Progress in Recrystallized SiC and Its Composites

Recrystallized silicon carbide（ RSi C）,a high purity Si C material sintered by the process of evaporation-condensation without any additives,is one of the most important structural materials in the fields of high temperatures. However,its low density and porous structure caused by the sintering mechanism in the absence of shrinkage,restrict its wide applications in engineering.This paper reviews the research progress and related technologies on the preparation of high-density RSi C and its composites. RSi C with relative high density up to 2. 75g·cm- 3can be obtained by a combination of pretreatment to Si C raw materials such as reshaping,modification and particle size distribution,and appropriate forming method. Post treatments such as cyclic pyrolysis and impregnation- recrystallization,and slurry impregnation- recrystallization are needed for the further density increase of RSi C（ 2. 99 g·cm- 3）. In addition,high performance RSi C- Mo Si2 and RSi C- Al composites obtained by melt infiltration are also reviewed.

KD-S SiC_f/SiC composites with BN interface fabricated by polymer infiltration and pyrolysis process

Continuous silicon carbide fiber reinforced silicon carbide matrix(SiC_f/SiC) composites are attractive candidate materials for aerospace engine system and nuclear reactor system. In this paper, SiC_f/SiC composites were fabricated by polymer infiltration and pyrolysis(PIP) process using KD-S fiber as the reinforcement and the LPVCS as the precursor, while the BN interface layer was introduced by chemical vapor deposition(CVD) process using borazine as the single precursor. The effect of the BN interface layer on the structure and properties of the SiC_f/SiC composites was comprehensively investigated. The results showed that the BN interface layer significantly improved the mechanical properties of the KD-S SiC_f/SiC composites. The flexure strength and fracture toughness of the KD-S SiC_f/SiC composites were evidently improved from 314±44.8 to 818±39.6 MPa and 8.6± 0.5 to 23.0±2.2 MPa·m^(1/2), respectively. The observation of TEM analysis displayed a turbostratic structure of the CVD-BN interface layer that facilitated the improvement of the fracture toughness of the SiC_f/SiC composites. The thermal conductivity of KD-S SiC_f/SiC composites with BN interface layer was lower than that of KD-S SiC_f/SiC composites without BN interface layer, which could be attributed to the relative low thermal conductivity of BN interface layer with low crystallinity.

Novel C_(sf)/SiBCN composites prepared by densifying C_(sf)/MA-SiBCN with the PIP process:Oxidation behavior and damage mechanism

To improve the oxidation resistance of short carbon fiber(C_(sf))-reinforced mechanically alloyed SiBCN(MA-SiBCN)(C_(sf)/MA-SiBCN)composites,dense amorphous C_(sf)/SiBCN composites containing both MA-SiBCN and polymer-derived ceramics SiBCN(PDCs-SiBCN)were prepared by repeated polymer infiltration and pyrolysis(PIP)of layered C_(sf)/MA-SiBCN composites at 1100℃,and the oxidation behavior and damage mechanism of the as-prepared C_(sf)/SiBCN at 1300–1600℃ were compared and discussed with those of C_(sf)/MA-SiBCN.The C_(sf)/MA-SiBCN composites resist oxidation attack up to 1400℃ but fail at 1500℃ due to the collapse of the porous framework,while the PIP-densified C_(sf)/SiBCN composites are resistant to static air up to 1600℃.During oxidation,oxygen diffuses through preexisting pores and the pores left by oxidation of carbon fibers and pyrolytic carbon(PyC)to the interior of the matrix.Owing to the oxidative coupling effect of the MA-SiBCN and PDCs-SiBCN matrices,a relatively continuous and dense oxide layer is formed on the sample surface,and the interfacial region between the oxide layer and the matrix of the as-prepared composite contains an amorphous glassy structure mainly consisting of Si and O and an incompletely oxidized but partially crystallized matrix,which is primarily responsible for improving the oxidation resistance.

Mechanical Properties and Oxidation Behaviors of Self-Healing SiCf/SiC-SiBCN Composites Exposed to H_(2)O/O_(2)/Na_(2)SO_(4)Environments

The oxidation behaviors and their influence on the mechanical properties of self-healing SiCf/SiC-SiBCN composites were investigated in H_(2)O/O_(2)and H_(2)O/O_(2)/Na_(2)SO_(4)environments at 1200‒1350℃for 100 h.As the temperatures increase from 1200 to 1350℃,the oxidation rate constants increase from 0.45×10^(–7)to 1.58×10^(–7)mg^(2)/(mm^(4) h)in H_(2)O/O_(2),and from 1.02×10^(–7)to 3.42×10^(–7)mg^(2)/(mm^(4) h)in H_(2)O/O_(2)/Na_(2)SO_(4).The involvement of Na_(2)SO_(4)leads to the formation of a loose lamellar oxide layer,the breakage of the SiBCN/CVI-SiC interface and the decrease in the oxide viscosity,thus accelerating the oxidation of the composites.The composites show the maximum retention rate of strength(102%,535.71 MPa)after oxidation in H_(2)O/O_(2)at 1200℃due to the good self-healing capacity of the produced glass,while the minimum(82%,430.56 MPa)in H_(2)O/O_(2)/Na_(2)SO_(4)at 1350℃caused by the severe microstructural corrosion derived from Na_(2)SO_(4).

Enhanced mechanical property and tunable dielectric property of SiC_(f)/SiC-SiBCN composites by CVI combined with PIP

The SiBCN matrix via chemical vapor infiltration(CVI)or/and polymer infiltration pyrolysis(PIP)technologies was orderly introduced to SiC_(f)/SiC composites to optimize the mechanical property and electromagnetic(EM)shielding effectiveness simultaneously.The BN interface with the thickness of 350 nm was designed to obtain a little stronger interface bonding.The flexural strength of SiC_(f)/SiC-SiBCN composites reached 545.45±29.59 MPa thanks to the crack deflection between the CVI SiC and CVI SiBCN,as well as CVI SiBCN and PIP SiBCN matrix because of the modulus difference between them.The fracture toughness(KiC)with the value of 16.02±0.94 MPa·m^(1/2) was obtained owing to the extension of crack propagation path.The adverse effect of stronger interface bonding was eliminated by the design of matrix microstructure for SiC_(f)/SiC-SiBCN composites.The thermal conductivity in the thickness direction was 7.64 W·(m·K)^(-1) at 1200℃and the electric resistivity decreased to 1.53×10^(3) Ω·m.The tunable dielectric property was obtained with the coordination of wave-absorption CVI SiBCN matrix and impedance matching PIP SiBCN matrix,and the total shielding effectiveness(SE_(T))attained 30.01 dB.It indicates that the SiC_(f)/SiC-SiBCN composites have great potential to be applied as structural and functional materials.