Fundamental understanding of relationships between coating microstructure and thermal conductivity is important to be able to understand the influence of coating defects, such as delaminations and pores, on heat insulation in thermal barrier coatings. Object-Oriented Finite element analysis (OOF) has recently been shown as an effective tool for evaluating thermo-mechanical material behaviour, because of this method’s capability to incorporate the inherent material microstructure as an input to the model. In this work, this method was combined with multi-variate statistical modelling. The statistical model was used for screening and tentative relationship building and the finite element model was thereafter used for verification of the statistical modelling results. Characterisation of the coatings included microstructure, porosity and crack content and thermal conductivity measurements. A range of coating architectures was investigated including High purity Yttria stabilised Zirconia, Dysprosia stabilised Zirconia and Dysprosia stabilised Zirconia with porosity former. Evaluation of the thermal conductivity was conducted using the Laser Flash Technique. The microstructures were examined both on as-sprayed samples as well as on heat treated samples. The feasibility of the combined two modelling approaches, including their capability to establish relationships between coating microstructure and thermal conductivity, is discussed.

Widely studied in the 1980s, the insulation of pistons in engines aimed at reducing the heat losses and thus increasing the indicated efficiency. However, those studies stopped in the beginning of the 1990s due to NOx emission legislation, and also due to acceptable oil prices. Nowadays, with the improvement of exhaust after treatment systems (Diesel Particulate Filter, Selective Catalytic Reduction, and Diesel Oxidation Catalyst) and engine technologies (Exhaust Gas Recirculation), there are more trade-offs for NOx reduction. Besides, the fast rise of the oil prices tends to come back to insulation technologies in order to save fuel. This paper deals with the realization of a 1 mm thick plasma sprayed thermal barrier coating with a graded transition between the topcoat and the bondcoat on top of a serial piston for heavy-duty truck engines (11L displacement – Exhaust Gas recirculation – Single Stage Turbocharger with Variable Geometry Turbine and intercooler). The effects of the insulated pistons on the engine performance are also discussed.

Thermally sprayed Inconel 718 coatings have been deposited by high velocity oxy-fuel (HVOF) spraying on Inconel 718 substrates. The aim of the on-going study is to understand and control the adhesion mechanisms and the residual stress state of the deposit/substrate system, in order to build up thick coatings for maintenance purposes. The coating adhesion strength was evaluated by the standard ASTM C633 tensile test. Coating shear strength was evaluated by the recently developed prEN15340 Shear Test. A modified Layer Removal Method (MLRM) test was carried out to measure residual stresses. The work is a part of an ongoing study for evaluation of relationships between process parameters, residual stress distribution and adhesion strength.

Plasma spraying of thermal barrier coatings (TBCs) on gas turbine parts is widely used today either to enable higher turbine inlet temperatures with consequent improvement of combustion efficiency or to reduce the requirements for the cooling system and increase components life-time. Development of low conductivity TBCs, which allows us to further increase gas turbine efficiency and availability, is an ongoing challenge. In order to get low thermal conductivity values an experimental program was conducted. Two zirconia powders were used for coating deposition: yttria partial stabilised zirconia (YPSZ) and dysprosia partial stabilised zirconia (DyPSZ). Microstructure evaluations were performed to evaluate the influence of the spraying parameters on the coating morphology and porosity level. Two methods were utilised to evaluate the coatings thermal conductivity: Laser Flash (LF) and Transient Plane Source (TPS). A comparison between the two methods was made as well as a correlation study between coating microstructure/composition and thermal conductivity (TC).

Plasma spraying operations performed with high carrier gas flow rate may improve the coating properties but they can also lead to lump formation and thus coating defects. The damaged work piece must then be stripped and recoated, which implies a considerable waste in terms of coating powder, energy and time. The aim of this study was to determine the cause of the lumps, and propose process modifications for avoiding their formation while keeping the coating quality. Numerical simulations based on 3D turbulent Navier-Stokes equations in local thermal and chemical equilibrium were carried out to understand the problem and estimate the feasibility of the proposed solutions. The computational results were supplemented by experiments for validation. A first set of investigations was focused on the location and orientation of the powder port injector. It turned out that it was not possible to keep the coating quality while avoiding lump formation by simply moving the powder injector. A new geometry of the nozzle exit was then designed and successfully tested for a first application with Ni-5Al powder used in production.

