Chromium Coatings Research Articles

Enhanced Mechanical and Tribological Capabilities of a Silicon Aluminum Alloy with an Electroplated Ni–Co–P/Si3N4 Composite Coating

Ni–Co–P/Si3N4 composite coatings were fabricated over an aluminum–silicon (Al–Si) substrate using a pulse-current electroplating process, in which the rapid deposition of an intermediate nickel–cobalt layer was used to improve coating adhesion. The microstructure, mechanical, and tribological behaviors of the electroplated Ni–Co–P/Si3N4 composite coating were characterized and evaluated. The results revealed that the electroplated Ni–Co–P/Si3N4 composite coating primarily consisted of highly crystalline Ni–Co sosoloid and P, and a volumetric concentration of 7.65% Si3N4. The electroplated Ni–Co–P/Si3N4 composite coating exhibited hardness values almost two times higher than the uncoated Al–Si substrate, which was comparable to hard chrome coatings. Under lubricated and dry sliding conditions, the electroplated Ni–Co–P/Si3N4 composite coating showed excellent anti-wear performance. Whether dry or lubricated with PAO and engine oil, the composite coating showed minimum abrasive wear compared to the severe adhesive wear and abrasive wear observed in the Al–Si substrate.

Зносостійкі покриття на алюмінієвих сплавах

Aluminum alloys are widely used in industry. Recently, they are used as a replacement for steel for the manufacture of pulleys, gears, columns of hydraulic rotary transmissions of grabs, etc. This reduces the weight of the parts and reduces carbon emissions into the atmosphere. However, aluminum alloys have low abrasive wear resistance, so their use is possible with coatings on the surface. High-speed gas flame spraying (HVOF) was carried out on the equipment (Diamond Jet Hybrid gun). Arc coatings with a thickness of 500 µm were applied using FMI metallizer and FMI-2 powder wire (Cr6Al6B3Fe-base). PEO (oxido ceramic) coatings were synthesized on D16T aluminum alloy in an electrolyte of 3 g/l KOH + 2 g/l liquid glass (sodium silicate) with a pulsed current at a frequency of 50 Hz, with the ratio of the densities of the cathode and anode currents Jc/Ja+15/15 A /dm2. The duration of the PEO process is 60 min. The open porosity of the coatings was determined by the method of hydrostatic weighing. The phase composition of the surface layers was studied using a DRON-3.0 diffractometer. HVOF (VC) and PEO coatings have been found to have 3 to 4 times higher wear resistance than high carbon hardened steel 100Cr6 and 2 times higher than galvanic chromium coatings, but have high energy consumption. Two methods have the lowest energy consumption: electric arc spraying of coatings and hard anodizing. At the same time, the wear resistance of such coatings is significantly lower than that of coatings obtained by the PEO and HVOF (VC) methods. For the restoration of worn parts, the most appropriate methods are HVOF (VC) and electric arc spraying, as they allow applying thick coatings up to 5 mm. Conclusions: It was established that the studied HVOF (VC), PEO, EDP and hard anodized coatings significantly increase the abrasive wear resistance of aluminum alloy parts. HVOF (VC) and electric arc spraying methods can be used to protect new and worn parts from wear. PEO and hard anodizing methods can be used to protect against wear on new parts only. The use of these methods increases the wear resistance of parts made of aluminum alloys to the level of steel and allows replacing steel and cast iron with aluminum alloys with coatings, reducing the weight of parts and carbon emissions into the atmosphere.

