Abstract

In this work, the dry sliding friction and wear properties of Ti₃AlC₂ and Ti₃AlC₂/Al₂O₃ composites against
AISI52100 steel ball were investigated using a reciprocating ball on flat configuration under different normal
loads. The results indicated that the friction/wear processes of both Ti₃AlC₂ and the composites against AISI52100 steel experienced two different stages with an abrupt transition between them under all test conditions. The first stage was characterized by low coefficient of friction ( μ) and neglectable wear rate. While the second stage was of much higher wear rate and . When the transition occurred, increased dramatically accompanied with formation of a mass of debris. In Ti₃AlC₂, the mainwear mechanisms during
the first stage involved surface materials transfer and oxidation accompanied with subsurface damages by grains kinking, delamination as well as transgranular and intergranular cracks. Accumulating of such contact damages under repeated sliding contact finally leaded to surface and subsurface microfracture of Ti₃AlC₂. Then microfracture controlled severe wear started. Incorporation of Al₂O₃ in Ti₃AlC₂ not only improved wear resistance of Ti₃AlC₂ but also extended the first mild friction/wear stage, because Al₂O₃ particles borne load and restrained large-scale deformation and microfracture of Ti₃AlC₂.

Abstract

Y₂SiO₅ is a promising candidate for oxidation-resistant or environmental/thermal barrier coatings (ETBC) due to its excellent high-temperature stability, low elastic modulus and low oxygen permeability. In this paper, we investigated the thermal properties of Y₂SiO₅ comprehensively, including thermal expansion, thermal diffusivity, heat capacity and thermal conductivity. It is interesting that Y₂SiO₅ has a very low thermal
conductivity (∼1.40 W/m K) but a relatively high linear thermal expansion coefficient ((8.36±0.5)×10⁻⁶ K⁻¹), suggesting compatible thermal and mechanical properties to some non-oxide ceramics and nickel superalloys as ETBC layer. Y2SiO5 is also an ideal EBC on YSZ TBC layer due to their close thermal expansion coefficients. As a continuous source of Y³⁺, it is predicted that Y₂SiO₅ EBC may prolong the lifetime of zirconia-based TBC by stopping the degradation aroused by the loss of Y stabilizer.

Abstract

In this work, we suggest a new and simple method named single gradient notched beam (SGNB) method for determining the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃ with four-point bending specimens. For the specimen with a gradient notch, a sharp natural crack will initiate and extends from the tip of the triangle under increasing load. Based on the straight through crack assumption or on the slice model, the stress intensity factor coefficient for this notched beam was derived. The fracture toughness can be calculated from the maximum load and the minimum of the stress intensity factor coefficient without knowing the crack length. To verify the feasibility and reliability of this suggested method, the SGNB method and two other conventional methods, e.g. the chevron notched beam (CNB) method and single edge notched beam (SENB) method, were performed to determine the fracture toughness of Ti₃Si(Al)C₂ and Al₂O₃. The measured fracture toughness values obtained from the SGNB method agreed well with those from conventional fracture toughness tests.

Abstract

The dispersion of aqueous γ-Y₂Si₂O₇ suspensions, which contain only one component but have a complex ion environment, was studied by the introduction of two different polymer dispersants, polyethylenimine (PEI) and polyacrylic acid (PAA). The suspension without any dispersant remains stable in the pH range of 9–11.5 because of electrostatic repulsion, while it is flocculated upon stirring due to the readsorption of hydrolyzed ions on the colloid surface. However, suspensions with 1 dwb%PEI exhibit greater stability in the pH range of 4–11.5. The addition of PEI shifts the isoelectric point (IEP) of the suspensions from pH 5.8 to 10.8. Near the IEP (pHIEP510.8), the stability of the suspensions with PEI is dominated by the steric effect. When the pH is decreased to acid direction, the stabilization mechanism is changed from steric hindrance to an electrosteric effect little by little. PAA also has the effect of reducing the hydrolysis speed via a ‘‘buffer effect’’ in the basic pH range, but the lack of adsorption between the highly ionized anionic polymer molecules and the negative
colloid particle surfaces shows no positive effect on hydrolysis of colloids and on the stabilization of Y₂Si₂O₇ suspensions.

Abstract

Abstract

Transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD) investigations were conducted on the hot-pressed Ti₂SnC bulk ceramic. Microstructure features of bulk Ti₂SnC ceramic were characterized by using TEM, and a needle-shaped b-Sn precipitation was observed inside Ti₂SnC grains with the orientation relationship: (0001) Ti₂SnC // (200) Sn and ［1210］ Ti2SnC // [001] Sn. With the combination of DSC and XRD analyses, the precipitation of metallic Sn was demonstrated to be a thermal stress-induced process during the cooling procedure. The reheating temperature, even as low as 400^oC, could trigger the precipitation of Sn from Ti₂SnC, which indicated the low-temperature instability of Ti₂SnC. A substoichiometry Ti₂SnxC formed after depletion of Sn from ternary Ti₂SnC phase. Under electron beam irradiation, metallic Sn was observed diffusing back into Ti₂SnxC. Furthermore, a new Ti₇SnC₆ phase with the lattice constants of a = 0.32 and c = 4.1 nm was identified and added in the Ti-Sn-C ternary system.

