Mullite Coatings Research Articles

Effect of Nano-Fly Ash Additive on the Mechanical and Microstructural Properties of Plasma-Sprayed Mullite Coatings

Effect of Nano-Fly Ash Additive on the Mechanical and Microstructural Properties of Plasma-Sprayed Mullite Coatings

Impedance Measurement and Demonstrator Tests of Developed Thermally Sprayed and Sealed Coating Systems for Bearing Applications

To avoid early stage failure of rolling bearings as components of train drives of electrical engines, the bearings must be protected. As state of the art, alumina coatings are applied by atmospheric plasma spraying and subsequently an organic sealer is used to enhance the service life of these bearings under harsh conditions. This includes high applied frequencies and high ambient air humidity. For further improvement, mullite coatings and a newly developed an epoxy‐ and silicon‐based sealer applied by different sealing techniques are investigated to further enhance the electrical insulation of the bearing coatings. Impedance measurements under dry and wet conditions are conducted to analyze the capacitive behavior of the coatings. X‐ray diffraction measurements are used to correlate the influence of the present crystallographic phases of the ceramic coating on the capacitive behavior. In a following step, selected coating systems are applied onto bearings and tested in demonstrator tests. For higher currents of roughly IBearing ≥ 0.6 A, all newly developed coatings exhibit a decreased number of electrical breakdowns compared to the reference. Consequently, choosing this coating system as electrical protection coating for bearings in electrical engines can reduce the damage occurring from electrical current breakdowns.

Plasma sprayed ceramic coatings for the thermal protection of carbon fiber reinforced plastics (CFRP): Thermal and mechanical properties of YSZ, aluminum titanate, cordierite and mullite coatings

Carbon fiber reinforced plastic (CFRP) is widely used in lightweight applications because of high values in specific strength and rigidity. However, the use is limited due to a low resistance against wear, low values in hardness and a low thermal resistance. The application of a ceramic TBC can protect the underlying CFRP and thus enhance the temperature resistance when a thermal load is applied.In the present study, Yttria stabilized Zirconia (7YSZ), Aluminum Titanate, Mullite and Cordierite were applied onto CFRP with cyanate ester resin based matrix by atmospheric plasma spraying. Aluminum was used as bond coat material to ensure adequate adhesion of the coatings on the CFRP.Thermal and mechanical properties of the coatings, such as Young's modulus, hardness, porosity, coefficient of thermal expansion, thermal diffusivity and phase composition were measured. The properties of the ceramic coatings were used to evaluate the materials as a potential thermal protection for CFRP substrates. The results show that Cordierite coatings contain a high amount of amorphous phase and have the lowest thermal conductivity as well as the lowest coefficient of thermal expansion among the investigated material systems.

Comparison of Hot Corrosion Behavior of YSZ, YSZ/Mullite and YSZ/Mullite Gradient Coatings on Nickel Base Superalloy Prepared by Plasma Spray Method

Three types of thermal barrier coatings, namely NiCrAlY/YSZ double-layer, NiCrAlY/YSZ/mullite three-layer and NiCrAlY/YSZ/gradient of YSZ and mullite coatings, were deposited on a nickel-base superalloy using a plasma spray method. Hot corrosion studies of the plasma-sprayed thermal barrier coatings were conducted in 45wt% Na2SO4 + 55 wt% V2O5 molten salt at 1050 °C. Microstructure and morphology of the coatings were examined and compared by scanning electron microscopy and X-ray diffraction. The addition of mullite in thermal barrier coating results in the mullite phase reacting with corrosive salt and oxygen to form NaAlSiO4 phase. In addition to this phase, alumina (Al2O3) and Na2O phases are also formed. The presence of alumina phase (Al2O3) along with Na2O leads to formation of NaAlO2. A dense layer of NaAlO2 can increase the resistance to hot corrosion of the coating compared to YSZ coating.

