Patrick Chartier

Site selectivity in Fe doped β phase NiAl

We have investigated site selectivity in iron doped β phase ternary alloys of the composition NixFeyAl(100−x−y), where the Ni concentration is within a few at. % of stoichiometry. Experimental techniques used include extended x‐ray absorption edge fine structure and Mossbauer measurements. The transition metal atoms in excess of 50 at. %, which occupy sites with transition metal nearest neighbors, are not chosen randomly among Ni and Fe, but are exclusively Fe, to within the accuracy of the measurements. This result requires a re‐analysis of previous magnetic susceptibility measurements on this system, which as re‐analyzed here imply more reasonable moment values for the Fe dopants. Our findings regarding Fe site selectivity are of significance in regards to recent studies showing a ductilization effect of Fe dopants microalloyed into Ni50Al50.

The effect of nanolayer thickness on the structure and properties of multilayer TiN/MoN coatings

The effect of nanolayer thickness on the structure and properties of nanocomposite multilayer TiN/MoN coatings is revealed. The multilayer (alternating) TiN/MoN coatings are prepared by the Arc-PVD method. The selected thickness of the nanolayers is 2, 10, 20, and 40 nm. The formation of two phases—TiN (fcc) and γ-Mo2N—is found. The ratio of Ti and Mo concentrations varies with varying layer thickness. The maximum hardness value obtained for different thicknesses of the layers does not exceed 28–31 GPa. The stability of TiN/MoN during cutting and tribological tests is significantly higher than that of products with TiN coatings. The nanostructured multilayer coatings with layer thicknesses of 10 and 20 nm exhibit the lowest friction coefficient of 0.09–0.12.

Site-projected electronic structure of two-dimensional Ti3C2 MXene: the role of the surface functionalization groups

The role of the surface groups T (T = OH, O or F) in the chemical bonding in two-dimensional Ti3C2Tx MXene is directly evidenced combining electron energy-loss spectroscopy in a transmission electron microscope and simulations based on density functional theory. By focusing on the 1s core electrons excitations of the C and (F, O) atoms, the site projected electronic structure is resolved. The Electron Energy-Loss Near Edge Structures (ELNES) at the C-K edge are shown to be sensitive to the chemical nature and the location of the T-groups on the MXenes surface and thereby allow for the characterization of the MXenes functionalization on the nanometre scale. In addition, the ELNES at the C and F-K edges are shown to be determined by the hybridizations of these atoms with the Ti d bands: these edges are thus relevant probes of the Ti d density of states close to the Fermi level which is of particular interest since it drives most of the Ti3C2Tx electronic properties. Finally, the crucial role in the MXenes functionalization of the etchant used for its synthesis is evidenced by locally determining the [O]/[F] concentration ratio using the corresponding K edges. This ratio is shown to be drastically increased from 1.4 to 3.5 when using HF or LiF/HCl respectively.

Composition, structure and tribotechnical properties of TiN, MoN single-layer and TiN/MoN multilayer coatings

Results of comprehensive investigations of nanostructed TiN and MoN single-layer coatings as well as multilayer coatings consisting of TiN/MoN alternating layers have been considered. The coatings have been deposited by a promising modern method of cathode-arc evaporation (vacuum-arc method). The elemental and phase compositions of coatings, their tribological and physico-mechanical properties: friction coefficient, wear, adhesion, hardness, and elastic modulus have been studied and the mechanisms of the coatings fracture have been discussed.

The effect of the deposition parameters of nitrides of high-entropy alloys (TiZrHfVNb)N on their structure, composition, mechanical and tribological properties

Nitrides of high-entropy alloys TiZrHfVNb produced using a vacuum-arc cathode evaporation have been studied using scanning electron and atomic force microscopies, energy dispersive, Rutherford ions backscattering, and X-ray diffraction analyses, microhardness measurements, and tribological tests. It has been found that the deposition parameters affect the structure, surface morphology, distribution of elements, mechanical and tribological properties of the coatings under study.

Mechanical properties of materials based on MAX phases of the Ti-Al-C system

By studies of materials based on the Ti3AlC2 MAX phase containing inclusions of titanium carbide it has been shown that as a titanium carbide content increases from 2 to 99%, the nanohardness and Young modulus of the material increase from 2.0 ± 0.4 to 23.6 ± 1.2 GPa and from 137 ± 21 to 447 ± 11 GPa, respectively. The exponent in the equation of creep for these samples has been found to vary from 104 to 140, which indicates that mechanical properties of the material and, hence, of the Ti3AlC2 MAX phase depend on the strain rate only slightly. The formation of broad hysteresis loops has been observed in the cyclic loading/unloading of the indenter for samples consisting mainly of the Ti3AlC2 MAX phase. This points to serious losses in elastic energy of the MAX phase in strain cycling and, hence, the prospects of the MAX phase application as a damping material. It has been found that the microhardness of a sample consisting of 98% Ti3AlC2 produced by sintering under a load of 4.9 N was 2.1 GPa and its fracture toughness was high (no cracks from the indent corners were observed even under a load of 149 N). Microhardness and fracture toughness of the material consisting of 71% Ti3AlC, 6% Ti2AlC, and 23% TiC were 3.0 ± 0.6 GPa and 4.3 ± 1.4 MPa·m1/2, respectively.

