Multilayer gradient rare earth disilicate coating prepared by in-situ melt infiltration and sintering method with high thermal shock resistance on porous Si3N4 ceramics

Dense multilayer gradient rare earth disilicate (γ-Y₂Si₂O₇ /β-Yb₂Si₂O₇ /β-Lu₂Si₂O₇) coatings were in-situ prepared by melt-infiltration /sintering procedure on porous Si₃N₄ ceramics for water resistance. Experimental and numerical simulation methods were used to study their thermal shock behavior. As a control, thermal shock behavior of pure γ-Y₂Si₂O₇ coatings were also compared. FEM results showed that the gradient design of modulus in ceramic coating can effectively avoid the mismatch of mechanical properties between coating layer and internal substrate, reducing the transient thermal stress in each layer during thermal shock. All of the pure γ-Y₂Si₂O₇ coatings were failed after thermal shock tests with ΔT ≈ 1200 ℃. However, when sintering temperature of multi-layer disilicate coatings were higher than 1400 ℃, the water absorption rates after thermal shock were all less than 5%, still showing good waterproof performance. The gradient design of modulus could effectively improve the structural stability of ceramic coatings.     

Effect of CMAS Deposits on MOCVD Coatings in the System Y2O3–ZrO2: Phase Relationships

Three metal‐organic chemical vapor deposition (MOCVD) coatings (ZrO₂, Y₂O₃·ZrO₂, Y₂O₃) were studied in contact with model CMAS to investigate the microstructural evolution and phase formation. The MOCVD coatings were covered with CMAS powder deposits and annealed for 1 h at 1250°C, respectively. The ZrO₂ coating was completely infiltrated by CMAS, whereas the yttria containing coatings show a higher resistance against CMAS infiltration. This is explained by the formation of a continuous oxyapatite layer in the reaction zone of the coating and the CMAS deposit. The Y₂O₃·ZrO₂ coating shows the best infiltration resistance despite the fact that the Y₂O₃–CMAS sample is the only completely crystallized. As crystallization products, oxyapatite, melilite, anorthite as well as new garnets bearing all available cations were formed. The garnet phase was confirmed by XRD and TEM. EDS measurements were used to calculate structural formula A₃B₂T₃O₁₂ (A = Ca, Y, Zr; B = Mg, Al; T = Al, Si).     

Effect of CrSi2 on the ablation resistance of a ZrSi2-Y2O3/SiC coating prepared by SAPS

To improve the ablation resistance of carbon/carbon (C/C) composites, a proportional amount of ZrSi₂-CrSi₂-Y₂O₃ mixed particles were deposited on the surface of SiC-coated C/C composites by supersonic air plasma spraying (SAPS) to form a ZrSi₂-CrSi₂-Y₂O₃/SiC coating. The microstructure and phase compositions of the coating were studied by SEM, EDS, XRD and its anti-ablation performance was tested by oxyacetylene torch. The experimental results showed that the ZrSi₂-CrSi₂-Y₂O₃ outer coating had a dense microstructure without obvious pores and microcracks, and the thickness reached approximately 150 μm. In the process of being eroded and scoured by the oxyacetylene flame, the coating exhibited excellent anti-ablation property, which was attributed to the mosaic microstructure formed by ZrO₂ and a Si-O-Cr liquid film on the coating surface. After experiencing an ablation time of 80 s, the linear ablation rate and the mass ablation rate of the coating were -1.0 ± 0.03 μm s^-1 and -0.16 ± 0.014 mg s^-1, respectively.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 1: Processing and characterization

