Properties of alumina coatings prepared on silica-based ceramic substrate by plasma spraying and sol-gel dipping methods

Silica-based ceramic cores are widely used in the manufacturing of hollow, nickel-based, superalloy turbine blades. However, elemental Hf, Ti, Al, and other active metals in the superalloy can react with silica-based ceramic cores during casting, resulting in a reduction in the quality of the turbine blades. In this study, both plasma spraying and sol-gel dipping methods were used to prepare alumina coatings on silica-based ceramic substrates to prevent the interfacial reaction. The performance of the alumina coatings prepared by both methods was evaluated by comparative analysis of the surface roughness, bonding interface morphologies, and the adhesive characteristics of the coating. The plasma-sprayed alumina coating has a roughness greater than 5 μm and peeled away from the substrate due to the difference in thermal expansion between SiO₂ and Al₂O₃ at temperatures above 1500 °C, rendering the silica-based substrate with the plasma-sprayed alumina coating unfit for the application requirements of the casting process. The alumina coating prepared by the sol-gel dipping method improved the roughness of the substrate from Ra 2.39 μm to Ra 1.83 μm, and no peeling was observed when heated to 1550 °C for 30 min due to the pinning characteristics of the coating on the substrate. Furthermore, the interfacial reaction between the DZ125 superalloy melt and the silica-based substrate coated with alumina by sol-gel dipping method were investigated. The alumina coating effectively inhibited the interfacial reaction and no reaction products were detected during the directional solidification with pouring temperature of 1550 °C and withdraw rate of 5 mm/min. While a uniform, 4–5 μm thick HfO₂ reaction layer formed between the uncoated substrate and the DZ125 alloy melt. Two dipping-drying cycles were required to ensure the alumina sol completely covered the surface of the substrate.     

Ferrochromium slag as a protective coating material against oxidation for caster rolls

In this study, ferrochromium (FeCr) slag, which is available as an industrial waste, is proposed as a protective surface coating material particularly for protection of continuous casting rolls against oxidation. FeCr slag was successfully deposited with atmospheric plasma spray (APS) method. Before the coating process, FeCr slag powder prepared in the particle size range of 5‐38 μm, was investigated using conventional characterization methods (XRD, SEM, TGA etc.). Thermal Barrier Coating (TBC) system was used as a basis for deposition processes. Accordingly, NiCoCrAlY (Amdry, ?45 + 5 μm) was firstly deposited as metallic bond coat layer onto the surface of AISI 420 substrate, and then FeCr slag layer was deposited as the top coating layer. After the deposition of FeCr slag powder, the resulting coating layer was found to have low porosity with a homogeneous microstructure. The deposited FeCr slag coatings were subjected to isothermal oxidation tests at different temperatures and test durations for determination of their oxidation behavior and upper operating temperature limits. The results obtained from this study indicate that FeCr slag can be considered as an alternative protective coating material for caster rolls which are subjected to high temperatures up to 800°C.     

Influence of the deposition energy on the composition and thermal cycling behavior of La2(Zr0.7Ce0.3)2O7 coatings

Lanthanum–zirconium–cerium composite oxide (La₂(Zr_0.7Ce_0.3)₂O₇, LZ7C3) coatings were prepared under different conditions by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphologies, cyclic oxidation behavior of these coatings were studied. Elemental analysis indicates that the coating composition has partially deviated from the stoichiometry of the ingot, and the existence of excess La₂O₃ is also observed. The optimized composition of LZ7C3 coatings could be effectively achieved by the addition of excess CeO₂ into the ingot or by properly controlling the deposition energy. Meanwhile, when the deposition energy is 1.15 × 10⁴–1.30 × 10⁴ J/cm², the coating has a similar X-ray diffraction (XRD) pattern to the ingot, and the thermal cycling life of the coating is also superior to other coatings. The spallation of the coatings occurs either within the ceramic layer approximately 6–10.5 μm above its thermally grown oxide (TGO) layer or at the interface between ceramic layer and bond coat.     

