Nanocrystalline Ni thin film exhibits poor thermo-mechanical properties due to its unstable microstructure at elevated temperature. Here, the paper endows a new approach to solve above issue via addition of nano-multilayers and incorporation of W for nanocrystalline Ni-based films, to provide novel Ni/Ni3Al-W nano-composite multilayered structure with high hardness and good thermal stability. The thermal evolution of microstructure and mechanical properties was investigated to reveal nanocrystalline stability and strengthening mechanisms for co-sputtered Ni/Ni3Al-W multilayers with varied W concentrations and annealing temperature. The lamellar structure and nonequilibrium phases are well maintained in 600 °C annealed multilayers, while nano-grains are further refined with increasing W addition. Annealing at 800 °C results in the appearance of elemental redistribution and phase separation in multilayers, leading to the layered structure dissolved and globular W-related particles precipitated. Annealing hardening is founded in most of annealed Ni/Ni3Al-W multilayers. Based upon microstructure observation, grain boundary relaxation and W-related phase precipitation are mainly responsible for the hardness enhancement of multilayers at 600 °C and 800 °C, respectively. Notably, the best hardness is achieved at the value of 15.6 GPa for 800 °C annealed 12.5 at% W doped Ni/Ni3Al-W multilayer, which shows the residual layer interfaces with larger precipitations in microstructure. This hardness increment for annealed Ni-based multilayers can be attributed that the high degree of strengthening is provided by a combination of hardening precipitation and survived lamellar structure via the Orowan mechanisms, offering a feasible insight to develop nano-metallic coatings for further increasing thermo-mechanical properties.Download high-res image (225KB)Download full-size image

During thermal exposure between 500 h and 7000 h at 850 °C and 950 °C, Ti(C,N), M6C-type (Ni,Co,Cr)3Mo3C, M23C6-type (Cr,Mo,Ni)23C6 carbides and non-topological close-packed phases (TCP) phases formed in the welds. M6C and M23C6 carbides formed due to both the reaction between C, Mo and Cr and the dissociation of M23C6, M6C and Ti(C,N). The coarsening (Ostwald ripening) rate of M6C carbides was greater than that of M23C6 carbides. The area fractions and average diameters of M23C6 carbides increased due to the long-range diffusion of Cr atoms at 950 °C. Hardness of weld metals decreased with increasing exposure time both at 850 °C and 950 °C due to a decrease of Mo and Cr in solution. From 500 h to 7000 h, the impact energy values of the weld metals exposed at 850 °C decreased due to an increment in area fractions and sizes of M6C carbides, while the impact energy values of weld metals exposed at 950 °C increased because of a decrease of Cr in solution and an increase in the sizes and spaces of GB M23C6 carbides.

The porosity produced in the 304 stainless steel laser welds was investigated. The laser welding process was conducted at a laser power of 10 kW. Different welding speeds and shielding gases were used. The porosity in the welds was characterized. The welding processes were observed using a high-speed video camera. The mechanism of the elimination of the porosity was revealed. The suppression of the porosity was made in N2 rather than Ar or He shielding gases. The porosity was mainly produced at the root and most of the porosity was less than 0.02 mm3. No significant differences of the metallic plume, liquid melt pool, and laser keyhole were found when Ar or N2 shielding gas was employed. The solubility of N2 in the liquid melt pool contributed to the reduction or elimination of the porosity in the 304-L laser welds.

Electroless Ni-P coating was plated on the surface of multi-walled carbon nanotubes (MWCNTs). The Ni-P coated carbon nanotubes (NiPCNTs) were dispersed into Inconel 718 (IN718) powder by ball milling. Then the powder mixtures were deposited into the IN718/NiPCNTs composite coatings by laser powder deposition process. The survivability of MWCNTs, susceptibility to heat affected zone (HAZ) liquation cracking and tensile mechanical properties of the coatings were studied. The results showed that the MWCNTs were successfully incorporated into the IN718 coatings but most of them were transformed into the porous carbon nano ribbons (CNRs), graphene nano sheets (GNSs) and diamond-like nano particles (DNPs). The CNRs were resulted by the inter-welding of the MWCNTs and GNSs. Large thin CNRs bridging with the Laves particles and interdendritically bonded regions improved the stress transfer across the interdendritic regions. The stress localized on the last remaining liquation film could be depressed and the susceptibility to HAZ liquation cracking was suppressed. Furthermore, the tensile strength of the IN718/NiPCNTs coatings was increased significantly whereas the ductility of the coatings was somewhat reduced, which was attributed not only to the addition of the carbon nano allotropes but also the increased formation of the hard but brittle Laves phase.

•W-doped carbon films exhibit low ICR value ranging from 6.25 to 7.21 mΩ-cm2.•Electrochemical results show carbon film with metallic W has better corrosion resistance.•Corrosion resistance during pulse polarization is greatly improved by proper W concentration.•Metallic W on surface is oxidized after polarization enabling self-passivating ability.•A proper W concentration is preferred while formation of WC by overdoping should be avoided.The effects of W doping on the microstructure, ICR, and corrosion resistance of carbon films are systematically investigated. The W-doped carbon film has a compact structure and the surface topography changes slightly with W concentration. W exists in mainly the metallic state when the concentration is small, but amorphous WC is formed in the CW2(A) and CW3(A) samples. The ICR at a typical compaction force of 150 N cm−2 increases marginally in the range of 6.25–7.21 mΩ-cm2 as the W concentration is varied. The carbon films with smaller W concentrations have better corrosion resistance and even the self-passivating ability. During pulse polarization, the bare SS316L shows a very large current density of 150 μA cm−2 due to breakdown of the passive film, but CW1(A) shows a stable and low current density of about 0.6 μA cm−2 due to the good self-passivating ability. The self-passivating ability originates from oxidation of metallic state W in the carbon film and so a proper W concentration can yield the desirable self-passivating effect.