A Novel Nanostructured Mullite Feedstock for Environmental Barrier Coatings via Atmosphere Plasma Spraying

A novel strategy is proposed to design an exclusive nanostructured mullite spherical feedstock for environmental barrier coatings (EBCs) via atmosphere plasma spraying (APS). The nanostructured mullite spherical feedstocks are successfully obtained by the procedure of spray drying, solid-phase sintering and flame spheroidization. The crystal grain size of nano-mullite increases with annealing temperature. When the temperature is as high as 1500 °C, the reaction of Al2O3 and SiO2 can be fully completed. The average size is 52 nm for the grain of mullite in feedstocks sintered at 1500 °C, which can be reduced from 52 nm to 48 nm by flame spheroidization treatment as well. Meanwhile, the nanostructured spherical feedstocks show high density and good flowability, which is suitable for APS. The mullite coatings is successfully prepared on a SiC substrate by APS, in which both the nanoscale crystals and nanostructures are perfectly kept as designed. Furthermore, there is no crack appeared in the nanostructured mullite coatings. Thus, the nanostructured mullite feedstock is one of promising candidates for the high performance EBCs.

Research status of bond coats in environmental barrier coatings

AbstractBond coats in environmental barrier coatings (EBCs) prevent oxidants from penetrating the substrate, mediate the mismatch of the coefficient of thermal expansion (CTE), and improve the adhesion strength between adjacent layers. However, the development of bond coats is rarely studied systematically. In this paper, the research status of the bond coats in EBCs is introduced in detail, including the materials and deposition methods. Thus far, Si, modified‐Si, mullite, etc., have been employed as bond coats. Nevertheless, visible drawbacks of each bond coat limit their application at high‐temperatures in extreme environments. Si bond coat is easily oxidized and forms thermally grown oxides that form cracks, resulting in delamination, spallation, and failure of EBCs. In the Si–HfO2bond coat, the optimal ratios of Si/HfO2, deposition methods, distribution of Si and HfO2, and oxidation of Si remain completely unsolved. For mullite bond coat, SiO2suffers selective evaporation in the water vapor environment, and the ratios of the Al2O3and SiO2in mullite coatings restrict its service lifetime. HfSiO4is a potential candidate acting as a next‐generation bond coat in EBCs is proposed. Furthermore, choosing reasonable deposition methods is beneficial to improve the performances of the bond coats in EBCs.

Electroless Ni–P Plating on Mullite Powders and Study of the Mechanical Properties of Its Plasma-Sprayed Coating

To effectively improve the properties of a mullite coating and its interfacial bonding with the substrate, a Ni–P layer is deposited on the surface of mullite powders by electroless plating. The original mullite powders and coated mullite powders are then deposited onto stainless-steel substrates by plasma spraying. The growth mechanism of the Ni–P layer during the plating, the microstructures of the coated powders and mullite coating and the properties of the mullite coatings are characterized and analyzed. The results indicate that the Ni–P layer on the surface of the mullite powder has cell structures with a dense uniform distribution and grows in layers on the surface of the mullite powder. The crystallization behavior of Ni-P amorphous layer is induced by heat treatment. Compared to the original mullite coating, the coating prepared by the coated mullite powders has better manufacturability, stronger adhesion to the substrate, lower porosity (7.40%, 65% of that of the original coating), higher hardness (500.1 HV, 1.2 times that of the original coating), and better thermal cycle resistance (two times that of the original coating). The method of preparation of high-temperature thermal barrier coatings with coated mullite powders has a high application value.

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.

Formation of Mullite Coating by Aerosol Deposition and Microstructural Change after Heat Exposure

Formation of Mullite Coating by Aerosol Deposition and Microstructural Change after Heat Exposure

Corrosion behavior of Y2O3-doped mullite-ZrSiO4 coatings applied on C/C–SiC composites in the presence of moisture at temperatures of 1373–1773 K

Commercial mullite powders show great potential for application in environmental barrier coatings. However, most of the commercial mullite powders contain amorphous silica, which produces volatile silicon hydroxide in hot steam. This leads to serious recession of mullite coatings in hot water vapor. In this study, a Y2O3-doped mullite-ZrSiO4 coating was fabricated on C/C–SiC composites using the slurry painting method. The addition of Y2O3 resulted in the elimination of the residual silica and derived-cristobalite in the commercial mullite powder, leading to the formation of a stable mullite-Y2Si2O7–ZrSiO4 coating. This indicates that Y2O3 can efficiently enhance the phase stability of commercial mullite at high temperatures. In addition, the corrosion behaviours of Y2O3-doped mullite-ZrSiO4 coatings were investigated at 1373, 1573, and 1773 K in a 95% H2O-5% O2 atmosphere. The coatings exhibited good corrosion resistance. The 15 mol% Y2O3 coating showed a weight loss rate of 1.62 wt % when exposed to hot steam for 10 h at 1573 K. This value is only 1/8th of that of the uncoated sample. Moreover, ZrSiO4 improved the water corrosion resistance of the Y2O3-doped mullite-ZrSiO4 coatings at 1573 K.

