Cr-based Coatings Research Articles

Effect of diffusion barriers on steam oxidation properties and interface evolutions of Cr coating for zirconium alloy at 1200 °C

Currently, Cr-based coatings are considered the most promising protective coatings for zirconium alloy claddings under the ATF design concept. However, the primary factor affecting the oxidation resistance of Cr coatings is the interdiffusion of Cr and Zr. In this study, Cr coatings with three refractory metal interlayers of Nb, Mo, and Ta were deposited on zirconium alloy surfaces using closed-field unbalanced magnetron sputtering. The study investigated the effects of different interlayers on the microstructure, oxidation properties, and interface evolution process of Cr coatings under steam conditions at 1200 ℃, and compared them with Cr coatings without intermediate layers. The results indicate that the refractory metal interlayer influences the orientation and growth rate of Cr coating grains. The formation of a Laves phase mixed layer in the interlayer during oxidation effectively hinders the diffusion of O and Cr to the zirconium alloy matrix, enhancing its oxidation resistance. Furthermore, the diffusion rate and vacancy concentration of Mo and Ta elements to the Zr layer exceed that of diffusion to the Cr layer. During the cooling process, a precipitate phase forms in the Zr-4 matrix, while the Nb coating forms an infinite solid solution in the β-Zr.

Novel Chromium–Silicon Slurry Coatings for Hot Corrosion Environments

Ni-based superalloys are commonly used in gas turbines because of their exceptional high-temperature mechanical properties. To secure a long service life, the materials must also have sufficient corrosion resistance. Therefore, diffusion coatings are widely used to enrich the surface in protective oxide scale-forming elements. For temperatures between 650 and 950 °C, where hot corrosion occurs, Cr-based coatings are advantageous. These are commonly applied via the laborious pack cementation process. Recently, a novel cost-effective Cr/Si slurry coating process has been developed which demonstrated resistance to oxidative high-temperature environments. Here, the protection of the slurry coatings against hot corrosion type I at 900 °C on the Ni-based superalloy Rene 80 is investigated and compared to coatings produced by pack cementation. Prior to the 300-h exposures in air containing 0.1% SO2 at 900 °C, 4 mg/cm2 of Na2SO4 was deposited on the material surfaces. The uncoated Rene 80 exhibited rapid dissolution of the initial oxide scale followed by catastrophic break away oxidation. In comparison, the slurry coatings showed significantly improved hot corrosion resistance compared to the uncoated alloy and a better protection than a Cr pack cementation coating. The Cr pack cemented Rene 80 showed improved hot corrosion resistance, but Cr depletion in the subsurface zone occurred with increasing exposure time, associated with the propagation of Al internal oxidation and increasing sulfidation. In contrast, the slurry coatings formed an external Cr2O3 scale coupled with an agglomeration of SiO2 underneath and a continuous Al2O3 subscale which offered a better diffusion barrier and leading to superior long-term protection against hot corrosion.

Enhancing the oxidation behaviors of Zr alloys for nuclear fuel cladding using nanolamellar Cr/CrN coating

Cr, CrN, and nanolamellar Cr/CrN coatings were deposited on Zr-4 substrate by multi-arc ion plating in Ar, N2, and Ar + N2, respectively. The oxidation behaviors of these various types of Cr-based coatings in steam under different temperature-time conditions were compared. The nanolamellar Cr/CrN coating had a smaller weight gain, a smoother oxide surface, fewer pores, no cracks, and could block the interdiffusion between Cr-Zr by generating a ZrN layer. The designed nanolamellar Cr/CrN coating can effectively combine the advantages of Cr and CrN coatings to show better resistance to oxidation. Oxidation mechanisms have also been discussed in detail.

