Durability Of Coatings Research Articles

Electrochemical Assessment of Galvanized Steel Corrosion in Alkaline Sulfate Solution

Corrosion of steel prestressing strand in bonded post-tensioned bridge tendons have been associated with elevated concentrations of sulfate ions in deficient grout that can develop due to grout segregation [1]. Previous research indicated that the early presence of elevated sulfate ion concentration in the deficient grout in contact with the embedded steel can impair steel passivity. Metastable pitting was observed for steel in saturated calcium hydroxide solutions with sulfate ion concentrations greater than 1,300 ppm and pitting was observed in solutions above 20,000 ppm [2-4]. In addition to steel strand corrosion, galvanized steel ducts in direct contact with the deficient grout with elevated sulfate ion concentrations were also observed to have severe localized corrosion [1]. Galvanized steel in alkaline solutions, such as in cement pore water, have been characterized to have greater resistance to chloride-induced corrosion with chloride threshold concentrations as much as 2.5 times greater than carbon steel [5]. Information relating to sulfate concentrations is less available. Electrochemical testing including the electrochemical noise (EN) technique was conducted to identify the role of early sulfate ion presence on corrosion of galvanized steel.EN provides a test method to assess the corrosion modality without the influence of an external excitation that can affect the electrode surface and electrolyte chemistry (such as metal oxidation and oxygen reduction) [6-7]. The testing aims to identify the corrosion mitigation characteristics of the galvanized steel such as zinc activity and protective barrier surface oxide layers in presence of sulfate ions as well as combined cases with chloride ion contamination below reported threshold concentrations for galvanized steel. In the testing here, steel specimen were exposed to alkaline solutions simulating pore solutions with pH 12.6 with varying sodium sulfate levels and sodium chloride. Supplemental electrochemical experiments included the steady-state condition (open circuit potential, linear polarization test, and electrochemical impedance spectroscopy). References K. Lau, and I. Lasa. "Corrosion of prestress and post-tension reinforced-concrete bridges." Corrosion of steel in concrete structures. Woodhead Publishing, 2016. 37-57. DOI: https://doi.org/10.1016/B978-1-78242-381-2.00003-1.S. Permeh, K. K . Krishna Vigneshwaran, K.Lau., and I.Lasa. “Corrosion of Post-Tensioned Tendons with Deficient Grout. Part 3: Segregated Grout with Elevated Sulfate and Vestigial Chloride Content.” Corrosion 75 (2019) pp.848-864.S. Permeh, K. Lau, M. Duncan, and R. Simmons. “Initiation of localized steel corrosion in alkaline sulfate solution.” Materials and Structures, 54 (2021), pp.1-17.S. Permeh, K. Lau, M. Duncan, and R. Simmons. “Identification of steel corrosion in alkaline sulfate solution by electrochemical noise.” Materials and Corrosion, 72(2021), pp.1456-1467.S. Yeomans. Galvanized steel reinforcement in concrete. Elsevier, 2004.S. Permeh, and K. Lau. "Electrochemical Behavior of Steel in Alkaline Sulfate Solutions with Low Level Chloride Concentrations." ECS Meeting s. No. 10. IOP Publishing, 2021.K. Lau, S. Permeh. “Assessment of durability and zinc activity of zinc-rich primer coatings by electrochemical noise technique.” Progress in Organic Coatings, 167(2022): 106840.

Effect of substrate roughness on NOx removal of poly heptazine imides coated cement pastes exposed to washing and weathering

Developing photocatalytic coatings onto cementitious materials is an attractive approach to combat air pollution. However, the low photocatalysis durability of the photocatalytic cementitious materials is the main challenge for their large-scale implementation in aggressive outdoor environments. Here, a coating composed of poly heptazine imides (PHI) was applied onto hardened cement pastes with different substrate roughness for nitrogen oxides (NOx) abatement under artificial washing and natural weathering. The apparent appearance, micromorphology and chemical composition of the PHI-coated cement pastes were characterized via visual inspection, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy and attenuated total reflectance–Fourier transform infrared analysis. It was found that increasing the substrate roughness improved the adhesion of PHI coating against the washing test, contributing to a higher photocatalytic efficiency of NOx removal at highly rough substrates after subjecting to the washing test. After the weathering tests, the NOx removal of PHI-coated cement pastes increased and then decreased slightly with the increasing substrate roughness; the highest photocatalytic efficiency was observed at a substrate roughness of 2.83 μm. The observations open a window for improving the durability of photocatalytic coatings applied onto cementitious materials under real-life conditions.

