Superior High-temperature Resistance Research Articles

An end-capping strategy for shape memory phthalonitrile resins via annealing enables conductivity and wave-absorption

Shape memory polymers (SMPs) with elevated switching temperatures and enduring thermal properties are highly desirable yet challenging in certain aerospace applications. In this study, high-temperature shape memory phthalonitrile resins (SMPNs) were developed using an end-capping strategy with the uniphthalonitrile (UPN) as the end-capping reagent to modulate the density of the cross-linked network. The resultant SMPNs exhibit a glass transition temperature (Tg) at approximately 300 °C, with a shape fixation rate of about 98 % and a shape recovery rate around 97 %. Importantly, these SMPNs also demonstrated exceptional thermal stability, with a thermal decomposition temperature exceeding 445 °C. Additionally, an oxyacetylene ablative test revealed the SMPNs' robust ablative resistance, as the linear ablation rate remained below 0.1 mm/s and the mass ablation rate was less than 0.1 g/s. The commendable shape memory performance and superior high-temperature resistance allowed the SMPNs to successfully undergo the shape recovery process when subjected to a butane flame. Furthermore, after a high-temperature annealing process, the SMPNs exhibited electrical conductivity due to graphitization, even possessing wave-absorbing properties within a specific frequency range. It is foreseeable that these multifunctional SMPNs will have extensive applications in the aerospace sector, given the advancement of smart structures.

Influence of high temperatures on the mechanical and microstructural properties of hybrid steel-basalt fibers based ultra-high-performance concrete (UHPC)

One of the major problem in ultra-high-performance concrete (UHPC) after exposure to high-temperatures is explosive spalling. Adding a single type of fibers alone is insufficient sometimes for suppressing such spalling, and therefore, hybridization of fibers has been adopted, particularly steel with organic fibers. However, the degradation of organic fibers at ambient and elevated temperatures is a significant challenge in the long term and needs to be addressed. Recently, inorganic basalt fibers have been considered as an alternative to organic fibers due to their better mechanical strength and high-temperature resistance characteristics. In this study, hybridizing steel with basalt fiber is proposed as a potential solution to the aforementioned problem. The spalling behavior, residual compressive strength, microstructure, porosity, hydration kinetics, phase decomposition, and quantification of hydration products were evaluated for hybrid steel-basalt fiber reinforced UHPC after exposure to high-temperatures. The study results indicated that hybrid steel-basalt fiber reinforced UHPC successfully prevented explosive spalling behavior. The use of inorganic mineral basalt fibers offers a promising solution for developing UHPC with superior high-temperature resistance.

Robust design of durable PTFE/graphene hollow fiber composite membrane for high-temperature lubricant recycling

Nowadays, the lubricant recycling becomes difficult due to its complex composition and high viscosity. Reinforced hollow fiber membranes (HFM) have been widely used in various separation fields due to their controllable separation accuracy, large specific area, backwashability, as well as low operation and maintenance requirements. Herein, a durable polytetrafluoroethylene (PTFE)/graphene (GE) hollow fiber composite membrane (HFCM) with high temperature resistance was designed through the laminated-sintering process to solve the lubricant issue. The membrane featured a solid three-layer reinforced structure and compared with the pure GE membrane, the adverse effects from GE brittleness are efficiently avoided, which ultimately improved the membrane flexibility and morphological stability. Moreover, this PTFE/GE HFCMs showed superior high-temperature resistance and lipophilicity with an oil contact angle of 0°. With a reasonable regulation of lubricant temperature and GE content, the membrane flux exceeded 917 L·m−2·h−1·bar−1 and stabilized at 458 L·m−2·h−1·bar−1 after 12 h of membrane operation at 150 °C. The membrane flux remained at a relatively high level of 103 L·m−2·h−1·bar−1 after four cycles, with a flux recovery over 83 %. In addition, the membrane separation-efficiency up to 99.02 % for activated carbon in lubricants. Overall, the PTFE/GE HFCMs presented great potential for recycling lubricants or other heavy crude oils at high temperatures.

