Degradation Of Coatings Research Articles

Vapor-Phase CMAS-Induced Degradation of Adhesion of Thermal Barrier Coatings

An investigation of the interface degradation and failure of EB-PVD thermal barrier coatings (TBCs) during burner rig cyclic oxidation in the presence of calcium–magnesium–alumino-silicate (CMAS) deposits has been conducted. The formation of two compounds of major CMAS constituents, specifically MgAl2O4 (spinel) and CaAl2Si2O8 (anorthite), was observed at the interface between the alumina scale and the TBC. Spallation occurred as a result of cracking and delamination primarily through the spinel layer. The tests showed more than a tenfold decrease in TBC life relative to the oxidation test without CMAS at the same temperature. It is demonstrated that this particular form of coating degradation occurs below the melting temperature of this particular CMAS composition and hence does not involve TBC infiltration by molten silicates. The likely mechanism of interface degradation, which involves vapor-phase transport of CMAS constituents as hydroxides, is proposed and discussed.

Water Vapor Effects on the CMAS Degradation of Thermal Barrier Coatings

A new damage mechanism of CMAS degradation of TBCs involving delamination through the thermally grown oxide (TGO) has been observed on burner rig tests of EB-PVD thermal barrier coatings. This mechanism operates in the absence of CMAS melting and is mediated by hydroxylation and volatilization of the constituent CMAS hydroxides, followed by vapor transport to the vicinity of the TGO through the TBC porosity. Analyses reveal the formation of spinel (MgAl2O4) and anorthite (CaAl2Si2O8) at the TGO/TBC interface, with concomitant evolution of porosity at the spinel/alumina interface. Tests were conducted in a controlled atmosphere tube furnace to validate the hypothesis and assess the factors that affect the evolution of the deleterious oxides. The microstructure of the reaction products formed in the lab test is compared to that formed in the burner rig.

A bulk and localised electrochemical assessment of epoxy-phenolic coating degradation

Abstract Water absorption is believed to be one of the main causes leading to the deterioration and degradation of protective organic coatings. Water uptake in coatings on metal may be measured, for example, by changes in electrical impedance (e.g. capacitance and resistance) as a function of time. Generally coating capacitance is expected to increase during initial stages of water uptake (due to the greater dielectric constant of water) while coating resistance is expected to decrease (due to the lower resistivity of water compared to most polymers). However, here we present evidence that water ingress is not the main determinant of damage in epoxy-phenolic can coatings. Thus, as the degree of cure is increased, the water saturated coating capacitance and resistance both increase while the time-to-failure also increases. We suggest that these observations are due, respectively, to an increase in the free polymer volume (permitting more water uptake) and to an increase in the charge transfer resistance at the metal-polymer interface (due to a higher density of polymer-to-substrate bonding). These results and interpretations are supported by local electrochemical impedance spectroscopy which has confirmed water absorption, coating failure and increased coating resistance for highly cured capacitive systems, at the microscale.

Electrochemical impedance spectroscopy (EIS) study on the degradation of acrylic polyurethane coatings

Degradation of acrylic polyurethane coatings with different NCO/OH values was evaluated by EIS technology.

Application of Plasma Erosion Technique for Faster Degradation of Coatings and Prediction of Their Durability

Assessment of durability of coatings is crucial for researcher especially for design of paints which would have service life warranty. As natural exposure takes several months for such studies, there is tendency today to use faster techniques like UV testers and Xenon arc weatherometers. Although these techniques are several times faster than natural exposure, it takes few weeks for coating chemist to observe any degradation. Among quicker techniques, plasma erosion is found to be useful one. Oxygen plasma produces extremely rapid degradation and few hours of exposure to plasma provides effects of several years of natural weathering. Advantages of the technique are speed and plasma equipments are cheaper than weatherometer. Results obtained with plasma erosion technique are found to have good correlation with results of exposure under Xenon arc source.