A Durable Nano-SiO2-TiO2/Dodecyltrimethoxysilane Superhydrophobic Coating for Stone Protection

Water can trigger freeze–thaw cycles, acid rain corrosion, and microbial colonisation, all of which destroy stone. Water is one of the most influential factors in the destruction of outdoor stone heritage. Therefore, materials with excellent hydrophobic properties and durability are urgently required to effectively retard long-term stone weathering. In this study, two nanoparticles, TiO2 and SiO2, were used to modify dodecyltrimethoxysilane (DTMS), a waterproof coating commonly used for stone heritage protection, to fabricate nanocomposite superhydrophobic coatings. The micromorphology, water repellence (water contact angle and capillary water absorption), suitability to protect stone heritage (color change and water vapor permeability), and durability (thermal, light, and chemical stability) of DTMS and nanocomposite coatings were evaluated. The scanning electron microscope (SEM) images revealed that adding 0.5% (w/w) SiO2 produced nanoscale roughness on the sandstone surface, leading to superhydrophobicity. The results of ultraviolet -visible (UV–Vis) spectrophotometer showed that adding 0.01% TiO2 shielded more than 90% of UV light but accelerated the decrease in the contact angle under UVA irradiation. The addition of SiO2 was able to avoid the detrimental effect of TiO2 under UV light. The thermogravimetric analysis (TGA) results showed that both SiO2 and TiO2 nanoparticles improved the thermal stability of the coatings. In particular, the fabricated nanocomposite coating, SiO2 and TiO2 co-modified DTMS, had excellent water repellence, low color change and outstanding durability, and retained about 85% of the water vapor permeability of the stone, showing promise for stone protection.

A reliable method by utilizing thermo-responsive palladium nanocomposite for fabricating Nickel coating on nylon 6 fabrics

Flexible conductive textiles are one of the best choices for smart textiles due to their comfortable wearing and good compliance with the human body. The great adhesion and durability of metal coatings onto substrate surfaces are important indicators for the high-performance requirements of smart textiles. The main bottleneck for the development of flexible conductive textiles is the poor adhesion between the metal coating and the substrate, so efforts have been made to improve it on a variety of fabrics. Herein, we reported a facile and economical method to fabricate flexible conductive fabrics using commercial nylon 6 fabrics as substrates. In brief, Styrene-co-N-Isopropylacrylamide/Pd (St-co-NIPAAm/Pd) nanocomposites acted as activators and adhesive interlayers for electroless plating to improve the adhesion of the nickel layer onto the nylon 6 surface. A highly conductive nylon 6 fabric was prepared by a coating of St-co-NIPAAm/Pd nanocomposite and followed by Ni-P electroless plating. The optimal electroless plating process parameters for fabricating adhesion-enhanced and highly reliable nickel coating were also investigated in the study. Its reliable conductivity and high durability mean it is promising for smart textiles, even via a series of repeated bending and washing tests. In addition, we explain the mechanism by which St-co-NIPAAm/Pd nanocomposites change at a temperature below or beyond the lower critical solution temperature (LCST). To sum up, the developed conductive nylon 6 fabric can lay the groundwork for the subsequent development of smart textiles.

RETRACTED: Efficiency and durability of carbon dots grafted TiO2-based coatings applied on concrete under peeling and washing trials

In engineering practice, the durability of photocatalytic coatings is a major concern. Here two types of unique visible light-responsive coatings are proposed and placed on the concrete surface, consisting of vinyl chloride/vinyl ester/ethylene copolymer (as a binder) and carbon-dots-grafted TiO 2 (TiO 2 -CDs) but different in fabrication. The first type is mono-layer coating (MLC), which involves directly spraying the TiO 2 -CDs suspension containing the binder onto the concrete. The second type was called double-layer coating (DLC), which involves applying a binder layer to the concrete surface before spraying the TiO 2 -CDs layer. These show that adding binder improves the coatings' anchoring on both MLC and DLC concrete substrates, along with desirable resistance to peeling and washing, when compared to the TiO 2 -CDs coated concrete without the binder. The well distributed binder in the TiO 2 -CDs-based coating inevitably decreases the photocatalytic efficiency of the MLC concrete due to the binder's masking effect on the coated surface. However, the DLC concrete fully exploits the binder adhesion by inserting a polymer matrix layer and therefore maintains a strong resistance to peeling and washing without compromising its photocatalytic efficiency. The proposed DLC approach enables the fabrication of highly cost-effective and durable photocatalytic coatings applied to cementitious materials. • The surface of TiO 2 nanoparticles was modified by APTM and integrated with CDs. • Two coating methods were used to bond the TiO 2 -CDs on the surface of substrate. • Effect of coating method on the stability of TiO 2 -CDs against peeling and washing. • Effect of environmental condition on removal HCHO of TiO 2 -CDs was studied. • DLCM of TiO 2 -CDs substrate had better stability compared with MLCM.