Abstract

The mechanical and thermophysical properties of quaternarylayered carbides, Zr₂[Al(Si)]₄C₅ and Zr₃[Al(Si)]₄C₆ have been investigated and compared with those of Zr₂Al₃C₄ and Zr₃Al₃C₅. These four carbides are generally alike in mechanical and thermophysical properties due to their similar crystal structures that consisting of alternatively stacked ZrC layers and Al₃C₂/[Al(Si)]₄C₃ slabs. The layer thickness of zirconium carbide and aluminum carbide has effects on their properties.  Thicker layer of zirconium carbide and/or thinner layer of aluminum carbides are in favor of stiffness, hardness, thermal, and electrical conductivities, but go against density, specific stiffness, Debye temperature, and coefficient of thermal expansion.

Abstract

In this work, we have fabricated a novel ternary aluminum nitride, Hf₃AlN, via a reactive hot pressing method using hafnium and aluminum nitride as starting materials. The crystal structure of Hf₃AlN was established by a combination of ab initio calculation, X-ray diffraction, and electron diffraction analyses.  The point group and space group of Hf₃AlN were determined as mmm and Cmcm, respectively. The lattice constants are a=3298 nm, b=1.135 nm, c=0.8842 nm and the atomic positions are Hf1 at 4c (0, 0.0441, 0.2500), Hf2 at 8f (0, 0.3701, 0.0437), Al at 4c (0, 0.7469, 0.2500), and N at 4a (0, 0, 0).  Electronic structure analysis demonstrated that Hf₃AlN should possess metallic conductivity and intrinsic damage tolerance.  We hope that this work will inspire future experimental research on this Hf-based ternary ceramic.

Abstract

MAX-phase carbides (M is an early transition metal, A is an A-group element) exhibit an interesting bonding characteristic of alternative stacking of strong M–C bonds and relatively weak M–A bonds in one direction. In the present first-principles total energy calculations, we establish the relationship between mechanical properties and electronic structure for ternary M₂AC (M = Ti, V, Cr, A = Al, Si, P, S) carbides. By systematically
tuning elements on the M and A sites, pronounced enhancements of bulk modulus, elastic stiffness, and ideal shear strength are achieved in V-containing V₂AC (A = Al, Si, P, and S) carbides. It is suggested that tailoring on the A site is more efficient than on the M site in strengthening the mechanical properties of studied serial carbides. The results highlight a general trend for tailor-made mechanical properties of ternary M₂AC carbides by control of chemical bonding.

Abstract

This article provides a review of current research activities that concentrate on Ti₃SiC₂. We begin with an
overview of the crystal and electronic structures, which are the basis to understand this material. Followings are the synthetic strategies that have been exploited to achieve, and the formation mechanism of Ti₃SiC₂. Then we devote much attentions to the mechanical properties and oxidation/hot corrosion behaviors of Ti₃SiC₂ as well as some advances achieved recently. At the end of this paper, we elaborate on some new discoveries in the Ti₃SiC₂ system, and also give a brief discussion focused on the \microstructure -property" relationship.

Abstract

Reciprocating ball-on-flat dry sliding friction and wear experiments have been conducted on singlephase
γ-Y₂Si₂O₇ ceramic flats in contact with AISI 52100 bearing steel and Si₃N₄ ceramic balls at 5–15N normal loads in an ambient environment. The kinetic friction coefficients of γ-Y₂Si₂O₇ varied in the range over 0.53–0.63 against AISI 52100 steel and between 0.51–0.56 against Si₃N₄ ceramic. We found that wear occurred predominantly during the running-in period and it almost ceased at the steady friction stage. The wear rates of γ-Y₂Si₂O₇ were in the order of 10−4mm³/(N m). Besides, wear debris strongly influenced the friction and wear processes. The strong chemical affinity between γ-Y₂Si₂O₇ and AISI 52100 balls led to a thick transfer layer formed on both contact surfaces of the flat and counterpart ball, which changed the direct sliding between the ball and the flat into a shearing within the transfer layer. For the γ-Y₂Si₂O₇/Si₃N₄ pair, a thin silica hydrate lubricant tribofilm presented above the compressed debris entrapped in the worn track and contact ball surface. This transfer layer and the tribofilm separated the sliding couple from direct contact and contributed to the low friction coefficient and wear rate.