Key Engineering Materials Vols. 602-603

Abstract: The combination of phenolic resin (PF) impregnation-pyrolysis (PIP) and MoSi₂-Si-Al alloy activated melting infiltration (AAMI) combination process were used to prepare MoSi₂-RSiC composites, in which the RSiC works as a matrix. Influence of infiltrated temperatures on the composition, microstructure, mechanical and electrical properties of the composites were studied by XRD, SEM, mechanical test et. al.. The results showed that an almost fully dense MoSi₂-RSiC composite with 3-D interpenetrated network structure was obtained, the compositions of it were mainly SiC, MoSi₂ , Mo (Si,Al)₂, and small amount of residual Si. With the increase of infiltrated temperatures, the flexural strength of the composites increased first and then decreased, it reached 171.40MPa when infiltrated at 1800°C, which was 107.63% higher than that of RSiC matrix. The fracture properties of the composites exhibited a typical brittle rupture. The composites possessed a volume resistivity of 2.90×10^-3 Ω·cm when infiltrated temperature equaled to 1900°C, which was nearly four order of magnitude lower than that of RSiC.

Abstract: Ternary carbide Ti₃SiC₂ is a good binder to make superhard composites with diamonds or cubic boron nitride. Superhard composites are normally made at high temperature and under high pressure around 5 GPa to avoid the phase transformation of diamonds or cubic boron nitride. This paper researched the synthesis of Ti₃SiC₂ from the powders of Ti, Si, and graphite by a cubic presser under 1 GPa to 4 GPa at 1400°C. The decomposition of Ti₃SiC₂ under 5GPa at 1400°C was also researched. From X-ray diffraction (XRD) and scanning electron microscopy (SEM) results, Ti₃SiC₂ was synthesized in 30 min under 1 GPa at 1400°C. The impurities were TiSi₂, Ti₅Si₃C_x, and TiC. As the pressure increased from 1GPa to 4GPa, less Ti₃SiC₂ more TiSi₂ was synthesized. Therefore, high presser > 1GPa is unfavorable for the synthesis of Ti₃SiC₂. After treated under 5GPa at 1400°C, pure Ti₃SiC₂ was decomposed.

Abstract: Titanium powder, silicon powder and carbon powder were used for the preparation of Ti₃SiC₂ matrix multiphase ceramic by self-propagating high temperature synthesis method (SHS) in air condition. The product was examined by XRD for qualitative and quantitative analysis, SEM for morphology analysis and infrared spectroscopy for transmittance analysis. It was shown in the research that product which was with titanium powder, silicon powder, and carbon powder as raw material was consist of three phases, Ti₃SiC₂ (76.6 wt%), TiC (21.2 wt%) and TiSi₂ (2.2 wt%). TiC phase and TiSi₂ phase distributed on the Ti₃SiC₂ phase matrix of the multiphase ceramic in the form of particles. Ti₃SiC₂ matrix multiphase ceramic which was with titanium powder, silicon powder, boron carbide powder and carbon powder as the raw material was also consist of three phases, Ti₃SiC₂, TiC and TiB₂. The infrared transmittance in the wave numbers of 1500-4000 cm^-1 is less than 15 %. The infrared transmittance of the product with a certain additive in the wave numbers of 600-4000 cm^-1 can be dropped below 10 %. Ti₃SiC₂ matrix multiphase ceramic may be a new high radiation infrared material, which has high temperature resistance, wear resistance and wide band.

Abstract: Room temperature friction and wear of Ti₃SiC₂-Ag sliding against Inconel 718 with a hemisphere-on-disc configuration were investigated in air. The effects of Ag content and TiAlN coating on Inconel 718 substrate were also included. Ti₃SiC₂/Inconel 718 tribo-pair showed high friction coefficient (0.6) and severe wear due to pullout of Ti₃SiC₂ grains was observed at a sliding speed of 1 m/s. Ti₃SiC₂-Ag composites had better tribological behavior than that of monolithic Ti₃SiC₂ in sliding against Inconel 718. At a sliding speed of 0.01 m/s, Ti₃SiC₂-Ag/Inconel 718 tribo-pairs exhibited moderate friction coefficient (0.32-0.4). At a sliding speed of 1 m/s, severe wear was not observed for Ti₃SiC₂-15vol.%Ag and Ti₃SiC₂-20vol.%Ag composites although the tribo-layer was not rich in Ag. When Ti₃SiC₂-Ag composites mated with TiAlN coating on Inconel 718 substrate, moderate friction coefficient (0.29-0.36) and low wear rate (10^-6 mm³N^-1m^-1) were obtained at 0.01 m/s. A transition from mild wear to severe wear of Ti₃SiC₂-Ag composites at 0.1 m/s can be attributed to the ploughing effect by hard asperities on TiAlN coating.

