Excess Carbon Atoms Research Articles

Hard yet tough and self-lubricating (CuNiTiNbCr)Cx high-entropy nanocomposite films: Effects of carbon content on structure and properties

The single-phase high-entropy alloy film is difficult to meet severe friction conditions due to its low hardness and high friction coefficient. Nano-composite structure film is composed of at least two separated phases, showing the properties of strength and toughness integration and excellent wear resistance. The design of nanocomposite structures can effectively improve the mechanical properties and tribological properties of high-entropy alloy films. In this study, the (CuNiTiNbCr)Cx nanocomposite high-entropy films (HEFs) integrated with high hardness, high toughness, and self-lubrication were synthesized by the double-target co-sputtering method. The effect of carbon content on microstructure, mechanical properties, and tribological properties of (CuNiTiNbCr)Cx films was studied. With the increase of carbon content in the HEFs, the carbon atoms preferentially react with Ti, Nb, and Cr to form a (TiNbCr)C ceramic-reinforced phase, and then the excess carbon atoms precipitate in the form of amorphous carbon (a-C) lubricating phase in the HEFs. The structure of the HEFs changes from an amorphous structure to a nanocomposite structure of amorphous (amorphous CuNiTiNbCr phase + a-C phase)/nanocrystalline (TiNbCr)C phase. When the carbon content is about 21.2 at.%, the carbide phase in the film reaches saturation and the hardness and modulus of the films are highest, which are 18 GPa and 228 GPa, respectively. The HEFs with a carbon content of 44.0 at.% show the best toughness and tribological properties with a friction coefficient of 0.16 and a wear rate of 2.4 × 10–6 mm3/(N m), which is mainly attributed to the excellent resistance to fatigue crack growth and the interfacial lubricating layer formed in the friction process. The nanocomposite (CuNiTiNbCr)Cx HEFs show very promising application prospect in the field of friction protection.

Novel bake hardening mechanism for bainite-strengthened complex phase steel

• Yield strength of materials can be greatly improved after pre-strain and baking. • Dislocation as fast channels for carbon diffusion promotes cementite formation. • Carbon clusters gathered at the dislocations act as precursors of cementite. • Cementite precipitation is main reason for increase in strength of bainite steel. In the automobile industry, stamping and paint baking processes are used to strengthen components, and this not only saves costs, but also further reduces the bodyweight of automobiles. In this work, the bake hardening mechanism of the complex phase (CP) steel CP980 was explored by comparing it with that of DP1180, which has a clear bake hardening mechanism and a carbon content similar to that of CP980. By analyzing the bake hardening response and the microstructural changes of the two steels, we found that the bake hardening process of CP980 was divided into three stages. In the first two stages, the carbon atoms diffused into dislocations to form Cottrell atmosphere pinning dislocations, and excess carbon atoms formed carbon clusters or low-temperature carbide pinning dislocations that were similar to DP1180. In the third stage, the dislocation acted as rapid channels for carbon diffusion, and fine cementite gradually formed when the carbon clusters gathered at the dislocations as precursors, resulting in precipitation hardening. This novel bake-hardening (BH) mechanism is crucial for obtaining a comprehensive understanding of the strain-baking behavior of advanced high-strength steals (AHSS).

Impacts of hydrogen annealing on the carrier lifetimes in p-type 4H-SiC after thermal oxidation

Thermal oxidation and hydrogen annealing were applied on a 100 μm thick Al-doped p-type 4H-SiC epitaxial wafer to modulate the minority carrier lifetime, which was investigated by microwave photoconductive decay (μ-PCD). The minority carrier lifetime decreased after each thermal oxidation. On the contrary, with the hydrogen annealing time increasing to 3 hours, the minority carrier lifetime increased from 1.1 μs (as-grown) to 3.14 μs and then saturated after the annealing time reached 4 hours. The increase of surface roughness from 0.236 nm to 0.316 nm may also be one of the reasons for limiting the further improvement of the minority carrier lifetimes. Moreover, the whole wafer mappings of minority carrier lifetimes before and after hydrogen annealing were measured and discussed. The average minority carrier lifetime was up to 1.94 μs and non-uniformity of carrier lifetime reached 38% after 4-hour hydrogen annealing. The increasing minority carrier lifetimes could be attributed to the double mechanisms of excess carbon atoms diffusion caused by selective etching of Si atoms and passivation of deep-level defects by hydrogen atoms.

