Jan Wigren

Influence of particle in-flight characteristics on the microstructure of atmospheric plasma sprayed yttria stabilized ZrO2

Abstract The atmospheric plasma spraying of a yttria stabilized ZrO 2 top-layer of a thermal barrier coating (TBC) produces a complex microstructure consisting of a wide variety of cracks and pores. These voids are known to influence the thermal conductivity and mechanical properties of the TBC. In this study, the influence of the plasma spray process on the microstructure of the coating and deposition efficiency was investigated with the aim of achieving better knowledge and control of the process. Eight process parameters to control the plasma process were employed in a fractional factorial designed experiment involving 16 different thermal barrier coatings. The microstructure of the coatings, characterized by seven features, in particular those of cracks and pores, was studied by means of scanning electron microscopy (SEM) and image analysis (IA) and the extent of the different features were quantified. For each sprayed coating, the particle velocity and particle temperature were measured prior to impact, using the optical measure system DPV 2000. The four spray gun parameters controlling the plasma plume were found to each have a significant influence on the particle properties. The remaining four parameters did not affect the particle properties, but instead influenced the coating microstructure directly. Multiple linear regression was used to find models describing how the particle properties and the other process parameters were related to the coating microstructure. The results showed particle velocity, particle temperature, spraying angle and substrate temperature to be the most important parameters influencing the coating microstructure. The influence of the different parameters and particle properties on the microstructure features varied, however.

Mechanical and thermophysical properties of thick PYSZ thermal barrier coatings : correlation with microstructure and spraying parameters

Abstract The thermophysical and mechanical properties of plasma-sprayed thermal barrier coatings are very strongly dependent on the microstructure, and this may be controlled by manipulating the parameters controlling the plasma spray process. A series of coatings was produced by plasma spraying ZrO 2 +8wt%Y 2 O 3 (PYSZ) to a thickness of about 2 mm, some six to eight times thicker then state of the art coatings currently in service. A controlled variation of plasma spray parameters was carried out to produce a range of different microstructures that could be related to material properties and these in turn correlated with the process variables. The particular properties studied were the thermophysical properties, diffusivity and expansion, and the mechanical properties, modulus, strain to failure and flexural strength measured in four point bend.

Thermal shock testing of burner cans coated with a thick thermal barrier coating

Thick (1.8 mm) thermal barrier coatings were air-plasma-sprayed onto two different substrate geometries, including small circular substrates and burner cans. Two different top-coating spray parameters were used, where the settings of the substrate temperature and the lamella thickness were varied. A segmentation crack network was found in the top coatings sprayed using a high substrate temperature and a high lamella thickness. The density of segmentation cracks was found to be independent of substrate geometry. No segmentation cracks were found in the top-coatings when a low substrate temperature and a low lamella thickness were used.In the segmented burner can, after 1000 thermal shock cycles, the segmentation crack network was still stable and no severe cracks had formed in the top coating. In the nonsegmented burner can, cracks were formed after only 35 thermal shock cycles. Among the crack types, horizontally oriented cracks were found in the top coating close to, and sometimes reaching, the bond coating. Cracks of this type are not tolerated in thermal barrier coatings because they can cause failure of the coating.Regarding the lifetime of the segmented burner can, it is believed the failure will be dependent on other mechanisms, such as bond-coating oxidation or top-coating decomposition.

Effect of grit blasting and spraying angle on the adhesion strength of a plasma-sprayed coating

A study of the effects of grit-blasting and plasma-spraying angles on the adhesion strength of an alloy (Tribaloy 800) that was plasma sprayed on a titanium-base alloy is reported. Five different spray and grit-blast angles were investigated: 45°, 55°, 65°, 75°, and 90°. The surface texture in different directions was characterized by the classic average roughness and by a fractal analysis number using a two-dimensional fractal analysis method. The grit residue was measured by an x-ray spectrometer. The study showed that the maximum adhesion strength was close to a 90° blasting and spraying angle. However, the grit residue reaches its maximum at a 75° blasting angle. From the image analysis of the interface in different directions, it was found that the nonperpendicular grit blasting produces an anisotropic surface. The fractal analysis method showed a rather good correlation with the blasting angle. However, no good correlation between the fractal number and the adhesion strength was found.

Experimental and numerical life prediction of thermally cycled thermal barrier coatings

This article addresses the predominant degradation modes and life prediction of a plasma-sprayed thermal barrier coating (TBC). The studied TBC system consists of an air-plasma-sprayed bond coat and an air-plasma-sprayed, yttria partially stabilized zirconia top layer on a conventional Hastelloy X substrate. Thermal shock tests of as-sprayed TBC and pre-oxidized TBC specimens were conducted under different burner flame conditions at Volvo Aero Corporation (Trollhättan, Sweden). Finite element models were used to simulate the thermal shock tests. Transient temperature distributions and thermal mismatch stresses in different layers of the coatings during thermal cycling were calculated. The roughness of the interface between the ceramic top coat and the bond coat was modeled through an ideally sinusoidal wavy surface. Bond coat oxidation was simulated through adding an aluminum oxide layer between the ceramic top coat and the bond coat. The calculated stresses indicated that interfacial delamination cracks, initiated in the ceramic top coat at the peak of the asperity of the interface, together with surface cracking, are the main reasons for coating failure. A phenomenological life prediction model for the coating was proposed. This model is accurate within a factor of 3.

