Degradation Of Coatings Research Articles

Hot corrosion behavior of Y4Al2O9 ceramics for thermal barrier coatings exposed to calcium-magnesium-alumina-silicate at 1250 °C

The degradation of thermal barrier coatings (TBCs) by calcium-magnesium-alumina-silicate (CMAS) attack has become increasingly dramatic. Y4Al2O9 ceramic, a new potential TBC candidate, has received an increasing attention. In this study, porous Y4Al2O9 ceramic pellets, instead of actual TBCs, are used to investigate the CMAS corrosion resistance at 1250 °C. Results indicate that Y4Al2O9 reacts with CMAS melt to form an impervious sealing layer mainly containing Ca-Y-Si apatite, which could mitigate CMAS further penetration. Once the sealing layer formed, further reaction would occur above the layer accompanying by the recession of sealing layer. This process is probably related to a solid state diffusion.

Numerical Study of the Decomposition Mechanism of Intumescent Coating

Intumescent coating represents an efficient way to increase the fire resistance of construction materials. During its degradation, intumescent materials expand and form a char layer that acts as an efficient thermal shield between the heat source and the materials.Encouraging predictions of thermal degradation and expansion of intumescent coatings in experimental conditions representative of fires were reported in the last decade by considering models involving conservation equations for mass and energy for the intumescent material coupled to approximate sub-models for describing the swelling mechanism. These degradation models used generally simplified decomposition mechanisms involving one, two or three reactions. In addition, the dehydration process that occurs in the early stage is usually ignored.The objective of this article is to assess the relevance of several multi-step reaction mechanisms to describe the mass loss and the mass loss rate of ICWB, an intumescent coating, in TGA at different heating rates in inert atmosphere. The kinetic parameters of the different reactions were optimized using the Shuffled complex evolution (SCE) technique.Results demonstrate that the drying process, which occurs up to 130°C for ICWB, cannot be neglected since the associated mass loss represents about 5%. In addition, the drying process can be described by a single reaction. The use of a simple mechanism, based on one or two reactions to model the degradation of dry intumescent coating, is insufficient and leads to significant discrepancies on both mass loss and mass loss rate. Considering three reactions improve considerably the predictions but discrepancies are still observed for temperature higher than about 420°C. Model results show that a four-step is required to capture accurately all the details of the degradation of dry intumescent coating under inert atmosphere. Simulations were run under air, showing that an additional reaction for oxidation is required to extend the four-step reaction mechanism for oxidative atmosphere, leading to a five-step reaction mechanism.

High-Throughput Fire Tests and Weathering-Induced Degradation Behaviour of Intumescent Coatings

In this work, the weathering-induced degradation of intumescent coatings was investigated by a systematic and comprehensive approach. A mechanism is revealed that is proposed to be responsible for the loss of function of intumescent coatings induced by weathering.First, the thermal decomposition of artificially weathered intumescent coatings was examined. To get a better understanding of the weathering and ageing phenomena, the degradation behaviour of the single ingredients during the weathering process was investigated, as well as their chemical and physical interactions. For the systematic approach, the materials that are essential for intumescence (ammonium polyphosphate, pentaerythritol, titanium dioxide, melamine and the binder) are treated with moisture, elevated temperature and UV radiation.Thermogravimetry (TG) and IR spectroscopy were used to compare the initial samples with their different grades of weathering. We demonstrate that ammonium polyphosphate, melamine and the binder are mainly responsible for the ageing process. Further, it was demonstrated that TG and IR spectroscopy are suitable measuring methods to detect the effects of weathering on intumescent coatings.Finally, a small-scale fire test procedure is introduced. Based on the reduction of the sample size, up to 50 samples can be tested in a single fire test. The results of this fire test have the same quality as the results from standard intermediate fire tests corresponding to DIN 4102-8.

