Oxidation Behavior Of Coatings Research Articles

Isothermal oxidation behavior of W-based alloy coatings doped with Ta, Cr, Ti, V

Tungsten-based multi-alloy coatings with the addition of Ta, Cr, Ti, and V were prepared by double glow plasma surface alloying (DGPSA) technology. The alloyed specimens showed bulges and pits on the surface with an island growth mechanism. The coating exhibits a single BCC phase. The hardness shows an increasing trend with the increase of alloyed elements. The maximum hardness value of WTaCrTiV is 1748 HV100g. The oxidation behavior of coatings at 800 °C was investigated, with emphasis on the effect of elemental doping on the antioxidant properties. With the addition of Ta, Cr, Ti, and V elements, the antioxidant properties of multi-alloy coatings were effectively improved compared the tungsten matrix. The WTaCrTi coating shows the optimum antioxidant properties with a dense Cr2O3 layer. As the number of elements in multi-alloy increases, the mixing entropy increases, leading to a decrease in the Gibbs free energy, which results in a more stable alloy.

Heat Treatment Induced Structural Transformations and High-Temperature Oxidation Behavior of Al21Co22Cr22Fe13Ni22 High-Entropy Coatings Produced by Non-Vacuum Electron Beam Cladding

In this study, Al21Co22Cr22Fe13Ni22 high-entropy coatings were produced on steel substrates by non-vacuum electron beam cladding of Co, Cr, and NiAl powders. The high-temperature oxidation behavior of the coatings was studied by holding the specimens at 900 °C in air. The microstructure and phase constitution of the samples were studied both in the as-cladded state and after the heat treatment. The microstructure was characterized using light microscopy (LM) and scanning electron microscopy (SEM). Synchrotron X-ray diffraction (SXRD) and energy-dispersive X-ray spectroscopy (EDX) were used to study the phase constitution of the coatings and the “coating-substrate” interface. The coating consisted of disordered bcc (A2), ordered bcc (B2), and disordered fcc (A1) phases. Annealing the coatings for 50 h at 900 °C led to the formation of fcc precipitates in the bcc dendritic grains and a mixture of fcc and σ-phase particles in the interdendritic regions. Needle-like nanosized B2-precipitates were formed due to annealing in the fcc grains at the coating/substrate interface. The microhardness at the top of the as-cladded coating was 585 HV and gradually decreased towards the substrate. A more uniform distribution of the microhardness was obtained after the annealing. Its average value was 441 HV. Rhomboid Cr2O3, needle-like Al2O3, and spinels of a different morphology were found on the surface of the samples after oxidation at 900 °C.

Effect of multi-arc current on the microstructure and properties of TiAlSiN coating on zircaloy-4 alloy

TiAlSiN coatings are deposited on zircaloy-4 by multi-arc ion plating at 50 A, 60 A and 70 A, respectively. The macro-morphology, micro-morphology, chemical composition and phases of the prepared coatings are observed and analyzed, and the high temperature oxidation behavior and adhesion strength of coatings are tested. The results demonstrate that the quantity of large particles on TiAlSiN coating increases gradually as the arc current increases, the coating becomes denser and denser with the porosity decreases gradually, and the content of Al elements in the coating increases gradually with the increase of multi-arc current. The phases of TiAlSiN coatings deposited at different multi-arc currents before oxidation mainly includes Ti3AlN, AlN, Ti2N. Meanwhile, the high temperature oxidation behavior of the coating is also improved gradually and the adhesion strength increases first and then decreases. When the arc current is 60 A, the adhesion strength is up to 23 N which is relatively large. The coatings deposited at 70 A shows the best high temperature oxidation resistance behavior, mainly due to the generation of a large amount of Al2O3 oxide in the coating after oxidation.

Oxidation Behavior of (Mo,Hf)Si2-Al2O3 Coating on Mo-Based Alloy at Elevated Temperature.

