Coatings For High-temperature Applications Research Articles

TiAlSiN(Ag) coatings for high temperature applications: The influence of Ag alloying on the morphology, structure, thermal stability and oxidation resistance

The influence of Ag content on the morphology, structure, hardness (H) and reduced Young's modulus (E), thermal stability, and oxidation resistance of TiAlSiN(Ag) coatings is investigated. The coatings were produced by DC reactive magnetron sputtering, with increasing Ag contents from 0 up to 15.9 at.%. All coatings exhibit a fcc type structure. The coating with 1.5 at.% Ag content displays the highest H and E values as a result of the coating densification and the grain refinement caused by the co-deposition of immiscible phases. Further Ag concentration degrades the H and E values of the coatings due to the incorporation of a softer Ag phase in the structure. Annealing at 800 °C in protective atmosphere for 2 h enhances the coatings crystallinity, with small increments in the H and E values. The dynamic thermal gravimetric oxidation curves reveal that Ag additions do not affect the starting point of oxidation of the coatings (~990 °C). However, it enhances their oxidation resistance. The improved oxidation resistance of the coatings with Ag incorporation is caused by the diffusion of Ag through the oxide scale to the surface, which modifies the composition of the Ti-Al-O layer, producing a more protective Al rich layer.

Effect of material characteristics on thermal, tribological and mechanical properties of high temperature cast iron coatings

Coating is a desideratum part of material science and effort must be entailed for obtaining desired high temperature properties. Properties of high temperature cast iron coatings depend on various factors and each of them must be precisely tailored for meeting the huge industrial demand. The unlimited architecture of novel materials has led to growth of various coating with tailored properties. Single/multi layered structure offers remarkable possibilities to improve coating properties. Cast iron has been increasingly used in gas turbine, internal combustion engine manufacturing industries due to its low cost and structural strength. Numerous researches have been done on cast iron coatings to study the thermal, tribological plus mechanical properties. Novel material for improving the thermal, mechanical and tribological properties of Thermal barrier coatings (TBCs) required an extensive property-characteristics connection realization. Yttria Stabilized Zirconia (YSZ) is the most established coating material for thermal barrier application. YSZ alone cannot withstand the huge industrial demand. YSZ in engine application suffers failure due to non-uniform thermal field, thermal stress and sintering during the cycle of operation. YSZ cannot withstand engine thermal properties including thermal cycle life and thermal durability. Wear rate gets affected as a result of brittle fracture, speed of operation and abrasion. High temperature exposure leads to microcracks and this affects the mechanical properties. TBCs are usually made of ceramic top coat and it suffers failure. Few material characteristics and factors include thermal aging, fracture toughness of the material. This paper summarizes confirmed experimental results by researchers concerning different factors including thermal conductivity, thermal shock resistance, thermal cycle life, wear resistance, coefficient of friction, hardness and material characteristics in regard with cast iron coatings for high temperature applications. This paper also recommends feasible materials and material combinations to overcome certain limitations and failures.

A Scalable Solution Route to Porous Networks of Nanostructured Black Tungsten

This paper studied the feasibility of a new solution-processed method to manufacture black tungsten nanostructures by laser conversion of tungsten hexacarbonyl precursor on the Inconel 625 substrate under argon atmosphere at ambient pressure. The results show that sublimation of the precursor can be prevented if the decomposition temperature (>170 °C) is achieved using the laser heating method. Three different laser powers from 60–400 W were used to investigate the role of laser parameters on the conversion. It was found that lower laser power of 60 W resulted in a mixture of unconverted precursor and converted tungsten. Higher laser powers >200 W resulted in α-W (BCC) in one step without further heat treatment. Different oxygen concentrations from 0.5 ppm to 21 vol% were used in the laser canister to investigate the effect of oxygen concentration on the conversion. It was found that the hard vacuum (>10−4 torr) or hydrogen is not necessary to obtain α-W (BCC). The solar absorptance varied from 63–97%, depending on the amount of precursor deposited on the substrate and oxygen content in the laser canister. This solution-based laser conversion of tungsten precursor is a scalable method to manufacture tungsten coatings for high-temperature applications.

