Microstructure design and mechanical properties of thermal barrier coatings with layered top and bond coats

The microstructure of layered thermal barrier coatings (TBCs) with three coating layers in the bond and top coats, respectively, prepared using a specialized coating system (TriplexPro™-200), was controlled and its mechanical properties were investigated, which were then compared with the common TBCs with a single layer in each coat. The bond and top coats were coated with 100 and 200 μm for each feedstock, resulting in 300 and 600 μm thicknesses in the bond and top coats, respectively. The microstructure of the top coat could be controlled by changing the feedstock and using a multiple hopper system—dense/intermediate/porous layers from surface to interface or reverse microstructure. In the case of the bond coat, a compositional gradient was achieved. The adhesive strength values of the top coats were strongly dependent on the microstructure, whereas the values for the bond coat were similar. The hardness and toughness values gradually changed from surface to interface, indicating that the mechanical properties corresponded well with the microstructure of the TBCs. The indentation stress-strain curves of both TBCs with the layered structure were located between the curves for TBCs with the single structure of relatively dense and porous microstructures. Damage on the surface and subsurface was strongly affected by the microstructure of the top coat, showing a similar trend with the stress-strain behavior. This evidence allowed us to propose an efficient coating in protecting the substrate from mechanical environments.     

Thermal diffusivity measurement by thermographic technique for the non-destructive integrity assessment of TBCs coupons

For thin (< 200 μm) air plasma spray (APS) and electron beam physical vapor deposition (EBPVD) ceramic thermal barrier coatings (TBCs), some non-destructive techniques indicate damage at the bond coat-TBC interface during either ageing or cyclic oxidation tests. However, no technique is available for thick (> 200 μm) APS TBCs.In this work, a semi-quantitative estimation of cracks at the interface of APS TBCs thicker than 300 μm is obtained from thermal diffusivity values measured by using a single side thermographic technique on coupons subjected to thermal cycling.In fact, during thermal cycling, two phenomena occur: sintering that promotes a significant increase of thermal diffusivity, and cracking that, representing an additional thermal resistance, causes an apparent decrease of thermal diffusivity.The idea presented hereinafter consists in removing the effects of sintering from apparent thermal diffusivity to estimate cracking at the interface.     

Microstructure Evolution and Interface Stability of Thermal Barrier Coatings with Vertical Type Cracks in Cyclic Thermal Exposure

In this study, the effects of intrinsic feature of microstructure in thermal barrier coatings (TBCs) with and without vertical cracks on the microstructure and mechanical properties were investigated in cyclic thermal exposure. The hardness values of TBCs with vertical cracks were higher than those without vertical cracks, showing a good agreement with microstructure. The TBC prepared without vertical cracks using the 204-NS was delaminated after 250 cycles in the cyclic thermal exposure test. The TBCs with and without vertical cracks prepared with 204 C-NS and the TBC with vertical cracks prepared with 204 NS showed a sound condition without any cracking at the interface or spalling of top coat. After the thermal exposure of 381 cycles, the hardness values were increased in the survived TBC specimens, and the thicknesses of TGO layer for the TBCs with 204 C-NS and 204 NS were measured as in the ranges of 5-9 and 3-7 μm, respectively. In the thermal shock test, the advantage of vertical cracks for thermal durability of TBC could be well investigated, showing relatively longer sustained cycles in the TBCs with vertical cracks. The TBCs with vertical cracks are more efficient in improving thermal durability than those without vertical cracks in cyclic thermal exposure.     

Preparation of YSZ coatings by thermal pressure and filtration of sol-gel paint (TPFSP) for thermal barrier application

Thick YSZ ceramic coatings were prepared by thermal pressure and filtration of sol-gel paint (TPFSP), a modified sol-gel composite coating technique. SEM results show that the coatings were dense and crack-free by using paints of high mass ratios of YSZ powder to Zr-Y oxide in sol and high pressures. And the cross-sectional detail of coatings exhibited that the microstructure was consisted of micro/nano-size ceramic particles and micro-pores. The thermal insulation tests indicated that mass ratio of YSZ powder to Zr-Y oxide in sol was in inversely linear relationship with temperature drop per micron thickness. The coating showed good adherence with alloy substrate and maintained structural integrity when exposed at 1050 °C for 200 h. The cyclic oxidation test also indicated that both of oxidation resistance and spallation resistance for YSZ coated specimens were greatly improved. The TPFSP process could be a promising method to prepare TBCs for wide applications.     

