Thermal spraying involves high temperatures which can be a serious problem for some applications. Among different possibilities existing to cool down a specimen during spraying, the use of liquid CO 2 is one of the most attractive. However, care has to be taken to obtain good results due to the specific characteristics of CO 2 . Moreover, there are still some remaining problems which limit the growth of such system. One of the most important is the condensation of air humidity at the exit of the atomising nozzle. Therefore, a new design of the nozzle has been developed. This design avoid ice building up and can be easily fixed on any kind of spraying set up. Examples of working conditions are presented.

This paper examines ways to control the porosity of thermally sprayed deposits. All spraying was done with a water-stabilized plasma system using different combinations of alumina, zircon, Ni, and Al powders. Sandwiched structures with alternating ceramic and metal layers were sprayed as were thick deposits consisting of metal and ceramic mixtures. Porosity was characterized by methods such as gas permeability, water immersion, MIP, SEM, and SANS. In addition, several post-processing methods were tested to determine their effect on porosity volume. For example, removing metallic phases by leaching or by annealing at temperatures above the melting point was found to effectively increase porosity, while the use of sealing materials proved effective at reducing porosity. Another method tested was calcination, which resulted in an increase or decrease of porosity depending on the deposit's chemistry and annealing conditions.

Properties of MCrAlY coatings obtained by High Velocity Oxy-Fuel (HVOF) thermal spray process operated in a standard configuration were compared with those obtained using a gas shroud attachment to the HVOF gun. Our measurements show that the attached gas (nitrogen) shroud nozzle considerably reduces the oxygen content in the coating without an appreciable change in the microstructure. The particle temperatures were decreased by an average of 100 °C at a standoff distance of 0.275 m (11 inches). There was also a large reduction in the particle velocity at this distance. Both these effects were related to the excessive amount of nitrogen used for shrouding.

A new plasma spray process was developed for the rapid deposition of very dense electrolyte layers for solid oxide fuel cells (SOFCs). The dense yttria-stabilized zirconia (YSZ) film was prepared by a center-injection low pressure plasma spraying (CI-VPS) process on various substrates in a triple-torch reactor. For deposition on porous substrates, an intermediate layer was applied using conventional atmospheric plasma spraying (APS) to close the large pores in the substrate. The films were characterized by XRD, SEM, and EMPA. The porosity of the film was analyzed by computerized image analysis of the micrographs. The film was also fractured by four-point bending to characterize the nature of bonding of layer-to-layer and within the deposit. The film analysis showed that YSZ layers with porosities of 0.3 % could be obtained at very high deposition rates with the CI-VPS process, with a very good functional performance of the layer as an electrolyte. Building of a complete SOFC by successive deposition of an atmospheric pressure sprayed porous cermet film, the dense YSZ electrolyte layer, and a porous perovskite film is discussed.

Reactive plasma spraying (RPS) with hydrocarbon (HC) gas has been studied as a way to improve the mechanical properties of Ni-Cr alloys and reduce the oxygen content of MCrAlY coatings. A conventional dc plasma torch has been modified by attaching a conical graphite tube (reactor) onto the end of the gun and spraying parameters were adjusted accordingly. Significant improvements have been achieved in terms of both objectives. As test results indicate, the hardness of Ni-Cr alloys has been doubled and the oxygen content in MCrAlY coatings has been reduced by an order of magnitude.

Thermal spray techniques can fulfill numerous industrial applications. Coatings are hence applied to resist against wear, corrosion, or to modify the surface characteristics of the substrate (e.g., conductivity, etc.). However, many of these applications remain inhibited by some deposit characteristics, such as a limited coating adhesion or pores, or by industrial costs since several non-synchronized steps (i.e., degreasing, sand-blasting and spraying) are needed to manufacture a deposit. The Protal process was designed to reduce the aforementioned difficulties by implementing simultaneously a Q-switched laser and a thermal spray torch. The laser irradiation is aimed to eliminate the contamination films and oxide layers, to generate a surface state enhancing the deposit adhesion and to limit the contamination of the deposited layers by condensed vapors. From Protal arises the possibility to reduce, indeed suppress, the preliminary steps of degreasing and sand-blasting. In addition, in some cases, a significant increase in the deposit adhesion versus standard preparation, a decrease of the porosity level and the increase of the deposit cohesion represent important additional effects of the process.

