APS Thermal Barrier Coatings Research Articles

The effects of TGO growth stress and creep rate on TC/TGO interface cracking in APS thermal barrier coatings

To design reliable thermal barrier coatings (TBCs), it is essential to elucidate the interface failure mechanism and establish a better life-prediction model. Up to now, how the variations of thermally grown oxide (TGO) growth stresses and creep rates affect the interfacial cracking behavior between top coat (TC) and TGO has not been clearly studied. In this work, the thermos-mechanical model with simplified cosine curve interface morphology in TBCs is developed utilizing numerical simulation software (ABAQUS). According to the calculated stress states of TC/TGO interface, the possible damage scenarios are inferred. On this basis, the interface crack nucleation and propagation is simulated by the surface-based cohesive behavior. General features of the interface crack are in good coincidence with previous conjectures. Furthermore, large TGO growth stresses will trigger off premature cracking, conversely, the formation of TC/TGO interface crack can be postponed due to large TGO creep rates. In addition, the dominant fracture mode of TC/TGO interface is impacted by the variations of TGO growth stresses as well as its creep rate. The position of interface fracture mode transition has been shifted to a certain extent.

Effectiveness of lanthanum zirconate and Yttria stabilised zirconia freestanding APS thermal barrier coatings against natural CMAS attack at high temperatures

ABSTRACT CMAS-resistant thermal barrier coating is essential for durability of turbine components in advanced aero-engines. They are mostly evaluated using synthetic model CMAS powders. However, the degradation by natural CMAS can be higher considering their multiple constituents, nanostructure and low fusion temperature. The study compares the resistance of APS LZ and 8-YSZ freestanding disks to natural CMAS up to 1450°C and 100 h. The readily formed sealing layer of apatite, stable at 1450°C for 100 h, restricts further damage to LZ. But YSZ undergoes extensive destabilisation from metastable tetragonal (t´) to monoclinic and cubic phases after 1 and 10 h at 1450°C and 1250°C, respectively. Wetting, spread of CMAS and thermal diffusivities of the coatings are assessed. A 4–10 fold increase in thermal diffusivities is noticed. The paper attempts to bring some insight on CMAS reactivity with coating constituents based on fundamental considerations of ionicity, basicity, bond strength and diffusivities of species.

Real-time Detection of CMAS Corrosion Failure in APS Thermal Barrier Coatings Under Thermal Shock

Calcium-magnesium-alumina-silicate (CMAS) corrosion has been regarded as the most important factor that leads to the degradation of thermal barrier coatings (TBCs). The failure mechanism of TBCs attacked by CMAS corrosion in the actual service conditions is still not clear due to the lack of an environmental simulator and nondestructive testing techniques. To solve the above problems, a real-time acoustic emission method combined with infrared thermography are developed to investigate the failure mechanism of TBCs attacked by CMAS corrosion. The results show that the acoustic emission signal spectrum only depends on the failure mode of the TBCs, and five failure modes are identified: surface vertical cracks, sliding interfacial cracks, opening interfacial cracks, substrate deformation and noise. The lifetime of TBCs attacked by CMAS corrosion is 40 thermal shock cycles, which is nearly six times lower than that of TBCs without CMAS corrosion (350 cycles). Conclusions: The failure mechanism of the former is interlaminar cracking and delamination in the ceramic coating; while that for the latter is interfacial delamination in the vicinity of thermal growth oxide.

Direct Correlation of the Crack Distribution in APS Thermal Barrier Coatings with the Measured Synchrotron XRD Residual Stress Distribution

Residual stress distributions as a function of depth in APS TBCs after different heat treatment times have been measured by synchrotron XRD. Three types of residual stress distribution were observed. The first type was a low stress state from the as-received sample; the second type was a compressive stress state increasing from the surface to the interface with a discontinuous feature in the trend near the interface; the third type of residual stress distribution has two jump in the trend, one was observed near the sample surface and the other near the interface. To understand the double jump in the third kind of trend, the 3D microstructures of the measured samples were systematically measured X-ray μ-CT. Vertical cracks and edge cracks were observed. We have shown by image based modelling that the vertical cracks do not have a large effect on the residual stress distribution and that edge cracks are related to the features near the sample surface. The edge crack can be observed in all the measured samples with the third kind of the residual stress trend; and the measured samples with the second kind of the residual stress trend are crack free. The relationship between the middle and the through edge cracks and the residual stress distribution was also investigated by image based modelling. It was found that the middle crack can also cause a feature near the sample surface while the through edge crack seems to not have a large effect on the residual stress distribution. A semi-destructive method was developed to determine the existence and the position of horizontal cracks in the APS TBC samples based on the above findings.

Experimental and numerical study of thermal conductivity of plasma-sprayed thermal barrier coatings with random distributions of pores

Numerical and experimental investigations were conducted to find the effect of percentage and distribution of porosity and heat treatment on the conductivity value in the plasma-sprayed thermal barrier coatings (TBC). The numerical analysis of the SEM images of six plasma-sprayed samples showed an explicit relation between changes in the porosity percentage and the conductivity. Also, with a similar analysis, it was shown that the distribution of porosity and its orientation do not have a significant impact on the conductivity magnitude. A correlation was established for the variation of conductivity with respect to the material porosity. The experimental results indicated that applying a heat treatment at 1070 °C leads to an increase in the conductivity for 5–30% in the APS TBCs.