This paper presents a method to increase deposition rate of thermal plasma spray operations through the use of multiple injection ports. Numerical simulations indeed revealed a major energy loss in the process when using only one port. The influence of the carrier gas and the particle stream on the heat flow coming from the plasma torch was found very local and small compared to the total amount of energy. To take as much as possible advantage of the energy available in the plume, we thus propose to use a multiple number of injectors around the flame. Computational simulations are carried out to estimate the feasibility. They are based on the 3D Navier-Stokes equations coupled with a turbulence model. The gases (plasma gas, surrounding air and carrier gas) are supposed to be in local thermal and chemical equilibrium and loading effects are accounted for. The numerical results are supplemented by experimental results showing that multiple injectors can significantly increase deposition rate while preserving or even slightly improving the deposition efficiency. Characterisation of the microstructure, evaluated for all tests, is similar and no obvious differences can be detected apart from the porosity. This method thus results in a substantial reduction of the production cost.

The introduction of sensor techniques has lead to a major change in the possibility to on-line control the plasma spraying process. By the use of such techniques reproducibility of the process can be enhanced by a more precise process control of flame and particle in-flight characteristics. In this paper a comparative study of four different on-line diagnostic systems and their sensitivity is presented. The systems evaluated are: the Particle Flux Imaging (PFI) from Zierhut Messtechnik GmbH, the PlumeSpector from Tecnar Automation Ltee., the Spray and Deposit Control (SDC) from the University of Limoges/Snecma Inc. and the DifRef M from Flumesys GmbH. Tests were performed using plasma spraying of a Nickel- Aluminium alloy. A statistical design of experiments (DOE) was defined using a ± 5% variation of the five process parameters current, argon flow rate, hydrogen flow rate, powder feed rate and carrier gas flow rate. System principles, sensitivity results are given and the systems suitability for use in industrial production is discussed.

In order to ensure reproducibility of thermal sprayed coating characteristics, process control by a sensor system is essential. Volvo Aero has more than 5 years experience of using different sensor systems in plasma and flame spraying. This article presents two examples where such systems have been used for quality control in production. In the first example the DPV2000 system was used in plasma spraying to control the tensile strength of an abrasive application. The study showed that the particle in-flight properties and the tensile strength were very sensitive to the carrier gas flow rate. In the second example the PlumeSpector system was used to control erosion and tensile strength in flame spraying of a composite powder composed of a core of Bentonite clay coated with a shell of NiCrAl alloy. The results show that the capability could be increased by a factor of two using on-line diagnostic control.

A process control tool has been developed for air plasma spraying of a NiAl (bond coat) and Al 2 O 3 (top coat) coating systems. The process is employed at Volvo Aero Corporation for abrasive purposes, such as knife-edge applications on compressor parts. In-flight particle temperatures, velocities and diameters were measured by the DPV2000 system. Several samples were sprayed and the coating microstructures were evaluated using Image Analysis techniques on optical and scanning electron microscope images. Top and bond coat thickness, oxides, porosity, grit blast residues, delaminations, surface roughness (on top, bond and substrate) and tensile strength were evaluated. Statistical regression analysis was then used to establish relationships between process parameters (i.e. current and primary gas flow), particle in-flight characteristics (i.e. velocity and temperature), microstructure properties, and mechanical properties. The equations derived were finally used for development of a tool, which can be used by the operator for on-line monitoring and control of the coating characteristics based on information of the current particle inflight characteristics. The tool makes it possible to continuously adjust the process set points, ensuring a high reproducibility and stability of the process.

Revealing the true properties is of utmost importance for the optimized use of thermal sprayed coatings and for the realization of the ultimate spray booth. The necessity of the true properties for a successful correlation as well as development of process windows for on-line control cannot is emphasized. Only the true properties can be expected to correlate to the spray parameters or the particle properties in the flame. What is actually meant when stating the following? - The coating has 5% porosity - Tensile bond is 50MPa - Hardness is 1000HV - Coating thickness is 100 nm Different loopholes during the steps of metallographic preparation, hardness and tensile bond strength testing are highlighted and discussed. The microstructure of a thermally sprayed coating typically consists of a multiphase matrix (often a mix between hard, soft and amorphous), pores, oxides, delaminations, cracks, grit residues and unmelted particles. Due to this complexity in the structure there are a number of possible errors that often can be made in metallographic laboratories. Several examples and explanations are given. It is shown that improper handling in the materials laboratory may lead to smearing and pullouts etc. This will ruin the results and effectively hide the true microstructure. Furthermore the use of different epoxies and curing methods effectively varies the tensile bond strength test results by more than 300% for various coatings. By using different preparation routines the micro hardness can be varied by more than HV 0.3 100.

