In this study, the in-situ technique was used to observe crack formation and growth in multilayer suspension plasma spray (SPS) thermal barrier coatings (TBCs). Utilizing synchronized three-point bending (3PB) and scanning electron microscopy (SEM), coupled with digital image correlation (DIC), we provide real-time insights into strain field dynamics around cracking zones. Bending-driven failure was induced in both single and composite-layer SPS coatings to investigate the crack behavior in these columnar-structured multilayer TBCs. The real-time observations showed that columnar gaps can facilitate crack initiation and propagation from the coatings' free surface. The composite-layer SPS coating exhibits lower susceptibility to vertical cracking than the single-layer SPS coating, possibly due to the presence of a gadolinium zirconate (GZ) dense layer at the coating's free surface that enhances the bonding strength within the coating's columnar structure. The splat structure of the bond coat (BC) layer contributes to the crack path deflection, thereby potentially improving the SPS coating' fracture toughness by dissipating the energy required for crack propagation. Moreover, it was revealed that grit particles at the BC/substrate interface seem to promote crack branching near the interface, localized coating delamination, and serve as nucleation sites for crack development. Hence, optimizing the grit-blasting process of the substrate before BC layer deposition is crucial for minimizing the possibility of crack formation under operational conditions, contributing to enhanced durability and prolonged lifespan. This study underscores the critical role of in-situ observation in unravelling the complex failure mechanisms of multi-layered coatings, paving the way for the design of advanced coatings with enhanced structural complexity and improved performance for more extreme environments.

Hybrid plasma spraying combines deposition of coatings from coarse powders and liquids (suspensions or solutions) so that the benefits of both routes may be combined. In this study, failure evolution of early-stage thermal barrier coatings (TBCs) with hybrid YSZ-YSZ and YSZ-Al 2 O 3 top-coats deposited by hybrid water/argon-stabilized plasma torch was evaluated. In-situ bending experiment was carried out in SEM to assess potential influence of the secondary miniature phase addition on the coating failure during mechanical loading. Adapted high-resolution open-source strain-mapping code GCPU_Optical_flow was used to track evolution of the local coating failure. For the tested coatings, addition of miniature phase did not weaken the hybrid coating microstructure as the crack propagation was practically insensitive to the presence of the secondary phase and dissimilar splat boundaries. Main micromechanisms of the top-coat failure were thus splats cracking, loss of cohesion (splat debonding), and mutual splat sliding.

Thermally sprayed wear resistant coatings have proven their effectiveness in many applications. Their benefit is unquestionable in the case of mutual sliding contact or abrasive stress caused by hard particles. However, for the case of dynamic impact loading, either single or cyclic, the lifetime of different types of coatings is rarely described, probably due to the complex influence of many parameters. The paper deals with the evaluation of resistance to dynamic impact loading of two types of HVOF-sprayed Cr3C2-rich binary hardmetal coatings (Cr3C2-42%WC-16%Ni and Cr3C2-37%WC-18%NiCoCr) with respect to the variation of their deposition parameters and compares them to a well established Cr3C2-25%NiCr coating. For each coating, a Wohler-like curve was constructed based on a failure criterion of sudden increase in impact crater volume. Besides, coatings deposition rate, residual stress, microstructure and hardness were evaluated. Differences in the coatings dynamic impact wear resistance was found, related to their residual stress. The failure mechanism and crack propagation mode are analyzed using SEM of impact surface and cross-sections. Deformation and related stress changes in coated systems during dynamic impact loading are described using FEA analyzes.

The influence of air plasma sprayed alumina coating geometry, microstructure, interface roughness on its delamination and crack propagation resistance during low temperature thermal cycling, i.e. thermal mismatch stress, is investigated both numerically and experimentally. Previous studies on thermal cycling loading concentrate on flat, numerically designed locally curved specimens and/or mathematically modeled roughness without extension towards real coating morphology, which renders the conclusions less practically driven. Results show that arbitrarily oriented cracks originate predominantly near the coating/substrate interface and propagate along zones of high tensile and shear residual stress. The crack path deflection was attributed to the complex stress concentration structure resultant from the intricate microstructural porosity and coating general convex geometry. Microstructural features such as porosity increase the interfacial and coating tensile stress, which may lead to important delamination processes even during low temperature thermal cycling.

