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.

Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.

Twin wire arc is a commonly used thermal spray technology for application of steel coatings to cast iron components. Hardness and adhesion strength are critical properties of such coatings, and significant research is available reporting these properties. However, residual stresses and the anisotropic structure of the coatings leads to significantly different behavior in bending applications than in the purely tensile loading of the standard adhesion test. In addition, microstructural features that are controlled by certain process parameters during deposition of the coating can have a significant effect on these properties. This work seeks to relate the hardness and pull-off adhesion strength to the coating microstructure, and to assess the related bending strength and failure mode. Comparisons between bend tests and pull-off adhesion tests show significant differences to consider when evaluating twin wire arc coatings.

ASTM C633 has been an industry standard for determining thermal spray coating adhesion and cohesion strengths for nearly 40 years. The test, however, has several drawbacks that can greatly affect the results. The epoxies used cannot withstand stresses greater than 15,000psi, producing data that may suggest coatings cannot function beyond the epoxy threshold under uniaxial tensile loading, resulting in data that can only be used for general quality control or acceptance testing. Previously published data shows coatings functioning beyond C633 limits, yet there is no standardized test to show true functional stress limitations. A four-point bend test method with an instrumented strain-gage has been used to show coating adhesion well beyond the yield point of the steel substrates and beyond the C633 limits for three materials and thermal spray processes: electric arc sprayed aluminum bronze, plasma sprayed alumina, and HVOF WC/Co/Cr. A strain-gage is applied to a prepared coating surface on a bend bar and loaded under tension or compression. The MTS universal load frame force data is used to calculate the stress at the coating/substrate interface by beam theory equations, allowing for stress and strain vs displacement curves to be generated and directly compared against C633 data for coating adhesion strengths. The resulting data can indicate microscopic coating behavior (cracking, de-bonding) as a result of the test sensitivity and can ultimately be used as design data for the practicing engineer.

Cyclic oxidation failure of Atmospheric Plasma Sprayed Thermal Barrier Coatings systems (APS TBCs), commonly used to insulate hot sections in gas turbines, usually results from the spallation of the ceramic top coat. Consequently, in order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barrier coatings is of utmost concern for engine-makers. Failure of the TBC is strongly related to the thermal and mechanical properties of each component of the multi-materials system (substrate, bond coat and ceramic) but also to the response of the TBC as a whole. The purpose of the work is to assess the mechanical behaviour of thick TBC using experimental approach for TBC standard lamellar, porous and microcracked microstructure (classically obtained through APS coatings). The experimental characterisation of the mechanical behaviour of the ceramic top coat of the TBC is addressed on specifically designed and prepared free-standing specimens using three points bending (3PB) tests and Small Punch Testing (SPT). The tests are performed on free-standing top coats made of YSZ in the as deposited states and for specimens that undergone isothermal aging at 1100°C for various durations (1h, 10h and 100h). The results of test performed at room temperature using both mechanical testing techniques are compared. This allows to show the evolution of mechanical properties after thermal aging. Tests performed at 850°C in the SPT ring show that the evolution of properties resulting from this aging may be different at room temperature as compare to 850°C.

A Vickers indentation method has been applied to determine the interfacial fracture toughness of modern multilayer thermal barrier coatings. The delamination behavior of four types of coating systems will be discussed and compared with results based on modified four-point bending (4PB) tests. The investigated multi-layer coating system consists of a CoNiCrAlY-bond coat applied via low-pressure plasma spray (LPPS) on a nickel-based superalloy and an atmospheric-plasma sprayed (APS) top layer of type gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ). A conventional YSZ mono-layer system is used for reference. The effects of GZO and YSZ microstructure were investigated using top coats with low and high porosities for both (multi- and single-layer) coating systems. Isothermal oxidation tests at 1100 °C up to 500 hours were performed to study the interaction between thermal aging and fracture behavior. Investigations of microstructure and sintering behavior show a significant influence of the annealing conditions on fracture toughness. It has been observed, that with increasing annealing time, the stiffness and thus the crack driving force of the GZO layer is increased due to sintering effects and healing of submicron defects. The lower stiffness and higher defect density of GZO seem to be the main reason for the reduced fracture toughness of the YSZ / GZO interface compared to the YSZ / CoNiCrAlY interface. As a result, the delamination of the top coat is observed to shift from the top coat / bond coat interface into the top coat double-layer.