To determine the effect of bond coat oxidation on the coating life, thermal shock testing were performed, using three different thermal cycles. The failure mode and crack paths were investigated in scanning electron microscope. A finite element model was developed to simulate the thermal shock tests. First, transient temperature fields during the thermal cycling were calculated. Second, stresses and strains evolving in the coatings due to thermal expansion mismatches and temperature gradients during the cycling were computed. The stress concentration at the interface due to the roughness of the bond coat was accounted for by using an ideal sinusoidal interface in the model. By adding an oxide layer with and without residual stresses to the model, the influence of the bond coat oxidation was determined. Both the experimental and numerical results revealed that the TBC failed by crack initiating in the ceramic top coat very close to the grown oxide layer at the interface followed by coating fatigue failure. Numerical simulation indicated that bond coat oxidation led to stress concentration at the peak of the asperity of the interface proceeding crack growth. It also showed that bond coat inelasticity and ceramic creep might further enhance the crack growth. There was little effect on coating behavior due to the residual stresses in the oxide layer.

Plasma spraying is a very complex process, controlled by a large number of process parameters. The spray gun parameters control the plasma plume and thereby the velocity and temperature of the particles in the plasma. Some of the spray gun parameters are difficult or impossible to control, but variations of them give rise to fluctuations in the microstructure of the sprayed thermal barrier coating and thereby low reproducibility. By movement of the control from the spray gun to direct control of the particle properties in the plasma this problem will be avoided, and it should result in better process control, higher quality of the final coating and thus improved reproducibility. In this study, the influence of the plasma spray process on the coating microstructure was investigated. An orthogonal factorial designed experiment was performed, where eight process parameters were varied, resulting in 16 different coatings. The particle properties were observed in-situ with the optical measurement system DPV 2000. The microstructure of the coatings was studied using optical microscopy and the amount of different features, i.e. cracks and pores, was quantified. Multiple linear regression was used to find models describing the relation between the spray gun parameters and the particle properties, between the spray gun parameters and the microstructure, and between the particle properties and the microstructure. The results showed that the spray gun parameters well describe the variation in particle velocity and particle temperature. Further, it was found that particle velocity, particle temperature, spray angle, and substrate temperature are the most important parameters concerning influence on the coating microstructure. However, their influence on the different microstructure features varied. The study implies that focus can be set on one or two particle properties measured in the plasma, instead of the numerous spray gun parameters.

This article attempts to predict the temperature, state and velocity of Tribaloy 800 particles by means of numerical modeling. It aims to identify parameters that have a significant influence on the inflight particle characteristics for Argon/Hydrogen plasma sprayed Tribaloy 800, and to compare predicted air entrainment and particle residence times between Argon/Hydrogen and Argon/Helium plasma gas mixtures. The effect of spray parameters (primary-, secondary- carrier gas mass flows, current, spray distance and nozzle diameter) on the particle in-flight characteristics (velocity and temperature) and their interactions are studied by a two level fractional factorial experiment applied on the simulations. A comparison between argon Argon/Hydrogen and Argon/helium plasm gas mixtures is made in order to investigate whether the coating oxidation level can be reduced using Argon/Helium. Finally, the correlation between the modeled parameters and the application microstructure is studied. Paper includes a German-language abstract.

The plasma spray deposition of a zirconia thermal barrier coating (TBC) on a gas turbine component has been examined using analytical and experimental techniques. The coating thickness was simulated by the use of commercial off-line programming software. The impinging jet was modelled by means of a finite difference elliptic code using a simplified turbulence model. Powder particle velocity, temperature history and trajectory were calculated using a stochastic discrete particle model. The heat transfer and fluid flow model were then used to calculate transient coating and substrate temperatures using the finite element method. The predicted thickness, temperature and velocity of the particles and the coating temperatures were compared with these measurements and good correlations were obtained. The coating microstructure was evaluated by optical and scanning microscopy techniques. Special attention was paid to the crack structures within the top coating. Finally, the correlation between the modelled parameters and the deposit microstructure was studied.