Corrosion of NiCoCrAlY Coatings and TBC Systems Subjected to Water Vapor and Sodium Sulfate

Thermal barrier coating (TBC) systems are commonly used in gas turbines for protection against high-temperature degradation. Penetration of the ceramic top coat by corrosive species may cause corrosion damage on the underlying NiCoCrAlY bond coat and cause failure of the TBC system. In the current study, four oxidation/corrosion conditions were tried: (i) lab air, (ii) water vapor, (iii) sodium sulfate deposited on the specimens, and (iv) water vapor + sodium sulfate. The test was done at 750 °C in a cyclic test rig with 48 h cycles. The corrosion damage was studied on NiCoCrAlY-coated specimens, thin APS TBC specimens, and thick APS TBC specimens. Water vapor was found to have very minor influence on the oxidation, while sodium sulfate increased the TGO thickness both for NiCoCrAlY specimens and TBC-coated specimens; the influence of the TBC thickness was found to be very small. Sodium sulfate promoted thicker TGO; more Cr-rich TGO; the formation of Y oxides, and internally, Y sulfides; pore formation in the coating as well as in the substrate; and the formation of a Cr-depleted zone in the substrate.

Effect of oxygen content in NiCoCrAlY bondcoat on the lifetimes of EB-PVD and APS thermal barrier coatings

The effect of oxygen content in NiCoCrAlY bondcoat on the cyclic oxidation lifetimes of EB-PVD and APS thermal barrier coatings (TBC) has been studied. The EB-PVD TBC system with an oxygen content of 0.05 wt. % in NiCoCrAlY bondcoat shows five times longer lifetimes compared to a TBC system with 0.2wt% oxygen in the bondcoat. In the bondcoat with the high oxygen content the minor (0.3wt.%) yttrium addition was found to be tied up by oxygen into fine precipitates of yttrium aluminates. Thereby the beneficial effect of yttrium onto the adherence of the alumina scale was significantly reduced. The critical scale thickness at failure was by about a factor of two lower for the high oxygen bondcoat than for the low oxygen bondcoat. In contrast to EB-PVD TBC systems, no detrimental effect of increasing oxygen content on the lifetime of APS-TBC systems was observed. This can be explained by a different failure mechanism of APS-TBC systems, whereby the lifetime is mainly determined by the rate of crack propagation through the ceramic topcoat.

The effect of moisture on the delayed spallation of thermal barrier coatings: VPS NiCoCrAlY bond coat+APS YSZ top coat

A fundamental understanding of failure mechanisms for thermal barrier coatings (TBC) is important for accurate life-time prediction and hence of much interest for industry. Failure (i.e. spallation or cracking) of the TBC usually occurs immediately upon cooling the specimen. However, in some cases spallation of the TBC is observed with a delay of several hours or even days after cooling, when the specimen is at ambient temperature and exposed to laboratory air. Because laboratory air contains water vapour, one hypothesis is that water plays a role in delayed failure of TBCs. This hypothesis is strongly supported by experiments in which the application of liquid water to a pre-oxidized TBC leads to spontaneous spallation/delamination at room temperature. The aim of this work is to study the effect of moisture on TBC systems in more detail. A series of experiments including acoustic emission techniques for in situ detection of cracking within the specimen and nuclear reaction analysis to determine hydrogen concentration depth profiles support the proposed hypothesis. Optical micrographs of APS TBCs isothermally oxidized at 1100°C show increased inward growing oxidation in cauliflower-like structures for specimens oxidized in moist atmospheres.

Thermal Fatigue of Thermal Barrier Coatings by Atmospheric Plasma Spraying

Turbine vanes and blades are the most intensively loaded elements in that they are subjected to a large variety of mechanical and high temperature loads. The thermal barrier coatings (TBCs) are widely used on different hot components of gas turbines, as blades and vanes, for both, power engineering as well as aeronautical applications. Currently, two methods are used for depositing TBCs on substrate, which are plasma spray (PS) and electron beam-physical vapor deposition (EB-PVD). A typical TBCs system consists of two thin coatings, including a ceramic coating and a metallic bond coat. Despite considerable efforts, the highly desirable prediction of their life time is still a demanding task. The PS coating was focused on in this work. Firstly, the TBCs systems are multiplayer material systems. The material properties are not easily determined, such as Young’s modulus of the top-coating of TBCs. Using the resonant frequency and the composite beam theory, the Young’s modulus of APS TBCs was gotten under from room temperature to 1150°C. Then using a commercial finite-element program, the model geometry is that of a cylinder specimen. The interface region between bond coat and top coating is modeled and meshed with a sinusoidal geometry. The temperature was designed and cycled over a range from room temperature to 1050°C. The force-air-cooling was designed to form temperature gradient across the thickness of TBCs. Finally, the fatigue life of TBCs was predicated. The maximum relative error is 20.1%.