Mechanical properties of solution-precursor plasma-sprayed thermal barrier coatings

The microstructure of thermal barrier coatings (TBCs) of 7 wt.% Y₂O₃ stabilized ZrO₂ (7YSZ) deposited using the solution-precursor plasma spray (SPPS) method has: (i) controlled porosity, (ii) vertical cracks, and (iii) lack of large-scale “splat” boundaries. An unusual feature of such SPPS TBCs is that they are well-adherent in ultra-thick forms (~ 4 mm thickness), where most other types of ultra-thick ceramic coatings fail spontaneously. Here a quantitative explanation is provided as to why as-deposited ultra-thick SPPS TBCs are so well-adherent. The mode II toughness of thin (0.2 mm) SPPS TBCs has been measured using the “barb” shear test, which is found to be 66 J m^− 2. Residual stresses in SPPS TBCs of thickness 0.2, 1.5, and 4.0 mm have been estimated using a microstructure-based object-oriented finite element (OOF) method. These stresses are found to be low, as a result of the strain-tolerant microstructure of the SPPS TBCs. The corresponding strain energy release rates that drive mode II cracks in the three different thickness SPPS TBCs have been found to be less than the mode II toughness.     

Evaluation of microhardness,fracture toughness and residual stress in a thermal barrier coating system: A modified Vickers indentation technique

The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67 MPa m^1/2, and the corresponding residual stress near the indented region varies from − 36.8 to − 243 MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81 MPa m^1/2, and residual stress varies from − 5 to − 30 MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.     

Investigation of interfacial properties of atmospheric plasma sprayed thermal barrier coatings with four-point bending and computed tomography technique

A modified four-point bending test has been employed to investigate the interfacial toughness of atmospheric plasma sprayed (APS) yttria stabilised zirconia (YSZ) thermal barrier coatings (TBCs) after isothermal heat treatments at 1150 °C. The delamination of the TBCs occurred mainly within the TBC, several to tens of microns above the interface between the TBC and bond coat. X-ray diffraction analysis revealed that the TBC was mainly tetragonal in structure with a small amount of the monoclinic phase. The calculated energy release rate increased from ~ 50 J/m^− 2 for as-sprayed TBCs to ~ 120 J/m^− 2 for the TBCs exposed at 1150 °C for 200 h with a loading phase angle about 42°. This may be attributed to the sintering of the TBC. X-ray micro-tomography was used to track in 3D the evolution of the TBC microstructure non-destructively at a single location as a function of thermal exposure time. This revealed how various types of imperfections develop near the interface after exposure. The 3D interface was reconstructed and showed no significant change in the interfacial roughness after thermal exposure.     

Thermal stability and mechanical properties of thick thermal barrier coatings with vertical type cracks

The thermal stability and failure mechanism of thick thermal barrier coatings (TBCs) with and without vertical type cracks were investigated through the cyclic thermal exposure and thermal-shock tests. The TBC systems with thickness of about 2000 µm in the top coat were prepared by an air plasma spray (APS) on the bond coat of about 150 µm in thickness prepared by APS. The adhesive strength values of the as-prepared TBCs with and without vertical type cracks were determined to be 24.7 and 11.0 MPa, respectively, indicating the better interface stability in the TBC with vertical type cracks. The TBC with vertical type cracks shows a better thermal durability than that without vertical type cracks in the thermal cyclic exposure and thermal-shock tests. The hardness values of the as-prepared TBCs with and without vertical type cracks were found to be 6.6 and 5.3 GPa, respectively, which were increased to 9.5 and 5.5 GPa, respectively, after the cyclic thermal exposure tests. These results indicate that the vertical type cracks developed in the top coat are important in improving the lifetime performance of thick TBC in high temperature environment.     