Insights into the microstructure characteristics, mechanical properties and tribological behaviour of gas-phase chromized coating on GCr15 bearing steel

In this study, a high quality chromized coating was produced on GCr15 steel substrate by vapor phase chromizing technique for sprag clutch application. The microstructure characteristics, mechanical properties and sliding friction behaviour were systematically investigated. The results show that the chromized coating consists of 1) an outmost layer of columnar Cr7C3 grains with excess carbon atoms (55 at.%) and considerable amount of iron atoms (6.2 at.%); and 2) an inner layer of coarse Fe3C grains. The carbide phase formation can be well explained via the thermodynamics evaluation for the Fe-Cr-C system, while the origin of carbon contamination is likely originated from the side reactions of precursor gases. The high surface hardness (16.6 GPa) and interfacial adhesion (~50 N) endow the chromized coatings with superior wear resistance against steel counterface under ambient conditions. The wear rates are found to decrease with the applied load (10–25 N). Scanning electron microscopy (SEM) analysis of the worn surfaces indicates that a chromium oxide tribolayer tends to be formed under a higher loading which displays lubrication and hence protects the contacting material during the friction process.

500 °C high-temperature reliability of Ni/Nb ohmic contact on n-type 4H-SiC

The reliability of Ni/Nb ohmic contact on n-type 4H-SiC at 500 °C was investigated. The current–voltage characteristics showed that, while the Ni(50)/Nb(50)/4H-SiC sample without applying the CF4:O2 etching process degraded just after 25 h and lost ohmic behavior after 50 h aging, the Ni(75)/Nb(25)/4H-SiC contact undergone CF4:O2 surface treatment still showed excellent stability after aging for 100 h at 500 °C. Though X-ray diffraction results indicated that the chemical compounds remained stable during the aging process, transmission electron microscopy showed that there was a redistribution of the chemical compounds at the interface of the contact after 500 °C aging. The depth distribution of the elements and energy-dispersive X-ray analyses revealed that the contribution of carbon agglomeration at the interface accounted for the degradation of the sample without applying the etching process. Whereas the well-controlled excess carbon atoms of the contact undergone CF4:O2 treatment ensured the stability of this contact when operating at a high ambient temperature.

Carbon transformations in rapidly solidified nickel–carbon ribbon

We report the structural transformations of carbon via the melt spinning and subsequent annealing of nickel-carbon alloys. We attained metastable solid solubility of carbon in nickel ribbon by achieving a rapid solidification rate of up to 1.6 × 106 K/s. Excess carbon atoms were found to be dissolved in the nickel lattice causing up to 1.4% strain for an alloy spun at 70 m/s tangential wheel speeds. High temperature heat treatments led to precipitation of carbon from the nickel lattice on the ribbon free surfaces but also led to growth of spherical precipitates within the nickel matrix, an effect consistent with bulk diffusion-driven Ostwald ripening. Carbon was excavated from the ribbons via chemical dissolution of the metal and characterized by electron microscopy and Raman spectroscopy. We found that the microstructure of carbon precipitated from the rapidly quenched ribbon could be tuned by varying the carbon content from 4 to 12 at. % in the precursor and annealing the ribbon at temperatures that ranged from 400 to 1200 °C. Via the step-wise variation of these two parameters, we sequentially transformed amorphous carbon nanospheres with a high BET surface area of 203 m2/g into thick, highly crystalline flakes of graphite that conformed to the shape of the as-spun ribbon.

