Abstract

γ-Y₂Si₂O₇ is a promising candidate both for high temperature structural applications and as thermal barrier coatings due to its unique combination of properties, such as high melting point, good machinability, high thermal stability, low linear thermal expansion coefficient (3.9×10⁻⁶ K⁻¹, 25–1400 ^◦C) and low thermal conductivity (<3 W/mK above 300 ^◦C). In this work, the hot corrosion behavior of -Y2Si2O7 in strongly basic Na2CO3 molten salt at 850–1000 ^◦C for 20 h in flowing air was investigated. In the employed conditions, multi-layer corrosion scales with total thickness less than 90 m were formed. At 850–900 ^◦C, the outmost layer of the scale was composed of the reprecipitation of Y2O3, the bottom of a Si-rich Na2O·xSiO2 (x > 3.65) melt layer, and the middle of a NaYSiO4 layer. At 1000 ◦C, the corrosion products turned out to be a mixture of NaY9Si6O26 and Si-rich Na2O·xSiO2 (x > 3.65). In all cases, a thin layer of protective SiO2 formed under the Na2O·xSiO2 melt and protected the bulk material from further corrosion.

Abstract

The influence of water vapor on the oxidation of Ti3AlC2 and Ti2AlC (TACs) was investigated at 500–1200 ^oC in controlled humidity atmospheres. Breakaway oxidation occurred in wet atmospheres at 500–600 ^oC because water vapor induced cracks in the oxide scales and prevented the formation of a protective Al2O3 scale. At elevated temperatures, water vapor slightly accelerated the oxidation of TACs due to the enhanced mass transportation process through the increased oxygen vacancy.

Abstract

Ti2AlC was predicted to bear Al-vacancy down to a sub-stoichiometry of Ti2Al0.5C. The phase instability beyond a critical Al content was attributed to occupation of the Ti–Al anti-bonding orbital, which reduces the coupling strength between Ti2C slab and Al atomic plane. The migration energy barrier of Al self-diffusion along the (0001) plane was low, 0.83 eV, resulting in rapid out-diffusion of Al during oxidation and decomposition of Ti2AlC at high temperatures.

Abstract

In this work, the effects of grain size, notch width, and testing temperature on the fracture toughness of two typical MAX phases, Ti3Si(Al)C2 and Ti3AlC2, were investigated using the chevron-notched beam (CNB) method. The high-fracture toughness in the range of 6.43–10.19MPam1/2 was determined. The critical notch width is about 250 m for the valid fracture toughness measurements of Ti3Si(Al)C2 and Ti3AlC2. For a fixed
notch width and testing temperature, the fracture toughness of coarse-grained (CG) samples is higher than that of fine-grained (FG) samples, and the toughness of Ti3AlC2 is higher than that of Ti3Si(Al)C2. Furthermore, the high-temperature fracture toughness of Ti3Si(Al)C2 and Ti3AlC2 samples show a similar trend that the measured toughness is nearly a constant when the testing temperature is before the ductile–brittle transition temperature (DBTT) and then it declines fast over DBTT. The mechanism for the high-fracture toughness of Ti3Si(Al)C2 and Ti3AlC2 is also discussed.

Abstract

Putting Ti3SiC2 in sandwich of Al2O3 will produce the hard–soft–hard sandwich composites of Al2O3/Ti3SiC2/Al2O3 with improved oxidation resistance and high hardness. The relative method has been established to evaluate the elastic modulus and strength of the symmetrical coating.  However, it is difficult to make the thickness of the Al2O3 coatings on both sides exactly the same during the synthesis process and this results in deviation inevitably. It is significant to develop a modified relative method to suit for the unsymmetrical coated sandwich samples. In this work, the sandwich composites of Al2O3/Ti3SiC2/Al2O3 with strong interfaces were prepared by in situ hot pressing process and a modified relative method was derived for determining the elastic modulus and strength of the Al2O3 coating. Experimental results revealed that the modulus and strength of Al2O3 were 398.2 GPa and 313.2MPa, respectively, which demonstrated the validity and convenience of the modified relative method.  The mechanism for the strong interface between Ti3SiC2 and Al2O3 was also discussed.

Abstract

A multilayer CrAlN coating of Cr0.58Al0.42N/Cr0.84Al0.16N/Cr0.51Al0.49N has been fabricated by a reactive magnetron sputtering method. It consists of a bonding layer, a Cr-rich intermediate layer and an Al-rich outer layer. The multilayer structure provides the coating with good protection against different types of high temperature corrosion, i.e., high temperature oxidation and hot corrosion. The outer Al-rich layer gives
the coating good oxidation resistance at 1000 and 1100 °C due to the formation of a continuous alumina scale. The parabolic rate constants of the coated samples decrease by about 2 orders of magnitude compared with that of the bare alloy samples. The intermediate Cr-rich layer can form a Cr2O3 scale to provide good protection under the hot corrosion conditions in the Na2SO4 salt fluxing at 900, 950 and 1000 °C. The incubation period of the hot corrosion extends several times longer when the alloy was coated by the multilayer coating at the three selected temperatures.

