Hard Cr Research Articles

Super hardening of Fe[sbnd]W alloy plating by phase transformation of amorphous to metal carbides-dispersed nanocrystalline alloys and application as promising alternative for hard chromium plating

Despite the superior hardness, wear resistance, and corrosion resistance of hard chromium (Cr) plating using toxic hexavalent chromium (Cr6+), the gradual restriction on the use of Cr6+ in industrial fields necessitates the development of an alternative, harmless plating material with excellent performance. Here, we report the fabrication of super-hard plating by the simple annealing of electrodeposited FeWC plating as a promising alternative for hard Cr plating. Amorphous-structured FeWC plating, including co-deposited citric acid-derived carbon, forms uniformly dispersed ternary metal carbides inside the plating during annealing, which changes the plating structure from amorphous to nano-carbide-dispersed alloys. The phase-transformed, nanocrystalline FeWC alloy annealed at 700 °C exhibited a maximum hardness of 20.5 GPa, which surpassed the hardness of hard Cr plating (12.5 GPa). The ball-on-disc tribo-test and corrosion test results further demonstrated the superior wear and anti-corrosion properties of the carbide-dispersed FeWC alloy plating compared with hard Cr plating. This study develops the applicability of this super-hard plating as a new hard coating material that overcomes the limitations of conventional metal plating without using any toxic chemical.

Effects of load and speed on high-temperature tribological performance of electrodeposited hard Cr coating in trivalent-Cr bath

ABSTRACT A chromium (Cr) coating was electrodeposited on 45 steel in trivalent-Cr bath. The morphologies and element distributions of the obtained coating were analysed using a scanning electronic microscope (SEM) and its configured energy dispersive spectrometer (EDS), respectively. The effects of normal load and rotation speed on the tribological performance of Cr coating at 300°C were analysed using a high-temperature friction tester. The results show that the Cr coating is mainly composed of Cr and Cr2O3 phases, and its average bonding force with the substrate is 9.20 N by the scratch method. Under the normal load of 2 N, the average coefficients of friction (COFs) of Cr coating at the rotation speeds of 300, 400 and 500 rpm are 0.70, 0.68 and 0.82, respectively; and the corresponding wear rates are 53.99, 97.38, and 46.17 μm3· N–1·mm–1, respectively, showing that the wear resistance of Cr coating at 500 rpm is the best among the three different rotational speeds. Moreover, the average COFs of Cr coating under the normal loads of 2, 4 and 6 N at 500 rpm are 0.82, 0.58 and 0.55, respectively; and the corresponding wear rates are 46.17, 84.54 and 71.19 μm3·N–1·mm–1, respectively. The wear mechanism of Cr coating at the rotation speeds of 300, 400 and 500 rpm is mainly adhesive wear; while that under the normal loads of 4 and 6 N is dominated by abrasive wear.

An effective strategy for improving the performance of electrodeposited Fe-W alloy plating: Tailoring microstructure by graphene incorporation

Despite the excellent performance of hard Cr plating using hexavalent chromium (Cr6+), its use is being restricted owing to recent environmental regulations. Electrodeposited Fe-W alloy is considered a possible alternative, but its high coefficient of friction (COF), high wear rate owing to tribo-oxidation, and high corrosion rate hinder its practical application. In this study, we attempted to solve these problems by modifying the microstructure of Fe-W alloy by incorporating 2D-structured graphene. The effect of graphene incorporation on the structure of the Fe-W alloy and its properties, especially microhardness, wear, and corrosion resistance, were investigated. The results showed the graphene effectively modified the columnar structure of the Fe-W alloy by inhibiting the vertical growth of clusters during electrodeposition, leading to fragmentation of the vertical cluster boundaries. This improved the hardness by as much as 17 % up to 9.1 GPa, and the corrosion resistance by changing the corrosion mechanism from localized corrosion to uniform corrosion. In addition, the graphene reduced the COF and wear rate by reducing the Fe2O3 formed by tribo-oxidation to a lower oxidation state. The structural modification of the plating by graphene incorporation thus provides an effective solution to improving the performance of Fe-W alloys.