This study describes an oxidation and corrosion resistant environmental barrier coating (EBC) applied to an AISI 441 stainless steel substrate. For this purpose, four polymer-derived ceramic (PDC) coating systems were developed. These coating systems consisted of a bond coat applied by dip coating, and a top-coat that was loaded with passive fillers and deposited by spray coating. The microstructures of the coatings were investigated using optical microscopy and scanning electron microscopy, including energy dispersive spectroscopy (EDS). X-ray powder diffraction (XRD) was used to investigate the phase composition of the coatings. The optimized composite top coatings were prepared from the preceramic polymer HTT1800, filled with yttria-stabilized zirconia and a specially tailored Al₂O₃–Y₂O₃–ZrO₂ (AYZ) passive filler, and commercial barium silicate glasses were used as sealing agents. After thermal treatment in air at 750°C, uniform and crack-free composite coatings on stainless steel substrates were developed, with thicknesses of up to 93 μm. Oxidation tests, which were performed at 850°C in synthetic air, showed that every tested coating system remained undamaged by oxidation and showed good bonding to the metal substrate.     

Antibacterial activity and corrosion resistance of Ta2O5 thin film and electrospun PCL/MgO-Ag nanofiber coatings on biodegradable Mg alloy implants

Biodegradable magnesium (Mg) alloys have drawn considerable attention for use in orthopedic implants, but their antibacterial activity and corrosion resistance still require improvement. In the present work, functional Ta₂O₅ (tantalum pentoxide) compact layers and PCL/MgO-Ag (poly (ε-caprolactone)/magnesium oxide-silver) nanofiber porous layers were subsequently deposited on Mg alloys via reactive magnetron sputtering and electrospinning, respectively, to improve anticorrosion and antibacterial performance. Sputter coating of the Ta₂O₅ resulted in a thick layer (～1?μm) with an amorphous structure and high adhesive strength. The nanostructure exhibited bubble-like patterns with no obvious nano-cracks, nano-porosities, or pinholes. The electrospun PCL/MgO-Ag nanofiber coating was porous, smooth, and plain with no obvious beads. In vitro corrosion tests demonstrated the PCL/MgO-Ag nanofiber-coated alloy had greater corrosion resistance than a Ta₂O₅ sputter-coated alloy or uncoated Mg alloy. The additional electrospun PCL/MgO-Ag nanofiber coating also had greater antibacterial behavior toward Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria than the Ta₂O₅-coated or uncoated alloy specimens. Increasing the MgO-Ag concentration of the nanofibers from 1 to 3?wt% increased antibacterial activity. The combination of Ta₂O₅ and PCL/MgO-Ag nanofiber coatings on Mg alloys may therefore have potential applications for reducing bone infection as related to orthopedic implants for bone repair.     

Comparison of corrosion behaviors and wettability of CMAS on Ta2O5-Y2O3 co-stabilized ZrO2 and YSZ thermal barrier coatings

High-temperature degradation of the plasma sprayed 16 mol% TaO_2.5 + 16 mol% YO_1.5 co-stabilized ZrO₂ (YTZ) and YSZ (7.6 wt% Y₂O₃-stabilized ZrO₂) coatings under calcium-magnesium-aluminon-silicate (CMAS) attack at 1200 °C and 1250 °C were comparatively investigated. Results indicated that the coatings were insensitive to the infiltration of CMAS after 10 h corrosion at 1200 °C. At 1250 °C, the entire YSZ cross-section completely failed and also underwent serious chemical corrosion after 3 h hot corrosion. Even after 10 h corrosion, the penetration depth of CMAS into the YTZ was only about 80 µm. For YTZ coating, the YTaO₄ stabilizer could not easily dissolve in CMAS and precipitated out of the YTZ crystal lattice owing to the strong chemical interaction between Ta⁵⁺ and Y³⁺. The wettability of CMAS on YTZ coating was worse than that on YSZ coating. Compared with YSZ coating, the YTZ coating showed better resistance to CMAS corrosion.     

Comparative study of the corrosion resistance of thermally sprayed ceramic coatings and their bulk ceramic counterparts

A comparative study of the corrosion properties of thermally sprayed ceramic coatings (Al₂O₃, Al₂O₃–TiO₂ with different ratios, mullite, and ZrSiO₄) and their sintered bulk ceramic counterparts was performed. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying processes. The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄ at 85 °C, respectively. The coating microstructures and phase compositions, as well as the corrosive environment were shown to have a strong effect on the corrosion resistance of the coatings. Al₂O₃–coatings were more sensitive to these factors than Al₂O₃–TiO₂ coatings were.The corrosion resistance of the bulk ceramics was superior to that of the thermally sprayed coatings. This is mainly because the coatings exhibited specific microstructure and contained amorphous and/or metastable phases not appearing in the bulk ceramics.     

