Atmospheric Plasma-sprayed Thermal Barrier Research Articles

Prediction of Porosity in Atmospheric Plasma Sprayed Thermal Barrier Coatings Using Terahertz Pulse and Hybrid Machine Learning Algorithms

Prediction of Porosity in Atmospheric Plasma Sprayed Thermal Barrier Coatings Using Terahertz Pulse and Hybrid Machine Learning Algorithms

CMAS infiltration behavior of atmospheric plasma-sprayed thermal barrier coating with tailored pore structures

The degradation of atmospheric plasma spraying (APS) thermal barrier coatings (TBCs) due to calcium-magnesium-alumina-silicate (CMAS) corrosion is considered as a critical issue. In present work, mitigation of CMAS infiltration of APS TBCs has been achieved by tailoring pore characteristics based on a novel APS structure regulation method, and its thermal conductivity, thermal shock lifetime and infiltration behavior were evaluated. The results revealed that the laminar pore distributed TBC formed by horizontally aligned porous embedded particle clusters (PEPC) showed longest thermal shock lifetime and lowest thermal conductivity. Characterization of CMAS corrosion suggested that whilst all the TBCs samples with different pore structures suffered corrosion by CMAS melt, the severity fluctuated greatly. Compared with randomly distributed PEPC areas, these horizontally aligned PEPC areas have a noticeable potential for restricting CMAS infiltration. This unique structure consumes a large amount of CMAS melt and creates anti-CMAS filling pores, thus improving the overall CMAS infiltration resistance of the coating. The alternating dense layers outside the porous layers also provide a layer-by-layer barrier to CMAS infiltration. This study emphasizes that the modulation of the pore structure of APS TBCs is a promising strategy to mitigate CMAS infiltration.

Systematic Approach to Optimize Technological and Economical Aspects of Atmospheric Plasma Sprayed Thermal Barrier Coatings

Plasma‐sprayed yttria‐stabilized zirconia coatings have been used in gas turbines for decades. They are applied for thermal insulation to increase operating temperature and hence efficiency and component's lifetime. To keep manufacturing costs low, especially deposition efficiency is important. However, increasing it is also related to a reduction in porosity, affecting the insulating properties of the layer. To find an optimal combination of efficiency and technological performance, a systematic study of the most affecting parameters of the atmospheric plasma spraying process is conducted, using response surface methodology. In detail, the influence of current, spraying distance, and hydrogen gas flow is investigated with respect to the deposition efficiency, porosity, microstructure, and mechanical properties of the coatings. Characterization is carried out by scanning electron microscopy, microindentation tests, and three‐point bending tests. The models generated based on these measured properties allow predictions of the system responses for any parameter variation in the investigated design space. In addition, a numerical model is developed for targeted optimization of the coating properties. This can be used to produce optimized coatings for load‐flexible gas turbines with high deposition efficiency, high porosity, and at the same time advantageous mechanical properties.

Crack Driving Forces of Atmospheric Plasma-Sprayed Thermal Barrier Coatings

High-temperature operation service conditions can be used to evaluate the durability of Atmospheric Plasma-Sprayed Thermal Barrier Coating systems (APS-TBCs). To evaluate the durability of TBCs within their life span, two different thermal cycling testing results, i.e., isothermal furnace cycling and burner rig cycling tests, are utilized to numerically investigate possible crack driving forces that might lead to the failure of TBCs. Although there are many studies on failure and life prediction, there is still a lack of quantitative evaluation and comparison on the crack driving forces under these two different thermal cycling schemes. In this paper, by using modified analytical models, strain energy release rates (ERRs) are estimated and compared between these two testing approaches using experimental data. A new residual stress model was developed to study the position where the maximum residual stress occurs due to coefficient of thermal expansion (CTE) mismatch at different thermally grown oxide (TGO) thicknesses. The main crack driving forces are identified for two types of thermal cycling. A possible cracking route is found based on the calculated equivalent ERRs with respect to distance from the interface between the topcoat (TC)/TGO layers. The relationship between crack driving force of isothermal furnace and burner cycling tests is also elaborated.

