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High-hardenability steels with high alloy content typically contain strong carbide-forming elements such as chromium and molybdenum. During low pressure carburization (LPC), strong carbide-forming elements can form stable carbides on or [...]

The carbide formation during the solid-state reaction between thin films and the solid-state reaction between 11 transition metals and an amorphous carbon layer are examined. In situ x-ray diffraction is used to obtain the activation energy for the formation of the various carbide phases.

Reaction between tantalum (Ta) and carbon (graphite or amorphous carbon) in tap-compacted powder mixture state under solar radiation heating was investigated. Although mono-carbide TaC is the final equilibrium product at 1600[degrees]C under presence of excess carbon, formation of sub-carbide [Ta.sub.2]C as the interim product was detected under certain circumstances. After 30 min reaction at 1600 [degrees]C reached by solar radiation heating, Ta powders compacted with graphite powders by pressing into pellet were fully converted to TaC but loosely tap-compacted powder mixtures of Ta and graphite yielded [Ta.sub.2]C coexisted with TaC. On the other hand, Ta powders tap-compacted with amorphous carbon were fully converted to TaC under the comparable reaction condition reflecting the high reactivity and chemical carbon activity of amorphous carbon compared with those of graphite (reference state of carbon). Keywords: Tantalum sub-carbide; Tantalum mono-carbide; Solar furnace

Thermal characteristics of Nb2SnC, a ternary carbide, were studied.

Experimental evidence is provided for the formation of neutral tantalum-carbide clusters in a supersonic expansion following the reaction of acetylene and tantalum in a laser ablation source.

Density functional calculations were performed on a series of M(OH3)-substituted (M = Ti, V) cyclopolyenes as simple models of metal carbides and oligomerization of the metal acetylide complexes {M(OH)(sub 3)}(sub 2)(mu-C2) as a possible precursor of these [(OH)(sub 3)MC](sub n) (n = 4, 6) hypothetical species are studied. The comparison of the electronic structure of these metal substituted cyclopolyenes with those of the corresponding cyclopolyenes has shown that these species should be more reactive toward both electrophilic and nucleophilic attacks.

Nanoscale friction was observed at the interface between a boron carbide film and an atomic force microscope tip.

Results show that water adsorbs onto titanium carbide and vanadium carbide at molecular level in a dissociative way. Data further indicate that differences in the electronic structures of the metal carbides result in different surface bonding and desorption profiles.

Density functional theory computations were performed on the titanium carbide clusters Ti3C8, Ti4C8, Ti6C13 and Ti7C13 and their anions, in relation to a previous study by L.-S. Wang et al. The layer-by-layer growth pathway suggested by Wang et al. seems susceptible to account for the formation and stability of high nuclearity titanium carbide clusters. However, the presence of multicage structures cannot be ruled out with other metals. The absence of the MetCar species Ti8C12- and Zr8C12- from the mass spectrum of anionic clusters, as observed by Wang under certain experimental conditions, can be tentatively explained by the formation of unstable molecular Rydberg states.

The extended Huckel tight binding method was used in determining the electronic structures of the carbides Tm2Fe2Si2C, Th2Re2Si2C, ThFe2SiC and Ho2Cr2C3 and the d-electron counts of their transition metal atoms Cr, Fe, Re and the bonding of their linear M-C-M units where M = Cr, Fe and Re. Results indicate that the M-C bonds of the M-C-M units were observed to be double bonds. The 14 MOs of the linear M-C-M units in R2Fe2Si2C and R2Re2Si2C are completely filled because the Si 3p level function as acceptor levels for these MOs.

The interaction of water with the nonpolar TiC(100) face was investigated as a function of surface temperature over the range 98-773 K. Temperature-programmed desorption has been used to monitor the desorption products as a function of coverage while high-resolution electron energy loss spectroscopy was used to probe the nature of adsorbed species as a function of surface temperature. Findings show the presence of both molecularly and dissociatively adsorbed water in the monolayer regime at cryogenic temperatures.

The structures and energies of the binary dititanium tricarbide Ti2C3 in the lowest singlet, triplet and quintet states were investigated by density functional theory using the hybrid B3LYP functionals. Geometries and frequencies for a number of isomeric structures were presented at the B3LYP level. Findings revealed that cyclic structures are energetically favored over the linear structures. These findings were substantiated by a comparison with the penta-atomic group of silicon-carbon clusters.

