Failure Mechanisms Of Coatings Research Articles

Performance of intumescent fire protection coatings applied to structural steel tension members with circular solid and hollow sections

Intumescent fire protection coatings are used to improve the fire resistance of steel members. Due to slender cross-sections typically used for tension members, high demands are placed on the effectiveness and reliability of the intumescent coatings. This paper describes real-scale fire tests of intumescent coatings applied to steel tension members with circular solid and hollow sections. First, the current regulations for the application of intumescent coatings on steel tension members are discussed. After that, a comparison between the foaming behaviour and failure mechanisms of intumescent coatings applied to steel tension members with different cross-sections and loading conditions is given. Next, the experimental setup and procedure of the performed fire tests are presented. In addition, the influences of various parameters on the performance of the intumescent coating are analysed, including the geometry of the steel profile, dry film thickness of the intumescent coating, load utilisation level as well as the orientation of the member. Based on the theoretical considerations and results of the fire tests, it is evidenced that mechanically loaded fire tests are essential to assess accurately the contribution of intumescent fire protection coatings to the fire resistance of steel tension members with circular solid sections.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes.

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an “electrochemical ion (proton) pump cell” mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20–60 localized sites across a ≈0.003 mm2 area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites “opening” and a small number of sites “closing” under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure–property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

In situ real time observation of tribological behaviour of coatings

The functional performance of coatings can be transformed using coatings to form an engineered surface system where resistance to wear and friction are improved relative to the uncoated substrate. Understanding the failure mechanisms of coatings in tribological contacts is difficult to achieve. This paper shows results from real time in situ measurements of tribological behaviour of coated surfaces.Two different test systems have been used. These are a microtribometer fitted inside an SEM which provides sequences of high resolution images of the deformation processes occurring in coatings, and a ball on disc tribometer fitted with imaging and profilometric systems giving real-time information on the tribological response of coatings.DLC and TiN coatings on tool steel substrates were tested in the experiments reported here. Delamination failure of the DLC coatings was observed with both test system. A new rotating scratch test is also described that can provide real-time in situ information on the response of the coatings to repeated abrasion. The in situ results are supported by post-test surface examination.

Degradation of thermally sprayed Al2O3 coatings in reactor-grade liquid-sodium and its mitigation by laser treatment

Laser treatment (LT) to enhance the performance of atmospheric plasma spray (APS) deposited Al2O3 thermal barrier coatings (TBCs) was attempted for applications in extreme liquid-sodium environments of a sodium-cooled fast reactor (SFR). Thermally sprayed sacrificial ceramic TBCs are being proposed and investigated for installation on the stainless steel (SS) 316LN core catcher assembly of future SFRs to combat the core disruptive accidents (CDAs), wherein the primary criterion to be satisfied by the TBC is its long-term compatibility with the liquid-sodium coolant over a whole reactor life. Sacrificial ceramic coatings are susceptible to liquid-metal induced degradation by reactor-grade sodium. This study provides new insights into the failure mechanisms of APS Al2O3 coatings in liquid-sodium environments. Subsequently, an attempt is made to improve the performance of coatings by a surface modification approach using a laser treatment of the TBC topcoat. APS coatings exposed to liquid-sodium failed by spallation and delamination of the incrementally deposited topcoat, which is attributed to the formation and growth stress of ternary Na–Al oxides between the lamellar layers of the APS coatings. Compared to the APS coatings, LT coatings successfully retarded the liquid-sodium ingression and thereby exhibited improved degradation resistance and structural stability in reactor simulated sodium environments. The enhanced performance of LT coatings is attributed to its carefully re-engineered architecture of the APS TBC that could beneficially control the kinetics of interaction with liquid-sodium.