Oxidation and carburization behaviour of two type 316H stainless steel casts in simulated AGR gas environment at 550 and 600 °C

• Duplex oxide and carburized layers form on steels exposed to AGR reactor coolant • Cracking in the oxide layer increases the oxidation and carburization rates • Carbon locates in the austenite lattice and predominately M 23 C 6 carbides • Diffusion of substitutional solutes is the rate-limiting mechanism of carburization • Small precipitates lead to a reduced carburization rate at 550ºC The time-dependent oxidation and carburization behaviour of two type 316H austenitic stainless steels with varying Mn content and differing average austenite grain size of 142 µm in ‘Low Mn’ (0.98 wt.% Mn) and 81 µm in ‘High Mn’ (1.52 wt.% Mn), were assessed at 550 °C and 600 °C in the presence of simulated reactor primary gas coolant containing 500 vppm H 2 O, 100 vppm H 2 , 300 vppm CH 4 and 1 vol.% CO, balanced with CO 2 . Rupture and spallation of the initial protective chromia layer occurs in the ‘High Mn’ steel after 2000 h at 550 °C, and leads to the formation of a magnetite/spinel oxide layer that reaches 75% surface coverage only after 8000 h. In contrast, ‘Low Mn’ steel reaches ~ 85% coverage after only 1000 h at the same temperature. The development of an inner carburized layer occurs gradually in both steels once the duplex oxide layer is forming. The differences in steel behaviour are reduced at 600 °C, where both oxidation and carburization are significantly accelerated. Only 0.02 wt.% carbon remains in solid solution in the austenite lattice in the carburized layer, the excess carbon atoms precipitating out in the form of Cr-rich M 23 C 6 particles, present both inter- and intragranularly. The experimental values of the activation energy for carburization suggest that diffusion of substitutional solutes as the rate-limiting mechanism of the process. Previously observed carburization depths of ≥ 200 µm in ex-service austenitic stainless steels, may be achieved by pre-conditioning 316H steel in simulated reactor gas coolant for ≥ 6000 h at 600 °C.

Strategies for Scalable Gas-Phase Preparation of Free-Standing Graphene

The preparation of graphene with high quality serves as a prerequisite for its usage. Traditional methods of graphene production, represented by liquid-phase exfoliation and chemical vapor depositi...

CF4:O2 surface etching for the improvement of contact resistance and high-temperature reliability in Ni/Nb ohmic contacts on n-type 4H-SiC

The influence of CF4:O2 surface etching on the formation of Ni/Nb ohmic contact to n-type 4H-SiC substrates was investigated. The specific contact resistivity extracted from the transmission line model showed that the contact with CF4:O2 etching had lower contact resistance than that of the sample without the etching process. The X-ray diffraction results showed that a higher concentration of Ni2Si formed at the interface of the contact after the fabrication process resulted in a lower specific contact resistivity. Moreover, the depth distribution of the elements revealed that under the CF4:O2 surface etching, a low density of the excess carbon atoms at the interface of the contact was observed. Transmission electron microscopy also confirmed that the low concentration of excess carbon atoms and the high density of Ni2Si at the interface play an important role in low specific contact resistivity as well as the high temperature reliability of the Ni/Nb/4H-SiC contacts undergoing CF4:O2 treatment.

Influence of Ni and Nb thickness on low specific contact resistance and high-temperature reliability of ohmic contacts to 4H-SiC

The influence of Ni and Nb thickness on the electrical properties and high-temperature reliability of Ni(x)/Nb(100 – x)/n-type 4H-SiC ohmic contacts, where x = 25, 50 or 75 nm, was investigated. Two-dimensional X-ray diffraction, hard X-ray photoelectron spectroscopy and transmission electron microscopy analyses indicated that a relatively high Ni concentration had a strong influence on low specific contact resistance due to the greater formation of Ni2Si at the contact interface, whereas a higher density of Nb improved the ability to collect excess carbon atoms by the formation of Nb6C5 at the interface, ensuring high-temperature reliability of the contacts. The experimental results showed that the Ni(50)/Nb(50)/4H-SiC ohmic contact is a potential candidate for high-temperature and harsh environment applications.