SOL-GEL synthesis of mullite coatings

Metals and their alloys, though having a broad spectrum of applications, cannot fulfill the requirements for the high temperature applications which are being and have been conceived these days. The higher operating temperatures pave the way for further technological advances. In order to sustain temperatures of the order of 1200 °C and higher, ceramic materials come to the fore. While popular ceramics like alumina, silicon carbide, silicon nitride have shown extremely good stability and durability under dry oxidizing conditions, when used in environments found in gas turbine applications, these materials have some limitations. To protect the component from this corrosion, the SiO, has to be protected. With this purpose in mind, Environmental Barrier Coatings have been introduced. These EBCs are primarily corrosion resistant coatings for high temperature applications. For this project, efforts are put together to create a mullite upon SIC EBC substrate utilizing sol–gel creation. For the synthesized mullite characterization has been conducted. It was done through X-ray diffraction (XRD) as well as Scanning Electron Microscopy (SEM). Results of differences in deciding parameters have also been elaborated in detail.

Thermal barrier coatings for high temperature applications – A short review

This review paper aims to portray the work done by various researchers in the domain of thermal barrier coatings. Thermal barrier coatings (TBCs) are provided to minimize the temperature of components in the field of aviation and power generation sector to increase the components performance and efficiency. The review was made in the aspects of methodology adapted for the coating process, thickness of various coatings, materials chosen for coating, anatomy of TBC and mechanism of thermally grown oxides (TGO). Based on the survey, majority of the articles are found employed the process of spray coating for TBC. On the hand, literatures depicts that the preferred materials for TBS is ceramics due to lack of thermal conductivity and high corrosion and wear resistance. The review expose that the TBCs are more resistant to oxidation, corrosion, erosion and it act as a good thermal insulator to protect the material high temperature environment.

Nano-Grain Ni/ZrO2 Functional Gradient Coating Fabricated by Double Pulses Electrodeposition with Enhanced High Temperature Corrosion Performance

Functional gradient materials (FGM) have many excellent properties, and high-performance gradient coating exhibits good prospective application. In this paper, the nano-grain Ni/ZrO2 gradient coating was prepared by double pulse electrodeposition (BP). The surface morphology, crystal structure and electrochemical corrosion resistance of the nano-grain Ni/ZrO2 coating and Ni coating, annealed at different temperatures (400–800 °C), have been compared. In the vertical direction to the substrate surface, the content of ZrO2 increases from 0% to 34.99%. X-ray diffraction (XRD) revealed that the average crystal size of Ni/ZrO2 gradient coating gradually increases from 13.75 to 27.75 nm, and the crystal structure is a face-centered cubic (FCC). The main crystal orientation faces are (111) and (200), while the (200) face exhibited a stronger preferred orientation. Compared with the Ni coating by scanning electron microscopy, the surface morphology of double pulse fabricated Ni/ZrO2 gradient coating was revealed as being smoother, denser, and having fewer pores, and the crystal particle size distribution became narrow. X-ray photoelectron spectroscopy (XPS) shows that the chemical binding states of elements Ni and Zr have been altered. The binding energies of 2p3/2 and 2p1/2 for Ni have been increased, resulting in a higher electron donor state of Ni. The binding energy of 3d5/2 and 3d3/2 of Zr4+ in ZrO2 is decreased, thus becoming better electron acceptors. Chemical bonding has been formed at the Ni/ZrO2 interface. This study demonstrated that double pulse electrodeposition is a promising fabrication method for functional gradient coatings for high temperature applications.

Thermally and photocatalytically active multifunctional porous coatings for high temperature applications.

In this study, the thermal catalytic powder mixtures of Mn, Ce and Co oxides supported on Al2O3 were synthesized. In order to obtain thermally active porous coating on the steel substrate, a binder solution produced via sol-gel method was used to bind the synthesized powder. Additionally by incorporation of TiO2 nanoparticles to previously described thermally active coating, both thermally and photocatalytically active multifunctional surfaces were obtained. The self-cleaning property and the rate of volatile organic compounds (VOC) degradation of the coatings were studied, in which pyrazine, thiazole, maltol, vanillin and toluene were applied as VOC. In TiO2 applications UV illuminations were applied. In the self-cleaning performance tests using soybean oil, the thermally active coating was slightly better than the TiO2 containing coatings. Under UV illumination, the high rates of VOC (photo) degradation was achieved with the thermally and photocatalytically active coating.