ПОВЫШЕНИЕ ИЗНОСОСТОЙКОСТИ И КОРРОЗИОННОЙ СТОЙКОСТИ ГАЛЬВАНИЧЕСКОГО ПОКРЫТИЯ ХРОМА ПРИМЕНЕНИЕМ НАНОРАЗМЕРНЫХ МАТЕРИАЛОВ

The article presents the results of testing nanocomposition electroplating coatings based on chromium for wear resistance and corrosion resistance. The studied nanocomposite coatings were obtained by co-deposition with nanoscale aluminum oxide particles. Nanoscale particles had a spherical shape and dimensions of 10... 100 nm. The production of nanoscale particles with such parameters was achieved by plasma recondensation of the feedstock. Electroplating chromium coatings were applied from a self-regulating chromium plating electrolyte in solid chrome plating modes. In order to ensure high sedimentation resistance of the electrolyte-suspension, nanocomposition coatings were applied under constant exposure to ultrasound at a frequency of 18 kHz. Tribological tests of the samples were carried out on a friction machine according to the "roller – pad" scheme according to GOST 23.224–86. Corrosion tests of the studied coatings were carried out on a laboratory installation by immersion of the studied samples in a saline solution according to GOST 9.308–85. As a result of laboratory tests for wear resistance, it was found that nanocomposite electrolytic coatings based on chromium have a wear resistance 1.5–1.8 times higher than the wear resistance of chromium base coatings and 2.1–2.2 times higher than the wear resistance of steel SHX15. The corrosion resistance of the chromium-based nanocomposition coating is 1.86 times higher than the corrosion resistance of the chromium base coating and 6.86 times higher than the corrosion resistance of 15X steel. Given the high wear resistance and corrosion resistance of chromium-based nanocomposite electroplating coatings, they should be recommended for use in machine-building and repair production as a way to restore worn parts and strengthen new ones.

ВЛИЯНИЕ НАНОРАЗМЕРНЫХ ЧАСТИЦ ОКСИДА АЛЮМИНИЯ НА МОРФОЛОГИЮ, СТРУКТУРУ И ХИМИЧЕСКИЙ СОСТАВ ГАЛЬВАНИЧЕСКОГО ХРОМА

The article presents the results of studies on the morphology, structure and chemical composition of chromium electroplating obtained by co-deposition with nanoscale aluminum oxide particles. Nanoscale particles were obtained by plasma recondensation of the feedstock. Electroplated chromium coatings were applied from a self-regulating chromium plating electrolyte in solid chrome plating modes with constant exposure to ultrasound on the electrolyte. The morphology and structure of the surfaces of the obtained coatings were studied using a Mira II Tescan scanning electron microscope. As a result of the conducted research, it was found that the introduction of nanoscale aluminum oxide particles into the chromium plating electrolyte leads to a decrease in the grain size of the coating, which entails an increase in the dislocation density and, as a consequence, the microhardness of the resulting coating. The average grain size of the chromium base coating was 1.1 × 10-4 cm, which corresponded to a dislocation density of 2.5 × 108 cm-2 and a microhardness of 9.6 GPa, while the nanocomposite electrolytic coating based on chromium had an average grain size of 7.2 × 10-5 cm, which corresponded to a dislocation density of 5.7 × 108 and a microhardness of 14.32 GPa. The analysis of spectrograms obtained as a result of emission analysis of samples with coatings showed that the nanocomposition electroplating of chromium contains elements of nanoscale particles used to produce it, which are absent in the base coating. This indicates the introduction of nanoscale particles into the coating during its application. The obtained results create prerequisites for further research and development of an algorithm and methodology for controlling the physico-mechanical properties of the coatings obtained by introducing a given number of certain nanoscale particles into them.

Evaluation on the Microstructure and Thermal Infrared Emissivity of Calcium-Doped Ytterbium Chromate Coating Deposited by Saucer-Shaped Porous Feedstocks for High-Temperature Thermal Protection

Evaluation on the Microstructure and Thermal Infrared Emissivity of Calcium-Doped Ytterbium Chromate Coating Deposited by Saucer-Shaped Porous Feedstocks for High-Temperature Thermal Protection

Oxidation properties and microstructure of a chromium coating on zircaloy-4 fuel cladding material applied by atmospheric plasma spraying

To achieve a full protective coat on welding joints at both ends of zircaloy cladding tubes, we studied the rapid deposition of a thick chromium coating on zircaloy-4 cladding tubes by atmospheric plasma spraying. Microstructure, mechanical properties and high-temperature oxidation tests of chromium coatings were also studied. The initial screening oxidation test of the specimen was carried out at 1100 °C, 1200 °C and 1300 °C in dry air, and the oxidation properties of the specimen were explained. The nanohardness and modulus of the sprayed chromium coating were higher than those of the zircaloy-4 substrate. The nitride phase of Cr2N was formed after the high-temperature oxidation test. After oxidation at 1300 °C, there were multilayer structures: surface Cr2O3, residual Cr coating, thin (Zr,Cr)O2 layer, Cr-Zr layer and zircaloy substrate. There were bubble folds on the surface of the chromium coating and voids in the Cr2O3 layer. The oxidation test showed that the chromium coating can effectively slow the severe oxidation of zircaloy-4 cladding tubes. In summary, an APS-thick chromium coating has the potential to improve the oxidation resistance of zircaloy-4 cladding tubes in light water reactors.