Optimization of Ni/Nb Ratio for High-Temperature-Reliable Ni/Nb Silicide Ohmic Contact on 4H-SiC

Low specific contact resistivity and high-temperature reliability of the Ni (x)/Nb (100-x) (where x = 25, 50, 75 nm) ohmic contact to 4H-SiC were investigated. After the annealing process at 1000°C for 3 min in N2 ambient, the I-V curves indicated that all the contacts exhibited the ohmic behaviors. Based on the transfer length method, the specific contact resistivity of the contacts were extracted. High concentration of Ni was responsible for low specific contact resistance of the Ni (75)/Nb (25)/4H-SiC sample by the formation of Ni2Si compound after the fabrication process. However, this contact lost the ohmic behavior at low temperature of 150°C. Whereas, both Ni (50)/Nb (50)/4H-SiC and Ni (25)/Nb (75)/4H-SiC contacts remained the ohmic behavior for 100-hour aging at 400°C. Two-dimensional X-ray diffraction analyses showed that the presence of carbon agglomeration formed at the interface of the Ni (75)/Nb (25)/4H-SiC contact caused the degradation of this sample when being aged at high temperature environment. Meanwhile, higher concentration of Nb in the Ni (50)/Nb (50)/4H-SiC and Ni (25)/Nb (75)/4H-SiC samples improved the ability to collect the excess carbon atoms and thus enhanced the high temperature reliability of these contacts when operating in high temperature ambient. Considering both low specific contact resistivity and high temperature reliability, the Ni (50)/Nb (50)/4H-SiC contact can be a good candidate for harsh environment applications.

High-temperature reliability of Ni/Nb ohmic contacts on 4H-SiC for harsh environment applications

High-temperature stability of the Nb/Ni and Ni/Nb Ohmic contacts to n-type 4H-SiC was investigated and compared after being aged at 400 °C for 100 h in N2 ambient. The electrical properties and surface morphologies of the contacts were characterized by using the combination of the current-voltage measurements and microscopic imaging. While the Nb/Ni/4H-SiC ohmic contact failed after 10-h aging, the Ni/Nb/4H-SiC contact still exhibited good ohmic characteristics with a slight increase of specific contact resistance after being aged for 100 h. According to the microscopic imaging results, though the Ni/Nb/4H-SiC contact has a rougher surface morphology compared to that of the Nb/Ni/4H-SiC contact, the Ni/Nb/4H-SiC has a better ability to collect excess carbon atoms at the interface, ensuring stable operation in a high temperature environment. The ability to collect excess carbon atoms of the Ni/Nb/4H-SiC is revealed by the formation of Nb6C5 which is confirmed by two-dimensional X-ray diffraction, hard X-ray photoelectron spectroscopy, and transmission electron microscopy analysis. The experimental results indicated that the Ni/Nb/4H-SiC ohmic contact is a potential candidate for high temperature and harsh environment applications.

Spheroidization of Nickel Powder and Coating with Carbon Layer through Laser Heating.

We developed a simple and efficient process, laser heating of nickel powder in ethanol, to produce carbon-encapsulated nickel microspheres. Long-pulse-width laser heated nickel powder suspended in pure ethanol into liquid droplets. In turn, the latter droplets became sphere-like, pyrolyzed surrounding ethanol and dissolved the produced carbon atoms. Because of their lower solubility in solid nickel, excess carbon atoms were then expelled from the metal core after solidification, thus forming graphite-like shells on the laser-modified Ni spheres. Hence, after pyrolysis the transformation of carbon was found to follow the dissolution-precipitation mechanism. The produced carbon-encapsulated nickel microspheres exhibited higher oxidation resistance compared with the initial nickel powder, while keeping their magnetic properties essentially unchanged.

