One of the promising thermal barrier coatings (TBC) options for use above 1250 °C has been La 2 Ce 2 O 7 (LC). This work explored the role of dual layered ceramic coatings in the top layer of the TBC system that has been prepared using atmospheric plasma spraying (APS). Above the NiCrAlY bond coat, 8 mol.% yttria stabilized zirconia (8YSZ) coating has been deposited with optimized APS parameters. Over the top layer (8YSZ), another layer that comprises composite with LC and 8 wt.% of 8YSZ (spray dried) has been deposited. Investigations into the hot-corrosion behavior of 8YSZ-LC based TBC subjected to Na 2 SO 4 +V 2 O 5 salt at 950 °C for 4 hours. A porous layer made mostly of LaVO 4 , CeO 2 , CeO 1.66 and YVO 4 was developed on the LC+8wt.% YSZ layer after being subjected to a hot corrosion test in Na 2 SO 4 +V 2 O 5 salt. Dissociation of LC and 8YSZ leads to the formation of new phases, such as CeO 1.66 , CeO 2 , LaVO 4 and YVO 4 as the corrosion by-products in the extreme environment. The findings indicated that delamination has occurred due to the phase transformation, cavities and cracks in the 8YSZ-LC based TBCs. The molten salt's hot corrosion mechanisms of the 8YSZ-LC based TBC are discussed in detail. Further, the potential use of 8YSZ-LC based dual coatings and scope for the future work have been derived from the current study.

Thermal barrier coatings have provided a revolution in the industry as they allow a higher operating temperature of equipment, improving the efficiency of gas turbines. However, one of the biggest challenges in terms of increasing the lifespan of TBC systems is the attack of fused silicates or simply CMAS (Calcium-Magnesium-Alumina-Silicate). CMAS are particles from the environment that can penetrate the TBC structure and cause delamination of the coating when exposed to high temperatures during thermal cycling. In this study, a plasma sprayed YSZ coating in the as coated and surface treated condition were given CMAS depositions from various preparation methods, and then subjected to thermal cycles at different evaluation temperatures and exposure times. The permeability of the ceramic layer and the penetration path of CMAS at different temperature levels were evaluated, as well as the penetration characteristics in relation to the microstructure of the ceramic layer. X-Ray diffraction and Scanning Electron Microscopy were used to characterize the applied CMAS and the penetration kinetics and conditions. Samples with longer exposure time had a considerable volume increase. The conditions to guarantee the formation of the silicate and its consequent wettability are also discussed.

In the study, Axial Suspension Plasma Spray (SPS) was used to produce a range of columnar microstructures from Yttria Stabilized Zirconia (YSZ) suspension after an extensive experimental design. The optimized microstructure was applied to a multi-layer GZ/YSZ system, in which both layers were sprayed with SPS. In addition to SPS, a new GZ coating using Axial Solution Precursor Plasma Spray (SPPS) was developed and deposited on top of the SPS GZ coating. The durability in the furnace cycling test (FCT), as well as the consequences of CMAS infiltration into the columnar coatings was extensively studied on different microstructures. Preliminary CMAS test on the SPS coatings infiltrated them completely, leading to delamination. To minimize the detrimental effect of CMAS on the underlying SPS, the dense solution precursor GZ layer was aimed to act as a sealant to protect the underlying columnar SPS-GZ layer from molten CMAS infiltration.

In automotive industry, thermal spray process is used to reduce engine weight by replacing cast iron liners inserted in cylinder bores. Especially, twin wire arc spray is one of widely used thermal spray processes with inexpensive cost and high deposition rate. In this study, two kinds of wire materials, low carbon steel (0.07 wt.%C) and high carbon steel (0.80 wt.%C) were deposited by twin wire arc spray process using two kinds of process gas (i.e., compressed air and nitrogen) in order to elucidate effects of carbon contents of ferrous coating and process gas type on the hardness and wear resistance of coating. In case of hardness, low carbon steel coatings had higher hardness when air was used as process gas whereas high carbon steel coatings had higher hardness when nitrogen was used, which was caused by the counter effects of carbon loss and oxide formation. The results of sliding wear test in lubricated condition indicated that coatings with higher hardness have better wear resistance and oxides improve wear resistance by playing a role as solid lubricant. The main wear mechanism was splat delamination induced by inter-splat crack, and traces of other wear behaviours such as splat tip fracture and abrasive wear were also observed.

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.