To improve the oxidation resistance of Mo-based alloys, a novel (Mo,Hf)Si2-Al2O3 composite coating was fabricated on a Mo-based alloy by the method of slurry sintering. The isothermal oxidation behavior of the coating was evaluated at 1400 °C. The microstructure evolution and phase composition of the coating before and after oxidation exposure were characterized. The anti-oxidant mechanism for the good performance of the composite coating during high-temperature oxidation was discussed. The coating had a double-layer structure consisting of a MoSi2 inner layer and a (Mo,Hf)Si2-Al2O3 outer composite layer. The composite coating could offer more than 40 h of oxidation-resistant protection at 1400 °C for the Mo-based alloy, and the final weight gain rate was only 6.03 mg/cm2 after oxidation. A SiO2-based oxide scale embedded with Al2O3, HfO2, mullite, and HfSiO4 was formed on the surface of the composite coating during oxidation. The composite oxide scale exhibited high thermal stability, low oxygen permeability, and enhanced thermal mismatch between oxide and coating layers, thus improving the oxidation resistance of the coating.

Effect of bias voltage on the microstructure and high‐temperature steam oxidation behavior of Cr coatings prepared by multi‐arc plating on Zircaloy‐4 alloy

AbstractCr coatings were deposited on the Zircaloy‐4 alloy by multi‐arc ion plating under different bias voltages from −50 to −110 V, and the microstructure, mechanical properties, and high‐temperature steam oxidation behavior of these coatings were systematically investigated. The results indicate that the bias voltage significantly affects the crystal orientation, surface morphology, deposition rate, nanohardness, and bonding strength of Cr coating on the Zircaloy‐4 alloy. The high‐temperature oxidation behavior of the Cr coating is closely related to its crystallographic orientation. Compared with the Cr coating with relatively random grain orientations, the Cr coating with (211) preferred orientation has a dense and uniform oxide scale and has the best oxidation resistance in the steam environment at 1200°C.

Formation process and oxidation behavior of MCrAlY + AlSiY composite coatings on a Ni-based superalloy

An AlSiY coating and two MCrAlY + AlSiY composite coatings with different thickness of MCrAlY interlayer were prepared by arc ion plating (AIP) and vacuum annealing. The isothermal oxidation behavior of coatings at 1100 °C for 300 h was also investigated to characterize the microstructure evolution of coatings during annealing. The composite coatings exhibited a better high-temperature oxidation resistance at 1100 °C. The reason is that the addition of MCrAlY layer can greatly contribute to prevent the diffusion of refractory elements to the outer layer. The inhibition of Al inward diffusion can be much stronger, as the Si content increases in the outer layer during annealing.

Oxidation behavior of NiCrAlYSi coatings with Re-based diffusion barriers on two superalloys

The NiCrAlYSi coatings with or without Re-based diffusion barriers (DB) were prepared on K438 and K465 superalloys by electroplating Re-Ni layer, arc ion plating (AIP) NiCrAlYSi coating and following vacuum annealing. Isothermal oxidation behaviors of all coatings were investigated at 1100 ℃ in static air. The results showed that better oxidation resistance was achieved for NiCrAlYSi coatings with Re-based DB and for NiCrAlYSi coatings on K438 than that on K465. Re diffused into NiCrAlYSi coatings during isothermal oxidation and increased spallation resistance of oxide scale. The role of Re-based DB and the effects of substrate compositions were discussed.

Airflow Dynamics and Aluminum Coating Oxidation Behavior under Electric-Arc Spraying with Airflow Pulsations

The paper aims to investigate the airflow dynamics of electric-arc spraying (EAS) with airflow pulsation. The study is focused on the dynamic structure of the airflow with an obstacle in the form of crossed electrodes at the steady and the pulsating air supply (with a frequency up to 120 Hz). The work was fulfilled using a computer simulation, the airflow “shadow” photo visualization, and the microstructure characterization of the coatings formed. It was found that when air flows along the crossed electrodes with a gap of 2 mm, a depression zone appears in the flow with a pressure drop from 0.56 MPa to 0.01 MPa. The air pulsation resulted in a change in a flow’s dynamic structure towards an increase in the length of the depression zone, which covers most of the arc, affecting the liquid metal oxidation. It is established that the frequency of a droplet formation should match the frequency of the airflow pulsation to minimize the metal oxidation. With the air pulsating at about 65 Hz, the oxide volume fraction in the aluminum coating was reduced by 3.6 times compared to the steady airflow. EAS with airflow pulsation has the potential for technological cost reduction.