Influence of Al and Y content on the oxidation resistance of CrAlYN protective coatings for high temperature applications: New insights about the Y role

CrAlYN hard coatings with two different average Al contents: ∼16 at.% and ∼25 at.%, and Y concentration varying between 1.2 and 5.7 at.% were deposited by direct current reactive magnetron co-sputtering of mixed Cr-Al and Y targets on commercial M2 steel substrates. The samples were heated to 1000 °C in air during 2 h to study their oxidation resistance and thermal stability. The Y content is critical and the coatings present different behaviour depending on the Al content. The best oxidation resistance and thermal stability are obtained for the coating with ∼16 at.% Al and 3.4 at.% Y. The initial film microstructure and the cubic phase (fcc-CrAlN) were retained, and a thin (Cr,Al)2O3 oxide protective scale was formed. At lower Y content (1.2 at.%) iron, from the substrate crosses the coating, while a higher content (4.6 at.%) avoided the iron diffusion at the expense of a thicker oxide scale with new oxide phases. The coatings with higher Al content (∼25 at. %) were not thermally stable at 1000 °C. A good oxidation resistance was obtained for 2.6 at.% of Y although new phases (hcp-AlN and Cr-Fe) were formed. Higher amount of yttrium (∼5.7 at. %) led to the complete oxidation of the coating.

Multi-layer solar selective absorber coatings based on W/WSiAlNx /WSiAlOyNx/SiAlOx for high temperature applications

A simulated and an experimental design of multilayer solar selective absorber coatings for high temperature applications is presented in this study. The optical tandem is composed of four layers deposited by magnetron sputtering on stainless steel substrates at room temperature. The first is a back-reflector tungsten layer, that is followed by two absorption layers based on WSiAlNx/ WSiAlOyNx structure for phase interference. The final layer is an antireflection layer of SiAlOx. The design was conducted with the help of SCOUT software creating a multilayer model based on transmittance (T) and reflectance (R) spectra of individual thin layers deposited on glass substrates. The final design shows simultaneously high solar absorptance α = 96.0% and low emissivity ε = 10.5% (calculated at 400 °C) together with high thermal stability at 450 °C, in air, and 600 °C in vacuum for 400 h and 300 h, respectively.

NiCrSiBC coatings: Effect of dilution on microstructure and high temperature tribological behavior

Protective coatings for high temperature applications have to exhibit good oxidation resistance and microstructure stability. The effective protection of parts with hardfacing techniques depends on the impact of the substrate on the deposited alloy, typically assessed by dilution. This investigation aims to evaluate the effect of dilution on microstructure and high temperature wear of NiCrSiBC coatings processed by plasma transferred arc on a stainless steel substrate. The microstructure was characterized by EDX, XRD, SEM and hardness. High temperature wear behavior was assessed by ball on disc tests with an alumina sphere sliding against the NiCrSiBC coating. Results showed that an increase on dilution suppressed the formation of Cr5B3 borides reducing hardness. Wear behavior was ruled by the combined effect of dilution and test temperature. The effect of dilution is two fold, altering the microstructure and oxidation resistance. At low wear temperatures, the microstructure rules the performance and harder coatings exhibited a better resistance. At high temperature, wear depends on the tribolayer formed. The low oxidation resistant Fe rich coatings form more debris during sliding. The more compact tribolayer formed under these conditions covers most of the area of the wear track and accounts for the enhanced performance of high dilution coatings.

Behaviour of cold sprayed superalloy in incinerator at 900°C

The present study tested the ability of a novel, facile and relatively less reported cold spray technique to develop the coatings for high temperature applications in the boiler tubes. The behaviour of the coating depends upon its microstructure, which further is decided by the coating deposition technique. The coating–substrate system material composition also affects its behaviour. For current investigation, 75Cr3C2–25(Ni–20Cr) coating is cold sprayed on Superni 75 superalloy and its behaviour is studied in a hot corrosive chlorine based aggressive environment of actual waste incinerator at 900°C. The coating microstructure obtained by cold spray process is noticed to be suitable for preventing the transport of environmental corrosive species towards the superalloy substrate.

Characterization of Oxidation Protection Coatings for High Temperature Applications by Means of Nanoindentation and Scanning Electron Microscopy Methods

AbstractOxidation protection coatings are required for thermally highly stressed components such as turbine blades in aircraft engines. Cyclic oxidation experiments were performed on a NiCoCrAlY protective coating of a nickel-based superalloy and hardness and modulus of elasticity (mechanical properties) were determined by nanoindentation before and after the experiments. Microstructure and chemical composition were characterized by means of scanning electron microscopy. Here, the focus is on the phase identification by combining electron backscatter diffraction and energy dispersive X-ray spectroscopy. Findings indicate that the chemical composition strongly influences the mechanical properties.

