Low Nitrogen Partial Pressure Research Articles

Effect of nitrogen pressure on the microstructure, conductivity, and corrosion resistance of ZrN-coated stainless steel as bipolar plate for proton exchange membrane fuel cell

Zirconium nitride (ZrN) coatings were deposited on the stainless steel using multi-arc ion plating under varying nitrogen partial pressures (PN) ranging from 0.5 to 3.5 Pa. The effect of phase composition, microstructure, and surface morphology on the corrosion resistance and conductivity of the ZrN-coated stainless steel in a simulated proton exchange membrane fuel cell (PEMFC) cathode environment was systematically explored. XRD and TEM revealed that the columnar-structured ZrN coating mainly comprised the fcc-ZrN phase. The Zr3N4 phase was discovered near the substrate during coatings prepared at low nitrogen partial pressures. Surface defects were reduced at nitrogen partial pressures exceeding 2.5 Pa. The ZrN coating exhibited enhanced corrosion resistance and conductivity (ZrN-3.5, Ecorr = 0.307 V, Icorr = 2.085 × 10−7 A/cm2, Rct = 1.125 × 105 Ω·cm2) compared to bare stainless steel in the simulated PEMFC cathode environment. The presence of surface defects influenced the corrosion resistance, while differences in conductivity were related to the phase composition and microstructure of the coating. Thus, the findings indicate that ZrN coatings prepared under optimal deposition conditions can have potential applications in PEMFC bipolar plates.

THE INFLUENCE OF PLASMA ASSISTANCE MODES ON CHARACTERISTICS AND COMPOSITION OF PVD ZIRCONIUM NITRIDE COATINGS

Zirconium nitride coatings are deposited by a vacuum-arc method at a low (0.01-0.04 Pa) nitrogen partial pressure in the modes without and with plasma assistance. The plasma assistance is provided by the operation of a gas plasma source based on a non-self-sustained arc discharge with thermionic and hollow cathodes. This new low-inertia method has not been practically studied in terms of formation of nitride coatings with different architecture (single-layer, multi-layer, gradient coatings). We note that the effect of plasma assistance (an increase of the nitrogen concentration in the nitride coating with the increasing nitrogen ion fraction in the gas-metal plasma) is practically unobservable in the ZrN coatings formed under the selected conditions. Furthermore, their phase compositions do not noticeably change and their physical, mechanical and tribological properties are not significantly improved.

Influence of Oxygen Flow Rate on the Properties of FeOXNY Films Obtained by Magnetron Sputtering at High Nitrogen Pressure

Fe-O-N films were successfully deposited by magnetron sputtering of an iron target in Ar-N2-O2 reactive mixtures at high nitrogen partial pressure 1.11 Pa (Q(N2) = 8 sccm) using a constant flow rate of argon and an oxygen flow rate Q(O2) varying from 0 to 1.6 sccm. The chemical composition and the structural and microstructural nature of these films were characterized using Rutherford Backscattering Spectrometry, X-ray diffraction, and Conversion Electron Mössbauer Spectrometry, respectively. The results showed that the films deposited without oxygen are composed of a single phase of γ″-FeN, whereas the other films do not consist of pure oxides but oxidelike oxynitrides. With higher oxygen content, the films are well-crystallized in the α-Fe2O3 structure. At intermediate oxygen flow rate, the films are rather poorly crystallized and can be described as a mixture of oxide γ-Fe2O3/Fe3O4. In addition, the electrical behavior of the films evolved from a metallic one to a semiconductor one, which is in total agreement with other investigations. Comparatively to a previous study carried out at low nitrogen partial pressure (0.25 Pa), this behavior of films prepared at higher nitrogen partial pressure (1.11 Pa) could be caused by a catalytic effect of nitrogen on the crystallization of the hematite structure.