Effect of temperature on the fretting wear behavior of Cr-coated Zircaloy cladding in high-temperature pressurized water

Cr-based coatings are promising candidates to protect the claddings on accident-tolerant fuel (ATF) rods in pressurized water reactors (PWRs). In this study, the effect of water temperature on the fretting wear behavior of an N36 Zirconium alloy cladding was investigated. In addition, the effect of applying a protective Cr coating to the cladding was also analyzed. Fretting wear tests were carried out using a unique apparatus that simulated the high-temperature and high-pressure (HTHP) water environment in a real PWR. Results show that changes in temperature alter the wear mechanism of cladding tubes. Furthermore, a Cr-coating improves the wear resistance of an N36 cladding significantly. The main wear mechanisms of Cr-coated cladding in HTHP water were observed to be abrasive and adhesive wear. The protective effect of a Cr coating on the cladding in a low-temperature water environment (25 °C, 90 °C) was less than it was in a high-temperature water environment (260 °C, 315 °C), indicating that Cr coatings can improve wear resistance in HTHP water.

Effect of Al content on 1200 °C steam oxidation behavior of Cr-based coatings on TZM alloy

In this paper, the 1200 °C high temperature steam oxidation behavior of Cr-based coatings with different Al content (0, 15 at%, 30 at%, 50 at%) deposited on titanium-zirconium-molybdenum (TZM) alloy has been investigated. The oxidation resistance of TZM alloy is significantly increased by the Cr-based coatings. The Cr70Al30 coating exhibits the best performance due the protectiveness of a triple oxide layer formed on coating surface, including outer Al2O3 and Cr2O3 mixed layer, intermediate Cr-rich oxide (Cr2O3) layer and inner Al-rich oxide (Al2O3) layer. The effect of Al content on the oxidation resistance of Cr-based coatings is discussed in detail.

Influence of a Mo interlayer on the oxidation behavior of a Cr coating on a Zr alloy substrate

Cr-based coatings with or without a Mo interlayer were deposited on a Zr substrate by multi-arc ion plating, and the effect of the Mo interlayer on the oxidation behavior of the coatings at different time lengths (maximum 90 min) at 1160 °C was investigated. Atomic scale characterization confirms that, on the one hand, the Mo interlayer prevents the Cr coating from reacting with the Zr substrate to form the ZrCr2 Laves phase and, on the other hand, the solid solution structure generated due to Mo doping effectively guarantees the integrity of the Cr2O3 oxide layer and the residual Cr coating.

Enhancement of sliding wear and scratch resistance of two thermally sprayed Cr-based coatings by ultrasonic nanocrystal surface modification

This study aims to identify whether mechanical surface treatment can improve the properties of thermal spray coatings for use in fuel-lubricated sliding conditions. In this study, we investigated the effects of ultrasonic nanocrystal surface modification (UNSM) technology on thermally sprayed chromium oxide (Cr2O3) and chromium carbide-nickel chromium (Cr3C2–NiCr) coatings and the resulting changes in sliding wear in fuels and scratch resistance. The improvement in coefficient of friction (COF) and specific wear rate (SWR) of the coatings after UNSM treatment was attributed to the reduced surface roughness, increased surface hardness, and the formation of tribo-layers (oxide) having lower shear strength with good lubricating properties. The wear mechanisms of the coatings involved plastic deformation leading to abrasive and oxidation modes.

Sol-Gel Materials for Electrochemical Applications: Recent Advances

This review article emphases on the modern approaches to the types of sol-gel materials that are beneficial for electrochemistry, monitored by a report of recent advances in the numerous fields of sol-gel electrochemistry. Modified electrodes for sensors and supercapacitors as well as anti-corrosion are described. Sol-gel synthesis expands the capabilities of technologists to obtain highly porous, homogeneous, and hybrid thin-film materials for supercapacitor electrode application. The widespread materials are transition metal oxides, but due to their low conductivity, they greatly impede the rate capability of electrochemical supercapacitors. The way to optimize their properties is the production of complex oxides or different composites. Among the new materials, a special place is occupied by perovskites and materials with an olivine-type structure, which can be easily obtained by the sol-gel method. The sol-gel coating process has demonstrated excellent chemical stability to advance the corrosion resistance of the various metal alloy substrates. Furthermore, the sol-gel process is a user-friendly technique for applying a hybrid sol-gel coating to provide corrosion resistance. The hybrid sol-gel coating technique is the most attractive, easy to prepare at a lower temperature, and has shown the potential to swap Cr-based coatings. The hybrid sol-gel coating has exhibited promising properties of adherent and uses chemically inert to enhance the corrosion resistance of the metal and alloys. Hence, this review article emphases on the recent advances and approaches in the sol-gel coating processes that influence the belongings of its hybrid sol-gel coating for protecting metal substrates and their alloys from corrosion. In addition, the author discusses the current problem and challenges of hybrid anti-corrosion sol-gel coatings. Metal oxides and composites based on them are actively used to create electrochemical sensors. They synthesized, including the anhydrous and citrate sol-gel methods. Such materials are widely used as glucose biosensors and harmful gas sensors.