Study on the Characteristics of a TBC System Containing a PVD-Al Interlayer under Isothermal Loading

In this work, the oxidation behavior of an atmospheric plasma-sprayed thermal barrier coating (TBC) system with a thin Al physical vapor deposition (PVD) film deposited over the bond coat is discussed. The TBC consisted of: (i) CoNiCrAlY bond coat sprayed on the Inconel 600 substrate; (ii) a thin Al interlayer deposited by direct current DC magnetron sputtering; and (iii) yttria-stabilized zirconia (YSZ) sprayed as the top coat. Such thermal barrier coatings (Al-TBC) were isothermally oxidized at 1150 °C with different holding times, and then they were compared with the reference TBC (R-TBC) systems without an Al interlayer (R-TBC). Scanning electron microscopy with energy-dispersive X-ray analysis was used to study the oxide formation along the bond coat (BC) and top coat (TC) interface, as well as crack formation in the yttria-stabilized zirconia top coat. Then, using Image Analysis, the oxide formation and crack formation were characterized in all specimens after a slow heating and cooling cycle, and after 100, 300, and 600 h of isothermal exposure. The results showed that the Al-TBC system proposed here exhibits higher oxidation resistance at the bond coat and top coat interface, less crack formation in the YSZ top coat, and enhanced mechanical stability compared to the conventional TBCs. It was found that enrichment of the bond coat and top coat interface with Al limited the formation of detrimental transition metal oxides during isothermal loading. Finally, the corresponding failure caused by thermally grown oxide (TGO) phenomena is “mixed failure mode” for both studied TBCs.

Erosion Performance of Atmospheric Plasma Sprayed Thermal Barrier Coatings with Diverse Porosity Levels

Thermal barrier coatings (TBCs) prolong the durability of gas turbine engine components and enable them to operate at high temperature. Several degradation mechanisms limit the durability of TBCs during their service. Since the atmospheric plasma spray (APS) processed 7–8 wt.% yttria stabilized zirconia (YSZ) TBCs widely utilized for gas turbine applications are susceptible to erosion damage, this work aims to evaluate the influence of their porosity levels on erosion behavior. Eight different APS TBCs were produced from 3 different spray powders with porosity ranging from 14% to 24%. The as-deposited TBCs were examined by SEM analysis. A licensed software was used to quantify the different microstructural features. Mechanical properties of the as-deposited TBCs were evaluated using micro-indentation technique. The as-deposited TBCs were subjected to erosion tests at different angles of erodent impact and their erosion performance was evaluated. Based on the results, microstructure-mechanical property-erosion performance was correlated. Findings from this work provide new insights into the microstructural features desired for improved erosion performance of APS deposited YSZ TBCs.

A study of suspension plasma-sprayed insulated pistons evaluated in a heavy-duty diesel engine

Thermal barrier coatings can be used to reduce the heat losses in heavy-duty diesel engines. A relatively new coating method for thermal barrier coatings is suspension plasma-spraying. Single-cylinder engine tests have been run to evaluate how heat losses to piston, cylinder head and exhausts as well as the specific fuel consumption are influenced by coating pistons with two different suspension plasma-sprayed thermal barrier coatings and one atmospheric plasma-sprayed thermal barrier coating, and comparing the results to those from an uncoated steel piston. The two suspension plasma-sprayed thermal barrier coatings showed reduced heat losses through the piston and less heat redirected to the cylinder head compared to conventional atmospheric plasma-sprayed thermal barrier coating, while one suspension plasma-sprayed coating with yttria-stabilized zirconia as top coat material showed increased exhaust temperature. However, the indicated specific fuel consumption was higher for all tested thermal barrier coatings than for an uncoated engine. The best performing thermal barrier coating with respect to indicated specific fuel consumption was a suspension plasma-sprayed coating with gadolinium zirconate as top coat material.

