Thermal barrier coatings (TBC) are normally based on yttria partially stabilised zirconia (YPSZ) coatings and are commonly used coatings in the high temperature, combustion region of gas turbines. TBC permit to increase the temperature of combustion, increasing the thermodynamic efficiency of the engine. Therefore, an engine equipped with TBC can produce a larger amount of energy over its lifetime. This increase in produced energy can be compared with the energy needed for the manufacturing and installation of TBC. The comparison can be performed in terms of the “energy return” (or “energy returned for energy invested”, EROI or EROEI). The qualitative analysis performed in the present study indicates that this return is large in comparison to that of other energy producing systems.

Thermal barrier coatings (TBCs) are widely used in gas turbines to reduce thermal exposure of structural components and increase turbine efficiency. They typically consist of a MCrAlY bond coat and a YSZ topcoat. At high temperatures, a thermally grown oxide (TGO) layer forms between the bond coat and topcoat. If this layer is a continuous scale of alumina, it will act as a diffusion barrier to suppress the formation of other detrimental oxides, thus helping to protect the substrate from further oxidation. It has been reported, however, that other oxides, such as chromia, spinel, and NiO, may form along with the TGO layer, ultimately leading to TBC failure. To investigate such claims, coatings of comparable thickness were deposited by various spraying methods onto a superalloy substrate using a powder of the same composition. Samples were isothermally oxidized at 1273 K for different periods up to 3000 hours. The samples were examined before and after furnace tests and the results are presented and discussed.

In order to improve gas turbine performance it is possible to decrease back flow gases in the high temperature combustion region of the turbo machine reducing shroud/rotor gap. Thick and porous TBC systems and composite CoNiCrAlY/Al 2 O 3 coatings made by Air Plasma Spray (APS) and composite NiCrAlY/graphite coatings made by Laser Cladding were studied as possible high temperature abradable seal on shroud. Oxidation and thermal fatigue resistance of the coatings were assessed by means of isothermal and cyclic oxidation tests. Tested CoNiCrAlY/Al 2 O 3 and NiCrAlY/graphite coatings after 1000 hours at 1100°C do not show noticeable microstructural modification. The oxidation resistance of new composite coatings satisfied Original Equipment Manufacturer (OEM) specification. Thick and porous TBC systems passed the thermal fatigue test according to the considered OEM procedures. According to the OEM specification for abradable coatings the hardness evaluation suggests that these kinds of coatings must be used with abrasive tipped blades. Thick and porous TBC coating has shown good abradability using tipped blades.

High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. This paper addresses the development of thick TBCs, focusing attention on the microstructure and the porosity of the Yttria Partially Stabilized Zirconia (YPSZ) coating, in relation to its resistance to thermal cycling fatigue. Thick TBCs, with different grade of porosity, were produced by means of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat, both sprayed by Air Plasma Spray. The thermal fatigue resistance of new TBC systems and the evolution of the coatings before and after thermal cycling were evaluated. The limit of thermal fatigue resistance increases with amount of porosity in the top coat. Raman analysis shows that the compressive in-plane stress increases in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat.

The present paper is addressed to the study of chemical stripping processes used in order to remove Thermal Spray deposited Thermal Barrier Coatings. Thermal Barrier Coatings consist in a bond coat of MCrAlY alloy (where M means for Ni, Co or a combination of both), that can be obtained generally by Vacuum Plasma Spray or High Velocity Oxygen Fuel and in a top coat of Yttria Partially Stabilized Zirconia obtained by Air Plasma Spray. These coatings are applied to gas turbine components in order to improve their hot corrosion and oxidation resistance and their service life time through a reduction of the service temperature. The paper focuses on the removal of NiCrAlY bond coat performed by chemical attack (based on hydrochloric acid). Characterization of the blade and vane surfaces after removal of NiCrAlY coatings has been performed from the point of view of surface morphology, metallurgical structure and chemical composition. The efficiency of the acid solution in NiCrAlY removal has been investigated and the behaviour of two Ni based alloys substrates in aggressive environment has been tested. The HCl based stripping solution shows good performances in Vacuum Plasma Sprayed NiCrAlY coatings removal from Ni superalloys. The tested stripping procedure is fast and safe because no damages to base materials have been noted.