Tribological Characterization of Electrolytic Hard Chrome Coatings

Electrochemical chromium plating is one of the most widely used surface treatments for metallic materials. This treatment results in a surface layer highly resistant to mechanical wear, temperature and corrosion. In this paper, the experimental material is in the form of a tube made of AISI 304 stainless steel, on the surface of which chromium layers have been deposited by three different companies dealing with electrochemical plating. Tribological tests were performed on a UMT Tribolab device with dry friction of balls made of AISI 52100 steel and ZrO2 ceramics, both with a diameter of 4.762 mm. Measurements were performed at different loads. In the experiments, the courses of friction coefficients were evaluated, and after the measurement, the amount of material removed during friction was determined from the cross-sections of the formed grooves. Furthermore, the influence of the hardness of individual chromium layers and counterparts on tribological behaviour was investigated. The hardness of the counterpart proved to be important, since in the case where the counterpart is softer than the surface layer, the wear was greater than in the opposite situation.

Effects of sample bias on adhesion of magnetron sputtered Cr coatings on SiC

Swelling of SiC at 300 ∘C due to in-service neutron irradiation causes tensile residual stresses in coatings which are expected to adversely affect the performance of coated SiC composite fuel cladding for light water reactors. Matching the coating swelling with the substrate, a solution common for thermal expansion, is not practical in the case of neutron irradiation. Biasing samples during magnetron sputtering deposition induces compressive residual stress which may counteract this. In this study, chromium coatings were deposited on SiC by DC magnetron sputtering with no external heating at bias voltages of –50V, –75V, and –100V. The effects of the bias voltage on morphology, residual stress, microstrain, texture, and adhesion are shown. The low deposition temperature resulted in the coating microstructure evolution following an energetic particle bombardment dominated trend. At the two lower bias voltages knock-on implantation dominated increasing the residual stress and microstrain while at the highest bias voltage, thermal spike migration allowed for defect relaxation. When the knock-on induced compressive residual stress exceeded 0.8 GPa microcrack formation in the SiC substrate decreased coating adhesion. While no microcracks formed at the lowest bias voltage, insufficient atomic mobility during coating growth lead to voids forming in the coating. A balance is needed to form void-free coatings that have high compressive residual stress.

Resource-saving processing of technological solutions and wastewater by electrochemical method (methods overview)

The paper summarizes the data on the wastewater formation from a number of plants of chemical, engineering, energy and leather industries and its processing in electrolyzers of various designs to obtain the substances that can be used in various production processes. In the ethylenediamine production, wastewater occurs at the stages of amines rectification and of ethylenediamine dihydrochloride neutralization. Recycling of effluents generated during the amine rectification in a two-chamber electrolysis cell allows the ethylenediamine and ammonia to be returned to the production process. The processing of ethylenediamine dihydrochloride solution in the membrane electrolyzers saves alkali for the neutralization process. Electrochemical processing of distilled liquid in soda production makes possible to obtain calcium hydroxide, sodium hydroxide and hydrochloric acid or chlorine, which can be used in various industrial processes. Wastewater generated during the demineralized water production contains a significant amounts of sodium sulfates and chlorides. The processing of these effluents in a membrane electrolyzer yields sulfuric acid and alkali, which are suitable for the regeneration of ion-exchange filters. After processing in an electrodialysis apparatus the wastewater generated during the chromium and nickel coatings, is concentrated to values that allow them to be used to prepare an electrolyte adjustment. The leather industry effluents contains an amount of trivalent chromium and sulphides. Electrochemical processing of these effluents leads to almost complete extraction of chromium and sulfides.

