Oxidation Behavior of ZrB<Subscript>2</Subscript>–SiC Composites at 1650 °C

The oxidation behavior of ZrB₂–SiC composites in air was studied at 1650 °C. Diffusion-controlled oxidation kinetics were found for the composites studied. A parabolic rate constant of 1.2 × 10^?8 g² cm^?4 s^?1 was measured for ZrB₂–10 % SiC composite. A transition in the oxidation kinetics was observed for ZrB₂–30 % SiC composite with the initial parabolic rate constant being 1.3 × 10^?8 g² cm^?4 s^?1. After exposure for 60 min, the parabolic rate constant was found to be 0.3 × 10^?8 g² cm^?4 s^?1. A single ZrO₂-rich oxide layer was found in the oxide scale structure of ZrB₂–10 % SiC composite. On the other hand, three-layer oxide structures, namely SiO₂-rich top layer, followed by ZrO₂-rich oxide scale and SiC-depleted layer, were found for ZrB₂–30 % SiC composite. The outer layer in the oxide scale structure of ZrB₂–SiC composite was tapered with enhanced oxidation at the corners of the sample. Vortex formation during the viscous flow of B₂O₃–SiO₂–ZrO₂ liquid near the corners on the surface was proposed as the root cause for enhanced oxidation at the corners of the sample.     

Microstructure and Oxidation Behavior of Al and Al/NiCrAlY Coatings on Pure Titanium Alloy

To improve the oxidation resistance of Ti alloys, a NiCrAlY coating was deposited as diffusion barrier between aluminum overlay coating and pure Ti substrate by air plasma spraying method. The microstructure and oxidation behavior of Al coatings with and without NiCrAlY diffusion barrier were investigated in isothermal oxidation tests at 800 °C for 100 h. The results indicate that the weight gain of the Al/NiCrAlY coating was 4.16 × 10^?5 mg² cm^?4 s^?1, whereas that of the single Al coating was 9.52 × 10^?5 mg² cm^?4 s^?1 after 100 h oxidation. As compared with single Al coating, the Al/NiCrAlY coating revealed lower oxidation rate and excellent oxidation resistance by forming thin Al₂O₃ + NiO scales at overlaying coating/diffusion barrier and diffusion barrier/substrate interfaces. Meanwhile, the inward diffusion of Al and the outward diffusion of Ti were inhibited effectively by the NiCrAlY diffusion barrier.     

Surface morphology evolution and ablation mechanism of SiC–Si multiphase ceramic coating on graphite under oxy-acetylene flame

A dense SiC–Si coating with average thickness of 580 μm was prepared on graphite by pack cementation. The surface morphology evolution and ablation mechanism of the coating at about 1780 °C were investigated by oxy-acetylene flame. After applying SiC–Si coating, the ablation surface temperature dropped about 300 °C. The ablation products SiO₂ presented high viscosity and provided good protection in the early-to-middle stage of the ablation. The mechanical denudation gradually became severe with the ablation time increasing. Finally, the convection between the gases formed in the subsurface and the ablative gases turned out to be the primary ablation barrier.     

Oxidation and Hot Corrosion Behavior of Plasma-Sprayed MCrAlY–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr₂O₃ and CoNiCrAlY–Cr₂O₃ coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na₂SO₄–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10^?2 and 1.36 × 10^?2 mg/cm² h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr₂O₄ spinel on the surface, while as-deposited Cr₂O₃ in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr₂O₃ coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr₂O₃ coating was found to be more protective than the NiCoCrAlY–Cr₂O₃ coating under hot corrosion condition.     

