Operating parameters which affect the droplet generation characteristics for a single wire arc spray, SWAS, torch have been studied. The droplet size distribution, velocity, axial divergence, and deposition efficiency can be controlled by the selection of wire size, wire feed rate, arc voltage, and gas jet back pressure. Coatings have been formed under a variety of operating conditions with structures from 3 mm to 4.5 mm in width. The average deposition efficiency is greater than 80 % and the average axial divergence angle is 2.4°. The SWAS torch has been shown to operate for extended periods of time with minimal erosion of non-consumable electrode.

The high velocity combustion wire (HVCW) sprayed coatings have unique structures and properties which are different from the conventional wire / powder HVOF coatings. This paper studies the coatings of 0.8% C Steel formed by the HVCW system. 0.8% C Steel coatings formed by the HVCW system were studied for their wear resistance (pin on desk wear test) and phase composition. Methods like SEM with Wavelength Dispersion Spectroscopy (WDS) attachment were utilized for determining the composition of the coatings. Microhardness and tensile bond strength of the coatings were also ascertained. A set of conventional oxy-acetylene wire flame spray coatings of 0.8% C Steel were also prepared and these coatings were then compared with the HVCW coatings. Possible applications of the HVCW coatings are discussed based upon the properties of the coatings.

Thermal spray processes using wires as feedstock are widely used to produce wear and corrosion protective coatings of nickel, cobalt or iron based alloys. In general, these coatings are processed by flame or arc spraying. In view of using massive wires as spraying material, the hardness and wear resistance of layers is limited by the possibility to produce the corresponding wires of such materials. In addition, the performance of wire sprayed coatings can be restrained by the amount of defects in the microstructure, like pores, oxides and cracks, which are particularly evident in the cases of flame and arc spraying. New High Velocity Combustion Wire (HVCW) systems open the opportunity to reduce the amount and size of the defects by an increased particle velocity. Also, improvements on wear resistance may be achieved by using cored wires. The paper gives an overview on recent developments in HVCW spraying using massive and cored wires.

The paper presents the basic principle and application of High Velocity Arc Spraying technique. Through adopting convergent-divergent nozzle, air circulation cooling and computer aided design technique, particle velocity was enhanced, which surpasses velocity of sound, and atomization effect was improved, so HVAS fabricated high quality coating. Experiment results show that HVAS has high particle velocity, high atomization effect, high bond strength and low porosity. Average velocity of atomized Aluminum particle is 373m/s. Bond strength, porosity and average particle size of 3Cr13 coating are 60MPa, 0.9% and 4.32 m respectively. For high coating quality, HVAS is widely used in maintenance, corrosion resistance and surface strengthening.

By means of Schlieren photography, enthalpy probe, mass spectrometry and the particle measuring system DPV 2000 the influence of the internal and external anode nozzle and torch geometry, on plasma jet quality for atmospheric plasma spraying was investigated. It turned out that there is a strong geometrical effect of the inner contour and that with a proper expansion of the hot core of the plasma jet a considerable improvement of the melting and deposition quality can be obtained. Also the outer torch contour is of influence on the spray process because it controls the formation and the intensity of turbulence and the interaction of the plasma jet with its surrounding and hence the cold gas entrainment.

Thermal plasma spray processes with their various operating parameters can be considered as flexible technique to carry out appropriate ceramics coatings. This work deals with plasma spraying of several ceramics powders (hydroxyapatite (HA), Al 2 O 3 -TiO 2 , Al 2 O 3 , ZrO 2 -Y 2 O 3 (YSZ) and Cr 2 O 3 ) with suitable parameters using a CAPS system ("Controlled Atmosphere Plasma Spraying"). The HPPS (High Pressure Plasma Spraying), APS (Air Plasma Spraying) and IPS (Inert Plasma Spraying) modes were applied in order to obtain the suitable microstructure. The microstructures and phase compositions allowed to establish that surrounding high-pressure in the CAPS chamber is leading to a good heating of the powder and a good quality for the coatings.

Materials in the system Si-C-N feature excellent properties for wear and corrosion protection applications even at elevated temperatures and an excellent thermal shock resistance among the ceramics. As these materials have no melting point, they cannot be processed purely by conventional spraying techinques, but need to be synthesized. Plasmajet CVD processes with single and triple DC torches and HF torches with supersonic nozzles have successfully been applied to produce Si-C(-N) coatings on different steel, aluminum, titanium and copper alloys as well as on graphite. Various liquid single precursors with suitable structure have been tested and evaluated with regard to the morphology and structure of the produced coatings. The processes are compared taking into account their characteristics concerning the injection modes, gas temperature and velocity profiles. Emission spectroscopy is used to determine the mechanisms of the coating formation. Guidelines for the optimum production of Si-C(-N) coatings by Plasmajet CVD are deducted.

The properties of thermal sprayed coatings depend mainly on the thermal and kinetic energy of the spray particles. Increase of thermal energy of sprayed particles can be realized using exothermic reactions between components in sprayed particles. Self propagating high temperature synthesis (SHS) is especially suitable to benefit from released energy in the spraying process. At present most commonly used spray material with exothermal reaction is Ni+Al. However, the highest amount of heat is produced in the reactions of aluminium and metal oxides. Of special interest are Cr 2 O 3 , NiO, CuO and V 2 O 5 because they obtain high reaction energies. Furthermore products of the reaction are of special, functional interest like NiAl as bonding agent or alumina as a wear resistant coating. To assure good contact between reacting substances (Al/Oxides) powders for plasma spraying were prepared by mechanical alloying. Calorimetric investigations of plasma sprayed coatings prove that during spraying Al reacts exothermically with oxides. Increase of oxide contents improves coating adhesion/ cohesion properties, hardness, and reduction of porosity. Results are discussed on the base of light microscopy, scanning electron microscopy (SEM) and X-ray structure analysis (XRD).

