Hard Cr Coatings Research Articles

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.

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.

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.

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.

PVD coatings for mill rolls for cold rolling of high carbon steel strips—Laboratory tests

Increasing the productivity of rolling mills requires improving surface properties of roll surface and prolonging roll life. The properties of interest are anti-adhesion to avoid catastrophic adhesive transfer, and wear resistance. Hard Cr coatings provides such properties for Al and carbon steel rolling, but should be replaced in the long run due to health/environmental issues. Physical Vapor Deposition (PVD) and Plasma-Enhanced Chemical Vapor Deposition (PECVD) of ceramic coatings are promising substitutes. In the context of high carbon steel strip rolling, TiN, TiBN, and CrN are tested here in terms of adhesive properties under large sliding conditions (plate-on-ring test) at increasing load. They are compared with a naked High Speed Steel (HSS) surface and a hard chromium coating. A 3-stage evolution is found as load increases, the last stage being characterized by massive adhesive transfer (severe wear) and high, irregular friction. TiN is shown to perform poorly under these conditions and does not bring an alternative solution to HSS steel. On the contrary, TiBN and CrN provide anti-adhesion protection up to the maximum load of the test, like Cr.

Establishing relationships between bath chemistry, electrodeposition and microstructure of Co–W alloy coatings produced from a gluconate bath

Electrodeposited Fe group: W and Mo alloys have the potential to replace hard Cr coatings for use in engineering applications where wear and corrosion resistance are needed. Electrochemical studies have concentrated in the past on Ni–W alloy deposition, but now interest in Co–W alloys has developed as they possess lower coefficients of friction when in contact with another metal. The most attractive coating composition is in the range 14–20 at.% W, if controlled deposition promotes crystalline alloys of high hardness, rather than softer amorphous alloys containing >20 at.% W. This paper employs ammonia free baths with low concentrations of cobalt and sodium tungstate and varying additions of sodium gluconate to produce alloys at close to 50% efficiency. Voltammetry, UV and visible spectrometry, and potentiostatic deposition have been performed on such baths, whilst XRD, SEM and TEM observations have been made on the deposits. This aims to optimise the process and to understanding the relationships between bath contents, electrochemical kinetics and alloy composition. Efficient deposition of coatings with hardness values up to 1000 kgf mm −2 occurred from a bath containing a high concentration of gluconate. Such deposits arise from concentrations of Co–W–gluconate complexes which promote the formation of nanoscale alloy grains. Current densities up to 2.75 A dm −2 in the agitated bath promoted deposition kinetics to form these highly orientated structures. These kinetics produced nano-segregation of W which may be assisted by the migration of Co–W clusters to boundary sites during the growth of the deposit.

Preparation of Cr hard coatings by ion beam assisted electron beam vapor deposition on Ni and Cu substrates

Hard Cr coatings on Ni and Cu surface were deposited by means of ion beam assisted deposition (IBAD) for the replacement of electroplated chromium in order to avoid the problems of polluting water and hydrogen-embrittlement films from the electroplating. The morphology and mechanical properties of films were examined using SEM, AFM, optical metallographic microscopy, microhardness tester, scratch tester and profile meter. Results showed that adding Al interlayer can improve the adhesion and decrease the residual stress between the coating and substrate due to atom diffusion between the interfaces. The thickness of the interlayer is about 100–200 nm. It concludes that the microhardness of the IBAD samples can reach or even surpass that of the electroplated. It is feasible to obtain Cr hard coatings on some metal substrates by IBAD with optimized equipment and processing.

Increased wear resistance of electrodeposited chromium through applications of plasma source ion implantation techniques

Three plasma source ion implantation (PSII) processes, using either ammonia (NH3), methane (CH4), or oxygen (O2) gases, are shown to increase the surface hardness and wear resistance of electrodeposited hard chromium. Characterization of the implanted surfaces indicates the formation of hard compounds, namely polycrystalline chromium-nitride (CrN), chromium-carbide (Cr3C2), and chromium-oxide (Cr2O3). For wear resistant applications of hard Cr coatings, PSII processes could extend the wear lifetime of coated components. By extending the lifetime and thus reducing the need for re-coating, PSII processes could also reduce the amounts of carcinogenic Cr+6 released into the environment.