Increasing Oxidation Time Research Articles

Effect of Pulse Duty Cycle and Oxidation Time on Microstructure and Properties of Micro-arc Oxidation Coatings on Ti6Al4V Alloy

Micro-arc oxidation (MAO) technology can in-suit generate coatings with high hardness and strong adhesion on the surface of titanium alloy, thereby greatly improve the application range of titanium alloy. The morphology and properties of coatings are controlled by changing electrical parameters such as duty cycle and oxidation time. This work mainly aims to study the influence of different duty cycles and oxidation times on MAO coatings microstructures and properties of Ti6Al4V alloy under the condition of constant voltage and pulse frequency. The scanning electron microscopy (SEM) and X-ray Diffractometer (XRD) were used to investigate into MAO coatings microstructure and phase composition. The roughness, porosity, thickness, and microhardness of MAO coatings were evaluated respectively by roughness tester, thickness gauge, and microhardness tester. The results show that surface morphology of the MAO coatings change from crater-like micropores to foam-like protrusions with the enhancement of duty cycle and oxidation time. When duty cycle is 50% and oxidation time is 30 min respectively, the ejection of the molten material made the MAO coating surface appear foam-like protrusions, and the surface morphology is seriously damaged. When the oxidation time and the duty cycle are 30 min and 10% respectively, the rutile phase intensity is the maximum, and further increasing the duty cycle will lead to a rutile phase decrease. In a limit range, the hardness rise with increase of oxidation time and duty cycle. The highest hardness is 668.85 HV.

High-temperature Oxidation Properties of TiC-reinforced Mo Matrix Composites with Cr Element Added

This work studied Mo-TiC-xCr (x = 0.0, 0.5, 1.0 wt.%) matrix composites with different component gradients prepared by Spark Plasma Sintering (SPS). The high-temperature oxidation test was conducted at 1200 ℃ in the atmosphere to study the influence of Cr on the high-temperature oxidation behavior of TiC-reinforced Mo matrix composites. The phase composition and morphology of the oxide film were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS), and the goal was to determine the effect of the Cr content on the high-temperature oxidation properties of the Mo-TiC-xCr (x = 0.0, 0.5, 1.0 wt.%) matrix composites. The results showed that with increasing oxidation time, the oxide film on the surface of Mo alloy will crack and fall off easily, and the porosity gradually increases. Oxygen underwent vigorous oxidation through the pores and the inside of the matrix, and a protective MoO2 internal oxide film disappeared. With increasing Cr content, the high-temperature oxidation performance of the Mo alloy improved, the thickness of the oxide film decreased, warping of the surface oxide film was inhibited, the porosity decreased, and volatilization of MoO3 was inhibited. After oxidation, the surface oxide film was mainly composed of TiO2, MoO3, Cr2O3 and Cr2(MoO4)3 phases. The TiC particles dispersed in the matrix were oxidized to TiO2 to inhibit the oxidation of the alloy. The Cr formed a protective oxidation film of Cr2O3, which effectively reduced the porosity and delayed the volatilization of the MoO3, thus improving its oxidation performance.

High temperature oxidation resistance of TNM alloy coated with/without 8YSZ/NiCoCrAlY thermal barrier coatings

The oxidation resistance of the TNM alloy (Ti-44Al-4Nb-1.5Mo (B, Y) (at. %)) coated with/without 8YSZ/NiCoCrAlY thermal barrier coating (TBC) was conducted through cyclic oxidation at 950 ℃. The formation mechanism of oxide scale, interface interdiffusion mechanism, and evolution law of thermal growth oxide (TGO) were systematically investigated. Results show that the weight gains of the TNM alloy coated with/without TBC are 2.45 mg/cm2 and 6.98 mg/cm2, respectively. The Ti-Al-N layer and the (Nb, Mo)-rich layer near the surface play a role in blocking the internal diffusion of air and the external diffusion of metal ions. After a comprehensive analysis of EPMA and SEM results, it is concluded that the diffusion rate of elements in NiCoCrAlY is Ni > Co > Cr. At the same time, the relationship between the interface diffusion layer thickness and oxidation time is built based on Fick’s first law. In addition, as the oxidation time increases, TGO evolves from a dense Al2O3 layer to CS (chromia, spinel oxide) layer, and CSN (chromia, spinel, and nickel oxide) cluster oxides. The mixed oxides induce crack nucleation and growth, thereby reducing TBC lifetime.