Electrochemical Properties and Electrochemical Milling of (TiB + TiC)/TC4 Composites

The (TiB + TiC)/TC4 composite is a new material with superior high-temperature resistance properties and structural strength. However, it cannot be easily machined by conventional methods. Electrochemical machining is an ideal method for the processing of titanium-matrix composites. In this study, the electrochemical properties of (TiB + TiC)/TC4 composites were investigated. The polarization curve of (TiB + TiC)/TC4 in a NaCl solution with a volume fraction of 20% was measured. The decomposition voltage was approximately 7.2 V. Basic electrochemical dissolution experiments were carried out on TC4 and (TiB + TiC)/TC4. The surface morphology was measured to investigate the dissolution mechanism. It was demonstrated that the reinforcing phase will fall off with the dissolution of TC4 and will not participate in the electrochemical dissolution reaction. Finally, electrochemical milling experiments on (TiB + TiC)/TC4 with different voltages and cathode feed rates were carried out to study the removal rate and surface roughness of the material. The material removal rate (MRR) increased, while the surface roughness decreased with the increases in the voltage and feed rate. A maximum MRR of 171.11 mm3/min and minimum surface roughness of 4.281 μm were obtained at a voltage of 50 V with a feed rate of 20 mm min−1.

Research on hole depth in femtosecond laser deep micropore processing technology based on filament effect

Ceramic-based composites have been applied to turbine components of new-generation aerospace engines due to their excellent properties. Despite the superior high-temperature resistance of SiC/SiC composites, it is necessary to combine with film cooling technology to guarantee normal function of turbine components. However, SiC/SiC composites are typical difficult-to-process materials，and the depth of film cooling holes have increasingly high requirements, which have become one of the major problems in the manufacturing of aerospace engine turbine parts. In this paper, the spatial energy density distribution based on filament effect of femtosecond laser was theoretically studied, it is revealed that the filament effect of femtosecond laser with high single pulse energy will lead to the peak of energy density shift towards the negative defocus direction. Based on the theoretical research, the experimental study on the processing of deep micropore in SiC/SiC composites by femtosecond laser was carried out, main results show that in equal conditions, the depth of micropore in feed trepanning is deeper than that in trepanning, and that in trepanning is deeper than that in direct punching. This research provides both theoretical and technical support for processing of micropore with high aspect ratio by ultrafast laser on the difficult-to-process materials, which have high practical value in the aerospace field.

Effect of alum sludge ash on the high-temperature resistance of mortar

The effect of high temperature on the mechanical, durability, and microstructural properties of mortar containing alum sludge ash (ASA) was investigated in this paper. The ASA was derived from grinding and calcinating alum sludge, a typical by-product of the drinking water treatment processes. Four different mortar mixtures with ASA content at weight percentages of 0%, 10%, 20%, and 30% (as cement replacement) were exposed to high temperatures of 300 °C, 550 °C, and 800 °C, respectively. The experimental results showed that mortar samples containing up to 20% of cement replaced with ASA exhibited superior high-temperature resistance to the reference ones (without ASA), especially after exposure to 800 °C. The thermal analysis determined the portlandite consumption because of ASA pozzolanic reaction, and the x-ray diffraction pattern showed that the ASA reaction might contribute to the formation of aluminum-bearing phases with excellent refractoriness in the binder matrix. In addition, crack examination conducted by backscattered electron images evidenced that the ASA addition mitigated the binder paste degradation.

