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.

•A Mg-alloy coating was deposited on Kapton substrate by magnetron sputtering.•AO erosion yield of coating was two orders of magnitude lower than that of Kapton.•The formation of a thin MgO scale resulted in excellent AO erosion resistance.In this paper, a continuous and uniform Mg-alloy coating was deposited by magnetron sputtering to protect flexible polyimide substrate from atomic oxygen (AO) erosion in the low earth orbit. AO exposure experiments indicated that the erosion yield of the as-deposited Mg-alloy coating was 8.897 × 10−26 cm3 atom−1, about two orders of magnitude lower than that of blank polyimide. During AO exposure, a relatively compact and continuous thin MgO scale formed on the surface of the Mg-alloy coating, resulting in the enhanced AO erosion resistance of the polyimide with Mg-alloy coating.Download high-res image (148KB)Download full-size image

•Breakaway oxidation of Ti3AlC2 occurs after oxidation at 1100 °C in air for 3171 h.•The oxidation law changes from cubic to linear after the transition time.•The critical Al content for selective oxidation is 5.99% lower than initial value.•Ti3AlC2 maintains its crystal structure after oxidation for 3250 h.•Pores generated at the Al2O3/Ti3AlC2 interface reduce the oxide scale adhesion.Catastrophic breakaway oxidation of Ti3AlC2 occurs during long-term oxidation at 1100 °C in air up to 4000 h. Correspondingly, the oxidation kinetics transforms from cubic law to linear law. The transition time is about 3171 h, and the corresponding critical Al content is about 5.99% lower than the initial content. Afterwards, Ti and C start to be oxidized instead of selective oxidation of Al, and Al2O3 scale loses its protectiveness. Meanwhile, voids generated at the Al2O3/Ti3AlC2 interface reduce the oxide scale adhesion. After 3250 h oxidation, Ti3AlC2 still maintains its crystal structure, indicating the capability of bearing great Al lack.

The critical Al content required to form protective Al2O3 scale on Ti3AlC2 at 1100 °C in air has been investigated by using a scale removal cyclic oxidation method. During the first five cycles, Al2O3 scale forms predominantly, and Ti3AlC2 keeps a similar low mass-gain rate. With increasing the cycle number, the Al content in Ti3AlC2 decreases. During the sixth cycle, the breakaway oxidation happens, and non-protective multilayer oxide forms. It is determined that the critical value of Al/Ti mole ratio is in the range of (0.970–0.977)/3 for the selective oxidation of Ti3AlC2, the corresponding atomic percent of Al is in the range of 16.25–16.34 at.%. Meanwhile, a part of Ti3AlC2 substrate decomposes to TiCx because of de-intercalation of enough Al from Ti3AlC2 during the sixth cyclic oxidation.

The oxidation resistance and thermal stability of Ti2AlC at 1600–1800 °C in air were studied by using induction heating method. The results showed that Ti2AlC could survive with relatively low oxidation rate at temperatures up to 1650 °C for a short period of time due to the formation of an Al2O3 inner layer with certain protectiveness. However, at 1700 and 1800 °C, severe oxidation of Ti2AlC happened, the entire Al2O3 inner layer no longer existed, and the whole oxide scale became porous, cracked and voluminous. In the oxidation processes, the Ti2AlC substrate decomposed to TiCx at 1700 °C, and transformed to Ti3AlC2 due to the reaction with TiCx at 1800 °C, indicating that the massive consumption of Al in Ti2AlC exceeded its deficiency tolerance.

•Electrochemical measurement on steels with various Cr contents is performed.•NaHSO3 solution is used as electrolyte to simulate industrial atmosphere.•Increasing Cr content leads to notably positive shift of corrosion potential.•Increasing Cr content results in decline of corrosion current density.•Electrochemical behavior is relevant to the characteristics of rust layer.To investigate the influence of chromium content on corrosion characteristics of weathering steels, the electrochemical measurements were performed on the steels containing 0–9% Cr (wt.%) in NaHSO3 aqueous solution. The results indicated that the open circuit potential of these steels shifted to the positive direction remarkably, because the additions of Cr improved the passivation capability of the steels. The corrosion current density of the steels containing more than 7% Cr (wt.%) decreased significantly after pre-rusted treatment, implying the corrosion resistance could be enhanced by the formation of protective goethite rust layer.

Highlights•The oxidation behaviour of C–ZrB2–SiC at 1600–2000 °C in air was investigated.•The oxidation kinetics was characterized via in situ dimension monitoring.•ZrB2 + SiC further improved the oxidation resistance of graphite compared to ZrB2.•The passive/active oxidation of SiC determined the oxide scale microstructure.The effects of ZrB2 and ZrB2 + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB2 + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB2 single addition, because the oxide scale formed on C–ZrB2–SiC is denser and thinner due to the existence of glassy SiO2. As the oxidation temperature increases, the oxidation rate of C–ZrB2–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.

Porous alumina ceramic was prepared by freeze casting method using tert-butyl alcohol as the solvent. The as–prepared porous alumina ceramic possessed long straight porous structure. The non-dendrite pore feature was quite distinguished from that prepared based on common solvents such as water and camphene. The porosity of the ceramic could be regulated through the solid loading. When the solid loading in the slurry was 20 vol%, the porosity of the alumina ceramic was 65%. With decreasing the solid loading, the porosity of the alumina ceramic increased linearly. The relationship between the total porosity (P) and initial solid loading (X) can be expressed as P = 98.8−1.7X. The ultra-high porosity of 82% could be achieved when the solid loading was 10 vol%. Moreover, the density of the porous alumina ceramic with the porosity of 82% was even lower than water’s. The compressive strength of the porous alumina ceramic with the porosity of 63 and 82% was determined to be 37.0 and 2.6 MPa, respectively.