Residual stresses as a factor in the selection of tungsten carbide coatings for a jet engine application

Tungsten carbide thermal spray coatings are important to the aerospace industry for the mitigation of midspan damper wear on jet engine fan and compressor blades. However, in some cases the coating can fail due to spallation and cracking, and in other situations the fatigue life of a fan or compressor blade is reduced when a coating is applied. Coating failures can result in decreased engine performance and costly maintenance time. A comprehensive experimental research program was conducted to evaluate coating crack resistance in bending, low-cycle fatigue properties of the coating and substrate, coating performance in jet engine tests, and microstructures for a wide range of coating compositions and application processes. Coating residual stress distributions also were evaluated. Eleven coatings were ranked according to their performance relative to the other coatings in each evaluation category. Results from the bend and low-cycle fatigue evaluations were compared to the experimentally evaluated residual stresses. Comparisons of rankings indicate a strong correlation between performance and the residual stresses in the coatings. Results from the program were used to select a suitable coating system for final in-service use based on two important criteria: (1) the coating must not fail while in service, and (2) the coating must not induce crack propagation into the substrate of the midspan damper.

Technical note: Grit blasting as surface preparation before plasma spraying☆

Abstract One of the major reasons for grit blasting before plasma spraying is to create enough surface roughness to ensure a strong mechanical bond between coating and substrate. However, this treatment also leaves a certain amount of grit residue, normally referred to as contamination, trapped in the surface. These residues are known to weaken the adherence of the coating, especially for those coatings operating at high temperatures and exposed to mechanical loading. The current work presents a total survey of the influence of grit-blasting parameters (automatic equipment) on surface roughness and contamination level for various alloys. Surface roughness was determined by using traditional measuring techniques whereas a method for measuring contamination levels with X-ray spectrometry was developed. The latter were correlated with the results of point counting in an optical microscope. It was found that maximum surface roughness is obtained at a stage where the contamination level increases rapidly. By using the above survey the grit-blasting parameters for the adherence of a plasma-sprayed Co-Mo-Cr-Si alloy on Inconel 718 were studied. The assessment of adherence was made using an improved tensile bond strength test method for both “as-coated” samples and samples exposed to simulated service conditions.

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria stabilized ZrO2: Part 2. modeling

Yttria stabilized ZrO2 particle in-flight characteristics in an Ar-H2 atmospheric plasma jet have been studied using analytical and experimental techniques. In the previous article,[1] the primary gas flow, plasma composition, current, and powder feed rate were systematically varied and particle surface temperatures, velocities, and size distributions measured and statistically analyzed. In this paper, a mathematical model for the plasma flow and particle characteristics is presented. Model predictions are compared with the experimental results in Ref 1 and a reasonable correlation is found. A statistical investigation (composite cubic face (CCF)) is performed on the particle predictions, giving fast and simple relationships between gun parameters and particle in-flight properties. The statistical and theoretical models that are presented here combine to form a powerful and cost-effective tool, which can be used in the evaluation and optimization of spray parameters off-line.

Long crack behavior in a thermal barrier coating upon thermal shock loading

The behavior of macroscopic long cracks in the ceramic top coat of a thermal barrier coating (TBC) system subjected to thermal shock loading and the influence of the cracks on the coating durability were investigated experimentally and numerically. Thermal shock testing was conducted until coating failure. Comparisons were made with coating samples without macroscopic cracks. The experimental results revealed that the presence of macroscopic cracks reduces the life of the TBC. The finite-element method, with a fracture mechanics approach, was applied to analyze preexisting long cracks, and the calculations correlate well with the experimental findings. It was found that the life of the coating is reduced with crack length as well as with maximum cycle temperature. It was also found that the stress-intensity factors for long cracks are initially high and decrease with the number of temperature cycles, which indicates that rapid crack growth occurs during the first number of cycles.

Development of a process window for a NiCoCrAlY plasma-sprayed coating

Abstract Some recent research has shown the importance of the substrate temperature during spraying on the properties of thermal barrier coatings. In the future, applications like small combustor cans for stationary gas turbines will be sprayed at high temperatures. For that purpose, the Metco Sulzer SM-F-100 Connex Gun which can be used at higher temperatures, has been tried at Volvo Aero. In a first step, which is described in this paper, a NiCoCrAlY bondcoat was studied. A L4 Taguchi matrix was sprayed to improve the microstructure of the coatings. Varying parameters were current intensity, argon and hydrogen flow rates. The DPV2000 diagnostic system was used to measure the velocity, temperature and diameter of the in-flight particles in order to search for a process window. The microstructures of the coatings, i.e. the number of unmelted particles, the percentage of pores, delaminations and oxides were evaluated and linked both to the spray parameters and to the in-flight properties of the particles. It is shown that the temperatures of particles are directly related to the amount of unmelted particles, oxides and delaminations in the microstructure. Tensile strength measurements also show a clear correlation to the amount of delaminations in the coating and therefore to the in-flight particle temperature.