Isothermal oxidation behavior and kinetics of thermal barrier coatings produced by cold gas dynamic spray technique

The cold gas dynamic spray (CGDS) method was employed to deposit the CoNiCrAlY bond coats of thermal barrier coating (TBC) system. The oxidation behavior of the coatings were investigated under isothermal oxidation at 1000°C, 1100°C and 1200°C for 8, 24, 50 and 100h. Recent studies on TBCs have concentrated on the CGDS process and its properties under working conditions. The motivation of this study is to investigate the oxidation behavior of TBCs produced using CGDS technique under service conditions and to determine the oxidation growth kinetics of thermally grown oxide (TGO). The results show that the isothermal degradation of coatings was considerably influenced by microstructure of coating, interfacial oxide growth rate, oxidation temperature and time.

Microstructure and performance analysis of organic coating under high temperature and high CO2 partial pressure

To evaluate the property of the organic coatings in oil and gas plants, the aging process was studied in high temperature and high CO2 partial pressure environment. Correlations were developed between the macroscopic properties and microstructure of the organic coatings. The surface appearance, mechanical properties, and permeability of the organic coatings were measured. Furthermore, the crystal structure of the organic coatings was investigated through synchrotron radiation grazing incidence X-ray diffraction (GIXRD) on the BL14B1 beam line in Shanghai Synchrotron Radiation Facility. Combined with the Fourier transform infrared spectroscopy, the molecular structure of the organic coatings was investigated. The experimental results indicate that the thickness variation and weight loss of the organic coatings increase with the immersion time, and the penetration resistance of the coating obviously decreases as the temperature rises. Moreover, the degradation of the organic coatings with immersion time in high temperature and high CO2 partial pressure environment is caused by the amorphization of the organic coatings as the groups and bonds of the organic coatings were not damaged.

Influence of heat-stable filler on the thermal shielding performance of water-based intumescent fire-resistive coating for structural steel applications

Intumescent coatings are the newest passive fireproofing materials which maintain structural integrity of high-rise buildings in fire events. The present work focuses on the influence of zirconium silicate as a heat-stable filler in intumescent coatings. Different coatings were formulated by varying the zirconium silicate concentration from 1, 3, 5, 8, and 10 on parts per hundred basis (phr). Fire performance of the coatings was then determined by fire test using a Bunsen burner fire flame at 950°C for 1 h. The degradation of coatings was examined by thermogravimetric analysis (TGA). The morphology of the intumescent chars was analyzed by environmental scanning electron microscopy. The char was also examined by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy. XRD confirmed the inertness of zirconium silicate with intumescent ingredients at high temperatures. TGA showed an increase in the weight residue of char at high temperature. The incorporation of zirconium silicate into intumescent coating forms a thermally stable char with better substrate adhesion. EDS analysis confirmed an increase in the antioxidation property of the char, and the fire test also confirmed an increase in char strength of coatings by the incorporation of zirconium silicate.

Técnica de medição de saliva utilizada no CECOM em pacientes com halitose

A saliva é um fluido aquoso e transparente secretado pelas glândulas salivares diretamente na cavidade bucal. É composta, principalmente, de água (99%) e componentes orgânicos e minerais. Uma de suas principais funções é a autolimpeza da boca. Quando há diminuição do fluxo salivar, observa-se maior estagnação de matéria orgânica sobre a língua, como células epiteliais descamadas e bactérias, formando a Saburra Lingual. A degradação da Saburra Lingual pelas bactérias provoca a formação de compostos sulfurados voláteis que causam o mau hálito. Uma boa quantidade de saliva é um dos fatores importantes para manter o hálito agradável. Neste trabalho, vamos demonstrar a técnica de medição de saliva que utilizamos no Cecom para tratamento da Halitose. Para esta técnica, utilizamos um tubo de Proveta com medições, um tubete de silicone de 1,5 cm de comprimento e um cronômetro. O paciente deve mastigar o tubete de silicone, durante 5 minutos, e ir cuspindo toda a saliva que for sendo produzida para dentro do tubo de Proveta. Ao final dos cinco minutos, medimos a quantidade de saliva dentro do tubo e dividimos este valor por 5. Desta maneira, obtemos o fluxo salivar do paciente. Com a saliva dentro do tubo de Proveta, avaliamos os padrões salivares do paciente, como quantidade, coloração, turbidez e viscosidade. Se for encontrada alguma alteração, realizamos o tratamento adequado para reestabelecer os padrões normais, através de Sialogogos, que estimulam a produção da saliva de diversas maneiras.