HVOF Sprayed Mullite Coatings for Use In Extreme Environments

Products used in industry and marine applications are exposed to extreme environments like high heat, humidity, acidic or alkaline or hyper saline environment, UV and IR radiation. Metals with good corrosion and oxidation resistance may be used but are restricted to Ni and Cr alloys, titanium and super alloys etc., which are costly and have their limitations. Hence ceramic coatings on low cost metals may be an answer to this problem. Ceramics are inherently chemically inert, high temperature resistant, corrosion and oxidation resistant. Flame spraying of ceramics is a good and reliable method for applying ceramic coatings on metallic substrates with good bond strength (> 80 MPa) and 1% porosity. In this work, HVOF technique is applied to obtain 100 microns thick mullite coatings on MS substrates with a NiCr bond coat. Mullite has a high oxidation and corrosion resistance. It is chemically inert. It has high temperature resistance even at 2000 C. These properties are ideal for industrial components exposed to salty environments. Characterization studies like XRD, SEM/EDS, Corrosion tests using polarization technique, coating thickness and surface roughness have been studied and reported. A corrosion rate of 1.55 mm/ year has been achieved in a sea water environment.

Formation of Mullite Coating by Aerosol Deposition and Microstructure Change after Heat Exposure

Formation of Mullite Coating by Aerosol Deposition and Microstructure Change after Heat Exposure

Magnetic Submicron Mullite Coatings with Oriented SiC Whiskers.

Addressing the challenge of making ceramic thin films with the in-plane-oriented nanorods, we propose to decorate the nanorods with magnetic nanoparticles and orient them using the external magnetic field. As an illustration, the mullite thin films with embedded and oriented SiC nanorods were synthesized. The SiC nanorods were decorated with the Fe3O4 nanoparticles. A two-step processing route was developed when the nanorods are first oriented in a sacrificial polymer layer. Then, the polymer film with the aligned nanorods was removed by heat-treatment. In the second step, a sol-gel/dip-coating method was applied to produce the mullite composite film. The main challenge was to guarantee that all of the nanorods that were initially randomly distributed in the polymer would have time to rotate toward the field direction before complete solidification of the sacrificial layer. Theoretical and experimental analyses of the orientational distribution of the nanorod axes were conducted to identify a relationship between the polymer viscosity and processing parameters of the system. In contrast to the ferromagnetic nanorods, the rate of rotation of paramagnetic nanorods and their time of alignment are more sensitive to the magnetic field. This methodology allows manufacturing of different ceramic films with aligned nanorods and making nonmagnetic ceramic coating magnetic.

Nanostructured mullite steam oxidation resistant coatings for silicon carbide deposited via atomic layer deposition

AbstractOxidation resistant, thin, pinhole‐free, crystalline mullite coatings were deposited on zirconia and silicon carbide particles using atomic layer deposition (ALD). The composition of the films was confirmed with inductively coupled plasma optical emission spectroscopy (ICP OES), and the conformality and elemental dispersion of the films were characterized with transmission electron microscopy (TEM) and energy dispersive X‐ray spectroscopy (EDS), respectively. The films are deposited on the particle surface with a deposition rate of ~1 Å/cycle. The elemental concentration of aluminum relative to silicon in the film was determined to be 2.68:1 which agrees closely with the ratio of stable 3:2 mullite (2.88:1). A high‐temperature anneal for 5 hours at 1500°C was used to crystallize the films into the mullite phase. This work represents the first deposition of mullite films by ALD. The mullite and alumina‐coated particles were exposed to high‐temperature steam for 20 hours at 1000°C to assess the oxidation resistance of the films, which reduced the oxidation of silicon carbide by up to 62% relative to uncoated particles under these conditions. The activation energy of oxygen diffusion in the films was determined with density functional theory, and the computational results aligned well with the experimental outcomes.

Effect of deposition temperature on the mechanical, corrosive and tribological properties of mullite coatings

Thermal-sprayed ceramic coatings are used worldwide to provide protection from corrosion and wear on components. Nevertheless, they exhibit very poor tribological performance in corrosive conditions, primarily because the high porosity and low cohesive strength, resulting in severe corrosion of metal materials and wear of coatings. Herein, a series of mullite coatings are deposited on the surface of titanium alloys under different deposition temperatures and the splats behavior, mechanical properties, corrosion and wear performance of mullite coatings are studied. Results show that adjusting the deposition temperature can modify the splats behaviors, improve the microstructure and degree of crystallinity and enhance the hardness and cohesive strength. Owing to the improvements, the specimens deposited at higher temperatures have the excellent anti-corrosion and wear-resistant ability.

