HiPIMS复合热处理制备高纯Ti2AlC MAX相涂层

Ti₂AlC MAX 相涂层是一类兼具金属和陶瓷特性的具有密排六方结构的高性能陶瓷涂层,在电接触、高温防护、宽温域摩擦等领域具有广阔的应用前景。然而 MAX 相涂层的成相成分窗口窄,性能受杂质相影响大,实现高纯、致密 Ti₂AlC MAX 相涂层的制备目前仍存在挑战。考虑沉积气压与溅射等离子体能量密切相关,采用高功率脉冲复合直流磁控溅射技术在钛合金基体上制备了 TiAl / Ti-Al-C 涂层,经后续热处理退火得到高纯 Ti₂AlC MAX 相涂层,重点研究不同沉积气压对涂层退火前后的成分、微观结构以及力学性能的影响和作用机制。结果表明,随着气压不断增大,沉积态涂层厚度先增加后减少。其中低沉积气压下沉积态涂层退火后,结构中除了 Ti₂AlC MAX 相外,还含有一定量杂质相;而在高气压下沉积态涂层退火后几乎全部转变为 Ti₂AlC MAX 相,呈现高纯、表面光滑致密的 MAX 相涂层特征。相较于沉积态涂层,退火后的涂层硬度变化不大,但由于生成了 Ti₂AlC MAX 相,涂层弹性模量有所提高。     

部分MAX相在NaOH、H2SO4和HCl中的电化学性质

用热压的方法合成了若干MAX相化合物,包括相（Ti₂AlC和Ti₂AlN）和312相（Ti₃SiC₂和Ti3AlC2）;研究了它们在1 mol/L HCl、1 mol/L NaOH和1 mol/L H₂SO₄中的电化学性质及其结构与其稳定性的关系.结果表明:在所有溶液中,312型MAX相比211相更稳定;Ti₃SiC₂ 和Ti₃AlC₂几乎在所有溶液里都发生钝化,而Ti₂AlC和Ti₂AlN在1 mol/L HCl中活跃地溶解,还伴有大量气泡产生;Ti₃SiC₂比Ti₂AlC、Ti₂AlN 和Ti₃AlC₂更稳定.     

Oxygen incorporation in M2AlC (M = Ti,V, Cr)

Oxygen incorporation in the MAX phases Ti₂AlC, V₂AlC and Cr₂AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti₂AlC and V₂AlC, but is incorporated interstitially in the Al layer of Cr₂AlC, even for carbon-deficient Cr₂AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr₂AlC and Cr₂Al were investigated in order to study oxygen incorporation in carbon-deficient Cr₂AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr₂AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation.     

MAX/金属基自润滑复合涂层研究现状与展望*

MAX / 金属基自润滑复合涂层具有优异的力学性能和摩擦学性能,MAX 相的加入拓宽了金属基复合涂层的研究和应用范围。首先分析 MAX / 金属基复合涂层在摩擦磨损过程中自润滑特性是如何起作用的,分别从 MAX 相的本质结构说明自润滑性能的存在,摩擦过程中润滑膜的生成说明提高减摩润滑性能的原因。随后阐述近年常见几种 MAX 相涂层以及 MAX / 金属基复合涂层的制备和特性,包括 Ti₂AlC、Cr₂AlC 涂层、高低温金属基体下的 MAX 复合涂层。最后归纳总结 MAX / 金属基复合涂层常见应用领域和表面防护效果,并对 MAX / 金属基复合涂层目前存在的问题和涂层质量的提升进行展望,为 MAX / 金属基自润滑复合涂层的推广应用提供参考。     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Corrosion behavior of select MAX phases in NaOH, HCl and H2SO4

