Influence of Ti3AlC2 content and load on the tribological behaviors of Ti3AlC2p/Al composites

In this study, Ti₃AlC₂ particles doped aluminum matrix composites were prepared by ultrasonic agitation casting method. Microstructure, mechanical properties, and tribological properties of pure aluminum and Ti₃AlC_2p/Al composites were characterized. Influence of different loads (10, 20, 30, and 40 N) and Ti₃AlC₂ contents (1.0, 2.0, 3.0, and 4.0 wt%) on the tribological behaviors of the composites were studied. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Energy dispersion spectroscopy (EDS), and 3D laser confocal were used to assist the analysis. The results indicated that fine and uniformly microstructure and the optimum comprehensive mechanical properties were exhibited on 2.0 wt%-Ti₃AlC_2p/Al composites. The abrasive grooves were widened and deepened with an increase in the load. The abrasion performance of composites improved distinctly with the addition of the Ti₃AlC₂ particles, which changed the wear mechanism from adhesive wear to abrasive wear. The 30 N load and the composites of 2.0 wt% Ti₃AlC₂ revealed the optimum tribological properties. The improvement of the tribological behavior of composites was attributed to the refinement of microstructure, the improvement mechanical properties and the three dimensional layered Ti₃AlC₂ phases with self-lubricating properties.     

Synthesis and enhanced mechanical properties of compositionally complex MAX phases

A novel strategy of fabricating compositionally complex MAX phases was successfully developed. Multicomponent 413 MAX phase solid solutions (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 and (Ti_0.28Nb_0.26Ta_0.28V_0.18)₄AlC_2.9 simultaneously containing 3 and 4 transition-metal elements at M site were experimentally synthesized via hot pressing equimolar mixture of 211 type MAX phase powders. By elemental analysis and structural characterization, it can be verified that those uniform compositionally complex solid solutions can be obtained only in the presence of Cr₂AlC in raw powders. Those compositionally complex MAX phase solid solutions exhibit typical layered structures with distinct elongated grains. This discovery further enriches the MAX phases family and provide a new avenue for tailoring the properties of these materials. MAX phase composites containing around 87.5 vol.% (Ti_0.36Nb_0.27Ta_0.37)₄AlC_2.8 exhibit a high flexural strength of 720 MPa and a high fracture toughness of 9.5 MPa m^0.5.     

Reducing the self-healing temperature of Ti2AlC MAX phase coating by substituting Al with Sn

In an effort to overcome the property degradation of Ti₂AlC MAX phase coating used in harsh environments, we fabricated a solid solution Ti₂(Al_0.6Sn_0.4)C coating with amount of Ti₅Sn₃ (20 wt.%) by a combined technique composing of magnetron sputtering and post-heat treatment. The cracks induced by Vickers indentation on coating surface were self-healed at 700 ℃, which is the lowest self-healing temperature among the Al-based MAX phase coatings till now. The structural evolution and kinetic diffusion revealed that the formation of SnO₂ is the key factor to achieve the crack self-healing at such a low temperature for Al-based MAX phase coatings. Additionally, the self-healed Ti₂(Al_0.6Sn_0.4)C coating exhibited better oxidation resistance compared to the unhealed one at 800 ℃. The results provide a novel and facile strategy to develop the protective MAX phase coatings with high performance at high temperature by partially substituting Al with Sn.     

Influence of Mo doping on the tribological behavior of Ti3AlC2 ceramic at different temperatures

(Ti_0·8Mo_0.2)₃AlC₂ solid solutions were successfully synthesized from Ti, Al, TiC, and Mo powders using the in situ hot-pressing sintering method. The tribological properties of (Ti_0·8Mo_0.2)₃AlC₂ and the reference Ti₃AlC₂ in the temperature range 25–800 °C were evaluated in ambient air with the counterpart of Al₂O₃ balls. The results show that (Ti_0·8Mo_0.2)₃AlC₂ has improved lubricating properties and wear resistance above 400 °C compared with Ti₃AlC₂. This can be contributed to the formation of tribo-oxidation films containing MoO₃ and MoO_3-x. Structural characterization of the tribo-oxidation films was conducted using SEM, EDS, Raman spectroscopy, and XPS to evaluate the effect of Mo doping on the wear mechanisms of Ti₃AlC₂ in detail.     

Synthesis and physical properties of (Zr1−x,Tix)3AlC2 MAX phases

MAX phase solid solutions physical and mechanical properties may be tuned via changes in composition, giving them a range of possible technical applications. In the present study, we extend the MAX phase family by synthesizing (Zr_1?xTi_x)₃AlC₂ quaternary MAX phases and investigating their mechanical properties using density functional theory (DFT). The experimentally determined lattice parameters are in good agreement with the lattice parameters derived by DFT and deviate <0.5% from Vegard's law. Ti₃AlC₂ has a higher Vickers hardness as compared to Zr₃AlC₂, in agreement with the available experimental data.     