Preparation and characterization of hydroxyapatite coating by magnetron sputtering on Mg–Zn–Ag alloys for orthopaedic trauma implants

The aim of the present paper was to evaluate the effect of hydroxyapatite coatings on the two types of Mg–Zn–Ag alloys as a possible solution to control magnesium alloy degradation. The coatings were prepared by the radio frequency magnetron sputtering method at a deposition temperature of 300 °C. To perform this evaluation, the coated alloys were immersed in a simulated body fluid solution at body temperature (37 ± 0.5 °C) to determine the corrosion resistance through electrochemical and immersion tests. Moreover, the investigation also consisted of the evaluation of microchemical, mechanical, and morphological properties. The deposition temperature of 300 °C was enough to obtain a crystalline hydroxyapatite structure with a Ca/P ratio close to the stochiometric one. The adhesion of coatings was not influenced by the nature of Mg–Zn–Ag alloys, so similar values for both coated alloys were found. The results showed that the coating was homogonous deposited on the Mg–Zn–Ag alloys and the corrosion resistance of uncoated magnesium alloys was improved.     

Influence of V-containing species on formation and corrosion resistance of PEO coatings developed on AZ31B Mg alloy

The defects, micropores, and microcracks on the plasma electrolytic oxidation (PEO) coating severely affect the long-term anti-corrosion feature. Inclusion of additives in the electrolyte is a feasible and effective strategy to attain the more intact PEO coating. The PEO coatings are fabricated on AZ31B Mg alloys in an silicon-based electrolyte with Na₃VO₄ (OPEO), NaVO₃ (MPEO), and V₂O₃ (VPEO), respectively, to explore the influence of additives on the corrosion resistance. MPEO sample has the smallest poriness and pore size unfolding the best anti-corrosion resistance, which is also consistent with the electrochemical analysis. Even after 336 h salt spray test, the MPEO coating exhibits the least corrosion. The excellent anti-corrosion performance of MPEO sample is the synergistic effect of inorganic salts and multiple metal oxides generated in the formation of PEO coating. Inclusion of inorganic salt with valence change metal is a feasible pathway to achieve the compact PEO coating since multiple metal oxides could be produced and deposited in the PEO coating during the fabrication.     

Corrosion and friction resistance of TiVCrZrWNx high entropy ceramics coatings prepared by magnetron sputtering

High entropy alloy coatings have attracted much attention because of their high hardness, low-level fault energy, and chemical stability. Nevertheless, this type of coating would inevitably suffer from wear, corrosion, aging, and so on. Hence, a novel coating with corrosion and friction resistance would be constructed for broadening its application scenarios. In this work, TiVCrZrWN_x high entropy ceramics coatings were prepared by reactive magnetron sputtering. The microstructure, mechanical properties, friction, and corrosion resistance of the coatings deposited at different nitrogen flow rates have been studied. The microstructure of TiVCrZrWN_x coatings is strongly dependent on the nitrogen flow rate and forms a stable FCC structure when the nitrogen flow rate reaches 24 sccm. The pure TiVCrZrW coating is 15.65 GPa, with the increase of nitrogen flow rate (24 sccm), the coating hardness reaches 21.27 GPa. The corrosion resistance of the coatings also increases continuously. According to the results of the impedance spectrum and polarization curve, the charge transfer resistance value of the coating gradually increases with the content of nitrogen, the current density rapidly decreases to a minimum as the potential increases. In terms of tribological behavior, the formation of V₂O₅ during the sliding in seawater could significantly reduce the coefficient of friction from 0.603 to 0.383. Therefore, TiVCrZrWN_x HECs coatings simultaneously possess high hardness, toughness, and excellent resistance to friction and corrosion, which is expected to provide a new and reliable method for the research field of coatings in the maritime field.     