Effect of thermal oxidation on helium implanted 316L stainless steel

The effect of thermal oxide layer on He implanted 316L stainless steel was studied to evaluate experimentally how thermal oxidation affects the diffusion and distribution of He in the material. In the case of thermal oxidation of a He implanted sample, with an increase in oxidation time, the max swelling height increases logarithmically as a function of time and finally saturates for all samples except for the lowest dose of implanted He. Concerning TEM results, two void regions are identified. Similar to the calculation, the total irradiated depth was around 250 nm and the large void region was formed around 100–150 nm depth. On the other hand, the small void region was observed immediately under oxide layer from the thermal oxidation. In contrast, there were no voids in the altered zone near the metal/oxide interface in the non-thermal oxidized/He implanted sample. This description of the phenomena was justified using the Kirkendall effect and the Point Defect Model.

Effect of oxide layers formed by thermal oxidation on mechanical properties and NaCl-induced hot corrosion behavior of TC21 Ti-alloy

In the current study on TC21 Ti-alloy (6.5Al–3Mo–1.9Nb–2.2Sn–2.2Zr–1.5Cr), the thermal oxidation formed oxide layers that considerably influenced mechanical properties (hardness and wear). TC21 specimens were oxidized at 600, 700, 800, and 900 °C for 5, 20, and 50 h. NaCl-induced hot corrosion testing was carried out on raw (un-oxidized) and oxidized specimens at 600 and 800 °C for 50 h. The cyclic testing was performed at 600 °C for durations of 5, 10, 20, 30, 40, and 50 h. The average thickness of the layer grew with increasing oxidation time and temperature. A thin oxide layer (average 0.16 µm) was generated by oxidation at a temperature of 600 °C for a duration of 5 h, and at 800 °C, a large oxide layer of 10.8 µm thickness was formed. The most significant surface hardness of 1000 ± 150 HV0.05 was produced for the layer oxidized at 900 °C. On the other hand, the lowest hardness of 360 ± 150 HV0.05 was recorded for the raw materials. Best wear resistance had been achieved for specimens oxidized at 800 °C. During NaCl hot corrosion test, the weight loss of the raw specimen was 6.4 mg/cm2 due to the flaking off of the corrosion product. However, for specimens oxidized at 600 °C for 50 h, weight loss after corrosion testing was 0.54 mg/cm2, less than that of the specimen before corrosion. Oxidized specimens at 800 °C exhibited the best mechanical characteristics and corrosion resistance.

Surface modification of activated carbon cloth with calcium silicate and hydroxyapatite: bioactive composite material

In this study new compounds consisting of activated carbon cloths (ACC) modified with calcium silicate (CaSiO3) were prepared for hydroxyapatite (HAP) generation. ACC samples were oxidized with 8 M HNO3 at different times (15 min and 2 h), to increase oxygenated functional groups. The CaSiO3 fine powders were prepared by chemical coprecipitation using Ca(NO3)2∙4H2O and Si(OC2H5)4, and 5 M NaOH was used as precipitant. The resulting powders were mixed with ethanol by ultrasound stirring and the previously oxidized activated carbon fibers were placed leaving under stirring for 30 min to allow particle dispersion. Once the formed compounds were dried, the samples were immersed in a simulated body fluid (SBF) solution for 21 days in conical tubes at 36.5 °C to allow the HAP formation on the ACC/CaSiO3 composite surface. The results indicated that the increase in oxidation time improves HAP formation on the surface from ACC/CaSiO3 compounds and this bioactive composite may be a potential material for bone regeneration.