Development of Wireless and Passive Corrosion Sensors for Material Degradation Monitoring in Coastal Zones and Immersed Environment

Wireless and fully energetically passive corrosion sensors inspired from the chipless Radio-Frequency IDentification (RFID) technology are presented for the monitoring of degradation of infrastructures located in coastal zones and underwater environments. The device consists of a reader and a remote passive sensor. This study presents two radio-frequency (RF) corrosion-sensitive resonators that can be integrated in such a device. The first is sensitive to a loss of metal due to its degradation, and the second is sensitive to the corrosion potential of metals, with respect to a reference electrode. The RF characterization of these two resonators demonstrates unambiguously the sensitivity of the method to corrosion of metal, passivation of steel as well as degradation of organic coatings. Then, their integration in a demonstrator including antennas is considered. The main parameters that influence the RF wave propagation and thus the reading distance are discussed. Following the presented results, the proposed method is considered as a reliable and versatile tool which should be able to be deployed in immersed, tidal, and splash zones, for example, for offshore facilities monitoring.

Control of the degradation of silica sol-gel hybrid coatings for metal implants prepared by the triple combination of alkoxysilanes

Hybrid materials obtained by sol-gel process are able to degrade and release Si compounds that are useful in regenerative medicine due to their osteoinductive properties. The present work studies the behavior of new organic-inorganic sol-gel coatings based on triple mixtures of alkoxysilanes in different molar ratios. The precursors employed are methyl-trimethoxysilane (MTMOS), 3-glycidoxypropyl-trimethoxysilane (GPTMS) and tetraethyl-orthosilicate (TEOS). After optimization of the synthesis conditions, the coatings were characterized using 29Si nuclear magnetic resonance (29Si-MNR), Fourier transform infrared spectrometry (FT-IR), contact angle measurements, hydrolytic degradation assays, electrochemical impedance spectroscopy (EIS) and mechanical profilometry. The degradation and EIS results show that by controlling the amount of TEOS precursor in the coating it is possible to tune its degradation by hydrolysis, while keeping properties such as wettability at their optimum values for biomaterials application. The corrosion properties of the new coatings were also evaluated when applied to stainless steel substrate. The coatings showed an improvement of the anticorrosive properties of the steel which is important to protect the metal implants at the early stages of the regeneration process.

A Comparative Study of Two Coating Systems Exposed for 2 Years in the Field and in Accelerated Atmospheric Corrosion Chamber Environments

Analysis of a magnesium rich and a full chromate coating system on AA2024-T3 panels exposed at two field sites, Pt. Judith, RI and a University Oceanographic Laboratory Ship (UNOLS) based out Seattle, WA for two years were compared to identical samples exposed in a B117 test chamber (2000 hours), and a modified salt fog chamber with ultra-violet radiation and ozone gas (2000 hours). Both coating systems utilized a topcoat of polyurethane. The analysis was performed utilizing FT-IR ATR on multiple locations for each panel as well as scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). When comparing the IR spectra for the UV/ozone chamber and field exposure samples to the baseline data for one formulation of topcoat in the magnesium rich coating system, it was observed that a decrease in the % reflectance peaks for various components of the polyurethane had occurred suggesting the degradation of the urethane component of the topcoat. The change in the FT-IR spectra indicating topcoat degradation on the field and modified chamber exposures is in contrast to the spectra from the samples exposed to the B117 chamber, which looks more like that of the baseline data. Therefore, the FT-IR data indicates that degradation of the topcoat components of the magnesium rich coating system exposed in the modified chamber after 2000 hours was much more similar to the identical coating exposed for 2 years at the Pt. Judith and the Seattle-based UNOLS ship, whereas the coating exposed to the B117 chamber was much more similar to the baseline sample that had not been exposed. When considering the IR spectra for the full chromate coating system, there were some slight differences in the spectra, this presumably being attributed to different formulations between the topcoat for the Mg rich system and the topcoat for the full chromate system. In terms of observable differences in the spectra between the exposures and the baseline samples, all major peaks were diminished under the UV/ozone exposure after 2000 hours. In terms of the B117 exposure, all of the major peaks are larger in magnitude than the baseline or field and UV/ozone chamber exposures; Therefore, the FT-IR data for the full chromate coating system formulation tested in the UV/ozone indicates that it is more aggressive in causing the degradation of the coating system components than the B117 chamber test. For the SEM-EDS results, the coated test panels that were subjected to the modified chamber exposure with UV/ozone/sodium chloride solution had higher relative percentages of oxidation in the scribe over the duration of exposure when compared to the B117 exposure chamber. It was observed for the coating surfaces analyzed for the fully chromated and Mg rich coating systems that longer exposures in the modified chamber contribute to not only increased oxidation of the scribe but also of the coating as compared to the baseline analyses of the full chromate or Mg rich coatings. Furthermore, higher percentages of aluminum, titanium, and especially silicon are noted with time in the modified chamber. These results are significant because the UV/ozone conditions in the modified chamber are at or higher than the cumulative values observed in the field and, in this instance, are an indication of degradation of the advanced performance coatings in the modified chamber that are normally very resistant to weathering, chalking, and color changes in the field. It also must be noted that the modified and B117 chamber exposures were in constant spray conditions, whereas the field sites experienced intermittent wet and dry exposure cycles. These results suggest, therefore, that the coating resin is becoming more enriched in pigment particles at the coating surface and are associated with coating system degradation over time, and that it may be possible to tailor the chamber exposure conditions to yield coating degradation specific to an exposure site in the field.