Reverse V-shaped wear rate vs. friction velocity curve of hard chromium coating and its inspiration for anti-wear coating design

In this research, the friction and wear behaviors of hard chromium (Cr) coating under harsh accelerated friction test conditions (unlubricated reciprocating friction with hard Al2O3 ceramic ball as counterpart) were investigated. The results show that hard Cr coating exhibits comparatively consistent tribological performance through the combination of oxidation wear, abrasive wear and plastic deformation mechanisms in general; it shows relatively low rate of wear in a wide range of friction frequency, and even at higher sliding frequencies its wear rate decreases with increasing frequency; its wear rate-friction velocity (or frequency) curve is in reverse V shape. It has been revealed that these surprising features of hard Cr coating are closely associated with its relatively high micro-hardness and compact frictional oxide thin film. This study enlightens a desirable direction for anti-wear coating design.

High Performance Accelerated Tests to Evaluate Hard Cr Replacements for Hydraulic Cylinders

To prolong the lifetime of hydraulic cylinders, a wear-resistant low-friction surface is required. Until now, hard Cr coatings were the best materials for this. However, in recent years, there has been an increasing pressure on the manufacturing of hard Cr plating and plated products, because of environmental and health hazards. The replacement of these coatings by alternatives has not been highly successful yet, because it requires extensive component testing, which is costly and time-consuming and thus not appropriate for material development. For this reason, there is a high need to develop tribological methods that simulate hydraulic cylinders’ component-testing closely. In addition, these new methods should also provide additional information (e.g., friction evolution) that can assist in the further development and optimization of alternative coatings. Having the above in mind and building on an existing method from the American Society for Testing and Materials (ASTM G133), a new test method that allows users to test directly on hydraulic cylinders was developed. This method can provide a relative ranking of both the wear resistance and frictional performance of alternative coatings in direct comparison to state-of-the-art hard Cr. Importantly, the method is repeatable and has a much shorter test duration than full-scale component tests, thereby accelerating material development significantly.

Synthesis of plasma-nitrided Cr coatings on HT9 steel for advanced chemical barrier property in a nuclear cladding application

Chromium–chromium nitride multi-coating barriers were synthesized on HT9 steel disks using the electroplating and radiofrequency (RF) plasma nitriding techniques, respectively, to significantly mitigate fuel–cladding chemical interactions in sodium fast reactors. It was revealed that plasma nitriding not only synthesized an additional chromium nitride layer at the outer surface of the Cr coating but also had a significant crack-closure effect on hard Cr coatings. Further, Cr coatings nitrided for durations ranging from 0 to 72 h were investigated to observe crack-closure stages; notably, nitriding for 72 h at 600 °C and an RF power of 180 W resulted in the elimination of most visible interior and penetration cracks. Moreover, an additional 0.75–1 μm thick nitride layer at the outer surface of the Cr coating included columnar grains of CrN and Cr2N phases aligned with the growth direction. Diffusion couple tests of Ce–Nd alloys at 650 °C for 25 h revealed that the 72-h-nitrided Cr coating possessed an excellent barrier property without any visible inter-diffusion, in contrast with the untreated Cr coating, for which a substantial inter-diffusion area of up to 17,500 μm2 was observed.