Oxidation behaviors and compressive strength evolution of DD6 Ni-based single-crystal superalloy at 1100 ℃

In this work, the oxidation behaviors of DD6 Ni-based single-crystal superalloy and its compressive strength evolution after oxidation were investigated. The results showed that the thermal exposure time has a significant effect on the oxidation products, oxidation kinetics, Al elements diffusion and compressive strength evolution of DD6 superalloy. Firstly, the relationship between grain size change of (Ni0.9,Co0.1)O, the thickness of γ′-free zone and the exposure time was approximate power exponential law, while the relationship between weight change of DD6 superalloy and exposure time was approximate parabolic law since the densification of inner continuous Al2O3 layers hindered the diffusion of oxygen ions. Moreover, the black phases of AlN in the γ′-free zone formed during the oxidation process, and its quantity and size increased first and then decreased with the oxidation time extending, since the inner Al2O3 not only blocked the ion but also reacted with AlN. In addition, with the increase of oxidation time, the compressive yield strength of DD6 in [001] direction first decreased rapidly and then gradually stabilized at about 740 MPa.

Effect of cathode pulse square-wave on the microstructure and corrosion resistance of PEO coatings of AZ91D magnesium alloy prepared in the Na2C4H4O6–KF–Na2SiO3 system

ABSTRACT Plasma electrolytic oxidation coatings were formed in Na2C4H4O6–KF–Na2SiO3 system on AZ91D magnesium alloy by controlling the single-pulse square-wave and bipolar pulse square-wave, respectively. Potentiodynamic polarisation (PD), electrochemical impedance spectroscopy (EIS) and electrochemical noise measurement (EN) were employed to investigate their corrosion behaviour in 3.5% NaCl solution. The results show that with bipolar pulse mode for PEO process, the growth rate of PEO coating on magnesium alloy first decreased and some micropores were present inside the PEO coating. With the increasing oxidation time, the PEO coating became much homogeneous and the porosity of the coating decreased largely, and the corrosion resistance of PEO coating was enhanced. The use of the bipolar pulse mode is beneficial to the improvement of the compactness and corrision performance of magnesium alloy with PEO treatment for the higher voltage stage and longer oxidation time.

Interdiffusion mechanism at the interface between TiAl alloy and NiCoCrAlY bond coating

Surface coatings are the best ways to protect materials from deterioration due to various aspects including corrosion and wear. However, after long-term oxidation, the high temperature oxidation resistance of TiAl-4Nb-1.5Mo (B, Y) alloy coated with NiCoCrAlY bond coating is weakened by the interfacial interdiffusion. Herein, the microstructure evolution, elements interdiffusion, and mechanical properties at the interface between TiAl alloy and NiCoCrAlY bond coating are analyzed through an isothermal oxidation experiment at 950 °C. Results show that the coarsening and coalescence of the β phase occur in the bond coating during high temperature oxidation. Meanwhile, the thickness of the bond coating decreases linearly before oxidation for 100 h. As the oxidation time increases, the consumption rate of the bond coating slows down. Based on the Layer-by-layer phase composition analysis, it is found that the interdiffusion zone forms three layers in sequence: NiAl and Al (Ni, Co)2Ti, AlNi2Ti(Nb, Mo) and Ti3Al, and Al3NiTi2 and TiAl. The TiN phase segregated around the α-Cr as well as the enriched Y element led to generation of microcracks. The TiN phase forms at the phase boundary between the β and γ phases after 250 h of oxidation. The layer composed of AlNi2Ti(Nb, Mo) and Ti3Al phase exhibits a severe hardening phenomenon, which may reduce the long-term service life of the TiAl alloy coated with NiCoCrAlY bond coating.