The paper presents the influence of laser-beam remelting of Al 2O3 -Ni ceramic layers spread on creep-resisting alloy by means of plasma spray upon the quality of ceramic coatings, which form thermal barrier and high temperature corrosion shield of these alloys. The examination showed that 0.103.10 9 W/m2 power density laser-beam scanning ensures good quality of coatings at beam moving rate 1 to 2 m/min. At smaller scanning rate, laser remelted ceramic layer spalls and chips. Better quality of the ceramic coatings can be obtained by diffusive chromoaluminizing applied prior to laser remelting. Laser-beam remelting conditions of such layers, elaborated in the course of examination, ensure pores fading, decrease of remelted layer thickness and increase of base adhesion without cracks, chips and spalls. Additionally remelted zone features either strong break up of structure or the structure is amorphic with unchanged chemical composition as to the matrix chemical composition. Obtained results are the base for the elaboration of thermal barrier technology and the technology of anticorrosion shield for creep - resisting alloys applied in high temperature power engineering.

In the article the results of set of experiments concerning the changes in microstructure of plasma sprayed Al 2 O 3 , Cr 2 O 3 and W 2 C coatings of tribological test pieces obtained with use of locally applied laser beam treatment are given and discussed. The improvement of sprayed on coating layer adhesion to the steel base was ascertained and evaluated. The possibilities of the application of the described technology for development and production of slide elements of tribological joints are noticed.

This paper explains how laser-beam remelting influences the quality of ceramic coatings on superalloy substrates. The coatings studied consist of Al2O3-Ni ceramic layers. Test results showed that a laser power density of 0.103 x 109 W/m2 is ideal for a beam rate of 1 to 2 m/min. It was also found that coating quality could be further improved by adding a diffusive Cr-Al layer prior to laser remelting. This added step reduces pore volume and layer thickness and increases adhesion without cracks, chips, or spalls. The examination also showed that, depending on cooling rate, it is possible to obtain layers that are similar in composition but different in structure or even amorphic.

Hydroxyapatite (HA) was plasma sprayed onto titanium alloy substrates. One set of samples had a crystalline content of 25%, the other set, 30%. The coatings were then subjected to CO2 laser treatments at different power levels and scanning rates while surface temperatures ware monitored with a pyrometer. XRD analysis verified the crystallinity of HA and the content of amorphous calcium phosphate as well as foreign phases. Optical microscopy revealed the microstructure and depth of the laser modified zone.

This paper examines the properties of thermal spray coatings produced by electrothermal explosion. Three refractory carbides (TaC, WC, and TiC) were sprayed onto metal substrates then analyzed using FESEM, EPMA, XRD, and microhardness testing. The new spray technique required the development of a special powder container to facilitate melting as well as the jetting process and mixing of deposit and substrate materials. Crystal structure and phase changes were also observed, which are attributed to decarburization.

The Directed Light Fabrication (DLF) process uses a laser beam and metal powder, fed into the laser focal zone, to produce free-standing metal components that are fully dense and have structural properties equivalent to conventional metal forming processes. The motion of the laser focal zone is precisely controlled by a motion path produced from a 3-dimensional solid model of a desired component. The motion path commands move the focal zone of the laser such that all solid areas of the part are deposited and the part can be built (deposited) in its entirety to near net shape, typically within +/-0.13mm. The process is applicable to any metal or intermetallic. Full density and mechanical properties equivalent to conventionally processed material are achieved.

Perovskite-type LaMnO 3 powders and coatings have been prepared by a novel technique, the reactive suspension plasma spraying (SPS) using an inductively coupled plasma of about 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO 2 powder in a saturated ethanol solution of LaCl 3 with a 1:1 molar ratio of La and Mn. Low reactor pressure was helpful in order to diminish the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70-90%. After a post-treatment with an 80% oxygen plasma an almost pure LaMnO 3 deposit was achieved in the center of the incident plasma jet.

