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.

One candidate alternative to chrome plating and hard anodizing is a tungsten carbide (WC) coating applied by the High Velocity Oxy-Fuel (HVOF) process. HVOF WC coatings are currently being evaluated in many service life tests, including fatigue. The purpose of this paper is to compare the fatigue life of HVOF WC coated specimens with the fatigue life of hard anodized and bare aluminum specimens. This work examines WC thermal spray coatings as candidates for replacement of hard chrome plating and hard anodizing in aircraft and helicopter applications such as landing gear. In fatigue testing, the results showed an expected fatigue deficit for hard anodizing as compared to bare aluminum. Paper includes a German-language abstract.

Our previous experiments with low-cost steel substrates confirmed that individual steps of conventional thermal barrier coating (TBC) deposition may influence fatigue properties of the coated samples differently. In this study, testing was carried out for TBC samples deposited on industrially more relevant Hastelloy X substrates. Samples were tested after each step of TBC deposition process: as-received (non-coated), grit-blasted, bond-coated (NiCoCrAlY) and bondcoated + top-coated (yttria-stabilized zirconia - YSZ). Conventional atmospheric plasma spraying (APS) with gas stabilized plasma torch was used for deposition of both bond coat and top coat. In addition, for one half of the samples, bond coat was prepared by consecutive combination of HVAF (High Velocity Air Fuel) and APS processes. Samples were tested both in as-sprayed condition and after 100 hours annealing at 980 °C, which simulated in-service conditions. Obtained results showed that different fatigue performance may be expected for various stages of the TBC deposition as well as due to the variation of the deposition process and sample temperature history.

The aim of this work is to study the adhesion of WC-Co coatings using acoustic emission testing. The coatings were produced by HVOF spraying then subjected to four-point bending while in situ acoustic emission measurements were made. The test specimens were then examined under a microscope, revealing regularly spaced transverse cracks on the coating surface as well as interfacial cracking. An analysis of the acoustic emission data revealed two types of acoustic events based on absolute energy and amplitude. This appears to be linked to the cracks that were observed forming the basis for an interfacial cracking model.

The future demands of diesel engines require new options for low-friction and wear-resistant materials in order to increase efficiency and achieve environmentally sound solutions. Efforts are made to improve the performance and reduce the weight of engine blocks by coating the Aluminium cylinder bores with thermal-spray processes. Thus beside other means today nanocrystalline coatings are currently discussed, which should allow for the desired combination of structural, productional, and topographical properties. Beside sufficient tribological properties it is important that the composite (base material and coating) allows for an elongated endurance under cyclic mechanical and thermal stresses. In this work a four-point-bending test was used to examine deleterious failure mechanisms during fatigue such as spalling of the coating or delamination from the substrate. Therefore various thermally sprayed coatings were bent in tension and compression. The results were analysed in relation to the coating microstructure.

The theory of residual stresses and of residual bending of plates with relatively thick sprayed coatings is summarized and some methods to minimize the bending are proposed. The particular case of an aluminium alloy substrate with a relatively thick plasma sprayed ceramic coating, with a large difference of thermal expansion coefficients, is discussed.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.

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.

In this study, a design of experiment (DOE) approach is used to investigate the influence of powder size and mixing ratio on the microstructure and properties of thermal spray coatings. Nano and micro sized WC-12Co powders combined in different proportions with Inconel 625 were deposited on carbon steel substrates by HVOF spraying. The resulting coatings are examined and the effect of different powder combinations on hardness and yield strength is assessed. Spraying procedures and test methods are described and the findings are discussed. Of the various coatings produced, one shows great promise for wear protection, particularly in oil and gas applications.