Influence of silsesquioxane addition on polyurethane-based protective coatings for bronze surfaces

Two-component solvent-born polyurethane coatings for the protection of bronze from corrosion were prepared. Trisilanol-heptaisooctyl polyhedral oligomeric silsesquixane (POSS) nanoparticles were exploited to increase the coating protective efficiency. Its improvement was confirmed through a combination of characterisation techniques. In particular, the POSS addition caused an increase of the water contact angle, and an enhancement of the elastic connotation and abrasion resistance of the polyurethane coating. Potentiodynamic polarisation measurements also indicated that the coating containing POSS nanoparticles has an improved protection efficiency. Impedance spectroscopy revealed that the magnitude of low-frequency impedance of polyurethane coatings decreased more for the coating without POSS during exposure in electrolyte for thirty days. Ex situ IR reflection-absorption spectroelectrochemistry was exploited to get insight into the degradation of coatings during chronocoulometric charging at anodic potentials. IR reflection-absorption technique was also used to evaluate any possible effect of different stripper solutions on the bronze substrate.

Corrosion Monitoring of Steel Structure Coating Degradation

An important aspect regarding the sustainability of steel structures is to ensure the structure is protected from corrosion. A number of surface coatings are available that play an important role in protecting these structures. An important part of the management of these structures is reliable and regular inspection along with methods for early detection of corrosion processes. In this paper, a development and application of sensors for monitoring the steel coating degradation and corrosion damage to steel substrate are presented. An encapsulated corrosion kit with integrated EIS sensors and ER probes was developed. To test its efficiency, steel probes were coated with selected coatings in the laboratory and their performance was assessed under various aggressive atmospheres, including; salt, industrial and humid atmosphere.

Thermal Spray of Cemented Carbide Coatings in Off-Angle Spraying: Correlations Between Process, Coating Features/Characteristics and Performance

Oxidation or decomposition of thermally sprayed coatings during deposition may noticably impair the coating features and characteristics, i.e. the surface integrity of coated components, including coating porosity, phase, microhardness, elastic modulus and toughness, etc., having a profound influence on the final performance of components. Further degradations along with enhanced oxidation of the coatings were observed for an off-angle spraying scheme in industrial practice, where inconsistent results were previously reported for spray angle-dependent degradation of various thermal spray coatings, with the mechanism not being fully understood with the less controlled surface integrity. An improved off-angle HVOF thermal spray has been developed for cemented carbide coatings, using pretreated WC-Ni feedstock powders covered with a nanoscale capsulizing nickel layer. A comprehensive study on the formation of multiple surface integrity parameters and the correlative interactions between them is thus facilitated with controllable surface integrity generation using the capsulized powders. Decarburization is mainly attributed to the exposure of pristine WC grains causing enhanced porosity in inter-splats regions, and then microhardness depends mainly on porosity and partly on phase composition, while the elastic modulus depends on intra-splats cohesion and the indentation fracture toughness on inter-splats cohesion, respectively. Abrasive wear from mild to severe regime transition could be interpreted by the inter-splats and intra-splats cohesion correlation. It has been demonstrated by the improved off-angle thermal spray that a manufacturing process could be designed and optimized on the identified unique correlations between the processes, surface integrity and the final performance.

Stability of acrylic polyurethane coatings under accelerated aging tests and natural outdoor exposure: The critical role of the used photo-stabilizers

The effect of photo-stabilizers on the crosslinking and weathering durability of acrylic polyurethane coatings based on the HSU 1168 acrylic polyol and the Desmodur N-75 polyisocyanate has been investigated. The aging of these coatings under accelerated weathering conditions and natural outdoor conditions upto a decade has been compared. Our findings showed that photo-stabilizers had a negligible effect on the conversion of isocyanate groups during the photocroslinking but enhanced the aging resistance. Coatings containing photo-absorbers exhibited an excellent weathering durability, upto 72 cycles upon accelerated aging and 10 years in natural outdoor exposure, while the unstabilized coatings appeared serious cracks, damages and chalking after 48 cycles of accelerated aging and after 5 years in outdoor weathering exposure. The degradation of coatings was also investigated by monitoring atomic force microscopy, weight loss and gloss loss, cracking, blistering and flacking of the coating. The degradation of coatings in the surface of natural and accelerated was compared by using Atomic force microscopy coupling with naonoscale infrared spectroscopy (AFM- IR) by following oxidized products on the coating surface.