In this paper we report on the electrochemical corrosion of select MAX phases, namely Ti₂AlC, (Ti,Nb)₂AlC, V₂AlC, V₂GeC, Cr₂AlC, Ti₂AlN, Ti₄AlN₃, Ti₃SiC₂ and Ti₃GeC₂ in 1 M NaOH, 1 M HCl and 1 M H₂SO₄ solutions. Polarization characteristics recorded in 1 M NaOH show that V₂AlC, V₂GeC and Cr₂AlC undergo active dissolution at potentials more positive than the corrosion potential, while Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate. In the 1 M HCl solutions, Ti₂AlC, V₂AlC and V₂GeC actively dissolve; Ti₃SiC₂ and Ti₃GeC₂ passivate. Depending on potential, (Ti,Nb)₂AlC and Cr₂AlC showed trans-passive behavior. In 1 M H₂SO₄ solutions, Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate, V₂AlC and V₂GeC show active dissolution, while Cr₂AlC exhibits trans-passive behavior. Ti₂AlN and Ti₄AlN₃ were passive in all solutions except in 1 M HCl, where Ti₂AlN showed trans-passive behavior. Given that the corrosion behavior of (Ti,Nb)₂AlC is unlike either Ti or Nb, the behavior of the former cannot be predicted from that of the latter.     

Processing and characterization of porous Ti2AlC with controlled porosity and pore size

In this work, we demonstrate a simple and inexpensive way to fabricate porous Ti₂AlC, one of the best studied materials from the MAX phase family, with controlled porosity and pore size. This was achieved by using NaCl as the pore former, which was dissolved after cold pressing but before pressureless sintering at 1400 °C. Porous Ti₂AlC with samples a volume fraction of porosity ranging from ～10 to ～71 vol.% and different pore size ranges, i.e. 42–83, 77–276 and 167–545 μm, were successfully fabricated. Fabricated samples were systematically characterized to determine their phase composition, morphology and porosity. Room temperature elastic moduli, compressive strength and thermal conductivity were determined as a function of porosity and/or pore size. For comparison, several samples pressureless-sintered without NaCl pore former, or fabricated by spark plasma sintering, were also characterized. The effects of porosity and/or pore size on the room temperature elastic moduli, compressive strength and thermal conductivity of porous Ti₂AlC are reported and discussed in this work. It follows that porosity can be a useful microstructural parameter to tune mechanical and thermal properties of Ti₂AlC.     

Ultra-High-Temperature Oxidation and Thermal Stability of Ti<Subscript>2</Subscript>AlC in Air at 1600–1800 °C

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 TiC_x at 1700 °C, and transformed to Ti₃AlC₂ due to the reaction with TiC_x at 1800 °C, indicating that the massive consumption of Al in Ti₂AlC exceeded its deficiency tolerance.     

Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air

Ti₂AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures, excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti₂AlC over conventional Al₂O₃-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti₂AlC and α-Al₂O₃, which could result in superior spallation resistance and make Ti₂AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti₂AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al₂O₃ scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in situ lift-out. Above a continuous and adherent α-Al₂O₃ layer, a discontinuous-transient rutile-TiO₂ scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al₂TiO₅ scale was identified at 1400 °C. The continuous α-Al₂O₃scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C, yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al₂O₃ scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive residual stress may be responsible the high spallation-resistance of the protective α-Al₂O₃ scale developed on Ti₂AlC, despite the absence of reactive element additions.     

Fabrication and mechanical properties of Ti2AlC/TiAl composites with co-continuous network structure

Ti₂AlC/TiAl composites with different volume fractions were prepared by hot pressing technology, and their reinforced structural characteristics and mechanical properties were evaluated. The results showed that when the reinforced phase volume fraction of Ti₂AlC was 20%, three-dimensional interpenetrating network structures were formed in the composites. Above 20%, Ti₂AlC phase in the composites accumulated and grew to form thick skeletal networks. The microplastic deformation behavior of Ti₂AlC phase, such as kink band and delamination, improved the fracture toughness of the composites. Comparative analysis indicated that the uniform and small interconnecting network structures could further reinforce the composites. The bending strengths of composites prepared with 20 vol.% Ti₂AlC reached (900.9±45.0) MPa, which was 25.5% higher than that of TiAl matrix. In general, the co-continuous Ti₂AlC/TiAl composite with excellent mechanical properties can be prepared by powder metallurgy method.     

