Polymeric materials used in spacecraft require to be protected with an atomic oxygen (AO)‐resistant layer because AO can degrade these polymers when spacecraft serves in low earth orbit (LEO) ...environment. However, mechanical damage on AO‐resistant coatings can expose the underlying polymers to AO erosion, shortening their service life. In this study, the fabrication of durable AO‐resistant coatings that are capable of autonomously healing mechanical damage under LEO environment is presented. The self‐healing AO‐resistant coatings are comprised of 2‐ureido‐41H‐pyrimidinone (UPy)‐functionalized polyhedral oligomeric silsesquioxane (POSS) (denoted as UPy‐POSS) that forms hydrogen‐bonded three‐dimensional supramolecular polymers. The UPy‐POSS supramolecular polymers can be conveniently deposited on polyimides by a hot pressing process. The UPy‐POSS polymeric coatings are mechanically robust, thermally stable, and transparent and have a strong adhesion toward polyimides to endure repeated bending/unbending treatments and thermal cycling. The UPy‐POSS polymeric coatings exhibit excellent AO attack resistance because of the formation of epidermal SiO2 layer after AO exposure. Due to the reversibility of the quadruple hydrogen bonds between UPy motifs, the UPy‐POSS polymeric coatings can rapidly heal mechanical damage such as cracks at 80 °C or under LEO environment to restore their original AO‐resistant function.
Self‐healing atomic oxygen (AO)‐resistant coatings are fabricated by depositing 2‐ureido‐41H‐pyrimidinone (UPy)‐functionalized polyhedral oligomeric silsesquioxane (POSS) on polyimides. The coatings composed of hydrogen‐bonded UPy‐POSS supramolecular polymers are mechanically robust, thermally stable, and transparent and exhibit excellent AO resistance. The AO‐resistant coatings can autonomously heal mechanical damage under a low‐earth‐orbit environment to restore their AO‐resistant function.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The crystal structure of the newly synthesized quaternary MAX phase (Cr2/3Ti1/3)3AlC2 was systematically characterized by various techniques. The space group of (Cr2/3Ti1/3)3AlC2 is determined to be ...P63/mmc by a combination of selected-area and convergent-beam electron diffraction techniques. Rietveld refinements of the neutron diffraction and X-ray diffraction data show that in (Cr2/3Ti1/3)3AlC2, Ti and Cr are ordered with Ti in the 2a and Cr in the 4f Wyckoff sites of a M3AX2 lattice. It is interesting to find that when the order of the magnetic moment of Cr atoms is considered, the ferromagnetic configuration of (Cr2/3Ti1/3)3AlC2 becomes the ground state. Meanwhile, the Raman-active mode wavenumbers of (Cr2/3Ti1/3)3AlC2 were calculated, and the theoretical data are quite consistent with the experimental data, further proving the ordered crystal structure of this phase. The formation of (Cr2/3Ti1/3)3AlC2 with a unique crystal structure may be related to the distinctly different electronegativities and covalent radii of Cr and Ti atoms.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The influence of alloyed chromium on atmospheric corrosion resistance (ACR) of weathering steels in simulated industrial atmosphere was investigated. The ACR of weathering steels was evaluated by ...determining the relative corrosion rate after periodic immersion wet/dry cyclic corrosion for 72h. The presence of Cr improves the ACR of weathering steels. The beneficial effect of alloyed chromium is attributed to both its promotion of the formation of a protective rust layer, which acts as a barrier against corrosion process, and its enhancement of passivation capability, which decreases the anodic dissolution of substrate alloys.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Short-carbon-fibers (C
sf
) reinforced Ti
3
SiC
2
matrix composites (C
sf
/Ti
3
SiC
2
, the C
sf
content was 0 vol%, 2 vol%, 5 vol%, and 10 vol%) were fabricated by spark plasma sintering (SPS) using ...Ti
3
SiC
2
powders and C
sf
as starting materials at 1300 °C. The effects of C
sf
addition on the phase compositions, microstructures, and mechanical properties (including hardness, flexural strength (
σ
f
), and
K
IC
) of C
sf
/Ti
3
SiC
2
composites were investigated. The C
sf
, with bi-layered transition layers, i.e., TiC and SiC layers, were homogeneously distributed in the as-prepared C
sf
/Ti
3
SiC
2
composites. With the increase of C
sf
content, the
K
IC
of C
sf
/Ti
3
SiC
2
composites increased, but the
σ
f
decreased, and the Vickers hardness decreased initially and then increased steadily when the C
sf
content was higher than 2 vol%. These changed performances (hardness,
σ
f
, and
K
IC
) could be attributed to the introduction of C
sf
and the formation of stronger interfacial phases.