Functional composite electrochemical coating Ni-Co-Al2O3 -an alternative to chromium plating

The physico-mechanical properties of composite electrochemical coatings (CEC) nickel-cobalt-aluminum oxide were investigated depending on the electrolyte parameters and electrolysis conditions. The previously developed low-concentration chloride electrolyte for nickel plating was used as an electrolyte to replace environmentally hazardous chromium plating electrolytes containing hexavalent chromium, which is prohibited by the laws of many countries. The wear resistance of the obtained CEC was determined on a three-ball friction machine. This made it possible to establish that the wear resistance of the CEC exceeds the wear of chrome coatings in dry friction mode by 2-2,5 times and is comparable to chromium in the friction mode with lubrication. In the dry friction mode, the higher wear resistance of the nickel-cobalt-alumina coating is explained only by the higher hardness of the latter. Probably, upon destruction of the coating, the particles of the alloying addition act as a solid lubricant, which causes an increase in the resistance of the CEC during wear. The microhardness of the CEC was determined using a PMT-3 microhardness tester and amounted to 6-25 GPa. The microhardness value was influenced by the concentration and properties of the dispersed phase, as well as the electrolysis conditions - the temperature and pH of the electrolyte, and the cathode current density. “Corrodcote” test was used in the study of corrosion. According to its data, the corrosion resistance of CEC is 2-3 times higher than the corrosion resistance of chromium deposits. The results obtained make it possible to recommend the developed functional CEC of nickel-cobalt-alumina instead of chromium coatings as corrosion- and wear-resistant.

Unravelling the Fe Effect on the Corrosion of Chromium Coatings: Chemical Composition and Semiconducting Properties

A model trivalent chromium-based electroplating bath doped with different concentrations of Fe was used to obtain different metallic coatings. The impact of the Fe was investigated on both the Cr layer and its native passive film by a detailed characterisation using X-ray Photoelectron Spectroscopy (XPS), Angle Resolved XPS and Auger Electron Spectroscopy. Moreover, the semiconducting properties of their oxide layers were explored by Mott-Schottky and the corrosion performance of the coating by linear polarisation resistance and kinetics of the oxide formation. Results revealed not only a homogeneous Fe distribution in the Cr layer but also the presence of an iron-chromium duplex oxide layer for concentrations ≥ 100 mg l−1 Fe in the bath. The Mott-Schottky analysis showed a p-n junction for such coatings due to the presence of an iron oxide layer on top of a chromium oxide one, which increased the total amount of point defects (charge carrier density) and drastically affected to their corrosion resistance (the polarisation resistance decreased by one order of magnitude and their oxide layer showed slower kinetics and a higher passivation current). In contrast, coatings with a single chromium oxide layer showed a p-type semiconducting behaviour as well as the best corrosion performance.

Corrosion and mechanical properties of nanostructure electrolytic Co-W and Fe-Co-W alloys

It has been shown that the corrosion-electrochemical behavior of coatings with iron (cobalt) tungsten alloys depends on the content of the refractory component, and an increase in chemical resistance in the acidic medium is due to the formation of acidic tungsten oxides on the surface. The use of artificial neural networks allowed us to identify the most significant parameters that determine the corrosion resistance of alloys. The microhardness of the coatings under study depended on the content of tungsten and exceeds the characteristics of individual metals, which makes it possible to recommend such materials as an alternative to hard chromium coatings. An increase in corrosion resistance and microhardness of coatings by alloys is associated with the formation of a fine-crystalline and X-ray amorphous structure of the surface layers when using non-stationary modes of electrodeposition.

Evolution of Texture, Strain, and Grain Boundary Constitution in Copper–Chromium Coatings and its Effect on Coating Corrosion Behavior

Evolution of Texture, Strain, and Grain Boundary Constitution in Copper–Chromium Coatings and its Effect on Coating Corrosion Behavior

Al-Cr Diffusion Coatings Processed on a Cast and a Heat-Treated Ni-Base Alloy

Abstract Diffusion coatings are used to protect components against oxidation and corrosion in high-temperature applications. The processing route and parameters determine the coating characteristics for each substrate. When more than one element is deposited, the number of processing variables increases, as single-step or two-step deposition can be used. With a two-step procedure, the element deposited first affects subsequent diffusion. Under these conditions, a better understanding of the phenomena involved is required, as the substrate plays an important role. To this end, a systematic assessment of chromium and aluminum diffusion coatings processed by out-of-pack cementation on a nickel alloy was carried out. The precipitation-hardened alloy was supplied as-cast and in a heat-treated condition. Three processing routes were used: a single-step procedure and two two-step procedures (AC, aluminizing+chromizing, and CA, chromizing+aluminizing). Characterization of the as-processed coatings showed that the nonuniform structure of the as-cast nickel alloy allowed preferential diffusion paths to form, reducing coating thickness and minimizing differences caused by the processing route. Synthesis of nickel aluminide (NiAl) occurs on coatings processed with the CA route, as aluminum diffuses in a nickel matrix that is richer in chromium, favoring the synthesis of the aluminide. Regardless of the processing route used, exposure to 1,100°C results in parabolic growth of the coating. At 1,100°C, partitioning of aluminum occurs between the upper surface of the coating and the nickel alloy, forming a diffusion barrier.

