Compound coatings of MCrAlY and alumina shows a sandwich structure with good wear-resistance and intensity at elevated temperature, the coating are usually applied to the furnace roll in modern continuously annealing line or continuously galvanizing line, to prevent the pickup forming on the roll surface. Coatings were prepared by the detonation spraying and the effect of spraying parameter of the denotation gas composition on the microstructure of coatings was investigated. The percentages of MCrAlY and alumina in the coating were determined by the composition of detonation gas mixture. Alumina content in the coating increased with the reduction of the nitrogen dilution in the gas mixture, which resulted in the hardness enhancement of the coating. The microstructure of coatings is different from that prepared by thermal plasma spraying or HVOF.

Continuously graded yttria stabilized ZrO 2 (YSZ) /NiCoCrAlY coatings were prepared using a high efficiency supersonic plasma-spray system and the thermal shock properties of the coatings were studied. The specimens were so prepared that the two kinds of powders with different melting point were fed to the different regions of the plasma jet by using two powder feeders. The two powders melted perfectly at the same power, and the overheating of the powder with lower melting point was avoided. In this way, a continuously graded transition layer was obtained. The Results show that the continuous change of the coefficients of thermal expansion and thermal conductivity in the transition layer leads to a excellent thermal shock resistance of the totally 0.9mm thick TBCs. The thermal shock cycles of the specimens which underwent heating by oxygen-acetylene flam to 1200 and then quenching into water reached more than 200. The coatings’ surface was still perfect without any visible cracks after the thermal shock test. The analysis shows that the dense structure and the sufficient plastic deformation of the particles depressed the formation of TGOs, which, together with the continuously graded thermal expansion coefficient and thermal conduction coefficient, contributes to the long thermal shock resistance of the coatings.

Formation of splats from hollow YSZ droplets deserves a special attention for thermal spraying. In this case, immediately prior to particle – substrate collision, we have a drop that consists of a liquid shell enclosing a gas cavity heated to a temperature close to the melt temperature. As the drop impinges onto the substrate, it suffers considerable deformation resulting in a rise of the gas pressure and in a decrease of gas temperature. The paper presented includes the results of the model experiments carried out under full control of the key physical parameters (KPPs) - temperature, velocity and size of droplet, and temperature of polished substrate, in which the starting powder was a specially prepared powder consisting exclusively of the hollow spherical YSZ particles. The main goal of the paper is to discuss and study the possible splat formation scenarios under above-mentioned conditions.

Increasing the combustion temperature in diesel engines is an idea which has been pursued for over 20 years. Increased combusting temperature can increase the power and efficiency of the engine, decrease the specific fuel consumption, CO and (possibly) the NOx emission rate. At the same time, TBCs should protect the metallic substrate against the corrosive attack of fuel contaminants (Na, V, and S). The most common system used is Yittria Partially Stabilized Zirconia (Y-PSZ). However, in diesel engines Y-PSZ TBCs have not met with wide success. To reach the desirable temperature of 850-900°C in the combustion chamber from the current temperature of 350- 400°C, a coating with a thickness of at least 1mm is required. This introduces different considerations than in the case of turbine blade coatings, which are on the order of 100µm thick. The design of a multilayer coating employing relatively low cost materials with complementary thermal properties is described. Numerical models were used to optimize the thickness for the different layers to yield the minimum stress at the operating conditions while achieving the desired temperature gradient. Abstract only; no full-text paper available.

In previous works, laser irradiation was associated to thermal spraying in the so-called MELTPRO process to improve the performance and the durability of TBCs. This study aims at presenting results concerning the thermal behavior of TBC's manufactured according to this process. Two types of severized thermal cycling were implemented: (i) “oxidizing” test: isothermal shocks were performed at different temperatures, ranging from 900°C to 1050°C down to 0°C; (ii) “quite non-oxidizing” test: thermal shocks were implemented from a temperature of 1100°C down to 50°C. Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon. TBC microstructure and its evolution during heat treating were characterized using image analysis, Knoop micro-indentation and XRD analysis. MELTPRO process was shown to increase twofold the lifetime of TBCs during isothermal shock tests. This is attributed to the fact that the columnar structure and the pore-crack architecture of remelted coatings improve the compliance property and decrease the permeability of TBCs. XRD analyses show that, in the Y-PSZ TBCs, the main phase is the metastable tetragonal (t’) phase both for as-sprayed and MELTPRO processed coatings. Moreover, remelted TBCs show a higher phase stability than as-sprayed TBCs during thermal shock tests: it seems that the remelted coatings have higher phase stability thanks to their pore architecture, which lead to a better compliance in relation to the thermo mechanical stresses, and so to a decrease in the stress variations undergone by the structure during the thermal cycles. During thermal annealing, it seems that MELTPRO processed coatings are less affected by sintering than as-sprayed coatings. Sintering phenomenon primarily concerns inter-lamellar cracks of non remelted areas. Besides, whatever the coating manufacturing process, the main remaining phase is the metastable tetragonal phase whatever the heat treating duration.