The Influence of Prolonged High-Concentration Ozone Exposure on Superhydrophobic Coatings in Static and High-Speed Flow Atmospheres.

The durability of superhydrophobic coatings under exposure to adverse factors that accompany their exploitation in natural and industrial environments remains a key problem in materials science. One such factor is a notable ozone concentration which can be generated as a result of corona discharge, dielectric barrier discharge, piezoelectric direct discharge, UV light photochemical processes, and others characteristic of the power industry. In this work, the mechanisms of degradation of the superhydrophobic coatings under prolonged exposure to high ozone concentrations at dynamic and nearly static conditions were studied. Our results indicate that in dynamic conditions, when the coatings are subjected to joint action of erosion loads initiated by the high-speed flow of the atmospheric air enriched with ozone, with ozone oxidation activity, the superhydrophobic state degrades quite rapidly. At the same time, in nearly static atmospheric conditions with the same ozone content, the degradation is substantially lower. Our study reveals the role of various factors such as the degradation of the layer of the hydrophobic agent, mechanical deterioration of texture, adsorption of ozone, and contact with water in the discovered behavior of the superhydrophobic coatings.

Experimental studies of the possibility of laser processing as a cleaner method of achieving a surface with good adhesion

Abstract In manufacturing processes many technological operations are designed, in which the adhesive properties of the treated surface are very important. These are processes related to application of any coating on the surface, such as gluing, painting, varnishing and others. Durability of coatings depends on proper preparation of the surface to which they are going to be applied. Conventional methods, such as grinding, sandblasting with subsequent washing and degreasing, as well as galvanic treatment applied to e.g. aluminium alloys - require the use of not only specific equipment but also chemical substances. They often lead to a significant burden on the environment due to their harmful properties. In an experimental study, attention was drawn to the significant environmental aspects of such a technological process and work was carried out to demonstrate whether it is possible to eliminate toxic and hazardous substances and to create good adhesion conditions by laser processing. To this purpose, samples were made out of two representative materials: X6Cr17 steel and AW-2024 aluminium alloy, abrasive surface treatment or in a galvanic bath and then washed, degreased and dried. Laser surface treatment without the use of additional chemicals was proposed as an environmentally cleaner technology. Surface roughness and adhesion of the test polymer coating were measured for comparative evaluation of the applied treatment methods. Obtained results were discussed in terms of the possibility of eliminating harmful influences and implementing laser treatment as a cleaner technology in the production of components requiring coating.

A Skin-Inspired Design Integrating Mechano-Chemical-Thermal Robustness into Superhydrophobic Coatings.

Designing scalable coatings with a wide spectrum of functions such as liquid repellency, anticorrosion, and antiflaming and a high level of mechano-chemical-thermal robustness is crucial in real-life applications. However, these individual functionalities and robustness are coupled together or even have conflicting requirements on the interfacial or bulky properties of materials, andthus,simultaneously integrating all these individual features into one coating has proved challenging. Herein, an integral skin-inspired triple-layered coating (STC) that resolves conflicting demands imposed by individual features on the structural, chemical, mechanical, and thermal properties of materials is proposed. Specifically, the rational design of multiple gradients in roughness, wetting, strength, and flame retardancy and the formation of continuous interfaces along its triple layers endow a sustained liquid repellency, anticorrosion, and flame retardancy even under harsh environments, as well as strong antiabrasion on surfaces and adhesion with the substrate. Such an all-in-one design enhances the durability and lifetime of coatings and reduces the maintenance and repair, thereby contributing to cost and energy saving. Together with a facile spraying fabrication process, this STC provides a feasible and sustainable strategy for constructing energy and resource-saving materials.