Effect of spraying ceramic powder pore structure on thermophysical properties of plasma-sprayed thermal barrier coatings

Pore structure is one of the major factors influencing the failure mechanisms of thermal barrier coatings (TBCs) during high-temperature service. Powders exhibiting different pore structures have been produced using the plasma spheroidization technique. To improve the strain tolerance of an aircraft engine's hot sections, the preparation method of the TBCs material—8-wt% yttria-stabilized zirconia powders—has been continuously optimized. In this paper, the constrained effects of pores on the ceramic top coating of plasma-sprayed lamellar TBCs were investigated. Furthermore, the crystal phase structures, thermophysical properties, porosity, microstructures, and synthesis mechanism of powders and coatings were analyzed. Results revealed that the top ceramic coating, which was made from powders cooled by air and distilled water, can influence the coating porosity. Moreover, thermal insulation was substantiated by the finite element analysis method. Consequently, dense powders cooled by water can be used on the top surface in direct contact with high temperature flame flow to resist the diffusion of oxides, and the porous powder cooled by air can be applied in the middle coating to decrease the thermal conductivity. Hence, TBCs with various layers of different porosities can prolong the service life of thermal barrier coatings.

Interfacial bonding mechanism and properties of HVOF-sprayed Fe-based amorphous coatings on LA141 magnesium alloy substrate

Due to poor corrosion and wear resistance, Mg-Li alloys often experience the premature failure during their service. In order to improve the corrosion and wear resistance of Mg-Li alloys, Fe-based amorphous coatings were firstly prepared on the LA141 magnesium alloy substrate by high velocity oxygen-fuel spraying technology (HVOF) in this study. It is noticed that by introducing the Ni60 interlayer with the thickness of 150 μm, the bonding strength of the Fe-based amorphous coating on LA141 alloy substrate can reach 56.9 MPa, which is currently the highest value of Fe-based amorphous coatings on magnesium alloys reported so far. It is found that the improvement of bonding strength is attributed to the localized metallurgical bonding at the interfaces of Fe-based amorphous coating/Ni60 and Ni60/LA141 and the compactness of Fe-based amorphous coatings. Moreover, the corrosion and wear resistance of the LA141 magnesium alloy with the Fe-based amorphous coatings are 34 times and 170 times higher than those of the bare LA141 magnesium alloy substrate, respectively. Finally, the failure mechanisms of the Fe-based amorphous coatings during tension and wear are discussed. This study can provide an alternative method for the surface protection of Mg-Li alloys.

Effect of pre-oxidation on the failure mechanisms of EB-PVD thermal barrier coatings with (Ni,Pt)Al bond coats

In order to clarify the effect of pre-oxidation for (Ni,Pt)Al bond coat on failure modes of TBCs, cyclic oxidation behavior of thermal barrier coatings system with both pre-oxidized and normal (Ni,Pt)Al bond coats was investigated at 1100 °C. The results indicated that thermal cycling resistance of TBCs was dramatically enhanced by pre-oxidation, where thinner and lower residual stress oxide scale, lighter rumpling of the BC/TGO interface were achieved pre-oxidized. The failure modes of these two TBCs were completely diverse. Besides, the lower oxygen partial pressure effectively enhanced transformation from θ to α-Al2O3 during pre-oxidation.

Fracture mechanism of TiN coatings on Ti-6Al-4V substrates: Role of interfaces and of the residual stress depth profile

The performance and integrity of coated engineering components rely on the time required for the appearance of defects and cracks and their propagation, leading to delamination of the coating and degradation of the substrate. Optimizing the coating's mechanical properties such as hardness, residual stress (RS), and adhesion is essential to delay crack onset and propagation. In the present work, we investigate the mechanical properties, and the failure mechanisms of model TiN coatings reactively sputtered onto Ti-6Al-4V substrates while comparing three interface engineering approaches to enhance the system durability: a) Argon plasma treatment, b) plasma surface nitriding, and c) Titanium implantation. The TiN coatings possessed a hardness of ~29 GPa and a Young's modulus of ~350 GPa. Multi-reflection grazing incidence X-ray diffraction was used to assess the compressive RS depth profile. Each interface engineering process induced RS variation in the coating and the adjacent interfacial area and in the substrate's near-surface layer ranging from −1 GPa to −2.2 GPa with different gradients. Cohesive failure, crack evolution and fracture toughness were studied using micro-scratch and micro-tensile tests, while the surface grain deformation behaviour and surface cracks on fractured Ti-6Al-4V with various interface treatments and TiN coatings were identified by SEM image and elasto-plastic property analyses. We found a close correlation between the RS of the coatings or the interface layers and the fracture mechanism. Specifically, higher compressive RS led to an enhanced interfacial shear strength and a critical energy release rate of around 550 to 790 MPa, and of ~18 J/m2, respectively.