Interface reactions between rutile coatings and molten aluminium or AlSi7Mg0.6 alloy

Rutile coatings deposited on corundum substrates are considered as promising functional elements improving the efficiency of the filtration of oxide inclusions out of aluminium melts. This contribution describes the reactions between rutile and two kinds of the aluminium melts and discusses the consequences of these reactions for the filtration process. It was found that the contact of rutile coatings with molten aluminium leads to the formation of a corundum layer at the solid/liquid interface. The exposure of the rutile coatings to molten AlSi7Mg0.6 alloy produces an interface layer of MgTiO3. The interface layers possess defined orientation relationship to rutile which is characteristic for locally heteroepitaxial growth. The density functional theory calculations revealed that the TiO2/α-Al2O3 and TiO2/MgTiO3 interfaces with the orientation relationships observed experimentally have low interface energies. The mechanisms of the interface layer formation and the impact of these layers on the degradation of the rutile coatings are discussed.

Temperature induced structure degradation of yttria-stabilized zirconia thermal barrier coatings

The structural aspects of yttria-stabilized zirconia (YSZ) thermal barrier coatings grown on a polycrystalline alumina substrate have been studied as a function of annealing time at 1250 °C with the use of electron microscopy, X-ray diffraction, and Raman spectroscopy. Upon the high temperature exposure, the feathery morphology of the columnar grains of the as-deposited coatings smoothens out with the concurrent development of surface undulations and grain coarsening. It is shown that the grain coarsening results in significant voiding along the merged grain boundaries and inside YSZ grains. The nucleation and growth of the high density of voids is related to the clustering of vacancies supplied from the merged gaps between columnar grains and subgrains within the columnar grains. The continued development of the voids leads to the formation and propagation of large cracks within the columnar grains, which weakens the overall mechanical integrity of the coating.

Thermal degradation and pyrolysis analysis of zinc borate reinforced intumescent fire retardant coatings

This work aims at evaluating the potential of nano-sized zinc borate as substitution of boric acid for thermal degradation and gaseous products in expandable graphite based intumescent fire retardant coating IFRC systems. The thermal degradation and pyrolysis of intumescent flame retardant coatings was characterized by bunsen burner fire test, thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDX), X-ray diffraction (XRD), fourier transformed infra-red analysis (FTIR), X-ray photoelectron spectroscopy (XPS) and gaseous emission analysis (Py-GC). Bunsen burner fire test reveals that the partial substitution of zinc borate (6.61 mass%) imparts a substantial improvement in thermal stability and reduces steel substrate temperature to 124 °C. From thermogravimetric analysis results, it was shown that this composition increases char residual mass from 39 to 46.14 mass%. The morphological structures of char residue were investigated by FESEM and it indicates that zinc borate promotes more continuous and compact char layers that hinder the heat diffusion and oxygen transmission effectively. XRD and FTIR results show that zinc borate develops a zinc-based glassy intumescent shield i.e. zinc bis (hydroxyanthrapyrimidine) dehydrate (C30H14N4O4Zn·2H2O) that strengthens the char. The new chemical species enhances the thermal stability of the freshly formed char at high temperature and provides an enhanced fire protection. XPS analysis shows the higher carbon contents in formulation IF-5 (6.6 mass%) and endorses high char residue. The Py-GC analysis confirms release of less toxic gaseous products in IF-5 formulation, considering their type and concentration, as compared to control formulation, and is considered as an environmentally safe intumescent formulation.

Diffusion cell investigations into the acidic degradation of organic coatings

Protective organic coatings work by preventing contact between an aggressive environment and a vulnerable substrate. However, the long required lifetime of a barrier coating provides a challenge when attempting to evaluate coating performance. Diffusion cells can be used as a tool to estimate coating barrier properties and lifetime. In this work, a diffusion cell array was designed, constructed, and compared to previous designs, with simplicity being the most important design parameter. Sulfuric acid diffusion through five different coatings was monitored using a battery of cells, and a mathematical model was developed to simulate the experimental data and to study diffusion mechanisms. The diffusion cells allowed an objective and fast analysis of coating barrier properties. It was found that sulfuric acid deteriorated these properties as it diffused through the films. This was also expressed in the modeling results, where a three-step time dependency of the diffusion coefficient was required to simulate both acid breakthrough time and the subsequent steady-state flux. A vinyl ester-based coating proved to be the most effective barrier to sulfuric acid diffusion, followed by a polyurethane coating. Amine-cured novolac epoxies provided the least effective protection.

