Microstructural characterization of plasma sprayed Al2O3-40 wt.% TiO2 coatings on high density graphite with different post-treatments

Graphite is one of the candidate materials proposed for application in pyrochemical reprocessing plants involving aggressive molten chloride environment. Post treatments are promising techniques for the improvement of properties of thermal spray coatings for different industrial applications. In the present work, the effect of post treatments like vacuum annealing (VA) and laser melting (LM) on the microstructure and chemical modification of plasma sprayed Al₂O₃-40 wt.% TiO₂ coatings over high density (HD) graphite substrates has been investigated. When compared with sprayed coatings (SC), VA coatings showed cluster morphology and LM coatings exhibited homogenous microstructure. On laser melted surfaces networks of cracks were observed. XRD studies showed that the metastable γ-Al₂O₃ phase present in the SC is transformed to stable α-Al₂O₃ after post treatments. In LM coatings Al₂TiO₅ phase was more predominant in contrast to SC and VA coatings. The microhardness enhancement was observed in case of LM coating compared to the VA and SC. Due to elimination of coating defects in LM samples, there is a considerable reduction in the surface roughness.     

Fabrication and Properties of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZrO<Subscript>2</Subscript> Composite Coatings

Al₂O₃ /xZrO₂ (where x = 0, 3, 13, and 20 wt.%) composite coatings were deposited onto mild steel substrates by atmospheric plasma spraying of mixed α-Al₂O₃ and nano-sized monoclinic-ZrO₂ powders. Microstructural investigation showed that the coatings comprised well-separated Al₂O₃ and ZrO₂ lamellae, pores, and partially molten particles. The coating comprised mainly of metastable γ-Al₂O₃ and tetragonal-ZrO₂ with trace of original α-Al₂O₃ and monoclinic-ZrO₂ phases. The effect of ZrO₂ addition on the properties of coatings were investigated in terms of microhardness, fracture toughness, and wear behavior. It was found that ZrO₂ improved the fracture toughness, reduced friction coefficient, and wear rate of the coatings.     

Carburization Behavior of Electrodeposited Ni3 Al–CeO2-Base Coatings on Fe–Ni–Cr Alloys

The cyclic carburization of electrodeposited pure and CeO₂-dispersed Ni₃Al intermetallic coatings on Fe–Ni–Cr alloys has been investigated at 850 and 1050°C for periods up to 500 h in a reducing 2%CH₄–H₂ atmosphere. At 850°C, all Ni₃Al-base-coating samples showed excellent carburization resistance and slow mass increases due to the formation of a thin γ-Al₂O₃ scale and a low carbon activity (a _c = 0.73). At 1050°C and a high carbon activity (a _c = 3.21), all coatings are superior to the uncoated Fe–Ni–Cr alloy in terms of carburization resistance. A thin α-Al₂O₃ scale slowly formed on all Ni₃Al coatings effectively blocked the carbon attack. The addition of CeO₂ particles in the Ni₃Al coatings significantly mitigated the cracking of the α-Al₂O₃ scale and the resultant internal oxidation and carburization. For all coatings, Ni-rich particles were found to be formed on the α-Al₂O₃ scale during oxidation, which had led to the deposition of catalytic coke.     

Microstructure of Suspension Plasma Spray and Air Plasma Spray Al2O3-ZrO2 Composite Coatings

Al₂O₃-ZrO₂ coatings were deposited by the suspension plasma spray (SPS) molecularly mixed amorphous powder and the conventional air plasma spray (APS) Al₂O₃-ZrO₂ crystalline powder. The amorphous powder was produced by heat treatment of molecularly mixed chemical solution precursors below their crystallization temperatures. Phase composition and microstructure of the as-synthesized and heat-treated SPS and APS coatings were characterized by XRD and SEM. XRD analysis shows that the as-sprayed SPS coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases, while the as-sprayed APS coating consists of tetragonal ZrO₂, α-Al₂O₃, and γ-Al₂O₃ phases. Microstructure characterization revealed that the Al₂O₃ and ZrO₂ phase distribution in SPS coatings is much more homogeneous than that of APS coatings.     