Ti3AlC2掺杂SPS法制备Ti2AlC/TiAl复合材料的研究

通过2TiC-Ti-1.2Al体系的原位热压反应制备了Ti₃AlC₂陶瓷,然后以59.2Ti-30.8Al-10Ti₃AlC₂(wt%)为反应体系,采用放电等离子烧结技术制备出Ti₂AlC/TiAl基复合材料。借助XRD、SEM分析了产物的相组成和微观结构,并测量了其室温力学性能。结果表明：原位热压烧结产物由Ti₃AlC₂和TiC相组成,Ti₃AlC₂呈典型的层状结构,TiC颗粒分布在其间。SPS法制备的Ti₂AlC/TiAl基复合材料主要由TiAl、Ti₃Al和Ti₂AlC相组成,Ti₂AlC增强相主要分布于基体晶界处,表现为晶界/晶内强化作用。力学性能测试表明：Ti₂AlC/TiAl基复合材料的密度、维氏硬度、断裂韧性和抗弯强度分别为3.85 g/cm₃、5.37 GPa、7.17 MPa?m_1/2和494.85 MPa。     

Formation of TiC/Ti2AlC and α2+γ in in-situ TiAl composites with different solidification paths

In order to improve the mechanical properties of TiAl alloys, TiAl composites with different solidification paths were synthesized by metallurgical method. Results show the TiC disappears and Ti₂AlC increases when the Al content is more than 42% (at.%, similarly hereinafter). Small TiC particles are located in Ti₂AlC grains with irregular shapes when the Al content is 40%, and they translate into clubbed Ti₂AlC with increasing of Al. This metallurgy method can solve the defects of the Al lacking and the residual TiC. The γ phase increases between lamellar colonies with the increasing of Al. When the Al content is 48%, the fully lamellar structure transforms into a duplex microstructure and there are small Ti₂AlC phases in γ phases, because the forming of Ti₂AlC phase must consume Al. The compressive strength increases up to 1678.68 MPa as Al content is 46 at.%, and then decrease to 1460.22 MPa, the compressive strain increases and then keeps stabilization with the increasing Al. The maximum strength improves 38.82% and the maximum strain improves 121.37%. The Ti₂AlC/TiAl composites fracture behaviors are load transferring behavior, crack deflection, trans-lamellar cracking and extraction of carbide reinforcements. The Ti₂AlC phase and the fully lamellar structure improve the mechanical properties.     

Oxidation behaviour of Ti–Al–C films composed mainly of a Ti2AlC phase

This paper addresses the oxidation behaviour of Ti–Al–C films composed mainly of a Ti₂AlC phase. The films exhibited rather low oxidation rates at 600 and 700 °C, with an oxygen-rich zone or a thin oxide layer appearing on the film surfaces. Much faster oxidation rates were observed at 800 and 900 °C. The Ti₂AlC phase was quickly consumed by oxidation. From the film surface to the inner zone, TiO₂-rich layer, Al₂O₃-rich layer, and TiO₂ + Al₂O₃ mixed layer was observed, respectively. The oxidation mechanism of the Ti–Al–C film is discussed based on the experimental results.     

