In this work, metal-based thermal barrier coatings (MBTBCs) for use in low heat rejection diesel engines have been produced, using high frequency induction plasma spraying (IPS) of iron-based nanostructured alloy powders. Important advances have been made over recent years to the development of ceramic-based thermal barrier coatings (TBCs) for diesel engines, but they are not yet applied in mass production situations. Besides the important economic considerations, the reliability of ceramic TBCs is also an issue, being associated with the difficulty of predicting their “in-service” lifetime. Through engineering of the nano/amorphous structure of MBTBCs, their thermal conductivity can be made as low as those of ceramic-based TBCs, with reduced mean free paths of the electrons/phonons scattering. In this work, nano/amorphous structured coatings were deposited by IPS using the following spray parameters: spraying distance (200mm), plasma gas composition (Ar/N 2 -85/15, by volume %), IPS torch power (25kW), and powder feed-rate (16g/min.). The structure and properties of the deposited layers were characterized through SEM (Scanning Electron Microscopy) observations. The thermal diffusivity (α) properties of the MBTBCs were measured using a laser flash method. Density (ρ) and specific heat (Cρ) of the MBTBCs were also measured, and their thermal conductivity (k) calculated (k =αρCp). The thermal conductivity of MBTBCs, with 7.5% total porosity, was found to be 1.22 W/m/K. The heat treatment study showed that phase transformation started at 650oC, and grain size growth from nano- to micron- scales occurred at around 1000°C under static exposure conditions. Thermal expansion coefficient (TEC) of MBTBCs was 15E-6 /K, which is close to the TEC of cast iron and thus, closer to the TEC values of aluminium alloys than are conventional TBCs. Fracture toughness of MBTBCs has also been assessed by use of Vickers hardness tests, with a 100 g load for 15 s, and the results show that there are no measurable crack developments around “indented” areas on all samples of MBTBCs tested.

Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often shown improved mechanical properties comparing to coatings produced from convention feedstock. Bimodal structured ceramic deposits have been reported to demonstrate better properties such as wear resistance, adhesion strength and toughness. For thermal barrier coatings, high temperature performance is a key point especially creep/sintering. In this study creep/sintering rate of plasma sprayed, bimodal structured, yttria stabilized zirconia coatings were investigated. The creep behaviour was investigated using free standing thick (3 mm) coating layers loaded in the four point bending setup at two temperatures : 800 and 1000 ºC in air. Under the same test conditions the creep results for nanostructured coatings and conventional plasma spray coatings have been compared together and the former one in both temperatures showed a lower creep rate.

The aim of this work, devoted to yttria stabilised zirconia (YSZ) thermal barrier coatings, is to produce by DC plasma spraying a single thick pass macro cracked orthogonally to the substrate. YSZ was plasma sprayed in air atmosphere on Hastelloy X substrates, with a NiCrAlY bond coat. A three-zone microstructure is observed, where lamellae and columns are present. The measurements of deposition stresses during spraying allow explaining the macrocrack formation.

High performance Thermal Barrier Coating (TBC) has been expected for use under extream conditions in advanced industrial fields. A high performance of zirconia (ZrO 2 ) composit coating was achieved by gas tunnel type plasma spraying. The zirconia-alumina (ZrO 2 -Al 2 O 3 ) composite coating has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness, and is expected to use as a Thermal Barrier Coating (TBC). TiN thick coatings which has excellent properties have also been formed at high deposition rate by means of the gas tunnel type plasma reactive spraying. In this study, the fundamental characteristics of this method were investigated by measuring the properties of the titanium nitride (TiN) coatings formed on traversed stainless steel substrate. Some coating characteristics of TiN which depend on the spraying distance, the environmental gas, traverse number etc. were then clarified. The functionally graded property of the ZrO 2 -Al 2 O 3 composite coating was clarified to make TiN-Al 2 O 3 -ZrO 2 Composite Coatings. The TiN coating was overcoated on the ZrO 2 -Al 2 O 3 coating, in order to enhance the performance of such high hardness composite coating. The Vickers hardness of the TiN layer was increased at the coating surface, which corresponded to the result that the coating surface became dense. The graded functionality of the hardness in the thickness direction was increased remarkably in the TiN-Al 2 O 3 -ZrO 2 Composite Coatings by Gas Tunnel Type Plasma Spraying. In addition, the performance of TiN thick composite coating was discussed as a heat resistant TBC.

