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 Characteristics and Oxidation Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and subsequently heat-treated at 1050 °C for 4 h in a vacuum environment. The microstructural characteristics and oxidation behavior of CoNiCrAlY coatings were investigated. The as-sprayed coating exhibited low porosity and oxygen content. The high plastic deformation of the sprayed particles led to significant refinement of γ-matrix and dissolution of β-(Ni,Co)Al phase in the as-sprayed coating. After heat treatment, the single phase (γ) in the as-sprayed coating was converted into a γ/β microstructure, and a continuous single α-Al₂O₃ scale was formed on the coating surface. Vacuum heat treatment can postpone the formation of spinel oxides within 100 h. After being oxidized at 1050 °C for 400 h, the heat-treated coating exhibited better oxidation resistance than the as-sprayed coating. The reduced growth rate of the oxide scale and the suppression of the formation of spinel oxides can be attributed to the vacuum heat treatment, as well as the intrinsic microstructure of the cold-sprayed coating. Finally, the effects of the microstructural changes induced during the cold spraying process on the growth of the thermally grown oxide and the oxidation mechanisms of the CoNiCrAlY coatings were discussed.     

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

In this study, high velocity oxy-fuel (HVOF) and vacuum plasma spraying (VPS) coatings were sprayed using a Praxair (CO-210-24) CoNiCrAlY powder. Free-standing coatings underwent vacuum annealing at different temperatures for times of up to 840 h. Feedstock powder, and as-sprayed and annealed coatings, were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The hardness and Young’s modulus of the as-sprayed and the annealed HVOF and VPS coatings were measured, including the determination of Young’s moduli of the individual phases via nanoindentation and measurements of Young’s moduli of coatings at temperatures up to 500 °C. The Eshelby inclusion model was employed to investigate the effect of microstructure on the coatings’ mechanical properties. The sensitivity of the mechanical properties to microstructural details was confirmed. Young’s modulus was constant up to ~200 °C, and then decreased with increasing measurement temperature. The annealing process increased Young’s modulus because of a combination of decreased porosity and β volume fraction. Oxide stringers in the HVOF coating maintained its higher hardness than the VPS coating, even after annealing.     

Characterization of Fe-Cr-B based coatings produced by HVOF and PTA processes

Two Fe-Cr-B based gas atomized powders, Armacor M and 16, were thermally sprayed on a low carbon steel substrate, using the HVOF (High Velocity Oxygen Fuel) process. Armacor M was also weld-surfaced with the PTA (Plasma Transferred Arc) process. The resultant deposits were subsequently characterized, using X-ray diffraction, scanning electron microscopy, and microhardness measurement. The effects of heat treatment were also studied for HVOF-sprayed coatings. The wear performance of the coatings was investigated by two-body abrasive wear tests. The results of microstructural analysis of as-sprayed deposits revealed oxide and boride phases such as Fe₃O₄ and Cr_1.65F_0.35B_0.96 in an α matrix for the HVOF-sprayed Armacor 16 coating, and only the boride phases (Cr_1.65F_0.35B_0.96 and Cr₂B) in an α matrix for the HVOFsprayed Armacor M coating. PTA weld-surfaced Armacor M coating contains needle-type long precipitates of Cr₂B) and Cr_1.65F_0.35B_0.96, in the α matrix. The hardness of the HVOF-sprayed Armacor 16 coating after heat treatment was substantially less than that of the as-sprayed coating due to the phase transformation from α to γ phase. Heat treatments of the HVOF-sprayed Armacor M coating did not produce changes in phase and its hardness decreased as compared to that of the as-sprayed coating. While HVOF-sprayed and PTA weld-surfaced Armacor M coatings have the same hardness, the latter shows better abrasive wear resistance because of the size and orientation of its boride phases. The broadening of the XRD patterns and the increase in hardness after wear testing suggest that the transformation from the crystalline to the amorphous structure occurred on the uppermost layer during wear testing.     

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr₂O₃ and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.     

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.     

Tantalum oxide coatings as candidate environmental barriers

Tantalum (Ta) oxide, due to its high-temperature capabilities and thermal expansion coefficient similar to silicon nitride, is a promising candidate for environmental barriers for silicon (Si) nitride-based ceramics. This paper focuses on the development of plasma-sprayed Ta oxide as an environmental barrier coating for silicon nitride. Using a D-optimal design of experiments, plasma-spray processing variables were optimized to maximize coating density. The effect of processing variables on coating thickness was also determined. X-ray diffraction (XRD) was use to ascertain that the as-sprayed coatings were comprised of α- and β-Ta₂O₅, but were fully converted to β-Ta₂O₅ after a 1200 °C heat treatment. Grain growth of the Ta₂O₅ followed a time dependence of t ^0.2 at 1200 °C.     

