Microstructure and mechanical properties of plasma sprayed TiC/Ti5Si3/Ti3SiC2 composite coatings with Al additions

The mass production of MAX phase coatings such as Ti₃SiC₂ and Ti₃AlC₂ using the plasma spraying method is highly challenging due to its ultra-high temperature and short reaction time. In this study, agglomerate powders of 3Ti/SiC/C/xAl with various Al contents (x = 0–1.5) were prepared to form TiC/Ti₅Si₃/Ti₃SiC₂ composite coatings using the plasma spraying technique. The effect of the Al addition on the microstructures and mechanical performances of the as-sprayed coatings was investigated. The addition of Al decreased the TiC content of the coatings while increasing their Ti₃SiC₂ content significantly. The addition of even small amounts of Al improved the MAX phase fraction of the coatings from 8.95 wt% (x = 0) to 34.05 wt% (x = 0.2) and 41.60 wt% (x = 0.5). Excess Al did not affect the Ti₃SiC₂ content of the coatings. The composite coatings showed a lamellar structure with pores and microcracks. With the addition of Al, the microhardness of the coatings increased slightly, while the fracture toughness improved significantly. The composite coatings with Al showed better wear resistance than those without Al. The wear mechanism of the coatings was a combination of adhesive wear, abrasive wear, and oxidative wear.     

Synthesis of MAX phase-based ceramics from early transition metal hydride powders

MAX phase ceramics are typically prepared by the reactive sintering of elemental powders that are often coarse, expensive, and prone to oxidation. The temperature-driven dehydrogenation of metal hydride powders offers an alternative synthesis approach, as the hydrides decompose into phase-pure, dimensionally fine elemental powder particles. The increased reactivity of these in situ formed, fine powder particles drastically reduces the formation temperature of the antecedent intermetallic phases, without forming excess binary carbides or facilitating powder oxidation in the Ti-Al-C and Zr-Al-C systems. This work elucidates the effect of metal hydrides on the sequence of formation reactions in MAX phase ceramics. In the Zr-Al-C system, the use of coarse, oxidation-prone elemental Zr powders prevented MAX phase formation, whereas spark plasma sintering of ZrH₂ powders at 1500 °C produced ceramics containing 60 wt% Zr₃AlC₂. Similarly, in the Ti-Al-C system, spark plasma sintering of TiH₂ powders at 1200 °C produced phase-pure Ti₃AlC₂ ceramics.     

Effects of 211 and 413 ordering on the corrosion behavior of V-Al-C MAX phases prepared by spark plasma sintering

In the present study, two V-Al-C based MAX phases, i.e., V₂AlC and V₄AlC₃ having two types of ordering were successfully manufactured by spark plasma sintering and the corrosion behavior of sintered samples was evaluated. Al, V and C metal powders were mixed with the desired molar ratios by a mixer mill, and sintered at 1300 °C. The relative density calculation revealed almost full densification for both prepared MAX phases. The measurements of mechanical properties showed a low increase in bending strength and Vickers hardness of V₄AlC₃ compared to V₂AlC MAX phase. Evaluation of corrosion behavior of developed MAX phases was carried out in 6.5 M HCl solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. Corrosion current density and corrosion potential of V₂AlC (5.3 ± 0.21 μA/cm² and -0.451 ± 0.01 V, respectively), and V₄AlC₃ (1.07 ± 0.22 μA/cm² and -0.091 ± 0.02 V, respectively) were measured and no passivation behavior was observed in their potentiodynamic polarization curves. However, EIS tests at open circuit potential confirmed more corrosion resistance of V₄AlC₃compared to V₂AlC. These tests also revealed the active dissolution of MAX phases in 6.5 M HCl solution at anodic potential of +0.1 V, while the impedance values of V₄AlC₃ were larger than those of V₂AlC. Microstructural investigation revealed the preferential dissolution of V₂AlC phase in grain boundaries after corrosion test. Moreover, the layered structure of V₂C MXenes was observed in some regions. After corrosion test, V₄C₃ MXene layers had larger thickness compared to V₂AlC. It was found that V₄AlC₃ with higher amount of Al₂O₃ and thicker layers has more corrosion resistance than V₂AlC MAX phase.     

Effects of Al substitution with Si and Sn on tribological performance of Ti3AlC2

Since their discovery, the MAX phases have elicited engineering interest as potential choices for wear resistant parts. One such compound is Ti₃AlC₂ with nano-layered structure, low density (4.25 g/cm³), good oxidation resistant and self-lubrication properties. The purpose of this investigation was to evaluate the dominant effect of the A-site solid solution elements addition on dry sliding characteristics of Ti₃AlC₂ against 0.45% C steel (S45C) disk, a material which is widely used in wear-critical applications such as impeller, gear and axles. Dry sliding tribological behaviors of hot-pressed Ti₃Al_0.94C₂, Ti₃Al_0.78Sn_0.22C₂ and Ti₃Al_0.67Si_0.28C₂ solid solutions were conducted using a block-on-disk type tester at surface sliding speed range from 10 to 30 m/s and in the normal load range from 20 to 80 N. The results show that friction coefficient of Ti₃Al_0.94C₂ is higher than that of Ti₃Al_0.78Sn_0.22C₂, but lower than that of Ti₃Al_0.67Si_0.28C₂. However, the change in wear rate as a function of normal load for different sliding speed shows almost reversed trend. Difference in the phase composition of friction films were found to be responsible for observed change in tribological behaved after partial substitution of Al in Ti₃AlC₂ with Sn and Si. This study shows that friction coefficient of Ti₃AlC₂ can be adjusted from 0.2 to 0.38 by partially substituting Al with Si and/or Sn.     