Impact testing appears as a most promising tool for gaining information on coating behavior in load-bearing applications. During dynamic impact test an indenter impacts successively the surface of the coating with constant force and frequency. The deformation of the coated specimen during impact testing is affected by the mechanical properties of both the substrate and the coating. Varying the impact load and the number of impacts, the evolution of coating surface deformation and contact fatigue failures can be observed. In the paper, the influence of dynamic impact load and number of impacts on the resulting impact crater volume and morphology is analysed, and the interpretation of the results in form of Wohler-like dependance is suggested and demonstrated on two types of HVOF sprayed Co-based alloy coatings. The low-number impact craters evolution and subsurface cracks propagation of HVOF sprayed Co-based alloy coatings is analyzed in more detail, by means of 3D optical microscopy and SEM. The results showed, that the higher ability to deform plastically increased the coatings dynamic impact fatigue lifetime. The cracks, responsible for coatings destruction, spread predominantly along the intersplat boundaries in the pile-up area.

Cold spray deposition is being investigated for mitigation of chloride-induced stress corrosion cracking (CISCC) in dry cask storage systems (DCSS) for spent nuclear fuel. Welded regions of austenitic stainless-steel canisters in DCSS are under tensile stress and susceptible to environmental chloride corrosion, which can potentially lead to the formation of CISCC. The low thermal input and high throughput nature of cold spraying make it a viable repair and mitigation option for managing potential CISCC. Cold spray coatings are under compressive stress and act as a barrier in Cl-rich environments. Characterization data including microstructure, hardness, and corrosion resistance are presented for cold spray coatings on stainless steel substrates.

Fatigue crack growth in self-standing plasma sprayed tungsten and molybdenum beams with artificially introduced notches subjected to pure bending was studied. Beams width, thickness and length was 4 mm, 3 mm and 32 mm respectively. Fatigue crack length was measured using the differential compliance method and fatigue crack growth rate was established as a function of stress intensity factor. Unusual crack opening under compressive loading part of the cycle was detected. Fractographic analysis revealed the respective crack formation mechanisms. At low crack propagation rates, the fatigue crack growth takes place by intergranular splat fracture and splat decohesion for Mo coating. In W coating, intergranular splat fracture and void interconnection formed the fatigue crack. Frequently, the crack deflected from the notch plane being attracted to stress concentrators formed by porosity. At higher values of the stress intensity factor, the splat intergranular cracking become more common and the crack propagated more perpendicularly to the specimen surface.

In order to guarantee their protective function, thermal sprayings must be free from cracks, which expose the substrate surface to e.g. corrosive media. Cracks in thermal sprayings are usually formed because of tensile residual stresses. Most commonly, the crack occurrence is determined after the thermal spraying process by examination of metallographic cross-sections of the coating. Recent efforts focus on in situ monitoring of crack formation by means of acoustic emission analysis. However, the acoustic signals related to crack propagation can be absorbed by the noise of the thermal spraying process. In this work, a high-frequency impulse measurement technique was applied to separate different acoustic sources by visualizing the characteristic signal of crack formation via quasi-real-time Fourier analysis. The investigations were carried out on a twin wire arc spraying process, utilizing FeCrBSi as a coating material. The impact of the process parameters on the acoustic emission spectrum was studied. Acoustic emission analysis enables to obtain global and integral information on the formed cracks. The coating morphology as well as coating defects were inspected using light microscopy on metallographic cross-sections. Additionally, the resulting crack patterns were imaged in 3D by means of X-ray micro-tomography.

Yttria stabilized zirconia coatings were deposited by plasma spraying and heat treated in air at 1100 °C for 50-200 h. Residual stresses in the ceramic topcoat and the thermally grown oxide (TGO) layer were measured before and after thermal exposure. After 50 h of exposure, tensile stress in the as-sprayed topcoat changed to compressive, which then increased with additional exposure time up to 150 h. The average compressive stresses in the cross-section of the TGO layer are shown to be higher than those on the surface of the oxide. In addition to shedding light on the nature and evolution of stresses in plasma-sprayed thermal barrier coating (TBC) systems, the results of the study also provide insights on crack initiation and propagation in the ceramic topcoat and at the topcoat-TGO-bond coat interface and its role in TBC failures.

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the spatiotemporal localization of crack initiation and the dynamics of crack propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical TBC YSZ/NiCoCrAlY composites were tested. The strain distribution on the coating surface is evaluated by the digital image correlation method (DIC) through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e. the changes of material properties, crack initiation, and crack propagation are identified. The tested coatings strongly influenced the first two phases, the influence on the crack propagation was less significant.