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.     

EIS STUDY ON THE EFFECTS OF HEAT TREATMENT ON CORROSION BEHAVIOR OF NICKEL BASE ALLOY COATING

采用超音速火焰喷涂(HVOF)方法在碳钢基体上制备了NiCrBSi喷涂层, 对包覆样 品进行900 ℃保温2 h或10 min热处理, 利用电化学阻抗谱(EIS)研究了涂层在3.5%NaCl 水溶液中的腐蚀失效过程和耐蚀性的变化规律. EIS图谱分析表明, 喷态涂层抗介质渗透能力 差, 腐蚀20 h后介质可渗达碳钢基体;900 ℃, 2 h保温热处理涂层腐蚀 15 h后EIS谱发生明 显变化, 产生局部腐蚀;而900 ℃, 10 min处理涂层为均匀腐蚀, EIS谱形可长时间保持稳定. 利用等效电路拟合, 获取了涂层界面反应阻力(腐蚀抗力)随时间变化的关系, 显示高温短 时(10 min)热处理涂层的界面反应阻力高且稳定, 其耐蚀性和抗介质渗透能力远优于喷态 涂层, 但2 h保温热处理涂层的耐蚀性比喷态涂层的差. 利用组织结构分析解释了热处理影响 涂层腐蚀行为的原因.     

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.     

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.     

Electrochemical Corrosion of HVOF-Sprayed NiCoCrAlY Coatings in CO<Subscript>2</Subscript>-Saturated Brine

The effect of pre-oxidation treatment and surface preparation of optimized NiCoCrAlY coatings deposited by high-velocity oxygen fuel (HVOF) spraying and exposed to a low-temperature corrosive environment is reported herein. Coatings with two surface finish conditions (as-sprayed and ground) were heat treated under two different oxygen partial pressures (air and argon). The electrochemical corrosion behavior was evaluated in CO₂-saturated brine via potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The results show that the grinding process and pre-oxidation treatment in argon enhanced growth and formation of α-Al₂O₃ scale. The potentiodynamic polarization results show that both pre-oxidation and surface treatment had a positive influence on the corrosion resistance of the coating. The reduction of the porosity and the formation of a dense, uniform, and adherent oxide scale through pre-oxidation treatment led to an increase of the corrosion resistance due to a decrease in active sites and blocking of diffusion of reactive species into the coating. However, according to the results, complete transformation from metastable alumina phases to α-Al₂O₃ in addition to formation and growth of dense α-Al₂O₃ is required to ensure full protection of the coating and base material over long periods.     

The role of microstructure in the mechanism of high velocity erosion of Cr3C2-NiCr thermal spray coatings: Part 2 — Heat treated coatings

In Part 1 of this two part series the variation in erosion mechanisms as a function of as-sprayed coating microstructure was presented. The oxidation resistance of Cr₃C₂-NiCr coatings means that they are used in high temperature applications where WC-Co based systems are no longer suitable. High temperature exposure has been shown to generate microstructural development in these coatings, leading to variations in coating hardness. In this work the effect of such coating development on the high velocity erosion response is investigated. The HVAF and HVOF coatings of Part 1 were heat treated for up to 30 days at 900 °C to generate a range of coating microstructures up to steady state. Erosion was performed under the same conditions as in Part 1. Heat treatment increased the ductility of the NiCr phase, enabling ductile erosion deformation to occur. Intersplat sintering reduced the significance of splat based erosion mechanisms and forced mass loss to become dictated by the phase microstructure. Such developments improved the quantified erosion resistance of both coating systems relative to the as-sprayed conditions. The coating microhardness was shown to be a poor indicator of erosion response across the range of coating microstructures investigated.     

The Effect of Water Vapor on the Initial Stages of Oxidation of the FeCrAl Alloy Kanthal AF at 900 °C

The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 °C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O₂ or O₂ + 40% H₂O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing α-Al₂O₃ while the outer layer initially consists of outward-growing γ-Al₂O₃. A straight and narrow Cr-enriched band is present at the top of the lower (α-Al₂O₃) oxide, corresponding to the original sample surface. In dry O₂, the top (γ-Al₂O₃) layer is converted into a mixture of γ-Al_2−x (Mg,Fe) _x O_3−(x/2), MgAl₂O₄ and α-Al₂O₃. This transformation does not occur in O₂ + H₂O. The initial acceleration of oxidation by H₂O is attributed to the stabilization of the outward-growing γ-alumina layer by the hydroxylation of the γ-Al₂O₃ surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.     

