The effect of interlayer on corrosion resistance of ceramic coating/Mg alloy substrate in simulated physiological environment

Ceramic coatings are deposited on biodegradable magnesium alloys by physical vapor deposition to reduce the electrochemical activity in the simulated physiological environment. Although an interlayer is generally used to reduce the mismatch between the hard coating and soft substrate, the effects of the interlayer on the electrochemical corrosion behavior have seldom been explored. In this work, AlO_xN_y ceramic coatings were deposited on AZ31 magnesium alloys with Al or Ti interlayers. Polarization tests and electrochemical impedance spectroscopy (EIS) were conducted to evaluate the corrosion resistance in the cell culture medium. The AlO_xN_y ceramic coating significantly improved the bio-corrosion resistance of the magnesium alloy, but the Ti interlayer accelerated the corrosion rate. In comparison, although the addition of an Al interlayer led to smaller enhancement in the surface mechanical properties of the AlO_xN_y coating, corrosion could be impeded effectively. Our results indicate that an Al interlayer is preferred over Ti and the corrosion failure mechanism is discussed from the perspective of defects.     

Influence of substrate pre-treatments on the growth of SixNyHz thin films by plasma enhanced chemical vapor deposition

A two-step plasma enhanced chemical vapor deposition procedure has been developed to produce high quality Si_xN_yH_z films for quantum cascade laser applications. The procedure consists in exposing the GaAs substrate to a controlled N₂ plasma previous to the silicon nitride film deposition. The pre-treatment causes the formation of a thin GaN film that passivates the GaAs wafer. The method has been optimized varying RF power, N₂ flow rate and process time of the pre-treatments and monitoring their effects on the resulting chemical composition and dielectric properties of the nitride overlayers, by means of infrared spectroscopy, X-ray photoelectron spectroscopy and electric characterizations. A narrow window in the pre-treatment RF power, N₂ flux and time values, improves the composition, structural and dielectric properties of the silicon nitride overlayers. The best result has been found depositing the silicon nitride films on GaAs wafer after 2 min of N₂ plasma treatment with a power of 20 W and a 50 cm³/min flow rate.     

Corrosion performance and mechanical stability of 316L/DLC coating system: Role of interlayers

In the present work, the corrosion performance and mechanical stability of diamond-like carbon (DLC) coatings were investigated in the context of their biomedical applications. DLC was prepared by radio-frequency (RF) plasma-enhanced chemical vapor deposition (PECVD) onto medical grade 316L stainless steel. Interlayers of amorphous hydrogenated silicon-based materials such as a-Si, a-SiN_x, a-SiC_x, and a-SiC_xN_y, and a nitrided interlayer, were studied in order to optimize its adhesion strength. Potentiodynamic polarization tests were performed to evaluate the corrosion performance of the 316L/DLC coating system. Electrochemical impedance spectroscopy (EIS) was used to determine the stability of the coating system during long-term tests of exposure to a simulated body fluid solution. The evolution of EIS spectra was monitored during two years of immersion in Ringer's solution. In addition to providing the best adhesion, the a-SiN_x interlayer was found to significantly improve the corrosion resistance of the DLC system since it is highly impervious to the liquid. This is demonstrated by a two-order of magnitude improvement in the corrosion current density compared to the DLC with the nitrided interlayer. The a-SiN_x interlayer substantially enhances the mechanical stability of the DLC coating system in the simulated body fluid environment, indicated by a slight reduction (less than 20%) in the adhesion strength and fivefold increase in the charge transfer resistance after two years of immersion. Moreover, Si-doped DLC coatings show improved corrosion barrier properties, due to the formation of a passive silicon oxide film at the electrode/electrolyte interface.     