•Studied difference between post and in situ annealing on roughness of NiTi film.•Hydrophobic property of sputtered NiTi thin films was discussed.Surface morphology and hydrophobic property of NiTi thin films are investigated under various in situ and post annealing temperatures. The XRD and AFM results reveal that in situ annealed NiTi thin films crystallize at lower temperature and have larger surface roughness than post annealed samples. Heating during sputtering provides energy for atoms to move to position of equilibrium, thus more regular crystal structure is observed in in situ annealed NiTi thin films. Another reason for the smoother surface observed in post annealed thin films is that large amount of Ni4Ti3 precipitation in hinders the formation of B19′ phase. The contact angle becomes larger with the increase of surface roughness and the maximum contact angle of 123.5° is achieved in 600 °C in situ annealed thin film with 41.7 nm surface roughness.

•Ni + C composite coating was deposited on GW83.•Ni + C coating was dense with large number of disordered band in the carbon layer.•In Ni + C coating the carbon coating had good adhesion with nickel interlayer.•The corrosion resistance was improved by Ni + C coating.The carbon film with electroless nickel interlayer (Ni + C) was fabricated to improve the corrosion resistance of the magnesium alloy GW83. Compared to other coatings used in this study, the dense Ni + C coating improved adhesion with the substrate. The corrosion potential (Ecorr) of the Ni + C coated magnesium alloy was about −1.37 V vs saturated calomel electrode (SCE) in contrast to about −1.67 V vs SCE of the bare one in 3.5 wt% NaCl solution. The corrosion current density was reduced from 186 μA cm−2 to 11 μA cm−2. The 5 h immersion test revealed that the Ni + C coated magnesium alloy GW83 showed much less corrosion compared to the bare alloy. These evidences indicated that applying Ni + C coating could effectively improve the corrosion resistance of the magnesium alloy GW83.

•C/TiN and C/CrN multilayer coatings are denser than single-layer coatings.•C/CrN multilayer coating has the best corrosion resistance among these coatings.•C/CrN has the lowest ICR of 4.08 mΩ-cm2 at compaction force of 150 N-cm−2.•The corrosion in cathode environment is much more serious than that in anode one.Aluminum bipolar plates offer good mechanical performance and availability for mass production while allow up to 65% lighter than stainless steel. To improve the corrosion resistance and surface electrical conductivity of aluminum bipolar plates, several coatings, including TiN, CrN, C, C/TiN and C/CrN, are deposited on aluminum alloy 5052 (AA-5052) by close field unbalanced magnetron sputter ion plating. Scanning electron microscope (SEM) results show that the coatings containing carbon layer are denser than TiN and CrN. Although the potentiodynamic test results show improved corrosion resistance by all the coatings, the potentiostatic test results reveal different stability of these coatings in PEMFC environments. Comparing the SEM images of these coatings after potentiostatic test, C/CrN multilayer coating exhibits the best stability. C/CrN multilayer coated AA-5052 has the lowest metal ion concentration after potentiostatic test, being 11.12 ppm and 1.29 ppm in PEMFC cathodic and anodic environments, respectively. Furthermore, the interfacial contact resistance (ICR) of the bare AA-5052 is decreased from 61.58 mΩ-cm2 to 4.08 mΩ-cm2 by C/CrN multilayer coating at the compaction force of 150 N-cm−2. Therefore, C/CrN multilayer coating is a good choice for surface modification of aluminum bipolar plate.

The desirable properties of metallic bipolar plates in polymer electrolyte membrane fuel cells are good corrosion resistance and high electrical conductance. In this study, carbon-implanted SS316L stainless steel bipolar plates are evaluated by various ex situ and in situ methods. X-ray photoelectron spectroscopy and transmission electron microscopy reveal a carbon-enriched layer with a thickness of about 240 nm thick. The structure depends on the ion implantation fluence. The interfacial contact resistance and electrochemical behavior are determined using ex situ techniques. The interfacial contact resistance decreases with increasing ion implantation fluence. The results obtained by potentiodynamic tests, potentiostatic tests, and inductively coupled plasma optical emission spectrometry measurements are consistent with each other confirming that the corrosion resistance is significantly improved after carbon ion implantation. The carbon-implanted stainless steel bipolar plates are assembled into single cells to undergo in situ evaluation. The peak power density of the carbon-implanted bipolar plate increases from 566.5 mW cm−2 to 840.0 mW cm−2 and the power density at 0.6 V increases by a factor of two compared to those measured from a single cell made of unimplanted stainless steel bipolar plates.Highlights► Ex situ and in situ tests are performed to study the effect of carbon implantation on performance of SS bipolar plates. ► XPS and TEM results disclose different surface composition and microstructure after carbon implantation. ► Carbon implanted SS316L exhibits superior performance in terms of surface conductivity and corrosion resistance. ► The single cell test shows great improvement in peak power density and output voltage after carbon implantation.

A Ni–Cr enriched layer about 60 nm thick with improved conductivity is formed on the surface of austenitic stainless steel 316L (SS316L) by ion implantation. The electrochemistry results reveal that a proper Ni–Cr implant fluence can greatly improve the corrosion resistance of SS316L in the simulated PEMFC environment. The samples after the potentiostatic test are also analyzed by XPS and the ICR values are measured. The XPS results indicate that the composition of the passive film change from a mixture of Fe oxides and Cr oxide to a Cr oxide dominated passive film after the potentiostatic test. Hence, the ICR increases after polarization due to depletion of iron in the passive film. Nickel is enriched in the passive film formed in the simulated PEMFC cathode environment after ion implantation thereby providing better conductivity than that formed in the anode one.