The aim of this work is to evaluate the brittleness of suspension sprayed aluminum oxide coatings with various methods, including Vickers indentation fracture toughness, four-point bending, and high-velocity particle impact testing. Coatings were applied via high-velocity suspension flame spraying (HVSFS) using suspensions of isopropanol and water solvents. HVOF-sprayed Al 2 O 3 powder feedstock was used as a reference. The tests are described and the results are presented and discussed.

Failure is an inevitable consequence with thermal barrier coatings and failure modes are complicated due to irregular microstructure in the coating layers and wide range of external conditions. In this study, three-point bend tests are used to monitor damage evolution in YSZ-CoNiCrAlY TBCs on superalloy and stainless steel substrates. Coating samples, consisting of the bond coat and topcoat, were deposited by atmospheric plasma spraying on test specimens measuring 80 x 6 x 4 mm. The long, narrow specimens were subjected to three-point bend testing, using acoustic emission sensors to detect the formation and propagation of cracks in the coatings and plastic deformation in the substrates. The investigation results indicate that variations in acoustic emission signals correspond well with changes observed in the stress-strain curves of the coatings and substrates and that failure mechanisms can be systematically analyzed based on the amplitude, frequency, and energy of the acoustic emission signals. A detailed description of the actual failure process is provided.

The application of metallic foam core sandwich structures in engineering components has been of particular interest in recent years because of their unique mechanical and thermal properties. Thermal spraying of the skin on the foam structure has recently been employed as a novel cost-efficient method for fabrication of these structures from refractory materials with complex shapes that could not otherwise be easily fabricated. The mechanical behavior of these structures under flexural loading is important in most applications. Previous studies have suggested that heat treatment of the thermally sprayed sandwich structures could improve the ductility of the skins and so affect the failure mode. In the present study the mechanical behavior of sandwich beams prepared from arc sprayed alloy 625 skin on 40 ppi nickel foam was characterized under four point bending. The ductility of the arc sprayed alloy 625 coatings was improved after heat treatment at 1100°C and 900°C while the yield point was reduced. Heat treatment of the sandwich beams reduced the danger of catastrophic failure.

This study assesses the strength and adherence of VPS titanium coatings on ultrahigh molecular weight polyethylene (UHMWPE) substrates. Four-point bend tests show the existence of a critical tensile strain of 1% corresponding to the onset of cracking. For strains up to 6%, crack density increases with no observed debonding. Fatigue tests over 106 cycles reveal that the coating remains uncracked at a strain of 1% and stays in a stable cracked state without debonding as strain is increased to approximately 6%. A laser shock test developed specifically for titanium-polymer interfaces revealed the existence of a debonding threshold corresponding to the adhesion strength. The results serve as a guide for the design of orthopedic implants on which VPS titanium coatings are used and, more generally, open the way for systematic measurement of adhesion between metallic coatings and polymer substrates.

Previous work on thermal barrier coatings (TBCs) has shown that Ce-containing bond coats promote the formation of a wedge-like interface oxide that improves delamination resistance. The oxide was found to form at temperatures greater than 1100 °C, which in many applications, may not be reached. In this study, TBC samples consisting of a YSZ topcoat and various cold-sprayed bond coats were prepared. In order to obtain a wedge-like thermally grown oxide (TGO), pre-oxidation was carried out for 20 h at 1100 °C prior to high-temperature testing for 1000 h at 1000 °C. It was confirmed that the pre-oxidation treatment produced a wedge-like TGO that continued to grow at 1000 °C, which improved delamination resistance as four-point bend tests showed. A wedge-like oxide was also observed in some TBCs exposed to temperatures of 1000 °C, without pre-oxidation.