Reactive sputtering of TiOxNy coatings by the reactive gas pulsing process: Part II: The role of the duty cycle

The reactive gas pulsing process (RGPP) was used to deposit titanium oxynitride thin films by dc reactive magnetron sputtering. A titanium target was sputtered in a reactive atmosphere composed of Ar + O₂ + N₂. Argon and nitrogen gases were continuously introduced into the sputtering chamber whereas oxygen was injected with a well-controlled pulsing flow rate following a rectangular and periodic signal. A constant pulsing period T = 45 s was used for every deposition and the duty cycle α = t_ON/T was systematically changed from 0 to 100%. The operating conditions were investigated taking into account the poisoning phenomena of the target surface by oxygen and nitrogen. Kinetics of poisoning were followed from measurements of the total sputtering pressure and titanium target potential during the depositions. Deposition rate and optical transmittance of titanium oxynitride coatings were also analysed and correlated with the process parameters. Pulsing the oxygen flow rate with rectangular patterns and using suitable duty cycles, RGPP method allows working according to reversible nitrided-oxidised target conditions and leads to the deposition of a wide range of TiO_xN_y thin films, from metallic TiN to insulating TiO₂ compounds.     

Influence of air oxidation on the properties of decorative NbOxNy coatings prepared by reactive gas pulsing

In this work the oxidation resistance of DC reactive sputtered niobium oxynitrides and its influence on the properties of the films are studied. The depositions have been carried out by DC magnetron sputtering with a reactive gas pulsing process. The nitrogen flow was kept constant and the oxygen flow was pulsed. Pulse durations of 10 s produced multilayer coatings with a period of λ = 10 nm. Three sets of films with increasing duty cycle (= on-time of high oxygen flow / pulse duration) have been deposited. The films were subsequently annealed in air at 400, 500 and 600 °C, respectively.X-ray diffraction measurements showed a clear and progressive change from a roughly amorphous nature of the films to a crystalline oxide-type compound for those annealed at 600 °C, which was consistent with the composition analysis. For annealing temperatures of 500 and 600 °C, the coatings presented a significant reduction in hardness, approaching the values characteristic of Nb₂O₅-type films. Moreover, the residual stress measurements performed by using the deflection method revealed low values in all the coatings nearly independent on the annealing temperature.Color variation in the CIE − L^?a^?b^? color space and the reflectance in the UV-visible spectrum range of these niobium oxynitrides were investigated and correlated to their chemical composition and structural features. For both properties, the variation tendencies are quite similar, showing the transition from a nitride-type alloy to an oxide-type one with increasing annealing temperature.     

In situ IR pyrometric analysis during thermal treatment in air of TiOxNy coatings

IR pyrometry is an original diagnostic tool for in situ analysis of surface transformations of coatings or bulk materials during the thermal treatment under reactive atmosphere such as high temperature oxidation. Significant oscillations of the pyrometric signal were observed during annealing in air of TiO_1.5N_0.5 coatings in the temperature range 673-823 K. This is due to interferences resulting from multi-reflections at the interfaces of a transparent growing film. This reveals the formation of a TiO₂ thin film on the top of the TiO_1.5N_0.5 coating. Modeling of the time dependence of the IR pyrometric signal allows the determination of the oxide layer thickness, transformation rate and optical properties of the films under the growth conditions. The progressive oxidation of a compact amorphous TiO_1.5N_0.5 coating from the external surface to the substrate interface was supported by SIMS, XRD and reflectance analyses.     

The nanostructure, wear and corrosion performance of arc-evaporated CrBxNy nanocomposite coatings

The composition, nanostructure, tribological and corrosion behaviour of reactive arc evaporated CrB_xN_y coatings have been studied and compared to CrN. The CrB_xN_y coatings were deposited on a commercial Oerlikon Balzers RCS coating system employing 80:20 Cr:B targets. To vary the composition, the nitrogen fraction was adjusted (N₂ fraction = N₂/(Ar + N₂)) and a moderate bias voltage of − 20 V was applied during coating growth. The coating composition and nanostructure was determined using time-of-flight elastic recoil detection analysis (TOF-ERDA), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Ball-on-disc dry sliding wear tests were conducted using an alumina ball counterface both at room temperature and at 500 °C with the relative humidity controlled at 20%. Potentiodynamic corrosion tests were undertaken in 3.5% NaCl aqueous solution. The wear tracks were examined using optical profilometry and scanning electron microscopy (SEM); the surface composition inside and outside of the wear tracks were investigated using Raman spectroscopy and XPS. All coatings exhibit nanocomposite structures and phase compositions which are in fair agreement with those expected from the equilibrium phase diagram. The lowest wear rate at room temperature and 500 °C was found for CrB_0.14N_1.14, which was shown to exhibit the highest hardness and possesses a nanocomposite nc-CrN/a-BN structure. CrB_0.12N_0.84 coatings showed the lowest passive current density in potentiodynamic corrosion tests.     