Investigation on the microstructural evolution and oxidation resistance of Ru-doped NiAlHf coatings at 1200 °C

NiAlHf coatings with and without Ru-doping are deposited on Ni-based single crystal substrate by using arc ion plating (AIP) technology, of which microstructural evolution and oxidation resistance are comparatively investigated. The results reveal that oxide scale formed on NiAlHf coating surface has obvious ridge morphology and HfO2 is mainly distributed at grain boundaries. In contrast, the ridge structure of oxide scale on NiAlHfRu coating surface is not obvious and as-formed RuO2 is mainly distributed inside of grains. In addition, microstructure of oxide scale formed on both NiAlHf and NiAlHfRu coating surfaces is gradually evolved with increasing oxidation time, developing a rumpled surface. The formation of oxide composite, such as NiAl2O4 spinel and NiO, facilitates aggregation and growth of voids and cracks, accelerating premature failure of coating. Moreover, the growth stress and oxidation kinetics are also discussed to evaluate coating oxidation resistance properties. The purpose of research is to investigate microstructural evolution of NiAlHf coatings with assistance of reactive elements during oxidation process, which can contribute to improving oxidation resistance performance and service life of NiAlHf coating.

The anti‐oxidation mechanism of SiCf/SiC–B4C modified with Al2O3 in wet atmosphere based on machine learning

AbstractTo study the anti‐oxidation mechanism of SiCf/SiC–B4C modified with Al2O3 in wet atmosphere, the damage evolution of composites after oxidation was explored by unsupervised machine learning technology (k‐means). Results display that the mean feature values of cluster‐1 (some small cracks in oxidation layer and matrix as well as fiber debonding) in composites modified with Al2O3 are larger than that in virgin after oxidation. Meanwhile, as the oxidation time increases, the concentrated area of cluster‐2 (fiber breakage, cracks in axial and transverse yarns) in both composites gradually shifts toward the direction of high mean feature values. Because Al2O3 can protect the BN from oxidation, the increase of peak frequency and risetime of composites modified with Al2O3 is less than that of virgin. Moreover, the proportion of cluster‐3 (big transverse matrix cracks) in composites modified with Al2O3 is always more than that of virgin. Based on the research results, the corresponding relationship between each damage behavior and each cluster is thoroughly built.

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu3+) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu3+ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu3+ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu3+ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu3+ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu3+ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.

Microstructure evolution and fracture behavior of heat treated enamel coating: Experiment and finite element analysis

The enamel coating (CaO-BaO-MgO-SiO 2 -Al 2 O 3 ) is prepared and performed isothermal oxidation at 900 °C. With the increase of oxidation time, the microstructure and phase evolution of enamel coating are analyzed. Meanwhile, the fracture toughness of heat treated enamel coatings are characterized using the micro-cantilever bending method. The experimental result indicates that the heat treatment can promote the precipitation of oval crystal particles from the enamel itself. With increase of crystallinity, the fracture toughness of heat treated enamel coatings increases from 2.35 to 4.20 MPa·m 1/2 . In addition, finite element method (FEM) with inserting global cohesive elements is adopted to estimate the fracture of micro-cantilever for enamel including elliptical crystals. Both load-displacement curves and failure modes from FEM analyses agree well with those from experiments. Meanwhile, the fracture mechanisms including crack deflection, crack bridging, intergranular and transgranular fracture of the enamel can be revealed. • With the increase of oxidation time, the microstructure evolution and phase of enamel coating are analyzed. • The crystallization of SiO 2 -Al 2 O 3 -CaO-BaO-MgO based enamel coating can improve its fracture toughness. • Finite element method (FEM) with inserting global cohesive elements can simulate the authentic fracture process of enamel coating including crystals. And, this method aids in understanding the fracture behavior of glass-ceramic coating.