Long-term and high-bioavailable potentially toxic elements (PTEs) strongly influence the microbiota in electroplating sites

Multiple potentially toxic elements (PTEs) wastes are produced in the process of electroplating, which pollute the surrounding soils. However, the priority pollutants and critical risk factors in electroplating sites are still unclear. Hence, a typical demolished electroplating site (operation for 31 years) in the Yangtze River Delta was investigated. Results showed that the soil was severely polluted by Cr(VI) (1711.3 mg kg−1), Ni (6754.0 mg kg−1) and Pb (2784.4 mg kg−1). The spatial distribution of soil PTEs performed by ArcGIS illustrated that the soil pollution varied with plating workshops. Hard Cr electroplating workshops (HCE), decorative Cr electroplating workshops (DCE) and sludge storage station (SS) were the hot spots in the site. Besides, the toxicity characteristic leaching procedure (TCLP) - extractable Cr and Ni contents in different workshops were significantly related (P < 0.05) to their bioavailable fractions (exchangeable fraction (F1) + bound to carbonate fraction (F2)), which pose potential risk to humans. Although the soil total Pb concentration was high, its mobility was very low (<0.007%). Moreover, the soil microbial community dynamics under the stress of long term and high contents of PTEs were further revealed. The soil microbiota was significantly disturbed by long term and high concentration of PTEs. A bit of bacteria (Caulobacter) and fungi (Cladosporium and Monocillium) showed tolerance potential to multiple metals. Furthermore, the canonical correspondence analysis (CCA) showed that the bioavailable fractions (F1 + F2) of Cr and Ni were the most critical environmental variables affecting microbiota. Therefore, remediation strategies are required urgently to reduce the bioavailability of soil Cr and Ni. The results of this study provide an overview of the pollution distribution and microbial dynamics of a typical plating site, laying a foundation for ecological remediation of electroplating sites in Yangtze River Delta of China.

Nacre‐Inspired, Liquid Metal‐Based Ultrasensitive Electronic Skin by Spatially Regulated Cracking Strategy

AbstractThe realization of liquid metal‐based wearable systems will be a milestone toward high‐performance, integrated electronic skin. However, despite the revolutionary progress achieved in many other components of electronic skin, liquid metal‐based flexible sensors still suffer from poor sensitivity due to the insufficient resistance change of liquid metal to deformation. Herein, a nacre‐inspired architecture composed of a biphasic pattern (liquid metal with Cr/Cu underlayer) as “bricks” and strain‐sensitive Ag film as “mortar” is developed, which breaks the long‐standing sensitivity bottleneck of liquid metal‐based electronic skin. With 2 orders of magnitude of sensitivity amplification while maintaining wide (&gt;85%) working range, for the first time, liquid metal‐based strain sensors rival the state‐of‐art counterparts. This liquid metal composite features spatially regulated cracking behavior. On the one hand, hard Cr cells locally modulate the strain distribution, which avoids premature cut‐through cracks and prolongs the defect propagation in the adjacent Ag film. On the other hand, the separated liquid metal cells prevent unfavorable continuous liquid‐metal paths and create crack‐free regions during strain. Demonstrated in diverse scenarios, the proposed design concept may spark more applications of ultrasensitive liquid metal‐based electronic skins, and reveals a pathway for sensor development via crack engineering.

Effects of Boron Addition on the Structure and Properties of Cr–Al–Ti–N Coatings Obtained Using the CFUBMS System

The influence of boron on the structure and on the mechanical and tribological properties of protective coatings of the Cr–Al–Ti–N alloys obtained by magnetron sputtering in a closed unbalanced magnetic field using segmented planar ceramic SHS targets has been studied. The obtained coatings were examined using glow discharge optical emission spectroscopy, scanning electronic microscopy, X-ray diffraction phase analysis, nanoindentation, and tribological tests according to the pin-on-disk scheme. It has been shown that the introduction of B results in the suppression of the columnar structure and the reduction of the roughness of the coatings. Hardness, elasticity modulus, elastic recovery, wear resistance, and the coefficient of friction of coatings of the Cr–Al–Ti–B–N alloys have been determined. The introduction of 2.3 at % B leads to a high hardness (H = 15 GPa), stable friction coefficient (f = 0.65), and a decrease of the reduced wear (Vw = 7.5 × 10–6 mm3/(N m)). It has been shown that the hard Cr–Al–Ti–B–N coatings of the optimal composition in their mechanical and tribological properties exceed the coatings without boron addition.