Formation of MoSi2 and Si/MoSi2 coatings on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy by hot dip silicon-plating method

MoSi₂ and Si–MoSi₂ coatings were deposited on TZM alloy by using a hot dip silicon-plating method. The composite coatings mainly consist of MoSi₂ and Si with a small amount of C/SiO₂/ZrSi₂. The composite coatings have fine MoSi₂ grain size and higher surface silicon concentration. The diffusion layer mainly consists of MoSi₂ layer and Mo₅Si₃C layer under the deposition time is 10 min. The diffusion layer is divided into three layers when the deposition time ranges from 15 to 25 min, which consists of Si–MoSi₂ layer, MoSi₂ layer and Mo₅Si₃ interface layer. The gradient structure can reduce the stress mutation between coating and the substrate, and further reduce the possibility of crack propagation. SEM and CLSM results show that a large amount of MoSi₂ grains are covered by a silicon layer when the deposition time is longer than 10 min, which results in a very low coating surface roughness. And the lowest values of Sa and Sq are 0.292 and 0.391 μm, respectively. Meanwhile, about 65.375%–79.898% of the coating surfaces are covered by silicon when the deposition time is 20–25 min.     

Fabrication of high-quality alumina coating through novel,dual-particle aerosol deposition

High-quality alumina (Al₂O₃) coating has an extensive demand in the fields of optoelectronics, solar cells, and corrosion/impurity resistant coatings and cutting tools. The quality of alumina coating depends on its hardness and transparency. To obtain hard and highly transparent alumina coating, which is also a widely used ceramic material, a novel, aerosol deposition approach is presented in which the starting powder, used to fabricate Al₂O₃ ceramic coatings, is composed of angular and spherical Al₂O₃ particles. Films fabricated using angular:spherical Al₂O₃ particle mixtures with ratios of 10:0, 7:3, 5:5, 3:7, and 0:10, showed significant variation in surface roughness and microstructure. The dramatic morphology modulation of the 3:7 angular:spherical Al₂O₃ mixture film, resulting from the superposition hammering effect caused by the aerosol mixture, improved transmittance (84.7%) and hardness (13.6 GPa). Previous studies used high-energy approaches to optimize Al₂O₃ film properties. This dual-particle approach, however, produces Al₂O₃ film with excellent transmittance and hardness while achieving a fast coating speed (32 mm² × μm/min) without additional thermal treatment. Our proposed approach provides a novel and energy efficient method to produce transparent Al₂O₃ films with superior durability.     

Effect of silica sol on performance and surface precision of alumina ceramic shell prepared by binder jetting

Using 3D printing technology to prepare ceramic shell used for precision investment casting can realize short process and efficient preparation of the ceramic shell, which has a great application potential in the casting field. However, the 3D printed ceramic shells often have the problems of low strength and accuracy. In this paper, a silica sol room temperature dip coating treatment combined with high temperature sintering method was proposed to improve the strength and surface precision of the ceramic shell prepared by the binder jetting. The effects of silica sol concentration and dip coating time on performance and surface precision of the alumina ceramic shell were studied. The mechanical properties and surface precision of the alumina ceramic shell prepared by the binder jetting were improved significantly with the increases of the sol concentration and dip coating time. With the dip coating time of 90 s and sol concentration of 30%, the maximum bending strength of the alumina ceramic reached 44.8 MPa, which was 18.9 times higher than that of the untreated alumina ceramic. The top surface roughness and side roughness of the alumina ceramic decreased from 6.87 μm to 5.70 μm and 7.55 μm–6.46 μm, respectively, compared to those of the untreated alumina ceramic.     

Sol-gel alumina coating of wire mesh packing

Wire mesh packings have seen increasing applications to multiphase processes in recent years. Despite the high surface area, open structure and thermal and chemical resistance, wire mesh packings have a complex geometry which hinders some chemical applications, including changes of surface properties through application of a uniformly adhesive coating. In this work, the sol-gel deposition method of alumina coating ceramics was investigated for the first time on stainless steel wire mesh by using Dixon rings as example. The kinetics of deposition during the hydrolysis and polycondensation was followed for a range of initial composition of the coating such as the ratios of Al₂O₃ to water, acid content, polyethyleneimine binder content and the number of deposition cycles. Well-adhered alumina with a thickness up to 20 μm was successfully deposited. The molar ratios of acid to alumina and alumina to water of 0.25 and 0.01, respectively, 48 h of the aging time, 96 h of the mixing time and 2.1 g/L of polyethyleneimine binder formed a free of cracks coating of controlled thickness alumina on the Dixon rings.     