Role of KOH and NaNO2 on the structural and corrosion properties of anodic spark electrolysis coatings produced on aluminium

Doping light elements into ceramic coatings on different metal substrates by anodic-spark electrolysis (ASE) to improve their properties, such as wear and corrosion resistance, has recently attracted a lot of attention. In this study, nitrogen-doped Al₂O₃ composite ceramic coatings had been fabricated in eco-friendly KOH–NaNO₂ electrolytes using the anodic-spark electrolysis (ASE) method after 9 min at a fixed applied ASE voltage (75 V higher than the breakdown voltages). To deposit a nitrogen-doped coating with high amounts of oxynitride phases possible, we thoroughly studied the ASE coatings deposited in different total variable salts concentrations (KOH+NaNO₂) and NaNO₂/KOH ratios of ASE electrolytes. The coating properties were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier-transform infrared spectroscopy (FT-IR), and the electrochemical impedance spectroscopy (EIS) tests. The results indicated that the coating produced in the KOH–NaNO₂ electrolyte with a low total variable salts concentration (2 gr.L⁻¹) and a high NaNO₂/KOH ratio value (3) is optimum in the investigated conditions. It has the highest percentage of nitrogen-doped phases, such as N-doped γ-Al₂O₃ and γ-AlON (γ-Al_2.78O_3.65N_0.35), and a homogeneous morphology of surface with the smallest average size of pores (<14 μm²). This coating showed the significantly higher corrosion resistance with a 4.101104 × 10⁶ Ω cm² value compared to the uncoated aluminium substrate with a corrosion resistance value of 0.094195 × 10⁶ Ω cm² after 48 h of immersion in the 3.5 wt% NaCl solution. The approach presented herein provides an attractive way to modify the surface of aluminium alloys to improve corrosion behaviour.     

Microstructure and tribological properties of multilayered ZrCrW(C)N coatings fabricated by cathodic vacuum-arc deposition

In this study, a series of ZrCrW(C)N multilayer coatings with various transition layers were deposited on AISI304 stainless steel using cathodic vacuum-arc deposition in N₂–C₂H₂ gas mixture. The tribological behaviors of sliding against Al₂O₃ balls under dry friction and lubricant conditions were investigated using a reciprocating tribometer. The results demonstrated that the ZrCrW(C)N coatings comprised (Zr, Cr, W) (C, N) crystallites and an amorphous carbon phase. It possessed a nano-hardness of 35.4 GPa and an elastic modulus of 417.7 GPa. The friction coefficient of the coating was reduced by 14% compared to that of the 304 matrices, and the wear mechanism changed from adhesive wear to slight abrasive wear under the lubrication steady state. Under dry friction conditions, the ZrCrW(C)N coatings with the entire CrWN transition layer exhibited wear rates of 1.27 ± 0.04 × 10^?8 mm³ (N m)^?1, which were one order of magnitude lower than that of the 304 steel. Compared with the untreated AISI304 stainless steel, the ZrCrW(C)N coating exhibits excellent mechanical and tribological properties under lubricated and dry friction conditions, which are crucial for engineering applications.     

Electrochemical investigation of chromium nanocarbide coated Ti–6Al–4V and Co–Cr–Mo alloy substrates