Polyethylene glycol derived carbon quantum dots nanofluids: An excellent lubricant for diamond-like carbon film/bearing steel contact

Polyethylene glycol derived carbon quantum dots nanofluids were synthesized via a slow thermal oxidation process. The size of carbon quantum dots was ca. 2 nm and had a decreasing trend with the increase of oxidation time. When used as lubricant in a diamond-like carbon film/bearing steel interface, the nanofluids achieved an ultra-low friction coefficient (μ ≈ 0.02), much lower than that of original polyethylene glycol (μ = 0.12). The worn surface analyses revealed that the nanofluids could effectively inhibit the tribo-oxidation of steel counterpart that occurred under original polyethylene glycol lubrication, and hence reduced the abrasion component of friction. Especially, the poly-hydroxyl carbon dots and oxidized polyethylene glycol species in nanofluids induced a hydroxyl-rich sliding interface via their tribochemical reactions with friction surfaces, which promoted the adsorption of polyethylene glycol molecules on sliding surfaces. Along with the mild corrosion wear of steel counterface, this shifted the boundary lubrication to a mixed/film lubrication regime, thereby achieving an ultra-low friction coefficient. The above results suggest that the polyethylene glycol derived carbon quantum dots nanofluids should be a quite excellent candidate lubricant for solid-liquid synergy lubrication based on diamond-like carbon films.

Study on Oxidation Behavior of Industrial Basic Oxygen Furnace Slag and Recovery of Magnetic Iron‐Containing Phase

The BOF slag contains a lot of iron‐containing species and its efficient recovery is a difficult problem in metallurgy. Herein, the oxidation behavior of industrial BOF slag under air atmosphere is investigated to explore the optimal oxidation conditions for transforming nonmagnetic FeO into magnetic MgFe2O4, including oxidation temperature, oxidation time, and airflow rate. The raw slag and oxidized slags are analyzed using chemical analysis, X‐ray diffraction (XRD) analysis, scanning electron microscope (SEM)‐energy dispersive spectroscopy (EDS) analysis, and Factsage thermodynamic simulation. The results show that the formation of MgFe2O4 phase is closely related to the oxidation temperature, oxidation time, and airflow rate. With the increase in temperature, the conversion rate of MgFe2O4 increases, and reaches the maximum at 1050 °C. However, the conversion rate of MgFe2O4 decreases and part of β‐C2S is converted to α‐C2S above 1050 °C. When the oxidation time and airflow rate increase, the conversion rate of MgFe2O4 increases and reaches the maximum value at 100 min and 1.25 L min−1, respectively. Correspondingly, the grade increased from 16.3% of the raw slag to 27.86% of the magnetic slag, an increase of 11.5%. This is a more convenient and effective method, which can not only bring economic benefits for relevant enterprises but also reduce environmental pollution.

Preparation and performance of (Co, Mn)3O4 spinel coating on Crofer alloy by composite electrodeposition and step‐heating thermal conversion

The preparation and performance improvement of the spinel coating on the surface of ferritic alloy is of wide interest for its application in the metallic interconnects of the solid oxide fuel cells (SOFCs). The CoMn2O3 composite coating is prepared on the surface of the Crofer alloy by the composite electrodeposition method. A step‐heating thermal conversion process is subsequently used to convert the composite coating into a spinel coating, while a direct‐heating process is implemented as the control experiment. Isothermal oxidation tests are then carried out for the prepared samples in order to present the high temperature performance. Scanning electron microscopy (SEM), X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and area‐specific resistance (ASR) are used to analyze the properties of the matrix and coatings. The experimental results reveal that the coatings by step‐heating thermal conversion exhibit better performance of electrical conductivity and oxidation resistance than the coatings by direct‐heating process. Furthermore, with the increase of oxidation time, the atomic proportion of Cr element diffusing to the surface of the matrix is maintained at about 3%–4% for the samples with spinel coatings by step heating, which effectively prevent the Cr volatilization in the matrix. The preparation of spinel coatings on the ferritic alloy by composite electrodeposition and step‐heating thermal conversion is helpful to stimulate new ideas for the development of reliable and cost‐effective metallic interconnect.