AUTOMATED PREPARATION GRADE JUDGEMENT SYSTEM FOR WEATHERING STEEL BRIDGES BY DEEP LEARNING

The quality and durability of repair coatings for steel bridges are significantly affected by the surface preparation prior to repair coating. Surface preparation by blasting stipulated by ISO Sa2 1/2 or higher is required as surface treatment before repair coating. The weathering steel bridge have unevenly corroded surface (rust grade D), and it is difficult to evaluate the surface condition by comparing the photographic example. Therefore, there is a high possibility that the difference in evaluation occurs by the engineer. In this paper, the authors attempted to develop a support system to determine preparation grade from the image of blasting. The purpose of this paper is to discuss the accuracy of the trained CNN classifier by the difference in the size of the teaching image data, and to evaluate the performance of the classifier by test data. It was considered that the high value of 83% or more was able to be obtained by the result of all evaluation indices, and the classifier with high performance was constructed by this method.

MOISTURE RESISTANCE OF EARTHEN COATINGS STABILIZED WITH GLYCEROL

One of the fundamental components of earth architecture is its coatings because they protect structures from atmospheric and anthropic agents. However, due to the hygroscopic nature of the earth, the plasters are highly vulnerable to rain and groundwater absorption. To deal with these agents of decay, experts and artisans have tested with different stabilizing substances throughout history. This article reviews a series of experiments based on verifying the use of small glycerol fractions to increase the durability and resistance of the earth coatings. Glycerol is a byproduct of various industrial processes, such as the production of biodiesel. Recently, there has been a significant increase in its production, making it a viable resource to consolidate and protect both historical building components and contemporary buildings. We analyzed comparatively mortars of natural earth, and others stabilized with glycerol to verify the cohesiveness, quantity of water required, and drying time. Subsequently, we performed flat tests and plasters applied on walls to undergo capillary absorption conditions and rain impact. An increase in the cohesiveness of the mortars added with glycerol of 12%, decreases the amount of water required for the mixtures by 18% and the drying time rises by almost 7 hours. The stabilized specimens absorbed 26% less water by capillarity, and the exposed coatings to weather conditions maintain their integrity after eight months of staying outdoors. Earthen mortars that were enriched with small volumes of glycerol developed a highly positive response to these factors, which are the most likely to provoke the earth plasters and the structures they protect. It is possible to have a material that improves the mixtures of soil and is sustainable from an economic, ecological, and cultural point of view.

Structure, mechanical and tribological properties of thick CrNx coatings deposited by HiPIMS

To address the high desirability of enhanced load capacity and durability of thick protective coatings in harsh working conditions, Cr/CrNx multilayer coatings with various thicknesses of 12–78 μm were deposited using high power impulse magnetron sputtering (HiPIMS). The microstructure, mechanical and tribological properties of Cr/CrNx coatings with different thicknesses were investigated by XRD, XPS, AFM, SEM, nano indentation, Vickers indentation and micro-tribometer, respectively. The coatings were deposited at a higher deposition rate up to 16.8 μm/h. With an increase in coating thickness, the coatings with the mixture CrNx phase containing Cr2N and CrN promote grain refinement and intrinsic stress. The increased thickness led to the improved mechanical property and the tribological properties. 78-μm-thick coating exhibits the highest value of the hardness of 26.6 GPa, H/E* of 0.075, H3/E*2 of 0.15, Rockwell C adhesion (above HF1 level) and fracture toughness of 6.3 MPa/m2, respectively. During dry wear test at a normal load of 10 N for the sliding time of 3 h, adhesion failure on worn surface were detected for 12 μm-thick coating with high friction coefficient (COF) of 0.75 and specific wear rate of 3.27 × 10−5 mm3N−1m−1. However, 78 μm coatings show smooth worn surface with a few abraded and blurry grooves with low COF of 0.55 and specific wear rate of 6.9 × 10−7 mm3N−1m−1 owing to the enhanced hardness, H/E*, H3/E*2, adhesion and fracture toughness. The problem of the balance between the influence of residual stress and mechanical/tribological properties is a key point in deposition of super-thick CrNx coating and that can be realize by the proportion control of Cr2N to CrN phases.

김 서림 방지 렌즈코팅의 내구성 시험 방법 개발

Recently, the interest in technologies for anti-fog lens coating has increased, owing to the inconvenience of lens fog occurrence while wearing a mask daily or at work. Consequently, many ocular products with anti-fog capability have been introduced into the market. However, their durability is poor or not specified, owing to the absence of a test method or standard for evaluating the durability of anti-fog coatings. In this study, a test method for evaluating the durability of anti-fog coating for lenses was proposed. The proposed method combines the durability test for anti-reflective coatings for lenses and the test for resistance to ocular fogging. The test results obtained for different coated lenses using the proposed method indicate that this method effectively evaluates anti-fog coating durability.

