Ceramic coating on ceramic with metallic bond coating

The change in structure and adhesion strength of the interface by heating in air has been investigated for a plasma- sprayed alumina coating on a ceramic substrate with a 50Ni- 50Cr alloy bond coating. A veined structure composed of NiO, NiCr ₂O₄, and NiAl₂O₄ oxides grew from the bond coating into cracks or pores in the top coating and the alumina substrate after heating at 1273 K for 20 h in air. The NiAl₂O₄ spinel may have formed by the oxidization of nickel, which subsequently reacted with the alumina coating or the substrate. The mechanism of the penetration of the spinel oxides into the cracks or pores is not clear. The adhesion strength of the coating is increased to about 15 MPa after heating at 1273 K for 20 h in air, compared to an as- sprayed coating strength of only 1.5 MPa.     

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.     

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.     

Noncontaminating plasma arc sprayed crucible coatings for containing molten ceramic oxides

A study was performed to assess the suitability of several plasma arc sprayed coatings applied to graphite crucibles for melt processing AI₂O₃ZrO₂, AI₂O₃ Y₂O₃, and AI₂O₃ ceramics. Coatings of W, Ta over W, and Re over W were evaluated. Pressed compacts of AI₂O₃ ZrO₂, AI2O3Y2O3, and AI₂O₂ were each placed in refractory metal-coated graphite crucibles and heated to 2040,2150, and 2200 ‡, respectively. Compatibility of the coating/ceramic oxide systems was evaluated by optical and scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and combustion chromatography. The Ta over W coating system was chemically nonreactive with all three molten oxides studied.     

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Al-Al₂O₃ composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al₂O₃ particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al₂O₃ particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al₂O₃ content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al₂O₃ bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.     

Sintering behavior and electrical property of la0.68ca0.32cr0.97o3

La_0.68Ca_0.32Cr_0.97O₃ separator films were prepared under various sintering conditions. Their bending strength, relative density, electrical conductivity and sintering behavior were investigated by a 4-point bending apparatus, X-ray diffraction (XRD), four terminal method, scanning electron microscooy (SEM) and energy dispersive X-ray spectroscopy (EDX). The bending strength of La_0.68Ca_0.32Cr_0.97O₃ at room temperature increased with the increase of sintering temperature and time. A relative density of more than 94% was obtained by sintering at 1673 K for 5 hours. Thus the present investigation revealed that the sintering of La_{0 68}Ca_0.32Cr_0.97O₃ at low temperatures was greatly assisted by the formation of Ca_a(CrO₄)_b. Ca_a(CrO₄)_b, and incongruently melts and reacts with La_1-xCa_xCrO₃-δ, which enhanced mass transport and resulted in rapid grain growth and densification of the films. Also, the electrical conductivity of La_0.68Ca_0.32Cr_0.97O₃ at 1273 K was more than 100 S/cm after heating at 1673 K for 7 hrs.     

Friction welding of pure titanium and pure nickel

Abstract

The joint tensile strength and metallurgical properties of a friction welded joint of commercially pure Ti and pure Ni has been investigated in as welded and post-weld heat treated conditions. While friction pressure did not significantly impinge on joint tensile strength, joint tensile strength was affected by friction time. A 1–1·5 μm thick interlayer is essential to join pure Ti and pure Ni using friction welding. A maximum joint tensile strength of 450 MPa was achieved and the joint fractured in the Ti original (not heat affected zone) substrate, i.e. the joint efficiency was approximately 112% relative to Ti substrate and 94·5% relative to Ni substrate. The joint tensile strength abruptly decreased as heating temperature was increased to 873 K and/or the Larson-Miller parameter was increased to approximately 19–20 × 10³. The joint tensile strength rapidly decreased with increasing interlayer thickness up to approximately 10 μm, and then remained constant for further increase in interlayer thickness. Four layers occurred at the interface of joints heated to more than 873 K, namely Ti₂Ni, TiNi, TiNi₂, TiNi₃. The fracture of heated joints propagated mainly in the Ti₂Ni layer and/or at the interface between the TiNi and TiNi₃ layers.     