Microstructure transformation and high temperature oxidation properties of (FeCoNi)100-xAlx new-type high-entropy alloys

Oxidation properties of (FeCoNi)100-xAlx (x = 0, 5, 10, 15, and 20) high entropy alloys were investigated in air atmosphere at 800 °C. Combined scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) sur-face analysis and X-ray diffraction (XRD) studies revealed that the addition of Al transformed the alloy from a single FCC solid solution structure to a mixed structure of FCC and BCC, from an unevenly distributed rod-like structure to a uniformly distributed dendritic structure. With the increase of Al content, the oxidation scale mainly consists of Al2O3 and Co2O3 and the oxidation kinetic curve of (FeCoNi)80Al20 HEA reaches the minimum value, which clarifies the superior high temperature resistance.

Effect of Al content on high temperature oxidation resistance of AlxCoCrCuFeNi high entropy alloys (x=0, 0.5, 1, 1.5, 2)

The oxidation behavior of AlxCoCrCuFeNi (x = 0, 0.5, 1, 1.5, 2) high entropy alloys are discussed in this work. Five kinds of HEAs with nominal compositions of are prepared by vacuum arc melting and subjected to isothermal oxidation at 1000°C for 100 h in static air. The results indicate that three typical CuNi-rich, AlNi-rich and matrix phases emerge after oxidation. In all oxidized HEAs there is a tendency to generate the Cr depletion or Al depletion region directly beneath the oxidation layer. With the increase of Al content, the oxidation scale mainly consists of alumina instead of Cr2O3 and the parabolic rate constant of Al2CoCrCuFeNi HEA reaches the minimum value, which clarifies the superior high temperature resistance.

Antioxidant Metabolism, Photosystem II, and Fatty Acid Composition of Two Tall Fescue Genotypes With Different Heat Tolerance Under High Temperature Stress.

Tall fescue (Festuca arundinacea Schreb.) is a typical and widely used cool-season turf grass. High temperature is a key factor that limits its utility. The objectives of this study were to investigate the behaviors of fatty acid composition and its gene expression patterns in heat-resistant genotype “TF71” and heat-sensitive genotype “TF133” exposed to heat stress (40/35°C, 14/10 h), and to broaden our comprehension about the relationship between heat tolerance and fatty acid function. The result showed that heat stress increased the malondialdehyde (MDA) content and relative electrolyte leakage (EL), but decreased the level of chlorophyll and the activity of superoxide dismutase (SOD) and peroxidase (POD) when compared to the controls, to a greater extent in “TF133.” This result proved that “TF71” had superior high-temperature resistance. Furthermore, comparing the changes in the composition of fatty acid and the expression of the genes involved in its synthesis between the two different genotypes under heat stress, we found that heat stress increased the degree of unsaturation, UFA/SFA, and double bond index (DBI) in “TF71.” Moreover, quantitative RT-PCR revealed that heat stress altered the expression of the genes involved in fatty acid synthesis, including ACAC, FabD, FabF, FabH, FabI, and FatA. According to these findings, we can speculate that increasing the unsaturation degree of fatty acid or controlling the equilibrium ratio of UFA/SFA might be closely associated with the improving of the heat resistance in tall fescue.

Formation and oxidation resistance of Al/Ni coatings on low carbon steel by flame spray

In this work the effect of heat treatment on the structure and the durability of double layer Ni/Al coatings was investigated. An initial Ni–Cr layer was primarily deposited by thermal spray, followed by a successive Al metallic layer in a second stage. The specimens were then heat treated in inert atmosphere. Nickel aluminide compounds were formed as a result of the diffusion between the two layers upon annealing. Indeed, scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy examinations of the coated samples revealed that the as sprayed coatings contain two distinct Al and Ni–Cr layers while the heat treated coatings were gradually transformed to Al3Ni depending on the heat treatment parameters. The as sprayed and annealed specimens were tested by subjecting them in an air environment at 700°C and 800°C, for the evaluation of their durability and resistance. It was found that the annealed samples have superior high temperature resistance compared with the as sprayed coatings and bare steel, which is attributed to the presence of Ni–Al compounds, to the low coating porosity and to the formation of dense oxides on the coating surface.

