The role of combustion synthesis in the formation of slurry aluminization

Slurry processes have been investigated for several years as an alternative technique to conventional CVD-derived aluminizing to achieve similar diffusion coatings. This study investigates the coating formation mechanisms during heat treatment processes on pure nickel using slurries, which contain high amounts of micro-sized aluminium particles. At temperatures in the range of 550 °C–1000 °C, aluminium diffuses into the nickel substrate, promoting the formation of intermetallic nickel–aluminide layers. In order to control this process, it is important to understand the mechanisms that occur in the initial stages, when the metallic aluminium powder melts and reacts in contact with nickel. While a conversion of closely pressed nickel–aluminium into aluminide by combustion synthesis is well known, DTA measurements were undertaken to investigate if and when such processes occur in loosely packed powders. Two compositions of nickel with aluminium or eutectic aluminium–silicon alloyed particles were used in order to reveal a potential influence of the melting point of the aluminium alloy particles. The influence of the atmosphere was studied by comparing results during exposure in argon and air. Subsequently, for comparison with the more complex mechanism of slurry aluminizing, both powders were applied to pure nickel substrate and the coating formation during heat treatment at 600 °C, 650 °C, and 700 °C was investigated. The results clearly show the importance of combustion synthesis on the formation of slurry coatings on nickel. Based on the observations, four steps were identified, which lead to the formation of aluminides and the subsequent growth of the aluminide layer: After melting of the aluminium powder, a network of molten aluminium forms within and between the particles, followed by dissolution of nickel in the aluminium melt. If enough Al is available, combustion synthesis between Ni and Al takes place. After this highly exothermic reaction, solid state diffusion controls the further formation of slurry coatings on nickel. Finally, the mechanism was verified by coating industrially used superalloys with the Al-based slurry in air and argon.     

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.     

The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl₂) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.     

The Effects of Annealing on the Mechanical Properties of Electrodeposited Nickel

Attempts have been made to clarify the existing experimental evidence of the embrittlement of Watts nickel after heat treatment at temperatures in excess of 600°C. Electrodeposits have been prepared from a ‘purified’ solution maintained at pH 4 at current densities of 21·5, 270 and 540 A/m². It was found with 21·5 A/m² nickel that the ductility (measured at 20°C) was drastically reduced as the annealing temperature (1 h treatment) increased from 600° to 1000°C, whilst the higher current density materials only registered a small reduction in this property after treatment close to 1000°C. The UTS decreased with increasing annealing temperature in all cases. Metallographic observations revealed that the intergranular embrittlement observed in the low current density nickel was due to the presence of grain boundary discontinuities. These were identified as voids by scanning electron microscopy, and confirmed by specific gravity measurements. Grain boundary voids were also observed in the annealed high current density nickel, but to a far lesser degree. The formation of a gaseous phase within the deposit at temperatures above 600°C is thought to be responsible; hence the voids are in fact gas filled bubbles.     

Investigation of the surface properties of heat treated electroless Ni–P coating

Electroless nickel phosphorus (Ni–P) coatings were synthesised from an acid chloride electrolyte. The synthesised coatings were heat treated at different temperatures, and the surfaces of the heat treated coating were characterised using scanning electron microscopy and X-ray diffraction. Adhesion, wettability, hardness and corrosion behaviour of the coatings were measured. The surface morphology showed the formation of a nano crystalline nickel matrix under heat treated condition. X-ray diffraction analysis of the heat treated samples revealed the recrystallisation of nickel and formation of Ni₃P phase in the coatings. The wettabilty study showed that the as-deposited Ni–P coating is hydrophobic and wettability increases to a maximum of 70.8° contact angle for heat treated temperature of 400°C due to nano crystalline formation. The Rockwell C adhesion test revealed the presence of micro cracks with increase in heat treatment temperature, however the failure is within the acceptability limit. The micro hardness of the Ni–P coating increased with increase in heat treatment temperature. Corrosion potential of the Ni–P coating shifted to a positive potential under heat treated conditions owing to oxidation and precipitation of Ni₃P phase. Decreased corrosion rate and corrosion current density (7.37–0.21?µA?cm^?2) is attributed to heat treatment at 400°C.     