Phase stability,electronic structure and mechanical properties of ternary-layered carbide Nb4AlC3: An ab initio study

In this paper we calculated the phase stability, electronic structure and mechanical properties of Nb₄AlC₃ by means of a first-principles pseudopotential total energy method. Based on thermodynamical calculations of the two possible crystal structures of Nb₄AlC₃, α-type Nb₄AlC₃ is confirmed to be the preferred equilibrium phase at ambient conditions. The chemical bonding displays layered characteristics that have commonly been reported for MAX ceramics. The equation of state and compressibility of α-Nb₄AlC₃ were investigated. The material exhibits anisotropic elasticity under hydrostatic pressure: it is more compressible along the c direction than along the a and b directions. The second-order elastic coefficients, bulk modulus, shear modulus and Young’s moduli were reported and compared with those of Nb₂AlC. Since the salt-rock-type Nb–C slab is thicker in Nb₄AlC₃ than that in Nb₂AlC, the former material shows higher elastic stiffness than the latter one; at the same time, Nb₄AlC₃ may display quasi-ductility, which has been well documented for MAX ceramics.     

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti₂AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti₂AlC had excellent oxidation resistance. The oxidation of Ti₂AlC obeyed a cubic law with cubic rate constants, k_c, increasing from 2.38×10^-12 to 2.13×10^-10 kg³/m⁶/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner -Al₂O₃ layer and a discontinuous outer TiO₂ (rutile) layer formed on the Ti₂AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti₂AlC substrate during thermal cycling.     

In situ diffraction study of thermal decomposition in Maxthal Ti2AlC

The thermal stability of Ti₂AlC at elevated temperature (1000-1550 °C) in vacuum has been investigated using in situ neutron diffraction. At temperatures above 1400 °C, Ti₂AlC became unstable and began to decompose via sublimation of Al, resulting in a porous surface layer of TiC_x being formed. The apparent activation energy for Ti₂AlC decomposition was determined to be 85.7 ± 2.6 kJ mol⁻¹. The kinetics of isothermal phase decomposition was modelled using least-squares linear regression fitting and the Avrami equation. The corresponding least-squares regression exponent (R²) and Avrami constants (k and n) for isothermal decomposition were determined to be 0.89, 0.268 min⁻ⁿ and 0.1, respectively.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Effects of TiC and Al4C3 addition on combustion synthesis of Ti2AlC

Preparation of the ternary carbide Ti₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of Ti:Al:C = 2:1:1, TiC-containing samples with TiC of 6.67–14.3 mol%, and Al₄C₃-containing samples with Al₄C₃ of 1.96–10 mol%. Effects of TiC and Al₄C₃ addition were studied on combustion characteristics and the degree of phase conversion. Due to the growth of laminated Ti₂AlC grains, the reactant compact was subjected to an axial elongation during the SHS process. Because the addition of TiC and Al₄C₃ led to a decrease in the reaction temperature, the flame-front propagation velocity was correspondingly reduced for the TiC- and Al₄C₃-containing samples when compared with the elemental reactants. Based upon the XRD analysis, formation of Ti₂AlC along with a secondary phase TiC was identified in the synthesized products. The grains of Ti₂AlC are typically plate-like with a size of 10–20 μm and several laminated Ti₂AlC grains form a layered structure. The content of Ti₂AlC yielded from the elemental powder compacts is about 85 wt%. The addition of TiC was found to facilitate the formation mechanism and therefore to enhance the extent of Ti₂AlC conversion approaching 90 wt%. As a result of the reduced exothermicity of the reaction, however, the content of Ti₂AlC decreased slightly in the products synthesized from the Al₄C₃-added samples.     

Oxidation of Cr2AlC (0001): Insights from Ab Initio Calculations

We performed an investigation of the initial stage of oxidation onto a relevant Cr₂AlC (0001) surface by ab initio calculations. For the most energetically stable Al-terminated Cr₂AlC (0001) surface, a detailed model describing the oxygen-surface interaction is developed by exploring the adsorption energetics. Based on the evaluation of the energetics and the structural properties of the atomistic models generated, the results point to an initial stage of the Cr₂AlC (0001) surface oxidation with some similarities with those observed in the Al (111) layer. Our findings on the bonding mechanism of single O adsorption atoms of the surface may lead to further alloying strategies to enhance oxidation resistance in a wide range of refractory-metal-based MAX phases.