Three kinds of nanostructured alumina coatings were prepared by air plasma spraying. As-sprayed coatings were characterized by SEM and XRD. Their surface roughness, porosity and microhardness were also examined. Results indicated that the smaller the size of the starting powders, the better the properties of the coating. The differences in the phase composition of the as-sprayed coatings could be used to explain the differences observed in coating properties.

TiO 2 is the most promising photocatalyst to decompose industrial pollutants in water and air. Compared with TiO 2 powder, the immobilized form is more suitable for application. In the present paper, the nanostructured TiO 2 photocatalyst is formed by thermal spraying with liquid feedstock. The microstructure of the nanostructured TiO 2 is characterized by infrared spectrum, X-ray diffraction, scanning electron microscope, and transmission electron microscope. The photocatalytic performance is characterized with photodegradation of phenol in aqueous solution. The effects of annealing treatment on the crystalline structure of the TiO 2 deposit and the photocatalytic performance are investigated. The results showed that TiO 2 photocatalyst deposited by thermal spraying with liquid feedstock presents nanostructure and anatase crystalline structure mainly. The crystalline structure of TiO 2 deposit can be controlled by post annealing treatment. The as-sprayed TiO 2 coating can decompose phenol in water with the presence of H 2 O 2 and UV light. Phenol can be destroyed easily while the complete mineralization takes a little more time. Annealing treatment can improve the photocatalytic performance of TiO 2 coating. The present results revealed that the TiO 2 coating in rutile structure presents a better photocatalytic performance compared with as-sprayed TiO 2 coating in anatase structure.

A new class of composite powders for thermal spray on the base of recycled hardmetal powder is proposed. WC-Co hardmetal powder that is either alloyed/mixed, coated, agglomerated or cladded with binder metal - cobalt enhances the technological properties of spraying powders and enables the producing of sprayed composite coatings with double-cemented structure. The manufacturing technology of WCCo/Co composite powders composed of mixing, agglomerating, sintering and crushing was studied and characteristics of the produced spray powders by particle size, morphology of particles and specific surface area were evaluated. The experiments for deposition of coatings by high velocity spraying processes (HVS), detonation gun (DG) spraying and High Velocity Oxy-Fuel (HVOF) spraying were carried out. Comparison abrasive erosion wear tests of sprayed coatings were performed.

A perovskite suspension was air plasma sprayed (APS) with a direct current (dc) plasma jet. The process conditions including the standoff distance, plasma forming gases and their ratio influence on the perovskite deposits and splats characteristics were investigated. Particularly the molten state of particles, phases obtained, morphology and composition of splats were studied. The process consists in injecting a well dispersed stable suspension of submicrometric particles. The suspension was produced by attrition milling LaMnO 3 perovskite powder (mean particle size of 0.8 µm). The plasma forming gases influences drastically the obtained phases in deposits. When the plasma gas mixture contains hydrogen the heat transfer is too high and the perovskite phase is decomposed but when the plasma forming gases is Ar/He or only Ar, just a very little quantity of perovskite is found to be decomposed in the plasma jet. With Ar/He, perovskite splats are well melted, but with argon there is a high percentage of semi-melted and non-melted particles.

HVOF-sprayed coatings have become increasingly important for many different industries. Because of this, the ability to adjust coating process parameters according to a customer’s demands becomes a necessity. Several studies have been done on various powders, spray parameters, and the resulting coating properties such as coating hardness, wear resistance, and internal stress. On the other hand few studies have been published regarding post-treatment and finishing processes. This study is intended to investigate the entire production process, from thermal spraying to the ready-to-use finished layer; the primary points of consideration were hardness, surface quality (roughness), and resulting microstructure. The scope of this study is to present initial results of cylindrical grinding as a finishing process. Several different tungsten-carbide-based coatings of various compositions and particles sizes were produced by thermal spraying under varying kerosene-oxygen flow, spray distance, and powder feed rates. Machining parameters such as speed of workpiece rotation and longitudinal feed rate were tested as well.