This paper presents an assessment of the properties of three thermal sprayed coatings for a thermal barrier applied to the piston head of an internal combustion engine. Coating (1) has a topcoat of Al 2 O 3 and a bond coat of Ni-Cr. Coating (2) has a topcoat of YSZ and a bond coat of CoNiCrAlY. Coating (3) is 316 stainless steel. The investigators conducted thermal cycle tests and tensile pin tests, and evaluated the thermal barrier effect of the coatings. The results showed that coating (3) had no cracks or delamination after the thermal cycle test, while coatings (1) and (2) had partial cracks and delamination. Coating (2) had the highest adhesion strength, and its thermal barrier property was better than that of coatings (1) and (3). In terms of cost and overall evaluation, coating (3) was considered the most reasonable and appropriate thermal sprayed coating for the piston head application.

In this study, plain carbon steel wire and pure iron powder are deposited on grit-blasted Fe-Si alloy substrates using thermal (electric arc, plasma, and HVOF) and kinetic spraying techniques. The coatings obtained were then heat treated. Coating microstructures and mechanical properties were investigated and compared focusing on oxidation, delamination, and Si dilution from the substrate to the coating.

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 sprayed coatings are often used for high temperature applications and, per se, are subjected to transient temperature gradients during operation. The recurrent temperature changes generate stresses that damage the coating with time, and can even lead to its delamination. The most common methods to evaluate coating behavior under thermal cycling are furnace testing or burner rigs. Both approaches cannot match the conditions reached in service for several applications, in terms of the achievable heating rates for instance. As a consequence, a versatile and robust method to evaluate coating resistance to spalling under thermal cycles is still to be found. This paper presents the development of a thermal cycling rig where the heat input is provided by a laser. This rig allows easy testing of several samples jointly for heating rates as high as 55°C/s and for thousands of thermal cycles. Preliminary trials have allowed the development of different spalling criteria. Finally, it was found that SS430-based materials arc-sprayed on Al substrates exhibit higher delamination resistance (life) under rapid heating/cooling cycles than SS304 coatings on the same substrate. For such high heating rates, the thermal stresses generated in the coating would be more critical than the thermal mismatch at the interface coating/substrate.

The dry sliding wear behaviour of two HVOF-sprayed Fe-Cr-Ni-Si-B-C (Colferoloy) alloy coatings was studied by ball-on-disk tests performed at room temperature (against alumina and 100Cr6 steel balls), at 400 °C and at 700 °C (against alumina balls only). HVOF-sprayed Ni-Cr-Fe-B-Si-C and Cr 3 C 2 -NiCr layers were also tested for comparison. Under all test conditions, the wear rate of the Colferoloy coatings is lower than that of the Ni-Cr-Fe-B-Si-C coating but larger than that of the Cr 3 C 2 -NiCr cermet. Specifically, at room temperature, the Colferoloy coatings exhibit a combination of mild abrasion, delamination and tribo-oxidative wear against alumina, whereas, against steel, they undergo very limited delamination with negligible wear loss. By contrast, the Ni-Cr-Fe-B-Si-C coating suffers larger wear against steel and undergoes more severe abrasive grooving against alumina. Although the Colferoloy and Ni- Cr-Fe-B-Si-C coatings possess similar microstructure and micro-hardness, their scratch behaviours, which depend on cracking resistance and plastic deformability, differ, thus explaining the micromechanical reason for the different wear mechanisms. At 400°C and 700°C, all of the metal alloy coatings are softened and suffer more severe abrasive grooving; by contrast, the behaviour of the Cr 3 C 2 -NiCr layer at 700 °C is controlled by the formation and delamination of an oxidised layer.

In rocket engine combustion chambers the cooling channels are subjected to extremely high temperatures and environmental attack. Because of the good heat conduction the inner combustion liner is made of copper. Thermal and environmental protection can be provided by Thermal Barrier Coating Systems. The performance of an APS-sprayed standard coating system for nickel based substrates (NiCrAlY and YPSZ) on copper substrates is investigated. Because mechanical and thermal properties (e.g. the coefficients of thermal expansion) of the two substrates are different, known failure mechanisms for nickel based substrates can not be directly transferred to the new application. Thermal cycling and laser shock testing is performed to identify possible failure mechanisms. The laser shock setup consists of a high-power diode laser (3kW) and realizes surface temperatures of up to 1500°C. Furthermore, it is possible to realize high thermal gradients inside the specimen, similar to those in real service. Delamination of the thermal barrier coating at the interface between bond coat and substrate is observed. Usually, this interface is not failing in standard applications, which gives an important hint for further research. Furthermore, FEM analysis confirms that stresses are maximal at this interface.

With the modification of plasma spray parameters, porosity ratio of top coat can control along the cross-section. This improves the thermal cycle resistance and decrease the thermal conductivity. Plasma sprayed ZrO 2 /8 wt.–% Y 2 O 3 –NiCrAlY TBC systems which have different porosity (%8-12) and range of 250-350µm thicknesses of top coats, during thermal cycling tests with different hold times at 1350 °C have been performed. The main failure modes: delamination cracking, TGO growth rate and phase transformation are strongly dependent on the hold temperature and time. The correlation between TBC thermal cycle lifetimes and duration of high temperature hold time per cycle is shown and discussed with depending on thickness and porosity ratio.