Degradation of La2Zr2O7thermal barrier coatings

Degradation of La2Zr2O7thermal barrier coatings

Predicting Effect of Base Alloy Composition on Oxidation- and Interdiffusion-Induced Degradation of an MCrAlY Coating

The effect of two base alloys, viz. Ni-based superalloy Rene 80 and Co-based alloy ECY768, on the oxidation and interdiffusion behavior of a NiCoCrAlY Re-containing coating after exposure for up to 2000 h at 1050°C in synthetic air has been evaluated. Scanning electron microscopy with energy-dispersive x-ray spectroscopy was employed to measure the element concentrations, and phases were identified by electron backscatter diffraction. Significant depletion of β-NiAl within the coating, enrichment of γ′-phase, and formation of µ-phase in the alloy and Kirkendall porosity at the coating–alloy interface were observed in the coating on Rene 80. In contrast, the coating on the Co-base alloy ECY768 formed large amounts of Cr/Re-rich carbides and the depletion of β-NiAl phase was less pronounced than that observed on the Ni-base alloy. These findings could be interpreted using an in-house-developed thermodynamic–kinetic CALPHAD-based computer model.

Semitransparent Decorative Coatings Based on Optical Interference of Metallic and Dielectric Thin Films for High Temperature Applications

This paper introduces a thin film multilayer structure composed of dielectric and metallic layers that allows for a wide range of aesthetic appearances using the phenomenon of optical interference. In addition, this multilayer structure allows the reflection and transmission coefficients to be controlled independently. The application of these decorative coatings to induction stoves is also studied. The aim is to provide an attractive aesthetic appearance for the transparent glass-ceramic, and allow the visualization of blue and white lighting systems. Moreover, degradation of these decorative coatings is studied at high temperatures, so as to ensure that the coating does not change its aesthetic appearance during normal operation of the stove. It has been found to be necessary to use dielectric materials with low diffusion coefficients of oxygen, or not containing oxygen, to prevent oxidation of the metal layers when subjecting the coating to high temperatures.

Microstructural degradation of Electron Beam-Physical Vapour Deposition Thermal Barrier Coating during thermal cycling tracked by X-ray micro-computed tomography

The degradation of an Electron Beam-Physical Vapour Deposition Thermal Barrier Coating caused by thermal cycling at 1150 °C has been followed in 3D non-destructively by time-lapse X-ray micro-computed tomography (μ-CT). Quantitative analysis of X-ray μ-CT virtual cross-sections on small samples is validated by destructive cross-sectional scanning electron microscopy (SEM) micrographs of larger ones. The evolution of thermally-grown oxide (TGO) is quantified. The TGO/bond coat interface roughness is measured in 3D. No significant rumpling is observed. Undulations are found locally at the interface of the as-deposited sample. Such undulations can increase in amplitude during cycling providing locations for interfacial cracks to initiate.

Coatings and Thin Films for Spacecraft Thermo‐Optical and Related Functional Applications

AbstractAn overview of coatings and thin films for spacecraft applications is provided, focusing on coatings for thermal control and optical applications. An outline of the requirements imposed by the harsh conditions in space is given. Space applications of coatings are listed and their qualification is described, including testing in the ESA laboratories at ESTEC. Coatings degradation in space and environmental testing, including degradation due to vacuum, thermal cycling, radiation, atomic oxygen, space debris, and outgassing effects, is covered in detail. Recent advances in the field and perspectives for future development are also reviewed based on the published literature.