Experimental analysis of thermal behavior in cryogenic propellant tank with different pressurants

The thermal behavior of cryogenic propellant tanks is crucial issue in the operation of cryogenic propulsion systems. Herein, ground experiments were conducted in a 600-mm-diameter cryogenic tank filled with liquid nitrogen. As pre-pressurants, gaseous helium and gaseous nitrogen (of the same species as the liquid), were used to investigate its effect in accordance with actual propulsion systems. The tank was sealed after pre-pressurization to observe the self-pressurization. The evaporation rate and heat flow in the tank were estimated based on pressure and temperature measurements. In addition, the axial liquid temperature distribution was obtained through the liquid draining from the tank bottom, and a thermal stratification model was developed. These results demonstrated that the type of pre-pressurant significantly affected the thermal behavior in the tank. A higher evaporation rate and higher liquid internal energy rise rate with a thicker thermal layer were observed in the helium pre-pressurization case. The lower nitrogen partial pressure in the helium case enhanced the vaporization and growth of the thermal layer. Estimation of the power balance in the tank demonstrated that not only the ullage but also the heat mass of the tank provided heat for the evaporation and thermal layer. The evaporation occurs mainly at the contact point between the tank skin and liquid surface. The nitrogen vapor rises in a thin layer along the tank skin because of buoyancy under nitrogen pre-pressurization; however, buoyancy is lower under helium pre-pressurization, and a radial vapor flow is probably produced from the contact point instead, leading to a higher heat flux to the liquid.

Correlating elemental distribution with mechanical properties of TiN/SiNx nanocomposite coatings

Detailed structural and chemical analyses reveal that Ti-Si-N coatings prepared with high nitrogen partial pressure and temperature are composed of nm-sized (nearly equiaxed) TiN crystallites encapsulated by ~1.0 nm thin Si-rich tissue phase. This nanocomposite structure allows for superior hardness (37.6 ± 1.5 GPa) and fracture toughness (4.5 ± 0.6 MPa√m). Contrary, deposition at lower nitrogen partial pressure and temperature does not allow for such a clear Si segregation during film growth, leading to nm-sized solid solution Ti1-xSixN crystallites. Although having the same Si content of 8.5 at.%, their hardness (32.1 ± 0.9 GPa) as well as fracture toughness (3.0 ± 0.2 MPa√m) is significantly lower.

Reactive sputter deposition of CoCrCuFeNi in nitrogen/argon mixtures

Thin films of (CoCrCuFeNi)Nx were deposited by direct current reactive magnetron sputtering. Two different targets were used. Thin films deposited from a solid near-equimolar target showed a Cu excess with segregation of Cu in the film resulting in the formation of a nanocomposite film. It was also not possible to reach fully stoichiometric nitride thin films with this target. To overcome this problem, a powder target with adjusted composition was used in a second series of experiments. As derived from longitudinal and transversal sound velocity measurements, a transition from the metallic face-centred cubic lattice to the nitride B1 (NaCl) structure is observed. The observed transition corresponds with the process change from the metallic regime at low nitrogen partial pressure to the poisoned regime at high nitrogen partial pressure. TEM measurements show a typical zone T growth with a strong (100) texture in the nitride regime, and reveal a mixing of the constituent metals at the nanometer scale. This study demonstrates that the constituent metals do not form binary nitrides, but a complex nitride is formed.

Investigation of structure and mechanical properties of plasma vapor deposited nanocomposite TiBN films

TiBN coatings have huge potential applications as they have excellent properties with increasing modern industrial requirements. Nanocomposite TiBN coatings were synthesized on cemented carbide, high speed steel and Si substrates by using cathodic arc plasma ion plating from pure TiB2 ceramic targets. The structure and mechanical properties of the TiBN coatings were significantly influenced by the nitrogen partial pressure. Rutherford backscattering spectrometry demonstrates that the nitrogen content of the coating varied from 2.8% to 34.5% and high-resolution electron microscopy images reveal that all coatings have the characteristic of nanocrystals embedded in an amorphous matrix. The root-mean-square roughness of the coatings increases from 3.73 to 14.64 nm and the coefficients of friction of the coatings at room temperature vary from 0.54 to 0.73 with increasing nitrogen partial pressure. The microhardness of the coating increases up to 35.7 GPa at 10 sccm N2 flow rate. The smallest wear rate is 2.65 × 10−15 m3 N–1 m–1 which indicates that TiBN coatings have excellent wear resistance. The adhesion test revealed that the TiBN coatings have good adhesion at low nitrogen partial pressure.