Type of powder feedstock greatly affects properties of zirconia coatings since they interact differently in a plasma flame and hence influence microstructure development. In this study, ZrO 2 -8wt% Y 2 O 3 thermal barrier coatings (TBCs) have been produced by atmospheric plasma spraying fused and crushed (FC) and hollow sphere (HOSP) feedstock powders. The sprayed coatings contained segmentation cracks going through the coating thickness. High substrate temperature during spraying gave rise to increased segmentation crack density (Ds). The FC powder has the capability of providing coatings with high segmentation crack density compared with the HOSP one. At lower temperature, the HOSP coating was more porous than the FC coating sprayed at similar temperature and hence exhibited a much reduced thermal diffusivity. At high plasma temperature, the HOSP particles attained higher particle surface temperatures and velocities than the FC ones. The particles temperatures and velocities for the HOSP particles were influenced more significantly by spray condition than those for FC particles.

Thermal barrier coatings (TBCs) made of yttria stabilized zirconia (YSZ) are frequently used in gas turbines to improve their efficiency. Coatings typically applied on combustion chamber parts with a thickness of more than about 500 mm often contain segmentation cracks. This type of cracks considerably improve the thermal cyclic life of the thick TBCs. Segmentation cracks can be directly introduced into the coatings during the plasma-spraying process. The influence of the most important process parameters as substrate temperature and passage thickness on the segmentation crack density will be described in detail. In addition to the coatings with thickness values of about 1 mm also thin coatings with segmentation cracks have been developed. This type of coating might also be favourable for an application on blades and vanes of gas turbines. The thermal cycling performance of all coatings was tested in gas burner test facilities. The thick, segmented TBCs almost reached the performance of conventionally sprayed thin TBCs. The thin, segmented TBCs showed an excellent performance at surface temperatures above 1300°C which was considerably better than that of conventionally sprayed thin TBCs. Possible explanations for the observed improvement of the thermal cycling behaviour by the introduction of segmentation cracks will be given. Abstract only; no full-text paper available.

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.

This study on ceramic thermal barrier coatings (TBCs) presents baseline thermal conductivity data on as-deposited 7-8 wt.% YSZ and a paired-cluster rare-earth oxide doped YSZ, prepared using air plasma spray (APS). The thermal diffusivity for each coating was measured up to 1100°C using the laser flash method, and from these values, the thermal conductivity was calculated. The maximum benefit for thermal conductivity reduction in TBCs with a (GdO 2 , Yb 2 O 3 )-doped YSZ composition was highest for APS dense, vertically macrocracked microstructures, whereas in the case of low density APS TBCs, the reduction in conductivity was found to be more strongly influenced by horizontally-oriented, sub-critical defects and porosity within the coating microstructure.

It is very important to clarify the characteristic deformation behaviors of thermal barrier coatings. Monotonic and cyclic deformation behaviors of thermal barrier coatings under uniaxial compressive loading were examined. Specimens of plasma-sprayed ZrO 2 -8%Y 2 O 3 and CoNiCrAlY were fabricated to test the coating materials independent of the substrates. The specimen was fabricated by dissolving out the substrate only at the region of gauge length. Thicknesses of the coatings were 300 μm. The stress-strain response was measured using the laser speckle strain / displacement gauge (SSDG). ZrO 2 -8%Y 2 O 3 coating showed the nonlinear stress-strain response with a considerably lower elastic modulus which value was about 10% of sintering ceramics. The coating was found to leave permanent strain by compressive loading: the compliance of the coating decreased by compressive loading. The compliance was also found to decrease furthermore by undergoing cyclic compressive load. On the contrary, CoNiCrAlY coating deposited by low pressure plasma spraying was found to show the stress-strain response with insignificant nonlinearity. The compliance of the stress-strain curve didn’t also decrease with increasing the number of compressive stress cycles. It was found that coating with many defects and pores showed stress-strain response with large nonlinearity.