Novel Cr2AlC MAX-phase/SiC fiber composites: Synthesis,processing and tribological response

A new class of ceramic matrix composites based on Cr₂AlC MAX-phase containing 5 and 10 wt.% of SiC fibers was developed in this investigation. The Cr₂AlC/SiC composites were performed through two consecutives steps: i) synthesis of the pure Cr₂AlC phase from its elemental constituents by reactive method under argon atmosphere at 1400 °C and particle size refinement, and ii) processing of Cr₂AlC powder and SiC fibers followed by densification using the field assisted sintering technology/spark plasma sintering. Cr₂AlC/SiC composites presented high density (98.6%) with an excellent dispersion of the fibers within the matrix and a strong matrix/fiber interfase. Tribological behavior of the developed composites was studied under dry conditions to reveal the role played by the SiC fibers. Incorporation of the SiC fibers within the Cr₂AlC matrix reduced the friction coefficient up to 20% for low testing loads, while the wear resistance increased up to 70–80% independently of the applied load.     

Microstructure evolution,mechanical and tribological properties of Ti3(Al,Sn)C2/Al2O3 composites

In this paper, in situ formed Ti₃(Al,Sn)C₂/Al₂O₃ composites were fabricated by sintering the mixture of Ti₃AlC₂ and SnO₂. The Al atoms could diffuse out of the Ti₃AlC₂ layered structure to react with SnO₂, resulting in the formation of Ti₃(Al,Sn)C₂ solid solution and Al₂O₃. When the SnO₂ content was 20?wt.%, the sintered Ti₃(Al,Sn)C₂/Al₂O₃ composite exhibited the best overall mechanical properties, because of the optimized cooperative strengthening effect of solution strengthening and Al₂O₃ enhancement. When the SnO₂ content increased up to 30?wt.%, the flexural strength and fracture toughness of Ti₃(Al,Sn)C₂/Al₂O₃ composite dramatically decreased on account of the large accumulation of generated Al₂O₃. Moreover, according to the SiC ball-on-flat wear tests, it was found that the wear resistance of Ti₃(Al,Sn)C₂/Al₂O₃ composites was significantly improved as the SnO₂ content increased.     

Fabrication of graphene oxide-Ti3AlC2 synergistically reinforced copper matrix composites with enhanced tribological performance

Ceramic particles reinforced copper (Cu) matrix composites with good electrical conductivities, superior mechanical and tribological properties show great prospect in electrical contacts, thermal management and sliding bearing materials. A novel Cu matrix composite with low coefficient of friction (COF) and high wear resistance is rationally designed and prepared by hot-press sintering the core-shell structured Cu/graphene oxide (GO)/Cu composite powders and Cu decorated Ti₃AlC₂ particles to achieve homogenous dispersion of GO in the Cu matrix and good interfacial bonding of Cu matrix and GO and Ti₃AlC_2. Its tribological performance and corresponding anti-wear alongside with friction reduction mechanisms at room temperature are systematically investigated. The GO-Ti₃AlC₂ synergistically enhanced Cu matrix composite exhibits lower COF and wear rate than those composites reinforced with GO or Ti₃AlC₂ alone_, for GO and Ti₃AlC₂ synergistically bear the load and form continuous, compact and lubricating tribo-layer on the worn surface.     

A multicomponent porous MAX phase (Ti0.25Zr0.25Nb0.25Ta0.25)2AlC: Reaction process,microstructure and pore formation mechanism

A multicomponent porous MAX phase (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)₂AlC has been successfully synthesized by using pressureless sintering of mixed elemental powders. The microstructure and phase composition of the samples sintered at various temperatures have been characterized by using SEM, XRD, EDS and other analyses, from which conclusions regarding the reaction and pore forming processes could be drawn. During the whole sintering process, the pores did mainly arise from the diffusion related reactions between Al and other elements at low temperatures (below 1200 °C), and the formation reaction of the MAX phase took place at higher temperatures (above 1200 °C). An exception is the clearance holes that were left from the pressing. The optimum sintering temperature for the final MAX phase (Ti_0.25Zr_0.25Nb_0.25Ta_0.25)₂AlC was 1600 °C. A too high sintering temperature (1700 °C) caused a serious loss of Al atoms and a decomposition of the synthesized MAX phase.     