Alumina coating deposition by electron-beam evaporation of ceramic using a forevacuum plasma-cathode electron source

We have explored the formation of alumina coatings by electron beam evaporation of bulk alumina ceramic using a forevacuum-pressure plasma-cathode electron beam source at a pressure of 60 mTorr of air. The alumina e-beam target material is evaporated at a rate up to 4.2?g/h, and the coating deposition rate is up to 18?μm/h. The evaporated ceramic is partially ionized by the energetic electron beam, and we have measured the plasma composition as a function of beam power. The deposited alumina coatings were characterized by scanning electron microscopy and energy-dispersive x-ray spectroscopy and found to be uniform and pore-free with a composition that is over 99% aluminum and oxygen, with less than 1% impurities including carbon, silicon and calcium.     

Preparation of Pd-doped Y3Al5O12 thermal barrier coatings using cathode plasma electrolytic deposition

Here, noble metal Pd-doped Y₃Al₅O₁₂ thermal barrier coatings (TBCs) were efficaciously prepared by means of cathode plasma electrolytic deposition (CPED). The formation mechanism of the Y₃Al₅O₁₂ coatings and the difference in coating performance before and after doping with Pd were analyzed. The results indicated that the preparation of the Y₃Al₅O₁₂ TBCs by using the CPED method could be divided into three stages, and the phase compositions of the coatings obtained with different deposition times were different. A single-phase Y₃Al₅O₁₂ TBCs with a 115-μm thickness was obtained after a deposition time of 20 min. After Pd doping, the average surface roughness of the TBCs decreased from 27.72 to 13.84 μm, and the high-temperature oxidation resistance and thermal shock resistance at 1050 °C improved significantly.     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Improvement in heat resistivity of alkaline earth silicate fiber boards by Al4SiC4 coating

Alkaline earth silicate (AES) fiber ceramic board was immersed in a novel coating slurry consisting of Al₄SiC₄ particles and silica-sol, forming a 500-μm coating layer on the AES fiber board. Linear shrinkage of the uncoated AES fiber board was over 3.0% after heating at 1100°C or higher for 8 hours, whereas the linear shrinkage of the AES fiber board with an Al₄SiC₄ coating was below 2.0%. The coating layer of Al₄SiC₄ changed to a hard shell structure consisting of cristobalite, alumina, and mullite after heating. When AES fiber board with an Al₄SiC₄ coating layer was heated at 1200°C for 8 hours, the compressive strength of the board reached 0.45 MPa, 2.5 times greater than that of the original uncoated board.     

Atomic layer deposition onto carbon fiber fabrics

Carbon fiber fabrics, consisting of interwoven bundles of 3000 single fibers, were coated with Al₂O₃ using the atomic layer deposition (ALD) process, exposing the fabrics to alternating pulses of trimethyl aluminium and water vapors. The thickness and uniformity of the coatings were investigated using scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The obtained coatings were conformal, 84 ALD cycles gave rise to approximately 20‐nm‐thick coatings and 168 ALD cycles to approximately 40‐nm‐thick coatings. It was found, that a uniform coating can be obtained at a purge time of 40 seconds. Reducing purge times below 20 seconds gives rise to increased particle growth and thus the coating becomes inhomogeneous. Initially, the samples that were coated had a size of 2×10 cm (thickness 0.3 mm). The size of the fabric was subsequently increased up to 8×20 cm and a uniform coating of the same quality was obtained. By oxidizing the coated fabrics, fabrics composed of interwoven alumina microtubes were obtained. Infiltration of the microtubes with solutions of two distinguishable fluorescent dyes showed that interchange of the dyes between warp and weft microtubes occurs, but is absent at approximately 20% of the crossovers. Taking all our findings into account, we conclude that the majority of the fibers were separated from each other by the coating prior to the oxidation. This work demonstrates that ALD is a suitable method to produce thin, conformal coatings on the surface of carbon fiber fabrics.     