Preliminary study on the TriplexPro™-200 gun for atmospheric plasma spraying of yttria-stabilized zirconia

The recently introduced three-cathode TriplexPro™-200 atmospheric plasma spray gun was tested with yttria-stabilized zirconia. The effects on the particle characteristics (velocity and surface temperature) were measured by time-of-flight measurement and two-color pyrometry, respectively (DPV-2000 diagnostic system) while the arc current intensity, the plasma gas flow, and its composition were systematically varied.With typical spray parameters of the Triplex® II, using the TriplexPro™-200 with the 9.0 mm nozzle the particle characteristics were found to be almost the same while the nozzle still provides power reserves. In this case, helium may be dispensed with entirely.Using the 6.5 mm or the 5.0 mm nozzle operation is limited to a maximum 500 A current, which significantly increased particle velocities, but it was not possible to keep the particles fully molten. The investigated range of powder grain size (d₅₀ = 58 μm) is to be considered with these parameters. Further possibilities are to be expected with the 6.5 mm and 5.0 mm nozzle using smaller-sized or lower-melting powders.Operating the TriplexPro™-200 gun with the 11.0 mm nozzle and/or nitrogen as secondary plasma gas instead of helium, increased particle temperatures and velocities are achievable. This offers extended possibilities to spray high-melting oxide ceramics like yttria-partially stabilized zirconia.     

Improving stability of thermal barrier coatings on magnesium alloy with electroless plated Ni–P interlayer

Thermal barrier coatings (TBCs) of zirconia stabilized by 8 wt.% yttria (8YSZ) on MB26 rare earth–magnesium alloy with MCrAlY as bond coat were prepared by air plasma spraying (APS). In order to improve the thermal shock resistance of the coatings, an interlayer of Ni–P alloy between the substrate and bond coat was prepared by electroless plating. The preparation, microstructure, bond strength and thermal shock resistance of the coatings were investigated. The results indicate that Ni–P interlayer not only has favorable effects on the protection of Mg alloy substrate from thermal oxidation during thermal spraying, but also significantly improves the bond strength of TBCs. The thermal shock life of TBCs was enhanced from 5 cycles to longer than 130 cycles with the application of Ni–P interlayer. The failure of TBCs in thermal shock test was mainly induced by the corrosion of Mg alloy substrate.     

Atmospheric plasma sprayed thermal barrier coatings with high segmentation crack densities: Spraying process, microstructure and thermal cycling behavior

Thermal barrier coatings (TBCs) with high strain tolerance are favorable for application in hot gas sections of aircraft turbines. To improve the strain tolerance of atmospheric plasma sprayed (APS) TBCs, 400 μm-500 μm thick coatings with very high segmentation crack densities produced with fused and crushed yttria stabilized zirconia (YSZ) were developed. Using a Triplex II plasma gun and an optimized spraying process, coatings with segmentation crack densities up to 8.9 cracks mm^− 1, and porosity values lower than 6% were obtained. The density of branching cracks was quite low which is inevitable for a good inter-lamellar bonding.Thermal cycling tests yielded promising strain tolerance behavior for the manufactured coatings. Samples with high segmentation crack densities revealed promising lifetime in burner rig tests at rather high surface (1350 °C) and bondcoat temperatures (up to 1085 °C), while coatings with lower crack densities had a reduced performance. Microstructural investigations on cross-sections and fracture surfaces showed that the segmentation crack network was stable during thermal shock testing for different crack densities. The main failure mechanism was delamination and horizontal cracking within the TBC near the thermal grown oxide layer (TGOs) and the TBC.     

Tensile fracture behavior of thermal barrier coatings on superalloy

Tensile fracture behavior of thermal barrier coatings (TBCs) on superalloy was investigated in air at room temperature (RT), 650 °C and 850 °C. The bond coat NiCrAlY was fabricated by either high velocity oxygen fuel (HVOF) or air plasma spraying (APS), and the top coat 7%Y₂O₃-ZrO₂ was deposited by APS. Thus two kinds of the TBC system were formed. It was shown that the coating had little effect on tensile stress-strain curves of the substrate and similar tensile strength was obtained in two kinds of the TBC system. However, the cracking behavior in the two kinds of TBC system at RT was different, which was also different from that at 650 °C and 850 °C by scanning electron microscopy. The interface fracture toughness of the two kinds of TBC system was evaluated by the Suo-Hutchinson model and the stress distribution in the coating and substrate was analyzed by the shear lag model.     