A study relating to thermodynamic research on gaseous transition-metal carbides was conducted to determine the equilibrium molecular structures of dimetal carbide Y2Cn (N = 2-8) from which the partition functions and thermodynamic properties are computed. Among other results, it was found that the ground-state isomers of diyttrium carbides exhibit cyclic structures in contrast to pure carbon clusters which are linear for n less than 10.

Experiments revealed new prominent clusters in the anion mass spectra of titanium carbide clusters, TixC (super -) (sub y) where x/y = 3/8, 4/8, 6/13, 7/13, 9/15 and 13/22 which can not be localized in the positive ion mass spectra. The common components of the clusters were C2 dimers, cubic framework and layered structures. Moreover, two growth regions were identified in the clusters. A combined photoelectron spectroscopy and density functional theoretical studies were performed to understand the structure and bonding of the clusters.

The energies and structural properties of titanium carbide TiC(sub x) (x = 3-4) clusters were investigated for the first time using density functional theory based on the hybrid B3LYP functional. TiC(sub x) was analyzed in their singlet, triplet and quintet potential energy surfaces. Experimental results showed that the planar four-membered ring structure VIs with a transannular Ti-C bond is the global TiC3 minimum and 21.5 kcal/mol more stable than the transannular C-C bond structure VIIt. Both cyclic structures are energetically preferred over a linear quintet minimum.

Microcracks were mechanically induced in the CVD coating layers on two types of cemented carbides with different thermal expansion coefficients, and one type of cermet. The microcracks were found to have beneficial effects on residual stress, transverse rupture strength, and chipping resistance during interrupted cutting. Residual stress in the coating on cemented carbide is tensile. Tensile residual stress decreases with increasing microcrack width and decreasing microcrack distance. Induction of 20 [[micro]meter]-distant and 0.025 [[micro]meter]-wide cracks relieves tensile residual stress by about 0.5 GPa, increases transverse rupture strength by about 0.70 GPa, and almost doubles the chipping resistance. Residual stress in the coating on cermet is compressive. Microcracks in the coating layer do not change residual stress or transverse rupture strength.

Magnetic Fe3C and alpha-Fe ultrafine particles were produced by laser-induced pyrolysis of Fe(CO)5 and C2H4. It was seen that after passivation, the Fe3C particles had a high saturation magnetization of 132 emu/g compared to that of the alpha-Fe particle, 95 emu/g. It was also observed that the oxygen is in different states for the two ultrafine particles. The oxygen on the Fe3C particles was mainly in absorbed form, in contrast to the chemically combined oxygen in the alpha-Fe particles.

Carbide coarsening in a 12% CrMoV steel was studied. Investigations in the gauge length and in the head for both interrupted and ruptured creep specimens tested at an initial stress of 80 MPa at different temperatures and at 600 degrees C at various initial stresses were conducted. The contributions to the rise in strain rate in the tertiary creep were studied in terms of plastic deformation. The rise in strain rate was primarily due to the microstructural degradation by carbide coarsening and the cavitation accumulation.

Ti carbide layers are synthesized by multipulse excimer laser (lambda = 308 nanometers) irradiation of Ti samples in ambient CH4 gas at superatmospheric pressure. Nonstoichiometric carbides with low carbon content are observed by the bulk of the samples. The laser fluence/number of pulses' combination is optimum. Large perturbation of the sample's surface is observed by an increase in the number of applied pulses. A model is proposed to explain the properties of the laser.

Single-crystal X-ray counter data provided the crystal structure of Gd2B3C2. The boron-carbon structural alignment of the B-C net in Gd2B3C2 explains a formal charge of 5- per B3C2 unit. The electronic structure of the boron-carbon layers observed in ThB2C and alpha-UB2C structures is strongly associated with that of the anionic two-dimensional boron-carbon layers.

The development of n- and p-type capacitors on good quality SiC epitaxial layers facilitates routine capacitance and conductance measurements for large range of temperature. Conductance measurements applicable to metal-oxide-semiconductor, carried out in SiC epitaxial layers, indicate that trap time constant for good quality SiC epitaxial layers is equal to that of the Si-SiO2 interface.