Laser and furnace pyrolyzed organosilazane-based glass/ZrO2 composite coating system—A comparison

The laser pyrolysis of a ceramic coating system composed of an organosilazane (Durazane 1800) with monoclinic ZrO₂ and glass particles as fillers was investigated. The samples were produced by spray coating on pre-treated stainless steel substrates and subsequently pyrolyzed by Nd:YV0₄ laser radiation (λ =1064 nm). The interaction of laser radiation with the polysilazane-based glass/ZrO₂ coating system led to formation of semi-crystalline dense coatings with a thickness up to 20 μm in a short time.Laser pyrolyzed ceramic coatings were characterized and compared with furnace pyrolyzed coatings. The microstructure of the coatings was investigated by scanning electron microscopy (SEM). Additionally the corrosion resistance and mechanical properties, such as hardness, adhesive strength and the tribological behaviour have been investigated. Ceramic coatings generated by laser pyrolysis showing promising mechanical properties, a super-hydrophobic surface as well as a high corrosion resistance.     

High performance environmental barrier coatings, Part I: Passive filler loaded SiCN system for steel

A novel environmental barrier coating system for steel consisting of a perhydropolysilazane (PHPS) bond coat and a polysilazane-based glass/ceramic composite top coat has been developed. After stabilising the coating slurries, double layers were applied on mild and stainless steel substrates by the dip-coating technique. Parameters like pre-treatment of the steel substrates, filler systems, particle size of the fillers or coating thickness were varied to optimize the coatings. The thermal treatment was performed in air at temperatures up to 800 °C. Microstructural analysis by SEM and XRD revealed the formation of a coating system consisting of a SiNO bond coat and a ZrO₂-filled glass/ceramic top coat. A uniform, well adherent, dense and crack-free coating system with a noteworthy thickness up to 100 μm was achieved. Even after cyclic oxidation tests on coated samples at 700 °C the coating system was still undamaged and no oxidation occurred on the mild steel substrates.     

Polymer derived SiC environmental barrier coatings with superwetting properties

Surfaces with superwetting capabilities can be used for corrosion protection, self-cleaning and bio-fouling protection amongst other applications. In this work, we present a method to produce a SiC coating with an almost superhydrophobic behavior exhibiting water contact angles of 145±3°. Ceramic coatings were produced by the pyrolysis of polycarbosilane as a preceramic precursor of SiC. Aluminum and carbon powders were used as active and passive fillers to compensate for the volume shrinkage of polycarbosilane during pyrolysis. The effects of particle size (Al particles ranging from 0.8 to 10 µm) and concentration 10–30% wt.) C and Al of both fillers were studied to produce defect-free ceramic coatings. We have observed that the fillers used not only affected the microstructure but also the surface roughness. We show that the addition of carbon fillers can increase the water contact angle of the ceramic from 42° up to 141°. The combination of carbon and aluminum fillers resulted in water contact angles up to 145°.     

Investigation of the conditions for synthesis of Ce<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>2</Subscript> dense coatings

The conditions for preparation of Ce_0.9Y_0.1O₂ (CYO) oxide coatings on La_0.8Sr_0.2MnO₃ (LSM) ceramic substrates by screen printing were investigated. The CYO compound was synthesized by the pyrolysis of polymer-salt composites with the aim of producing submicron powders with a uniform size distribution. Transmission electron microscopy of the microstructure of the CYO compound synthesized with ethylene glycol revealed that the synthesis product consists of ultrafine crystalline particles with an average size of 5–15 nm. The use of CYO nanopowders made it possible to prepare rather dense single-layer coatings on LSM substrates. It was demonstrated that annealing of the coatings at high temperatures leads to the recrystallization and coarsening of particles.     