Opportunities for TBCs in the ZrO2-YO1.5-TaO2.5 system

An examination of the ZrO₂-YO_1.5-TaO_2.5 system reveals several promising attributes for use in thermal barrier coating applications. The rather unique presence of a stable, non-transformable tetragonal region in this ternary oxide system allows for phase stability to high temperatures (1500 °C). Selected compositions with high levels of yttria and tantala have also shown superior resistance to vanadate corrosion than the commercially utilized 7YSZ. In addition, Y + Ta stabilized zirconia compositions within the non-transformable tetragonal phase field exhibit toughness values comparable or somewhat higher than those of 7YSZ, which bodes well for their durability as TBCs. These promising attributes are discussed in this paper in the context of recent experimental work.     

Nanoscratching deformation and fracture toughness of electroless Ni-P coatings

In the present investigation electroless Ni-P coatings were prepared. Structural characterizations indicated that the as-deposited coating had an amorphous structure with a P content of 23 at.%. The deformation behavior of an electrolessly amorphous Ni-P coating was investigated by using the Vickers indentation and the Tribo-indenter instrumented nano-indentation technique. The hardness of the Ni-P coating is remarkably improved after proper heat-treatment and the hardness is as high as 12.7 GPa for the coating annealed at 400 °C for 1 h. However, the cracks were observed during the indentation of the Ni-P coatings annealed at 400 °C and 500 °C for 1 h. The corresponding fracture toughness was evaluated as 2.58 MPa m^0.5 and 1.33 MPa m^0.5, respectively. Nanoscratching tests indicated that the wear resistance of the Ni-P coatings was improved significantly with an increasing ratio of hardness (H) to elastic modulus (E). It was observed that the friction coefficient increased from 0.083 ± 0.006 for the Ni-P coating annealed at 300 °C up to 1.337 ± 0.009 for the IF steel substrate, while the H/E simultaneously decreased from 0.084 (10.7/128) to 0.009 (1.85/200). The study revealed that the electrolessly amorphous Ni-P coating had offered better corrosion resistance than the Ni-P coatings after heat-treatment. An annealing temperature of 300 °C is preferentially suggested for the trade-off between the wear resistance property and anti-corrosion property of the Ni-P coating.     

Low conductivity plasma sprayed thermal barrier coating using hollow psz spheres: Correlation between thermophysical properties and microstructure

Life and thermal properties of plasma sprayed TBCs - widely used in gas turbine engines - are closely related to the microstructure of the ceramic top coating. Especially, the thermal behaviour of this coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material.A specific hollow yttria partially stabilised zirconia powder was produced in a one-step process by spray drying and an experimental statistical design study was conducted to investigate the influence of spraying variables (primary and secondary gas flow rates, arc current, spraying distance, spraying angle and traverse speed) on structure and properties of resulting plasma sprayed coatings. The coatings were characterized with respect to deposition efficiency, roughness, porosity and thermal conductivity. A reduction of 25% of the thermal conductivity was achieved by improving the spray and powder parameters. A quantitative characterization of the porous structure using image analysis of polished cross-sections was implemented. The parameters that have relevant influence on the coating porous structure were identified, and their relative importance was determined. An attempt was made to identify morphological criteria of the porous network (coarse/fine porosity ratio, cracks total length, cracks orientation) correlating with the thermal conductivity values.     