Advances in the control of electrophoretic process parameters to tune the ytterbium disilicate coatings microstructure

Suspensions of ytterbium disilicate in isopropanol were prepared using iodine dispersant. Their zeta potential, electrical conductivity, and pH dependence with iodine concentration is detailed. Electrophoretic deposition was performed on silicon substrates at various voltages (100-200 V) and times (until 10 minutes) and the growth dynamic was investigated. It was observed that the deposited mass reaches a maximum value for [I₂] = 0.2 g/L, and the coating microstructure becomes porous at higher iodine concentrations. Current density and voltage measurements allowed to correlate this behavior to the increase of free protons concentration in the suspension. In these conditions, it was proved that porosity increases with the increase in applied voltage, and a compaction occurs as the deposition time increases. This has been related to the coating resistance increase and subsequent decrease in effective voltage in the suspension. The denser coatings (20% of porosity) were obtained in the case of suspension without iodine, at the minimum applied voltage and for the longest deposition times.     

Strongly adherent Al2O3 coating on SS 316L: Optimization of plasma spray parameters and investigation of unique wear resistance behaviour under air and nitrogen environment

Plasma spray deposition of Al₂O₃ is a well-established technique for thick ceramic coatings on various substrates to shield them from corrosion and wear. Owing to its high hardness, aluminum oxide is known to protect stainless steel substrates from wear. However, the plasma process requires optimization for desired coating thickness and adhesion strength. It is also necessary to understand the sensitivity of friction and wear resistance of the deposited coating on exposed environment for evaluation of service life. The study offers comprehensive investigation on plasma process parameters for the development of strongly adherent aluminium oxide coatings on SS 316L substrate. Impact of environment like dry air and dry nitrogen on tribological properties of the coatings was also investigated. Dense adherent coatings of alumina could be deposited on SS 316L at a plasma power of 20 kW with an intermediate bond coat of NiCrAlY to enhance the adhesion properties. The effects of stand-off distance and bond coat thickness on adhesion strength were additionally examined. Further, the coatings were characterised for phase composition, microstructure, microhardness and wear resistance potential. Reciprocating wear tests of the coatings were carried out using ball on disc reciprocating tribometer at different loading conditions (5, 10 and 15 N) at constant (5 Hz) sliding frequency. Unlike the coefficient of friction (COF), wear volume was found to increase with an increase in normal load. These adherent coatings revealed promising properties for the applications where the tribological failure of SS 316L in dry air or dry nitrogen environment is to be controlled.     

The morphology,thermal property,and failure mechanism of GdNdZrO thermal barrier coatings by EB-PVD

Nowadays, the Gd₂Zr₂O₇ thermal barrier coatings (TBCs) have been evaluated as a promising alternative to yttria-stabilized zirconia (YSZ). Thus, this investigation focuses on the thermal property, morphology, and failure mechanism of double ceramic layers (DCLs) GdNdZrO/YSZ advanced TBCs. The GdNdZrO coatings with columnar morphology have been deposited on NiCoCrAlYHf bond coating using an electron beam physical vapor deposition method. Material characterizations mainly include X-ray diffraction, scanning electron microscope, and transmission electron microscopy. The thermal conductivity of GdNdZrO ceramic material is 0.494 W/mK at 1200°C. The thermal shock life of GdNdZrO/YSZ TBCs shows an average shock life of 5235 cycles. The TBC degradation occurs on the crack area within thermally grown oxide layer leading to the interface instability. The interface broken might play an important role in the failure mechanism of TBCs.     

Thermal stability and performance of optimized ZrCx diffusion barriers in ceramic coating systems for ATF applications

Surface-modified Zr alloy claddings with advanced ceramic coatings are promising materials for accident-tolerant fuel (ATF) systems to meet stringent safety regulations concerning light water reactors. The applications of ceramic coatings are, however, limited as a result of inferior thermal stability when used in conjunction with Zircaloy-4 (Zry-4) substrates. Herein, the thermal stability of sub-stoichiometric zirconium carbide barrier layers as a function of composition was studied. Integrated ceramic coatings comprising ZrC_0.55 diffusion barriers and a Cr₂AlC top layer were synthesized via a magnetron sputtering method. After rapid thermal annealing, the ZrC_0.55 barrier layer having a thickness of 0.5 μm effectively prevented the inter-diffusion between Cr₂AlC and the Zry-4 substrate, thereby ensuring retention of the structural integrity of the integrated ceramic coating system for ATF applications.     