Fracture-based transient thermo-mechanical analysis of reciprocating engine thermal barrier coatings

The durability of thermal barrier coatings for reciprocating internal combustion engine applications was investigated using a coupled, unsteady fracture-based thermomechanics model. The intra-cycle engine heat flux varies substantially in magnitude and has frequency similar to that of the coating thermal timescale, giving rise to relatively large spatial temperature nonuniformity within the coating that varies temporally. A finite difference scheme was used to calculate the temperature field. An analytical model that assumes equi-biaxial stress followed by plane strain delamination was used to evaluate an energy release rate that represents the driving force for cracking. Because the location and time of the peak energy release rate cannot be established a priori, they were computed for all times in the cycle. The peak energy release rate was found to occur later in the cycle than the peak surface temperature, and was often located within the coating as compared to at the coating–substrate interface where there was a mismatch in coefficient of thermal expansion. Coating thickness was found to affect both the magnitude of the peak energy release rate and its location. The residual stress, which arises from the coating deposition process, was found to have a significant effect on the peak energy release rate, and may be a process parameter that can be used to promote coating life. The computational framework demonstrated insight regarding the trade-off between engine thermodynamic performance and coating durability.

Interface cracking behavior in high-temperature coatings with non-uniformly distributed segmentation cracks

This work numerically studies the interface cracking behavior in the high-temperature coatings that contain non-uniformly distributed segmentation cracks within the outermost coating layer. A representative cell model was proposed for representing the various distribution features of the segmentation cracks in the actual coatings. The evolutions of the delamination driving forces were obtained by calculating the energy release rates at the interface crack tips. It was found that the interface cracking behavior of a segment is weakly affected by the in-plane sizes and interface crack lengths of its neighboring segments. The durability of the segmented coatings may be overestimated if the non-uniformity of the segmentation crack distributions is neglected. An effective segmentation crack density was proposed to consider the influence of the non-uniformity on the delamination behavior, which can be further used for the evaluation of the durability. Moreover, the negative influences of the coatings’ elastic modulus, thickness, and thermal expansion mismatch on the durability can be alleviated by introducing dense segmentation cracks. It provides an alternative way to resolve the difficulties in weighting the different material properties of the newly developed coatings materials for achieving the best performance.

AUTOMATED PREPARATION GRADE JUDGEMENT SYSTEM FOR WEATHERING STEEL BRIDGES BY DEEP LEARNING

The quality and durability of repair coatings for steel bridges are significantly affected by the surface preparation prior to repair coating. Surface preparation by blasting stipulated by ISO Sa2 1/2 or higher is required as surface treatment before repair coating. The weathering steel bridge have unevenly corroded surface (rust grade D), and it is difficult to evaluate the surface condition by comparing the photographic example. Therefore, there is a high possibility that the difference in evaluation occurs by the engineer. In this paper, the authors attempted to develop a support system to determine preparation grade from the image of blasting. The purpose of this paper is to discuss the accuracy of the trained CNN classifier by the difference in the size of the teaching image data, and to evaluate the performance of the classifier by test data. It was considered that the high value of 83% or more was able to be obtained by the result of all evaluation indices, and the classifier with high performance was constructed by this method.

Tough and Strong Waterborne Polyurethane Network Combined with Sub‐Nanoscaled Calcium Phosphate Oligomers for Protective Coating

AbstractThe optimal design of high wear‐resistance and durability of coatings using organic–inorganic interpenetrating networks has been a long‐term goal of researchers. Although some progress has been made in the modification of waterborne polyurethanes (WPU), there are still significant challenges in fabricating more durable polyurethane‐based protective coatings from the perspective of structural design at a molecular level. Here, a sub‐nanoscaled (<1 nm) inorganic calcium phosphate oligomer (CPO) is used to carry out organic–inorganic hybrid cross‐linking in a WPU network and precisely regulate the structure of the macromolecular network at the molecular level. The herein‐produced CPO‐WPU hybrid coating shows excellent wear resistance and water resistance. Taber abrasion tests demonstrate that the CPO‐WPU coating can be capable of 15 500 turns at a load of 750 g, 2.2 times that of pure WPU coating, and the water‐resistance of CPO‐WPU coating is significantly improved compared with WPU coating due to its well‐integrated organic–inorganic network. Overall, the CPO‐WPU coating material significantly improves the mechanical strength, wear‐resistance, and water‐resistance, compared with the original WPU coating and other modified polyurethane. This hybrid coating can provide a potential application in paper‐making, textiles, and furniture due to its unique organic–inorganic network, low cost, and green process.

