Qingliang Shen

Annealing effect on microstructure and mechanical properties of magnetron sputtering Ti–Si–C thin film

Abstract To investigate the influence of temperature on nucleation and growth of Ti3SiC2, Ti–Si–C thin films were deposited by magnetron sputtering from elemental targets of Ti, Si and C on Si(100) and Al2O3 substrates at temperature <200°C. Subsequently, the as deposited films were annealed in vacuum at 800, 950, 1100 and 1200°C respectively. The as deposited films consisted of amorphous TiC, amorphous Si and free C, as determined by X-ray diffraction and X-ray photoelectron spectroscopy. Annealing in vacuum <950°C resulted in improved crystallinity of TiC and formation of SiCx and Ti5Si3 phases. However, the Ti3SiC2 phase forms in films at 1100°C owing to the increase in Si diffusion coefficient. Moreover, the evolution of hardness and elastics modulus with annealing temperatures was determined by nanoindentation. The results showed a continuous decline of film hardness with increasing annealing temperature due to the formation of Ti3SiC2 and Ti5Si3 phases.

Microstructure and mechanical properties of SiC fibres after exposure at elevated temperature

To understand the service behaviour of SiC fibres, the effects of ambient environment and temperature on the microstructure, mechanical property and oxidation behaviour of these fibres were investigated. The result shows that, the surface of SiC fibres becomes rough after exposure in air from 973 to 1573 K due to the formation of small SiO2 particles, and a smooth SiO2 film will be formed on the SiC fibre at 1773 K. In Ar atmosphere, SiC fibres will change into clusters of large SiC crystals after heat treatment for 2 h at 2373 K. The tensile strength of SiC fibres decreased by 66 and 95% when the fibres were exposed at 1773 K for 5 min in air and 2373 K for 2 h in Ar, respectively. This degradation is associated with the evaporation of CO and SiO from the fibres as well as with SiC grain growth in the fibres.

Simultaneously improving the mechanical strength and electromagnetic interference shielding of carbon/carbon composites by electrophoretic deposition of SiC nanowires

C/C composites with both high specific strength and excellent electromagnetic shielding efficiency are required for advanced space applications. For this purpose, SiC nanowires (SiCNWs) were introduced in C/C composites by electrophoretic deposition. The presence of SiCNWs in C/C induces the formation of a SiCNW/pyrocarbon coaxial structure, which refines the matrix and decreases the stress concentration of the matrix. The average flexural strength of C/C composites increases by ∼100% after adding SiC nanowires due to enhanced interlaminar bonding after introducing the nanowires. Meanwhile, the average electromagnetic shielding efficiency in the X band (8.2–12.4 GHz) increases from 35.6 dB to 60.5 dB with an optimized nanowire content. Reflection is the predominant mechanism for electromagnetic shielding while introducing SiC nanowires also enhances the electromagnetic absorption within the composites. As pyrocarbon in the SiCNW–pyrocarbon coaxial structure shows a higher degree of crystallization, the coaxial structure increases the electron loops in the matrix and greatly improves reflection at high frequency. Also, the absorption of electromagnetic waves inside the composites is enhanced by introducing SiCNWs due to the increase of electron hopping among interfaces. This work proves that introducing SiC nanowires is promising in fabricating lightweight C/C composites with excellent mechanical and electromagnetic properties.