The Evaluation of Durability of Plasma-Sprayed Thermal Barrier Coatings with Double-layer Bond Coat

The durability of atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) with a double-layer bond coat was evaluated via isothermal cycling tests under 1120 °C. The bond coat consisted of a porosity layer deposited on the substrate and an oxidation layer deposited on the porosity layer. Two types of double-layer bond coats with different thickness ratios of the porosity layer to the oxidation layer (type A: 1:2 and type B: 2:1, respectively) were prepared. The results show that the porosity layer was oxidation free, the oxidation layer included a fraction of well-distributed α-Al2O3. The coefficient of thermal expansion of the oxidation layer was about 11.2 × 10−6 K−1, which was rather lower than that of the porosity layer. Thus, the oxidation layer can be regards as a secondary bond coat between ceramic topcoat and traditional bond coat. The thermal cyclic lifetime of type A TBCs was about 60 cycles, which exceeded 1.2 times the durability of type B TBCs. The delamination cracks in both TBCs all propagated in the ceramic topcoat, which were all identical to those in traditional TBCs. Therefore, the design of the double-layer bond coat affected the stress level rather than the stress distribution in TBCs.

Oxidation behavior of thermal barrier coating systems with Al interlayer under isothermal loading

In the present study, the phenomena related to the Thermally Grown Oxides (TGO) in atmospheric plasma sprayed Thermal Barrier Coatings (TBCs) are discussed. CoNiCrAlY bond coatings were sprayed on Inconel 600 substrates. Subsequently, thin Al layers were deposited by DC-Magnetron sputtering. Finally, yttria-stabilized zirconia (YSZ) top coatings were deposited to form a three-layered TBC system. The thus produced aluminum interlayer containing thermal barrier coatings (Al-TBC) were subjected to isothermal exposure with different holding times at 1150 °C and compared with reference TBCs of the same kind, but without Al interlayers (R-TBC). The oxide film formation in the interface between bond coating (BC) and top coating (TC) was investigated by scanning electron microscope (SEM) after 100 and 300 h of high temperature isothermal exposure. The growth of this oxide film as a function of the isothermal exposure time was studied. As a result, the designed Al-TBC system exhibited better oxidation resistance in the BC/TC interface than the two-layered R-TBC system. This was lead back to the Al enrichment, which slows down the formation rate of transition metal oxides during thermal loading.

Numerical investigation on the cracking behaviors of thermal barrier coating system under different thermal cycle loading waveforms

The influence of a thermal cycle fatigue (TCF) loading waveform on cracking behaviors of an atmospheric plasma-sprayed thermal barrier coating (TBC) system was numerically studied using the commercial software. Results showed that the TCF loading waveform considerably affects stress distributions in the TBCs and is the leading factor for the initiation and propagation of cracks in TBCs samples. Under triangular load waveform, the cracks initiate and grow in top-coat (TC) layers. However, under trapezoidal load waveform, the cracks are likely to initiate at an off-valley location and propagate along the TC/thermally-grown-oxide (TGO) interface, causing the complete spallation of the TC layer. Furthermore, the influences of the prior time and holding time on the crack initiation and propagation under the trapezoidal loading conditions were discussed, respectively. Interfacial cracks are likely to initiate when the prior time is prolonged. Under the trapezoidal loading conditions, the location of crack initiation moves from the peak to the off-valley location of a TC/TGO interface as holding time increases. Under a long holding time of the trapezoidal loading conditions, an improved spray quality of TC/TGO interface can increase the service life of TBCs.

Failure Mechanism of Yttria Stabilized Zirconia Atmospheric Plasma Sprayed Thermal Barrier Coatings Subjected to Calcia-Magnesia-Alumino-Silicate Environmental Attack

The contribution focuses on the description of failure mechanism of atmospheric plasma sprayed multilayer thermal barrier coatings subjected to calcia-magnesia-alumino-silicate (CMAS) environmental attack. To identify exothermic and endothermic reactions which occurred during heating/cooling by means of calorimetry was also utilized initial yttria stabilized zirconia (YSZ) powder subsequently used for thermal spraying of multilayer thermal barrier coating system (TBCs), CMAS powder later on utilized for thin layer deposition and its mixture. Atmospheric plasma spray technique was used to produce the TBCs on a grit blasted nickel-based superalloy substrates, where CoNiCrAlY powder was used for deposition of a bond coat and YSZ powder was sprayed as a top coat. In accordance to the aerospace standard the thin layer of CMAS was deposited on as sprayed TBCs samples surface from its colloidal solution by paint brush method. Burner-rig test, utilizing direct propane-oxygen flame, was used for thermal cyclic exposition of the multilayer coated samples at the temperature of 1150 °C. Samples after thermal cyclic exposure test were investigated by means of materialographic analysis approaches. The significant reduction in life-time of CMAS coated YSZ top coat was observed due to lower melting point phase formation and molten silicate crystallization within the pores providing the spallation identified as a major mechanism of TBCs failure.