Structural and tribological properties of DC reactive magnetron sputtered titanium/titanium nitride (Ti/TiN) multilayered coatings

Ti/TiN multilayered coatings of 200 layers with the thickness of 1.5 μm were deposited by a reactive DC magnetron sputtering technique using a mixture of Ar and N₂ gas. XRD technique was employed to elucidate the structural parameters. The presence of different phases like TiN, TiO_xN_y and TiO₂ were confirmed by XPS analyses. The observation of longitudinal optic (LO) phonon modes in the Raman spectra confirmed the highly crystalline nature of the deposited films. A microhardness value of 25.5 GPa was observed for Ti/TiN multilayers. The observed lower friction coefficient value for the Ti/TiN multilayers on mild steel (MS) indicated that the stack layers have better wear resistance property. Results from the electrochemical polarization and impedance studies showed the favorable behavior of the Ti/TiN multilayers, which have improved the corrosion resistance property of MS in 3.5% NaCl solution. The results of this study demonstrate that these multilayers can improve the corrosion resistance of mild steel substrates.     

Raman spectroscopy investigations of TiBxCyNz coatings deposited by low pressure chemical vapor deposition

TiB_xC_yN_z coatings have been prepared applying LPCVD and characterized using SEM/EDX, XRD, and Raman micro-spectroscopy. It has been shown that first-order, defect-induced Raman spectra of good quality can be obtained from TiB_xC_yN_z coatings, even if buried within a multilayer stack. The Raman peak assignments fit well with previous work on TiC_1 − xN_x. Even small changes in the B:C:N ratio result in systematical shifts of the Raman peaks. With increasing nitrogen content, the acoustical phonons shift to lower frequencies. A high correlation of the Raman shifts with lattice constants derived from XRD has also been found. Additionally, intensity and FWHM of the Raman peaks also change going from carbon- to nitrogen-rich coatings. The sensitivity of the TA peak Raman shifts to changes of the investigated basic coating properties is largest for N-rich coatings. Looking at the full range of coatings the dependence of the Raman shifts is slightly nonlinear.The present work establishes Raman microscopy as a complementary non-destructive technique compared to XRD for studying coatings like TiB_xC_yN_z. Structural, optical and chemical properties can be determined with considerably higher spatial resolution.     

Corrosion behaviour of MoSx-based coatings deposited onto high speed steel by magnetron sputtering

MoS_x-based films were deposited using magnetron sputtering from a pure MoS₂ target. Alloying was accomplished by “co-deposition” from separate targets onto substrates having a two-fold rotation. An additional experiment had also a Cr⁺-ion etch for surface preparation, followed by a Cr adhesion layer, made using a Cr target mounted on a cathodic arc evaporation source. MoS_x and Al- and Ti-alloyed MoS_x coatings have been deposited onto high speed steel (HSS) and glass substrates for corrosion investigations.The coatings were characterised by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, hardness and adhesion measurements. The corrosion behaviour of the samples was electrochemically measured by open-circuit-potential (OCP) measurements and by potentiodynamic corrosion tests in 0.8 M NaCl solution (pH 7). Additionally the MoS_x-based coatings on HSS have been exposed to salt spray tests. The corrosion investigations revealed that the addition of Al and Ti to MoS_x shifts the open-circuit-potential of about 80 to 110 mV to lower values, i.e. the alloying elements make the MoS_x coating a little bit less noble. In agreement with the OCP measurements, the corrosion potential E_corr in potentiodynamic corrosion tests was the highest for non-alloyed MoS_x coatings on HSS substrates. After the potentiodynamic corrosion tests, a strong corrosive attack could be observed for all coated samples. In salt spray tests the lowest number of corrosion pits was found for the MoS_x-Al (Cr⁺) coating on HSS.     