Influence of temperature and concentration on tribological properties of silicon nitride in glycerol aqueous solution

Friction and wear behaviors of self-mated Si₃N₄ in glycerol aqueous were investigated by varying the temperature (30 °C, 50 °C, and 70 °C) and concentration (pure water, 5 vol%, 20 vol%, and 50 vol%) of glycerol aqueous solution. Friction tests were conducted on a ball-on-disk apparatus. Normal load and sliding velocity were fixed at 30 N and 0.5 m/s, separately. After each tests, friction coefficients and wear rates were measured to evaluate friction and wear behavior. The results showed that the period of running-in process reduces with the increase of concentration and decrease of temperature. Increase of temperature could intensify wear behavior, and when concentration is larger than 20 vol%, wear rate of glycerol aqueous solution is one order less than that of pure water. Our findings could also guide for the use of glycerol aqueous solution as lubricant at different temperature. At 30 °C, the higher the concentration was, the smaller wear volume and total wear rate were. However, at 50 °C and 70 °C, total wear rates of disk were the largest when concentration is 5 vol%, a concentration of glycerol larger than 20 vol% must be added into water to reduce the wear rate. Wear regimes at different conditions were also given in this paper based on lubrication state number.     

Synthesis,properties and thermal decomposition of the Ta4AlC3 MAX phase

The present work describes a synthesis route for bulk Ta₄AlC₃ MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta₂H, Al and C powder mixtures in the 1200–1650 °C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity α-Ta₄AlC₃ was obtained by hot pressing at 1500 °C for 30 min at 30 MPa. The β-Ta₄AlC₃ allotrope was observed in the samples produced by SPS. The Young’s modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta₄AlC₃ into TaC_x and Al vapour in high (˜10⁻⁵ mbar) vacuum at 1200 °C and 1250 °C was also investigated, as a possible processing route to produce porous TaC_x components.     

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)₂AlC MAX phase solid solution by co-sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)₂AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr₂AlC, Y₅Al₃, and Cr-carbides. In contrast to pure Cr₂AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr₂AlC. Calculations predicting the metastable phase formation of (Cr,Y)₂AlC and its decomposition are in excellent agreement with the experimental findings.     

Tailoring Magnetic Properties of MAX Phases,a Theoretical Investigation of (Cr2Ti)AlC2 and Cr2AlC

(Cr₂Ti)AlC₂ is a newly discovered MAX phase with ordered occupations of Ti and Cr atoms on M sites. The Cr‐containing MAX phase is expected showing magnetic property, which provides functional applications in spintronics and as self‐monitoring smart coating. The magnetic states of (Cr₂Ti)AlC₂ are predicted by GGA and GGA + U methods and compared to those of Cr₂AlC. The ground states are predicted as FM or AFM‐XX configurations depending on the calculation methods. Analysis of the electronic structure shows that the magnetic moments mainly originate from the net spins of Cr 3d valence electrons, whereas the contribution of other atoms is negligible. The calculated magnetic moments of Cr atoms in (Cr₂Ti)AlC₂ are higher than those in Cr₂AlC due to the larger distance between the out‐plane Cr atoms separated by the intercalated nonmagnetic Ti–C slab. This work provides an insight on tailoring magnetic properties of MAX phases by modifying the crystal structure.     

Self-lubricating behavior caused by tribo-oxidation of Ti3SiC2/Cu composites in a wide temperature range

Ti₃SiC₂/Cu composite, a new wide temperature range intelligent lubricating functional material, was fulfilled, for mechanical equipment components, by Spark Plasma Sintering process. The microstructure, composition and mechanical properties of the Ti₃SiC₂/Cu composites (TSC-Cu) were investigated. Additionally, the friction and wear behaviors of TSC-Cu sliding against Inconel 718 were conducted on a pin-on-disk configuration at a sliding speed of 0.5 m/s under a load of 5 N at 25–800 °C. For comparison, the tribological property of polycrystalline Ti₃SiC₂/Inconel 718 was measured in an identical condition. The worn surface of TSC-Cu was analyzed by SEM, EDS and XPS, respectively. The results indicated that TSC-Cu consisted of Ti₃SiC₂, TiC and Cu₃Si. It was worth noting that the as-formed Cu₃Si uniformly distributed along the grain boundary of Ti₃SiC₂. As for mechanical property, the addition of Cu increased the hardness, compressive strength of TSC-Cu but lowered its flexural strength. Compared with polycrystalline Ti₃SiC₂, the average friction coefficient of TSC-Cu was higher at 25–400 °C whereas it was lower at 600 °C and 800 °C. The lower friction coefficient was owing to the cooperative lubricating characteristic of tribo-oxidation films containing TiO₂, SiO₂ and CuO. Furthermore, the wear rate of TSC-Cu was absolutely lower than that of polycrystalline Ti₃SiC₂, which resulted from the effective surface strengthening effect of the as-formed hard TiC product. Moreover, the wear mechanism of the composite changed from three-body abrasion wear to adhesion wear and tribo-oxidation wear, with the temperature increasing from RT to 800 °C.     