Corrosion Inhibition of 304 Stainless Steel by Nano-Sized Ti/Silicone Coatings in an Environment Containing NaCl and Water Vapor at 400–600°C

The corrosion behavior of 304 stainless steel (SS) and its corrosion inhibition by brushing nano-sized Ti/silicone coatings on its surface in an environment containing a solid NaCl deposit and water vapor at 400–600°C was studied. Results indicated that water vapor or NaCl, especially water vapor plus NaCl accelerated the corrosion of the steel markedly. The corrosion scales of the uncoated steel had a duplex structure at 400–500°C and internal oxidation occurred for the uncoated steel at 600°C in an environment containing NaCl and water vapor. The corrosion of the 304SS was inhibited efficiently by the coatings at 400–500°C, and the coated steel suffered corrosion to some extent and most of the coatings were destroyed at 600°C. X-ray diffraction (XRD) indicated that the corrosion products of the uncoated steel were mainly Fe₂O₃, Cr₂O₃, NiO or Na₂CrO₄, and the coatings consisted mainly of TiO₂ and SiO₂ after exposure at 400–500°C. The good corrosion resistance of the nano-sized Ti/silicon coatings was attributed to the formation of SiO₂, and TiO₂ that resulted from the decomposition of the organic components in the coating and fast oxidation of nano-Ti powder respectively during the experiments, TiO₂ mixed together with SiO₂ and formed a new coating on the steel surface that played an important role in the protection of the steel.     

Effect of Graphite Addition on Structure and Properties of Ti(CN) Coatings Deposited by Reactive Plasma Spraying

Ti(CN) coatings with graphite addition ranging from 0 to 50 wt.% were prepared using reactive plasma spraying technology and their microstructure, mechanical, and tribological properties were investigated using scanning and transmission electron microscopy, x-ray diffraction analysis, x-ray photoelectron spectroscopy, Vickers microhardness testing, and block-on-ring wear testing. The results showed that graphite addition resulted in crystallite size refinement and an increase in the amount of amorphous phase. The Ti(CN) coatings consisted of a mixture of Ti(CN), graphite, CN _x , and amorphous phases. The hardness first increased then decreased as the graphite content was increased, with a maximum of 1450 HV_0.2 for 30 wt.% graphite addition. The fracture toughness decreased from 4.38 MPa m^1/2 to 2.76 MPa m^1/2 with increasing graphite content. The friction coefficient decreased due to unreacted graphite embedded in the matrix. Also, the wear rate first decreased then increased, with a minimum value of 2.65 × 10^?6 mm³ N^?1 m^?1 for 30 wt.% graphite addition. The wear mechanisms of the Ti(CN) coatings included abrasive, adhesive, and oxidation wear.     

Tribocorrosion Behavior of Fe-Based Amorphous Composite Coating Reinforced by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in 3.5% NaCl Solution

Although corrosion and friction/wear behavior of Fe-based amorphous coatings and their composites has been extensively studied during the past decade, there is very limited work related to tribocorrosion behavior. In this paper, the tribocorrosion behavior of a Fe-based amorphous composite coating reinforced with 20 wt.% Al₂O₃ particles was investigated in a 3.5% NaCl solution on a ball-on-disk tester and was compared to the monolithic amorphous coating and 316L stainless steel (SS). The results showed that the amorphous composite coating exhibited the highest tribocorrosion resistance among the three materials tested, as evidenced by the lowest coefficient of friction (~0.3) and tribocorrosion wear rate (~1.2 × 10^?5 mm³/N·m). In addition, potentiodynamic polarization measurements before and during tribocorrosion testing demonstrated that corrosion resistance of the amorphous composite coating was not influenced so much by mechanical loading compared to the amorphous coating and the 316L SS. Observations on the worn surface revealed a corrosion-wear- and oxidational-wear-dominated tribocorrosion mechanism for the composite coatings. The excellent tribocorrosion resistance of the composite coating results from the effect of chemically stable Al₂O₃ phase which resists oxidation and delamination during sliding, along with poor wettability with corrosive NaCl droplets.     