Yttria partially stabilized Zirconia (Y-PSZ) thermal barrier coatings (TBCs) have numerous applications as insulation layers onto gas turbine components. The main function of TBC is to enhance efficiency by increasing working temperature. These coatings are commonly manufactured using air plasma spraying. The characteristics of TBCs strongly depends on their pore and crack network architecture. Engineering the coating architecture by an adapted process is a prerequisite to modify TBC characteristics. Therefore, air plasma spray and in situ laser irradiation by diode laser processes were combined in this study to modify structural characteristics of TBCs. The coatings were remelted layer by layer during their deposition or alternately (i.e., an as-sprayed layer followed by an in situ remelted layer). This process was named MELTPRO. TBC apparent thermal conductivity was quantified implementing numerical computations based on 2-D real discrete structures. Coating thermal properties were correlated to their pore network architecture characteristics quantified by image analysis (i.e., nature, orientation, percentage, etc.). Moreover, thermal annealing at 1100°C were performed to compare sintering phenomenon of manufactured TBCs. Results show that the MELTPRO process permits: (i) to modify of the pore network architecture of as-sprayed TBCs. The porosity level increases by the in situ remelting process due to the formation of large horizontal cracks (as shown by image analysis). These horizontal cracks behave as thermal resistances and are responsible for the decrease of the coating thermal conductivity of about 30 % compared to the as-sprayed ones; (ii) to architecture differently the pore network for a constant thermal conductivity, because thermal conductivity and porosity level are not explicitly correlated; (iii) to improve the thermal insulation which remains unchanged after thermal annealing treatments: this seems to be due to the fact that the large cracks of remelted TBCs are less sensitive to sintering than inter and intra lamellar cracks of as-sprayed TBCs.

Thermal cycle lifetimes of two thermal barrier coating (TBC) systems with the same plasma sprayed yttria-stabilized- zirconia (YSZ) topcoat but different low pressure plasma sprayed (LPPS) bond coats, conventional and cryomilled NiCrAlY feedstock powder, were studied. Thermal cycling tests consisted of 50 min at 1121C followed by 10 min air-cooling to room temperature. The coating produced with the cryomilled powder showed a 300% increase in lifetime when compared to the conventional one. Both TBCs failed as a result of delamination and spallation of the ceramic top coats. Several factors like thermally grown oxide (TGO) thickness, TGO composition, CTE mismatch, creep resistance of the NiCrAlY bond coat, and others that affected the thermal cycling life of the system, were analyzed in this work. Abstract only; no full-text paper available.

This paper presents novel results of a series of experiments intended to study the role of the size of the feedstock powder on the microstructure of the deposits. For this purpose, Metco and the feedstock powder, yttria-stabilized (8% wt) zirconia, with number-weighted mean particle sizes of 32, 47, 56, and, 88 micrometer, are used. Small-angle neutron scattering and multiple small-angle neutron scattering (MSANS) methods are applied to determine the microstructure of the four deposits. Companion indentation measurements are performed to determine the elastic moduli of the deposits. The paper also discusses the MSANS 3-void model in relation to the anisotropic elastic properties. Paper includes a German-language abstract.

The melting of plasma-sprayed thermal barrier coatings (TBC) using highly concentrated energy sources is one of the methods associated with increasing the density of the sprayed material and modifying the phase composition in order to improve the properties of these coatings. This paper presents the results of the investigations into TBC melting using pulse and continuous carbon dioxide lasers. Paper includes a German-language abstract.