This paper presents the results of a study of the morphology of alumina splats deposited on stainless steel and alumina substrates. The substrates were either plasma sprayed or coated via plasma enhanced CVD. Substrates that were plasma sprayed were annealed if necessary to get specific phase structures, then polished to around 0.4 μm (Ra). CVD-coated substrates with an Ra ~6 nm and a columnar amorphous structure were sprayed as deposited. Splat studies show that the crystal structure of alumina substrates and the release of entrapped gas have a major influence on splat formation. For plasma sprayed coatings, disk-shaped splats with excellent adhesion properties were obtained on hot γ alumina, while on α alumina, splat shape and morphology were irregular and adhesion very poor. The effect of entrapped gas, on the other hand, can be seen in the splats that formed on the CVD-coated substrates. These splats were very porous and, in many, most of alumina flowed out to rim. As the paper explains, this is the result of gas release upon impact of molten particles, which reduces wettability and thermal contact between the splat and substrate.

Dc plasma torches typically use a mixture of inert and molecular gases when spraying high melting powders. The addition of molecular gases increases the enthalpy of plasma jets, but it also produces arc root fluctuations that can cause variations in injected powders. This paper describes an innovative plasma torch system characterized by a long nozzle and three parallel cathodes. The nozzle consists of several electrically insulated rings and a ring-shaped anode. By adding more rings, the arc gap and voltage can be increased along with the enthalpy of the plasma jet. The results of various tests, comparing the spray rates and deposition efficiencies of new and conventional torches, are presented in the paper.

The objective of this study is to develop viable surface-treatment technologies for offshore and underwater structures. The study focused on cavity formation, which is a prerequisite to underwater plasma spraying using a fluid-stabilized local dry-wet method. Factors affecting cavity formation that were investigated in the study include slit angles on protection tubes, jet velocity of water curtains, and clearances between protection tubes and plate surfaces. All plasma-sprayed coatings formed underwater were made with pure Ti wire and then assessed to determine the influence of various factors on cavity formation. Macroscopic examination of the coatings revealed neither cracks nor peelings. X-ray analysis confirmed that coatings consisted mostly of Ti phase.

Duplex processes consist in applying a treatment, like annealing, carburizing, or nitriding for example, to a plasma sprayed coating in order to improve particular physical characteristics of this coating. Trials of vacuum heat treatment or low pressure carburizing have been performed on APS NiCr coatings, previously sprayed on a steel type 16MnCr5. As a result, densification and adherence of the coating on the substrate have been significantly increased. To a lesser extent, the coating hardness has also gone up. The diffusion of chemical elements have been highlighted by X-ray analysis, showing the creation of a physicochemical bonding at the interface zone.

Current HVOF systems available on the world market are relatively complicated to operate and expensive. At the Bauman State Technical University in Moscow (BMSTU) a new portable HVOF spraying and cutting system MiniJet-30 has been developed. This rugged system offers the ease of operation of conventional gas welding equipment and produces high quality HVOF coatings. In this paper, the results of numerical simulation calculations of supersonic gas/powder flows, with and without a particle mask, are presented and compared with experimental outcomes. Additionally, typical HVOF coatings were sprayed and evaluated for bond strength, porosity, microhardness, abrasive wear, and phase composition (OM, XRD). The results are compared to those of other HVOF systems. Lastly, the cutting efficiencies for stainless steel, cast iron, and non-ferrous metals are discussed.

This paper summarizes the results of several studies on the velocity of powder particles during plasma spraying. It presents and compares experimental data collected by numerous authors for a wide range of materials. An analysis of the data shows how plasma jets are affected by stream velocity, process parameters, and the density and size of powder particles. It also sheds light on the maximum velocity that particles can attain in different plasma streams.

A composite was obtained by spraying ferromagnetic powder with a plasma arc torch onto an aluminum substrate. The influences of spraying conditions (substrate temperature, plasma gas composition, powder granulometry) on the efficiency of the induction heating by the composite are investigated.