Mechanical properties and corrosion resistance of nanocrystalline ZrNxOy coatings on AISI 304 stainless steel by ion plating

Transition metal oxynitrides have become emerging decorative coating materials due to their adjustable coloration and high hardness and corrosion resistance. This research studied the effect of oxygen content on the coloration, mechanical properties and corrosion resistance of ZrN_xO_y thin films deposited on AISI 304 stainless steel using hollow cathode discharge ion plating (HCD-IP). The Zr/N/O ratios of the ZrN_xO_y films were determined using X-ray photoelectron spectroscopy (XPS). The color of the ZrN_xO_y thin film changed from golden yellow to blue and then slate blue with increasing oxygen content. X-ray diffraction (XRD) patterns revealed that phase separation of ZrN and m-ZrO₂ occurred as the oxygen content reached 31.2 at.%. ZrN(O) (ZrN with dissolving oxygen) is dominant at oxygen content less than 18.1 at.%, while m-ZrO₂ phase was prevailed at oxygen content above 40.3 at.%. Phase separation lowered the hardness of the ZrN_xO_y films as the fraction of ZrO₂ was less than 40%. The residual stresses in ZrN phase was higher than that in ZrO₂, and the residual stress decreased for the specimen containing 30 to 37% ZrO₂. For the samples containing more than 44% ZrO₂, the average residual stress was close to that in ZrO₂ phase. The corrosion resistance was evaluated by salt spray test and potentiodynamic scan in two solutions: 0.5MH₂SO₄ + 0.05 M KSCN and 5% NaCl solutions. The results showed consistent trend in the two solutions. From the results of potentiodynamic scan, corrosion resistance increased as the packing density of the film increased, whereas the film thickness was not a crucial factor on corrosion current; moreover, the electrical conductivity of the film may be one of the significant factors in corrosion resistance. Results of salt spray tests suggested that the corrosion of ZrN_xO_y in NaCl may play an important role in corrosion resistance.     

Characterization and blood compatibility of TiCxN1 − x hard coating prepared by plasma electrolytic carbonitriding

A novel technique to form Ti(C, N) on titanium, named as plasma electrolytic carbonitriding (PEC/N) on cathode was successful used to prepare TiC_xN_1 − x coating. The structure, composition and morphology of the coating were characterized by XRD, XPS and SEM, respectively. The results indicated that TiC_0.3N_0.7 as a new species appears on the surface of the titanium plate, and the thickness of the coating with porous surface morphology increases with the treated time. The blood compatibility of the TiC_0.3N_0.7 coating was evaluated by haemolysis ratios, dynamic blood clotting test, plasma recalcification time and platelet adhesion. The results indicated that the blood compatibility of the plasma-treated titanium with TiC_xN_1 − x coating is significantly improved as compared to the original titanium. Additionally, the results derived from measurements of hardness and corrosion indicated that the coating has excellent mechanical and corrosion-resistant properties.     

The effect of molybdenum on the corrosion behaviour of the high-entropy alloys Co1.5CrFeNi1.5Ti0.5Mox in aqueous environments

The purpose of this study is to investigate the electrochemical properties of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x high-entropy alloys in three aqueous environments which simulate acidic, marine, and basic environments at ambient temperature (∼25 °C). The potentiodynamic polarisation curves of the Co_1.5CrFeNi_1.5Ti_0.5Mo_x alloys, obtained in aqueous solutions of H₂SO₄ and NaOH, clearly revealed that the corrosion resistance of the Mo-free alloy was superior to that of the Mo-containing alloys. On the other hand, the lack of hysteresis in cyclic polarisation tests and SEM micrographs confirmed that the Mo-containing alloys are not susceptible to pitting corrosion in NaCl solution.     