Electrophoretic deposition of ceramic coatings on ceramic composite substrates

Abstract

The applicability of electrophoretic deposition (EPD) for the fabrication of single layer and multilayer ceramic coatings on dense ceramic composite materials has been examined. Al₂O₃/Y-tetragonal zirconia polycrystal (TZP) functionally graded composites of tubular shape were successfully coated with a two layer coating comprising porous alumina and dense reaction bonded mullite layers. The dual layer coating structure was designed to eliminate the numerous cracks caused by volume shrinkage during sintering of the individual EPD formed layers. In another example, mullite fibre reinforced mullite matrix composites were coated with a thin layer of nanosized silica particles using EPD. The aim was to achieve a compressive residual stress field in the silica layer on cooling from sintering temperature, in order to increase composite fracture strength and toughness. The EPD technique proved to be a reliable method for rapid preparation of single layer and multilayer ceramic coatings with reproducible thickness and microstructure on ceramic composite substrates.     

Characterization and property of microarc oxidation coatings on open-cell aluminum foams

The ceramic coatings were prepared on open-cell aluminum foams by microarc oxidation (MAO) treatment in an alkaline-silicon electrolyte. The morphology, microstructure, elemental distribution, and phase composition of the MAO coatings were investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction, respectively. The corrosion behaviors of the coated and uncoated foams were evaluated by electrochemical polarization measurement. The results show that the MAO coatings cover the surface of open-cell aluminum foams. The coatings were composed of an external porous layer and an internal dense layer. The main phase of the MAO coating phase is γ-Al₂O₃. The coated aluminum foams exhibit more positive corrosion potential and lower corrosion current density compared with the uncoated aluminum foams.     

Ablation behavior of inorganic particle-filled polybenzoxazine composite coating irradiated by high-intensity continuous laser

Further utilization of aircraft structural materials is threatened by the fact that high-intensity continuous lasers are widely used in the field of military defense. To protect the aircraft structure from laser damage, ammonium polyphosphate filled polybenzoxazine composite coatings were prepared on the substrate. The anti-laser ablation characteristics of the coatings were investigated. Results showed that the addition of the inorganic filler improved the anti-laser ablation performance of polybenzoxazine. The back-surface temperature of substrates covered with the composite coatings was more 50% lower than that in the case of a pure polybenzoxazine coating after laser ablation. Further, the residue of the composite coating could be vertically divided into three distinct regions, with the dense surface char layer and the porous pyrolysis layer acting as shielding layers for the laser beam and preventing any heat-related transformations from occurring. The addition of the inorganic particles improved the surface reflectivity of the coatings resulting in much more laser energy dissipation. The decreased pyrolysis rate ensured that the pneumatic cooling effect of pyrolysis gas was more lasting and stable, owing to which the composite coatings could act as effective thermal protection layer for longer. These results confirmed that the inorganic filler modified polybenzoxazine coating exhibits excellent anti-laser properties and are suitable for protecting structural materials from laser-related damage.     

Passive filler loaded polysilazane-derived glass/ceramic coating system applied to AISI 441 stainless steel,part 1: Processing and characterization

This study describes an oxidation and corrosion resistant environmental barrier coating (EBC) applied to an AISI 441 stainless steel substrate. For this purpose, four polymer-derived ceramic (PDC) coating systems were developed. These coating systems consisted of a bond coat applied by dip coating, and a top-coat that was loaded with passive fillers and deposited by spray coating. The microstructures of the coatings were investigated using optical microscopy and scanning electron microscopy, including energy dispersive spectroscopy (EDS). X-ray powder diffraction (XRD) was used to investigate the phase composition of the coatings. The optimized composite top coatings were prepared from the preceramic polymer HTT1800, filled with yttria-stabilized zirconia and a specially tailored Al₂O₃–Y₂O₃–ZrO₂ (AYZ) passive filler, and commercial barium silicate glasses were used as sealing agents. After thermal treatment in air at 750°C, uniform and crack-free composite coatings on stainless steel substrates were developed, with thicknesses of up to 93 μm. Oxidation tests, which were performed at 850°C in synthetic air, showed that every tested coating system remained undamaged by oxidation and showed good bonding to the metal substrate.     