Pressureless sintering and properties of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics: The effect of pyrolytic carbon

A novel (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic was successfully prepared by pressureless sintering at 2200 °C. With increasing content of resin-derived-carbon, the density, and mechanical and thermal properties increased up to a maximum content of 2～4 wt% resin addition, after which further addition was detrimental. All specimens showed high strength (≥347±36 MPa), with the highest value achieving 450±64 MPa, and fracture toughness significantly higher (>20 %) than those of the corresponding monocarbides and Ta_0.5Hf_0.5C, (Ta_1/3Zr_1/3Nb_1/3)C. The thermal conductivity was approximately equivalent to the lowest value of the corresponding mono-carbides, which was assumed to be due to the lattice distortion effect.     

Synthesis of novel Zr-rich 312-type solid-solution MAX phase in the Zr-Ti-Si-C system

A novel 312-type MAX-phase solid solution series in the Zr-Ti-Si-C system has been synthesized by the vacuum carbosilicothermic reduction method using mixtures of TiO₂, ZrO₂, SiC, and Si powders as starting materials. The upper limit for Zr content in metal sublattice of the synthesized (Zr,Ti)₃SiC₂ MAX phase solid solutions was found to be as much as approximately 66 at%, closely corresponding to a hypothetical quaternary Zr₂TiSiC₂ MAX phase. A wide miscibility gap inside the interval of Zr content in metal sublattice ranging between 22 at% and 55 at% was found. Crystal structure of the synthesized MAX-phase solid solutions was studied by HR-STEM/HAADF and XRD Rietveld analyses. The lattice constants were determined to be linearly correlated with Zr content as predicted by Vegard's law. A significant inhomogeneity in distribution of metal atoms similar to that of out-of-plane ordered quaternary MAX phases has been established for both Ti-rich and Zr-rich MAX-phase solid solutions.     

Synthesis and mechanical and thermal properties of (Cr,Mn)2AlC solid solutions

(Cr,Mn)₂AlC solid solutions have attracted much attention due to their magnetic property which will be enhanced with increasing Mn solubility. So far, only 3?at.-% Mn solubility in (Cr,Mn)₂AlC bulk materials has been reported. This work reports on the synthesis of (Cr,Mn)₂AlC solid solutions via a new reaction route with AlCr₂, C and Mn as starting materials. The Mn solubility increases with increasing the starting Mn content. The maximum solubility of Mn in (Cr,Mn)₂AlC was 8.3?at.-%, corresponding to a resultant solid solution (Cr_0.83Mn_0.17)₂AlC. However, increasing the Mn content in the starting mixture caused the formation of impurities in the sintered samples. A dense (Cr_0.95Mn_0.05)₂AlC ceramic has been achieved by hot-pressing the mixture of AlCr₂/C/0.1Mn. The mechanical properties and thermal expansion coefficient of (Cr_0.95Mn_0.05)₂AlC were measured.     

(Ti,V)_3AlC_2/Al_2O_3复合材料的制备及力学性能

为了提高Ti_3Al C_2陶瓷的力学性能,本研究以Ti C粉、Ti粉、Al粉和V2O5粉为起始反应原料,采用原位热压技术在1350°C下反应烧结合成出了(Ti,V)_3AlC_2/Al_2O_3复合材料。利用X-射线衍射和扫描电子显微技术对合成产物的物相和微观结构进行了表征,并分析了复合材料的合成机制。最后,对(Ti,V)_3AlC_2/Al_2O_3复合材料的力学性能进行了研究。测试结果表明:(Ti_(0.92),V_(0.08))_3Al C_2/10wt%Al_2O_3复合材料具有最佳的力学性能,其硬度、断裂韧性及抗弯强度分别为5.56 GPa、12.93 MPa·m~(1/2)和435 MPa,相比于单相Ti_3Al C_2材料分别提升了60%、108%和31%。     

Synthesis,microstructure and mechanical properties of high-entropy (VNbTaMoW)C5 ceramics

High-entropy ceramics (HEC) with a fixed composition of (VNbTaMoW)C₅ were prepared by spark plasma sintering (SPS) from 1500 °C to 2200 °C. XRD, TEM, HRTEM, SAED and EDX were used to investigate effects of the sintering temperatures on compositional homogeneity, constituent phases and microstructure of the HECs. The results showed that single-phase HEC formed at a temperature as low as 1600 °C while ultimate elemental distribution homogeneity could be obtained at 2200 °C. Elemental distribution homogenization was accompanied by microstructural coarsening and oxide impurities aggregating at grain boundaries as temperature increased. SPS at 1900 °C for 12 min could yield uniform HECs (VNbTaMoW)C₅ with Vickers hardness, nanohardness, fracture toughness and Young’s modulus reaching 19.6 GPa, 29.7 GPa, 5.4 MPa m^1/2 and 551 GPa, respectively. The resultant HECs showed excellent wear resistance when coupled with WC at room temperature.