The fatigue performance of conventional structural steel with an applied thermal barrier coating (TBC) was evaluated via cyclic bending. Tests were carried out for as-received and grit-blasted substrates as well as for samples with thermally sprayed bond coats and topcoats. Failure mechanisms were identified and changes in fatigue resistance were assessed based on results obtained for different loading amplitudes supplemented by fractographic analysis.

This study investigates the effects of gas composition on cold-sprayed titanium coatings deposited under nine different spray conditions. Experiments show that higher levels of gas purity translate to higher particle velocities and measurable improvements in bending strength. The influence of gas temperature, pressure, and chemical composition is considered in the study along with interactions between carrier gases and sprayed particles. In addition to bending strength, the resulting coatings are assessed in terms of porosity and oxygen content.

Cold spraying is a promising process to fabricate functional coatings. Because of the solid-state particle deposition, the electrical and chemical properties of the coatings are similar to those of bulk materials. This is not the case with mechanical properties, however, which differ between the coating and bulk due to severe plastic deformation in the particles. The residual stress and bonding state of particles in cold-spray coatings are investigated in this study as to their influence on mechanical anisotropy. Bending tests on cold spray copper coatings show that loading direction has a significant effect, which may be related to the distribution of microvoids and pores. Traverse patterns and process gas temperature are also examined and are shown to play a role.

To answer current issues adequately considering technical, economic, as well as environmental requirements, material transformation and especially surface treatment industries must be source of innovations to be proactive. As a result, developing new alternative solutions to existing ones had become a top priority. Considering surface treatment processes, conventional ones (thermal spraying, plasma transferred arc) do not allow to consider this approach since the processes themselves (co-treatment of different powders) do not permit to guarantee the initial composition nor do they ensure a sufficient homogeneity to the coating structure. If indeed the dry surface treatment processes have already shown large potential, several limits remain such as an inefficient adhesion, an environmental impact over the life cycle or almost no materials on the market. To overcome these issues hybrid coating technologies (combining several processes) are likely to be developed. From all of them, laser technology seems to be very promising due to its high flexibility considering all the potential parameters (varying power, continuous or pulsed beam, etc.) and the localised treated area. For instance, combining simultaneously a laser with a thermal spray process enables the elaboration of a thick coating showing a good adherence. The ablation laser applied on the substrate surface just before the impacting particles as promoted in the PROTAL process permit to insure a suitable surface state favourable to the particles adhesion. The control of the coating microstructure was not so much studied. That is why, to complete the knowledge in this area, this work aims at studying the influence of laser technology in association with plasma spraying on the coating microstructure and more precisely on the coating mechanical properties. Coatings were characterized by SEM and void content was evaluated through image analysis and Archimedean porosimetry. Mechanical properties were assessed by the four points bending test for evaluating the coating apparent Young modulus.

The cold spray, for its peculiarity, is becoming increasingly in the reconstruction or repair of damage aluminium alloy components, especially in the aviation industry. Both thin (<0.5mm) and thick (up to centimeters) coatings are necessary in order to achieve dimensional recovery of the components. Contrary to thin, thick coatings can be deposited in single-pass or in multi-pass giving different thermal and stress contribution to the components and coatings itself. The thermal input, the amount and the type of residual stresses (compressive or tensile) confer appreciable or depreciable characteristics to the coatings adhesion, the crack propagation and the coating mechanical property. In this study two sets, single and multi-pass aluminium alloy coatings of different thickness are deposited into Al6061 substrate. The metallographic analysis by electronic and optical microscopes, the four-point bending test and the Vickers microhardness are performed; also the multi-pass coatings were characterized by fractographic analysis. Finally the different coating adhesions to substrate and cohesions are compared by standard ASTM C633 adhesion and cohesion tests.

