Experimental studies involving aluminum particles sprayed onto polished AISI304L substrates using the Valuarc 200 wire arc previously showed that there exists a transition temperature from splash to disk splats. Increasing the substrate temperature above the transition temperature was seen to increase the number of disc splats, thus producing coatings of improved properties. XPS test results have shown that increasing the substrate temperature also results in increased oxygen content on the surface of the substrate. Experiments also show that prolonged heating of a substrate at a particular (fixed) temperature further promotes oxidation of the substrate surface, thus increasing the surface roughness (Ra). Samples generated on substrates that were held at or above 350°C (above Tt) for prolonged periods of time (over 20 minutes) were seen to promote splashing. This is in contrast to the previous findings that showed substrate temperatures above Tt further promoted disc type splats and improved adhesion between the splat and substrate. Samples generated in this study consistently showed that splashing can be seen at temperature well above the transition temperature, if the substrate has been heated for too long a duration. The cause of splashing is believed to be related to increased surface roughness resulting from prolonged oxidation of the substrate surface. Abstract only; no full-text paper available.

This paper reports morphological study of coatings microstructure performed on two selected reference abradable materials: AlSi-hBN and NiCrAl-Bentonite. Scanning Electron Microscopy (SEM) was used to obtain adequate micrographs for analysis. The porosity and non-metal particles morphology was assimilated to elliptical inclusions within the metallic matrix. Accordingly, they were described by means of three different geometrical parameters related to the shape, size and orientation distribution, allowing additional statistical analysis on the coatings.

Abradable coatings are located on the stationary parts of gas turbines, in front of blades, which cut a track in them. This has to be achieved with minimum wear of the blades, in order to control the over-tip leakage. These coatings are generally deposited by thermal spraying of composite powders comprising a metal base, a polymer filler generating porosities and a dislocator such as hBN. The very demanding properties are nowadays adjusted using rig tests, where samples are rubbed by the contact of a dummy, simulating actual working conditions in an aircraft engine. Several types of behaviour are usually described, but few numerical data are produced from these tests. Only the blade wear (or metal transfer) is generally measured. As the understanding of contact phenomena is fundamental for the development of predictive models allowing the design of more performing materials, a comprehensive characterization process of the rub path was developed. The study was based on a topological survey made by laser profilometry, giving three-dimensional maps. These maps were then processed by image analysis and several parameters were computed, like surface roughness and parameters giving information on the shape and orientation of the holes or grooves in the rubbed surface of the samples.

The aim of the research was to compare the size, shape and distribution of the NiCrBSi and Fe 2 O 3 powder particles. Of interest were the chemical and phase compositions of both the powders and structures obtained by plasma- or HVOF-spraying them. The experiments have shown that although the powders differ in structure and particle size, it is possible to use them to obtain composite coatings containing both ingredients. A phase analysis was conducted to study the different phase composition for both spray methods.

WC-Co powders with nano-sized WC were deposited by dynamic powder deposition process. Microstructural characterization and phase analysis of feedstock powders with different compositions and as-deposited coatings with various substrates were carried out by SEM and XRD. The results show that there is no detrimental phase transformation and/or decarburization of WC by dynamic powder deposition. It is also observed that nano-sized WC in the feedstock powder is maintained in the deposited coatings. It is demonstrated that it is possible to fabricate the nano-structured WC-Co coatings with low porosity and very high hardness by dynamic powder deposition.

TiO 2 based coatings are promising photocatalyst as they are chemically stable and nontoxic. To develop more effective photocatalyst with cost effective plasma spraying technique, TiO 2 and TiO 2 -Fe 3 O 4 coatings were prepared using agglomerated anatase TiO 2 and composite TiO 2 -Fe 3 O 4 powders, respectively. The photocatalytic activity and photoelectrochemical characteristics of sprayed coatings were examined. The photo-response of the sprayed TiO 2 electrode was comparable to that of single crystal TiO 2 , but the breakdown voltage approximated to 0.5V (vs. SCE). The short-circuit current of TiO 2 was 1.05mA/cm 2 , which was 15 times comparing with that of TiO 2 -Fe 3 O 4 electrode under 30mW/cm 2 xenon light illumination. N-type TiO 2 semiconductor particle reacted with Fe 3 O 4 or Fe 2 O 3 particle and concurrently produced p-type FeTiO3 in plasma spraying processes. FeTiO3 compound obviously improved the photocatalytic activity of the TiO 2 coating for the formation of p-n junction between p-type FeTiO3 and n-type TiO 2 , which may spatially separate the photo-generated electrons and holes. As a result, plasma-sprayed TiO 2 -Fe 3 O 4 coatings are promising candidates for the applications to photocatalyst.

Deformation and solidification process of molten metal droplets was directly observed by high speed video camera system. These processes depend on the droplet property, and affect the microstructure of splats. Relationship between formation process and microstructure of splats was discussed. Abstract only; no full-text paper available.

