C/C-HfC-SiC composites prepared by precursor infiltration and pyrolysis process were ablated by oxyacetylene torch under two different flame conditions. The ablation performance of the composites was investigated in the heat flux of 2.38 MW/m2 (HF-L) and 4.18 MW/m2 (HF-H) for 60 s. The mechanical denudation in 4.18 MW/m2 (HF-H) was higher than that in 2.38 MW/m2 (HF-L), while the results indicated that the composites had a similar and good ablation property under two different flame conditions. C/C-HfC-SiC composites can adapt the heat flux from 2.38 MW/m2 to 4.18 MW/m2. The HfO2 was not melted completely in the heat flux of 2.38 MW/m2 (HF-L). So, both HfO2 and SiO2 layers acted as an effective barrier to the transfer of heat and oxidative gases into the underlying carbon substrate. SiO2 was severely consumed in 4.18 MW/m2 (HF-H), where the HfO2 molten layer played a more important role in protecting the inner composite.
Barium-strontium aluminosilicate (BSAS) and Si/BSAS coatings were fabricated on the surface of C/SiC composites via a two-step laser cladding process. The microstructure, mechanical properties, and the water vapor corrosion behavior of the samples were investigated. The BSAS coating was found to be tightly bonded to the substrate and only a few pores and microcracks were observed. The introduction of a silicon middle layer was revealed to reduce thermal stress and promote the healing of defects formed during the laser cladding process. To evaluate the corrosion resistance, the BSAS and Si/BSAS-coated C/SiC composites were exposed to an atmosphere of 50% H2O and 50% O2 at 1250?°C. The resulting weight change and flexural strength were measured as a function of the corrosion time. The addition of the silicon middle layer below the BSAS top layer resulted in a better resistance to water vapor corrosion. Furthermore, the Si/BSAS-coated samples showed a lower weight loss and a smaller reduction in flexural strength than the BSAS-coated and the uncoated samples during water vapor corrosion. Thus, laser cladding is demonstrated to be an effective and feasible method to fabricate high-quality ceramic coatings on C/SiC composites. The introduction of a silicon middle layer can inhibit defect formation during the laser cladding process and protect the composite from water vapor corrosion.
The effect of friction stir processing (FSP) on the pitting corrosion and the intergranular attack of 7075 aluminum alloy was investigated. Three friction stir processed samples were produced by employing a constant tool travel speed of 100?mm/min at the rotating speeds of 630, 1000 and 1600?rpm. It was demonstrated that the processed samples suffered from both pitting and intergranular corrosion. Also, the sample processed at 1600?rpm exhibited the best pitting corrosion resistance. For all FS processed samples, the corrosion attack in the heat affected zone was pitting corrosion, whereas no intergranular corrosion was detected in this area.
Titanium-aluminum-nitride (TiAlN) films were grown by plasma-enhanced atomic layer deposition (PEALD) on 316L stainless steel at a deposition temperature of 200?°C. A supercycle, consisting of one AlN and ten TiN subcycles, was used to prepare TiAlN films with a chemical composition of Ti0.25Al0.25N0.50. The addition of AlN to TiN resulted in an increased electrical resistivity of TiAlN films of 2800 µΩ cm, compared with 475 µΩ cm of TiN films, mainly due to the high electrical resistivity of AlN and the amorphous structure of TiAlN. However, potentiostatic polarization measurements showed that amorphous TiAlN films exhibited excellent corrosion resistance with a corrosion current density of 0.12 µA/cm2, about three times higher than that of TiN films, and about 12.5 times higher than that of 316L stainless steel.
C/C-ZrB2-ZrC-SiC composites were fabricated by polymer infiltration and pyrolysis (PIP) with a preform of Cf/ZrB2. The carbon fibers and the resin carbon were coated with ceramic layer after PIP in the composites. The composite presents a pseudo-plastic fracture due to deflection of cracks and pullout of fibers. The composite has a higher bending strength by this method in comparison with the conventional PIP process due to fewer heat treatment cycles. The static oxidation test shows that the mass loss of the composites is no more than 1% after 20 min oxidation at 1100 °C. The “core-shell” structure between ZrC-SiC ceramic and other phases plays a positive role in preventing the inward diffusion of oxygen. The ablation resistance of the C/C-ZrB2-ZrC-SiC composite samples was tested using a plasma generator. After ablation for 120 s, the mass and linear ablation rates of the composites are 4.65 mg cm-2 s-1 and 2.46 μm s-1, respectively. The short carbon layer shows a better ablation resistance than the nonwoven carbon fabric layer after the ceramic coating is peeled off because of its higher ceramic content.