The microstructure and mechanical properties of tantalum nitride coatings deposited by a plasma assisted bias sputtering deposition process

Abstract In this study, two tantalum nitride-based coatings were synthesized onto Ti-6Al-4V substrates with two different Ar/N 2 flux ratios using a form of plasma assisted bias sputtering deposition termed the double cathode glow discharge deposition technique. Their microstructures and mechanical properties were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy and nanoindentation tests. At a low nitrogen partial pressure, the tantalum nitride coating consists of a hexagonal Ta 2 N phase, with a preferred (101) orientation, while at a high nitrogen partial pressure, the as-deposited coating is composed of a face-centered cubic (fcc) TaN phase with a strongly (200) oriented texture. The two as-deposited coatings exhibited striated nanostructured composed of equiaxed grains about ~ 10 nm in diameter, embedded with an array of homogenously distributed nanopores. The mechanical properties and damage resistance of the coatings were evaluated by nanoindentation techniques. The hardness and elastic modulus of the Ta 2 N coating was higher than those of the TaN coating, indicating that the Ta 2 N coating may offer better protection ability for the underlying metal substrate under load-bearing conditions. In addition, the presence of nanopores is beneficial to the contact damage resistance for both coatings.

Structural and Mechanical Properties of Ti-W-N Thin Films by Dual Unbalanced Magnetron Sputtering

Ti-W-N thin films grown on Si (100) and AISI D2 steel substrates had been deposited by a d.c. magnetron sputtering with pure Ti and W targets in a mixture of Ar and N2 plasma. The nitrogen partial pressure was varied from 0% to 9% of total gas. All Ti-W-N films were formed in solid solution with determination by x-ray diffractrogram analysis. A strong preferred orientation TiN(111) was detected. Their mechanical properties were studied using nanoindentation with Berkovich tip. An increase in hardness was observed with increasing nitrogen partial pressure. The optimum protective coating for plastic deformation was Ti-W-N film grown at 9% nitrogen partial pressure. Chemical bonding of Ti-W-N films was investigated by x-ray photoelectron spectroscopy. Binding energy analysis showed that N was mainly in TiN and W2N. The corrosion behavior was studied in variation of nitrogen partial pressure. Ti-W-N films deposited on steel at low nitrogen partial pressure showed excellent corrosion resistance in NaCl solution.

Effect of Al doping on phase formation and thermal stability of iron nitride thin films

In the present work, we systematically studied the effect of Al doping on the phase formation of iron nitride (Fe–N) thin films. Fe–N thin films with different concentration of Al (Al = 0, 2, 3, 6, and 12 at.%) were deposited using dc magnetron sputtering by varying the nitrogen partial pressure between 0 and 100%. The structural and magnetic properties of the films were studied using x-ray diffraction and polarized neutron reflectivity. It was observed that at the lowest doping level (2 at.% of Al), nitrogen rich non-magnetic Fe–N phase gets formed at a lower nitrogen partial pressure as compared to the un-doped sample. Interestingly, we observed that as Al doping is increased beyond 3 at.%, nitrogen rich non-magnetic Fe–N phase appears at higher nitrogen partial pressure as compared to un-doped sample. The thermal stability of films were also investigated. Un-doped Fe–N films deposited at 10% nitrogen partial pressure possess poor thermal stability. Doping of Al at 2 at.% improves it marginally, whereas, for 3, 6 and 12 at.% Al doping, it shows significant improvement. The obtained results have been explained in terms of thermodynamics of Fe–N and Al–N.

Qualitative analysis of Zircaloy‐4 cladding air degradation in O2–N2 mixtures at high temperature

This article is devoted to the qualitative analysis of the Zircaloy‐4 degradation mechanism at 850 °C in oxygen/nitrogen partial pressure atmospheres. Thermogravimetry, optical microscopy, scanning electron microscopy, and energy dispersive X‐ray spectrometry, are used to provide some information regarding the oxidation kinetics and the various phases involved along the process. The kinetic curves reveal two stages: a pre‐transition and a post‐transition one. Oxide growth during the pre‐transition stage is controlled by oxygen vacancy diffusion in the oxide layer, since neither oxygen nor nitrogen partial pressure influences the kinetics. In the post‐transition stage, nitrogen has an accelerating effect in the corrosion reaction. The kinetic curves reveal two distinct behaviors after the transition according to the oxygen and nitrogen partial pressures: in the first case the corrosion rate rises substantially and leads to a rapid degradation of the metal at high oxygen and nitrogen partial pressures, in the second case the corrosion rate rises dramatically and reaches a plateau at low oxygen and nitrogen partial pressures. This paper describes the influence of both gases on the corrosion kinetics in relation with morphological observations, analyses the discrepancies found between high and low oxygen and nitrogen partial pressures; it also exhibits the complexity of the solid state transformations due to three distinct reactions that appear to take place during the corrosion process.