Roles of the micro‐arc oxidation coating on the corrosion resistance and mechanical properties of extruded Mg–2Zn–0.5Zr–1.5Dy (mass%) alloy

AbstractAiming to enhance the biodegradable magnesium alloys' poor mechanical properties and corrosion resistance in aggressive chloride mediums, micro‐arc oxidation coatings with oxidation times range from 3 to 15 min were prepared on the surface of extruded Mg–2Zn–0.5Zr–1.5Dy (mass%) alloy. Assessments were carried out using electrochemical experiments, hydrogen evolution experiments, and tensile tests, respectively. The results show that the micro‐arc oxidation coating is porous and mainly consists of the MgO phase. When the oxidation time increases from 3 to 15 min, the thickness and the pore size of the coating increase, while the coating's porosity decreases. The coating with the oxidation time of 15 min can effectively prevent the formation of pitting holes on the surface of the alloy, which remarkably reduces the decay rate of ultimate tensile strength, yield strength, and elongation of the alloy after immersion in simulated body fluid for 0–28 days, and keep the alloy with higher and more stable corrosion resistance. This increases the application possibility of extruded Mg–2Zn–0.5Zr–1.5Dy alloy as a biodegradable material.

A multi-scale prediction model of elastic modulus for ceramic matrix composites considering oxidation damage

The oxidation damage evolution law of ceramic matrix composites prepared by chemical vapor infiltration (CVI) process was analyzed. Considering the effects of temperature and oxidation time, prediction models of elastic modulus about fiber and cell were established based on the distribution of microcracks in matrix and the oxidation process of interface, fiber and matrix. The prediction results show that the tensile elastic moduli of carbon fiber (Cf)/SiC and SiC fiber (SiCf)/SiC composites decrease more obviously with the increase of temperature and oxidation time. The elastic prediction model was verified by the tensile tests of the composites after high temperature oxidation. The error between the prediction results and the test results of SiCf/SiC composites with BN interphase after oxidation at 1000 ℃ for different time is no more than 2%, and the error between the prediction results and the test results of Cf/SiC composites with PyC interphase after oxidation at 700℃ for different time is no more than 7%.

Study on the Pre-Oxidation and Resulting Oxidation Mechanism and Kinetics of Mo-9Si-8B Alloy.

Molybdenum silicon boron alloy is regarded as the next generation of superalloy that is expected to replace nickel-based superalloys. However, the high-temperature oxidation resistance of Mo-Si-B alloy has always been an issue worth studying. In this study, Mo-9Si-8B alloy was prepared via a plasma oscillatory pressure sintering process and pre-oxidized at 1300 °C while maintaining a certain balance of mechanical and oxidation properties. The influence of the oxide protective layer on its performance at high temperature of 1150 °C was explored, the micro-mechanism of its performance and its failure mode of the hinge-locking mechanism was illustrated, and finally, its oxidation kinetics was inferred. In conclusion, pre-oxidized Mo-9Si-8B (at.%) alloy did play a role in delaying the oxidation process during the initial period of cyclic oxidation. However, with the increase of cyclic oxidation time, the improvement of high-temperature oxidation resistance was limited.

Investigation of the oxidation mechanisms of superlattice Cr-CrN/TiSiN-Cr multilayer coatings on Zircaloy substrates under high-temperature steam atmospheres

Cr-CrN/TiSiN-Cr multilayer coatings were deposited on Zircaloy substrates by multi-arc ion plating, and the microstructure and oxidation behavior of the as-deposited coatings, specifically the middle CrN/TiSiN layer, were investigated. The CrN/TiSiN layer exhibited a superlattice structure. The multilayer coatings effectively protected the Zircaloy substrates from oxidation. (Cr1-xTix)2O3 and Cr2TiO5 phases were produced in the coating after oxidation at 1200 °C for 1500 s. With oxidation time increases, most of the Cr atoms in these two phases were replaced by Ti atoms. Oxidation mechanisms of the as-deposited coatings are discussed and a decomposition model of the superlattice CrN/TiSiN layer is proposed.