High‐temperature oxidation resistance of Al2O3‐forming heat‐resisting alloys with noble metal and rare earth additions

High‐temperature oxidation behavior of Al₂O₃‐forming heat‐resisting alloys with noble metal (palladium, platinum, gold) and rare earth (yttrium) additions was studied in oxidizing atmospheres (oxygen, oxygen‐water vapor) for 18 ks at 1473, 1573, and 1673 K, by mass gain measurements, amount of spalled oxide, observation of surface appearance, X‐ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Mass gains of all the Al₂O₃‐forming Fe‐20Cr‐4Al alloys increased with increasing oxidation temperatures in both oxidation conditions. After oxidation in oxygen, the mass gains of the alloys with noble metal were almost the same values after any oxidation temperature. The mass gain of the alloys with yttrium decreased with increase in yttrium addition up to 0.1 mass%, and then tended to increase with 0.5 mass% yttrium addition at all oxidation temperatures studied. The amount of spalled oxide from the Fe‐20Cr‐4Al (A4) alloy showed the biggest value at 1573 K‐oxidation, and then decreased in the order of 1473 K, 1673 K. On the other hand, the amount of spalled oxide from the other alloys decreased compared with the A4 alloy. No spalled oxide from 0.5Pt, 0.05Y, and 0.5Y alloys was observed at any oxidation temperature. After oxidation in an oxygen‐water vapor mixture (dew point: 353 K), the mass gain of all the alloys showed similar values to that obtained in oxygen after any oxidation temperature. The amount of spalled oxide from the A4 alloy was about the same after oxidation at 1473 and 1573 K in oxygen, but then was higher when oxidized at 1673 K. The amount of spalled oxide from the other alloys obtained in oxygen–water vapor increased compared with those obtained in oxygen. On the other hand, the amount of spalled oxide from the 0.5Y alloy was zero after any oxidation temperature, and that from the 0.5Pt alloy was also zero after 1673 K‐oxidation.     

Effects of heating temperature and duration on the microstructure and properties of the self-healing coatings

The present work investigates how the heating temperature and duration affect the properties of the self-healing coating on martensitic steels. The coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ is fabricated by means of air plasma spraying. The thermal shock test is performed at 600 °C, 700 °C and 800 °C, respectively, to evaluate the thermal-mechanical stability of the coating. The cross-section morphology of the samples after 1 h, 9 h, 18 h and 30 h of heat treatment shows that the porosity of the coating decreases with the increase of heating duration. The evaluation of electrochemical performance by electrochemical impedance spectroscopy shows that the corrosion resistance of the coating after being heated for 18 h is much better than the other samples due to the process of the inner layer being compacted in the coating. The adhesive tensile strength test between coating and substrate shows that the adhesive strength of the coatings is higher than 9 MPa within 40 h of heat treatment at 600 °C. The residual stress reaches a minimum value after the coating was heated for 9 h at 600 °C, then increases with the heating duration after 9 h. Energy dispersive X-ray analysis at the Vickers indentation indicates that the oxygen content at the crack position increases significantly after being heated for 30 h at 600 °C. These experimental results suggest that this coating can meet the requirement of application under the actual temperature conditions.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

High-temperature oxidation of nickel-based alloys and estimation of the adhesion strength of resulting oxide layers

The kinetics of isothermal oxidation (1100°C) of commercial nickel-based alloys with different content of sulfur (0.22–3.2 wt ppm) is studied. The adhesion strength in a metal/oxide system is estimated as a function of sulfur content and duration of high-temperature exposure. The scratch-test technique is proposed to quantitatively estimate the work of adhesion of resulting oxide films. It is found that the film microstructure is composed of an inner α-Al₂O₃ layer and an outer NiAl₂O₄ spinel layer, which are separated by discrete inclusions of TiO₂. Residual stresses in the oxide film are experimentally determined by X-ray diffraction.     