Effect of Al content on high-temperature oxidation behavior and failure mechanism of CrAl-coated Zircaloy

The application of coating to cladding materials can potentially improve the accident-tolerant fuel cladding for light water reactors. In this study, oxidation behavior of five CrAl-based coatings containing different amounts of Al deposited onto Zr-4 alloy exposed to high-temperature air is investigated. The results demonstrate that pure Cr coating exhibits excellent oxidation resistance due to the formation of dense and continuous Cr2O3 oxide. By contrast, loose coatings with high Al contents correspond to poor oxidation resistance, which is related to the reaction between Zr and O. This paper proposes that the oxidation failure mechanisms of coatings depend on Al content. Data availability statementThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

TiN/NbN Nanoscale Multilayer Coatings Deposited by High Power Impulse Magnetron Sputtering to Protect Medical-Grade CoCrMo Alloys

This study describes the performance of nanoscale multilayer TiN/NbN coatings deposited on CoCrMo medical-grade alloys by utilising novel mixed high power impulse magnetron sputtering (HIPIMS) and direct current unbalanced magnetron sputtering (UBM) technique in an industrial size vacuum coater. Scanning electron microscopy analysis showed that these coatings were extremely dense without any intercolumnar voids. The coating exhibited high hardness of 28 GPa, as well as low friction and wear coefficient of 0.7 and 1.4 × 10−14 m3·N−1·m−1, respectively, as compared to the bare material. Scratch tests revealed superior coating to substrate adhesion due to the HIPIMS etching prior to coating deposition. Energy-dispersive X-ray analysis of the wear debris generated during the impact test together with focused ion beam cross-section analysis in different locations of the impact crater revealed the coating failure mechanism and further confirmed the excellent coating to substrate bonding strength. Potentiodynamic polarisation tests in NaCl and Hank’s solutions revealed the clear passivation behaviour, several orders of magnitude lower corrosion currents, and high pitting potentials of the coating, which guarantee excellent protection to the base alloy in such aggressive environments. Inductively coupled plasma mass spectrometry analysis of Hank’s solution containing corrosion debris of the coated sample revealed that the leaching of harmful metal ions from the base material was reduced to below the detection limit of the technique, thus demonstrating the high barrier properties of the coating.

Failure and Phase Transformation Mechanism of Multi-Layered Nitride Coating for Liquid Metal Injection Casting Mold

Ti-Al-Si target and Cr-Si target are sputtered alternately to develop a multi-layered nitride coating on a steel mold to improve die-casting lifetime. Prior to the multi-layer deposition, a CrN layer is developed as a buffer layer on the mold to suppress the diffusion of reactive elements and enhance the cohesive strength of the multi-layer deposition. Approximately 50 nm CrSiN and TiAlSiN layers are deposited layer by layer, and form about three μm-thickness of multi-layered coating. From the observation of the uncoated and coated steel molds after the acceleration experiment of liquid metal injection casting, the uncoated mold is severely eroded by the adhesion of molten metallic glass. On the other hand, the multi-layer coating on the mold prevents element diffusion from the metallic glass and mold erosion during the experiment. The multi-layer structure of the coating transforms the nano-composite structured coating during the acceleration test. Since the nano-composite structure disrupts element diffusion to molten metallic glass, despite microstructure changes, the coating is not eroded by the 1,050 oC molten metallic glass.

The Oxidation Behaviour and Notch Wear Formation of TiAlN Coated Tools Using Different Oxidation Techniques.

This paper proposes a novel approach to assessing oxidation behavior of TiAlN coatings with defined stoichiometry on the rake and flank surfaces. This is based on the multi-parametric comparison of the oxidation effects detected on the coatings’ surfaces resulting from static diffusion couple tests. In this experimental study the diffusion couples consisting of Ti-based and Ni-based alloys and coated TiAlN cutting inserts are tested, respectively. The optimum oxidation temperature was determined by annealing the selected TiAlN coating in a high temperature chamber at temperatures: 700 °C, 800 °C, 900 °C and 1000 °C in air. Concurrently, the mass change and corresponding thickness of the Al2O3 oxidized layer were measured and computed. The comparison of oxides produced covers the surface morphologies, chemical elements and phases which were analyzed by means of SEM (scanning electron microscope), EDS (energy dispersive spectroscopy) and XRD (X-ray diffraction techniques). Additionally, scratch tests were performed to assess the penetration depth down to the substrate and coating failure mechanism after oxidation in diffusion couples. An acceptable similarity of Al2O3 films formed on the TiAlN coating surfaces in diffusion couples and machining processes was established.