Oxidation Mechanisms of Copper and Nickel Coated Carbon Fibers

Differential-Thermal Analysis (DTA) and X-ray diffraction analysis were applied to determine the mechanisms of high-temperature oxidation of copper- and nickel-coated carbon fibers. Both kinds of coatings were deposited by electroless plating onto the fiber surface. The as-deposited copper film was crystalline, whereas the nickel coating consisted of an amorphous Ni–P alloy. Coated fibers were heated from room temperature to 900 °C in air at 10 °C min^?1. For the copper coating, the main oxidation product formed at low temperatures was Cu₂O, while at higher temperatures was CuO. The crystallization of Ni–P took place at 280–360 °C with the formation of Ni and Ni₃P. The final compounds were NiO, Ni₂P and Ni₃(PO₄)₂. After complete oxidation of the carbon fibers, copper and nickel-oxidized microtubes were obtained. Besides, while copper reduced the temperature of the fiber oxidation, nickel coatings increased the minimum temperature needed for this reaction.     

A MoSi2/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a MoSi₂ outer coating was prepared on pack-cementation SiC coated C/C composites by a hydrothermal electrophoretic deposition. The phase composition, microstructure and oxidation resistance of the prepared MoSi₂/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free MoSi₂ outer coatings. The MoSi₂/SiC coating can protect C/C composites from oxidation at 1773 K for 346 h with a weight loss of 2.49 mg cm⁻² and at 1903 K for 88 h with a weight loss of 5.68 mg cm⁻².     

Dual-Layer Oxidation-Protective Plasma-Sprayed SiC-ZrB<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Carbon Nanotube Coating on Graphite

Graphite is used in high-temperature gas-cooled reactors because of its outstanding irradiation performance and corrosion resistance. To restrict its high-temperature (>873 K) oxidation, atmospheric-plasma-sprayed SiC-ZrB₂-Al₂O₃-carbon nanotube (CNT) dual-layer coating was deposited on graphite substrate in this work. The effect of each layer was isolated by processing each component of the coating via spark plasma sintering followed by isothermal kinetic studies. Based on isothermal analysis and the presence of high residual thermal stress in the oxide scale, degradation appeared to be more severe in composites reinforced with CNTs. To avoid the complexity of analysis of composites, the high-temperature activation energy for oxidation was calculated for the single-phase materials only, yielding values of 11.8, 20.5, 43.5, and 4.5 kJ/mol for graphite, SiC, ZrB₂, and CNT, respectively, with increased thermal stability for ZrB₂ and SiC. These results were then used to evaluate the oxidation rate for the composites analytically. This study has broad implications for wider use of dual-layer (SiC-ZrB₂/Al₂O₃) coatings for protecting graphite crucibles even at temperatures above 1073 K.     

The Effect of Hf on the Oxidation and Corrosion Behavior of Ti0.7Al0.3N Coating Prepared by Arc-Ion Plating

Ti_0.7Al_0.3N and Ti_0.68Al_0.30Hf_0.02N coatings were deposited on 1Cr–11Ni–2W–2Mo–V stainless steel by arc-ion plating (AIP), and their oxidation and corrosion performance were characterized using TGA, TEM, SEM/EDS, EPMA and XRD. The oxidation behavior of the coatings at 800 °C for up to 100 h was investigated, and the results showed that the introduction of hafnium into the Ti_0.7Al_0.3N coating dramatically improved the oxidation-resistance of the coating in air. Compared to the Ti_0.7Al_0.3N coating, the presence of Hf in the nitride coating promoted the outward diffusion of Al, and suppressed the outward diffusion of Ti and inward diffusion of O. The Ti_0.7Al_0.3N coating was completely oxidized and formed a layered scale after oxidation at 800 °C for 20 h in air. Meanwhile, local serious oxidation of the substrate occurred. Corrosion tests of the coatings with a NaCl deposit in wet oxygen at 650 °C for 10 h were also conducted, and the results showed that the Ti_0.7Al_0.3N coating suffered serious local corrosion, while a thin and dense scale formed on the surface of the Ti_0.68Al_0.30Hf_0.02N coating, and its anti-corrosion performance was remarkably enhanced.     