Corrosion behaviour of dc magnetron sputtered Fe1−xMgx alloy films in 3 wt% NaCl solution

Fe_1−xMg_x alloy films (with x ? 43.4 at.% Mg) were deposited by dc magnetron sputtering onto glass slide substrates. The objective of this study was to characterise the corrosion properties of these alloys in saline solution for application as new friendly environmentally sacrificial coatings in the protection of steel structures. The morphological and structural properties of the alloys were systematically studied prior to electrochemical experiments, and then the degraded surfaces were analysed to determine the composition and nature of corrosion products. Alloys with <25  at.% Mg were single-phase body-centred cubic (bcc) with enlarged lattice parameters, whereas for magnesium contents above 25 at.%, amorphisation occurred. The reactivity of the alloys in saline solution is strongly dependent on the Mg content and the alloy structure. The incorporation of magnesium leads to an open circuit potential shift of the alloy towards more negative values, that confers an attractive interest of these alloys as sacrificial coatings. A transition in corrosion activity is observed at 25 at.% Mg from which the reactivity decreases with the magnesium content increase. The evolution of the alloy corrosion behaviour is discussed in terms of structural and corrosion products evolution versus magnesium content.     

Mechanical properties, deformation behaviors and interface adhesion of (AlCrTaTiZr)Nx multi-component coatings

In this study, (AlCrTaTiZr)N_x multi-component coatings with quinary metallic elements were developed as protective hard coatings for tribological application. The mechanical properties, creep behaviors, deformation mechanisms and interface adhesion of the (AlCrTaTiZr)N_x coatings with different N contents were characterized. With increasing the N₂-to-total (N₂ + Ar) flow ratio, R_N, during sputtering deposition, the (AlCrTaTiZr)N_x coatings transformed from an amorphous metallic phase to a nanocomposite and finally a crystalline nitride structure. The hardness of the coatings accordingly increased from 13 GPa to a high value of about 30 GPa, but the creep strain rate also increased from 1.3 × 10^− 4 to 7.3 × 10^− 4 1/s. The plastic deformation of the amorphous metallic coating deposited with R_N = 0% proceeded through the formation and extension of shear bands, whereas dislocation activities dominated the deformation behavior of the crystalline nitride coatings deposited with R_N = 10% and 30%. With increasing R_N, the interface adhesion energy between the coatings and the substrates was also enhanced from 6.1 to 22.9 J/m².     

纳米Si3N4 对TC4钛合金微弧氧化层微观结构和性能的影响

在基电解液中加入氮化硅纳米颗粒,对TC4钛合金进行微弧氧化（MAO）处理,研究了Si₃N₄浓度对微弧氧化层表面形貌、耐蚀性和耐磨性的影响。添加Si₃N₄的MAO层呈现多孔结构,当Si₃N₄浓度为1 g/L时,涂层厚度最大,且经过7 d的酸腐蚀试验,该涂层的耐蚀性良好,腐蚀速率最低,约为0.057 mg·cm^-2·d^-1。随着Si₃N₄的加入,MAO涂层的抗菌性能先升高后降低。当Si₃N₄的添加量为1 g/L时,该MAO层的抗菌性能最好。Si₃N₄的加入能明显提高涂层在模拟海水中的耐磨性。当Si₃N₄的添加量为3和4 g/L时,所得涂层的摩擦系数低且稳定,且添加3 g/L Si₃N₄制备来的MAO涂层表现出优异的耐磨性。     

Pulsed laser deposition of antifriction thin-film MoSex coatings at the different vacuum conditions