The Wettability and Corrosion Behaviors of CMAS on M-YTaO4 at 1350 °C

CMAS corrosion is one of the significant failure mechanisms for thermal barrier coatings (TBCs). In this work, the corrosion behaviors and wettability of CMAS on M-YTaO4 at 1350 °C are studied to evaluate the M-YTaO4 ceramics’ potential to replace YSZ. The results show that the contact angles of CMAS on M-YTaO4 with different time (1 min, 0.5 h, 4 h, 10 h and 24 h) are around 14.3° (14.4°14.4°, 14.4°, 14.3° and 14.2°, respectively), and larger than that on YSZ (8.0°, 1 min), which means the wettability of CMAS on M-YTaO4 is worse than that on YSZ and which is benefit to improve its corrosion resistance. The thickness of reaction layer is increasing with elapsed time, but the increasing rate is decreased, and the maximum reaction thickness is 31.42 μm at 24 h. There are four main phases after reaction, which are M-YTaO4, M’-YTaO4, Ca2Ta2O7 and Y2Si2O7. For the effects of lattice distortion and thermodynamics reason, M’-YTaO4 can retain steadily at room temperature. Besides, the chemical reaction process between CMAS and yttrium tantalite is also be discussed.

Comparative study on the tensile cracking behavior of CrN and Cr coatings for accident-tolerant fuel claddings

In this work, a comparative study was conducted on the tensile behaviors of two representative accident-tolerant cladding coatings, i.e., CrN coating and Cr coating, at both room temperature and 400 °C by in situ tensile testing. The surface and interfacial cracking behaviors of the coatings were experimentally tested and analyzed, with the surface crack densities predicted by a modified shear-lag model. The results showed that CrN coating exhibited brittle fracture at both room temperature and 400 °C, while the failure of Cr coating showed brittle-to-ductile transition when increasing the temperature from room temperature to 400 °C. Moreover, Cr coating exhibited better crack resistance than that of CrN coating under mechanical loading at both temperatures, due to: (i) the higher fracture toughness and ductility of Cr; (ii) better deformation compatibility between Cr coating and Zr-4 substrate. In particular, at 400 °C, Cr coating with excellent plastic deformability exhibited a protective or prohibitive effect on the crack initiation process of the Zr-4 substrate. It is indicated that the mechanical deformation properties and failure mechanisms of Cr and CrN coatings are important factors that should be considered in the selection and evaluation of accident-tolerant coatings.

Comparative study on the degradation of a zinc-rich epoxy primer/acrylic polyurethane coating in different simulated atmospheric solutions

The degradation of a zinc-rich epoxy primer/acrylic polyurethane coating system on carbon steel was studied in the simulated marine atmospheric solution (0.05% NaCl), industrial atmospheric solution (0.35% (NH4)2SO4), and marine-industrial atmospheric solution (0.05% NaCl + 0.35% (NH4)2SO4), by electrochemical impedance spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy methods. The influences of the soluble contaminants in the atmospheres, including chloride ions, sulfate ions, and ammonium ions, on the coating failure mechanism were compared and analyzed. The results showed that the degradation speed of the coating system in three simulated atmospheric solutions is in the following order: simulated marine atmosphere > simulated marine-industrial atmosphere > simulated industrial atmosphere. The degradation is mainly related to the permeation rate of the water through the coating. The aggressive ions, basic cations, and the property of the corrosion products also have influences on the degradation process. The alternating temperature environment (45°C 12 h + 25°C 12 h) could increase the degradation speed of the coating and the difference of the speed in the three solutions.