Overview on Recent Developments of Bondcoats for Plasma-Sprayed Thermal Barrier Coatings

The performance of MCrAlY (M = Ni, Co) bondcoats for atmospheric plasma-sprayed thermal barrier coatings (APS-TBCs) is substantially affected by the contents of Co, Ni, Cr, and Al as well as minor additions of Y, Hf, Zr, etc., but also by manufacturing-related properties such as coating thickness, porosity, surface roughness, and oxygen content. The latter properties depend in turn on the exact technology and set of parameters used for bondcoat deposition. The well-established LPPS process competes nowadays with alternative technologies such as HVOF and APS. In addition, new technologies have been developed for bondcoats manufacturing such as high-velocity APS or a combination of HVOF and APS for application of a flashcoat. Future developments of the bondcoat systems will likely include optimization of thermal spraying methods for obtaining complex bondcoat roughness profiles required for extended APS-TBC lifetimes. Introduction of the newest generation single-crystal superalloys possessing low Cr and high Al and refractory metals (Re, Ru) contents will require definition of new bondcoat compositions and/or multilayered bondcoats to minimize interdiffusion issues. The developments of new bondcoat compositions may be substantially facilitated using thermodynamic–kinetic modeling, the vast potential of which has been demonstrated in recent years.

Characterization of the mechanical properties of thermal barrier coatings by 3 points bending tests and modified small punch tests

The Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS TBCs) is commonly used to insulate hot sections in gas turbines. Cyclic oxidation failure usually results from the spallation of the ceramic top coat. In order to predict such spalling phenomena, understanding the mechanisms for cracks initiation and propagation in thermal barriers is a major issue 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 present work is to assess the mechanical behavior of a complete TBC using comparative experimental (uniaxial and biaxial loading) and Digital Images Correlation (DIC) analysis approaches for classical TBC microstructures obtained through APS coatings.The experimental characterization of the mechanical behavior of the TBC systems was studied on as deposited specimens. Three Points Bending (3PB) tests were performed at room temperature, with the ceramic coating either under tension or compression. Additionally, in situ observations during 3PB tests by a camera, associated to a DIC analysis, allow determining the evolution of the strain field of surface sample correlated with the damage evolution. Location of crack initiation and crack propagation paths up to macroscopic failure were investigated. These tests highlighted the strong differences in mechanical behavior and fracture mode depending on the tension or compression stress state in ceramic coating. Small Punch Tests (SPT) were also performed at room temperature using both geometries (tension/compression). This allows pointing out the similarities of failure modes between uniaxial solicitation and biaxial flexure. Tests performed at 850°C in the SPT ring show that when temperature varies, different mechanical properties can be observed.

Life Prediction of Atmospheric Plasma-Sprayed Thermal Barrier Coatings Using Temperature-Dependent Model Parameters

The failure analysis and life prediction of atmospheric plasma-sprayed thermal barrier coatings (APS-TBCs) were carried out for a thermal cyclic process. A residual stress model for the top coat of APS-TBC was proposed and then applied to life prediction. This residual stress model shows an inversion characteristic versus thickness of thermally grown oxide. The capability of the life model was demonstrated using temperature-dependent model parameters. Using existing life data, a comparison of fitting approaches of life model parameters was performed. A larger discrepancy was found for the life predicted using linearized fitting parameters versus temperature compared to those using non-linear fitting parameters. A method for integrating the residual stress was proposed by using the critical time of stress inversion. The role of the residual stresses distributed at each individual coating layer was explored and their interplay on the coating’s delamination was analyzed.