Microstructure and Oxidation Behavior of NiCoCrAlY Coating With Different Sm2O3 Concentration on TiAl Alloy

In order to improve the oxidation resistance of TiAl alloys, NiCoCrAlY coatings with different amounts of Sm2O3 were prepared by laser cladding on TiAl alloys. The microstructure and oxidation behavior of the coatings were investigated by isothermal oxidation tests at 900°C. The results indicated that the grains of the coatings were refined by Sm2O3. The fine grain reduced the crack sensitivity of the doped coatings and promoted the transformation of θ-Al2O3 to α-Al2O3. Therefore, the internal oxidation of the coatings and the growth rate of the oxide films were reduced. The segregation of Sm at grain boundary inhibited the outward diffusion of Ti, thus reducing the excessive oxidation of Ti. In addition, the oxidation mechanism of the coating was changed from simultaneous diffusion of Al and O to predominant inward diffusion of O. The oxidation resistance of the doped coatings was significantly improved. However, excessive Sm2O3 is detrimental to the improvement of the oxidation resistance. The oxidation resistance of 3 wt% Sm2O3 is the best.

Preparation and high‐temperature oxidation resistance of multilayer MoSi2/MoB coating by spent MoSi2‐based materials

AbstractSpent MoSi2 and MoB were used as raw materials to prepare multilayer MoSi2/MoB coating on molybdenum by the two‐step method of slurry deposition and spark plasma sintering. The results showed dense MoSi2/MoB coating after sintering while penetrated cracks appeared in MoSi2 coating due to coefficient of thermal expansion mismatch between the Mo substrate and coating. After the sintering of MoSi2/MoB coatings, MoB and Mo2B diffusion layers were formed between MoB transition layer and Mo substrate without defects, exhibiting good metallurgical bonding. The high‐temperature oxidation behavior of coatings (1500°C) was also explored. After oxidation of 50 h at 1500°C, lowest mass gain (0.035 mg/cm2) was obtained for MoSi2/MoB coating, and the oxide scale was dense and complete without voids, making the oxygen diffusion at elevated temperature inhibited. Compared with MoSi2 coating under the same oxidation conditions, relatively thinner silica oxide scale was acquired by MoSi2/MoB coating because of the reduction of cracks, and the multilayer coating exhibits better anti‐oxidation properties at high temperature.

Effect of γ' Phase Elements on Oxidation Behavior of Nanocrystalline Coatings at 1050 °C.

To study the effect of γ′ phase elements on the oxidation behavior of nanocrystalline coatings, two comparable nanocrystalline coating systems were established and prepared by magnetron sputtering. The oxidation experiments of the nanocrystalline coatings on the K38G and N5 superalloys were carried at 1050 °C for 100 h, respectively. The chemical composition of the above coatings is the same as the substrate alloy, including the γ′ elements, such as Al, Ta, and Ti. After serving at a high temperature for certain periods, their oxides participated and then affected the oxidation behavior of the coatings. The Al2O3 scale can be formed on the N5 coating, which cannot be formed on the K38G coating. Tantalum and titanium oxides can be detected on the oxide scale, which ruin its purity and integrity.

Preparation, characterization and oxidation behavior of CeO2-gradient NiCrAlY coatings applied by HVOF thermal spraying process

Conventional and CeO2-gradient NiCrAlY coatings (0.5–2.0 wt% of nano-CeO2 from the inner layer to layers adjacent to the outer surface) were produced by the high-velocity oxy-fuel spraying (HVOF) process. Microstructural characteristics, phases analysis and high-temperature oxidation behavior of the functionally graded (FG) NiCrAlY/nano-CeO2 coatings were investigated and compared with those monolayer (ML) NiCrAlY coatings with without CeO2 nanoparticles. The microstructure of the ML- and FG-coatings, as well as processed powders, were characterized using a transmission electron microscope (TEM), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), microhardness testing and Raman spectroscopy. On account of the calculation of oxidation behavior of the coating, the ML- and FG-coatings were exposed to laboratory's air at 1000 °C up to 100 h (short-term exposure) and 5000 h (long-term exposure) and moreover, the weight gain versus oxidation time and oxide growth rate were subsequently examined. Obtained results indicated that FG-NiCrAlY/nano-CeO2 coating had a better oxidation resistance as well as a lower oxide growth rate compared to the rest of the ML-coatings. In addition, increasing oxidation resistance of FG-NiCrAlY/nano-CeO2 coating can be attributed by controlling Al and O diffusion due to the formation of relatively thin, dense, and sticky Al2O3 oxide scale. Besides, all cases of the oxidized ML- and FG-coatings had a parabolic rate behavior owing to their diffusion-controlled oxidation under short- and long-term oxidation tests.