High temperature oxidation behavior of the Cr2AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al2O3 scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr2AlC coating resulted in the transformation of Cr2AlC to Cr–C phases. Compared with bulk Cr2AlC, the Cr2AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.Highlights► High temperature oxidation behavior of Cr2AlC coating was investigated. ► Cr2AlC coating can improve the high temperature oxidation resistance of Ni-based M38G alloy. ► The good oxidation resistance of the Cr2AlC coating was due to the formation of Al2O3 oxide scale. ► Columnar grain boundaries in Cr2AlC coating lead to internal oxidation and nitridation. ► The internal oxidation and nitridation accelerate the degradation of the Cr2AlC coating.

By using surface sol–gel method with perhydropolysilazane (PHPS) as a precursor, a silica coating was prepared on a Kapton substrate as an atomic oxygen (AO) protective coating. The AO exposure tests were conducted in a ground-based simulator. It is found that the erosion yield of Kapton decreases by about three orders of magnitude after the superficial application of the coating. After AO exposure, the surface of the coating is smooth and uniform, no surface shrinkage induced cracks or undercutting erosion are observed. This is because that during AO exposure the PHPS is oxidized directly to form SiO2 without through intermediate reaction processes, the surface shrinkage and cracking tendency are prohibited. Meanwhile, this PHPS derived silica coating also presents self-healing effect due to the oxidation of free Si. Compared with other kinds of silica or organic polymer coatings, this PHPS derived silica coating exhibits a superior AO erosion resistance.

In order to improve the erosion resistance of spacecraft materials in atomic oxygen (AO) environments, a kind of polydimethylsiloxane (PDMS)/SiO2 hybrid coatings has been prepared on Kapton by a sol–gel process. The tetraethoxysilane (TEOS) and silanol-terminated PDMS were used as two main precursors. The content of inorganic silica in the PDMS/SiO2 hybrid coatings varied, achieving a maximum value of 40 wt% under the addition of a coupling agent. The PDMS/SiO2 hybrid coatings possessed good adhesion and flexibility, improved thermal stability. The AO resistance of the hybrid coatings was investigated by exposure tests in the AO simulator. The results showed that the hybrid coatings had excellent AO resistance. The PDMS/SiO2 (20 wt%) hybrid coating had an erosion rate two orders of magnitude slower than the polyimide film, and did not crack during AO exposure. The coating performance was also found to improve with increasing SiO2 content.

Based on the structure characteristic of Ti3AlC2 and the easy formation of Ti3Al1 − xSixC2 solid solution, a Si interlayer was selected to join Ti3AlC2 layered ceramic by diffusion bonding method. Joining was performed at 1,300–1,400 °C for 120 min under 5 MPa load in an Ar atmosphere. The phase composition and interface microstructure of the joints were investigated by XRD, SEM and EPMA. The results revealed that Ti3Al(Si)C2 solid solution formed at the interface. The mechanism of bonding is attributed to silicon diffusing inward the Ti3AlC2. The strength of joints was evaluated by a 3-point bending test. The jointed specimens exhibit a high flexural strength of 285 ± 11 MPa, which is about 80% of that of the Ti3AlC2; and retain this strength up to 1,000 °C. The high mechanical performance of the joints indicates that diffusion bonding via a Si interlayer is effective to bond Ti3AlC2 ceramic.

The phase transformation of alumina formed during oxidation of β-NiAl coating prepared on M38G alloy by pack cementation was investigated. Oxidation experiments were conducted at 950 °C for various times from 2 to 180 min. The phase composition and microstructure of the oxide scales were investigated by using glancing angle XRD, AFM and SEM. The results showed that at the initial oxidation stage needle-like θ-Al2O3 was formed and then it covered the sample surface rapidly. The formation of α-Al2O3 grains beneath the θ-Al2O3 layer was favored by depletion of Al in the β-NiAl coating during oxidation. α-Al2O3 preferred growing on the top (ridge) of β-NiAl grains, which resulted in the formation of net-like α-Al2O3 inner layer. With increasing time, θ-Al2O3 transformed to α-Al2O3 gradually. After 180 min oxidation, most of θ-Al2O3 grains transformed into α-Al2O3. A mechanism of excessive voids' formation at the oxide/coating interface was also proposed in this paper.

Al2O3 thin films were deposited on a Ti3Al based alloy (Ti–24Al–14Nb–3V–0.5Mo–0.3Si) by sol–gel processing. Isothermal oxidation at temperatures of 900–1000 °C and cyclic oxidation at 800–900 °C were performed to test their effect on the oxidation behavior of the alloy. Results of the oxidation tests show that the oxidation parabolic rate constants of the alloy were reduced due to the applied thin film. This beneficial effect became weaker after longer oxidation time at 1000 °C. TiO2 and Al2O3 were the main phases formed on the alloy. The thin film could promote the growth of Al2O3, causing an increase of the Al2O3 content in the composite oxides, sequentially decreased the oxidation rate. Nb/Al enriched as a layer in the alloy adjacent to the oxide/alloy interface in both the coated and uncoated alloy. The coated thin film decreased the thickness of the Nb/Al enrichment layer by reducing the scale growth rate.