Mechanical and Thermo-physical Properties of Plasma-Sprayed Thermal Barrier Coatings: A Literature Survey

Atmospheric plasma-sprayed thermal barrier coatings (APS TBCs) have been studied from an extensive review of the dedicated literature. A large number of data have been collected and compared, versus deposition parameters and/or measurement methods, and a comparison was made between two different microstructures: standard APS coatings and segmented coatings. Discussion is focused on the large scattering of results reported in the literature even for a given fabrication procedure. This scattering strongly depends on the methods of measurement as expected, but also—for a given method—on the specific conditions implemented for the considered experimental investigation. Despite the important scattering, general trends for the correlation of properties to microstructure and process parameters can be derived. The failure modes of TBC systems were approached through the evolution of cracking and spalling at various life fractions.

Improvement of strain tolerance of functionally graded TBCs through laser surface micro-texturing

Atmospheric plasma-sprayed thermal barrier coating made up of YSZ and LaMgAl11O19/YSZ was plasma-sprayed over nickel-based superalloy. The coated surfaces are textured using a picosecond pulsed laser for different groove geometries. The scanning parameters were optimised to minimise the occurrences of re-cast layer and horizontal cracks. The textured samples were subjected to thermal shock cycles to study their thermal stability. The width-to-depth ratio (Wd) and the groove spacing between the adjacent texture were varied to analyse and correlate their geometrical influence in providing thermal stress–strain tolerance. The textured samples exhibit higher lifetime compared to the YSZ and LaMgAl11O19/YSZ as-sprayed surface. The induced thermal stress and minimal strain tolerance in the as-sprayed surfaces result in the traditional interface delamination failure where the failure occurs at the bond coat–ceramic layer interface. The textured grooves having higher Wd restrain the propagation of crack across the coating thickness and provide improved strain tolerance. The horizontal cracks initiated at the edge propagates across the textured layer chipped the groove segments within the ceramic bulk layer. The LaMgAl11O19/YSZ-based textured sample having Wd of 0.8 and 300 µm groove spacing exhibits higher lifetime of 219 thermal cycles. This implies the significance of groove density in improving the thermal shock resistance of TBCs.

Investigation of Crack Propagation Behavior of Atmospheric Plasma-Sprayed Thermal Barrier Coatings under Uniaxial Tension Using the Acoustic Emission Technique

Uniaxial tension is a common technique to characterize the adhesive strength of plasma-sprayed thermal barrier coatings (TBCs). In this work, the crack initiation, growth, and propagation behavior of atmospheric plasma-sprayed TBCs during uniaxial tension testing was investigated using the acoustic emission (AE) technique, x-ray diffraction analysis, scanning electron microscopy, and the finite-element method (FEM). The experimental results indicated that the position of crack initiation was usually located within the ceramic layer, and the crack tended to propagate along the tension direction, with some key horizontal cracks reaching the metallic layer/ceramic layer interface, after which vertical cracks initiating at the middle and lower segments of the horizontal cracks propagated along the interface. When some critical cracks were formed at the interface and a series of assembled splats separated from the coating, the coating failed completely. The AE signal could be divided into three typical stages, corresponding to the three stages of the stress–stain curve under uniaxial tension. Detailed analysis of the AE signal associated with the failure behavior was performed. The dynamic propagation patterns of the key cracks in the ceramic layer during the tension process were simulated using the FEM, whose results further confirmed the conclusions drawn from the experimental results.

Critical Weak Points of Atmospheric Plasma Sprayed Thermal Barrier Coatings

The 8 wt. % yttria stabilized zirconia top coat (TC) and the CoNiCrAlY bond coat (BC) were sprayed onto the surface of newly developed fine-grained cast polycrystalline nickel-based superalloy Inconel 713LC by means of atmospheric plasma spraying (APS). As-prepared samples were isothermally exposed at the temperature of 1050 °C for 200 hours in an ambient atmosphere. Structural changes in the thermal barrier coatings (TBC) system after thermal exposure were studied by means of scanning electron microscope equipped with an energy dispersive microanalyzer. Critical weak points were identified on both the substrate-bond coat and bond coat-top coat interfaces.