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Densification and Phase Formation Behavior of Alumina Infiltrated with Polycarbosilane
Abstract
Conventional powder processing is commonly used to fabricate Al2O3/SiC nanocomposites, in which SiC is added in a powder form with sizes of tens to hundreds nanometres. Recently a new processing route using a SiC polymeric precursor such as polysilazane, polysilastyrene, or polycarbosilane has been also attempted, where a polymeric precursor solution is mixed with alumina powder. The resulting powder is dried and heat-treated or sintered to form nano-sized SiC particles in situ by the pyrolysis of the polymeric precursor. In this study, porous alumina bodies were infiltrated with the hexane solution of polycarbosilane and sintered at 1600°C for 2 h in Ar or N2. The densification and phase formation behavior was studied as was the effect of SiO2 addition.
... The 15R phase has already been reported to form from a Si-depleted (C-rich) amorphous matrix provided by a polycarbosilane, aimed at yielding SiC matrix composites with Al 2 O 3 as secondary phase. 37 The overall reaction system is very complex and at this stage of the investigations cannot yet be discussed in full detail. In any case, the low temperature oxygen contamination and the Si depletion are key points. ...
The use of preceramic polymers in the synthesis of Sialon ceramics has been scarcely discussed in the literature. In this article we report the production of virtually phase‐pure β′‐Sialon ceramics from a mixture of commercially available polysilazanes and γ‐Al2O3 nanopowder, pyrolized in N2 atmosphere in the 1300°C–1600°C range. This approach combines the advantage of embedding nano‐sized fillers in preceramic polymers, in terms of their reactivity towards the Si–N based ceramic pyrolysis residue, with the complex interactions with residual carbon, also present as a secondary phase in the same ceramic residue. Starting from a polymer (PSZ20) yielding a SiCN amorphous ceramic after pyrolysis, the Sialon phase purity is greatly affected by the residual C content: for an optimized polymer/filler ratio (PSZ20/Al2O3 = 2), β′‐Sialon can be produced possesing only small quantities of quasi‐amorphous SiC as a secondary phase. Additional improvements based on the partial replacement of PSZ20 with a polymer (PHPS) not containing C in the backbone, lead to the production of pure nanocrystalline β′‐Sialon powders (average grain size of 100–200 nm).
Conversion of rice husk precursors (raw husks, pulverized black ash and mixtures of white ash and carbon black) to silicon carbide through vacuum pyrolysis over a wide temperature range, has been studied. The products of the pyrolysis (SiC whiskers and particles) were analyzed with the help of XRD, SEM and chemical analysis. The present study indicates that a temperature range of 1250–1370°C, carbonaceous substrate, sufficient void space, controlled release of SiO vapour, and the presence of CO are favourable conditions that tend to maximize the formation of SiC whiskers. Under similar experimental conditions, the formation of SiC (w) from rice husk precursors has been observed to increase in the following order: the lowest from the pulverized black ash, intermediate from the white ash and carbon black mixtures, and the highest from the raw rice husks.
Alumina/10 wt% SiC nanocomposites were prepared by using a number of processing techniques, in order to produce different microstructures while keeping a uniform distribution of the SiC particles in the alumina matrix. Basically, this study used three techniques, i.e. mechanical mixture of powders, the use of an inorganic precursor for alumina precipitated onto SiC particles and the use of different polymeric precursors for SiC precipitated on alumina particles. Hot pressing at 1700 °C was necessary to produce fully dense nanocomposites. The alumina precursor route produced the smallest matrix grain size and the SiC precursor route produced the smallest reinforcement particle size. Despite the differences in microstructure between the nanocomposites, the flexure strength remained the same, and no distinct improvement over the monolithic alumina was observed. The fracture mode, however, changed from intergranular to transgranular with the presence of the SiC particles.
The pyrolysis of a PCS precursor has been studied up to 1600 C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 T
p < 800="" c)="" leading="" to="" an="" amorphous="" hydrogenated="" solid="" built="" on="" tetrahedral="" sic,="">2 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 T
p < 1200="" c)="" resulting="" in="" sic="" nuclei="" (less="" than="" 3="" nm="" in="" size)="" surrounded="" with="" aromatic="" carbon="" layers,="" and="" (3)="" a="" sic="" grain-size="" coarsening="">T
p > 1400 C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.
Silicon carbide ceramics containing 20 wt% polycarbosilane was fabricated by hot-pressing with various additives (B, Al, AlN). The addition of polycarbosilane resulted in a considerable increase in flexural strength up to 1050 MPa for the 1 wt% AlN and 0.5 wt% B-doped specimens and fracture toughness up to 4.0 MPa m1/2 for the 1 wt% Al-doped specimen. The improved fracture strength was a result of liquid-phase sintering and the improved toughness was a result of crack deflection along the grain boundaries. Crack branching was also observed in 1 wt% AlN and 0.5 wt% B-doped specimens.
The synthesis of SiC (both powder and whiskers) was carried out from rice husks with and without the use of catalysts (iron, cobalt and nickel). The introduction of the catalyst increased the reaction rate, the yield becoming up to three times that for the uncatalysed reaction, and increased the proportion of -phase from 95% to 99%. The general behaviour of the three catalysts was very similar, although nickel was the most effective from the point of view of reaction rate, and cobalt in producing larger crystal size. Analysis of the evolution of reaction rate, morphology of the whisker formed, evolution of gases during reaction, crystal size and intermetallic phases, led to a reaction mechanism based on the formation of an M-Si-C liquid phase which is essential for the nucleation and growth of the SiC whiskers.
Structural ceramic nanocomposites are reviewed with emphasis on the and systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.