Synthesis of ternary compounds of the Ti-Al-C system at high pressures and temperatures

Ternary carbides have been synthesized from a mixture of Ti, Al, and C powders taken in the stoichiometric relation 3/1.2/2 under quasihydrostatic pressures and temperatures. The amount of the MAX phase in samples and the material density has been increased by the two-stage synthesis at high pressures (1–2 GPa) and temperatures followed by homogenizing. The material, which had been produced at a pressure of 2 GPa, a temperature of 1200°C and annealed in an argon atmosphere, contained 94.3 Ti3AlC2-4.3 TiC-1.4 Al2O3 wt %. The material density and Vickers microhardness at the load of 4.9 N have been measured to be 4.27 g/cm3 and 3.3 GPa, respectively.

Thermal Stability and Mechanical Characteristics of Densified Ti3AlC2-Based Material

The mechanical properties and temperature stability in air and hydrogen of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) have been investigated. At room temperature the samplesexhibited microhardness HV = 4.6 GPa (at 5 N), hardness HV50 = 630 MPa (at 50 N ) and HRA=70 (at 600 N), Young modulus was 140 ± 29 GPa, fracture toughness K1C=10.2 MPa·m0.5compression strength 700 MPa and bending strength 500 MPa. After 1000 hours of exposition at 600 °C the oxide film (containing mainly Al2O3 and TiO2) formed on the surface and material demonstrated a higher oxidation resistance than chromium ferrite steels. Due to the surface oxidation the defects self-healing took place and the bending strength of the porous Ti3AlC2 (22% porosity) after exposition for 3 h at 600 oC in air slightly (for 3%) increased as compared to that at 20 oC. Besides, the porous Ti3AlC2 material resisted to high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 %.

Study of the Thermal Stability and Mechanical Characteristics of MAX Phases of Ti-Al-C(N) System and their Solid Solutions

The DTA and TG study in air of Ti2Al (C1-xNx) and Ti3AlC2 synthesized under Ar 0.1 MPa pressure and densified in thermobaric conditions at 2 GPa, 1400 °C, for 1 h showed that the increase of the amount of TiC layers in Ti-Al-C MAX phases structures leads to the increase of their stability against oxidation: 321 MAX phase Ti3AlC2 are more stable than Ti2AlC and Ti2Al (C1-xNx) solid solutions both before and after thermobaric treatment. The oxide film formed on the surface of the highly dense (ρ=4.27 g/cm3, porosity 1 %) material based on nanolaminated MAX phase Ti3AlC2 (89 % Ti3AlC2, 6 % TiC, 5 % Al2O3) manufactured by hot pressing (at 30 MPa) made the material highly resistant in air at high temperatures: after 1000 hours of exposition at 600 °C it demonstrated a higher resistance to oxidation than chromium ferrite steels (Crofer GPU and JDA types). Due to the surface oxidation self-healing of defects took place. Besides, the Ti3AlC2 material demonstrated resistance against high-temperature creep and after being kept in H2 at 600 °C for 3h its bending strength reduced by 5 % only. At room temperature the Ti3AlC2 bulk exhibited microhardness Hμ = 4.6 GPa (at 5 N), hardness HV50 = 630 (at 50 N ) and HRA = 70 (at 600 N), Young modulus was 140 ± 29 GPa, bending strength =500 MPa, compression strength 700 MPa, and fracture toughness K1C=10.2 MPa·m0.5.

Evidence for Symmetry Reduction in Ti3(Al1−δCuδ)C2 MAX Phase Solid Solutions

Ti3[Al1-δCuδ]C2 MAX phase solid solutions have been synthesized by sintering compacted Ti3AlC2-Cu composites produced by mechanical milling. Using X-ray and neutron diffraction techniques, it is demonstrated that the Cu mixing into the Al site is accompanied by lattice distortion, which leads to symmetry reduction from a hexagonal to a monoclinic structure. Such symmetry reduction likely results from this mixing through deviation of the A-site position from the special (0, 0, 1/4) position within the P63/mmc space group of the original Ti3AlC2 structure. Moreover, it is demonstrated that the Cu admixture into the A site can be adjusted from the composition of the reactant mixture. The lattice parameter variation of the solid solution compounds, with 10-50 atom % Cu in the A site, is found to be consistent with Vegards law.