MAX phase Zr2SeC and its thermal conduction behavior

The elemental diversity is crucial to screen out ternary MAX phases with outstanding properties via tuning of bonding types and strength between constitutive atoms. As a matter of fact, the interactions between M and A atoms largely determine the physical and chemical properties of MAX phases. Herein, Se element was experimentally realized to occupy the A site of a MAX phase, Zr₂SeC, becoming a new member within this nanolaminated ternary carbide family. Comprehensive characterizations including Rietveld refinement of X-ray Diffraction and atom-resolved transmission electron microscopy techniques were employed to validate this novel MAX phase. The distinct thermal conduction behaviors emerged are attributed to the characteristic interactions between Zr and Se atoms.     

基于QoS保证的多种频谱资源联合分配算法

提出了基于静态、半静态和动态频谱资源共性特征指标的频谱可用性定义方法,并借鉴改进的最大加权时延优先(modified largest weight delay first,M-LWDF)算法的QoS需求定义方法,定义了用户的业务需求指标.在此基础上,提出了结合频谱可用性和用户业务需求的频谱分配方法和二次分配方法,其核心思想是将用户需求和频谱资源的可用性按照一定的方式量化,再采用简单的按大小一一对应的方式进行分配.与随机分配方案、先静后动的频谱分配方案的仿真对比表明,本方案在平均失败次数、平均总速率等性能方面具有明显的改善.     

Interface modification of carbon fibers with TiC/Ti2AlC coating and its effect on the tensile strength

A uniform TiC/Ti₂AlC gradient coating was obtained on carbon fibers via an in-situ reaction in molten salts. The results indicated that in-situ growth of TiC/Ti₂AlC coating caused strong interfacial bonding and surface defects. In this case, evident stress concentration was induced and cracks penetrated the fiber easily during tensile loading. Thus the tensile strength of carbon fibers was dramatically decreased to 78?±?13?MPa. In order to improve the performance of the as-prepared TiC/Ti₂AlC-coated carbon fibers, a pyrolytic carbon layer was pre-fabricated on carbon fibers. By introducing pyrolytic carbon layer, the interfacial bonding strength and surface defects were reduced accordingly. These improvements lead to a decrease of stress concentration and cracks propagation, and facilitate the interfacial debonding during tensile loading. As a result, the tensile strength of the fiber was significantly increased to 550?±?72?MPa. This fact indicates that pre-fabricating a pyrolytic carbon layer on carbon fibers is an effective method to improve the reliability of the TiC/Ti₂AlC-coated carbon fibers. The present work also provides a feasible way to fabricate TiC/Ti₂AlC interphase for high-performance Cf/SiC composites.     

Oxidation behaviour of the MAX-phases Ti2AlC and (Ti,Nb)2AlC at elevated temperatures with and without fluorine treatment

MAX-phase materials have shown great potential for different technical applications due to their mechanical properties. If the main group element is aluminium their excellent oxidation resistance also makes them attractive for several high temperature applications. As an example the thermodynamically stable MAX-phase Ti₂AlC forms a thin, protective alumina layer in oxidising atmospheres at elevated temperatures. This alumina layer is formed due to the high Al activity within the material and prevents further attack by the environment. However, high temperature oxidation tests at 900 °C in air of “technical” Ti₂AlC which is not pure single-phase Ti₂AlC led to the formation of a non-continuous alumina scale which is intersected by a mixed TiO₂/Al₂O₃ scale. Furthermore, internal oxidation was observed. This “technical” material consists of two phases namely Ti₂AlC plus γ-TiAl due to the manufacturing route. Such γ-TiAl-grains are preferentially oxidised. This type of internal attack can be suppressed by a preceding fluorine treatment.     

High temperature compressive creep of dense and porous Cr2AlC in air

The creep behaviours of the dense and porous Cr₂AlC ceramics were investigated, being the first study on creep behaviour of not only Cr₂AlC but also porous MAX phase. Creep rates of rather dense (2% of porosity) and porous Cr₂AlC (53% and 75%) were measured in the temperature range 1073–1473 K during heating and subsequent cooling after high temperature exposure. The compressive creep tests were performed under stress range of 1–12 MPa. Creep rates of porous Cr₂AlC were higher than the ones of the rather dense material at lower temperature but lower at higher temperature, which can be attributed to effects of the formed oxide scale and some crack healing associated with the scale formation. A comparison of the creep rates with other refractory materials reveals favourable properties of the porous Cr₂AlC.     

Recent advances in MXenes: From fundamentals to applications

The family of MAX phases and their derivative MXenes are continuously growing in terms of both crystalline and composition varieties. In the last couple of years, several breakthroughs have been achieved that boosted the synthesis of novel MAX phases with ordered double transition metals and, consequently, the synthesis of novel MXenes with a higher chemical diversity and structural complexity, rarely seen in other families of two-dimensional (2D) materials. Considering the various elemental composition possibilities, surface functional tunability, various magnetic orders, and large spin–orbit coupling, MXenes can truly be considered as multifunctional materials that can be used to realize highly correlated phenomena. In addition, owing to their large surface area, hydrophilicity, adsorption ability, and high surface reactivity, MXenes have attracted attention for many applications, e.g., catalysts, ion batteries, gas storage media, and sensors. Given the fast progress of MXene-based science and technology, it is timely to update our current knowledge on various properties and possible applications. Since many theoretical predictions remain to be experimentally proven, here we mainly emphasize the physics and chemistry that can be observed in MXenes and discuss how these properties can be tuned or used for different applications.