Structure and fracture mechanism of a two-phase chromium–nickel alloy during high-temperature deformation

The structure and mechanical properties of a two-phase Kh65N33V2FT alloy has been studied after tests at room and high temperatures. The morphology of the main phases, namely, solid solutions of nickel in chromium (α) and chromium in nickel (γ), is changed depending on temperature. The lattice parameters of the main phases have been determined. The main mechanism of deformation for this alloy is shown to be grain-boundary sliding. Bulk and grain-boundary diffusion creep and self-regulating diffusion-viscous flow is possible in the γ phase during high-temperature deformation. The heat resistance of this alloy is restricted to 1000°C because of the formation of a γ-phase percolation cluster.     

Cyclic Oxidation of Heat Resisting Steels

Standard cast, heat resisting steels containing 25–29 w/o (weight percent) chromium and 30–36 w/o nickel together with cast alloys containing 45 or 60 w/o nickel plus low levels of aluminium were subjected to cyclic oxidation in air at 1000 and 1150°C. The standard materials suffered rapid weight loss which was somewhat mitigated by the presence of cerium. The 45 w/o nickel alloys were much more resistant and the 60 w/o nickel alloys showed superior resistance to cyclic oxidation. This improvement was due to alumina formation at or near the alloy surface. In the absence of aluminium, alloys underwent subsurface chromium carbide oxidation at a rate independent of alloy chromium content. This effect is shown to be a consequence of rapid oxygen diffusion along internal phase boundaries.     

TC4钛合金表面化学镀Ni-P合金耐磨层研究

利用化学镀方法在TC4钛合金表面成功制备结合力良好的Ni-P合金耐磨层,研究了提高镀层结合力的方法,结合SEM、XRD、EDS等现代物理分析方法分析了不同温度热处理后镀层的组织结构,从而建立不同热处理温度、镀层结构与镀层硬度和耐磨性能的关系。结果表明:二次浸锌活化方法和热处理能显著提高镀层与基体的结合强度,经600℃热处理后镀层结合力达到35N。基材的硬度HV为3780MPa,磨损量为9.6mg,镀态镀层的硬度HV为5760MPa、磨损量为7.7mg。随着热处理温度升高Ni3P相增多,该相的弥散分布使镀层硬度增加,最高硬度HV达到9790MPa,但400℃后硬度降低,这是由于Ni3P相随着热处理温度的继续升高而发生偏聚,使弥散强化程度下降;镀层的磨损量随着热处理温度的升高而减小,说明耐磨性能随着热处理温度的升高而增强,600℃热处理后,虽然镀层晶粒长大、粗化及镀层硬度降低,但此时镀层晶格的完整性最佳,镀层塑性和韧性提高,所以耐磨性能最好。     

Properties and tribological performance of ceramic-base chromium and vanadium carbide composite coatings

In the current study, the surface of AISI D2 steel was coated with the powder blends of ferro-vanadium (Fe-V) and ferro-chromium (Fe-Cr). The coatings were performed using a thermo-reactive diffusion (TRD) treatment by the pack cementation method at three different temperatures (900 °C, 1000 °C, and 1100 °C) and three different durations (1 h, 2 h, and 3 h). The structural and mechanical characteristics of the coatings were compared between the treatment groups. For this aim, the types of the formed phases, the microstructure, the microhardness, the surface roughness, and the wear and friction performance of the coated samples were examined. XRD analysis found composite carbide coatings including chromium carbide (Cr-C), vanadium carbide (V-C), and chromium vanadium carbide (Cr-V-C). The coatings' thickness was 11.3–23.2 μm, hardness was 2100–2500 HV, and average surface roughness (R_a) was 0.286–0.550 μm, depending on the treatment condition. The vanadium containing phase contents of the coatings increased with the elevating coating temperatures. The formed composite coating layers caused a change in the appearance of wear track and wear mechanism on the material surface. After the coating process, there found to be a decrease in the friction coefficient as well as an improvement in the wear resistance up to 7 times. In the composite coating layers, the increase in V-C content in comparison to Cr-C led to an enhancement in wear resistance on the material surface.     