Abstract

Dense bulk Ta₂AlC ceramic was fabricated by in-situ reaction/hot pressing of Ta, Al and C powders.   The reaction path and effects of initial composition on the purity were investigated.  It was found that Ta₂AlC formed through the reactions between AlTa₂ and graphite, pr between Ta₅Al₃C, TaC and graphite at 1500 ^-1550 ^oC.  By modifying the molar ratio of the initial Ta, Al and C powders, single-phase Ta₂AlC was prepared at 1550 ^oC under an Ar atmosphere with an optimized composition of  Ta:Al:C = 2:1.2:0.9.  The lattice parameter and a new set of X-ray diffraction data of Ta₂AlC were obtained.  In addition, Ta₂AlC was reported unstable above 1600 ^oC and decomposed to Ta₄AlC₃, and then to TaC_x.

Abstract

The thermal stability of bulk Ti₃SiC₂ in high-purity nitrogen was investigated. It was surprising to observe that Ti₃SiC₂ underwent rapid and catastrophic disintegration above 1300 ^oC, although this material was thermally stable below this temperature.  This degradation was unexpected and extremely serious, and has been termed ‘‘Ti₃SiC₂ pest.’’ This phenomenon was related to the volume change associated with the formation of mixtures of TiCx, Ti(C, N)x, and TiN, which caused internal tensile stresses and cracked the resulting layers. ‘‘Ti₃SiC₂ pest’’ could be prevented by increasing oxygen partial pressure in nitrogen.

Abstract

TV₄AlC₃, a new MAX phase, was synthesized by reactive hot pressing of a V, Al, and C powder mixture at 1700 ^oC. Using a combination of Rietveld refinement with X-ray diffraction data and ab initio calculations, the crystal structure was determinated.  It was found that V₄AlC₃ has a Ti₄AlN₃-type crystal structure. The lattice constants are a=0.29310 nm and c=2.27192 nm. And the atomic positions are V1 at (4f) (1/3, 2/3, 0.0544), V2 at (4e) (0, 0, 0.1548), Al at (2c) (1/3, 2/3, 1/4), C1 at (2a) (0, 0, 0), and C2 at (4f) (2/3, 1/3, 0.1080).

Abstract

The isothermal oxidation behavior of Zr₂Al₃C₄ in the temperature range of 500 to 1000 °C for 20 h in air has been investigated. The oxidation kinetics follow a parabolic law at 600 to 800 °C and a linear law at higher temperatures. The activation energy is determined to be 167.4 and 201.2 kJ/mol at parabolic and linear stages, respectively.  The oxide scales have a monolayer structure, which is a mixture of ZrO₂ and Al₂O₃.  As indicated by x-ray diffraction and Raman spectra, the scales formed at 500 to 700 °C are amorphous, and at higher temperatures are -Al₂O₃ and t-ZrO₂ nanocrystallites. The nonselective oxidation of Zr₂Al₃C₄ can be attributed to the strong coupling between Al₃C₂ units and ZrC blocks in its structure, and the close oxygen
affinity of Zr and Al.

Abstract

Bulk Si₂N₂O/ β-cristobalite composites have been fabricated by a hot-pressing method using Si₃N₄, SiO₂ and Li₂CO₃ as starting materials. β-Cristobalite in the as-sintered composites is successfully stabilized to room temperature through incorporating N and Li into its structure. The introduction of β-cristobalite significantly improves the dielectric properties. Si₂N₂O/62 vol.% -cristobalite composite shows a low dielectric constant of 4.8 at 1 MHz. In addition, the density, Young’s and shear modulus, and strength of the composites decrease with the increase of β-cristobalite content. When the β-cristobalite content is up to 62 vol.%, the flexural strength of the composite reaches 212MPa. The bulk Si₂N₂O/ β- cristobalite composites show the combination of low density, excellent mechanical performance, low dielectric constant and loss tangent, indicating that they are promising high-temperature structural/functional materials.

Abstract

Material removal and surface damage of Ti₃SiC₂ ceramic during electrical discharge machining (EDM) were investigated. Melting and decomposition were found to be the main material removal mechanisms during the machining process. Material removal rate was enhanced acceleratively with increasing discharge current, ie, working voltage, ui, but increased deceleratively with pulse duration, te. Microcracks in the surface and loose grains in the subsurface resulted from thermal shock were confirmed, and the surface damage in Ti₃SiC₂ ceramic led to a degradation of both strength and reliability.

Abstract

Two new ternary aluminum carbides, Hf₃Al₄C₆ and Hf₂Al₄C₅, were identified and their crystal structure was determined by a combination of X-ray diffraction, Z-contrast scanning transmission electron microscopy and density-function calculations. Theoretical second-order elastic constants, bulk modulus, shear modulus and Young’s modulus of Hf₃Al₄C₆ and Hf₂Al₄C₅, as well as Hf₃Al₃C₅, Hf₂Al₃C₄, HfC and Al₄C₃, were calculated and compared. These new carbides show promisingly high elastic stiffness.

Abstract

Y₂SiO₅ has potential applications as a high-temperature structural ceramic and environmental/thermal barrier coating. In this work, we synthesized single-phase Y₂SiO₅ powders utilizing a solid–liquid reaction method with LiYO₂ as an additive.  The reaction path of the Y₂O₃/SiO₂/LiYO mixture with variation in temperatures and the role of the LiYO₂ additive on preparation process were investigated in detail. The powders obtained by this method have good sinterability. Through a pressureless sintering process, almost fully dense Y₂SiO₅ bulk material was achieved with a very high density of 99.7% theoretical.