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.

The most commonly used structural materials for blades and other high temperature components of gas turbines are nickel base superalloys. A TBC protection coating system consists of a top coat of yttria partially stabilized zirconia and an underlying bond coat, usually MCrAlY (where M stands for Ni, Co or a combination of both). MCrAlY is normally deposited by the thermal spray processes: air plasma spray (APS), vacuum plasma spray (VPS/LPPS) or high velocity oxygen fuel (HVOF). The adhesion between the bond coat and the substrate, and therefore of the whole thermal barrier system, strongly depends upon the surface roughness. A high level of roughness generally denotes better adhesion, especially with the HVOF thermal spray process, where it is a necessity. Generally the roughness is reached by means of grit blasting with an abrasive media; this results in a certain level of surface contamination due to the entrapment of abrasive particles. The aim of this work was to set up a new surface preparation process in order to obtain a completely clean surface with a suitable roughness, which can be coated afterwards with HVOF or VPS/LPPS thermal spray technology. The tests carried out by this process on turbine blades, coated with a HVOF system, led to obtaining a coating/base material interface without any contamination caused by the surface preparation.

High temperature thermal fatigue causes the failure of Thermal Barrier Coating (TBC) systems. Due to the difference in thickness and microstructure between thick TBCs and traditional thin TBCs, they cannot be assumed a-priori to possess the same failure mechanisms. Thick TBCs, consisting of a CoNiCrAlY bond coat and Yttria Partially Stabilised Zirconia top coat with different values of porosity, were produced by Air Plasma Spray. Thermal fatigue resistance limit of TBCs was tested by Furnace Cycling Tests (FCT) according to the specifications of an Original Equipment Manufacturer (OEM). TBC systems were analyzed before and after FCT. The morphological and chemical evolution of CoNiCrAlY/TGO microstructure was studied. Sintering effect, residual stress, phase transformation and fracture toughness were evaluated in the ceramic Top Coat. All the tested samples passed FCT according to the specification of an important OEM. Thermal fatigue resistance increases with the amount of porosity in the top coat. The compressive in-plane stresses increase in the TBC systems after thermal cycling, nevertheless the increasing rate has a trend contrary to the porosity level of top coat. The data suggest that the spallation happens at the TGO/Top Coat interface. The failure mechanism of thick TBCs subjected to thermal fatigue was eventually found to be similar to the failure mechanism of thin TBC systems made by APS.

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.

Invar alloy (Fe – 36%Ni) is used in industrial applications which require high dimensional stability because of its exceptionally low thermal expansion coefficient. Purpose of this work is to enhance the performance of molds for the production of carbon fiber reinforced plastic (CFRP) components. Four different kinds of commercial powders were coated on an Invar substrate: Al 2 O 3 - 12TiO 2 , Cr 2 O 3 and ZrO 2 - 8Y 2 O 3 by Air Plasma Spray (APS) and WC - CoCr by High Velocity Oxygen Fuel (HVOF). Metallographic microscopy observation and SEM analysis were carried out and microhardness and fracture toughness were evaluated by means of the micro - indentation method. Friction behaviour and wear resistance were evaluated in dry sliding conditions with Pin On Disk apparatus for not coated Invar substrate and for the different coated substrates. Chromium oxide and tungsten carbide coatings exhibited higher mechanical characteristics respect to the other coatings: chromium oxide had the higher hardness value and tungsten carbide the higher fracture toughness. Tungsten carbide coating had the lower average coefficient of friction and together the chromium oxide the lower wear mass loss and wear rate. Among APS ceramic coatings, Cr 2 O 3 exhibited the best mechanical and tribological behavior while the HVOF cermet coating exhibited the best behavior among all the coatings.