Fabrication of High-Temperature Heat Exchangers by Plasma Spraying Exterior Skins on Nickel Foams

Thermal-sprayed heat exchangers were tested at high temperatures (750 °C), and their performances were compared to the foam heat exchangers made by brazing Inconel sheets to their surface. Nickel foil was brazed to the exterior surface of 10-mm-thick layers of 10 and 40 PPI nickel foam. A plasma torch was used to spray an Inconel coating on the surface of the foil. A burner test rig was built to produce hot combustion gases that flowed over exposed face of the heat exchanger. Cooling air flowed through the foam heat exchanger at rates of up to 200 SLPM. Surface temperature and air inlet/exit temperature were measured. Heat transfer to air flowing through the foam was significantly higher for the thermally sprayed heat exchangers than for the brazed heat exchangers. On an average, thermally sprayed heat exchangers show 36% higher heat transfer than conventionally brazed foam heat exchangers. At low flow rates, the convective resistance is large (~4 × 10^?2 m² K/W), and the effect of thermal contact resistance is negligible. At higher flow rates, the convective resistance decreases (~2 × 10^?3 m² K/W), and the lower contact resistance of the thermally sprayed heat exchanger provides better performance than the brazed heat exchangers.     

Growth Kinetics of In Situ Fabricated Dense NbC Coatings on Gray Cast Iron

In the present study, dense niobium carbide (NbC) coatings are fabricated by in situ techniques on gray cast iron (Fe) substrates at 1150 °C for 5 min, followed by a heat treatment at 990, 1010 and 1030 °C for 5, 10, 15 and 20 min. The microstructure, element composition and metallographic phase of the coating are characterized by scanning electron microscope, energy dispersive spectral and x-ray diffraction, respectively. Results show that the coating consists of NbC and α-Fe phases. NbC coating thickness ranges from 12.51 ± 1.4 to 29.17 ± 2.0 µm depending on the heat treatment temperature and time. In addition, the growth kinetics of dense niobium carbide coatings are estimated. A diffusion model based on Fick’s laws is used to explore the carbon diffusion coefficients of the dense NbC coating in the range of heat treatment temperatures in which the experimental results of the kinetics of the niobium carbide coating are in good agreement with those estimated using diffusion model.     

Fundamental study of generation of interfacial temperatures with metal surfaces and coatings under conditions of sliding friction and mechanical impact

The measurement of surface temperatures generated by sliding friction is discussed and experiments are carried out using dynamic thermocouples to determine surface temperatures arising from sliding friction and mechanical impact. Experimental results over a very wide range of loading and velocity conditions show an appreciable degree of similarity with calculated values using the equations given in part one of this study. Impacts with a 7·3 kg projectile with nickel and nickel coated test heads onto an angled steel anvil, have shown that very high transient surface temperatures can be reached. At velocities above 5 m s^?1, surface temperatures in excess of 1000°C are obtained within a contact duration of less than 2 ms. The lower temperatures recorded in the case of impacts involving nickel plated steel test heads correspond to the temperature difference across the thickness of the coating rather than to the difference in surface temperature and the cold junction.     

Performance of thermal barrier coatings on γ‐TiAl

The current development of new generation gamma titanium aluminides is expected to result in alloy chemistries and microstructures capable of operating at temperatures in excess of 850 °C. Under these conditions, environmental and thermal protection becomes a concern since oxidation might eventually limit the maximum service temperatures achievable. Therefore protective coatings are necessary to exploit the full potential of gamma titanium aluminides at moderately elevated temperatures; however, as yet no coating system tested has proven sufficient performance for long‐term use in automotive and aerospace applications. Thermal barrier coatings (TBCs), typically applied to nickel‐based alloys, offer the potential to increase the service temperature of components by lowering the metal surface temperature in combination with cooling systems. The paper is focussed on development of thermal barrier coatings for gamma titanium aluminides. Different coatings were used for oxidation protection and bond coat application. Substrate specimens were either pre‐oxidized or coated with PVD‐Al₂O₃, TiAlCrYN, or diffusion aluminides. Yttria‐stabilized zirconia TBCs were deposited applying electron‐beam physical vapour deposition. Cyclic and quasi‐isothermal oxidation tests were carried out at 900 °C in air. Post‐oxidation analysis of the coating systems was performed using scanning electron microscopy with energy‐dispersive X‐ray spectroscopy. Zirconia top coats offer a promising thermal protection concept to be applied on γ‐TiAl components. However, high oxidation resistance has to be supplied by protective coatings. Diffusion layers of the TiAl₃ aluminide provided excellent environmental protection because of the formation of a continuous alumina scale. No spallation of the thermal barrier coatings was observed on aluminized specimens during 1000 1‐h cycles and 3000 h of cyclic and isothermal oxidation testing, respectively.     