In order to obtain nano-structured ceramics composite coating for high temperature application, pre-mixed Al 2 O 3 /Y 2 O 3 powders were plasma-sprayed in this study. Plasma spraying of spray-dried Al 2 O 3 /Y 2 O 3 powder resulted in the formation of amorphous coating of metastable Al 2 O 3 -Y 2 O 3 solid solution. After the heat treatment, α-Al 2 O 3 /YAG nano-structured composite coating was successfully obtained via eutectic reaction between Al 2 O 3 and Y 2 O 3 . It was possible to control the sizes of Al 2 O 3 and YAG particles widely by heat treatment with proper condition. Hardness of the coatings showed close relationship with their microstructure.

Stainless steel particles have been covered with an alumina shell by the mechanofusion process in order to reinforce stainless steel coatings by uniformly distributed alumina particles. Two stainless steel particle size distributions (PSD) in the range of 65 µm and 120 µm were tested. It was found that the mechanical energy input induced a spherical shape of the final composite particles with a controlled shell thickness (3 µm and 2 µm respectively) without forming new phases that usually take place during the mechanofusion process. The new spherically-shaped composite particles were sprayed in air with a D.C. plasma torch working with an Ar/H 2 mixture as plasma forming gas. At mid-flight, two types of composite particles were detected : the first case corresponded to well molten particles where all the alumina shell has flowed to the tail of the particle ; the second case was related to particles which still retained some evidence of the alumina shell uniformly distributed around the stainless steel core. When the mechanofused particles were sprayed onto a cold smooth substrate (stainless steel 316L, Ra<0.05 µm), the resulting splats were extensively fingered and became disk shaped when the substrate surface was preheated over 300°C. However, alumina was either spread exactly on the stainless steel splat corresponding to well molten particles or dispersed in fingers and frozen over the surface of the stainless steel splat corresponding to particles covered by the broken alumina shell. An important effect of fine particle size on in-flight droplet behavior is detected because the center of gravity is more decentred than that of coarse particles influencing the deposit build-up. The composite stainless steel/alumina coatings sprayed on a rough stainless steel substrate (Ra = 6.7 ± 0.3 µm) preheated to 200 °C are compared to those of pure stainless steel. Hardness and adhesion/cohesion of deposits formed with fine particles were found to be improved comparatively to a pure stainless steel deposit. However, when coarse particles are used, the value of hardness is decreased and works is in progress to understand this phenomenon.

The effect of methods and conditions of thermal spraying on structure and phase composition of coatings of the Al-Cu-Fe system alloy powders has been investigated. It is shown that the maximum ø-phase content of the coatings can be produced by the method of air-gas plasma spraying. In this case the thermal spray coatings inherit the multi-phase nature of initial powders. Preheating of the substrate prior to spraying allows the ø-phase content of the coatings to be increased.

Silicon nitride and sialons are very attractive materials for thermal spaying, but the high temperatures of spray processes lead to their decomposition instead of melting. Therefore, the use of these materials as protective coatings has been very restricted. Nevertheless, researchers have tried to provide silicon nitride-based coatings using metallic or oxide binders. Oxide binder additions to silicon nitride have been quite successful. In this paper, mixtures of silicon nitride and oxides were prepared for the thermal spraying of silicon nitride-based materials by using a detonation gun. Powders for the spraying were prepared through mixing, sintering, crushing and sieving. To get an oxide binder of low melting point, three components of oxides, Al 2 O 3 -ZrO 2 -TiO 2 , were selected; the ratio of oxides was determined to have a low melting point. When the sintering temperatures were below 1400°C, phases of the powders and coating layers were composed of α-Si 3 N 4 and oxides and any of sialon phases were not found. By sintering at the temperatures between 1400 and 1600°C in a nitrogen gas environment, χ(chi)-sialon (Si 6 Al 10 O 21 N 4 ) and β’-sialon (Si 3 Al 3 O 3 N 5 ) were formed. The ratio of β’-sialon increased as the sintering temperature increased. TiO 2 was transformed to a nitride, TiN. During the spraying procedure χ-sialon was decomposed to amorphous binder, but β’-sialon was not totally decomposed. Finally a coating layer composed of tetragonal-zirconia and β’-sialon was made.