Ni-Al and NiO-Al Composite Coatings by Combustion-Assisted Flame Spraying

A new, cost-efficient and on-site-applicable thermal spraying process for depositing NiAl metallic overlay or bond-coat coatings for high temperature applications by synthesizing the desired intermetallic phases in-flight during oxy-acetylene flame spraying is presented. Base-metal powders were used for spraying and, by adjusting the spraying conditions, excellent NiAl-based coatings were achieved on various substrates, including mild steel, stainless steel and aluminium alloys. Expensive, pre-alloyed or agglomerated powders are avoided and the method is very promising for in-situ work and repairs. We call the new method “Combustion-Assisted Flame Spraying” (CAFSY) and its viability has been demonstrated at a pre-industrial level for coating metallic substrates. The NiAl-based coatings produced by CAFSY exhibit very high integrity with good adhesion, very low porosity, high surface hardness and high erosion resistance at a substantially lower cost than equivalent coatings using pre-prepared alloy powders.

Protective high temperature coatings with intrinsic depletion sensor

A novel approach for non-destructive residual life-time estimation of coatings for high temperature applications is presented. Coatings of ferromagnetic Cr-doped AlN are applied on a paramagnetic metallic substrate (Ni-base alloy) using a diffusion treatment. The Cr-content in AlN achieved by this procedure is about 2at.%. Applied on a metallic plant component (turbine blade, chemical reactor, boiler tubes etc.) these coatings serve simultaneously two functions. On the one hand they deliver protection against high temperature corrosion because they act as a reservoir phase for alumina and chromia scale formation, while on the other hand due to their magnetic properties they can be used as a depletion sensor for the reservoir of the protective elements. Knowledge about the oxidation kinetics combined with monitoring of the coating's magnetic moment before and after oxidation can allow prediction of its residual life-time. Magnetic investigations of the coating indicate soft ferromagnetic behaviour. The ferromagnetic domains are visualised with the help of magnetic force microscopy (MFM). During oxidation of the coating a change in the microstructure of the alloy subsurface zone takes place. This is characterised by a gradual decrease of the amount of the ferromagnetic Al(Cr)N-phase and by the formation of new ferromagnetic precipitates of Ni and Fe throughout the diffusion zone. Their influence on the net magnetic moment of the coating–alloy system is discussed.

MCrAlY Bondcoats by High-Velocity Atmospheric Plasma Spraying

MCrAlY bondcoats (M = Co, Ni) are used to protect metallic substrates from oxidation and to improve adhesion of ceramic thermal barrier coatings for high temperature applications, such as in land-based and aviation turbines. Since MCrAlYs are prone to take up oxygen during thermal spraying, bondcoats often are manufactured under inert gas conditions at low pressure. Plasma spraying at atmospheric conditions is a cost-effective alternative if it would be possible to limit the oxygen uptake as well as to obtain sufficiently dense microstructures. In the present work, high-velocity spray parameters were developed for the TriplexPro 210 three-cathode plasma torch using MCrAlY powders of different particle size fractions to achieve these objectives. The aims are conflictive since the former requires cold conditions, whereas the latter is obtained by more elevated particle temperatures. High particle velocities can solve this divergence as they imply shorter time for oxidation during flight and contribute to coating densification by kinetic rather than thermal energy. Further aims of the experimental work were high deposition efficiencies as well as sufficient surface roughness. The oxidation behavior of the sprayed coatings was characterized by thermal gravimetric analyses and isothermal heat treatments.

Lubricious zinc titanate coatings for high temperature applications

Processing–structure–tribological property interrelationships are reported for as-deposited and ex situ annealed atomic layer deposition (ALD) nanocrystalline zinc titanate (ZnxTiyOz) coatings. Annealed coatings at 550°C exhibited improved friction (steady-state friction coefficient of ~0.12) and wear rate (1×10−7mm3/N·m) compared to the as-deposited coatings. The ilmenite structure ZnTiO3 (104) texture, which is present at temperatures up to 550°C, was responsible for improved friction and wear compared to other phases/structures, e.g., Zn2TiO4 cubic spinel structure, annealed at T>550°C. Nanoscale sliding-induced plastic deformation was possible when TEM observed ZnTiO3 (104) stacking faults were sheared parallel to the sliding direction resulting in an intrafilm shear velocity accommodation mode. This subsurface aided in shear accommodation and prevented brittle fracture. Since there is predominately (104) slip, i.e., no interacting slip systems that lead to localized hardening and resistance to shear, this further translated to low friction and wear. Thus, it is feasible to generate high temperature lubricious oxides through nanostructural control of coating growth texture and defects.