(Invited) EIS Analysis of the Water Uptake Process in Model Epoxy Coatings and Free Films

Electrochemical Impedance Spectroscopy (EIS) is a widely used technique to study the organic coatings degradation. Thanks to capacitance measurements in the high frequency (HF) domain, EIS allows measuring the water uptake into the coating, using different models. Usually, a three electrodes cell is used to perform such measurements but if the sole HF domain is considered to assess the water uptake during the beginning of the immersion period, then the reference electrode (RE) can be deleted and a two electrodes cell can be used. In this work, model epoxy coatings (DGEBA/DAMP), with or without pigments (TiO2), were prepared as free films or were applied onto steel panels (Q-Panel). The different samples were immerged in a NaCl 3 wt% saline solution and water uptake was followed by EIS using a two or three electrodes cell. EIS spectra obtained without RE show new time constants in the HF domain which can be related to intrinsic properties of the system. This leads to a better understanding of the water uptake phenomenon into organic coatings and offers the possibility to modify well-known water uptake models.

Degradation of conformal coatings on printed circuit boards due to partial discharge

A series of experiments has characterised the nature of damage to the surface of silicone-coated printed circuit boards resulting from partial discharge. A potential difference was applied between tracks and observations indicate that partial discharge activity occurred above the coating surface. Analysis of damage development indicates that coatings 70 μm or less in thickness are more severely degraded and thus offer reduced protection from active partial discharge. Coating degradation is reduced with increasing thickness, with coatings of 180 μm and greater showing the least degradation. Finite element analysis indicated surface electric field strengths of 7.5 to 8.0 kV mm-1 could be achieved before partial discharge damage was observed in the 180 μm coatings. Inconsistent and fluctuating partial discharge inception voltages may be a consequence of at least two competing factors that alter surface electric fields, namely the accumulation of surface pollution of a finite conductivity which reduces surface fields, and the surface field-strengthening effects of developing surface defects.

Controlled Drug Release: Photocatalytic Degradation of Cell Membrane Coatings for Controlled Drug Release (Adv. Healthcare Mater. 12/2016)

Controlled Drug Release: Photocatalytic Degradation of Cell Membrane Coatings for Controlled Drug Release (Adv. Healthcare Mater. 12/2016)