Fantastic behavior of near zero wear of Cr-based coatings

In this research, the long- and short-term tribological properties of three Cr-based coatings were investigated, and their wear characteristics were analyzed. The Cr-based coatings refer to a Cr coating (CR3) electrodeposited from a Cr(III) electrolyte, a Cr coating (CR3H) from the same Cr(III) electrolyte with subsequent heat-treatment and a Cr coating (CR6) from conventional Cr(VI) electrolyte. The results show that no matter what kind of Cr-based coatings via different processes or preparation steps, and whether in long- or short-term wear, they all revealed the behavior of near zero wear at the later stage of wear or after entering a certain wear phase. The zero-wear behavior is probably one of the intrinsic characteristics of Cr-based coatings, the intricate mechanisms of which need to be explored in detail and further analyzed in the future.

Cr-Based Sputtered Decorative Coatings for Automotive Industry.

The present work aims to study the impact of O and N addition on Cr-sputtered coatings on plastic (polycarbonate, PC) used in automobile parts, as a promisor alternative for auto part metallization, while eliminating the usage of toxic hexavalent chromium. The coatings were deposited using DC magnetron sputtering from a single pure Cr target in a reactive atmosphere (N2 and/or O2). The deposition of the coatings was performed maintaining the total pressure constant and close to 1 Pa by tuning Ar pressure while reactive gases were added. The target current density was kept at JW = 20 mA·cm−2. Structural characterization revealed a mixture of α-Cr, δ-Cr, β-Cr2N, and CrN crystalline structures as well as amorphous oxides. The coating hardness ranged from 9 GPa for the CrON coating to 15 GPa for the CrN coating. All deposited coatings showed a particularly good interface adhesion; adjusting the amount of O and N made it possible to tune the optical properties of the Cr-based coatings as desired. The promising results open future industrialization of sputtered Cr-based coatings for automotive industries.

Enhanced plasticity of the oxide scales by in-situ formed Cr2O3/Cr heterostructures for Cr-based coatings on Zr alloy in 1200 °C steam

The mechanical maintenance of oxides scales was studied for the chromium (Cr) coated zirconium (Zr) alloy coupons that proceeded oxidation in the 1200 °C steam. During the oxidation process, Cr/CrAlSiN coatings evolved into a structure of a thin and high-quality oxides scale growing over a thick Cr-rich layer, which retained a long-term structural integrity and barrier effect. Moreover, multiple toughening mechanisms could operate in the oxidized specimens when subjected to superimposed stresses. The finite element simulation revealed that the thin scale would generate less growth stress and the thick Cr-rich layer would effectively relieve the stress.

Microscratching and fretting of electro-co-deposited Cr-based composite coatings with BN, graphene, and diamond reinforcements

Herein, enhanced damage tolerance of electrochemically processed Cr-based coatings reinforced with BN (Cr–BN), diamond (Cr–D) and graphene (Cr–Gr) is presented. The effect of micro-meter sized BN and diamond particles as well as 2D graphene nano-flakes in Cr matrix is studied due to its excellent mechanical properties and lubricating nature. The highest hardness of Cr–D (~ 23 GPa) is ascribed to the highest dislocation density of ~ 18×1011 cm−2, estimated through Taylor’s model. Scratch test revealed minimum wear volume and coefficient of friction of ~ 27 ± 0.47 × 10–14 m3 and 0.14, respectively, for Cr–D coating. Fretting wear results were concurrent with the scratch test, with Cr–D exhibiting minimum coefficient of friction (COF ~ 0.53) and wear loss (wear volume ~ 0.222 × 10–3 mm3). Diamond particles promote strong bonding due to mechanical interlocking, and efficient load transfer that restricts wear, and passes as a suitable reinforcement of Cr-coating for tribological applications subjected to high load and temperature conditions.

