide (SiC) and 15% silicon nitride, determined by 29 Si MAS NMR in relation to the total silicon sites, and under argon atmosphere, SiC.

Synthesis of a polycyclic silazane network and its evolution to silicon carbonitride glass

Thermodynamic equilibria were calculated to determine optimum fabrication conditions for SiC-whisker-reinforced Si{sub 3}N{sub 4} composite ceramics. Results of calculations indicate that pressureless sintering in a nitrogen atmosphere of 0.1 MPa may be performed at temperatures up to 2,123 K (1,850{degree}C) without whisker or matrix degradation. In addition, gas pressure sintering at temperatures up to 2,373 K (2,100{degree}C) requires a nitrogen pressure of 1 to 10 MPa. Hot isostatic pressing (total pressure {ge} 100 MPa) at 2,273 K (2,000{degree}C) can yield undegraded composite materials only when the nitrogen partial pressure in a gas mixture with an inert gas is kept between 1 to 10 MPa.

Günther Petzow

Papers

Thermodynamic calculations for the formation of SiC-whisker-reinforced Si sub 3 N sub 4 ceramics