SiCp/Al2O3 Ceramic Matrix Composites Prepared by Directed Oxidation of an Aluminium Alloy for Wear Resistance Applications

SiCp/Al2O3 ceramic matrix composites were fabricated using DIrected Metal OXidation (DIMOX) process. Continuous oxidation of an Al-Si-Mg-Zn alloy with appropriate dopants along with a preform of SiC particulate has led to the formation of alumina matrix surrounding silicon carbide particulates. The fabricated composites were investigated for wear resistance on a pin-on-disc tribometer against gray cast iron. Wear surfaces were examined under scanning electron microscope in order to identify the wear mechanisms involved. SiCp/Al2O3 matrix composites were found to possess enhanced mechanical properties such as flexural strength, fracture toughness and wear resistance, all at an affordable cost of fabrication. The superior high temperature resistance of SiCp/Al2O3 based ceramics makes them suitable for tribological applications above room temperature or in high speed non-lubricated sliding.

Strength and Resistance of Alkali-Activated Slag Concrete to High Temperature

This study presents an investigation into high-temperature resistance of alkali-activated slag concrete (AASC). Sodium oxide (Na2O) concentrations of 4%, 5% and 6% of slag weight and liquid sodium silicate (SiO2) with modulus ratio of 0.8 ( mass ratio of SiO2 to Na2O ) were used as activators to activate granulated blast furnace slag (GBFS). All cylindrical specimens with the same binder content and liquid/binder ratio of 0.5 were cast and cured in the air, under the saturated limewater and in a curing room at relative humidity of 80% RH and temperature of 60 °C, respectively. Test results demonstrate that the high-temperature resistance of AASC decreased with an increase of temperature. The compressive strength and high-temperature resistance of AASC improved with an increase dosage of Na2O and AASC cured at relative humidity of 80% RH and temperature of 60 °C has the superior performance, followed the AASC by air curing and saturated limewater curing. The higher compressive strength and superior high-temperature resistance have been obtained in AASC than comparable OPC.

Effects of EB-PVD Process TGO Formation and Growth within Thermal Barrier Coatings

It has been found that under oxygen partial pressure of ~2×10-6 kPa, the high-temperature oxidation of thermal barrier coatings (TBCs) occurred during an electron beam physical vapor deposition (EB-PVD) process for producing the TBCs top ceramic coating. In the present investigation, two modified bond coats (BCs) of NiCrAlY with Si addition, and NiCrAlY with Co and Hf additions, were developed by Arc Ion-plating technique to study the effects of the EB-PVD process on thermally grown oxide (TGO) formation and growth. The isothermal and cyclic oxidation tests were conducted and the cross-sectional morphologies of the specimens were examined to compare the high-temperature oxidation behaviors of the two TBCs. It was found that a mixed oxide layer have been developed in the as-deposited TBCs with a NiCrAlYSi BC. The mixed oxide layer mainly included Cr2O3, NiO, Al2O3 and their spinel. With the mixed oxide layer, TBCs with the NiCrAlYSi BC showed a superior high-temperature resistance on later high-temperature exposure to TBCs with NiCoCrAlYHf BC, where no mixed oxide layer was observed. The pre-formed mixed oxide layer apparently shortened the time to fully develop a protective α-Al2O3 layer and therefore restrained the TGO growth in TBCs.