Experimental research on micro-cutting wear of 38CrSi steel caused by TiAlN- and TiCrAlN-coated tools

A comparative study was carried out on the wear of 38CrSi high-alloy steel using TiCrAlN- and TiAlN-coated tools. The presence of elemental Cr in the TiCrAlN coating resulted in formation of a Cr–O film on the tool, which provided a lower friction coefficient. This also mitigated transfer of cutting heat to the tool substrate and inhibited elemental diffusion between the tool and the workpiece. Simultaneously, the hard Cr–O film, which formed outside the Al–O film, provided excellent wear resistance and protected the Al–O film. Thus, elemental Cr played an active role in enhancing the wear resistance of the tool, prolonging its life. A workpiece cut using a TiCrAlN-coated tool had a surface roughness of 0.79 µm, which was about 62% lower than that obtained using a TiAlN-coated tool. Although wear of the TiCrAlN-coated tool was reduced, reaching 0.2 mm over the standard test time of 8 min, this time was 300% longer than that observed with the TiAlN-coated tool. Furthermore, the cutting force components were smaller, at up to 8 N, and the cutting force was 50% lower than with the TiAlN-coated tool. The wear and failure mechanisms of micro-milling 38CrSi alloy were the product of a combination of forms of wear and followed the wear law in terms of tool tip breakage, coating damage, abrasive wear, and adhesive wear; coating damage had the most important effect on tool life.

A comparison of the galling wear behaviour of PVD Cr and electroplated hard Cr thin films

Electroplated hard chromium (EPHC) is used in many industries as a wear and corrosion resistant coating. However, the long term viability of the electroplating process is at risk due to legislation regarding the toxic chemicals used. The physical vapour deposition (PVD) process has been shown to produce chromium and chromium-based coatings that could be a possible alternative for EPHC in some applications. This study investigates the microstructure and properties of two PVD chromium coatings as a possible alternative to EPHC to provide resistance to galling. Two PVD deposition processes are investigated, namely electron beam PVD (EBPVD) and unbalanced magnetron sputtering (UMS). Galling wear tests were performed according to ASTM G98-17. The results show that the two PVD coatings are of similar hardness, surface roughness and exhibit similar scratch behaviour. However, the galling wear resistance of the coating deposited by UMS is approximately ten times that of the EBPVD coating, and similar to that of the EPHC. X-ray diffraction reveals that the EBPVD chromium coating has a strong preferred orientation of the {200} planes parallel to the coating surface whilst in the UMS PVD coating, preferred orientations of the {110} and {211} planes parallel to the surface are observed. The EPHC does not exhibit relative peak intensities which conform to the International Centre for Diffraction Data (ICDD) powder diffraction pattern consistent with chromium. The crystal orientation of the PVD chromium coatings appears to play a significant role in influencing galling resistance.

Very Hard Corrosion-Resistant Roll-Bonded Cr Coating on Mild Steel in Presence of Graphite

The present work discusses the development of very hard Cr and Cr-carbide coating by roll bonding of Cr powder on a mild steel followed by annealing at 800, 1000, 1100 and 1200 °C with and without the presence of graphite powder packing in argon environment. In addition, the effect of a roll skin pass of 5% prior to the application of coating was studied. The presence of graphite allows diffusion of both carbon and Cr in the mild steel substrate, leading to the formation of Cr-carbide on the outer surface, making the surface very hard (VHN ~ 1800). Depending on the annealing temperature and processing condition, diffusion layer thickness of Cr is found to be in the range of 10-250 μm with Cr content of 12.5-15 wt.% across the diffusion layer. Excellent stable passivity of the coated surface is observed in 0.2 N H2SO4, which is comparable to a highly passivating 304 stainless steel, and very low corrosion rate of the coating is observed as compared to the substrate mild steel.