In this study, acoustic emission sensing is used to monitor interfacial cracking in thermal barrier coatings during uniaxial tensile adhesion testing. The TBCs consist of a ZrO 2 topcoat and a NiCrAl bond coat, both of which are applied by atmospheric plasma spraying. Tensile testing was performed to failure and the resulting fracture surfaces were examined by SEM and XRD analysis. Experimental results show that cracks usually initiate in the ceramic layer then propagate toward the metallic-ceramic interface where failure occurs. Finite element simulations were also conducted, confirming the experimental findings.

The standard technique for applying zirconium (Zr) to uranium (U) is roll bonding, where a thin foil of Zr is placed on each side of a U plate which is then encased in steel and rolled at high temperatures. This study evaluates an alternative approach in which Zr layers are plasma sprayed on U and then clad with aluminum (Al) by hot isostatic pressing. The interface region between the Zr and Al is examined by SEM, revealing a reaction layer consisting of Al, Zr, and Si. SEM images show good conformance between the Al sheet and Zr surface along with the presence of Al in the porous Zr. Initial test results indicate that increased interface roughness and Al penetration into the plasma-sprayed Zr have the potential to improve bond strength by impeding crack propagation in the reaction layer.

The efficiency of aero-engines combustion chambers with thermal barrier coating (TBC) is improved when numerous cooling holes are laser drilled with inclined angles. However, during the laser drilling process, especially in the percussion mode, a detrimental crack can be generated at the TBC interface. Thus, each hole could be edged with a non-visible delaminated area underneath the ceramic top-coat. The present work is focused on the thorough study of the delamination induced by laser percussion drilling when interrupted drilling conditions are presented. Shallow angle drilling was applied on separated holes with 1 to 4 laser pulses respectively and various acute incident angles. Crack length was assessed by conventional metallographic preparation. A special experimental method was carried out in order to inspect the delaminated interface and the lateral edge of a semi-hole. This non-destructive assessment of the delamination of laser drilled TBC was complemented by a 3D imaging of a semi-hole using X-Ray microscopy. Results are presented with attention on both crack initiation and propagation during the laser percussion drilling of plasma-sprayed TBC.

In this paper evaluation of sealed and unsealed thermally sprayed aluminum (TSA) for the protection of 22%Cr duplex stainless steel (DSS) from corrosion in aerated, elevated temperature synthetic seawater is presented. The assessments involved general and pitting corrosion tests, external chloride stress corrosion cracking (SCC), and Hydrogen induced stress cracking (HISC). These tests indicate that DSS samples which would otherwise fail on its own in a few days do not show pitting or fail under chloride SCC and HISC conditions when coated with TSA (with or without a sealant). TSA-coated specimens failed only at very high stresses (>120% proof stress). In general, TSA offered protection to the underlying or exposed steel by cathodically polarizing it and forming a calcareous deposit in synthetic seawater. The morphology of the calcareous deposit was found to be temperature dependent and in general is of duplex nature. The free corrosion rate of TSA in synthetic seawater was measured to be ~5-8 µm/year at ~18°C and ~6-7 µm/year at 80°C.

NiCrAl/ZrO 2 -8Y 2 O 3 coatings deposited on SUS304 stainless steel and 45 carbon steel substrates were prepared by APS at different preheating temperatures, of which thickness exceeded 1mm. This study analyzed the coatings’ separation from different preheated substrates in the cooling process after spraying due to residual thermal stress. The Young’s modulus of the porous YSZ coatings was calculated and also measured by Knoop indentation methods for comparison purposes. The result indicated that the failure of porous thick YSZ coatings is mainly caused by the cracks nucleation, propagation and coalescence, which is related to the thermal-expansion coefficient difference between substrate and coatings, preheating temperature, porosity of coatings and so on. Due to their increased porosity, the porous and thick YSZ coatings had much lower calculated and measured Young’s modulus values than the sintered YSZ coatings.

In thermal barrier coating (TBC) systems, a continuous alumina layer developed at the ceramic topcoat/bond coat interface helps to protect the metallic bond coat from further oxidation and improve the durability of the TBC system under service conditions. However, other oxides such as spinel and nickel oxide, formed in the oxidizing environment, are believed to be detrimental to TBC durability during service at high temperatures. It was shown that in an air-plasma-sprayed (APS) TBC system, post-spraying heat treatments in low-pressure oxygen environments could suppress the formation of the detrimental oxides by promoting the formation of an alumina layer at the ceramic topcoat/bond coat interface, leading to an improved TBC durability. This work presents the influence of post-spraying heat treatments in low-pressure oxygen environments on the oxidation behaviour and durability of a thermally sprayed TBC system with high-velocity oxy-fuel (HVOF)-produced Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat. Oxidation behaviour of the TBCs is evaluated by examining their microstructural evolution, growth kinetics of the thermally grown oxide (TGO) layers, as well as crack propagation during low frequency thermal cycling at 1050°C. The relationship between the TGO growth and crack propagation will also be discussed.