Microstructural characterization of radio frequency and direct current plasma-sprayed Al2O3 coatings

Microstructures of radio frequency (RF) and direct current (DC) plasma-sprayed Al₂O₃ coatings deposited onto steel substrates were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), electron microprobe analysis (EMPA), polarizing optical microscopy (OM), and transmission electron microscopy (TEM). Because RF and DC plasmas produce different particle heating and acceleration, the morphology, phase structure, and fracture modes of the coatings vary substantially. In the case of RF coatings, a clear lamellar microstructure with relatively thick lamellae was observed, which is due to the large particles and the low particle velocities, with α-Al₂O₃ as the predominant phase and with delamination type of fracture detected on the fracture surface. In contrast, the DC coatings consisted of predominantly metastable γ-Al₂O₃ as well as amorphous phases, with a mixed fracture mode of the coating observed. In spite of limited interfacial interdiffusion detected by EMPA, TEM showed an interfacial layer existing at the interface between the coating and the substrate for both cases. For RF coatings, the interfacial layer on the order of 1 μm was composed of three sublayers, each of which was different in composition and morphology. However, the interfacial layer for the DC coating consisted primarily of an amorphous phase, containing both coating and substrate materials with or without platelike microcrystals; although in some regions a thick amorphous Al₂O₃ layer was in direct contact with the substrate.     

TEM Study of the Initial Oxidation of Ti42Al48Cr8Ag2

A microstructural investigation of oxide scales was carried out by analytical electron microscope (AEM) after the as-cast Ti₄₂Al₄₈Cr₈Ag₂ alloy was exposed to air at 1,173 K for 5, 30 and 180 min, respectively. The results indicated that the formation of oxide scale experiences a transformation from ternary-layer system consisting of an outmost rutile (TiO₂-rich) layer, mid amorphous alumina and inner nitrides to a double-layer of TiO₂ and Al₂O₃-rich with increasing time from 5 to 30 min. In the subsurface zone, a change was also found a metastable Cr-rich new phase with bcc structure of a = 1.371 nm formed in addition to the oxidation for X-phase during 30 min. After 180 min, a continuous Laves-phase layer formed instead at the same position. In addition, the process revealed that amorphous alumina changed to a metastable structure before stable α-Al₂O₃ formed.     

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), K _p, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in K _p following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower K _p values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al₂O₃ layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al₂O₃. The formation of a thin α-Al₂O₃ layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.     

Microstructure and Residual Stress of Alumina Scale Formed on Ti2AlC at High Temperature in Air

Ti₂AlC ternary carbide is being explored for various high temperature applications due to its strength at high temperatures, excellent thermal-shock resistance, and high electrical conductivity. A potential advantage of Ti₂AlC over conventional Al₂O₃-forming materials is the near-identical coefficient of thermal expansion (CTE) of Ti₂AlC and α-Al₂O₃, which could result in superior spallation resistance and make Ti₂AlC a promising option for applications ranging from bondcoats for thermal barrier coatings to furnace heating elements. In this study, isothermal and cyclic oxidation were performed in air to examine the oxidation behavior of Ti₂AlC. Isothermal oxidation was performed at 1000, 1200 and 1400 °C for up to 25 h and cyclic oxidation consisted of 1,000 1-hour cycles at 1200 °C. Characteristics of the oxide scale developed in air, including mass change, residual stress in the α-Al₂O₃ scale, phase constituents and microstructure, were examined as functions of time and temperature by thermogravimetry, photostimulated luminescence, x-ray diffraction, scanning electron microscopy, and transmission electron microscopy via focused ion beam in situ lift-out. Above a continuous and adherent α-Al₂O₃ layer, a discontinuous-transient rutile-TiO₂ scale was identified in the oxide scale developed at 1000 and 1200 °C, while a discontinuous-transient Al₂TiO₅ scale was identified at 1400 °C. The continuous α-Al₂O₃scale thickened to more than 15 μm after 25 h of isothermal oxidation at 1400 °C, and after 1,000 1-hour cycles at 1200 °C, yet remained adherent and protective. The compressive residual stress determined by photoluminescence for the α-Al₂O₃ scale remained under 0.65 GPa for the specimens oxidized up to 1400°C for 25 hours. The small magnitude of the compressive residual stress may be responsible the high spallation-resistance of the protective α-Al₂O₃ scale developed on Ti₂AlC, despite the absence of reactive element additions.     

低气压等离子喷涂TiO2涂层结构的研究

本研究分析了在低气压等离子喷涂条件下,TiO_2涂层的结构受等离子弧喷涂过程中氧分解量所影响。氧分解量很大程度上依赖于离子气中氢流量的大小,此外喷涂室压力也有一定影响。而等离子弧功率和喷涂距离对TiO_2涂层的氧分解量没有多大影响。在离子气中氢流量不变的条件下,涂层中Ti_3O_5量随喷涂室压力(从100×133.322Pa至400×133.322Pa)增加而增加。     