Evaluation of aluminide diffusion coatings for thermal cyclic oxidation protection of a nickel‐base superalloy

Active element modified aluminide diffusion coatings on IN738 substrates were produced by a new route using continuously cast, aluminum alloy wires consisting of Al‐Y, Al‐Ce, Al‐La and Al‐Si‐Y. The cast wires were used as evaporation sources for ion‐vapour deposition followed by diffusion heat treatments to form nickel aluminide coatings. In order to examine the oxidation resistance of these coatings at elevated temperatures, thermal cyclic oxidation experiments were carried out in air at 1050°C. While all coatings were found to provide significant protection, the Al‐La modified coatings provided the greatest resistance to cyclic oxidation. On the other hand, with coatings based on Al‐Si‐Y alloys, while silicon has a strong ability to reduce the outward diffusion of aluminum, the adverse effect of silicon on mechanical properties of the coating, together with the formation of volatile silicon monoxide, led to catastrophic localized oxidation of the protective coatings.     

Improvement of oxidation resistance in graphite for MgO–C refractory through surface modification

Graphite, used as a carbon source in a conventional magnesia–carbon (MgO–C) refractory, was modified with an acid reagent, resulting in a negative charge on the surface of graphite, to enhance the coating efficiency of aluminum (Al) phase, which was compared to the pristine graphite through its dispersibity and oxidation behavior. The graphite particles with and without surface modification were added, respecticely, in an Al(NO₃)₃ suspension used as a coating reagent, and then filtered at room temperature. The modified graphite shows better disperbility than the pristine graphite, indicating that the coating efficiency of Al precursor is enhanced in the modified graphite. With respect to oxidation behavior, the modified graphite without the coating layer is totally reacted with oxygen at heat treatment of 900 °C in air. However, the Al-coated graphite starts to react with oxygen at heat treatment of 900 °C and fully reacted with oxygen at heat treatment of 1000 °C, showing the gray and white colors, respectively. It is verified that the Al layer is individually and uniformly formed on the surface of graphite and the oxidation resistance of graphite is enhanced owing to the increased coating efficiency of Al precursor.     

Salzschmelzkorrosion gerichtet erstarrter eutektischer CO-Cr-C-Basis Superlegierungen bei hohen Temperaturen

Corrosion of directionally solidified eutectic Co-Cr-C-Superalloys by molton salts at high temperatures The corrosion behaviour of various directionally solidified 73C-class eutectic alloys (Co-Cr₇C₃) and the conventionally cast nickel base alloy IN 738* were investigated using a eutectic sulphate melt (sodium, calcium, and magnesium with 2% sodium chloride). As these materials are designed for high temperature applications, tests were carried out at 900°, 1000°, and 1100°. The additions to 73C were nickel, aluminium, and manganese. Corrosion surface attack for 73C and IN 738 was found to be similar. The grain boundary formation of sulphides and oxides in IN 738 is shown up as a disadvantage when compared with 73C as 73C has no grain boundaries perpendicular to the surface. This could possibly be compensated by directionally solidifying IN 738. A 10% nickel addition to 73C was found to increase the corrosion resistance, a 2% aluminium addition showed a minor improvement, and a 4.7 or 10% manganese addition to 73C to influence the corrosion resistance considerably.     