This study investigated the electrochemical behavior of chromium nano-carbide cermet coating applied on Ti–6Al–4V and Co–Cr–Mo alloys for potential application as wear and corrosion resistant bearing surfaces. The cermet coating consisted of a highly heterogeneous combination of carbides embedded in a metal matrix. The main factors studied were the effect of substrate (Ti–6Al–4V vs. Co–Cr–Mo), solution conditions (physiological vs. 1 M H₂O₂ of pH 2), time of immersion (1 vs. 24 h) and post coating treatments (passivation and gamma sterilization). The coatings were produced with high velocity oxygen fuel (HVOF) thermal spray technique at atmospheric conditions to a thickness of 250 μm then ground and polished to a finished thickness of 100 μm and gamma sterilized. Native Ti–6Al–4V and Co–Cr–Mo alloys were used as controls. The corrosion behavior was evaluated using potentiodynamic polarization, mechanical abrasion and electrochemical impedance spectroscopy under physiologically representative test solution conditions (phosphate buffered saline, pH 7.4, 37 °C) as well as harsh corrosion environments (pH ～ 2, 1 M H₂O₂, T = 65 °C). Severe environmental conditions were used to assess how susceptible coatings are to conditions that derive from possible crevice-like environments, and the presence of inflammatory species like H₂O₂. SEM analysis was performed on the coating surface and cross-section. The results show that the corrosion current values of the coatings (0.4–4 μA/cm²) were in a range similar to Co–Cr–Mo alloy. The heterogeneous microstructure of the coating influenced the corrosion performance. It was observed that the coating impedances for all groups decreased significantly in aggressive environments compared with neutral and also dropped over exposure time. The low frequency impedances of coatings were lower than controls. Among the coated samples, passivated nanocarbide coating on Co–Cr–Mo alloy displayed the least corrosion resistance. However, all the coated materials demonstrated higher corrosion resistance to mechanical abrasion compared to the native alloys.     

Corrosion behavior of air plasma spraying zirconia-based thermal barrier coatings subject to Calcium–Magnesium–Aluminum-Silicate (CMAS) via burner rig test

Three different kinds of thermal barrier coatings (TBCs) — 8YSZ, 38YSZ and a dual-layered (DL) TBCs with pure Y₂O₃ on the top of 8YSZ were produced on nickel-based superalloy substrate by air plasma spraying (APS). The Calcium–Magnesium–Aluminum-Silicate (CMAS) corrosion resistance of these three kinds of coatings were researched via burner rig test at 1350 °C for different durations. The microstructures and phase compositions of the coatings were characterized by SEM, EDS and XRD. With the increase of Y content, TBCs exhibit better performance against CMAS corrosion. The corrosion resistance against CMAS of different TBCs in descending was 8YSZ + Y₂O₃, 38YSZ and 8YSZ, respectively. YSZ diffused from TBCs into the CMAS, and formed Y-lean ZrO₂ in TBCs because of the higher diffusion rate and solubility of Y³⁺ in CMAS than Zr⁴⁺. At the same time, 38YSZ/8YSZ + Y₂O₃ reacts with CAMS to form Ca₄Y₆(SiO₄)₆O/Y_4·67(SiO₄)₃O with dense structure, which can prevent further infiltration of CMAS. The failure of 8YSZ coatings occurred at the interface between the ceramic coating and the thermally grown oxide scale (TGO)/bond coating. During the burner rig test, the Y₂O₃ layer of the DL TBCs peeled off progressively and the 8YSZ layer exposed gradually. DL coatings keep roughly intact and did not meet the failure criteria after 3 h test. 38YSZ coating was partially ablated, the overall thickness of the coating is thinned simultaneously after 2 h. Therefore, 8YSZ + Y₂O₃ dual-layered coating is expected to be a CMAS corrosion-resistant TBC with practical properties.     

A highly refractory material based on zirconia stabilized with yttrium and aluminum oxides

Conclusions With combined additives (Y₂O₃ + Al₂O₃), zirconium dioxide forms solid solutions which resist thermal decomposition.A material was synthesized containing from 90–93 mol. % ZrO₂, from 3.5 to 5 mol.% Y₂O₃, and from 3.5 to 5% mol Al₂O₃, possessing an average coefficient of thermal expansion which is lower, and a thermalshock resistance which is higher, than the double solid solutions in the system ZrO₂-Y₂O₃ and ZrO₂-CaO.Translated from Ogneupory, No. 4, pp. 42–45, April, 1973.     