Potential of zinc based-graphene oxide composite coatings on mild steel in acidic solution

This study introduces varying concentrations of graphene oxide (GO) as a filler into zinc chromate in forming composite coatings to improve the corrosion protection of mild steel. The purity of synthesized GO was inferred through the application of complementary characterization techniques, including FT-IR, XRD, Raman, SEM-EDX, and TEM analyses. GO doped zinc chromate coatings were deposited on the surface of mild steel through the brushing method. Electrochemical studies, i.e., electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PD) were conducted to elucidate the anticorrosion properties of the coated mild steel exposed to 0.5 ​M HCl solution. It was revealed that the highest anticorrosion protection was attained at low doping amount of 0.5% of GO with a corrosion rate of 0.036 mpy. Surface analyses revealed that incorporating GO into zinc chromate coating can effectively improve the anticorrosion properties and adhesion strength.

The Mechanism for the Effect of Nanocarbons Added to Electroplating Electrolytes on the Tribotechnical Properties of Chromium Coatings

The Mechanism for the Effect of Nanocarbons Added to Electroplating Electrolytes on the Tribotechnical Properties of Chromium Coatings

Trivalent Chromium Electrolytes with Organic and Inorganic Ligands: Use of Pulsed Currents

Hexavalent chromium salts are targeted by health authorities and in particular by the European regulation REACH because of their high toxicity for humans and the environment. Trivalent chromium salts are a promising alternative to hexavalent ones in the perspective of hard chromium coatings and have been extensively studied [1]. However, trivalent chromium ions bind with water molecules to form complexes resistant to electrochemical reduction, requiring ligands addition to evolve toward more favorable chemical equilibrium. This addition leads to a decrease of the coating properties, due to the incorporation of impurities such as carbon and oxygen. If carbon can be removed by the use of inorganic ligand, the presence of oxygen is more difficult to avoid [2], mostly due to hydroxide species incorporation induced by the pH increase at the interface. This phenomenon is the direct consequence of the important current part used to reduce protons during the plating process. It exists different ways to limit the pH elevation [3] such as the temperature, the electrolytes pH modification or the addition of specific chemical compounds, but an attractive way consists in current modulation i.e. using pulsed currents. The relaxation of the diffusion layer is known to reduce the competition between the diffusion of electrochemical species from the bulk and the proton discharge, which will limit the pH increase and restore the chromium concentration near the surface close to the initial one.Parameters of the pulse sequences tested in the present work were chosen from pH measurements [4] undertaken at the surface vicinity (less than 100 µm), and from the behavior of copper solutions working at high potentials where the competition between copper reduction and hydrogen takes place. Potentials were monitored at two-time scales: ms for the responses to current modulation (transient curves) and min for the average potential evolution all along the process. Finally, a comparison was made between inorganic and organic based trivalent chromium electrolytes, with a special care to cations presence, because of their influence on hydrogen discharge.[1] P. Benaben, Plating and Surface Finishing, p. 8, 2011.[2] A. Baral, R. Engelken, J. Electrochem. Soc., vol. 152, no 7, p. C504, 2005[3] J. Ji, W. C. Cooper, D. B. Dreisinger, E. Peters, J Appl Electrochem, vol. 25, no 7, p. 642-650, juill. 1995[4] M. Marcelet, Y-S. Chao, B. Vuillemin, M-P. Gigandet, C. Gleyzes, R. Oltra, J. Tardelli, J-Y. Hihn, ECS Meet. Abstr., vol. MA2019-02, no 21, p. 1046, sept. 2019

In Situ Investigation of Migration of Cations during Electrolyte Spreading on Chromium Coated Steels Electrodeposited from Trivalent-Chromium Electrolyte