Preparation and oxidation behaviour of electrodeposited Ni–CeO2 nanocomposite coatings

Ni–CeO₂ nanocomposite coatings with different CeO₂ contents were prepared by codeposition of Ni and CeO₂ nanoparticles with an average particle size of 7 nm onto pure Ni surfaces from a nickel sulfate. The CeO₂ nanoparticles were dispersed in the electrodeposited nanocrystalline Ni grains (with a size range of 10–30 nm). The isothermal oxidation behaviours of Ni–CeO₂ nanocomposite coatings with two different CeO₂ particles contents and the electrodeposited pure Ni coating were comparatively investigated in order to elucidate the effect of CeO₂ at different temperatures and also CeO₂ contents on the oxidation behaviour of Ni–CeO₂ nanocomposite coatings. The results show that the as-codeposited Ni–CeO₂ nanocomposite coatings have a superior oxidation resistance compared with the electrodeposited pure Ni coating at 800 °C due to the codeposited CeO₂ nanoparticles blocking the outward diffusion of nickel along the grain boundaries. However, the effects of CeO₂ particles on the oxidation resistance significantly decrease at 1050 °C and 1150 °C due to the outward-volume diffusion of nickel controlling the oxidation growth mechanism, and the content of CeO₂ has little influence on the oxidation.     

Effect of surface condition and bonding pressure on quality of diffusion bonded high purity copper for linear collider accelerator structures

Abstract

Tests have been carried out to assess the feasibility of diffusion bonding as a fabrication technology for vacuum tight joints in linear accelerator cells for the Next Linear Collider. High purity copper specimens were diffusion bonded over a range of temperatures from 400 to 1000°C, under high (3.45 MPa) and low (3.45 kPa) bonding pressures, and at two different diamond machined surface finishes. Experiments showed that diffusion bonds with strengths equal to, or greater than, that of silver brazed joints could be made at temperatures ≥700°C at the 3.45 MPa bonding pressure, or ≥800°C at the 3.45 kPa bonding pressure. Partial strength diffusion bonds were made at temperatures as low as 400°C at the high bonding pressure, whereas no bonding (zero strength) was observed at temperatures below 700°C at the low bonding pressure. Observations of the fracture surfaces of the diffusion bonded specimens showed that bonding begins by point asperity contact. At low bonding pressures, surfaces created by diamond turning of annealed copper specimens produce higher strength bonds than those created by diamond flycutting of unannealed surfaces, whereas at higher bonding pressures the effect of surface finish was less important.     

Transient Oxide Formation on APS NiCrAlY After Oxidation Heat Treatment

In this study, the transient surface oxide formation on APS NiCrAlY samples was examined after oxidation heat treatment at temperatures between 1000 and 1100 °C. The surface oxides observed on the NiCrAlY surface included a sporadic top layer of NiO and a continuous layer of alumina immediately adjacent to the NiCrAlY coating. Cr-rich oxide in smaller quantities than alumina was also found surrounding the NiO and dispersed within alumina. The alumina assumed whisker-shaped morphology when being observed from the surface but formed continuous film along the NiCrAlY surface. Although the formation of alumina has been observed on all the samples examined in this study, the NiCrAlY sample heat treated at 1050 °C for 5 h generated more continuous α-alumina layer and also contained less surface NiO and Cr-rich oxide. Based on the results, it is believed that NiO developed first upon exposure to an oxidizing environment at high temperature, and stable alumina began to form with the increase in heat treatment temperature and time.     

Ni–Al composite coatings: diffusion analysis and coating lifetime estimation

The interdiffusion of Ni matrix/Al particle composite coatings and nickel substrates was studied using electron probe microanalysis (EPMA) and a one-dimensional diffusion model. The initial coating microstructure was a two-phase mixture of γ(Ni) and γ′(Ni₃Al). The coating/substrate assemblies were aged at 800 to 1100°C for times up to 2000 h. It was found that aluminum losses to the substrate are significant at 1000°C and above. The experimental results for the diffusion of Al into the substrate were compared with model predictions based on a diffusion equation for a finite layer on an infinite substrate. Using combined experimental and model results, the effects of temperature and coating thickness were determined and a rationale was developed for coating lifetime prediction.