A favorable chromium coating electrodeposited from Cr(III) electrolyte reveals anti-wear performance similar to conventional hard chromium

Decades of application has proved that electrodeposited chromium (Cr) coating is a perfect coating material. Unfortunately, almost all of these excellent coatings for anti-wear, i.e., hard Cr coatings, are produced by traditional electrodepositing process based on highly toxic Cr(VI). In this research, we reported a promising Cr coating embedded with graphite nanocrystals, which was electrodeposited from a Cr(III) electrolyte and reveals favorable anti-wear performance similar to that of conventional hard Cr coating in a range of sliding velocities. In addition, the developing direction of researches on electrodepositing thick Cr coating from Cr(III) electrolyte was elucidated via comparative investigation of the friction and wear behaviors of three typical Cr coatings.

Synergistic effect of Cu/Cr co-doping on the wettability and mechanical properties of diamond-like carbon films

By choosing carbide-forming element Cr and non-carbide-forming element Cu as co-doped metal elements, we firstly fabricated Cu and Cr co-doped diamond-like carbon (Cu/Cr-DLC) films using a facile hybrid ion beam deposition system. The effect of Cu/Cr co-doping on wettability and mechanical properties of DLC films were focused. The resultant Cu/Cr-DLC films were shown to afford good hydrophobicity and superior mechanical performance. In particular, the film with Cu11.88%Cr6.57% (at.%) exhibited a relatively high water contact angle of 103.6°, lower residual stress of 0.89GPa and high hardness of 13.44GPa. The combined structural analysis demonstrated that the synergistic Cu/Cr co-doping resulted in the critical and significant relaxation of distorted CC bond length, which ultimately caused the reduction in residual stress. Due to the formation of hard Cr carbide nano-particles in amorphous carbon matrix, the films maintained the high value of hardness. In addition, noted that the interesting wetting variety from hydrophilic to hydrophobic state was attributed to the enhanced surface graphitization and emergence of copper oxidation phases, and the film topographical microstructure could also promote the hydrophobicity of film when the maximum height of roughness was in a certain range. These results provide an expected robust synthesis method to make DLC film a promising protective coating for long lasting hydrophobic application and related harsh fields.

Development of a Composite Technique for Preconditioning of 41Cr4 Steel Used as Gear Material: Examination of Its Microstructural Characteristics and Properties

Commercial 41Cr4 (ISO standard) steel was treated by a composite technique. An intermediate layer was introduced firstly at the 41Cr4 steel surface by traditional carburizing and nitriding. Then a hard Cr coating was brush-plated on the intermediate layer. Finally, the coating layer was modified by high current pulsed electron beam (HCPEB), followed by quenching and subsequent tempering treatment. The microstructure, mechanical properties, and fracture behavior were characterized. The results show that a nanocrystalline Cr coating is formed at the 41Cr4 steel surface by the treatment of the new composite technique. Such nanocrystalline Cr coating has acceptable hardness and high corrosion resistance performance, which satisfies the demands of the gears working under high speed and corrosive environment. The composite process proposed in this study is considered as a new prospect method due to the multifunction layer design on the gear surface.

チャンネル型微細溝を有したPVD硬質厚膜の形成とその摩擦特性

A PVD thick film forming technique has been developed to produce a channel-type microgroove exhibiting a lubricant retention effect. This technique uses hard Cr plating with a channel-type microgroove prepared by etching as a PVD film underlayer. Surface morphology of the hard Cr plating with a channel-type microgroove is inherited by the PVD film. This study clarified that this technique can form a channel-type microgroove that penetrates from the underlayer surface to the surface of a PVD thick film with thickness greater than 10 μm. In Scratch tests and Rockwell indentation tests, the CrN thick film with the channel-type microgroove showed a higher adhesion than the conventional CrN thick film. In Pin-on disks test under a liquid paraffin and MoS2 lubricant environment, the CrN thick film with the channel-type microgroove maintained a low friction coefficient without seizing in conditions where general CrN thick film seized to the pin. In Cupping tests, the CrN thick film with the channel-type microgroove showed a lower drawing force and a higher lubricant retention capability than the conventional CrN thick film.