Plasma-sprayed nanostructured Al2O3/TiO2 powders and coatings

Air plasma spray has been used to produce metastable oxide-ceramic powders and coatings, starting with commercially available Al₂O₃/13TiO₂ powder feed. The feed material undergoes rapid melting and homogenization in the high-temperature zone of the plasma jet. A metastablex-Al₂O₃·TiO₂ phase is formed when the molten droplets are quenched on a chilled substrate. The metastable phase has a defect spinel structure and a nanocrystalline grain size. When heated, it decomposes into an equilibrium two-phase structure, consisting ofα-Al₂O₃ andβ-Al₂O₃·TiO₂. Both types of ceramic materials have potential as hard, wear-resistant coatings.     

The hot corrosion behaviour of HVOF sprayed MCrAlX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900°C using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na₂SO₄ salt films. In the presence of NaCl in the Na₂SO₄ salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower Al content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat‐treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short‐term corrosion kinetic measurements and SEM analyses.     

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.     

Properties of Cr3C2-NiCr cermet coating sprayed by high power plasma and high velocity oxy-fuel processes

The structure, hardness, and shear adhesion strength have been investigated for Cr₃C₂-NiCr cermet coatings sprayed onto a mild steel substrate by 200 kW high power plasma spraying (HPS) and high velocity oxy-fuel (HVOF) processes. Amorphous and supersaturated nickel phases form in both as-sprayed coatings. The hardness of the HVOF coating is higher than that of the HPS coating, because the HVOF coating contains more nonmelted Cr₃C₂ carbide particles. On heat treating at 873 K, the amorphous phase decomposes and the supersaturated nickel phase precipitates Cr₃C₂ carbides so that the hardness increases in the HPS coating. The hardness measured under a great load exhibits lower values compared with that measured with a small load because of cracks generated from the indentation. The ratio of the hardnesses measured with different loads can be regarded as an index indicating the coating ductility. The ductility of the HVOF coating is higher than that of the HPS coating. Adhesion strength of the HVOF coating was high compared with the HPS coating. The adhesion of the coatings is enhanced by heat treating at 1073 K, and that of the HVOF coating is over 350 MPa.     

On the Early Stage Isothermal Oxidation of APS CoNiCrAlY Coatings

The aim of this study is to analyze the evolution of microstructural and room temperature mechanical properties of air plasma sprayed (APS) CoNiCrAlY coatings before and after early stage high-temperature oxidation. To this purpose, selected samples were isothermally heat treated at 1110 °C for different durations. Phase analysis and oxide scale characterization were performed using x-ray diffraction. Morphological and microstructural features of as-sprayed and oxidized CoNiCrAlY coatings were analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. After heat treatment, a duplex oxide scale, composed of an inner α-Al₂O₃ layer and an outer spinel-type oxide layer, was observed on coating top-surface. The nanoindentation technique was employed to study the evolution of the mechanical properties. An increase in Young’s modulus and hardness with increasing the aging time was observed, this effect was mainly addressed to the partial densification of coating microstructure.     

Development of a Pre-Oxidation Treatment to Improve the Adhesion between Thermal Barrier Coatings and NiCoCrAlY Bond Coatings

High-temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC), and a yttria (7 wt%)-stabilized zirconia (YSZ) thermal barrier coating (TBC), were thermally cycled to failure for seven different controlled pre-oxidation treatments and one commonly employed industrial pre-oxidation treatment to establish the preferred microstructures of the thermally-grown oxide (TGO) on a NiCoCrAlY bond coating after pre-oxidation. It was found that the failure of the coating system occurred along the TGO/BC interface when the TGO attained a critical thickness, except if a NiAl₂O₄ spinel layer developed contiguous to the TBC/TGO interface. Then, the coating system failed at a smaller TGO thickness along the NiAl₂O₄/α-Al₂O₃ interface. The value for the TGO thickness at failure increased for a larger area fraction of Y-rich oxide pegs at the TGO/BC interface after pre-oxidation. A desired slow-growing oxide layer on the BC surface was promoted when the presence of the oxides NiAl₂O₄, θ-Al₂O₃, Y₃Al₅O₁₂ at the TGO surface after pre-oxidation was avoided. The α-Al₂O₃ layer, which developed adjacent to the BC upon thermal cycling, grew at a low rate if the initial oxide at the onset of oxidation consisted of θ-Al₂O₃ instead of α-Al₂O₃. Based on these results a pre-oxidation treatment is proposed for which the lifetime of the entire coating system during service is enhanced.