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 evaluation of residual stress distributions of a thermal sprayed plate is an important problem from the viewpoint of preventing the cracking and/or delamination of coatings. Though the well-known method for estimating the residual stress has been widely used based on Stoney’s type equations, the accuracy of estimation has not always been made clear. In this paper, accurate estimation of residual stresses of coated components using finite element analysis was implemented and polynomial expression of the relationship between residual stress and deflection is proposed. It was confirmed that the residual stress estimated by the newly proposed equation was in good agreement with the experimental results measured by X-ray diffraction method in case of a CoNiCrAlY coating applied on an IN738LC substrate using plasma spraying in air atmosphere.

The influence of the high velocity air fuel (HVAF) sprayed coating on the fatigue behavior of the low alloy steel was studied at different stress levels. It was observed that only one single main crack initiator existed in the substrate after fatigue at low stress levels, but there were multi-cracks on the substrate surface at high stress level. Detailed investigations showed that the cracks in the HVAF coatings sinuously extended to the interface and deflected thereat along the interface. Consequently, free-standing coating was formed due to its limited bond strength to the substrate and the lower elastic modulus than that of the substrate. The gap between the free-standing coating and the substrate surface was found to be correlated with the stress level. The high stress can greatly degrade the adhesion to the substrate causing the delamination of the coating. The cracks in the HVAF coating had no significant effect on the fatigue life of the substrate.

The adhesion at the interface between coating and substrate is a key factor for the reliability and performance since the main problem in the application of coating systems is the delamination at the interface. Finite element models are developed to find a way to predict the adhesion strength of a thermal sprayed coating system from simply designed indentation test. Large depth indentation behavior is simulated to study shear induced delamination beneath the indenter. The interface between the coating and the substrate is modeled by three different bonding characters to investigate the effect of interfacial bonding on indentation test. Pressure-strain relations based on Tabor’s suggestion are observed for various combinations of material properties and interfacial bonding characteristics. The growth of a crack in the interface plane leaves a clear imprint on indentation load-depth curve in case of soft coating on hard substrate. Tabor curve also shows potential ability to detect interface bonding strength under indentation test.

Performance and reliability of TS coatings are strongly dependent upon their inelastic behavior. To that end, continuum-level flow stress and hardening have been explored by recourse to e.g., indentation, substrate curvature and tensile measurements. While the results of such efforts have great implication for process and quality control, further refinement is necessary for incorporation into models of wear, fatigue or delamination. Here we present results and analysis of mechanical testing on metallic TS coatings deposited by different methods, emphasizing particular characteristics under localized and cyclic loading.

The delamination wear mechanism of the thermally sprayed coatings was studied by analyzing coatings structural feature and stress distribution on the warm surface, and the influencing factors on the delamination wear were discussed. And the delamination wear mode of coating was developed. The results show that, the thermally sprayed coatings have typical aspect of lamellar structure. There are oxide layers between splats, and there also exist porosity and micro-crack in the coatings. The coating surface was subjected to alternately tensile stress and compression stress caused by normal load and friction force during sliding. In a certain depth below the surface, there exists maximum shear stress. Therefore fatigue damage will take place at subsurface of the coating under alternate stress. The adhesion strength between splats of coating prepared by HVAS is by far lower than casting material because of lamellar structure. And the adhesion strength between splats is further weakened due to the defects (such as porosity and micro-crack) appearing mostly on the boundaries between thin oxide sheets and splats. When the fatigue damage accumulates to a certain value, micro-cracks initiate at the defects between splats. Then these micro-cracks grow, connect, and propagate along the defects between splats. Finally, these cracks shear to the coating surface leading to spallation of the splats, and thus wear debris is generated. By repeating the above process delamination of the coatings will occur. Reducing friction coefficient, increasing coatings hardness and adhesion strength between inter-splats are the basic methods to improve the wear resistance of thermally sprayed coatings. Abstract only; no full-text paper available.

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.

Microcracks in thermal barrier coatings are inherent from the plasma spraying process. Such cracks might constitute a threat to the coating. The influence of pre-existing cracks in the global direction of the interface between the bond coat and the top coat on the risk of delamination is addressed through finite element simulations. Stress concentrations at the interface due to the roughness of the plasma sprayed bond coat are accounted for by a sinusoidal interface. The effect of oxidation of the bond coat is modelled by including a thin oxide layer between the ceramic coat and the bond coat. It was found that the crack tip position of pre-existing cracks, as well as the presence of an oxide layer, significantly influences the risk of delamination. As the oxide thickness increases, the risk of crack propagation increases. It is also found that not all pre-existing cracks can propagate. For some crack tip locations, the crack remains closed during the entire loading sequence.