Experimental quantification of carbon gradients in martensite and its multi-scale effects in a DP steel

In multi-phase steel grades such as ferrite-martensite dual phase (DP) steels, it is of the utmost importance to understand the underlying effects of solid phase transformations with respect to their consequences on the final mechanical properties. In the present research, by employing dilatometry experiments two equivalent DP microstructures have been designed which are identical in most primary aspects such as constituents’ volume fractions, the corresponding grain sizes and their morphologies. The difference, however, is the fashion in which carbon atoms in the martensite constituents are distributed. Electron probe microanalysis (EPMA) shows that excess carbon atoms which are rejected as a consequence of ferrite formation are accumulated in the adjacent austenite grains close to the boundaries yielding sharp carbon gradients. During final quenching, the characteristics of martensite transformation are therefore severely influenced. Through electron backscatter diffraction (EBSD) analysis many coarse substructures were detected in the interior regions of the inhomogeneous martensite associated with low dislocation densities. However, the areas close to the ferrite/martensite boundaries demonstrate finer substructures. Moreover, by means of nanoindentation tests it was found that the interior areas with lower carbon contents and coarser substructures show lower local strength as compared to the marginal regions. Macroscopic stress-strain behavior revealed that a higher ultimate tensile strength and enhanced post-uniform elongation are achieved in the specimen with sharp carbon gradients in its martensite constituents.

Laser-assisted chemical vapor deposition setup for fast synthesis of graphene patterns.

An automatic setup based on the laser-assisted chemical vapor deposition method has been developed for the rapid synthesis of graphene patterns. The key components of this setup include a laser beam control and focusing unit, a laser spot monitoring unit, and a vacuum and flow control unit. A laser beam with precision control of laser power is focused on the surface of a nickel foil substrate by the laser beam control and focusing unit for localized heating. A rapid heating and cooling process at the localized region is induced by the relative movement between the focalized laser spot and the nickel foil substrate, which causes the decomposing of gaseous hydrocarbon and the out-diffusing of excess carbon atoms to form graphene patterns on the laser scanning path. All the fabrication parameters that affect the quality and number of graphene layers, such as laser power, laser spot size, laser scanning speed, pressure of vacuum chamber, and flow rates of gases, can be precisely controlled and monitored during the preparation of graphene patterns. A simulation of temperature distribution was carried out via the finite element method, providing a scientific guidance for the regulation of temperature distribution during experiments. A multi-layer graphene ribbon with few defects was synthesized to verify its performance of the rapid growth of high-quality graphene patterns. Furthermore, this setup has potential applications in other laser-based graphene synthesis and processing.

Coupling XRD, EXAFS, and 13C NMR to Study the Effect of the Carbon Stoichiometry on the Local Structure of UC1±x

A series of uranium carbide samples, prepared by arc melting with a C/U ratio ranging from 0.96 to 1.04, has been studied by X-ray diffraction (XRD), (13)C nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS). XRD determines phase uniqueness and the increase of the lattice parameter versus the carbon content. In contrast, (13)C NMR detects the different carbon environments in the lattice and in this study, clearly identifies the presence of discrete peaks for carbon in the octahedral lattice site in UC and an additional peak associated with excess carbon in hyperstoichiometric samples. Two peaks associated with different levels of carbon deficiency are detected for all hypostoichiometric compositions. More than one carbon environment is always detected by (13)C NMR. This exemplifies the difficulty in obtaining a perfect stoichiometric uranium monocarbide UC(1.00). The (13)C MAS spectra of uranium carbides exhibit the effects resulting from the carbon content on both the broadening of the peaks and on the Knight shift. An abrupt spectral change occurs between hypo- and hyperstoichiometric samples. The results obtained by EXAFS highlight subtle differences between the different stoichiometries, and in the hyperstoichiometric samples, the EXAFS results are consistent with the excess carbon atoms being in the tetrahedral interstitial position.

Graphene Growth from Spin-Coated Polymers without a Gas

We grew graphene films on polycrystalline metal substrates using spin-coated polystyrene or polyaniline films as a carbon source by heating the films in an Ar atmosphere or in a vacuum. In an Ar atmosphere of 6.7×104 Pa, precise control of the initial polymer film thickness is crucial for few-layer graphene growth. In a vacuum, few-layer graphene films were obtained regardless of the initial polymer film thickness, because excess carbon atoms are removed from the surface when the polymer is thermally decomposed. This latter method does not require any gas and is thus a very simple and easy way to grow few-layer graphene.