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 most commonly used structural materials for blades and other high temperature components of gas turbines are nickel superalloys such as Inconel 738, MAR M247M or Hastelloy. Thermal barrier coatings (TBCs) are widely used on these substrates as protection against high temperatures and oxidation. A TBC system consists of a top coat of yttria partially stabilized zirconia deposited by air plasma spray and an underlying bond coat (usually MCrAlY, where M is Ni, Co or a combination of both). MCrAlYs are normally deposited by thermal spray processes such as air plasma spray, vacuum plasma spray (VPS/LPPS) or high velocity oxygen fuel (HVOF). In general, the adhesion of the whole thermal barrier system is strongly dependent on the surface preparation of the substrate and it is generally believed that a certain degree of roughness promotes better adhesion. OEM’s (Original equipment manufacturer) procedure for preparation of substrates and analysis have been reviewed and considered as basis of this work. The scope of this work is to set up a new cleaning methodology in order to obtain a completely pollution free surface to be coated afterwards with HVOF or VPS/LPPS. The properties of this new methodology have been compared with standard surface preparation techniques such as blasting with corundum and silicon carbides. The obtained samples have been analysed by means of metallography and chemical composition of the interface in order to measure the interfacial pollution between substrate and coating. Finally adhesion of MCAlY coating have been tested and compared with specification of the main OEMs.

Coating technology is progressing at a steady rate with continuous significant improvements in the coatings performance. In the aerospace field, as well as in the stationary gas turbine field, coatings deposited by different processes (thermal spray, CVD, EBPVD) play an important role in order to increase the performances of the engines. In particular, in order to improve the resistance to oxidation and corrosion at high temperature, aluminium is deposited by several techniques (pack aluminising, above the pack and CVD) in alternative or addition to thermal spray coatings (mainly MCrAlY alloys where M stands for Co, Ni or CoNi). These MCrAlY coatings are generally deposited by Low Pressure Plasma Spray (LPPS) or Vacuum Plasma Spray (VPS), but also by High Velocity Oxygen Fuel (HVOF) and Air Plasma Spray (APS). This paper addresses the study of aluminium coatings deposited by CVD on CoNiCrAlY bond coats deposited by different processes: VPS with F4 gun, LPPS with EPI gun and HVOF. The aim is to verify if and how the different CoNiCrAlY coatings obtained by these three processes with different content of oxides and porosity could affect the deposition rate and quality of the Al coatings. The obtained samples have been characterized from the metallographic point of view in order to determine porosity, thickness and structure of both CoNiCrAlY and Al coatings. Al coating thickness has been taken as parameter in order to define the Al coating deposition rate on the three different CoNiCrAlY coatings. Further tests for the determination of aluminium content and chemical composition of the coatings are in progress.

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.

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.

The appropriate selection of bulk materials and coatings of valve components, is an important factor for the economic success of oil and gas production activities in petrochemical field. Materials and coatings are important because particle erosion and surface wear is associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problem and cost increases. The most popular solution of these problems is the deposition of hard material like tungsten carbide or chromium carbide by thermal spray. Particularly these coatings are deposed by HVOF (High Velocity Oxygen Fuel) to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt-chromium based coating, chromium carbide nickel-chromium coating as well as Inconel 625 are adopted actually in the specifications of the industrial petrochemical companies and their behavior and wear, erosion and corrosion properties are reported in literature. This paper addresses the study and surface analysis and characterization of alternative coatings such as NiAl and composite material WC / intermetallic compounds containing mainly Ni, Cr, Co and Mo. The best parameters to produce these coatings has been found by implementing a DOE and the obtained coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the thermal spray coatings in an atmosphere of H 2 S and CO 2 [1] and to wear and erosion test according to ASTM G75-95; removed material weight and usured surface damages have been determined. Furthermore the coatings have been completely characterized before and after the tests from the point of view of the structure (porosity, coating cohesion and adhesion, hardness, wear) and of the surface properties by means of a prototype 3- dimensional stylus micro-topography surface analysis system. Their corrosion and functional behaviour have been finally compared with the behaviour of the above mentioned coatings applied at present as standard in the petrochemical sector. The results state that WC/intermetallic compound could be a good substitute of IN625 for certain kind of application where good antierosion behaviour is requested.

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.

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.

In order to be suitable for food processing application, besides having the usual characteristics as high adhesion, high cohesion, high compactness and maximum hardness and wear resistance, the thermal spray coating should not release foreign substances, as prescribed by the international standards. This paper defines a very strict procedure according to valid EC and FDA standards in order to test the compatibility of the coating with the food. It discusses the applicability of this test method, in which a contact is created between a food-simulating solvent and the thermally sprayed coating to be analyzed. The inert nature of the drawn migration cell, the adopted time-temperature conditions and the characterization of the coating before and after the migration test are discussed. Paper includes a German-language abstract.