Application and Development Progress of Cr-Based Surface Coatings in Nuclear Fuel Element: I. Selection, Preparation, and Characteristics of Coating Materials

To cope with the shortcomings of nuclear fuel design exposed during the Fukushima Nuclear Accident, researchers around the world have been directing their studies towards accident-tolerant fuel (ATF), which can improve the safety of fuel elements. Among the several ATF cladding concepts, surface coatings comprise the most promising strategy to be specifically applied in engineering applications in a short period. This review presents a comprehensive introduction to the latest progress in the development of Cr-based surface coatings based on zirconium alloys. Part I of the review is a retrospective look at the application status of zirconium alloy cladding, as well as the development of ATF cladding. Following this, the review focuses on the selection process of ATF coating materials, along with the advantages and disadvantages of the current mainstream preparation methods of Cr-based coatings worldwide. Finally, the characteristics of the coatings obtained through each method are summarized according to some conventional performance evaluations or investigations of the claddings. Overall, this review can help assist readers in getting a thorough understanding of the selection principle of ATF coating materials and their preparation processes.

Antifouling potential of multi-walled carbon nanotubes-modified chlorinated rubber-based composites on the colonization dynamics of pioneer biofilm-forming eukaryotic microbes

Chlorinated rubber (CR)-based materials available for artificial surfaces have important commercial and industrial applications. However, CR-based materials are susceptible to diverse biofouling. In this study, different amounts (i.e. 0.2 wt % and 2 wt %) of multi-walled carbon nanotubes (MWCNTs) were incorporated into the CR matrix (CF0), respectively, in order to prepare the MWCNTs-modified CR composites (MCRCs, CF1 and CF2) with improved antifouling (AF) properties. The CR-based coatings were characterized using the SEM technique and then examined in the natural seawater via long-term field exposure assays. The impact of MCRCs on the colonization dynamics of pioneer eukaryotic microbes (PEM) in the natural biofilms with regard to their diversity, abundance, distribution patterns and succession process was investigated using the single-stranded conformation polymorphisms (SSCP) technique. MCRCs demonstrated differential surface structure features and enhanced AF properties compared with the unfilled CR matrix. The diversity and abundance of the PEM on the MCRC surfaces were found to be lower than that of the CR matrix, particularly reduced on the CF2 surfaces (P < 0.05); while the dominant population of PEM on the MCRCs was significantly higher than that of the CR matrix (P < 0.05). This indicated the colonization of the PEM on MCRC surfaces was greatly perturbed by the incorporation of MWCNTs in the CR matrix, as well supported by the reduced Evenness index and the clustering patterns. It seems that MWCNTs incorporated in the CR matrix would result in differential surface structures, enhanced AF properties and reinforced perturbation effect on PEM, which would further impede the development of natural biofilms and conduce to the prevention of macrofouling formation. MCRCs would have potential applications in future coating industries for biofouling mitigation.

Effect of plasma nitriding and modulation structure on the adhesion and corrosion resistance of CrN/Cr2O3 coatings

This paper examines the corrosion resistance of high-speed steel (HSS) substrates subjected to plasma-nitriding (PN) before being coated with CrN (in a single layer) or CrN/Cr2O3 (in multiple layers). Low-temperature (∼300 °C) nitriding treatment was applied for various durations (1 h, 3 h, and 5 h) with the aim of maximizing the adhesion strength and corrosion resistance of the HSS substrate. The multilayer coatings were applied in various thickness ratios of CrN vs Cr2O3 using high-power impulse magnetron sputtering (HiPIMS). The purpose of this study was to investigate the influence of plasma nitriding and Cr-based coating layers on the microstructural characteristics as well as the adhesion and corrosion characteristics. Specimens that underwent PN treatment for 5 h presented Lc1 and Lc2 values of 40.2 N and 78.9 N, respectively. The adhesion strength of the CrN/Cr2O3 multilayer coated PN-HSS substrate (PN-5h) (Lc 1 = 93.8 N) greatly exceeded that of the CrN-coated specimens. These results demonstrate that plasma nitriding and the modulation period strongly affect the adhesion strength between Cr-based coatings and the underlying HSS substrate. Corrosion resistance was characterized in terms of corrosion current, as follows: CrN/Cr2O3 multilayer > CrN layer > base-HSS. These findings also demonstrate that the application of a CrN/Cr2O3 multilayer coating can greatly enhance the corrosion resistance of HSS.