Deposition of Al2O3-TiO2 Nanostructured Powders by Atmospheric Plasma Spraying

Al₂O₃-13%TiO₂ coatings were deposited on stainless steel substrates from conventional and nanostructured powders using atmospheric plasma spraying (APS). A complete characterization of the feedstock confirmed its nanostructured nature. Coating microstructures and phase compositions were characterized using SEM, TEM, and XRD techniques. The microstructure comprised two clearly differentiated regions. One region, completely fused, consisted mainly of nanometer-sized grains of γ-Al₂O₃ with dissolved Ti⁺⁴. The other region, partly fused, retained the microstructure of the starting powder and was principally made up of submicrometer-sized grains of α-Al₂O₃, as confirmed by TEM. Coating microhardness as well as tribological behavior were determined. Vickers microhardness values of conventional coatings were in average slightly lower than the values for nanostructured coating. The wear resistance of conventional coatings was shown to be lower than that of nanostructured coatings as a consequence of Ti segregation. A correlation between the final properties, the coating microstructure, and the feedstock characteristics is given.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Effects of a Steam Pre-treatment on the Formation and Transformation of Alumina Phases on Fe Aluminide Coatings

Several researchers have studied the transformation of metastable aluminas (γ- and θ-) to α-Al₂O₃ but very little is known regarding alumina scales formed under water vapour and their transformation to α-Al₂O₃. Some results have indicated that water vapour increases the oxidation rate of alumina-scale forming coatings but others have found the opposite, that is, that under water vapour the oxidation rates decrease as either transition aluminas do not form or the transformation to α-Al₂O₃ is accelerated. In addition, it was found that χ-Al₂O₃ is the only oxide that forms at the initial stages of oxidation under 100 % steam on Fe–Al coatings at 650 °C. Under these conditions, this oxide is very protective, and slowly transforms onto α-Al₂O₃. A preliminary study of the transformation of χ- to α-Al₂O₃ at 900 °C under laboratory air was carried out. χ-Al₂O₃ was generated by a steam pre-treatment on slurry Fe aluminide coatings deposited on P92.     

Microstructure development of Al2O3–13wt.%TiO2 plasma sprayed coatings derived from nanocrystalline powders

The development of constituent phases and microstructure in plasma sprayed Al₂O₃–13wt.%TiO₂ coatings and reconstituted nanocrystalline feed powder was investigated as a function of processing conditions. The microstructure of the coatings was found to consist of two distinct regions; one of the regions was completely melted and quenched as splats, and the other was incompletely melted with a particulate microstructure retained from the starting agglomerates. The melted region predominantly consisted of nanometer-sized γ-Al₂O₃ with dissolved Ti⁴⁺, whereas the partially melted region was primarily submicrometer-sized α-Al₂O₃ with small amounts of γ-Al₂O₃ with dissolved Ti⁴⁺. The ratio of the splat microstructure to the particulate microstructure and thus the ratio of the γ-Al₂O₃ to α-Al₂O₃ can be controlled by a plasma spray parameter, defined as the critical plasma spray parameter (CPSP). This bimodal distribution of microstructure and grain size is expected to have favorable impact on mechanical properties of nanostructured coatings, as has been observed before.     

Rapid Formation of α-Al2O3 Scale on an Fe–Al Alloy by Pure-Metal Coatings at 900 °C

Rapid formation of an α-Al₂O₃ scale on Fe–50 at.%Al by pure metal thin coatings of Ni, Al, Ti, Cr or Fe was investigated, and the effects of those elements on Al₂O₃-scale evolution were assessed. The oxidation behavior of samples with and without coatings could be divided into two groups: the samples with/without Ni and Al, and those with Ti, Cr and Fe. The mass gains of samples coated with Al and Ni were almost the same as that of non-coated Fe–50 at.%Al alloy. The mass gains of samples coated with Ti, Cr, and Fe were much lower than that of the Fe–50 at.%Al alloy. A stable α-Al₂O₃ scale was found to develop from the beginning of oxidation on the samples coated with Ti, Cr and Fe. However metastable θ-Al₂O₃ remained after long-time oxidation of non-coated and Ni- and Al-coated samples. The direct α-Al₂O₃ scale formation on the samples with Cr or Fe coatings was speculated to be due to sympathetic nucleation of α-Al₂O₃ on the surface of Al-supersaturated Fe₂O₃ for Fe-coated sample, and composition changes from (Cr,Al)₂O₃ to (Al,Cr)₂O₃ for the Cr-coated sample. Initial formation of an oxide having a corundum structure was inferred to provide a nucleation site for precipitation of α-Al₂O₃ without prior formation of a metastable Al₂O₃ scale.     