Laser ablation behavior of flake graphite and SiO2 particle-filled hyperbranched polycarbosilane matrix composite coatings

Composite coatings consisting of flake graphite and SiO₂ fillers in a hyperbranched polycarbosilane (HBPCS) matrix were designed and prepared to meet the requirements of laser protection. The laser ablation behavior of the composite coatings were investigated. Control experiments were designed to study the performance of SiO₂ during laser irradiation. The results show that the introduction of SiO₂ changes the anti-laser protective mechanism and can improve the anti-laser property of the coating. High power laser irradiation results in pyrolysis of HBPCS and the formation of SiC particles. Chemical reactions between SiO₂, graphite, and SiC play an important role in consuming energy, and provide an excellent cooling effect to the substrate, leading to decreased temperature. SiC particles formed on the surface of the laser ablation area act as a shield to prevent the laser from irradiating deeper layers of the coating. Due to the cooling effect and thermal stability of SiC, the proposed coating shows a good anti-laser property.     

Effects of suspension properties on the fabrication of Yb2Si2O7 coatings using electrophoretic deposition

This study examines the electrophoretic deposition of Yb₂Si₂O₇ particles on SiC substrates to produce Environmental Barrier Coatings. To prepare crack-free and homogeneous green coatings, the effect of the solvent, dispersant concentration, and pH were investigated. Ethanol provided a well-dispersed suspension and crack-free coating which was shown by sedimentation tests and microstructure analysis. The effect of the dispersant concentration was investigated with zeta potential measurement and microstructure analysis with a concentration above 0.5 g/L resulting in higher ionic strength and producing cracked and uneven coatings. The ionic strength was also associated with the powder packing density with larger indentation impressions measured for loosely packed coatings. The deposition rate depended on the suspension properties influenced coating integrity with delamination evidenced by analysing the current density drop during deposition. Sintering of the green coatings having different densities and microstructure showed their importance in the preparation of uniform and dense sintered coatings.     

Microstructure and mechanical properties of flame-sprayed PEEK coating remelted by laser process

Poly-ether-ether-ketone (PEEK) is one of the high-performance thermoplastics. It is being increasingly used for many industrial applications due to its excellent properties. In this paper, a flame spraying technique is used to deposit PEEK coating on 304L stainless substrates. CO₂ and Nd:YAG laser treatments are chosen to remelt the as-sprayed polymer coating to get a dense coating. The microstructures of the as-sprayed and remelted coatings are characterized by SEM and XRD. The results show that both CO₂ and Nd:YAG lasers are suitable for densifying the PEEK coating on stainless substrate. However, the remelted coatings present different crystalline structure due to their laser processing parameters. Hardness measurements, tribological and scratch tests are conducted to characterize the mechanical properties of remelted coating. The coatings’ mechanical properties are correlated with their structures.     

Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding

Al₂O₃-TiB₂-TiC ceramic coatings with high microhardness and wear resistance were fabricated on the surfaces of carbon steel substrates by laser cladding using different coating formulations. The microstructures of these ceramic coatings with the different coating formulations were investigated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometer. The wear resistance and wear mechanism were analyzed using Vickers microhardness and sliding wear tests. The results showed that when the amount of independent Al₂O₃ was increased to 30%, the ceramic coatings had a favorable surface formation quality and strong metallurgical bond with the steel matrix. The cladding layer was uniformly and densely organized. The black massive Al₂O₃, white granular TiB₂, and TiC distributed on the Fe substrate significantly increased the microhardness and wear resistance. The laser cladding ceramic coating had many hard strengthening phases, and thus resisted the extrusion of rigid particles in frictional contact parts. Therefore, the wear process ended with a “cutting-off” loss mechanism.     

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.     