An experimental investigation on thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings

The primary intention of this work is to investigate the thermo-mechanical buckling delamination failure characteristic of air plasma sprayed thermal barrier coatings (TBCs) under compression tests at high temperature. The TBCs samples with a pre-delamination were firstly designed and they had been successfully prepared by air plasma sprayed technique. The main novelty of this paper is that the first work to validate and obtain three kinds of the interface failure forms in TBCs system during compression tests, i.e. buckling delamination, edge delamination and global buckling failure. The effects of the initial delamination length, temperature gradient and applied mechanical load on the delamination resistance of the TBCs system were discussed in detail. It is difficult to observe buckling delamination or edge delamination failure phenomena until the initial delamination length in TBCs reaches or exceeds 4 mm or more. For edge delamination failure, the interface fracture toughness (Γ_i^II), energy release rate (G_ss^edge) and stress intensity factor (K_II) between the TBC/TGO interface were 35 J m^− 2, 38.8 J m^− 2 and 0.97 MPa at high temperature gradient, respectively. Using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX), it was inferred that the delamination fracture located within the ceramic coating close to the TBC/TGO interface. The results agree well with other experimental and theoretical results.     

Mechanical and tribological properties of TiN/Ti multilayer coating

A large area filtered arc deposition (LAFAD) technique was used to deposit TiN/Ti multilayer coatings with fixed TiN layer thickness and different Ti layer thickness. Nanoindention and pin-on-disk tribometer were used to characterize the hardness, elastic modulus, plasticity, friction coefficient, and wear rate of the multilayer coatings. The dependence of the mechanical and tribological properties of the coating on the Ti interlayer thickness was systematically studied. It was found that the increase in the Ti layer thickness resulted in a decrease in the effective hardness and elastic modulus, and an increase in the wear rate, plasticity, and toughness. The coatings with a Ti layer thicknesses of 0, 25 nm and 50 nm possess an excellent combination of high effective hardness (> 20 GPa), high plasticity (> 69%), low friction coefficient, and high wear resistance.     

Laser surface modification of AISI 410 stainless steel with brass for enhanced thermal properties

Brass coating was applied to AISI410 steel using high power laser in a laser engineered net shaping (LENS™) system. The influence of laser treatment on interfacial microstructure and thermal performance was evaluated as a function of coating thickness. Laser deposition resulted in a diffused and metallurgically sound interface between metallurgically incompatible brass coating and AISI410 steel substrate. The thermal conductivity of AISI410 steel increased from 27 W/mK to a maximum of 37 W/mK depending on the coating thickness, almost 50% gain. The absence of sharp interface between the coating and the substrate, as a result of laser processing, resulted in a low interfacial thermal contact resistance. Thermal performance tests showed that the brass coating can enhance the heat transfer rate of stainless steel substrate. These results show that novel and efficient feature based coatings can be exploited using laser-based advanced manufacturing technologies for various industrial applications.     

In situ deposition behavior of SiO2 on YSZ-TBC-coated IN738LC during a burner rig test

This work was planned as a preliminary test to obtain an optimal condition for in situ deposition of SiO₂ on zirconia-based thermal barrier coating (TBC)-coated IN738LC specimens using a burner rig. The effect of the in situ deposited SiO₂ on the long-term reliability of TBCs upon cyclic burner rig operations will be tested in the next study. Tetraethylorthosilicate (TEOS), the precursor for the deposition, was fed admixed with methanol into the combustion chamber of the burner rig at an exhaust flame temperature of 1530 °C. All five coating processes were selected by varying the concentration of TEOS in the mixture and applied to hollow type pin specimens infixed to the rotating carousel to face the downstream exhaust gas vertically. A total of 1500 cc of the mixture at a given vol.% TEOS was fed evenly to the burner rig for 54 min during the coating process.     

Thermo mechanical analysis of a partially ceramic coated piston used in an SI engine

The purpose of this paper is to determine the temperature and the stress distributions in a partial ceramic coated spark ignition (SI) engine piston. Effects of coating thickness and width on temperature and stress distributions were investigated including comparisons with results from an uncoated piston. It is observed that the coating surface temperature increase with increasing the thickness in a decreasing rate. Surface temperature of the piston with 0.4 mm coating thickness was increased up to 82 °C. The normal stress on the coated surface decreases with coating thickness, up to approximately 1 mm for which the value of stress is the minimum. However, it rises when coating thickness exceeds 1 mm. As for bond coat surface, increasing coating thickness, the normal stress decreases steadily and the maximum shear stress rises in a decreasing rate. The optimum coating thickness was found to be near 1 mm under the given conditions.