Synthesis of a novel nanostructured zinc oxide/baghdadite coating on Mg alloy for biomedical application: In-vitro degradation behavior and antibacterial activities

In this research, zinc oxide (ZnO) and zinc oxide/baghdadite (ZnO/Ca₃ZrSi₂O₉) were prepared on the surface of Mg alloy using physical vapor deposition (PVD) coupled with electrophoretic deposition (EPD). For this purpose, the nanostructured ZnO was prepared with a thickness of 900 nm and crystallite sizes of 64 nm as under layer while nanostructured baghdadite with a thickness of 10 µm was deposited on the Mg alloy substrate as an over-layer. Electrochemical measurement exhibited that the ZnO/Ca₃ZrSi₂O₉-coated specimen has a higher corrosion resistance and superior stability in simulated body fluid (SBF) solution in comparison with the ZnO-coated and bare Mg alloy samples. Antibacterial activities of the uncoated and coated specimens were evaluated against various pathogenic species (Escherichia coli, Klebsiella pneumoniae, and Shigella dysenteriae) via disc diffusion method. The obtained results showed that ZnO and ZnO/Ca₃ZrSi₂O₉ coatings have great zones of inhibition (ZOI) against E. coli, Klebsiella, and Shigella. However, less ZOI was found around the bare Mg alloy. Therefore, ZnO/Ca₃ZrSi₂O₉ is a promising coating for orthopedic applications of biodegradable Mg alloys considering its excellent antibacterial activities and high corrosion resistance.     

Influences of electrolytes on tribocorrosion performance of MAO coating on AZ31B magnesium alloy in simulated body fluid

In this paper, the effects of electrolytes on the corrosion resistance and tribocorrosion performance of micro-arc oxidation (MAO) coatings on AZ31B magnesium (Mg) alloys in simulated body fluid (SBF) were studied. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilized to explore the microstructure, surface morphology, and phase components of the MAO coatings. Corrosion and tribocorrosion performance of MAO coated Mg alloys were evaluated by using potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and a ball-on-disk tribotester. It was found that MAO coating produced in electrolyte containing both Na₂SiO₃ and Na₂B₄O₇ exhibited superior corrosion resistance and tribocorrosion performance in the SBF.     

Conversion behaviour and resulting mechanical properties of polysilazane-based coatings

Polymer-derived ceramics exhibit a convenient route for the processing of low-dimensional ceramics like coatings or fibres. In previous investigations unfilled and composite coatings have been developed using ammonolysed bis(dichloromethylsilyl)ethane (ABSE) or perhydropolysilazane (PHPS) as precursors and BN, ZrO₂ or glass particles as filler materials. The coating systems provide excellent corrosion and oxidation resistance to underlying metals. This paper reports on the effect of the precursor system and the pyrolysis parameters on the conversion behaviour, shrinkage and mechanical properties, including hardness and Young's modulus, of ABSE- and PHPS-based coatings. Therefore the crosslinking and pyrolysis behaviour as well as the mechanical properties of the coatings were investigated up to pyrolysis temperatures of 1000 °C in nitrogen and in air by ATR-IR, SEM, profilometry and nanoindentation measurements. The coatings pyrolysed at 1000 °C in nitrogen, have hardness values of 13 GPa and Young's moduli up to 155 GPa.     

A novel non-skid composite coating with higher corrosion resistance

In this work, low-pressure cold-sprayed Ni-Zn-Al₂O₃ intermediate layers were deposited between supersonic–plasma-sprayed NiCr-Cr₃C₂ surface layers and underlying low-carbon steel layers to form a sandwich structure that enhances the corrosion resistance of non-skid NiCr-Cr₃C₂ coatings. The corrosion performance of these bi-layer non-skid coatings and that of a single-layer coating were investigated through electrochemical measurements and observations of their corrosion morphologies. The novel non-skid coating with a top layer possessing a fine powder grain size exhibited the best corrosion resistance because of the pseudopassivation of the interlayer and physical barriers created by the corrosion process. The intermediate layer substantially improved the corrosion resistance of the non-skid coatings.