Abstract: The surface polishing of Ti₃SiC₂ disk in fluids (water, ethanol, propanol, glycol, and glycerol) is conducted on a Buehler grinder/polisher and evaluated using surface roughness. Using Buehler automatic grinder/polisher, the Ti₃SiC₂ disks are grinded and polished in the as-mentioned lubricants by grinding disk of diamond with sizes of 45 μm to 3 μm. The surface roughnesses of Ti₃SiC₂ disks at each stage are measured by 3D surface profiler. The results show that the lowest surface roughness (Ra) of Ti₃SiC₂ disk obtained by mechanical polishing is 0.04 μm. The optimum polishing process of Ti₃SiC₂ disk is as follows: using water as lubricant, at a load of 5 N, for steps 1 to 4, the Ti₃SiC₂ and grinding disk rotates comparatively and the sizes of diamond particles on the abrasive disk are 45, 15, 9, and 3 μm, respectively. For step 5, the abrasive disk is woven cloth with no diamond particles. The duration of each step is 5 min. Using the same polishing process, the surface roughness of Ti₃SiC₂ disk by direct hot pressing is lower than that by in situ reactive hot pressing. Using the same polishing process but different lubricants, the surface roughness of the Ti₃SiC₂ disks increases in the order of water, ethanol, propanol, glycol, and glycerol. In water, the surface roughness of Ti₃SiC₂ disk decreases with the increasing quantity of water and polishing duration.

Abstract: As the new structural material, TiAl intermetallic compound has great potential application in aerospace engine, energy and automotive fields. But the bottleneck problems including poor room temperature ductility and high-temperature oxidation resistance limit its application. Ti₂AlC possesses an unusual combination of the merits of both metals and ceramics, which is considered the best reinforcement for TiAl intermetallic compound. In the present work, Ti₂AlC/TiAl matrix intermetallic compound was successfully fabricated by in situ hot pressing method from the mixture of Ti/Al/TiC. The phase transitions were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The microstructure of the product was studied by scanning electron microscopy (SEM). Ti reacted with Al liquids to form Ti-Al intermetallics below 900 °C firstly. With increasing temperature (above 900 °C), a part of TiAl intermetallics reacted with TiC to form Ti₂AlC reinforcement. The as-sintered product presented dense and typical lamellar structure. The in-situ synthesized fine Ti₂AlC contributed to improve the strength of TiAl matrix intermetallic compound.

Abstract: In this study, 5 wt. % Ti₃SnC₂/Cu composite was synthesized by hot pressed sintering, and its tribological properties against AISI52100 steel balls were investigated using a ball-on-disk wear tester. The effects of sliding speeds and applied loads on the tribological behavior of Ti₃SnC₂/Cu were studied. The results showed the wear rate of Ti₃SnC₂/Cu composite increased with the increase of applied load and decreased with increase of sliding speed. The main tribological mechanisms of Ti₃SnC₂/Cu were abrasive wear and slightly oxidative wear. The friction coefficient of Ti₃SnC₂/Cu composite was stable and much lower than that of Cu at the same conditions. The loads were effectively born by the Ti₃SnC₂ particles and the wear resistance of the matrix was obviously improved.

Abstract: Cu matrix composites with various contents of Ti₂AlN were fabricated by powder metallurgy using spark plasma sintering (SPS) method. Ti₂AlN ceramic particles were pre-treated by electroless copper plating, the result showed that Ti₂AlN reinforcement and Cu matrix were strongly bonded. The effects of Ti₂AlN content on microstructure, electrical resistivity and mechanical properties were systematically investigated. With the addition of low fraction of Ti₂AlN, the hardness and tensile strength of matrix were improved without losing the fracture toughness too much. Tensile strength of Ti₂AlN/Cu composites were about 380 MPa with the content of 7 wt. % reinforcing phase, and the conductivity of the composites remained about 61.5 % IACS. Moreover, the wear tests illustrated that the loads were effectively born by the Ti₂AlN reinforcement, the main tribological mechanism changed from adhesive wear to abrasive wear compared with Cu, thus the friction and wear resistance was also obviously improved.

Abstract: Recently, a number of graphene-like early transition metal carbides and nitrides named as MXenes were fabricated by exfoliating MAX phases in hydrofluoric acid at room temperature. From experiments results and theory calculations, MXenes are promising anode materials in batteries as well as in metal-ion capacitors. To the best of our knowledge, experimental or calculated evidence has been supported the existence of more than 70 MAX phases members. Therefore, many counterparts MXene may be exist. Herein, employing density functional theory (DFT) computations, we have systematically examined the relative stability, structure and electronic properties of a series of two-dimensional metal carbides and nitrides including M₂C (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Mo and Ta), M₂N (M=Ti, V, Cr, Zr, Hf), M₃C₂ (M=Ti, V, Nb, Ta), Ti₃N₂, M₄C₃ (M=Ti, V, Nb, Ta) and Ti₄N₃. The results demonstrate that all MXenes are metallic and have the similarly electronic structure with bulk transition metal carbides and nitrides, indicating that MXene may have superior catalysis and adsorption instead of expensive pure transition metal.

Abstract: A three-layer material based on B₄C/Al, B₄C/TiB₂ and B₄C composites had been successfully prepared. The microstructures and properties of B₄C/Al-B₄C/TiB₂-B₄C composite were investigated in detail. The three-layer porous preform was first prepared by hot pressing. Then the three-layer composite was fabricated by infiltrating aluminum into the porous preform in vacuum. The three-layer B₄C/Al-B₄C/TiB₂-B₄C composite showed good interfacial bonding. When using the B₄C ceramics layer as the load-bearing surface, the flexural strength, fracture toughness and Vickers hardness of the composite were 230 MPa, 3.4 MPa·m^1/2 and 38 GPa, respectively.