Structural and mechanical properties of room temperature sputter deposited CrN coatings

Chromium nitride (CrN) thin films were deposited at room temperature on silicon and glass substrates using DC reactive magnetron sputtering in Ar+N2 plasma. Structure and mechanical properties of these films were examined by using XRD, FESEM and nanoindentation techniques. XRD studies revealed that films are of mixed phase at lower nitrogen partial pressure (PN2) and single phase at higher (PN2). Microscopy results show that the films were composed of non-equiaxed columns with nanocrystallite morphology. The hardness and elastic modulus of the films increase with increasing nitrogen partial pressure (PN2). A maximum hardness of ∼29GPa and elastic modulus of 341GPa were obtained, which make these films useful for several potential applications.

TiN Protective Coating on NdFeB by DC Magnetron Sputtering

TiN protective coating was prepared on NdFeB by DC magnetron sputtering to improve the corrosion resistance and scratch resistance of the magnetic. During deposition, the nitrogen/argon mixture gas was introduced into the chamber with the nitrogen partial pressure at 20 % and 10 % and the bias of 200 V was applied to subtract, respectively. At a lower nitrogen partial pressure of 10%, TiN coating was composed of many particles with an irregular shape. With increasing the nitrogen pressure to 20%, composing particles become small and show as the regular trihedron shape. All TiN coatings have a cubic structure. The bias makes the depositing TiN coating denser and preferential growth become no obvious. By the nitrogen partial pressure of 20 % and bias of 200 V, the dense TiN coating has best corrosion resistance and excellent wear resistance.

Synthesis–structure–property relations for Cr–B–N coatings sputter deposited reactively from a Cr–B target with 20 at% B

Cr–B–N films were synthesized by unbalanced magnetron sputtering from a sintered Cr–B target with 20 at% B in an Ar–N 2 discharge at varying N 2 partial pressures ( p N 2 ) of up to 64% of the total pressure ( p Ar + p N 2 = 0.4 Pa). Coating composition and microstructure were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and wavelength-dispersive electron-probe microanalysis and correlated with mechanical and tribological properties measured by microindentation and dry sliding ball-on-disk tests. For low nitrogen partial pressures ( p N 2 ⩽ 22 % ), the XRD patterns are composed of broad overlapping peaks with low intensity. These films have hardness values of ∼15 GPa and indentation moduli of ∼150 GPa. Increasing p N 2 from 22% to 28% results in an increase of the N content from ∼38 to 50 at% where the films meet the quasi-binary CrN–BN composition. Thereby, an increase of the hardness from ∼15 to 32 GPa is obtained. A further increase in p N 2 up to 64% results in minor changes of the chemical composition, micro and bonding structure as well as mechanical properties. While in ball-on-disk testing early failure was observed for coatings grown at p N 2 ⩽ 22 % , higher nitrogen contents in the discharge yielded friction coefficients of ∼0.43 independent of chemical composition, microstructure and mechanical properties.

Effects of treatment temperature on the microstructure and magnetic properties of Fe-N thin films

Fe-N thin films were fabricated on both 100 si and NaCl substrates by RF magnetron sputtering under low nitrogen partial pressure. The microstructure and magnetic properties of Fe-N thin films were investigated with the increase of the substrate temperature (T s and the annealing temperature (T a ). It is more difficult for nitrogen atoms to enter the Fe lattice under higher T s above 150°C. The phase evolution is visible at higher T a above 200°C. The phase transformation of α ″ -Fe 16 N 2 occurred at 400°C. The change of crystal size with T a was clearly visible from bright and dark field images. The clear high-resolution electron microscope (HREM) images cf 110 α 111 γ , 112 α and 200 α , phases were observed. The interplanar distances from TEM (transmission electron microscope) and HREM match the calculated values very well. From the results of the vibrating sample magnetometer (VSM), the good magnetic properties of Fe-N films were obtained at 150°C of T s and 200°C of T α respectively.