Near infrared light responsive surface with self-healing superhydrophobicity in surface chemistry and microstructure

Self-healing superhydrophobic surfaces have attracted considerable attention for enhancing the durability of superhydrophobic coatings, which have applications in various fields. However, it is difficult to find safe and effective strategies for self-healing superhydrophobic coatings to recover both microstructure and surface chemistry simultaneously. Herein, we demonstrate a high-efficiency self-healing hydrophobic surface with remote control ability. Through near infrared light (NIR) irradiation, the collapsed surface microstructure and altered surface chemistry can be recovered easily and simultaneously in a very short period. The extraordinary performance is fully discussed and can be attributed to the excellent photothermal properties of candle soot (CS). In addition, the surface demonstrates repeatable self-healing ability thanks to the good shape memory of the epoxy-based shape memory polymer (SMP) and the continued release of fluorinated alkyl silane encapsulated in microcapsules. The presented surface shows great potential and broad impacts owing to its simplicity and efficiency.

The durability of superhydrophobic and slippery liquid infused porous surface coatings under corona discharge characteristic of the operation of high voltage power transmission lines

Application of polyfunctional water-repelling coatings for high voltage outdoor power lines wires, insulators, and line supports recently gathered large interest due to their ability to substantially boost performance. Such functionalization can mitigate the accumulation of water precipitates and atmospheric pollutions, decrease leakage current, increase flashover and corona ignition voltage. However, harsh conditions concomitant to a high voltage such as high energy ions and electrons, high ozone concentration, UV radiation impose strict requirements on the durability of applied coatings. In this paper, we focus on the analysis of the durability of superhydrophobic coatings (SHC) and slippery liquid infused porous surfaces (SLIPS) under prolonged exposure to industrial grade alternating current corona discharge. Reported results show that in contrast to previously reported low resilience of polymer-based SHC, proper metal-based SHC withstands many hours of exposure without notable degradation. The obtained data on the stability of SLIPS under corona discharge show notable degradation related to the high rate of infused layer depletion. Overall, reported results broaden the understanding of the effect of corona exposure on functional materials and reinforce the optimistic view on the applicability of SHC and SLIPS coatings in the power transmission industry.

On the Durability of Protective Titania Coatings on High-Voltage Spinel Cathodes.

TiO2‐coating of LiNi0.5‐x Mn1.5+x O4 (LNMO) by atomic layer deposition (ALD) has been studied as a strategy to stabilize the cathode/electrolyte interface and mitigate transition metal (TM) ion dissolution. The TiO2 coatings were found to be uniform, with thicknesses estimated to 0.2, 0.3, and 0.6 nm for the LNMO powders exposed to 5, 10, and 20 ALD cycles, respectively. While electrochemical characterization in half‐cells revealed little to no improvement in the capacity retention neither at 20 nor at 50 °C, improved capacity retention and coulombic efficiencies were demonstrated for the TiO2‐coated LNMO in LNMO||graphite full‐cells at 20 °C. This improvement in cycling stability could partly be attributed to thinner cathode electrolyte interphase on the TiO2‐coated samples. Additionally, energy‐dispersive X‐ray spectroscopy revealed a thinner solid electrolyte interphase on the graphite electrode cycled against TiO2‐coated LNMO, indicating retardation of TM dissolution by the TiO2‐coating.

Health and Durability of Protective and Thermal Barrier Coatings Monitored in Service by Visual Inspection

Protective and Thermal Barrier Coatings (TBC) applied on gas-turbine blades gradually degrade due to oxidation, aluminum depletion and impacts of environmental particles. Among various non-destructive coating testing methods (NDT), visual inspection can be undertaken regularly in service, but it provides little quantitative information, and only surface defects can be detected. This work aims at in-service monitoring of turbine blades with multilayer coatings applied by atmospheric plasma spraying (APS) in a few variants. They were validated during a series of accelerated mission tests of a retired military turbofan engine in a test cell together with five other technologies. The fifty-hour rainbow test focused on assessing coating durability. Between engine runs, 12 borescope inspections were conducted to monitor the health of the blades. Finally, the blades were disassembled and examined using computed tomography (CT) and metallographic methods. Throughout the testing, 31 newly-coated blades (66%) withstood the tests, producing results comparable to the reference blades. However, 16 blades suffered intolerable failures observed as increased roughness, gradual loss of the topcoat, spallation and minor foreign object damage. Visual inspection results were generally in agreement with subsequent laboratory tests.