Effect of Al Layer on Oxidation Behavior of NbCrAl Coating

NbCrAl coatings in the presence or absence of Al outer layer were prepared on C103 Nb based alloy and alumina substrate by direct current magnetron sputtering technique. The oxidation performance of coating systems was evaluated by isothermal oxidation tests. The element diffusion and oxidation behavior of the coating systems were investigated. The results indicate that the mass gains of NbCrAl and Al/NbCrAl coatings are 2.02 mg/cm2 and 0.79 mg/cm2 at 1 000 °C for short time. Al/NbCrAl coatings exhibit more effective protection than NbCrAl coatings. The addition of Al layer can improve the oxidation resistance of NbCrAl coatings, which is attributed to the Al layer offering enough Al content to react with oxygen to form a continuous and dense Al2O3 scale on NbCrAl coating and it can inhibit the further internal oxidation.

Microstructure and Oxidation Behavior of Cobalt Diffusional Coating Fabricated on AISI 429 Stainless Steel

The best candidates for interconnects in solid oxide fuel cells are chromia-forming alloys. However, the low oxidation resistance of these materials needs to be solved to obtain better performance of the interconnect. The aim of this study was to coat AISI 429 ferritic stainless steel with a cobalt-based layer using pack cementation technique. For investigating the oxidation behavior of the coated and uncoated AISI 429 steel, three types of oxidation tests were conducted: isothermal oxidation at 750 °C; cyclic oxidation at 750 °C; and oxidation at different temperatures (400–900 °C). The cobalt-based coating and oxide scales formed were studied by X-ray diffraction and scanning electron microscopy. The cobalt-based coating converted to Co3O4, CoFe2O4, and CoCr2O4 spinels during oxidation exposure. The results showed that the cobalt spinels improved oxidation resistance through the decrease in Cr2O3 growth.

High temperature anti‐oxidation behavior of in situ and ex situ nanostructured C/SiC/ZrB2‐SiC gradient coatings: Thermodynamical evolution, microstructural characterization, and residual stress analysis

AbstractNanostructured C/SiC/ZrB2–SiC oxidation protective gradient coating was prepared by a two‐step reactive melt infiltration method. In order to reduce production cost, ZrB2 phase was synthesized by the in situ reactive that included low‐cost ZrO2 and B2O3 powders as raw materials. High‐temperature oxidation behavior of coatings was evaluated by isothermal oxidation test at 1773 K in air for 10 hours. Thermodynamical behavior of the coatings at various temperatures during oxidation test and coating process was predicted by HSC Chemistry 6.0 software. Compressive residual stresses of 36.9 MPa and 41 MPa were calculated for in situ and ex situ coatings by Williamson‐Hall method. After 10 hours of isothermal oxidation at 1773K, in situ and ex situ coatings showed 12.84% and 15.69% of weight losses with oxidation rates of 1.87 × 10−2 g cm−3 h−1 and 0.91 × 10−2 g cm−3 h−1, respectively. These results indicated that the oxidation protection ability of the coating produced by the in situ method was very close to ex situ coating.

Processing and oxidation behavior of Pt-diffused coatings

Pt-diffused coatings were synthesized on Ni10Al alloy by electroplating a thin layer of Pt (1–5 μm) followed by a diffusion treatment in vacuum at 1000–1100 °C. Effects of processing parameters (Pt thickness and vacuum diffusion temperature) on the microstructures and chemical composition of Pt-diffused coatings were investigated. Also, the oxidation behavior of the coatings was studied at 1100 °C. Pt film reveals nano-sized grain size of 93–113 nm. Vacuum diffusion treatment leads to the grain growth. 1-μm Pt film with 1000 °C/1 h vacuum diffusion fabricates nano-sized Pt-diffused coating which exhibits the best oxidation resistance. And a single continuous Al2O3 scale forms on the coating with low Al (~ 9 at%) and Pt (~ 6 at%) contents. Increasing initial Pt thickness would introduce more defects and cracks, leading to a duplex scale and decreasing the oxidation resistance of Pt-diffused coatings.