Yttria Partially Stabilized Zirconia (YPSZ) coatings are used as Thermal Barrier Coatings (TBCs) because of their capability to improve gas turbine performance by allowing higher turbine inlet temperature and reduced cooling air flow. Usually, YPSZ layers are applied by plasma spray on Ni-based superalloys pre-coated with a bond coat layer of MCrAlY, where M is Co, Ni, or both. In this work we used Raman spectroscopy to study the performance of TBCs during thermal cycling at two different temperature (1273K and 1373K). Raman scattering is capable of detecting monoclinic zirconia and to analyze residual stresses of the coatings with an higher sensitivity than traditional X-ray diffraction measurements. Raman spectra were analyzed by deconvolution methods in order to study the evolution of the relative intensities and position of different bands. We found that the position of the 635 cm-1 band shifts to higher values of wave number during thermal cycling and by comparing this data with those reported in literature, it is possible to retrieve the value of residual stresses in zirconia coating. We also observed a relation between the intensities of the bands at 602 cm-1 and 635 cm-1 band and the number of thermal cycles performed.

Thermal barrier coating failures almost always occur by spallation due to interlayer stresses. During service, a thermally grown oxide forms between the bond coat and insulating ceramic. This oxide has a significant impact on the life of the coating. In this work, a number of innovative methods are used to study TBC bond coats, topcoats, and interface oxide layers. CoNiCrAlY bond coats produced by APS, VPS, and HVOF spraying are analyzed by X-ray photoelectron spectroscopy (XPS) and compared based on the presence of oxides. Zirconia powders and topcoat layers are examined by X-ray diffraction and Raman scattering in order to study the crystal structure and spatial distribution of different phases. The authors also use Raman microscopy to map the surface of the topcoat layer and XPS to determine the elemental composition. This provides useful data because surface and interface roughness affect the spallation resistance of the oxide layer and thus the expected life of the TBC. Paper includes a German-language abstract.

Thermal barrier coatings typically consist of a ceramic topcoat and a metallic bond coat that promotes adhesion and protects the substrate from corrosion. This study evaluates surface preparation processes used prior to the application of the bond coat layer. In the experiments, NiCrAlY bond coats are plasma sprayed onto Inconel substrates prepared by various methods, including dry and wet blasting and solid CO 2 cryogenic cleaning. At different points in the process, samples are extracted and characterized based on surface roughness, subsurface hardness, morphology, adhesion, interface contamination, and coating thickness and structure. Paper includes a German-language abstract.

Engine pistons are working components subjected to very high wear actions, mechanical and thermal stresses; moreover they can suffer damages due to pinking. Many solutions have been proposed, but there is still a large margin of improvement and strong efforts are made for improving performances and service life, taking into account for the requirements of fuel composition and of environment. Advantages can be obtained by the utilization of thermal spray coatings as protection against corrosion and pinking damages; on this matter the evolution of thermal spray processes and techniques offers suitable means. The aim of this paper is to evaluate the possibility to coat with NiCr alloys or with austenitic stainless steel (AISI 316L) the surface of engine pistons made by Al alloys. Coating layers, with thickness in the range of 200 ÷400 µm, have been sprayed, using Plasma Spray processes, on samples for metallographic investigation end test and on pistons directly. Optical and Scanning Electron Microscopy analyses were carried out on the cross sections to examine the microstructural features, while the hardness properties have been evaluated by means of both surface and cross-sectional measurements. Bend test is in progress to get information about the coating strain as well as about adhesion of the coating to the substrate. Finally the tested coatings have been applied directly on pistons and these are being tested on the test bench, evaluating the improvement of the service life.