Long-Term Failure Mechanisms of Thermal Barrier Coatings in Heavy-Duty Gas Turbines

Thermal barrier coatings serve as thermal insulation and antioxidants on the surfaces of hot components. Different from the frequent thermal cycles of aero-engines, a heavy-duty gas turbine experiences few thermal cycles and continuously operates with high-temperature gas over 8000 h. Correspondingly, their failure mechanisms are different. The long-term failure mechanisms of the thermal barrier coatings in heavy-duty gas turbines are much more important. In this work, two long-term failure mechanisms are reviewed, i.e., oxidation and diffusion. It is illustrated that the growth of a uniform mixed oxide layer and element diffusion in thermal barrier coatings are responsible for the changes in mechanical performance and failures. Moreover, the oxidation of bond coat and the interdiffusion of alloy elements can affect the distribution of elements in thermal barrier coatings and then change the phase component. In addition, according to the results, it is suggested that suppressing the growth rate of uniform mixed oxide and oxygen diffusion can further prolong the service life of thermal barrier coatings.

Nondestructive characterization of elastic modulus of APS Ni–5Al/10hBN coating on stainless steel 304 under high temperature

The failure mechanisms of coatings, under high thermo-mechanical loads, can be examined through finite element simulations. However, mechanical properties such as Young’s modulus and Poisson’s ratio are necessary to achieve an accurate model. In this study, nickel–aluminum with hexagonal boron nitride (NiAl/hBN) is deposited on stainless steel 304 substrate using atmospheric plasma spray technique. The effect of the hydrogen content in the plasma-formed gas on the microstructure, hardness, and porosity of the deposited layers is examined. Also, the mechanical properties of the coating are measured at ambient temperatures using non-contact laser ultrasound technique. Results indicated that the flow rate of hydrogen has a major impact on the coating structure and elastic properties. The inhomogeneity in the coating can be minimized by decreasing the hydrogen flow rate. The coating Young’s modulus was increased at room temperature, which slowly decreased while increase in atmospheric temperature. The behavior of Young’s modulus at high temperature causes decrease in flexural strength. In addition, the back calculations are well agreed with experimental results.

Failure Mechanism of Organic Coating with Modified Graphene Under Simulated Deep-sea Alternating Hydrostatic Pressure

Failure Mechanism of Organic Coating with Modified Graphene Under Simulated Deep-sea Alternating Hydrostatic Pressure

Evaluation of fracture toughness and residual stress for both single GYbZ and double layered GYbZ/8YSZ coatings by modified indentation tests

Single (Gd0.9Yb0.1)2Zr2O7 (GYbZ) and double layered GYbZ/yttria partially stabilized zirconia coatings were prepared with supersonic plasma sprayed technique. The microstructure and texture of the two coatings were analyzed by scanning electron microscope and X-ray diffraction. The elastic modulus, hardness, fracture toughness and residual stress of both coating systems were measured in indentation tests. The influence of residual stress on fracture toughness measurements was discussed with different indentation models. It is interesting to find that the absolute value of residual stress on the cross section of the coating becomes smaller, while the corresponding fracture toughness gets larger when the indented point gradually moves to the interface. The present results would be helpful for understanding the design optimization and failure mechanisms of advanced ceramic coatings.

Bending fracture behavior of freestanding (Gd0.9Yb0.1)2Zr2O7 coatings by using digital image correlation and FEM simulation with 3D geometrical reconstruction

It is important to investigate the mechanical performances of (Gd0.9Yb0.1)2Zr2O7 (GYbZ) materials deposited on irregular substrates for improving new thermal barrier coatings. Three-point bending fracture characteristics of freestanding GYbZ coating prepared by supersonic plasma sprayed (SPS) technique were investigated with the help of digital image correlation technique. The cracking time, crack propagation path, and mechanical properties of GYbZ coating were obtained. Meanwhile, the X-ray computed tomography technique was introduced to scan the microstructure of freestanding GYbZ coatings, which are used to establish three-dimensional (3D) finite element model by using the Avizo software. The brittle cracking criterion was applied to describe the bending fracture process of GYbZ coatings. The critical cracking strain was estimated as 0.36%±0.03% by repeatedly comparing the difference between the experimental and simulated curves. The results would be extended to predict the dangerous region and failure mechanisms of GYbZ coatings deposited on irregular substrate during finite element simulations.