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.

The Ti-6Al-4V alloy is widely used in aerospace applications for its excellent mechanical properties, however, it presents low wear resistance. It is often coated with a cermet using high-velocity oxy-fuel (HVOF) spraying to improve its wear performance. The Cr3C2-NiCr cermet becomes particularly interesting since it is non-carcinogenic, compared to traditional cermet coatings containing tungsten-cobalt compounds. While the improvement in wear resistance of Ti-6Al-4V with this coating has been demonstrated, its impact on the fatigue performance of the alloy remains to be studied. This is precisely the aim of this study, which focuses on the fatigue life of a Cr3C2-25NiCr-coated Ti-6Al-4V alloy. Among the various influencing factors, surface preparation represents a significant source of crack initiation, particularly in the case of sandblasted surfaces. Indeed, the inclusion of fragmented alumina particles can produce stress concentration zones. Thus, laser texturing, which is a method involving the creation of anchoring points through controlled ablation, can be considered today as a less harmful surface preparation technique. The results obtained from cyclic tensile fatigue tests with a stress ratio of 0.1 for these two surface preparation methods are presented in this paper.

In this work, finite element modeling is used to investigate the influence of segmentation cracks on stress distribution and failure in thermal barrier coatings deposited by atmospheric plasma spraying. The results indicate that the presence of segmentation cracks does not improve thermal insulation, but it may be beneficial in regard to thermal shock resistance, depending on crack density, and residual stress around crack tips, depending on crack length. It may also improve strain tolerance, which is affected by crack density as well as length. A model is proposed to explain the mechanism of failure in thick TBCs exposed to thermal shock. Damage caused by thermal shock can be attributed to the propagation of segmentation cracks and the formation of horizontal cracks at the bond coat-topcoat interface.

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.

The physical characteristics and volume growth of TGOs in TBC systems lead to TBC failure. It is proved that enriching the BC/TC interface with α-Al 2 O 3 is beneficial to an extended operational time by prolonging the steady-state growth stage of the TGO. The corresponding phase reactions in TBC systems with heightened Al activity, however, are not studied yet. In this work, the stage formation of TGO layers of TBC systems with PVD-Al interlayers is described. The study uses thermal cyclic loading with dwell time at maximum high temperature of 1,150 °C. The crack formation in the ceramic top coat and the TGOs thickness at the interface are investigated by SEM/EDS after 1, 6, 12, 24, 40 and 80 thermal cycles. The results plot the interface change and crack formation as a function of the thermal cycle number. The corresponding failure mode is discussed.

La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF), deposited on a metallic porous support by means of plasma spray-physical vapor deposition (PS-PVD) is a promising candidate for oxygen-permeation membranes. However, after O 2 permeation tests, membranes show vertical cracks leading to leakage during these tests. In this work, a feature leading to crack formation has been identified. More specifically; Membrane residual stress changes during thermal loading have been found to be related to a phase transformation in the support. In order to improve the performance of the membranes, the metallic support has been optimized by applying an appropriate heat treatment. Additionally, it has been found that coatings deposited at lower oxygen partial pressures consist of 70% cubic and 26% rhombohedral perovskite phases. This increases the non-stoichiometry, which drives the formation of non-perovskite phases during annealing, affecting the membrane stability and the ionic conductivity. The amount of oxygen added during spraying can be used to suppress the cubic to tetragonal phase transformation.

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 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.

Thermal barrier coatings (TBC) in general fail by delamination of the ceramic partially stabilised Zirconia (PSZ) top coat (TC) from the underlying metallic bond coat (BC). The process is initiated by crack initiation and growth either in the TC or in the thermally grown oxide (TGO) that will form at the interface between top and bond coat. The aim of the present paper is to describe the degradation due to crack growth in such a way that data can be used for FEM modelling work. Flat rectangular test coupons have been subjected to thermal cyclic fatigue (TCF) in air with a temperature range from 100°C to 1100°C. Identical samples were removed from the TCF furnace at different times of thermal cycling in order to achieve material with different degree of damage. After mounting, cutting and sectioning the specimen were investigated by light optical microscopy (LOM) and scanning electron microscopy (SEM) together with an energy dispersive spectrometer (EDS). Image analysis of LOM micrographs was used for measurement of crack distribution and degree of TC damage. A method for crack growth measurement based on the degree of TC / TGO damage has been developed. Furthermore, a measure of TBC damage as a function of elapsed fatigue cycles was introduced. The TBC material shows a mixed black and white fracture surface after TCF cycling. Delamination crack growth data are presented. Delaminated TC/BC interface surface as a function of fatigue cycles follows an S-curve behaviour.