Superior characteristics of the cold sprayed coating have led to many high-tech applications. Until recently, all these applications were carried out using ‘stationary’ systems only, while some applications such as in-situ repair of aircraft body/engine parts require a portable system. Recently a ‘Portable High Pressure Cold Spray System’ called KINETIKS 2000-2 has been developed. This system is capable of 400 C/20 bars nitrogen/helium jet, and produces dense coatings with clean interfaces of many materials. In order to establish the suitability of this process for producing aluminum alloy coatings for aerospace and other high tech industries, various aluminum alloys (CP-Al, HP-Al, 6061 Al, 7005 Al) coatings were produced over many substrate materials (2024 Al, 7005 Al, 4041 Steel, ZE41A Mg). Coatings were characterized using microstructure, bond strength, bend test, corrosion studies, etc. Microstructural study showed that dense coatings with about 2-4% porosity values were produced with clean and well bonded interfaces. Bond strength of these coatings varied between 20 to 35 MPa, Bend test results showed that the coatings have adequate strengths and could withstand severe strain conditions. Salt fog corrosion studies (ASTM B 117) showed that the coatings impart corrosion resistance to the substrates.

WC-Co/copper coatings with 8 layers were fabricated by warm spray deposition in order to investigate the effect of ductile layer inclusion on their fracture behavior. Bending strength, work of fracture, and surface hardness of freestanding coatings were examined by three point bending tests after removal of the substrates. The multilayered samples showed nonlinear stress-strain curves due to cracks in the WC-Co layers and plastic elongation of the copper layers. The multilayered samples with lower volume fraction of copper showed even lower bending strength than the monolithic samples of WC-Co and copper and no beneficial feature in mechanical performance was found. On the other hand, the samples containing higher volume fraction of copper exhibited more than twice higher work of fracture and moderately better bending strength than the monolithic WC-Co coatings, while the surface hardness was almost identical among all samples instead of the monolithic copper. The ductility of copper layers and the plastic constraint by the intact WC-Co layers attributed to enhance their mechanical properties. It has been concluded that cermet/metal laminate coatings can be one alternative approach to further improvement of the mechanical properties of thermal sprayed cermet coatings.

The thermo-mechanical properties of a thermal barrier bond coat (BC) play an important role in governing the life-time of a coating system. The presented work aims to determine these properties for NiCoCrAlY coatings sprayed on Hastelloy X substrates sprayed under different process conditions. Temperature dependent Young’s modulus values are determined for both Atmospheric Plasma Sprayed (APS) and HVOF sprayed coatings using the four-point bending test. Particular attention is paid to microstructure-property relationships during heating. Young´s modulus was determined up to 950°C and evaluated for coatings loaded in both tension and compression. Results are discussed in the context of the effect of feedstock material, process conditions and microstructure characteristics. The methods and results presented are attractive, particularly for the thermal spray industry, since these properties are a prerequisite when the BC is to be considered in component design.

In this work the high temperature mechanical properties of UHTC coatings deposited by plasma spraying have been investigated; particularly the stress-strain relationship of ZrB2 based thick films has been evaluated by means of 4-point bending tests up to 1500 °C in air. Results show that at each investigated temperature (500, 1000, 1500 °C) Modulus of Rupture (MOR) values are higher than the ones obtained at room temperature; moreover at 1500°C the UHTC coatings exhibit a marked plastic behaviour, maintaining a flexural strength 25 % higher compared to RT tested samples. The coefficient of linear thermal expansion (CTE) has been evaluated up to 1500 °C: obtained data are of primary importance for substrate selection, interface design and to analyze the thermo-mechanical behaviour of coating-substrate coupled system. Finally SEM-EDS analyses have been carried out on as sprayed and tested materials in order to understand the mechanisms of reinforcement activated by high temperature exposure and to identify the microstructural modifications induced by the combination of mechanical loads and temperature in an oxidizing environment.