Resistance to corrosion and inhibition of cathodic delamination of polymer topcoat makes chromium coated steels suitable for application in the packaging industry. These coatings were traditionally produced from hexavalent chromium electrolytes. These coatings are now being phased out due to the carcinogenic nature of hexavalent chromium used in the production process (by the REACH regulation in the European Union). Electrodeposition from trivalent chromium containing electrolyte is being considered as an alternative to replace these traditional hexavalent-chromium coated steels. The cathodic delamination behavior of polymer topcoat on these trivalent-chromium coated steels were studied in detail previously [1-3]. However, the factors that result in inhibition of cathodic delamination are still not well understood.The two important factors influencing the rate of cathodic delamination are the electron transport and cation migration on the surface. The aim of this study is to understand the influence of complex composition of these trivalent-chromium coatings on the electrical conductivity and ion migration on the surface of the coating at the microscopic scale. The electronic conductivity of the coatings was studied using current-sensing atomic force microscopy to understand electron transfer occurring at the cathodic delamination front. Cation migration during oxygen-reduction induced spreading of electrolyte on these trivalent-chromium coated steel was studied using in situ scanning Kelvin probe force microscopy. The coatings are found to highly resist electron transport. Heterogeneities observed in the microscopic scale using the current-sensing atomic force microscope are correlated to the behavior observed during electrolyte spreading. The kinetics of spreading observed at microscopic scales are similar to that observed using a scanning Kelvin probe at larger scales. The results demonstrate in situ scanning Kelvin probe force microscopy as an effective tool to study the migration of cations during electrolyte spreading and the local heterogeneities at the microscopic scale. Warren, David & N. McMurray, H & de Vooys, Arnoud. (2013). Localised SKP Studies of Cathodic Disbondment on Chromium/Chromium Oxide Coated Steel. ECS Transactions. 50. 67-78.Wint, N & de Vooys, Arnoud & Mcmurray, Hamilton. (2016). The corrosion of chromium based coatings for packaging steel. Electrochimica Acta. 203. 326-336.James E. Edy, Hamilton N. McMurray, Koen R. Lammers, Arnoud C.A. deVooys (2019). Kinetics of corrosion-driven cathodic disbondment on organic coated trivalent chromium metal-oxide-carbide coatings on steel. Corrosion Science. 157. 51-61.

Excellent Properties of Ni-15 wt.% W Alloy Electrodeposited from a Low-Temperature Pyrophosphate System

Electrodeposited Ni-W alloy coatings are considered to be one of the most suitable candidate coatings to replace carcinogenic hexavalent chromium coatings. In this work, Ni-W alloys are electrodeposited from pyrophosphate baths containing different concentrations of Na2WO4 2H2O (CW) at 40 °C. Both CW and the applied current density can affect the W content in the coatings. The effect of CW becomes weaker with the increased current density. The Ni-W alloys with 15 ± 5 wt.% W (Ni-15 wt.% W) are obtained from the bath containing 40 g L−1 CW at a high current of 8 A dm−2. The microhardness, corrosion resistance and hydrogen evolution reaction (HER) are measured with a microhardness tester and an electrochemical workstation. The modified properties are studied by heat treatment from 200 to 700 °C. The highest microhardness of 895.62 HV and the better HER property is presented after heat treatment at 400 °C, while the best corrosion resistance in 3.5 wt.% NaCl solution appears at 600 °C.

Effect of high-temperature oxidation on the surface properties as applied to quenching of high-temperature bodies

This paper contains the results of studying the surface properties before and after high-temperature oxidation. For this, the plate zirconium samples with chromium, gold and silver coatings were prepared. Cut profiles of the samples were obtained to study the structure of coatings and the thickness of the oxide layer. The measurements of contact angles were carried out. The results showed that a porous heterogeneous oxide layer was formed on the samples after high-temperature oxidation. At the same time, the wettability of the samples was improved. The thickness of the oxide layer on the chrome-coated zirconium sample was the smallest. Using of electroplated silver coating for experiments involving heating to high temperatures seems inappropriate because it was damaged after the oxidation tests. It is assumed that the main factor which influence on the rise of the transition temperature to the intensive cooling regime during quenching is the appearance of the oxide layer, rather than the improved wettability and wickability. High-temperature oxidation leads to the simultaneous formation of an oxide layer with a low thermal effusivity and to an improvement in wettability, therefore the contribution of each of these two effects on quenching can be confused or overestimated.