Study of Ni-20Cr Coatings for High Temperature Applications – A Review

Hot corrosion is a serious problem in various high temperature applications. The thermal spray coatings provides possible solution to counter the problem of hot corrosion in high temperature environment and enhances the life of equipment. Ni-20Cr alloy compositions require long initiation time at which the hot corrosion process moves from the initiation stage to the propagation stage. In this review, the performance of Ni-Cr coatings by different investigators has been presented to understand the mechanism of hot corrosion under different environments.

Effect of Hf on structure and age hardening of Ti–Al-N thin films

Protective coatings for high temperature applications, as present e.g. during cutting and milling operations, require excellent mechanical and thermal properties during work load. The Ti1−xAlxN system is industrially well acknowledged as it covers some of these requirements, and even exhibits increasing hardness with increasing temperature in its cubic modification, known as age hardening. The thermally activated diffusion at high temperatures however enables for the formation of wurtzite AlN, which causes a rapid reduction of mechanical properties in Ti1−xAlxN coatings. The present work investigates the possibility to increase the formation temperature of w-AlN due to Hf alloying up to 10at.% at the metal sublattice of Ti1−xAlxN films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti1−x−yAlxHfyN alloys, as well as the ternary Ti1−xAlxN, Hf1−xAlxN and Ti1−zHfzN systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100°C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti1−x−yAlxHfyN films starts at lower temperatures than the ternary Ti1−xAlxN coating. However, the formation of a dual phase structure of c-Ti1−zHfzN (with z=y/(1−x)) and w-AlN is shifted to ~200°C higher temperatures, thus retaining a film hardness of ~40GPa up to ~1100°C, while the Hf free films reach the respective hardness maximum of ~38GPa already at ~900°C. Additional annealing experiments at 850 and 950°C for 20h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti1−x−yAlxHfyN.

Electrosynthesis of Rare Earth Oxide Coatings for High Temperature Applications

Rare earth oxides are commonly employed as dopants or coatings to improve the development and adherence of alumina scales. However, for practical applications, doping is difficult to control and the use of coatings is preferred. Nevertheless the thickness of such coatings is relatively limited for long term exposures at high temperatures and thicker coatings are hence required. With this in mind, the cathodic electrodeposition technique has been investigated in this work. The results show that deposits of about 20 µm RExOOHycoatings can be obtained on a Ni superalloy in 20 min. The applied current density and time significantly influence the microstructure, thickness, crystallite size and number of oxygen vacancies of the coatings. Their needle-like microstructure is indicative of non negligible amounts of rare earth hydroxides. However, the hydroxide peaks overlap with the oxide peaks in the X-ray diffraction (XRD) patterns. XRD also suggests that the coatings are either amorphous or of nanocrystalline nature, as supported by Raman spectroscopy. Their multicracked morphology is related to the shear stresses between the coating and the substrate, hydrogen bubbling and mostly by drying of the coatings in air. The number of cracks is increased after a heat treatment which also allows full crystallization of the RExOycoating and pre-oxidation (α-Al2O3) of the superalloy. The combined effect of both oxides results in an improved oxidation resistance of the Ni-base superalloy at 1100°C in air.

Modification of NiAl Intermetallic Coatings Processed by PTA with Chromium Carbides

Abstract Equipment that operates under high-temperatures can be protected with NiAl intermetallic coatings because of their metallurgical stability. This study evaluates the effect of chromium carbides added to Ni–Al intermetallic coatings processed by plasma transferred arc (PTA). Mixtures of Ni and Al powders with and without different Cr23C6 fractions (15, 30, and 45 wt %) were deposited by PTA on an AISI 304 stainless steel plate, using two different currents (100 and 150 A). Coatings were evaluated for the presence of welding defects, and the developed microstructures were characterized by X-ray diffraction and scanning electron microscopy. Vickers microhardness and semiquantitative energy dispersive spectroscopy chemical composition were also determined. NiAl and Cr7C3 development was confirmed by X-ray diffraction analysis. A combination of NiAl/Cr–Fe–Ni phases was identified. The hardness was strongly related to the formed phases and volume fractions. The present study shows that PTA hardfacing can be used to produce reinforced intermetallic coatings for high-temperature applications, showing significant advances toward the development of coatings that can withstand severe operation conditions.