An Investigation into the Corrosion Behavior of MgO/ZrO<Subscript>2</Subscript> Nanocomposite Coatings Prepared by Plasma Electrolytic Oxidation on the AZ91 Magnesium Alloy

Plasma electrolytic oxidation (PEO) of AZ91 Mg alloys was performed in ZrO₂ nanoparticles containing Na₂SiO₃-based electrolytes. The phase composition and the microstructure of PEO coatings were analyzed by x-ray diffraction and scanning electron microscopy followed by energy dispersive spectroscopy. Pitting corrosion properties of the coatings were investigated using cyclic polarization and electrochemical impedance spectroscopy tests in a Ringer solution. The results showed the better pitting corrosion resistance of the composite coating, as compared to the oxide one, due to the thickened inner layer and the decrease in the surface defects of the composite coating. Also, the PEO process decreased the corrosion current density from 25.06 µA/cm² in the Mg alloy to 2.7 µA/cm² in the oxide coating and 0.47 µA/cm² in the composite coating.     

Effects of ZrB2 and SiC dual addition on the oxidation resistance of graphite at 1600–2000 °C

The effects of ZrB₂ and ZrB₂ + SiC additions on the oxidation kinetics of graphite at 1600–2000 °C in air were investigated. The ZrB₂ + SiC dual addition improves the oxidation resistance of graphite more effectively than the ZrB₂ single addition, because the oxide scale formed on C–ZrB₂–SiC is denser and thinner due to the existence of glassy SiO₂. As the oxidation temperature increases, the oxidation rate of C–ZrB₂–SiC gradually increases and oxide scales with layered microstructures form on its surface due to the greatly enhanced active oxidation of SiC at higher temperatures.     

Protective Al-rich mullite coatings on Si- based ceramics against hot corrosion at 1200 °C

The hot-corrosion behavior of uncoated SiC, bulk mullite and CVD grown mullite coatings in contact with Na₂SO₄ were investigated at 1200 °C. The range of thermodynamic activity of Na₂O (10^− 4 to 10^− 6) simulated in this study makes the oxide very basic leading to high reaction rates with SiO_2. Uncoated SiC corroded severely, forming various Na₂O·SiO₂ compounds with a significant weight gain. Even though the thermodynamic activity of silica is reduced in mullite, several compounds in Na₂O·SiO₂·Al₂O₃ system were formed in case of bulk mullite. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO₂ activity, offered protection to the underlying SiC in corrosive environments. As predicted correctly by thermodynamic calculations, the trend in weight gain with the coated SiC samples followed the theoretically calculated SiO₂ activity in Al-rich mullite.     

Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

Development and Investigation on New Composite and Ceramic Coatings as Possible Abradable Seals

To improve gas turbine performance, it is possible to decrease back flow gases in the high-temperature combustion region of the turbo machine by reducing the shroud/rotor gap. Thick and porous thermal barrier coating (TBC) systems and composite CoNiCrAlY/Al₂O₃ coatings made by air plasma spray and composite NiCrAlY/graphite coatings made by laser cladding were studied as possible high-temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al₂O₃ and NiCrAlY/graphite coatings after 1000 h at 1100 °C do not show noticeable microstructural modification. The oxidation resistance of the new composite coatings satisfied original equipment manufacturer (OEM) specifications. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specifications for abradable coatings, the hardness evaluation suggests that these kinds of coatings must be used with abrasive-tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.     