Thermal barrier coatings are gaining considerable importance for the improvement the energetic efficiency of turbines. These materials are often applied on the surface of blades and are based on a layer of antioxidation material (mainly MCrAlY alloys) and a top layer that acts as proper thermal barrier (normally Yttria partially stabilized Zirconia). Coating removal is an important aspect in the production of these blades. "Decoating" or "stripping" is needed for the production of new components as well as for the reconditioning of existing ones. The present paper is dedicated to the comparison of different stripping methods and to the characterization of the blades surface after removal of thermal spray coatings both of Zirconia and of MCrAlY. The results reported here show that chemical stripping is particularly suitable for MCrAlY coating removal and does not affect the substrate. Water jet stripping can successfully be used for Zirconia-MCrAlY system removal although care is needed to avoid substrate damage. Salt bath technologies have been formed to be effective for TBC removal but not for MCrAlY removal.

The present work has the purpose of comparing different thermal spraying techniques, namely axial plasma spray, standard air plasma spray and high velocity oxygen flame (HVOF), for depositing metal matrix composites, in this case chromium carbide nickel-chromium based. The quality of the coatings deposited by these three techniques has been assessed in terms of structural characteristics (porosity, oxide concentration, unmelted particles presence, etc.) and of mechanical characteristics (hardness, adhesion, etc.) as well as surface morphology. A specific efficiency test has been carried out to compare the three examined technologies. The results of the present study indicate that, against a slightly decrease in the quality of the film in terms of structural and mechanical properties, axial plasma sprayed coatings can be sprayed with a higher efficiency in comparison to the traditional technologies.

In the aerospace field as well as in the stationary gas turbine field, thermal sprayed coatings are used to improve the surface properties of Nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly Yttria partially stabilized Zirconia) In the present study, our purpose was to assess the properties of thermally sprayed bond coat CoNiCrAlY comparing the performance of three different techniques: Vacuum Plasma Spray (VPS), High Velocity Oxygen Flame (HVOF) and Axial Plasma Spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of structural (porosity, oxide concentration, unmelted particles presence) and mechanical characteristics (hardness, adhesion). Furthermore, a study of the surface composition and morphology has been carried out. Specific efficiency trial has been carried out to compare the efficiency of the three examined technologies. We observed that the highest quality films are obtained by VPS, but that also HVOF and AxPS sprayed films have interesting properties which can make their use interesting for some applications in view of the lower cost of HVOF and AxPS.

This paper compares two methods for determining the composition of Ti/TiN coatings deposited by reactive plasma spraying. The coatings were obtained by spraying titanium powder in a low-pressure N2/Ar atmosphere. The resulting film has a variable nitrogen content in the form of titanium nitrides, depending on gas partial pressure, total pressure, sample-source distance, and other parameters. The composition of the film was determined using X-ray diffraction and X-ray photoelectron spectroscopy. The two techniques provide similar results and either can be used for the compositional characterization of these coatings.

Thermal spray deposited Zirconium oxide (or Zirconia, ZrO2) is used in several industrial sectors for various purposes: as thermal barrier for turbine blades, as wear and corrosion resistant coating in industrial applications as well as biomedical applications. This paper reports the results of a study aimed at determining the relation between the plasma spray process conditions and the coating properties. A 24-1 fractional factorial Design of Experiments has been used and the coatings have been characterised in terms of chemical composition, crystal lattice structure and mechanical properties (morphology, porosity, roughness, hardness). Coatings with different characteristics have been obtained depending on the spray parameters combination. We found that it is possible to vary the spraying parameters in such a way to obtain dense, compact Yttria Partially Stabilised ZrO2 deposits which can be useful as wear and corrosion resistant coatings. In other conditions less dense and more porous layers can be obtained, useful for thermal barrier applications.

A reliable measuring tool of surface degradation can therefore result in substantial savings in time and money for the development of machine components, lubricants, wear resistant coatings, surface treatments process, and for the control/optimization of components life time during production sites maintenance. For this purpose, Thin Layer Activation (TLA) has much to offer since it makes possible to monitor in situ very small surface material losses under real operating conditions, often without the need of dismantling the working equipment. In this paper, an overview of the TLA technology is presented, which enables wear to be measured under real operating conditions by monitoring the loss of substance caused by surface degradation. It emphasizes the benefits that this technique can bring through already tested applications in the automotive, energy, and machinery industry. Paper includes a German-language abstract.