One of the methods used for the evaluation of coating adherence consists of determining an apparent interfacial fracture toughness through interfacial indentation in the cross-section of the coated system. In the present paper the interfacial crack propagation in a thermal spray coated system, during interfacial indentation test, was studied by finite element method. A Vickers indenter was applied at the coating/substrate interface to perform indentation test. In order to simulate the cracking, an initial semi-circular crack lying at the interface between the substrate and the coating was considered. Since the cracks resulting from interfacial indentation test have been reported to have a semi-circular shape, the crack was forced to advance by a uniform distance normal to the crack front in the plane of interface at each step of calculation. In these conditions it was shown that the interfacial crack has a mixed fracture mode. An apparent interfacial fracture energy Jca was calculated by this method to evaluate the interfacial adhesive toughness. The model was applied to study the crack propagation at the interface between a Cr 3 C 2 -NiCr HVOF coating and a grey cast iron substrate. A reasonable agreement between the calculation results and reported experimental data was found.

Plasma-sprayed alumina-waste glass composite coatings on ceramic substrates were produced. Two kinds of alumina powders, different alumina volume fractions, and two glass powders particle size distributions were tested. Post-process thermal treatments were performed. The coatings were characterized by SEM, XRD, Vickers microhardness, fracture toughness, abrasion resistance tests. Coatings superior to traditional tile glazes were obtained with as high as 50 vol.% of waste glass. Fine glass powders (<45µm) must be employed to achieve adequate toughness. A low-cost spray-dried alumina can be used instead of the expensive commercial powders. The thermal treatment enhances the coating properties. A FEM thermo-mechanical simulation was performed. Elastic modulus calculations show a definite coating anisotropy (higher mechanical properties in the longitudinal direction). Compressive residual stresses in the alumina and tensile ones in the glass are developed after the thermal treatment. Crack propagation studies based on Griffith model show cracks initiating from larger pores and propagating easily through the glass, thus explaining the coating toughening achieved through the employment of finer glass powders. Cracks are stopped by alumina; this effect is enhanced in the thermally treated coatings. The numerical and experimental (from indentation fracture toughness test) crack propagation patterns are in good agreement.

The crack propagation characteristics obtained by observing the microstructure of bent coatings at different stages of applied strain were conducted. At the same time, acoustic emission (AE) features of released events were recorded. Generated cracks were found to emit weak acoustic emission events at the initiation (creation) of the crack at the coating surface, and then become strong releasing a uniform acoustic emission energy per event, when propagating transversally in the coating. However, cracks propagating at the interface with the substrate could generate both weak and strong events. It was also found that increasing the thickness of the coating leads to a decrease in the number of events and an increase of their released acoustic emission energy.

The SUS316L stainless steel rod specimen coated with plasma-sprayed Al 2 O 3 deposits has been fatigued in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of its application to prosthetic implant materials. Push-pull loading fatigue tests were conducted at the stress ratio of R = -1, and at the frequency of 2 Hz. Pure titanium powder was selected for undercoat. Fatigue damage was examined on longitudinal section of the specimen and fracture surface by optical and electron microscopy from the microstructural viewpoints. The plasma spraying of Al 2 O 3 powder has significantly improved fatigue properties of the substrate metal in the longer range of fatigue lives, compared with the results of the non-coated steel specimen. It was found from electrochemical experiments that titanium for undercoat metal has acted as sacrificial anode to protect the substrate metal from corrosive attack and under lower stress amplitudes the plasma sprayed Al 2 O 3 coating kept the solution out at an early stage of fatigue lives. Fatigue cracks preferentially originated from flaws, which had been caused on the substrate metal surface through grit blasting, and extended into the bulk of substrate metal. Fatigue cracks appear not to develop into plasma-sprayed deposits while the deposits could accommodate themselves to the crack opening displacement at the surface of substrate metal. It was understood that the plasma sprayed coating has enhanced fatigue properties in the solution both by keeping the solution out during the early stage of fatigue lives and by electrochemical effects of the undercoat metal when the topcoat was cracked in macroscopic scale.