Corrosion, adhesion and erosion study of MZ and ML system using thermal plasma

Mullite processed through thermal plasma has been identified as promising material in ceramic industry for thermal barrier coating (TBC) applications. Addition of ZrO2 as impurities reduces the fracture toughness property of mullite coatings. In the current study, effort has been taken to compare the performance of La2O3 as an equivalent material to ZrO2. Here MZ (Mullite-ZrO2) and ML composites (Mullite-La2O3) have been prepared in different weight proportions of 10%, 20% and 30%. The MZ and ML composites are ball milled for 2 hr and sprayed over AISI 1045 steel specimen at 15 cm and 20 cm spray distance using 15kW thermal plasma torch. The MZ and ML coated samples were studied for SEM, XRD, adhesion test, roughness, wear, sliding test and coating thickness. Though the ML material has not been tested for exact application, but the study reveals that La2O3 can be used as cost effective material for TBC applications.

Corrosion, adhesion and erosion study of MZ and ML system using thermal plasma

Mullite processed through thermal plasma has been identified as promising material in ceramic industry for thermal barrier coating (TBC) applications. Addition of ZrO2 as impurities reduces the fracture toughness property of mullite coatings. In the current study, effort has been taken to compare the performance of La2O3 as an equivalent material to ZrO2. Here MZ (Mullite-ZrO2) and ML composites (Mullite-La2O3) have been prepared in different weight proportions of 10%, 20% and 30%. The MZ and ML composites are ball milled for 2 hr and sprayed over AISI 1045 steel specimen at 15 cm and 20 cm spray distance using 15kW thermal plasma torch. The MZ and ML coated samples were studied for SEM, XRD, adhesion test, roughness, wear, sliding test and coating thickness. Though the ML material has not been tested for exact application, but the study reveals that La2O3 can be used as cost effective material for TBC applications.

Dense and crack-free mullite films obtained from a hybrid sol–gel/dip-coating approach

Abstract

Parametric Appraisal of Slurry-Sprayed Mullite Coatings for Coating Thickness

The slurry spray technique (SST) has been employed to deposit mullite-based coatings on ASTM 1018 low-carbon steel substrate for environmental barrier coating applications. A Taguchi L18 orthogonal array is adopted for optimization of the identified process variables and material parameters, namely stamping pressure, fly-ash content, and sintering additive, time, and temperature. The measured thickness of the produced coatings was chosen as the response characteristic for the present study. The optimum values of the process variables were predicted by employing analysis of variance based on raw data and the signal-to-noise (S/N) ratio. Based on analysis of the experimental results, the effect of each parameter level on the coating thickness is discussed. It is observed that the sintering temperature had a strong influence on the maximum coating thickness of the slurry-sprayed coating. The as-sprayed coatings fabricated using SST demonstrated splat morphology with continuous interface, suggesting good adherence to the substrate.

Fabrication of Mullite Coating and its Oxidation Protection for Carbon Fiber Reinforced SiC Composites

To protect carbon fiber-reinforced SiC (C/SiC) composites against oxidation,mullite coating was prepared on C/SiC composites by dip-coating method with high solid content Al2O3-SiO2 sol as raw materials. X-ray diffraction and scanning electron microscopy were employed to analyze the phase and microstructure of the coating. The results show that the as-prepared coating is SiO2-rich, monolithic and well bonded with substrate without penetrating crack, giving rise to good oxidation-resistance. After soaked at 1400°C for 30min under static air, the coated C/SiC composites possess 87% of original flexural strength. As a result of sealing and filling of cracks and pores by viscous SiO2 in coating, the coated C/SiC composites exhibit improved oxidation resistance at 1500°C and 1600°C. There is no change in flexural strength after oxidized at 1500°C and 1600°C for 30min, respectively. Nevertheless, the carbothermal reduction between viscous SiO2 and free carbon in C/SiC substrate would occur obviously when oxidation temperature was elevated or oxidation time was prolonged, leading to local foaming in coating and decreasing in oxidation resistance.