The goal of this work is to have standardized quality control of thermal spray coatings incorporated into the Standard Practices Manuals of all aircraft engine original equipment manufacturer’s around the world. With the many manufacturers’ criteria for evaluation of thermal spray materials, laboratories are forced to have multiple criteria/systems to test and analyze coatings. As airlines/repair shops are moving towards overhaul of more variable engine types/models in their shops, the need to have a common evaluation system has been identified and is currently being addressed. A sub-committee of the European Airline Committee for Materials Technology is currently working to formulate this common system for thermal spray coatings evaluation. Participants are from GEAE, Pratt & Whitney, Rolls Royce, SNECMA and KLM Royal Dutch Airlines. The progress and goals of this group will be documented and reported.

The age-old problem of fixing a flaw in an airfoil of a vane or bucket that is otherwise serviceable, has been resolved by the careful blending of two known technologies, HVOF (High Velocity Oxy Fuel) and HIP (Hot Isostatic Pressing). This recently patented process facilitates the successful replacement of parent material as to allow a component which would otherwise be taken out of service to be reused in a “like new” condition. HVOF spraying of an airfoil employs the same material as the parent part. This process by itself will not provide sufficient adhesion to cause a complete marriage of materials. HIP’ing completes the union. Resulting “RECAST” repaired components display the same mechanical and physical properties as the parent material.

Abradable seals are used in compressors of aircraft and industrial gas turbines to decrease clearance between the stator casing and rotor blade tips and hence to increase compressor efficiency and decrease fuel consumption. The main interest of abradable materials producers has been concentrated on abradable seals for aircraft engines, and special requirements of industrial gas turbine manufacturers have not been met so far. The most significant requirement in industrial gas turbines is durability. This is driven by the need for several times longer periods between overhauls in industrial gas turbines compared to aircraft engines. Westaim Ambeon has developed a new composite powder, Durabrade2413, that meets these requirements. The new abradable seals fabricated by using this powder have been extensively tested over a prolonged period of time. This paper will present the results of an intensive development, evaluation and abradability testing of seal properties. This paper will also show that Durabrade2413 series coating properties can be altered in a broad range by changing spray parameters to tailor the coating to a particular application. The abradable seals are suitable to rub against steel and Ni alloy blades. The abradability results of Durabrade2413 are compared to Durabrade2222 (the Metco 307-NS equivalent), the well known 75Ni25 Graphite abradable that has been on the market for the last 30 years.

High temperature oxidation behavior of MCrAlY coatings was studied at several temperatures in the range from 800 to 1100°C. In this study the MCrAlY coatings were obtained by plating using CrAlY as precursor powders in an electrolytic bath containing nickel and cobalt salt in solution. The size of the precursor CrAlY powders used was generally below 10 um. As-plated coatings consisted of a random distribution of CrAlY particles in the Ni-Co matrix. The heat-treatment of the as-plated coatings at elevated temperature resulted in the development of a gamma and beta structure. Both as-deposited and oxidized coatings were characterized by optical, scanning electron microscope and electron beam microprobe. During oxidation the coatings formed alumina scale with a negligible amount of transient nickel and chromium oxides. The spallation resistance of the oxide scale was investigated by thermal shock testing. The test consisted of a rapid cooling from 1000°C to 100°C with a two- minute dwell time at the maximum temperature. The thermal shock test was conducted in a) as–deposited and heat-treated condition and b) after preoxidation at 800°C and 1050°C, respectively. The coatings retained the alumina scale during thermal shock cycling.

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.