During the past few years thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often demonstrated improved properties relative to coatings produced from convention powder. Nanostructured or bimodal structured ceramic coatings have been reported to exhibit better wear resistance and adhesion strength. For thermal barrier coatings, key properties include thermal conductivity and thermal stability. In this study thermomechanical properties of plasma sprayed, bimodal structured, yttria stabilized zirconia coatings were investigated. The thermal stability was examined by measuring the hardness and elastic modulus of free standing coating layers before and after heat treatments in air at 600-800 °C. The creep behaviour was investigated using free standing coating layers loaded in the four point bending configuration at temperatures of 1000 °C in air. The results for bimodal structured coatings have been compared with the behaviour of conventional plasma spray coatings under the same test conditions.

The high velocity oxy-fuel (HVOF) combustion spray technique previously has been shown to be an excellent solution for depositing nano-reinforced thermoplastic polymer coatings. Dense polymer coatings can be produced regardless of ceramic particle size with little change in the spray parameters. Composite powders with multiple scales of silica reinforcement, ranging from 12 nm to 100 µm, have been created. Preliminary testing was begun using melt processing. The multiple scales have shown improved scratch resistance relative to single-scale reinforcements.

In order to improve the efficiency of gas turbines, thermal barrier coatings (TBCs) have been applied to components in the hot sections of advanced gas turbines. During service, thermally grown oxide (TGO), which consists of an Al 2 O 3 layer and a mixed oxide layer, forms at the interface between the top coating and bond coating. It is supposed that the reason for failures of TBCs, such as cracking, delamination or spalling, is due to decreased bond strength caused by TGO growth or due to the formation of stress concentration sites caused by porosities in the mixed oxide. In this study, to inhibit the growth of TGO, plasma sprayed CoNiCrAlY bond coating was remelted with a YAG laser prior to spraying the top coating. A thin Al 2 O 3 layer formed at the top coating/bond coating interface, and the formation of porous mixed oxide during thermal aging tests was inhibited. Four-point bending tests showed that the bond strength of TBC with remelted CoNiCrAlY was superior to standard TBC.

An investigation was carried out into the effect of the interface roughness between the metallic bond coat and the ceramic topcoat on internal stresses in a thermal sprayed ceramic thermal barrier coating (TBC). To evaluate the effect of the interface roughness on the residual stress in the top coat, the specimens with two kinds of bondcoat roughness (rough type and smooth type) were prepared. The in-plane stresses of the specimens were measured with laboratory X-rays. The in-plane stresses for the both of the rough and smooth specimens were about 60MPa and independent of the roughness of the bond coat. Using high energy X-ray, the stress of the rough specimen was compressive and the stress of smooth specimen was tensile. This tendency is different from the result measured by laboratory X-ray. This difference in the stress value is coursed by the out-of-plane stress. Theses stresses in the topcoat were estimated by the hybrid method, that is to estimate out-of-plane stress using laboratory X-rays and high-energy synchrotron X-rays. As a result, the larger the roughness of the bond coat became the larger out-of-plane-stress become.

Thermal barrier coatings (TBC) in general fail by delamination of the ceramic partially stabilised Zirconia (PSZ) top coat (TC) from the underlying metallic bond coat (BC). The process is initiated by crack initiation and growth either in the TC or in the thermally grown oxide (TGO) that will form at the interface between top and bond coat. The aim of the present paper is to describe the degradation due to crack growth in such a way that data can be used for FEM modelling work. Flat rectangular test coupons have been subjected to thermal cyclic fatigue (TCF) in air with a temperature range from 100°C to 1100°C. Identical samples were removed from the TCF furnace at different times of thermal cycling in order to achieve material with different degree of damage. After mounting, cutting and sectioning the specimen were investigated by light optical microscopy (LOM) and scanning electron microscopy (SEM) together with an energy dispersive spectrometer (EDS). Image analysis of LOM micrographs was used for measurement of crack distribution and degree of TC damage. A method for crack growth measurement based on the degree of TC / TGO damage has been developed. Furthermore, a measure of TBC damage as a function of elapsed fatigue cycles was introduced. The TBC material shows a mixed black and white fracture surface after TCF cycling. Delamination crack growth data are presented. Delaminated TC/BC interface surface as a function of fatigue cycles follows an S-curve behaviour.