Diffusion characteristics of plasma nitrided hard chromium on AISI 1010 steel

Abstract In order to investigate the different Cr-N formation characteristics of plasma nitrided hard Cr coatings, Cr was electrodeposited on AISI 1010 steel and plasma nitrided at 600, 700 and 800 °C for 3 h, 5 h and 7 h, respectively. Phase analyses of resulting Cr-N phases and grain size of Cr layer before and after nitriding process were calculated by X-ray diffraction analysis. Structure of nitride layer and its thickness were analyzed using scanning electron microscopy micrographs. The micrographs indicated that samples consisted of three distinctive layers. In order to distinguish these layers, scanning electron microscopy and energy dispersive spectroscopy (EDX) analysis were used as well as elemental distribution versus depth was plotted. The Cr-N diffusion was investigated by layer thickness measurements, and diffusion coefficient as well as activation energies were calculated.

Effect of plasma nitriding time on surface properties of hard chromium electroplated AISI 1010 steel

Abstract Properties of steel can be enhanced by surface treatments such as coating. In some cases, further treatments such as nitriding can also be used in order to get even better results. In order to investigate the properties of nitride layer on hard Cr coated AISI 1010 steel, substrates were electroplated to form hard Cr coatings. Then hard Cr coatings were plasma nitrided at 700 °C for 3 h, 5 h and 7 h and nitride phases on the coatings were investigated by X-ray diffraction analysis. The layer thickness and surface properties of nitride films were investigated by scanning electron microscopy. The hardness and adhesion properties of Cr-N phases were examined using nano indentation and Daimler-Benz Rockwell C adhesion tests. The highest measured hardness was 24.1 GPa and all the three samples exhibited poor adhesion.

Role of chemical hardness in the adsorption of hexavalent chromium species onto metal oxide nanoparticles

We employed chemical hardness to explain the adsorption behaviour of Cr(VI) anions on metal oxide sorbents. In the absence of electrostatic attraction at pH values around the point of zero charge, silica and titania nanoparticles adsorbed a very small amount of Cr(VI) anions. This lack of adsorption is because the magnitude of the chemical hardness is too high to pair with the less hard Cr(VI) anions. In contrast, maghemite nanoparticles with relatively weak values of hardness adsorbed 0.021mol of Cr(VI) per one mole of OH groups on the particle surface, 2.7 times that of titania. The magnitude of the chemical hardness decreases in the order of SiO2≫TiO2>γ-Fe2O3, which is determined by the hardness of constituting metal cations. This result indicated that the adsorption onto an oxide surface is governed by metal cations in that their chemical hardness determines the basicity of surface OH groups, which regulates adsorption as well as protonation/deprotonation. The hardness match between a sorbent and a sorbate may be interpreted as their chemical affinity. We demonstrated that the chemical affinity is the principal driving force for adsorption of Cr(VI) anions onto the oxides.

Sliding contacts for the pharmaceutical industry: failure analysis and dry sliding tests for the replacement of hard Cr on AISI 316L steel

Several alternatives were compared for the replacement of hard electroplated Cr coating to improve the tribological properties of the AISI 316L austenitic stainless steel for pharmaceutical packaging applications, including low temperature carburizing (LTC), thermal spray coatings (Al2O3-13TiO2, WC-17Co), substitution of the AISI 301 reference counterface with polymeric materials (PTFE, UHMWPE, PEEK). In dry sliding block on ring tests, the LTC AISI 316L cylinders lead to the lowest wear rates of the AISI301 sliders under low loads (up to 10N). When considering the polymer vs. uncoated AISI 316L couple, PEEK and UHMWPE lead to lower friction and comparable wear rates with respect to the reference couple (AISI 301 vs. Hard Cr coated-AISI 316L) in the whole range of tested loads.