Deposition and characterization of MAX-phase containing Ti–Si–C thin films by sputtering using elemental targets

Deposition of Ti–Si–C thin films onto glass substrates by magnetron DC-RF sputtering was examined using elemental targets of titanium, silicon and carbon, in order to investigate effects of carbon content in the films on the formation of ternary carbide compounds such as Ti3SiC2, known as MAX-phase. The DC and RF powers supplied for sputtering the titanium target and the silicon one were respectively fixed at DC120 W and RF50 W, while the DC power supplied for sputtering the carbon target was varied from 400 W to 200 W to vary the carbon content in the deposited films. Based on XRD, it was found that Ti3SiC2 as well as TiCx were formed in the film obtained under an electric power of DC200 W while only TiCx formed under an electric power higher than DC300 W. According to Raman spectroscopy, it was found that excess carbon atoms precipitated out as fine graphite crystallites in the Ti–Si–C film, and it was assumed that its amount increased with increasing the carbon content in the film. The electrical resistivity of the obtained films decreased with the decrease of the electric power supplied to the carbon target, and the minimum electrical resistivity lower than 230 μΩcm was noteworthy.

Role of structure of C-terminated4H-SiC(0001¯) surface in growth of graphene layers: Transmission electron microscopy and density functional theory studies

Principal structural defects in graphene layers, synthesized on a carbon-terminated face, i.e. the SiC(000) face of a 4H-SiC substrate, are investigated using microscopic methods. Results of high-resolution transmission electron microscopy (HRTEM) reveal their atomic arrangement. Mechanism of such defects creation, directly related to the underlying crystallographic structure of the SiC substrate, is elucidated. The connection between the 4H-SiC(000) surface morphology, including the presence of the single atomic steps, the sequences of atomic steps, and also the macrosteps, and the corresponding emergence of planar defective structure (discontinuities of carbon layers and wrinkles) is revealed. It is shown that disappearance of the multistep island leads to the creation of wrinkles in the graphene layers. The density functional theory (DFT) calculation results show that the diffusion of both silicon and carbon atoms is possible on a Si-terminated SiC surface at a high temperature close to 1600{\deg}C. The creation of buffer layer at the Si-terminated surface effectively blocks horizontal diffusion, preventing growth of thick graphene layer at this face. At the carbon terminated SiC surface, the buffer layer is absent leaving space for effective horizontal diffusion of both silicon and carbon atoms. DFT results show that excess carbon atoms converts a topmost carbon layer to sp2 bonded configuration, liberating Si atoms in barrierless process. The silicon atoms escape through the channels created at the bending layers defects, while the carbon atoms are incorporated into the growing graphene layers. These results explain growth of thick graphene underneath existing graphene cover and also the creation of the principal defects at the C-terminated SiC(0001) surface

Effect of bainitic transformation on bake hardening in TRIP assisted steel

► Bainitic transformation in TRIP-assisted steel can lead to a very good bake hardening response as demonstrated by other researchers also. ► No extra deformation is needed. Dislocations can be generated in situ during the transformation itself. ► Detail characterisation and theoretical treatments showed bainite plates are sufficiently enriched with extra carbon atoms which can migrate and lock the dislocations. Bake hardening is a phenomenon where freshly generated dislocations get pinned down by the migrating carbon atoms under the influence of temperature employed in paint baking shop. Experimentally, a minimal 2% deformation is given to generate such new dislocations. On the other hand, after bainitic transformation, steel contains a large number of dislocations as well as excess carbon atoms in bainite, a combination of which is capable of producing bake hardening effect. In the current analysis, one grade of transformation induced plasticity aided steel was chosen to study the effect of isothermal bainitic transformation on subsequent bake hardening response, without giving any deformation assuming that the previous treatment would have generated sufficient dislocations which could be pinned down by the migrating carbon atoms under the influence of thermal treatment of the bake hardening process. The final microstructure was characterised by many techniques, using Thermo-Calc, optical microscopy, XRD analysis and 3-DAP. A good agreement was observed amongst all the techniques employed.