In this work, the atomic oxygen (AO) erosion-resistance effect and mechanism of the Perhydropolysilazane (PHPS) coating were investigated from the perspective of element distribution in the depth ...direction. The results revealed that the coating demonstrated good adhesion and intrinsic AO erosion-resistance, which was attributed to the composition gradient formed in the coating. Moreover, the oxygen ratio of the SiOx on top layer of the coating could be elevated during AO exposure, strengthening the Ar ion etching durability of the coating. According to these results, an AO erosion-resistance mechanism model of the PHPS-derived SiOx coating was finally obtained.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Dense HfCxN1−x carbonitride ceramics are very promising as potential ultra-high temperature ceramics (UHTCs) for application under extremely harsh environments. However, the thermophysical and ...mechanical properties of the HfCxN1−x carbonitrides have not been investigated clearly. The present work prepared HfCxN1−x (x = 0.3, 0.4, 0.5, 0.6, 0.7) ceramics at 1950 °C under 30 MPa in flowing Ar atmosphere by using hot pressing sintering method. The relative densities of the samples obtained reached above 96%. Thermal conductivity of the as-prepared HfCxN1−x carbonitrides ranged from 19 to 24 W m−1 K−1 at room temperature. The increased role of electrons in thermal conduction caused by both increasing nitrogen content and increasing temperature, resulted in improved thermal conductivity, varying from 32 to 39 W m−1 K−1. With increasing nitrogen content, the electrical conductivity also increased, ranging from 149 to 213 × 104 Ω−1 m−1. With the increase of nitrogen content, Hf-C covalent bonds are gradually replaced by Hf-N covalent bonds with lower bond strength, resulting in HfC0.7N0.3 exhibiting the highest room-temperature flexural strength and hardness, HfC0.3N0.7 exhibiting the highest fracture toughness. Their mechanical properties are greatly improved over the binary HfC and HfN. The high-temperature flexural strength of the HfC0.7N0.3 decreased from 324 MPa at 1000 °C, to 139 MPa at 1600 °C and 100 MPa at 2000 °C. Meanwhile, it was revealed that the high-temperature flexural strength decreased with increasing nitrogen content for the as-prepared HfCxN1−x carbonitrides, similar to the changing trend of room-temperature flexural strength. The HfC0.3N0.7 possessed high-temperature plasticity at 2000 °C, attributed to the ability of the coarser grain to produce numerous layer dislocations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The oxidation tests of Ti_3AlC_2 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 ...Ti_3AlC_2 were investigated. The microstructure, grain size and phase compositions of Ti_3AlC_2 substrate didn＇t change after oxidation, hence the oxide removed Ti_3AlC_2 substrate maintained its initial flexural strength and hardness. However, the flexural strength of oxide retained Ti_3AlC_2 decreased by about 5%. Acoustic emission monitoring indicated that during the process of three-point bending test, the formed Al_2O_3 scale on Ti_3AlC_2 surface fractured firstly in a cleavage manner, then the substrate/oxide interface cracked,and finally the Ti_3AlC_2 substrate fractured. The mechanical degradation was caused by the preferential formation of cracks in brittle Al_2O_3 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 Al_2O_3 scale on the surface of Ti_3AlC_2 substrate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Amorphous Cr-Al-C coating firstly transformed into solid solution, and then into Cr2AlC MAX phase during annealing.•All the coatings exhibited passivation behaviors, and amorphous coating possessed ...the lowest passive current density.•All the passive films exhibited p-type semiconductor characteristics.•The passive film on the amorphous coating was most protective for containing a relatively high content of Cr2O3.
The microstructure evolution of an amorphous Cr-Al-C coating prepared by magnetron sputtering was investigated. With increasing annealing temperature, three kinds of coatings, including amorphous coating, partially crystallized coating ((Cr,Al)2Cx solid solution), and fully crystallized coating (Cr2AlC MAX phase), were obtained. The electrochemical measurements in 3.5 wt. % NaCl solutions indicated that all of the coatings exhibited passivation behaviors, and amorphous coating possessed the lowest passive current density. All the passive films exhibited p-type semiconductor characteristics, and the passive film on amorphous coating provided the best protection due to the presence of a relatively high content of Cr2O3.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•The oxidation behavior of Ti3AlC2 is thickness-dependent.•Selective oxidation does’t occur for Ti3AlC2 foil with thickness of 85 μm.•The 140 μm thick foil possesses more stable Al2O3 scale and ...longer lifetime.•The mechanism of breakaway oxidation of Ti3AlC2 is discussed.
The oxidation behavior of Ti3AlC2 foils at 1100 °C in air has been investigated. It is found that the oxidation behavior of Ti3AlC2 is thickness-dependent. For the 85 μm thick sample, breakaway oxidation occurs initially. For the 95 μm thick sample, selective oxidation occurs initially and transforms to breakaway oxidation after 12.5 h. When the thickness exceeds 110 μm, Ti3AlC2 foils exhibit much longer lifetime. Continuous Al2O3 scale is formed on the 75−120 μm thick wedge-shaped sample, suggesting that diffusion rate of Al is high in the whole body during oxidation. The mechanism of breakaway oxidation of Ti3AlC2 is discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
ZrB2-20SiC and ZrB2-20SiC-10TaSi2 (volume fraction) coatings were prepared on siliconized graphite by slurry method. The as-prepared coatings were oxidized at 1700 °C in air for 30 min by induction ...heating method. The oxidation resistance of the coatings was evaluated based on the consumption of the coating. The addition of TaSi2 significantly improved the ultra-high temperature oxidation resistance of ZrB2-20SiC coating, due to the formation of a Ta dissolved oxide layer with much lower oxygen permeability.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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