Sliding speed-temperature wear transition maps for Si3N4/iron alloy couples

The superior high temperature resistance of silicon nitride (Si3N4) based ceramics makes them suitable for tribological applications above room temperature or in high speed unlubricated sliding. There are some published works on the wear behaviour of Si3N4/metal alloys. However, experimental data are shown in a form that is not of direct use for engineers involved in materials selection. In the present work, Si3N4 pins were tested against tool steel and grey cast iron on a pin-on-disc tribometer. Ceramics were produced by hot-pressing and tested without lubrication at variable temperature and sliding speed. SEM/EDS and XRD analysis were used for chemical and microstructural characterisation of worn surfaces and wear debris. At low speeds (0.05–0.5ms−1) and room temperature, Si3N4 surfaces are polished-like due to a combination of humidity-assisted tribo-oxidation and abrasive action of very fine wear debris. At high sliding speeds (2–3.5ms−1), as well as for temperatures in the range 400–600°C, an extensive coherent tribolayer mainly composed by iron oxides spreads over the ceramic surfaces. Polishing and protection by adherent tribolayers are the mechanisms responsible for observed severe and mild wear regimes, respectively. Wear maps are constructed showing the transition of wear regimes in Si3N4/iron alloys contacts determined by constant flash temperature curves. Equations for calculation of bulk and flash contact temperatures in tribocontacts between dissimilar materials are deduced.

Purifying effects and product microstructure in the formation of tic powder by the self-propagating high-temperature synthesis

In self-propagating high-temperature synthesis (SHS), the exothermic heats of reaction usually involving solid reactants are sufficient for the process to sustain itself once the reactant mixture is ignited. Much research has been done on the application of SHS for the synthesis of many high-temperature materials. Advantages of SHS include the simplicity of apparatus as well as the absence of the need for external heating other than the small amount of heat for ignition. furthermore, due to the fact that the SHS process typically takes place above 2,000 C, many of the impurities volatilize, often yielding a product containing less impurities than in the reactant mixture. Among the numerous materials synthesized by SHS, TiC has attracted much attention because of its superior high-temperature resistance and strength, hardness, and wear resistance. Conventional TiC production methods such as the carbothermic reduction of titanium dioxide, the titanium hydride process, and chemical vapor deposition suffer from the shortcomings of high oxygen or hydrogen content and low purity, which adversely affect the mechanical properties of the product. The main objectives of this work were to study the effect of SHS on lowering the impurity content of the product and to examine the effect of the reactantmore » mixing ratio on the product microstructure.« less

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al2O3 particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al2O3 had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al2O3 exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al2O3 particle clustering.

Aluminum-Matrix Aluminum Nitride Particle Composite as a Low-Thermal-Expansion Thermal Conductor

AbstractAluminum-matrix composites containing AIN or SiC particles were fabricated by vacuum infiltration of liquid aluminum into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AIN was superior to Al/SiC in thermal conductivity. At 59 vol.% AIN, Al/AlN had a thermal conductivity of 157 W/m. °C and a thermal expansion coefficient of 9.8 × 10−-6°C−1 (35–100 °C). Al/AlN had similar tensile strength and higher ductility compared to Al/SiC of a similar reinforcement volume fraction at room temperature, but exhibited higher tensile strength and higher ductility at 300–400°C. The ductility of Al/AlN increased with increasing temperature from 22 to 400°C, while that of Al/SiC did not change with temperature. The superior high temperature resistance of Al/AlN is attributed to the lack of a reaction between Al and AIN, in contrast to the reaction between Al and SiC in AI/SiC.

Poly(1,3,4‐oxadiazole) fibers: New fibers with superior high temperature resistance

AbstractA polyoxidiazole fiber, alt. poly[1,3‐/1,4‐phenylene‐2,5‐(1,3,4‐oxadiazole)], with excellent thermal stability was prepared from poly(isophthalic‐terephthalic hydrazide) fiber by thermal cyclodehydration. Conversion of this polyhydrazide fiber proceeded at 280°C. in a muffle furance under positive nitrogen pressure and was completed in 48–72 hr., as evidenced by elemental analyses. The resulting polyoxadiazole fiber had the following typical fiber properties: T/E/Mi/den. = 2.6/3.1/124/3.1. It was thermally stable when exposed for prolonged periods of time at temperatures of 400°C. During heat treatment between 300 and 400°C. the initial polyoxadiazole fiber undergoes a structural change and final properties were: T/E/Mi/den. ∼ 1.5/1.5/90/3.0. The polyoxadiazole structure was found to degrade severely at 450°C.