Y 2 O 3 stabilized ZrO 2 (YSZ) coating is widely applied to the electrolyte layer in solid oxide fuel cell. With regard to the application of YSZ coating to solid oxide fuel cell, the electrical conductivity of YSZ coating is of essential importance. In this study, YSZ coating was deposited by plasma spraying using a sinter-crushed powders containing 4.5mol% Y 2 O 3 . The electrical conductivity of the coating at both directions parallel and perpendicular to the coating surfaces was measured by a direct current approach. The measurement was carried out at a temperature range from 600 °C to 1000 °C. The effect of spray distance on the electrical conductivity was also examined. The test showed that 4.5YSZ layer yielded the electrical conductivities of 2.2 s·m-1 and 4.3 s·m-1 at a temperature of 1000 °C in both direction perpendicular and parallel to the coating surface, respectively. It is clear that the electrical conductivity of the coating was lower than the identical sintered bulk material. The electrical conductivity of YSZ coatings in this study was of one-fifth to one-fourth in perpendicular direction and about one-half in parallel direction compared with the sintered YSZ bulk. It was considered that the lower electrical conductivity of the coating compared with the identical sintered bulk results from the lamellar structure with the limited interface bonding.

The lamellar microstructure of plasma-sprayed ZrO 2 -8wt.%Y 2 O 3 (8YSZ) coatings was quantitatively characterized by a novel approach. The porous structure of the coating was visualized through infiltrating the tracer into voids in the coating combining with the energy dispersed X-ray line analysis of the tracer in the coating. The quantitative characterization parameters included the mean lamellar interface bonding ratio, mean lamellar thickness and width distribution of lamellar interface gaps. 8YSZ coatings were deposited by plasma spraying under different spray distances to reveal the effect of spray distance on the lamellar microstructure of plasma-sprayed 8YSZ coatings. The results showed that the maximum mean bonding ratio was 32% when the coating was deposited at spray distance from 65 to 80 mm. As spray distance was increased to 100-120 mm, the mean bonding ratio was reduced. The mean lamellar thickness was not significantly influenced by spray distance although it was found that the mean lamellar thickness tended to increase with the increase of spray distance from 80 mm. It was observed that lamellar interface gaps in the coating were mainly in submicrometer order. With the increase of spray distance, the mean width of interface gaps and fraction of wide interface gaps tended to increase.

Microstructural analysis has played, and continues to play, a key role in the development of thermally sprayed coatings. The advances in equipment and consumable technology for evaluating coatings have enabled laboratory personnel to accurately observe the coating characteristics in a cost effective manner and with great confidence. Microstructural analysis has been important in the development of new thermally sprayed coatings used for aerospace, automotive, electronic, bio-technology, petroleum and other high and low tech applications. The coatings that are being developed consist of a combination of materials that vary in hardness and general microstructural characteristics. Accurate microstructural analysis is more important than ever because these new, complex coatings cannot be produced successfully without control of the microstructure. Established metallographic techniques have not always been able to produce accurate results. A resin bonded diamond surface was developed which reduces the damage created during the grinding and lapping of a variety of coating types while maintaining the flatness and integrity of the overall coating microstructure.

One type of multimodal (comprised of a mixture of nanosized and micronsized WC particles) and two types of conventional WC-Co powders were thermally sprayed by HVOF. Three types of HVOF torches, JP5000, DJ2600- hybrid (H 2 ) and DJ2700-hybrid (C 3 H 6 ), were employed to produce a total of 66 different WC-Co coatings. For all the coatings produced, the in-flight particle temperature and velocity during deposition, deposition efficiency, hardness and abrasion resistance were measured. These measured properties and characteristics were used to construct process maps (via inverse distance weighting) in order to (i) establish relationships between the various process parameters, properties and performance characteristics and (ii) identify process windows where “optimized” coatings could be produced. It was observed that the multimodal coatings exhibited a much larger processing window for the highest performance, i.e., these coatings demonstrated high abrasion resistance over a broad range of particle temperature and velocity, a characteristic not observed for the conventional coatings. The maps revealed other interesting differences between the multimodal and conventional coatings and among the coatings produced using the various torches. The study demonstrates the value of process maps in producing information for engineering optimized thermal spray coatings.

Indentation tests were provided with various applied loads and indentors’ profiles to determine the mechanical properties of selected HVOF coatings. The four well-known wear resistant coatings, WC-17%Co, Cr 3 C 2 -25%NiCr, NiCrSiB and AISI 316L, sprayed by TAFA JP-5000, were tested. Besides the common indentation tests (e.g. coatings surface hardness and microhardness), the indentation fracture toughness (IFT) and Young modulus of elasticity (E) were measured using Vickers and Knoops indentors profiles, resp. From the Vickers indentation results, the IFT was evaluated in terms of 18 models for KIc determination. The big differences between results of used models were found. For evaluation of E, the Knoop indentation in three different orientations was used. The measurement results show the anisotropic behavior of sprayed coatings. Based on the measured data, the relation between coatings properties with respect to the coating unique microstructure was proved.