Development and testing of multicomponent fuel cladding with enhanced accidental performance

Abstract Accident Tolerant Fuels have been widely studied since the Fukushima-Daiichi accident in 2011 as one of the options on how to further enhance the safety of nuclear power plants. Deposition of protective coatings on nuclear fuel claddings has been considered as a near-term concept that will reduce the high-temperature oxidation rate and enhance accidental tolerance of the cladding while providing additional benefits during normal operation and transients. This study focuses on experimental testing of Zr-based alloys coated with Cr-based coatings using Physical Vapour Deposition. The results of long-term corrosion tests, as well as tests simulating postulated accidents, are presented. Zr–1%Nb alloy used as nuclear fuel cladding serves as a substrate and Cr, CrN, CrxNy layers are deposited by unbalanced magnetron sputtering and reactive magnetron sputtering. The deposition procedures are optimized in order to improve coating properties. Coated as well as reference uncoated samples were experimentally tested. The presented results include standard long-term corrosion tests at 360∘C in WWER water chemistry, burst (creep) tests and mainly single and double-sided high-temperature steam oxidation tests between 1000 and 1400∘C related to postulated Loss-of-coolant accident and Design extension conditions. Coated and reference samples were characterized pre- and post-testing using mechanical testing (microhardness, ring compression test), Thermal Evolved Gas Analysis analysis (hydrogen, oxygen concentration), optical microscopy, scanning electron microscopy (EDS, WDS, EBSD) and X-ray diffraction.

Understanding the deformation and cracking behavior of Cr-based coatings deposited by hybrid direct current and high power pulse magnetron sputtering: From nitrides to oxynitrides

The increasing demand for corrosion and wear reduction in modern tribological systems places high requirements on hard coatings deposited by physical vapor deposition (PVD) technology in numerous technical applications e.g., plastic processing, die casting or machining. One possible approach to withstanding the excessive wear is coating of tools with PVD hard coatings of the system Cr-Al-O-N. Each of the constituent elements can provide the coating with particular characteristics in point of its performance under different loading conditions. Therefore, the ongoing improvement in modern surface engineering requires comprehensive insights into the elastic-plastic deformation and cracking behavior of such coatings. For this purpose, several modifications and extensions of established experiments are required to overcome the limitations involved in characterization of thin hard coatings e.g., as a result of the μm ranges. In the presented work, three coatings CrN, (Cr,Al)N and (Cr,Al)ON deposited on quenched and tempered AISI 420 steel substrate were investigated. All coating systems were deposited through a hybrid technology, consisting of direct current and high power pulse magnetron sputtering (dcMS/HPPMS). The deformation and cracking behavior of the coatings and coating/substrate compounds were studied through application of static loadings by nanoindentation and Rockwell tests as well as dynamic loading conditions using nanoscratch tests. Complementary quantitative investigations were performed by means of depth profiling using confocal laser scanning microscopy (CLSM). Qualitative analyses of Rockwell imprints and nanoscratch tracks were conducted through scanning electron microscopy (SEM). Based on the results, precise analyses of nanoindentation force-displacement curves can improve the understanding about the possible crack formation in the coatings. Compared to (the binary) CrN, the ternary coating system (Cr,Al)N exhibits more promising characteristics in point of elastic-plastic deformation behavior and crack resistance. The crack resistance decreases through incorporation of oxygen into the ternary nitride and deposition of quaternary oxynitrides (Cr,Al)ON. Such character of oxynitrides can be overseen taking into consideration only the static loadings and therefore, applying of the both static and dynamic loading conditions are required to gain precise knowledge on the deformation and cracking behavior of thin hard coatings.