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 is the first of two papers concerning the intrinsic mechanical properties of arc-sprayed WC-FeCSiMn coatings. In part 1 the elastic and plastic forming behavior of the layers are investigated by indentation, bending and tensile tests. They were performed on coated mild steel substrates as well as freestanding as-sprayed samples with different geometries. Considering the coatings microstructure, element and pore distribution, as well as the local microhardness the results of the indentation, bending, and tensile tests were evaluated. The critical role of pores and inhomogeneities within the sprayed coating was examined in detail. Micro- and macrocracking were investigated by SEM after the indentation and tensile tests. In-situ surface observation by optical 3D-microscopy was used to study the onset of cracking during the 3-point bending test.

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.

HVOF coating finds wide applications in power generation industry to protect the hot path components from high temperatures gases. The mechanical properties influence the life span of such components. In the present study, mechanical properties of HVOF spray of Inconel-625 alloy onto stainless steel surface was examined. The three point bending tests are carried out to determine the load and the flexural displacements characteristics of the coated workpieces. The workpieces were subjected to aqueous static corrosion environment before and after the three point bending tests. The surface morphology and elemental composition of the coating after the bending tests are examined using SEM and EDS. It is found that the local corrosion site in the coating result in high stiffness of the coating and the cracks extending along the surface of the coating are resulted due to the shear on the surface of the coating.

Four-point bend test using acoustic emission (AE) was used to compare coating properties under mechanical solicitation, mainly the toughness and the spalling behaviour. Coatings are made from the same material; the 2005NS (WC-17Co), sprayed with an HVOF gun at various spray parameters. Coatings deposited on thin rectangular substrates were tested with the coating bent in tension and in compression. AE features like the event number, energy per event and cumulative energy were used to assess the damage in the coatings. The results are analyzed in relation with the coating microstructure.

In this study, Acoustic Emission (AE) signals are used to monitor the degradation of plasma sprayed Thermal Barrier Coatings (TBC) using cyclic four point bend tests. Signal analysis both in time and frequency domains is carried out in order to identify the key parameters which can be used to classify the acoustic emission signals as a function of the damage mechanisms. This classification offers a mean of prediction of the long-term behavior of the thermal barrier coating based on the acoustic emission signal signature at the early stages of bench testing. The samples consist of a Nickel-based alloy blade coated with a duplex TBC made of a 150 μm thick bond coat covered with a 300 μm thick partially-stabilized zirconia coating. Tests were performed on unnotched and perpendicularly notched samples in order to discriminate the AE from perpendicular cracks. Two broadband transducers are used for acquisition of acoustic emission signals. Measuring the time between signal detection by each of the two transducers provides a means of determination of the location of the source of the acoustic signals. A classification of the signals based on their energy and their maximum peak frequency is presented. A comparison is made between the degradation mechanisms of TBC under thermal cycling conditions that were presented elsewhere (1) and the results of four point bend tests presented here.

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.

Experimental designs have been used by the thermal spray community to improve and optimize spray parameters to produce coatings with desired properties. The influence of four spray parameters including top and bond coat thicknesses, substrate temperature, and spray distance on the mechanical properties of plasma sprayed thermal barrier coatings has been examined. Two experimental matrices; (i) a four by nine according to a Taguchi experimental design, and (ii) a four by seventeen according to a full factorial design of the experiment, were developed. Six samples from each group were tested using a four point bend arrangement. Yield strength and elastic modulus were calculated from the four point bend test. A multi-linear regression analysis on yield strength and elastic modulus values from each experimental matrix was carried out to determine the influence of each spray parameter on these properties. The multi-linear regression analysis results for these two experimental matrixes are compared.

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.

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.

Under load and stress thermal spray coatings have unique behaviour as compared to the bulk materials. The aim of this study was to define the mechanical properties of thermal spray coatings and consider the effect of external stress on coatings under test conditions. Five HVOF or Plasma spray coatings: NiCr, WC-CoCr, Al 2 O 3 -TiO 2 , Al 2 O 3 , and Cr 2 O 3 were studied. Mechanical properties, such as elastic modulus (E), and tensile strength (a) were measured. Behaviour of some coatings in different loading conditions (tension and compressive) was studied.