Twin Wire Arc Spraying of cored wires for wear resistant applications is being increasingly used for industrial applications such as FBC boilers. Two promising cored wires namely Alpha 1800 and Armacor M were selected and sprayed by Twin Wire Arc Spray Technique. The sprayed samples were tested at different temperatures ranging from 3000C to 6000C in a High Temperature Erosion Test Rig using quartz as erodent. The samples were tested at 300 and 900 angles. The erosion behaviour of Alpha-1800 coating was compared with that of base metal Carbon Steel and Armacor M coating. The results showed that the erosion resistance of Alpha-1800 coating was superior than those of Carbon Steel base metal and Armacor M coating at all temperatures and at impact angles of 300 and 900. SEM studies of few erosion tested samples were also carried out to understand the morphology of the eroded regions. Abstract only; no full-text paper available.

The knowledge of thermal fluxes transferred from impinging high temperature jets is always important. For example, during thermal spray applications, the impinging jet acts directly on the transient surface temperature of the substrate. For preheating applications, it acts on the time required to reach the required surface temperature. It is also important to estimate it for some other applications like thermal cycling tests of parts for example. In the present paper, both CFD computations and experiments were carried out in order to quantify thermal fluxes from an impinging HVOF jet. The case of a CDS gun fueled by natural gas was considered. Stand-off distances ranging from 50 mm up to 300 mm were investigated. The model incorporates a 2-layer extension to k-å models for a better estimation of thermal exchanges through the viscous layer on the front body surface. The experiments were performed using a system incorporating two square calorimeters positioned one beside the other in order to attempt defining a radial profile. Abstract only; no full-text paper available.

The behaviour of plasma sprayed powder particles at impact is not adequately understood so that the microstructure of thermal barrier coatings cannot be predicted with the required accuracy. This is due to the complexity of the spraying process in which the shape of the impacting particles is influenced by the spraying conditions and the structure of the previously formed parts of the coating. In this paper we present some techniques to improve the understanding of coating formation. We investigate the impact of a single molten powder particle on a surface by performing splat tests, measuring the particle’s temperature, velocity, and diameter at the appropriate stand-off distance before the spraying process starts. The measured values are assigned to the splats. SEM-images of the splats are analysed by means of a image analysis algorithm. Thus we are able to evaluate the influence of particle properties on the splat shape. In order to get a better understanding of the mechanism of coating build-up, a three-dimensional series of cross sections is created. With this kind of examination the shape of splats can be characterised in three dimensions.

Nitrogen-alloyed steels have outstanding properties, but are rarely used in thermal spraying. In this study, high-nitrogen duplex and austenitic steels are sprayed using APS and HVOF techniques and the resulting layers are evaluated based on microstructure, composition, and corrosion and wear properties. HVOF layers outperformed the APS layers in corrosion and wear testing, with austenitic steel having the highest corrosion resistance, and duplex steel the highest wear resistance. Paper includes a German-language abstract.

This paper discusses the performance of arc-sprayed composite layers produced using aluminum oxide-filled wires. In tribological property testing, the composite layers show good non-slip behavior with a high coefficient of friction. They also exhibit excellent wear resistance, four to six times better than that of aluminum layers, owing to the presence of aluminum oxide particles dispersed throughout the metal matrix. Paper includes a German-language abstract.

In this investigation, binary FeAl layers are produced by detonation spraying and examined by means of microscopy and X-ray diffractometry. The examinations show that the layers have a lamellar structure consisting of FeAl and Fe 4 Al 13 phases with a small amount of alumina. It is observed that with increasing amounts of fuel gas and decreasing detonation frequency, the proportion of FeAl phases decreases and variations in microhardness throughout the coating become irregular. Paper includes a German-language abstract.

The goal of this work is to develop a flame sprayed NiCr-Cr 3 C 2 coating that resists oxidation, corrosion, and erosion in boiler environments and to assess the influence of Cr 3 C 2 fraction and sealing on coating performance. In the experiments, NiCr cermet alloys with different proportions of chromium carbide were applied to boiler steel substrates by means of subsonic velocity flame spraying and some of the coatings were sealed with a silicon resin. The paper describes the various tests performed and presents the results. Paper includes a German-language abstract.

This study investigates the relationship between splat morphology and adhesion strength in plasma sprayed coatings. The shape and size of the splats are examined and found to differ, in some cases significantly, depending on whether the splats are in the center or on the periphery of the spray pattern. Splats in the center of the pattern are formed by particles with higher temperature and velocity than those on the periphery and they tend to produce higher quality layers. In an experiment in which alumina layers are sprayed through a graphite slit filter, coating adhesion is shown to be superior, as expected. Paper includes a German-language abstract.