The formation of protective multifunctional coatings on magnesium alloy MA8 using plasma electrolytic oxidation (PEO) in an electrolytic system containing nanosized particles of titanium nitride was investigated. Electrochemical and mechanical properties of the obtained layers were examined. It was established that microhardness of the coating with the nanoparticle concentration of 3 g l-1 increased twofold (4.2 ± 0.5 GPa), while wear resistance decreased (4.97 × 10-6 mm3 N-1 m-1), as compared to respective values for the PEO-coating formed in the electrolyte without nanoparticles (2.1 ± 0.3 GPa, 1.12 × 10-5 mm3 N-1 m-1).
The oxidation tests of Ti3AlC2 were conducted at 1100 and 1200 °C in air for 48 and 360 h, respectively, and the effects of high temperature oxidation on the flexural strength and hardness of Ti3AlC2 were investigated. The microstructure, grain size and phase compositions of Ti3AlC2 substrate didn't change after oxidation, hence the oxide removed Ti3AlC2 substrate maintained its initial flexural strength and hardness. However, the flexural strength of oxide retained Ti3AlC2 decreased by about 5%. Acoustic emission monitoring indicated that during the process of three-point bending test, the formed Al2O3 scale on Ti3AlC2 surface fractured firstly in a cleavage manner, then the substrate/oxide interface cracked, and finally the Ti3AlC2 substrate fractured. The mechanical degradation was caused by the preferential formation of cracks in brittle Al2O3 scale as well as at defective and lacunose grain boundaries of the substrate where stress concentration generated. The mechanical degradation was insensitive to oxidation temperature and time in the present conditions. In addition, the surface hardness increased significantly after oxidation due to the formed hard Al2O3 scale on the surface of Ti3AlC2 substrate.
The corrosion behavior of copper exposed in a simulated coastal-industrial atmosphere has been investigated using weight loss measurement, scanning electron microscopy, X-ray diffraction, potentiodynamic polarization and in-situ electrochemical impedance spectroscopy (EIS) with micro-distance electrodes. The results show that corrosion kinetics follows the empirical equation D = Atn. The main corrosion products are composed of Cu2O, Cu2Cl(OH)3 and Cu4Cl2(OH)6. A two-layer structure comprising a loose outer layer and a compact inner layer forms the corrosion products during corrosion process. SO2 has been found to promote the formation of Cu4Cl2(OH)6.
The CoFe2 alloy (CF) was prepared by reducing CoFe2O4 in the H2 ambient. Subsequently the CF sample was nitrided in the NH3 atmosphere to produce the composite of CoFe2N and CoFe2. The magnetostriction, thermal expansion, resistivity and corrosion resistance of CF sample and the nitrided sample (CFN) at 1000 °C were investigated. The saturation magnetostriction coefficiency λs and thermal expansion coefficient α at 300 K for the nitrided CFN were 50 ppm and 10 ppm/K, respectively, approximately equal to those for the CF sample. However, compared with CF, CFN presents a decrease in temperature coefficient Rλ (300 K) of magnetostriction by ~11%. The smaller resistivity and improved corrosion resistance in the H2SO4 solution may expand the applications of the CoFe2 in the fields needing lower resistivity or in the acidic environment.
The effect of copper addition to 2205 duplex stainless steel (DSS) on its resistance against pitting corrosion by the Pseudomonas aeruginosa biofilm was investigated using electrochemical and surface analysis techniques. Cu addition decreased the general corrosion resistance, resulting in a higher general corrosion rate in the sterile medium. Because DSS usually has a very small general corrosion rate, its pitting corrosion resistance is far more important. In this work, it was shown that 2205-3%Cu DSS exhibited a much higher pitting corrosion resistance against the P. aeruginosa biofilm compared with the 2205 DSS control, characterized by no significant change in the pitting potential and critical pitting temperature (CPT) values. The strong pitting resistance ability of 2205-3%Cu DSS could be attributed to the copper-rich phases on the surface and the release of copper ions, providing a strong antibacterial ability that inhibited the attachment and growth of the corrosive P. aeruginosa biofilm.