Microstructure and corrosion resistance of ceramic coating on carbon steel prepared by plasma electrolytic oxidation

Ceramic coating was prepared on Q235 carbon steel by plasma electrolytic oxidation (PEO). The microstructure of the coating including phase composition, surface and cross-section morphology were studied by X-ray diffraction (XRD), Fourier transform infrared spectroscope (FTIR) and scanning electron microscopy (SEM). The corrosion behavior of the coating was evaluated in 3.5% NaCl solution through electrochemical impedance spectra (EIS), potentiodynamic polarization and open-circuit potential (OCP) techniques. The bonding strength between Q235 carbon steel substrate and the ceramic coating was also tested. The results indicated that PEO coating is a composite coating composed of FeAl₂O₄ and Fe₃O_4. The coating surface is porous and the thickness is about 100 μm. The bonding strength of the coating is about 19 MPa. The corrosion tests showed that the corrosion resistance of Q235 carbon steel could be greatly improved with FeAl₂O₄-Fe₃O₄ composite coating on its surface.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.     

Strength and Corrosion Behavior of Fe–25Cr Alloys under Various Strain Rates in Ar and N2–0.1%SO2 at 973 K

The mechanical strength and corrosion behavior of Fe–25Cr alloys were studied in Ar and in N₂–0.1SO₂ at 973 K under strain rates of 2.7 × 10⁻⁴–10⁻⁶/s. A 0.1μm thick adhering Cr₂O₃ layer is formed on the alloy by pre-treatment in Ar. Scales formed in N₂–0.1SO₂ are thicker with poorer adherence to the metal substrate. The tensile strength is higher, about 9–19 MPa, in Ar than in N₂–0.1SO₂. However, the strain-rate sensitivities show similar values in both environments. Deformation is concentrated near the grain boundaries of the alloy, and it is more conspicuous at lower strain rates. Scale growth is rapidly increased by the deformation in N₂–0.1SO₂, and the scale growth rate increases with the increase in strain rates. The scale growth rate is higher at higher strain rates and the scale increases in sulfides.     

Transient Liquid-Phase Diffusion Bonding of Aluminum Metal Matrix Composite Using a Mixed Cu-Ni Powder Interlayer

In the present study, the transient liquid-phase diffusion bonding of an aluminum metal matrix composite (6061-15?wt.% SiCp) has been investigated for the first time using a mixed Cu-Ni powder interlayer at 560?°C, 0.2?MPa, for different holding times up to 6?h. The microstructure of the isothermally solidified zone contains equilibrium precipitate CuAl₂, metastable precipitate Al₉Ni₂ in the matrix of ??-solid solution along with the reinforcement particles (SiC). On the other hand, the microstructure of the central bond zone consists of equilibrium phases such as NiAl₃, Al₇Cu₄Ni and ??-solid solution along with SiC particles (without any segregation) and the presence of microporosities. During shear test, the crack originates from microporosities and propagates along the interphase interfaces resulting in poor bond strength for lower holding times. As the bonding time increases, with continual diffusion, the structural heterogeneity is diminished, and the microporosities are eliminated at the central bond zone. Accordingly, after 6-h holding, the microstructure of the central bond zone mainly consists of NiAl₃ without any visible microporosity. This provides a joint efficiency of 84% with failure primarily occurring through decohesion at the SiC particle/matrix interface.     

Effect of Substrate Temperature on Cold-Gas-Sprayed Coatings on Ceramic Substrates

This study focuses on cold-gas-sprayed deposition of metallic coatings onto ceramic substrates for application in power electronics. In order to achieve the required surface activation for bonding, the substrate is heated during spraying. The effects of substrate temperature on bond strength and coating properties are investigated for cold-gas-sprayed coatings of copper and aluminum on Al₂O₃. It is found that the adhesion strengths of the cold-gas-sprayed coatings and that of the single-impacting particles increase with the increasing temperature and roughness of the substrate. Coatings sprayed on heated substrates show relatively low compressive stresses and low hardness, while their electrical conductivity reaches high values of over 90% IACS. Overall, a higher substrate temperature is found to improve the coating properties significantly.     