Effect of SiC on the corrosion behaviour of chromium coatings

Abstract

Cr–SiC composite coatings were plated in Cr(VI) baths containing SiC green powder. Analysis of Cr–SiC coatings has demonstrated a relatively low percentage (about 0·1–0·9 wt-%) of the particles incorporated into the Cr coating. XPS data suggest that the surface of SiC was covered with a layer of SiO₂ formed as a result of oxidation of SiC in the Cr(VI) bath. The corrosion behaviour of Cr–SiC composite coatings in an acidic sulphate solution was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy techniques. Impedance spectra were recorded at different times of exposure to the sulphate solution and the data interpreted using various equivalent circuit models. The best fit of the experimental data corresponded to a R ₁(Q ₁ R ₂)(Q ₂[R ₃ W]) circuit. The simulation of EIS data with the proposed equivalent circuit model made it possible to obtain a quantitative evaluation of some parameters reflecting the corrosion behaviour of the samples studied at the solution/electrode interface. Bode plots have shown that at the beginning of immersion the phase angle showed two peaks. It is suggested that the high frequency peak represents the dielectric characteristic of the protective oxide film on the surface of coating and the low frequency peak corresponds to the substrate/electrolyte interface via pinholes. The data obtained suggest that the change in the phase angle with frequency applied qualitatively represents the processes occurring in the coating and at the steel/solution interface via pinholes. Analysis of the data suggests that SiC particles enhance the corrosion resistance of the chromium coating as a result of the structural characteristics of the coatings obtained; namely, the size and number of pores.     

Al, Si, and Al�CSi Coatings to Improve the High-Temperature Oxidation Resistance of AISI 304 Stainless Steel

Oxidation resistance of chromium steels is due to the formation of Cr₂O₃ on the surface. However, this surface layer destabilizes above 1,000 °C and does not protect the metal. In this study, three types of coatings were applied to AISI 304 stainless steel (SS), and the microstructure and oxidation resistance of the coatings were investigated. Aluminum coating, silicon coating, and the codeposition of Al and Si were deposited on an SS substrate by the pack cementation method. The microstructure of the samples was then examined by SEM and EDS, and phases were identified by XRD. The oxidation resistance of these samples was studied in air at 1,050 °C. The results showed that the best resistance to oxidation was obtained, in order, from the codeposition of Al?CSi, Al coating, and Si coating.     

Oxidation Resistant Ce-modified Silicide Coatings Prepared on Ti–6Al–4V Alloy by Pack Cementation Process

Cerium-modified silicide coatings were prepared on Ti–6Al–4V by pack cementation. The effects of different kinds of activators (NaCl, AlF₃, AlCl_3, and NH₄Cl) and pack CeO₂ concentrations (1, 3, and 5 wt%) on the coating structures were studied. The results show that the coatings were mainly composed of a TiSi₂ outer layer, a TiSi middle layer, a Ti₅Si₄ inner layer and a 1–2 μm thick Ti₅Si₃ interdiffusion zone. NH₄Cl was a more suitable activator for preparing the Ce-modified silicide coating on Ti–6Al–4V, based on the coating microstructure and growth rate. The coating thickness decreased with increasing CeO₂ concentration in the pack. Oxidation tests at 800 °C in air showed that the Ce-modified silicide coating showed improved oxidation resistance compared to both the uncoated alloy and the pure silicide coating. A dense, but thick oxide scale formed that was composed of a TiO₂ outer layer and a SiO₂ inner layer.     

Microstructures and Properties of the C/Zr-O-Si-C Composites Fabricated by Polymer Infiltration and Pyrolysis

A way to improve the ablation properties of the C/SiC composites in an oxyacetylene torch environment was investigated by the precursor infiltration and pyrolysis route using three organic precursors (zirconium butoxide, polycarbosilane, and divinylbenzene). The ceramic matrix derived from the precursors at 1200 °C was mainly a mixture of SiC, ZrO₂, and C. After annealing at 1600 °C for 1 h, ZrO₂ partly transformed to ZrC because of the carbothermic reductions and completely transformed to ZrC at 1800 °C in 1 h. The mechanical properties of the composites decreased with increasing temperature, while the ablation resistance increased due to the increasing content of ZrC. Compared with C/SiC composites, the ablation resistance of the C/Zr-O-Si-C composites overwhelms because of the oxide films which formed on the ablation surfaces. And, the films were composed of two layers: the porous surface layer (the mixture of ZrO₂ and SiO₂) and the dense underlayer (SiO₂).