High-temperature oxidation resistance of chromium-based coatings deposited by DLI-MOCVD for enhanced protection of the inner surface of long tubes

For nuclear safety issues, there is an international effort to develop innovative “Enhanced Accident Tolerant Fuels” (EATF) materials. EATF cladding tubes are of particular interest because they constitute the first barrier against radioactive fission species dispersal in case of accidental scenario such as LOCA (LOss of Coolant Accident). Actual nuclear fuel claddings are made from Zr-based alloys and to increase safety margins, both mechanical strength and resistance to high-temperature oxidation have to be improved. Several alternatives using high-temperature oxidation resistant coatings for outer-wall protection have been proposed worldwide but there is currently no solution for the inner-wall protection. In order to resist to high temperature steam environment upon LOCA transients, internal Cr-based coatings deposited by DLI-MOCVD (Direct Liquid Injection of MetalOrganic precursors) were investigated. These hard metallurgical coatings could also be used in high-temperature corrosive environments as those encountered in aeronautics and other industries to protect 3D complex components. Thanks to a suitable chemistry of the liquid Cr precursor, bis(ethylbenzene)chromium, different coatings were deposited including: metal Cr, chromium carbides CrxCy and mixed carbides CrxSizCy. The high-temperature behavior of these Cr-based coatings under oxidizing atmospheres has been studied using several techniques and various oxidation tests including pure steam environment followed by water quenching down to room temperature to be representative of LOCA situations. Amorphous CrxCy coatings showed the most promising properties. For instance compared to uncoated substrate, they shift the catastrophic oxidation towards higher temperatures and delay the complete oxidation of the substrate at 1473K of >2h. The results are discussed in terms of oxidation mechanisms and protection of the fuel claddings inner surface deduced from fine characterizations of the samples before and after oxidation tests.

Ti- and Cr-based hard coatings obtained at low temperatures by unbalanced magnetron sputtering

We prepared a set of Ti- and Cr-based ternary and quaternary hard coatings at low deposition temperatures (< 200 °C) by closed-field unbalanced magnetron sputtering in a gas mixture of Ar + N2. The morphological, structural and mechanical properties of the coatings were characterized by means of atomic force microscopy, scanning electron microscopy, X-ray photoelectron spectrometry, a micro-scratch tester and a nanoindentation tester. The results of the analyses performed revealed that the surface of the deposited ternary Ti-Al-N coatings was rougher than that of the CrTiAlN coatings. The metallic elements in the coatings react with nitrogen to form CrN, TiN and AlN nitride phases, as no Cr2N phase was detected in the Cr-based coatings. The optimized quaternary coating demonstrated mechanical properties superior to those of the ternary ones in terms of hardness; adherence strength of 32 GPa and elastic modulus of 418 GPa were measured for the multicomponent coating of composition Cr0.68Ti0.19Al0.13N.

Influence of Ag additions on the structure, mechanical properties and oxidation behaviour of Cr-O coatings deposited by HiPIMS

Here, we investigate the influence of Ag additions on the properties of Cr-O coatings deposited by magnetron sputtering. The deposition rate of Cr-O coating is 1.6 times higher than a single Cr coating, taken as reference, deposited with identical conditions. Incorporation of Ag in the Cr-O coating increases the deposition rate. Transition from a dominant columnar to a very compact morphology, as well as from crystalline to amorphous structure, was observed with the O addition. However, no morphological changes were observed with Ag additions. Annealing at temperature ≥ 500 °C promotes the crystallization of Cr-O and Cr-O-Ag coatings, i.e. the formation of a crystalline composite structure, formed by: Cr, Cr2O3 and Ag phases, the later in the case of Cr-O-Ag coatings. The increase of coatings crystallinity improved the mechanical properties. Thermogravimetric analysis showed that O and Ag additions to Cr have no influence on the onset point of oxidation. The oxidation kinetics of the Cr-based coatings is controlled by the outward diffusion of Cr ions through a top Cr2O3 layer. A dual oxide layer was formed for Cr-O and Cr-O-Ag coatings, when exposed to a continuous increase of the temperature up to 1200 °C. A compact layer of Cr2O3 grows on top of the coating, due to the Cr ions outwards diffusion, leaving behind a porous layer formed by Cr2O3 grains resulting from the recrystallization process, together with Ag agglomerates inside the empty porous zones.