Impermeability of Y3Al5O12 ceramic against molten glassy calcium-magnesium-alumina-silicate

Degradation of thermal barrier coatings (TBCs) caused by calcium-magnesium-alumina-silica (CMAS) glassy penetration is becoming an urgent issue for TBCs industrial applications. In this work, yttrium aluminum garnet (Y3Al5O12, YAG) nano-powders were synthesized through a chemical co-precipitation route. The resistance of YAG ceramic to glassy CMAS infiltration at 1250 °C was evaluated. YAG ceramic bulk sintered at 1700 °C for 10 h was comprised of a single garnet-type Y3Al5O12 phase. The molten CMAS glass was suppressed on the surface of the YAG ceramic at 1250 °C. A chemical reaction between YAG and the molten CMAS glass did not occur at 1250 °C for 24 h, suggesting that YAG could act as an impermeable material against glassy CMAS deposits in the TBC field.

Enhancement of UV-aging resistance of UV-curable polyurethane acrylate coatings via incorporation of hindered amine light stabilizers-functionalized TiO2-SiO2 nanoparticles

In this study, polymeric hindered amine light stabilizers (HALS)-functionalized silica coated rutile titanium dioxide (TiO2-SiO2) nanoparticles were prepared by encapsulating commercially available TiO2-SiO2 nanoparticles with methyl methacrylate (MMA) and 1,2,2,6,6-pentamethyl-4-piperidyl methacrylate (PMPM) copolymers via miniemulsion polymerization. The obtained functional (TiO2-SiO2/P(MMA-co-PMPM)) fillers have been added to polyurethane acrylate (PUA) oligomers to get UV-curable nanocomposite coatings. The functionalization of the TiO2-SiO2 nanoparticles with polymeric HALS has been confirmed by infrared spectra (IR), thermogravimetric (TG), and X-ray photoelectron spectroscopy (XPS) analyses. The scanning electron microscope (SEM) micrographs indicated that homogeneous dispersion of TiO2-SiO2/P(MMA-co-PMPM) composite nanoparticles resulted in improved transparency and mechanical properties of the UV-curable PUA coatings. Rhodamine B (Rh.B) photodegradation measurement confirmed the excellent UV-shielding performance of PUA nanocomposite coatings containing TiO2-SiO2/P(MMA-co-PMPM). The addition of TiO2-SiO2/P(MMA-co-PMPM) composite nanoparticles reduced the UV-curable PUA coatings degradation rate dramatically. The UV-aging resistance of PUA coatings was improved significantly. Over all, the combination of TiO2-SiO2 nanoparticles and polymeric HALS offers an attractive way to fabricate the multi-functional fillers, which can be used to improve the mechanical properties and UV-aging resistance of PUA coatings simultaneously.

Degradation of protective coatings in steel chimneys of flue gas desulfurisation systems

This paper presents problems associated with corrosion of steel stacks and changes in philosophy of corrosion protection when implemented into flue gas desulphurization systems. In-situ tests were taken by means of electrochemical impedance spectroscopy at the stack which discharges flue gas before wet lime/limestone desulphurization. A very quick electrolyte absorption by the spherical infill vinylester coating was found using the electrochemical impedance spectroscopy. An analysis of destruction shows a joint effect of mechanical and corrosion factors on the poor resistance of the coating. The poor resistance of the coating is also connected with the method of chimney construction and coating application.

Improving the Degradation Resistance and Surface Biomineralization Ability of Calcium Phosphate Coatings on a Biodegradable Magnesium Alloy via a Sol-Gel Spin Coating Method

Bioactive calcium phosphate (CaP) coatings have been applied on biodegradable magnesium (Mg) and its alloys to provide a suitable degradation rate for orthopedic applications. However, the porous CaP coatings formed in the aqueous solution owns poor degradation resistance and thus short in vivo valid life. This paper reports a sol-gel spin coating method to prepare hydroxyapatite (HA, Ca10(PO4)6(OH)2) and fluorine doping HA sealing coatings on a porous CaP coating. The effects of fluorine content on the degradation and biomineralization behaviors of the composite coatings were studied. It reveals that the homogeneous and stable sol-gel coating, especially the fluorapatite coating could significantly improve the degradation resistance and surface biomineralization ability of the porous CaP coating on the Mg alloy.