MoSe_x coatings were obtained by pulsed laser deposition in vacuum at the pressure of background Ar gas up to 10 Pa. The deposition temperature was 200 °С. The films were studied by means of X-ray diffraction, scanning and transmission electron spectroscopy, X-ray photoelectron spectroscopy, and Rutherford backscattering spectroscopy of helium ions. The tribological properties of thin-film coatings were investigated by pin-on-disk testing in air with 50% relative humidity. In addition, wear tracks were studied by micro-Raman spectroscopy. Chemical composition, structure, and tribological properties of the coatings were found to be sensitive to the presence of the inert gas. Thus, increasing the gas pressure from 10^− 4 to 10 Pa changes the chemical composition, so that the ratio of the atomic concentrations of Se and Mo (x = Se/Mo) increases from 1.5 to 2.4 in the principal deposition zone. The changing of the structure concerns the accumulation of distortions in the lattice of MoSe_x nano-crystals as increasing the distance between the basal planes and intensive formation of nano-sized inclusions of the amorphous phase and Mo nano-crystals in the volume of the coatings. At the optimal gas pressure (∼ 2 Pa), the composition of the coating was close to the stoichiometric one, and the layer adjacent to the substrate consisted of MoSe_x nano-crystals with the basal planes parallel to the substrate surface or oriented at small angles to the surface. The thickness of the oriented layer in such coatings was greater than the thickness of the similar layer in the coatings deposited in vacuum (10^− 4 Pa). The tribological properties of MoSe_x coatings deposited on substrates of stainless steel type 95 × 18 (18 at.% Cr) depend on the gas pressure. The friction coefficient in air decreases from 0.08 for deposition at the background pressure of 10^− 4 Pa to 0.04 for deposition at the optimal pressure. This change in the deposition conditions has only a marginal effect on the coating durability. Means to increase the durability are also considered.     

Interplay of microstructural features in Cr1−xAlxN and Cr1−x−yAlxSiyN nanocomposite coatings deposited by cathodic arc evaporation

The interplay between the deposition geometry, the chemical and phase composition, the crystallite size, the lattice strain and the direction and the degree of the preferred orientation of crystallites was investigated in the Cr_1−xAl_xN and Cr_1−x−yAl_xSi_yN nanocrystalline coatings and nanocomposites, which were deposited in cathodic arc evaporation process at different positions of substrates in the deposition apparatus. The different positions of the substrates affected primarily the distance between the samples and the cathodes and consequently the chemical and phase composition of the coatings, the crystallite size, the lattice strain and the preferred orientation of crystallites. In the Cr_1−xAl_xN coatings, the dominating cubic crystallites were preferentially oriented with their 〈111〉 direction perpendicular to the sample surface; this out-of-plane preferred orientation of crystallites was accompanied by a strong in-plane texture. In the Cr_1−x−yAl_xSi_yN coatings, a strong inclination of the {111} texture from the normal direction and a decay of the in-plane preferred orientation were observed in cubic crystallites with increasing silicon (and aluminium) contents.     

TiOxNy coatings grown by atmospheric pressure metal organic chemical vapor deposition

Titanium oxynitride coatings were deposited on various substrates by an original atmospheric pressure metal organic chemical vapor deposition (MOCVD) process using titanium tetra-iso-propoxide as titanium and oxygen precursors and hydrazine as a nitrogen source. The films composition was monitored by controlling the N₂H₄ mole fraction in the initial reactive gas phase. The variation of the N content in the films results in significant changes in morphological, structural and mechanical properties. When a large excess of the nitrogen source is used the resulting film contains ca 17  at % of nitrogen and forms dense and amorphous TiO_xN_y films. Growth rates of these amorphous TiO_1.5N_0.5 coatings as high as 14 μm/h were obtained under atmospheric pressure. The influence of the deposition conditions on the morphology, the structure, the composition and the growth rate of the films is presented. For the particular conditions leading to the growth of amorphous TiO_1.5N_0.5 coatings, first studies on the mechanical properties of samples grown on stainless steel have revealed a high hardness, a low friction coefficient, and a good wear resistance in unlubricated sliding experiments against alumina which make them very attractive as protective metallurgical coatings.     

Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al_xCrFe_1.5MnNi_0.5 alloys, obtained in H₂SO₄ and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al_xCrFe_1.5MnNi_0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.