Adhesion of Vacuum Deposited Optical Coatings on PMMA and Polycarbonate

Abstract

The deposition of interference coatings with optical functions (e.g., anti-reflective coatings, beam splitters and filters) on plastic substrates is becoming ever more important. A well-known deposition method for such coatings on polymers is plasma ion-assisted vacuum evaporation (plasma ion-assisted deposition or plasma IAD). However, the industrial production of well-adhering dielectric coatings on unlacquered polymer surfaces has not yet been developed to a satisfactory level. PMMA is known to be extremely difficult to coat with optical layers because of its inadequate adhesion characteristics. Polycarbonate is considered to be less problematic, but in many cases unexpected adhesion failures arise. This paper presents a study of the reasons for the poor adhesion to PMMA of oxide coatings deposited by plasma IAD. We show that in such a coating process it is only the exposure of the substrate to particles and short-wavelength radiation that determines its adhesion to the deposited layer. For polycarbonate, we found a dependence of the adhesion strength of the coating on its porosity. Considering these results, modified plasma IAD processes have been developed that enable the deposition of well-adhering optical coatings on these polymers.     

Electron beam physical vapour deposition and mechanical properties of c-ZrO2-ZTA-coatings on alloy 617 substrates

Multilayered zirconia toughened alumina (ZTA) and c-zirconia coatings were prepared using electron beam physical vapour deposition (EB-PVD). Characterizations of the morphology and chemical composition of the deposited coatings were performed using scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). Scratch resistance, nano-indentation and bending strength were used for the evaluation of the mechanical properties. X-ray diffraction of the top ceramic TBC surface showed that it consists entirely of cubic ZrO₂ phase. The energy-dispersive X-ray spectroscopy analysis (EDS) showed that α-Al₂O₃ is the only oxide phase present at the interface, while SEM indicated the presence of columnar c-ZrO₂ as the only phase of the top coat. Delamination over a large region was observed in the case of double layer (ZTA) coating. In contrast, the multilayered (ZTA1 + ZTA2 + c-Z) coating showed neither delamination nor cracking. The hardness and scratch measurements showed that the top coat c-ZrO₂ layer is harder than the ZTA layers. The thermal conductivity of the multilayer coatings was estimated using the theoretical density and thermal conductivity values of zirconia toughened alumina (ZTA) and cubic-zirconia (c-ZrO₂) together with their experimentally measured data.     

Structure characterisation and mechanical properties of crystalline alumina coatings on stainless steel fabricated via sol–gel technology and fibre laser processing

The surface modification of stainless steel by coating with alumina (Al₂O₃) was carried out using sol–gel coating technology in combination with laser processing. Alumina coatings have been synthesised via a sol–gel route and deposited on stainless steel substrates by dip coating. The coated substrates were then treated with pulsed ytterbium fibre laser radiation (λ = 1064 nm) in continuous wave mode with different specific energies. The composition and structure of the coated surfaces after laser processing were characterised by ATR-FTIR, XRD, SEM and contact angle measurements, whilst the mechanical properties of modified surfaces were determined using nano-indentation. The results showed that the alumina xerogel films coated on the substrates are successfully converted into crystalline alumina ceramic coatings by the laser irradiation, the structure of resulting coatings being dependent on the irradiation conditions, with increase of laser specific energy leading to the formation of initially γ-Al₂O₃ with increasing amounts of α-Al₂O₃ at higher energy. Nano-indentation results reveal that the laser processing results in significant improvement in hardness and Young's modulus of the alumina-coated surface and, at optimum, can achieve the mechanical properties at the same level as pure α-alumina ceramic, much higher than those of the as-dried xerogel coating.     

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.