In this paper, the authors use computational tools to simulate thermal and mechanical processes involved in plasma spraying that lead to crack formation and delamination. The substrate-coating system is represented by a metal strip (120 x 20 x 2 mm) and a ceramic layer with a thickness of 0.06 mm. The transient heat transport problem and stress-deformation state of the 3D sample is solved via stress relaxation due to plastic deformation and distributed displacements or cracking. Paper includes a German-language abstract.

This paper presents the results of long-term thermal cycling tests on plasma-sprayed thermal barrier coatings, including coating samples produced with functionally graded materials. The role of oxidation is also considered based on the results of elemental analysis. The authors explain how the coatings were produced and tested and present and analyze the test results. The thermal barrier coatings formed with functionally graded materials were found to be relatively unaffected after the long-term thermal cycling test and showed no signs of oxidation. Paper includes a German-language abstract.

A thermal cycling test rig and procedure was designed in order to predict the life expectancy of Thermal Barrier Coatings (TBC) under thermal cycling conditions similar to those meet in combustion chambers. Two 2kW-halogen lamps highly focused on the TBC were used to expose the surface of the coating to an intense heat flux. A 25x100 mm TBC is Air Plasma Sprayed (APS) centered onto a substrate 25x370 mm. The thermal cycling can be done either under inert or oxidizing atmosphere in order to separate oxidation-induced acoustic emissions from that resulting from the mismatch of the Coefficient of Thermal Expansion (CTE) of the coating compared to that of the substrate. Two transducers located at each end of the substrate monitor the Acoustic Emission (AE) signals emitted by crack initiation and/or propagation, were recorded and analyzed in order to deduce available information about TBC behavior under thermal load. The use of two transducers with a time of flight approach provides a valuable means of identifying both the crack formation and its location. This thermal cycling test is adequate for the study of various samples, like welded substrates coated with TBC or TBC coated around holes. The presence of cracks is observed using metallography preparation and microscopic observation.

Rotating bending fatigue tests have been conducted at room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6AI-4V) with HVOF sprayed coating of a cermet (WC-12%Co) and S45C with WFS sprayed coating of a 13Cr steel (SUS420J2). Plane bending fatigue tests were also conducted at stress ratios, R, of -1, -0.5 and 0 for S45C with WC-12%Co coating. The fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials, R and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that a crack was initiated in the coating and then cracks were initiated in the substrate due to the stress concentration of the crack in the coating. The fatigue strength of the sprayed materials was dominated by that of the sprayed coating. Therefore, the fatigue strength could be evaluated uniquely in terms of the true stress on the coating surface. The influence of compressive residual stress of the sprayed coatings on fatigue strength was discussed based on the fatigue mechanisms at different stress ratios.

Microscopic fracture mechanisms of thermal spray coatings under bending stress are investigated. Samples of thermally sprayed coatings were made using three distances. The sprayed powder was pure molybdenum. Vertical microcracks occur in lamellae and subsequently, these cracks join together and form vertical macrocracks in the samples sprayed with a short spraying distance. On the other hand, horizontal microcracks occur at the lamellae interfaces, and these cracks link together in the samples sprayed with a long spraying distance. These tendencies can be explained in terms of the hardness of the lamella and the bonding strength between each lamella. It is clarified that the bonding strength between each lamella corresponds to the applied strain at the point of rapid increase of the acoustic emission (AE) event. The amplitude and rate of AE beyond the point of rapid increase are high in the coatings which formed macrocracks. It is concluded that the coating which has high resistance to crack formation has a high point of AE increase, low AE amplitude and low AE increasing rate.

Low cycle fatigue tests were performed at room temperature (RT) and at 673 K for l%Cr-0.5%Mo steel comparing the specimens coated with chromium carbide by gas spraying and the ordinary uncoated specimens, and the mechanism of fatigue crack formation was investigated. Following observations and conclusions were made: (1) When sprayed with ceramic, the fatigue life suffers reduction at either temperature, but at 673 K, the degradation was so much smaller than that at RT that the fatigue life was actually, though slightly, longer than that at RT. (2) The cracks are initiated in the ceramic layer very early in the whole fatigue life, the crack initiation lifetime becoming the longer, the smaller the strain range. (3) The fatigue failure process can be viewed as comprising following steps: first, early initiation of fatigue crack at the surface of the ceramic coating, rapid propagation through it to the substrate metal, and initiation of crack in the metal, the initial rate of propagation of such a crack being a number of times (perhaps as much as one full order of magnitude) faster than that in uncoated steel.