The impact of storage conditions upon gentamicin coated antimicrobial implants

A systematic approach was developed to investigate the stability of gentamicin sulfate (GS) and GS/poly (lactic-co-glycolic acid) (PLGA) coatings on hydroxyapatite surfaces. The influence of environmental factors (light, humidity, oxidation and heat) upon degradation of the drug in the coatings was investigated using liquid chromatography with evaporative light scattering detection and mass spectrometry. GS coated rods were found to be stable across the range of environments assessed, with only an oxidizing atmosphere resulting in significant changes to the gentamicin composition. In contrast, rods coated with GS/PLGA were more sensitive to storage conditions with compositional changes being detected after storage at 60°C, 75% relative humidity or exposure to light. The effect of γ-irradiation on the coated rods was also investigated and found to have no significant effect. Finally, liquid chromatography–mass spectrometry analysis revealed that known gentamines C1, C1a and C2 were the major degradants formed. Forced degradation of gentamicin coatings did not produce any unexpected degradants or impurities.

Fabrication of ultra high temperature ceramic matrix composites using a reactive melt infiltration process

Fiber reinforced composites containing a ZrB2 matrix are prepared by reactive melt infiltration, using a Zr based intermetallic compound. The reactive melt infiltration process is further characterized by determining the contact angle of the different melt and preform elements. Two different methods for boron powder impregnation of the preforms, which form ZrB2 during Zr2Cu melt infiltration, are evaluated. Degradation of fibers and different fiber coatings for Zr2Cu melt are evaluated and the microstructure of the infiltrated samples is investigated by means of XRD and SEM.

Photocatalytic Degradation of Cell Membrane Coatings for Controlled Drug Release.

Biomimetic cell-membrane-camouflaged particles with desirable features have been widely used for various biomedical applications. However, there are few reports on employing these particles for cancer drug delivery due to the failure of the membrane coatings to be efficiently degraded in the tumor microenvironment which hampers the drug release. In this work, core-shell SiO2 @TiO2 nanoparticles with enhanced photocatalytic activity are used for controlled degradation of surface erythrocyte membrane coatings. The antitumor drug docetaxel is encapsulated into nanocarriers to demonstrate the controlled drug release under ultraviolet irradiation, and the drug-loaded nanoparticles are further used for enhanced cancer cell therapy. Here, a simple but practical method for degradation of cell membrane coatings is presented, and a good feasibility of using cell membrane-coated nanocarriers for controlled drug delivery is demonstrated.

Influence of biodegradable polymer coatings on corrosion, cytocompatibility and cell functionality of Mg-2.0Zn-0.98Mn magnesium alloy

Four kinds of biodegradable polymers were employed to prepare bioresorbable coatings on Mg-2.0Zn-0.98Mn (ZM21) alloy to understand the relationship between polymer characteristics, protective effects on substrate corrosion, cytocompatibility and cell functionality. Poly-l-lactide (PLLA), poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) or poly(lactic-co-glycolic) acid (PLGA) was spin-coated on ZM21, obtaining a smooth, non-porous coating less than 0.5μm in thickness. Polymer coating characterization, a degradation study, and biocompatibility evaluations were performed. After 4 w of immersion into cell culture medium, degradation of PLGA and PLLA coatings were confirmed by ATR-FTIR observation. The coatings of PLLA, PHB and PHBV, which have lower water permeability and slower degradation than PLGA, provide better suppression of initial ZM21 degradation and faster promotion of human osteosarcoma cell growth and differentiation.

Model of coating wear degradation in precision glass molding

Coatings in precision glass molding (PGM) experience severe thermo-chemical and thermo-mechanical loads during several hundred or thousand pressing cycles. Until now, little is known about the integrity loss of protective coatings for these applications. Gaining knowledge of fundamental mechanisms on the functional degradation of protective coatings is of great interest. The work presented in this paper investigates compositional changes in the surface-near region of precious metal PVD-coatings after being annealed in hot glass contact. Different characteristic wear zones identified on the samples have been analyzed by light microscopy, SEM/EDX, as well as X-ray photoelectron spectroscopy (XPS). The results are compared to the wear stages observed on molds used in industrial production. A qualitative model for the degradation of the substrate-coating system is proposed, ranging from the initial phases of material alteration to the final coating breakdown. In addition, indications that glass components might play a role in the overall degradation process were found but could not be revealed in detail. With regards to highly precise molding tools for PGM, further work should be focused on the initial phase of wear, since the end of service life time is reached at early degradation stages, far before the coating flake-off.