In this paper, plasma sprayed thermal barrier coatings (TBCs) with and without bond coat are stressed to various stress levels under four point bending with in situ acoustic emission (AE) to monitor any cracking activities. Micro- and macro-cracks occurring during the tests are investigated to better understand the failure mechanisms of TBCs. The results show that limited AE activities were detected in the first four stress-relief cycles, while plastic deformation and the greatest AE activity were observed when the applied load exceeded the yield point. In addition, they show that a TBC system that contained an adhesive layer had less AE activity (cracking events) than the TBC without an adhesive layer. In addition, the samples tested at a main speed of 5 micrometer/s resulted in a higher AE activity than the samples tested at 10 micrometer/s. With increasing plastic deformation, macro cracks and surface cracks also occurred. Paper includes a German-language abstract.

In this paper, the mechanical properties and cracking features of yttria stabilized zirconia with and without bond coat on steel substrates is investigated. Four point bending tests have been used to evaluate the mechanical properties, while acoustic emission (AE) has been used to in situ monitor the cracking behavior during the bend tests. In the sprayed state, the samples were characterized by four-point bending tests with local noise emission. In comparison with the substrate, the coated samples showed an increased flow pressure. It is also shown that the processing parameters, the cooling, and the adhesive layer have a significant influence on the flow pressure. It is proven that the cooling affects the behavior of the load-displacement curves. The AE analysis shows the different deformation behavior of the coating-substrate system for the different processing conditions. This is supported by the surface crack analysis. Paper includes a German-language abstract.

The formation of the microstructure is influenced by a number of parameters. The spray angle is considered to be one of the most important and difficult to control parameters, especially for substrates with complex shapes. In this paper, the influence of the spray angle on the microstructure of gray aluminum oxide and yttrium oxide stabilized (8% by weight) zirconium oxide applications is investigated. Plots are made from each material at four spray angles (90 degree, 70 degree, 55 degree, and 45 degree). Their microstructure is characterized by means of intrusion porosimetry and SEM. The results show clear differences between the two materials. The aluminum oxide microstructure is significantly influenced by the spray angle, while the YSZ microstructure is almost independent of the spray angle. Paper includes a German-language abstract.

This paper evaluates different techniques for measuring bond coat surface roughness, explores the influence of different thermal spray processes on the surface roughness of bond coats, and correlates bond coat surface roughness with spray parameters. The results of an evaluation of various techniques for measuring the surface roughness of the adhesive layer including the influence of the thermal spraying process, the powder size, the travel speed, and the coating thickness on the roughness of the thermal barrier coatings (TBCs) adhesive layers are presented. Light microscopy, a needle-tracking profile stylus, and non-contact interferometry with white light are used to characterize the adhesive layer surfaces. The results show that the measurement technique used had significant effects on some of the measured surface roughness values. Paper includes a German-language abstract.

Yttrium oxide-stabilized zirconium dioxide is mainly used in thermal barrier coatings. However, the desired higher gas inlet temperatures have initiated the search for novel materials. Low thermal conductivity and high melting points are important criteria for the selection of these materials. This paper investigates a zirconate material with a pyrochlore structure and a high melting point. In addition, it investigates the sintering behavior of the coatings at elevated temperatures. Dilatometer tests are carried out at 1200 deg C for at least 70 hours. For samples which had been annealed for 24 hours at 1250 deg C, the change in the porosity distribution is determined by means of mercury porosimetry. The paper presents the first results of thermal cycling tests on a plasma-sprayed coating. These first results are very promising. Paper includes a German-language abstract.

Thermal barrier coatings systems are composed of a zirconium dioxide-(6 to 8 wt.%) yttrium oxide ceramic top coat about 300-500 micrometer in thickness, deposited either by air plasma spraying or electron beam assisted physical vapour deposition, over an MCrAlY (M = Ni, Co or NiCo) bond coat, about 100 micrometer thick, deposited by vacuum plasma spraying. In this paper, the stiffness of as-sprayed zirconia is measured using three different techniques, namely cantilever beam bending, ultrasonic resonance and nanoindentation. The paper explores the effect of post-deposition heat treatment on the value obtained. The results show that the cantilever bend technique, employed with a high precision scanning laser method of displacement measurement, was found to be the most reliable procedure. Paper includes a German-language abstract.