Effect of substrate temperature on adhesion strength of plasma-sprayed nickel coatings

We plasma-sprayed nickel coatings on stainless steel and cobalt alloy coupons heated to temperatures ranging from room temperature to 650 °C. Temperatures, velocities, and sizes of spray particles were recorded while in-flight and held constant during experiments. We measured coating adhesion strength and porosity, photographed coating microstructure, and determined thickness and composition of surface oxide layers on heated substrates. Coating adhesion strength on stainless steel coupons increased from 10–74 MPa when substrate temperatures were raised from 25–650 °C. Coating porosity was lower on high-temperature surfaces. Surface oxide layers grew thicker when substrates were heated, but oxidation alone could not account for the increase in coating adhesion strength. When a coupon was heated to 650 °C and allowed to cool before plasma-spraying, its coating adhesion strength was much less than that of a coating deposited on a surface maintained at 650 °C. Cobalt alloy coupons, which oxidize much less than stainless steel when heated, also showed improved coating adhesion when heated. Heating the substrate removes surface moisture and other volatile contaminants, delays solidification of droplets so that they can better penetrate surface cavities, and promotes diffusion between the coating and substrate. All of these mechanisms enhance coating adhesion.     

Oxidation behaviour of the alloy IC-6 and protective coatings

In this work, NiCoCrAlY coatings were deposited on a new Ni-base alloy, IC-6. The oxidation kinetic curves of alloy IC-6, K17 and NiCoCrAlY coatings on alloy IC-6 at 900-1100 °C were obtained. The results indicated that the oxide scales consisted of α-Al₂O₃, NiAl₂O₄, NiO, as well as a small amount of NiMoO₄ and MoO₂. These scales occurred after alloy IC-6 exposure at 900 °C for 100 h. The weight loss occurred when alloy IC-6 were exposed at 1050 and 1100 °C due to the formation of volatile MoO₃. After the NiCoCrAlY coating was deposited, the scales mainly contained α-Al₂O₃, when the specimens were oxidized at 900 °C, and α-Al₂O₃and Cr₂O₃ at 1050 °C. The formation of α-Al₂O₃ and Cr₂O₃ scales on NiCoCrAlY coating was directly responsible for improving oxidation resistance of the alloy IC-6.     

Effect of Vacuum Annealing on the Characteristics of Plasma Sprayed Al2O3-13wt.%TiO2 Coatings

Adhesion strength is one of the critical properties for plasma-sprayed coating. In this study, the plasma-sprayed Al₂O₃-13wt.%TiO₂/NiCrAl coatings were annealed at 300-900?°C for 6?h in vacuum. The tensile bond strength and porosity of the coatings were investigated. The microstructure and the fracture were studied using optical microscopy, scanning electron spectroscopy, and x-ray diffraction. It was found that the tensile bond strength of coatings increased with the increase of annealing temperature until 500?°C, reaching the maximum value of 41.2?MPa, and then decreased as the annealing temperature continues to increase. All coatings presented a brittle fracture and the fracture occurred inside the ceramic coatings except for the coating annealed at 500?°C, which had a brittle-ductile mixed fracture and the fracture occurred at the interface of bond coating and the substrate.     

Analytical Studies on the Behavior of Nickel- and Cobalt-Base Shrouded Plasma Spray Coatings at Elevated Temperature in Air

Nickel- and Cobalt-base coatings were developed on boiler-tube steels by the argon-shrouded, plasma spray process. The cyclic behavior of bare and coated boiler-tube steels has been studied in air at 900 °C. Four types of coatings were used: Ni-22Cr-10Al-1Y (NiCrAlY), Ni-20Cr, Ni₃Al and Stellite-6 (St-6). The NiCrAlY has also been used as a bond coat of approximately 150 μm thick before the final coating in all the cases. Oxidation products analyzed in the scale are mostly oxides of the elements present in the coatings and substrate steels. NiCr₂O₄ and CoCr₂O₄ spinels are the most-common observed phases in the scale for nickel- and cobalt-base coatings. The internal oxidation of coatings and diffusion of iron from the base steel to the upper scale occurred during the study. Cracks formed in the scale and coating and may be due to differences in composition of coatings, bond coat, substrate and oxides formed.