Ablation behavior of C/SiC with YSZ and ZrB2 coatings under high-energy laser irradiation

Carbon fiber-reinforced silicon carbide (C/SiC) composites are important candidates for laser protection materials. In this study, ablation mechanism of C/SiC coated with ZrO₂/Mo and ZrB₂–SiC/ZrO₂/Mo under laser irradiation was studied. ZrB₂–SiC multiphase ceramic and ZrO₂ ceramic were successfully coated on C/SiC composite by atmospheric plasma spraying technology with Mo as transition layer. Phase evolution and morphology of composite were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Moreover, ablation behavior of the composite was investigated by laser confocal microscopy. Results showed that ablation mechanism of C/SiC composite was controlled by phase transformation, thermal reaction, and thermal diffusion, with solid–liquid transition of ZrB₂ and ZrO₂ being dominant factor. Endothermic reaction and good thermal diffusivity of coatings were also important factors affecting ablation performance. Reflectivity effect of ZrO₂ coating was limited under high-energy laser irradiation. Compared with ZrO₂/Mo single-phase-monolayer coating, designed ZrB₂–SiC/ZrO₂/Mo coating showed better ablation performance, and breakdown time of C/SiC increased from 10 to 40 s. The depletion of liquid phase in molten pool was identified as an important factor responsible for rapid failure of C/SiC. The coating failed when the entire liquid phase was consumed within molten pool, followed by rapid damage of C/SiC substrate. Results of this study can provide theoretical guidance and research ideas for design and application of laser protective materials.     

High emissivity MoSi2–ZrO2–borosilicate glass multiphase coating with SiB6 addition for fibrous ZrO2 ceramic

To develop a high emissivity coating on the low thermal conductivity ZrO₂ ceramic insulation for reusable thermal protective system, the MoSi₂–ZrO₂–borosilicate glass multiphase coatings with SiB₆ addition were designed and prepared with slurry dipping and subsequent sintering method. The influence of SiB₆ content on the microstructure, radiative property and thermal shock behavior of the coatings has been investigated. The coating prepared with SiB₆ included the top dense glass layer, the surface porous coating layer and the interfacial transition layer, forming a gradient structure and exhibiting superior compatibility and adherence with the substrate. The emissivity of the coating with 3 wt% SiB₆ addition was up to 0.8 in the range of 0.3–2.5 μm and 0.85 in the range of 0.8–2.5 μm at room temperature, and the “V-shaped grooves” surface roughness morphology had a positive effect on the emissivity. The MZB-3S coating showed excellent thermal shock resistance with only 1.81% weight loss after 10 thermal cycles between 1773 K and room temperature, which was attributed to the synergistic effect of porous gradient structure, self-sealing property of oxidized SiB₆ and the match of thermal expansion coefficient between the coating and substrate. Thus, the high emissivity MoSi₂–ZrO₂–borosilicate glass coating with high temperature resistance presented a promising potential for application in thermal insulation materials.     

Micro-/nanostructure evolution of C/SiFeO(N,C) polymer-derived ceramic papers pyrolyzed in a reactive ammonia atmosphere

SiFeO(N,C)-based ceramic papers were prepared via a one-pot synthesis approach by dip-coating a cellulose-based paper template with a polymeric perhydropolysilazane precursor modified with iron(III)acetylacetonate. The preceramic composites were subsequently pyrolyzed in ammonia atmosphere at 500, 700, and 1000°C, respectively, and the characteristics of the three resulting ceramic papers were comparatively investigated. Scanning electron microscopy revealed that in each sample, the morphology of the template is successfully transferred on the ceramic system, with the cellulose-derived fibers being converted to elemental carbon encased by a SiFeO(N,C) coating. Electron transparent cross-sectional samples for transmission electron microscopy (TEM) were prepared from the ceramic papers, employing a standard ultramicrotomy slice cutting procedure, allowing for a detailed characterization of their in situ generated micro-/nanostructure as well as occurring crystalline phases. TEM imaging and diffraction revealed that depending on pyrolysis temperature a different microstructure with a distinct phase assemblage is generated in the polymer-derived ceramic papers. Crystallization from the polymer precursor starts with the precipitation of wüstite (Fe_(1-x)O) nanoparticles at 700°C inside the ceramic coating and secondary ε-Fe_xN at the fiber-coating interface. Upon pyrolysis at 1000°C however, the sample primarily accommodates metallic α-iron nanocrystals that impart ferromagnetic characteristics to the ceramic paper. The results show that the template-assisted polymer-derived ceramic route is a feasible approach in the production of complex ceramic compounds with fibrous paper-like morphology. By adjusting the pyrolysis temperature, microstructure and phase composition of the ceramic paper can be conveniently tailored to the needs of its respective application.     