Magnetron Co-Sputtering of Nanostructured Chromium Aluminium Nitride Coatings

Ternary chromium aluminium nitride (Cr,Al)N coatings were produced by reactive magnetron co-sputtering technique at different nitrogen deposition pressures. Densified nanostructured coatings with grain size below 100 nm were obtained under critically controlled deposition conditions at low nitrogen partial pressures. The nanostructured coatings were generally of improved surface roughness and properties. Microhardness measurements showed that the coatings had much higher hardness than those of coarser grain sizes. It is believed that the refinement of the coating structure at low nitrogen pressures is associated with a larger number of atoms/molecules depositing on the substrate with higher energies, thus enhancing the adatom mobility and nucleated cluster formation in the coatings. The relationship between the grain size reduction and the deposition rate of the coatings was analysed.

Role of the composition and type of film-forming species in film structure formation

We present the results of an investigation into correlation between the type of film-forming species (FFS) (atoms and molecules) and the structure of the films deposited by reactive magnetron sputtering of W-30 at.%Ti alloy in an Ar–N2 working gas. Mass-spectrometric studies have revealed that the FFS flux contains both atomic (W, Ti, N) and molecular (WN, TiN) species and the flux composition depends on the nitrogen partial pressure in a working gas. At low nitrogen partial pressures (PN2<0.1 Pa) the FFS flux is mainly composed of W, Ti atoms and WN, TiN molecules and the film deposited from such flux is formed of biphase [W+(W,Ti)2 N] composite material with extremely small (approx. 10 nm) grain size. The films deposited from the flux containing mostly metal and nitrogen atoms (higher nitrogen partial pressures PN2>0.2 Pa) are formed from monophase (W,Ti)2 N material with 2–4 times larger crystallite size. The microhardness of the films deposited from the FFS flux containing mainly metal atoms and nitride molecules was noticeably higher than that of the films formed mostly from metal and nitrogen atoms.

Mechanism governing nitrogen absorption by steel weld metal during laser welding

The existence of monatomic nitrogen in the plasma just over the keyhole during CO2 laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO2 laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO2 laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO2 laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO2 laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.

Role of the composition and type of film-forming species in film structure formation

We present the results of an investigation into correlation between the type of film-forming species (FFS) (atoms and molecules) and the structure of the films deposited by reactive magnetron sputtering of W-30 at.%Ti alloy in an Ar–N 2 working gas. Mass-spectrometric studies have revealed that the FFS flux contains both atomic (W, Ti, N) and molecular (WN, TiN) species and the flux composition depends on the nitrogen partial pressure in a working gas. At low nitrogen partial pressures ( P N2<0.1 Pa) the FFS flux is mainly composed of W, Ti atoms and WN, TiN molecules and the film deposited from such flux is formed of biphase [W+(W,Ti) 2 N] composite material with extremely small (approx. 10 nm) grain size. The films deposited from the flux containing mostly metal and nitrogen atoms (higher nitrogen partial pressures P N2>0.2 Pa) are formed from monophase (W,Ti) 2 N material with 2–4 times larger crystallite size. The microhardness of the films deposited from the FFS flux containing mainly metal atoms and nitride molecules was noticeably higher than that of the films formed mostly from metal and nitrogen atoms.

Nitrogen Control During the Autogenous ARC Welding of Stainless Steel

This study deals with nitrogen absorption and desorption during the autogenous welding of stainless steel, investigating the influence of the base metal nitrogen and surface-active element concentrations and nitrogen partial pressure in the shielding gas. The weld nitrogen concentration increases with shielding gas nitrogen content at low nitrogen partial pressures, but at higher partial pressures nitrogen absorption is balanced by N2 evolution. This steady-state nitrogen content is not influenced significantly by the base metal nitrogen content in low sulphur alloys, but in high sulphur alloys, an increase in the initial nitrogen concentration causes higher weld nitrogen contents over the entire range of partial pressures evaluated. The weld metal saturation limit is reached at progressively lower shielding gas nitrogen contents as the base metal nitrogen level increases. It is postulated that less nitrogen is required in the shielding gas to reach the saturation limit in the high sulphur alloys because an appreciable fraction of the nitrogen already present in the base metal is prevented from escaping by a higher level of surface coverage. A kinetic model can be used to describe this behaviour. The desorption rate constant decreases with an increase in sulphur content, but the absorption rate constant is not a strong function of the sulphur concentration. The higher rate of nitrogen removal at the onset of steady-state behaviour causes higher-nitrogen alloys to require more supersaturation prior to bubble formation.