Effect of negative bias on TiAlSiN coating deposited on nitrided Zircaloy-4

TiAlSiN coatings were deposited on the nitrided Zircaloy-4 by multi-arc ion plating at −100 V, −200 V and −300 V. In this study, the high temperature oxidation behavior of coatings was tested by a box-type resistance furnace in air for 3 h at 800 °C; the macro-morphology of coatings was observed and analyzed by a zoom-stereo microscope; the micro-morphology of coatings was analyzed by a scanning electron microscopy (SEM), and the chemical elements of samples were analyzed by an energy dispersive spectroscopy(EDS); the adhesion strength of the coating to the substrate was measured by an automatic scratch tester; and the phases of coatings were analyzed by an X-ray diffractometer(XRD). Results show that the coating deposited at −100 V shows better high temperature oxidation resistance behavior, at the same time, Al elements contained in the coating is of the highest amount, meanwhile, the adhesion strength of the coating to the substrate is the highest, which is 33N. As the bias increases, high temperature oxidation resistance behavior of the coating weakens first and then increases, the amount of large particles on the surface of the coating increases first and then decreases whereas the density of the coating decreases first and then increases, and adhesion strength of the coating to the substrate increases first and then weakens. The coating's quality is relatively poor when the bias is −200 V.

Oxidation behavior of NiCrAlY coatings prepared by arc ion plating using various substrate biases: Effects of chemical composition and thickness of the coatings

NiCrAlY coatings with various Cr and Al contents were prepared on Ni-based superalloy K438G from a single NiCrAlY target by adjusting the substrate bias during high vacuum arc ion plating. The oxidation behavior of the coatings follows the oxide map laws proposed by Wallwork and Hed rather than the guide line based on Cr/Al=4. As the coatings degrade due to Al depletion, formation of Cr2O3 scales beneath the external α-Al2O3 scales was observed. The mechanism for the oxidation of Ni-Cr-Al alloys is discussed.

Isothermal oxidation behavior of HVAF-sprayed Ni and NiCr coatings in H2-H2O environment

The formation of a protective chromia scale on stainless steels is known to be suppressed by the presence of water vapor in reducing conditions. Thermal spray coatings present a promising approach to improve the durability of steels by transferring the first line of oxidation attack from the bulk steel to the coating. In the present work, isothermal oxidation behavior of Ni and NiCr coatings deposited by High-Velocity Air Fuel (HVAF) process on 304L stainless steel was investigated at 600°C for 168h. Ar-10%H2-20%H2O was selected as the oxidation environment to study the oxidation behavior of the coatings in a low pO2 environment containing H2 and H2O. BIB/SEM, EDS, and XRD techniques were used to characterize the as-sprayed coatings and to investigate the oxidation mechanisms in the coated samples. Results showed that both Ni and NiCr coatings imparted oxidation protection to the 304L substrate. The chromia-forming 304L steel presented a duplex but non-protective oxide scale comprising of an outer Fe3O4 layer on an inner (Fe, Cr)3O4-spinel oxide. In contrast, the NiCr coating presented superior oxidation behavior due to the formation of a continuous, thin, and slow-growing Cr2O3 scale. The Ni coating, too, protected the substrate owing to limited nucleation and growth of the deleterious NiO scale in the low-oxygen-activity environment.

Fabrication of Al–Si coating on Ti–6Al–4V substrate by mechanical alloying

ABSTRACTAl–Si coatings were synthesized on Ti–6Al–4V alloy substrate by mechanical alloying with Al–Si powder mixture. The as-prepared coatings had composite structures. The effects of Al–Si ratio, milling duration and rotational speed on the microstructure and oxidation behavior of coating were investigated. The results showed that the continuity and the anti-oxidation properties of the coating were enhanced with the increase of Al–Si weight ratio. The thickness of the coating largely increased in the initial 5-hour milling process and decreased with further milling. A rather long-time ball milling could result in the generation of microdefects in coating, which had an adverse effect on the oxidation resistance of coating. Both the thickness and the roughness of the coating increased with the raise of rotational speed. The low rotational speed would lead to the formation of discontinuous coating. The rotational speed had a limited effect on the coating oxidation behavior. Dense, continuous and high-temperature protective Al–Si coatings could be obtained by mechanical alloying with Al–33.3 wt.%Si powder at the rotational speed ranging from 250 to 350 rpm for 5 h.