Within a Brite Euram project thick thermal barrier coatings for combustor applications were produced by plasma spraying of yttria partially stabilised zirconia (ZrO2 + 8 wt.% Y2O3). The material properties of such coatings strongly depend on their microstructure which can be altered by manipulating the parameters controlling the plasma spraying process. Covering a variation of possible microstructures, the coatings considered had a thickness of about 2 mm and were six to eight times thicker than the coatings currently in service. This investigation was concerned with an evaluation of the thermophysical and mechanical properties of these coatings and their correlation with the microstructure and the plasma spray parameters. Particular attention was paid to the influence of coating segmentation, microcracking and porosity. The experimental work included the measurement of the thermal diffusivity using the laser flash technique, thermal expansion measurements, and the determination of flexural strength and Young's modulus by means of a specially constructed four-point bend rig. Since some of the samples considered were sprayed according to a partially factorial test plan a statistical evaluation of the material data was possible yielding the correlation between process parameters and material properties.

Plasma sprayed MCrAlY bondcoats play a major role in thermal barrier coatings. During service, oxide forms on both sides of the bond coat and must be minimized to prevent coating failures. Along with powder chemistry, coating microstructure significantly influences oxide growth. It is known that both coating microstructure and coating strength are strongly related to plasma spraying parameters. This present work examines the effect of inflight particle properties on the adhesion strength and microstructure of NiCrAlY and NiCoCrAlY bondcoats. The relation between particle velocity and temperature and coating properties is particularly important. Relatively small changes in spray parameters such as arc current and gas flows can have a major impact on sprayed particles and consequently coating microstructure. Through online control of particle states it is expected that the quality of plasma-sprayed MCrAlY coatings can be significantly improved.

Thermal barrier coatings are used in several industries to improve thermal efficiency. Examples are gas turbine engines and marine diesels. The performance and life of thermal barrier coated components depend on a variety of factors all related to the specific application. This paper gives an overview of some of the aspects to consider and put special attention to. The different features, in the microstructure, will be discussed with respect to their appearance and influence on the performance of the TBC. Thermal conductivity, microstructure, failure mechanisms and different applications are highlighted.

In the Brite Euram Project COMBCOAT Thermal Barrier Coatings for improved thermal protection with a thickness up to 2 mm have been developed for combustor applications. As a typical experiment to evaluate the quality of the coatings, life cycles to spallation by means of thermal cycling tests have been determined at ANSALDO Ricerche, BMW Rolls- Royce and Volvo Aero Corporation. To ensure that the ranking on the different cycling experiments are equivalent and to compare cycles to spallation reached in the different rigs, a comparison of 6 different coatings, highly porous as well as segmented ones, has been performed on the three different rigs. Results of the different tests demonstrated that a direct comparison of the result is not possible. The ranking and the number of cycles to spallation have been inconsistent for different specimen and rigs. Detailed simulation of experiments need to be performed to understand the differences between results.

A traditional plasma spray gun consists of an anode and a cathode. During spraying small particles of anode material of either copper or tungsten, depending on the brand of the gun, will be worn off and deposited in the coating. The size and frequency of the particles from a copper anode has generally a dramatic appearance (in the beginning or at the end of its life) whereas a tungsten nozzle normally behaves more randomly during its life. Tungsten particles can therefore be expected anywhere in a plasma sprayed coating. Unfortunately the material properties of tungsten is not very compatible with a thermal barrier coating of partially stabilized zirconia and it is shown that a contamination will cause a catastrophic failure, if located in, for a thermal barrier, a critical region. The behavior of tungsten at elevated temperatures is investigated and clearly show the detrimental effect of tungsten on the life and performance of a thermal barrier coating.

Reproducibility is a current challenge for the thermal spray industry. Reproducibility associated problems represent a large cost every year not only in terms of rejections and rework, but also in costs for destructive testing and decreased production flow. Thermal spray coatings are moving in the direction of being considered only as a "band aid" to becoming a design element. One of the prerequisites for such a development is an increase in reproducibility for thermal spray coatings. The purpose of this paper is to outline a vision aiming in the direction of a future "ultimate spray booth", where thermal spraying is as reproducible and reliable as machining, grinding or other production processes. A way to increase reproducibility and reliability in the future spray shop involves utilising major parts of IT - technology. This also includes active co-operation design-production in the pre-spray process. This paper will deal with areas such as: operation drawings and lists through multimedia techniques, education programs for operators and designers through multimedia techniques, CAD/CAM, Off-line programming and simulation, On-line diagnostics of flame (particle diagnostics) and coating (temperature & Acoustic emission measurements), on-line Statistical Process Control and Knowledge Based System techniques.