Incorporation mechanism of colloidal TiO2 nanoparticles and their effect on properties of coatings grown on 7075 Al alloy from silicate-based solution using plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was applied using a pulsed unipolar waveformto produce Al₂O₃−TiO₂ composite coatings from sol electrolytic solutions containing colloidal TiO₂ nanoparticles. The sol solutions were produced by dissolving 1, 3, and 5 g/L of potassium titanyl oxalate (PTO) in a silicate solution. Scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, and Raman spectroscopy were applied to characterizing the coatings. Corrosion behavior of the coatings was investigated using polarization and impedance techniques. The results indicated that TiO₂ enters the coating through all types of micro-discharging and is doped into the alumina phase. The higher level of TiO₂ incorporation results in the decrease of surface micro-pores, while the lower incorporation shows a reverse effect. It was revealed that the higher TiO₂ content makes a more compact outer layer and increases the inner layer thickness of the coating. Electrochemical measurements revealed that the coating obtained from the solution containing 3 g/L PTO exhibits higher corrosion performance than that obtained in the absence of PTO. The coating produced in the absence of PTO consists of γ-Al₂O₃, δ-Al₂O₃ and amorphous phases, while α-Al₂O₃ is promoted by the presence of PTO.     

A Study on the Deposition of Al2O3 Coatings on Polymer Substrates by a Plasma Spray/Micro-Arc Oxidation Two-Step Method

To increase the wear resistance of polymer matrix composites, alumina coatings were deposited on polymer substrates by a two-step method combining plasma spraying and micro-arc oxidation. The microstructures and phase compositions of the processed coatings were investigated for different treatment times. Uniformly distributed pores were found in addition to the presence of various coral-like structures and floccules on the surface of the coatings. The presence of α-Al₂O₃ and γ-Al₂O₃ phases was identified by XRD. The distribution of alumina was analyzed by EDS and is discussed. The maximum bond strength of the coatings was found to be 5.89 MPa. There was little thermal damage in the polymer substrates after the coatings were produced.     

Microstructural evolution during high-temperature oxidation of Ti2AlC ceramics

Microstructural development during high-temperature oxidation of Ti₂AlC below 1300 °C involves gradual formation of an outer discontinuous TiO₂ layer and an inner dense and continuous α-Al₂O₃ layer. After heating at 1400 °C, an outer layer of mixed TiO₂ and Al₂TiO₅ phases and a cracked α-Al₂O₃ inner layer were formed. After heating to 1200 °C and cooling to room temperature, two types of planar defect were identified in surface TiO₂ grains: twins with (2 0 0) twin planes, and stacking faults bounded by partial dislocations. Formation of planar defects released the thermal stresses that had generated in TiO₂ grains due to thermal expansion mismatch of the phases (TiO₂, α-Al₂O₃ and Al₂TiO₅) in the oxide scale. After heating to 1400 °C and cooling to room temperature, crack propagation in TiO₂ grains resulted from the thermal expansion mismatch of the phases in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and the volume changes associated with the reactions during Ti₂AlC oxidation. An atomistic oxidation mechanism is proposed, in which the growth of oxide scale is caused by inward diffusion of O^2? and outward diffusion of Al³⁺ and Ti⁴⁺. The weakly bound Al leaves the Al atom plane in the layered structure of Ti₂AlC, and diffuses outward to form a protective inner α-Al₂O₃ layer between 1100 and 1300 °C. However, the α-Al₂O₃ layer becomes cracked at 1400 °C, providing channels for rapid ingress of oxygen to the body, leading to severe oxidation.     

On the Microstructure of the Initial Oxide Grown by Controlled Annealing and Oxidation on a NiCoCrAlY Bond Coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ+β Ni–21Co–18Cr–22Al–0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO₂) in the range of 0.1–10⁵ Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al₂O₃ layer without the simultaneous formation of NiAl₂O₄ spinel was promoted for oxidation at low pO₂. The formation of metastable θ-Al₂O₃ was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y₂O₃ and Y₃Al₅O₁₂ within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y₂O₃ precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO₂.     

Tribological Behavior of Plasma-Sprayed Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-20 wt.%TiO<Subscript>2</Subscript> Coating

Al₂O₃-20 wt.% TiO₂ ceramic coatings were deposited on the surface of Grade D steel by plasma spraying of commercially available powders. The phases and the microstructures of the coatings were investigated by x-ray diffraction and scanning electron microscopy, respectively. The Al₂O₃-20 wt.% TiO₂ composite coating exhibited a typical inter-lamellar structure consisting of the γ-Al₂O₃ and the Al₂TiO₅ phases. The dry sliding wear behavior of the coating was examined at 20 °C using a ball-on-disk wear tester. The plasma-sprayed coating showed a low wear rate (~4.5 × 10^?6 mm³ N^?1 m^?1), which was <2% of that of the matrix (~283.3 × 10^?6 mm³ N^?1 m^?1), under a load of 15 N. In addition, the tribological behavior of the plasma-sprayed coating was analyzed by examining the microstructure after the wear tests. It was found that delamination of the Al₂TiO₅ phase was the main cause of the wear during the sliding wear tests. A suitable model was used to simulate the wear mechanism of the coating.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.