A study on the effect of presence of CeO2 and benzotriazole on activation of self‐healing mechanism in ZrO2 ceramic‐based coating

In this study, the effect of presence of CeO₂ and benzotriazole inhibitor agent on activation of self‐healing reactions and the corrosion behavior of ZrO₂ ceramic‐based coating are evaluated. The ZrO₂ and ZrO₂‐CeO₂‐benzotriazole ceramic‐based coatings were synthesized using sol–gel process and heat treated at 150°C. Afterward, X‐ray diffraction analysis (XRD), and Field Emission Scanning Electron Microscopy (FE‐SEM) were utilized to evaluate the phase analysis and morphology of these coatings. In addition, Energy Dispersive Spectroscopy (EDS) was used for elemental analysis of obtained coatings. Corrosion and self‐healing behavior of the coatings were investigated in 3.5 wt% NaCl solution using Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization tests. The results of XRD analysis revealed the amorphous nature of both coatings. FE‐SEM observations and EDS analysis results showed the presence of benzotriazole inhibitor agent and self‐healing reactions in the cracks and defects of ZrO₂‐CeO₂‐benzotriazole ceramic‐based coating. Moreover, results of electrochemical tests revealed that the presence of CeO₂ and benzotriazole inhibitor agent in the ZrO₂ ceramic‐based coating results in intense increases in the corrosion resistance of this coating by activating the self‐healing mechanism and forming passive layers.     

Influence of preheating processes on the microstructure of laser glazed YSZ coatings

Laser glazing is considered to be a promising surface sealing technique for thermal barrier coating. The dense top layer with reduced surface roughness and the segment cracks perpendicular to the surface are considered to be suitable for improving the thermal cycling and hot corrosion resistance of these kind of coatings. In present study, yttria stabilized zirconia ceramic coatings were manufactured by atmospheric plasma spraying and then subjected to a Nd-YAG pulsed laser source. During the laser glazing process, coatings were preheated to 600 °C and 800 °C in order to obtain different microstructure of the laser glazed coatings. The surface morphologies and cross-sections of the coatings were examined by scanning electron microscopy and microhardness measurements of coatings were carried out. The results indicate that preheating process induces a reduction of the grain size of laser glazed coatings in conjunction with an increasing of microhardness and toughness. In addition, preheating also decreases the substrate-coating interface tensile stress which leads to a reduction of crack surface density.     

Microstructure and ablation resistance of ZrCxNy-modified ZrC-SiC composite coating for carbon/carbon composites

Nitrogen-modified ZrC-SiC coatings were prepared by thermal evaporation, in situ reaction, and nitriding process, and the microstructure and ablation property of the coatings were studied. The results showed that nitrogen atoms could replace the carbon atoms and fill the vacancies of ZrC. In addition, the interface of the ZrC phase was optimized. The nitrogen atom solid solution was limited on the coating surface, and the interior of the coating was composed of high-melting point ZrC and SiC ceramics. The ablation test showed a reduction in the ablation rate of the coating after nitriding due to the formation of a dense ZrO₂ layer.     

Solution precursor high-velocity oxy-fuel spray ceramic coatings

For the first time, the solution precursor high-velocity oxy-fuel spray process was used to deposit Al₂O₃–ZrO₂ ceramic coatings. X-ray diffraction analysis and transmission electron microscopy characterization show that the as-sprayed coating is composed of mixed nanocrystalline ZrO₂ and γ-Al₂O₃ as well as amorphous phases. The as-sprayed coating consists of ultrafine splats with diameters ranging from 2 to 5 μm. Few spherical particles, hollow-shell structures are also observed on the coating surface. Polished cross-section shows that the coating is quite dense with a thickness of 40 μm.