Zirconia (ZrO 2 ) coating formed by plasma spray method is widely used industrially as a thermal barrier coating (TBC). But it has still problems such as spallation and cracks inside the coating. The solution will be given by the development of new spaying processing. The zirconia-alumina (ZrO 2 -Al 2 O 3 ) composite coating formed by gas tunnel type plasma spraying has a high hardness layer at the surface side of the coating, which shows the graded functionality of hardness, and is superior as a TBC. In this paper, the performance of such high hardness ZrO 2 - Al 2 O 3 composite coating was investigated and the merit as TBC was clarified. The Vickers hardness of the high hardness layer near the coating surface increased by the thermal process of high energy plasma, which corresponded to the result that the coating became denser. Also, the effect of alumina mixing was discussed about the microstructure of this composite coating. The combination of high hardness of Al 2 O 3 with the low thermal conductivity of ZrO 2 resulted to the development of high performance TBC. The thermal conductivity of such ZrO 2 - Al 2 O 3 composite coatings was proved to be much smaller than that in the longitudinal direction. Moreover the adhesive strength of such high hardness zirconia composite coatings was investigated as well as its mechanical properties. The adhesive strength of such high hardness coatings to the substrate was weakened as the increase in the coating thickness.

Splats are the building blocks of thermal sprayed coatings, and thus the mechanical properties of such coatings are directly related to splat behavior. Elastic and elastoplastic properties of coatings have been measured on the macro-scale, but are not yet quantitatively predictable using process parameters as inputs. Coating mechanical properties represent contributions from intrinsic splat properties, splat-splat interfaces, and other microscopic defects. In this study, we investigated the intrinsic properties of Ni and Ni based alloy splats on substrates, for a variety of process methods and input parameters. Residual stresses were measured via X-ray microdiffraction and elastic and elastoplastic properties were studied via nano-indentation. From a purely scientific standpoint, splat studies provide insight into rapidly cooled small-volume structures that often exhibit extra ultrafine- or nano-crystalline structure. Thus, accordingly, nano-indentation response was also compared to the prediction for bulk counterparts of the splat materials.

Thermal barrier coatings (TBCs) are capable of protecting hot-section engine components from the hot gas stream, and thereby can provide improvements in component durability and engine efficiency. Thick TBCs can provide further improvements in durability and efficiency, especially for static components. The main commercial coating methods for TBCs are electron beam physical vapor deposition (EB-PVD) and air plasma spray (APS). These processes have limitations for depositing thick TBCs: for EB-PVD, the deposition rates are low and the cost is high; for APS, durability is reduced with increased thickness. Inframat Corporation, in collaboration with the University of Connecticut, is developing a new plasma spray process, namely, solution precursor plasma spray (SPPS), for the formation of TBCs and also functional films from liquid precursor feedstock, instead of the solid powder feedstock used in conventional APS. SPPS TBCs have many unique microstructural features, including: ultra-fine splats, vertical micro- and macrocracks, micrometer- and nanometer-size porosity. These unique microstructural features provide a TBC with high thermal cycling spallation life and bond strength. These coatings have been made in thickness up to 2 mm and show excellent durability. In this paper we present microstructural characteristics and thermal cycling performance of SPPS-formed 7YSZ thick coatings varying in the range of 0.5-2 mm.

The mechanical behavior of nanostructured yttria stabilized zirconia (YSZ) thermal spray deposits was examined and compared to conventional zirconia coatings. Young’s modulus was measured using indentation techniques. The anisotropy of the deposits was estimated by indenting the deposits in the perpendicular and parallel directions to the substrate. Statistical distribution of the mechanical properties was correlated with the microstructure. The effective Young’s moduli of nanostructured and standard YSZ were also modeled by means of 2D eXtended FEM; whereby actual microstructures were assessed. The simulation was based on micrographs by employing a standard meshing program combined with an in-house developed XFEM package, which incorporates the crack structure into the model. The effect of nano-scale features on the effective Young’s modulus were predicted and compared to experimental observations.

This work evaluates the microstructure and composition of zirconia films produced by thermal plasma chemical vapor deposition (TPCVD). The results show that TPCVD has the potential to produce durable ceramic films with columnar structure, even in open air. Paper includes a German-language abstract.

This work assesses the properties of vacuum plasma sprayed YSZ coatings for potential use in solid oxide fuel cells. The results of the investigation show that low-porosity layers of yttria-stabilized zirconia can be produced by using a fine powder and by adjusting plasma gas composition. Under optimized spraying conditions, YSZ layers with a thickness of 10-20 μm, a porosity less than 1%, and an average roughness of 1 μm are achievable. Paper includes a German-language abstract.