In the present work thin Al 2 O 3 coating was obtained mainly by low pressure plasma spray (LPPS). Low-porosity and low-roughness deposits resulted from the optimized spray conditions, i.e. plasma parameters, grit blasting, powder feed rate and specimen rotation speed. Results showed that LPPS processing was highly beneficial for densifying the ceramic coatings, especially when coupled to a moderate powder feed rate. Coating average surface roughness (Ra) ranged from 1.5 to 2.5 µm for a coating thickness of less than 30µm and an original substrate Ra of 1.1µm. The spray conditions were optimized particularly for a low feed rate and a high specimen rotating speed to lower surface roughness. Moreover, a specific atmosphere/temperature control device was developed (using local gas injection close to the specimen to be coated). This resulted in improving cooling efficiency, which reduced microcracking in the deposits. Mechanical pull-off adhesion test was also carried out to evaluate these low-roughness thin coatings. Adhesion was shown to be satisfactory for direct coating (i.e. without any bond coat) of a low-roughness (Ra=1.1µm) AISI 316L substrate.

Ni powders prepared by mechanical milling under liquid nitrogen for 15 hr were sprayed using two stoichiometric ratios of the oxygen-fuel mixture in an effort to promote the formation of fine oxide phases. The oxide phases were introduced in an effort to improve mechanical properties and thermal stability of the coatings, via chemical reaction between oxygen and milled powders during flight and after impingement. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy, transmission electron microscopy and nanoindentation. The average grain size of the milled powders was 15.7 ± 5.1 run and ultrafine NiO and Ni 3 N particles with a size less than 5 run were distributed in the milled powders. These fine oxide and particles distributed in the powders were formed as a result of interaction between Ni, N from the milling slurry, and O from the surrounding environment under the energetic milling conditions. The coating microstructure was composed of nanocrystalline grains with an average grain size of 92.5 + 41.6 nm and extremely fine NiO particles of ~5 nm distributed homogeneously inside the grains. Ni 3 N phase was not found in the coating as it appears to have decomposed during HVOF thermal spraying. The coating sprayed with higher oxygen fraction in a hydrogen-oxygen mixture showed no significant increase in hardness and elastic modulus when compared to those of the coating sprayed with lower oxygen fraction in hydrogen-oxygen mixture. This was attributed to the small difference in the volume fraction of NiO particles between the coatings. These results indicate that new techniques of ultrafine dispersoid introduction in nanocrystalline coatings are potentially attractive as a means to improve the mechanical properties of the coating through reactive HVOF spraying.

HVOF spraying process is widely used to improve component life in service due to the high bond strength of the coatings, which is a result of the high particle velocity upon impact, and consequent low coating porosity. However, many parameters can affect metallic coatings properties, especially unmelted particles and oxidation level. Flame parameters, such as calorific power, combustion ratio and temperature, are of prime importance. Moreover, the fuel gas employed in this spraying process can lead to various coating properties and deposition efficiency. The aim of this work was focused on the influence of some fuel gases, namely propane, propylene (LPG) and hydrogen, on stainless steel coating characteristics. A specific domain common for those three gases was determined in order to effectively compare those gases with the same flame parameters. Flame characteristics were computed using a simple model for all the fuel gases considered. Temperature as well as calorific power were fixed. For different substrate temperatures, obtained through a special CO 2 cooling nozzle system, richness was varied from 1.4 to 1.6. Microstructure investigation as well as oxide content and microhardness measurements were conducted. For the same kinetic torch parameters, thickness-per pass gave an idea of the deposition efficiency. In the range studied, deposits properties were quite similar for both LPG fuel gases. Hydrogen led to better characteristics in term of oxide content, although its deposition efficiency was a bit lower. A general law was established to link oxide content within the coatings to the flame parameters. A reasonable regression analysis was obtained for all the coatings sprayed. The combination of cooling efficiency (i.e. CO 2 flow rate) and flame characteristics (i.e. interaction of the particle in flight) led to a good correlation. These correlations were further verified by spraying another metallic powder, namely Inconel 625.

Oxidation of HVOF sprayed 316L stainless steel coatings was studied experimentally. Oxygen content in the sprayed coatings was analyzed and its dependence on several spray parameters such as spraying distance, mixture ratio of fuel to oxygen, and composition of atmospheric gas on the substrate was studied. The oxygen content in the original powder was about 0.03 wt%, which typically increased to 0.3 % in the HVOF sprayed coatings under the standard spraying conditions. Reduction of spray distance significantly increased the oxygen level due to the excessive heating of substrates by the flame. The sprayed deposits were analyzed by XRD and the oxides within the coatings were identified as magnetite Fe 3 O 4 or chromite FeCr 2 O 4 . By using a nitrogen-gas shield attached to the substrate, it was revealed that the oxidation during flight is around 0.2 wt%. Control of oxidation by attaching a gas shroud to the HVOF nozzle has been attempted and oxygen content below 0.15 % has been achieved so far while maintaining deposition efficiency over 73 %.

Thermal spray usually involves high temperatures in short times. This phenomenon is enhanced with the use of an HPPS system (Plazjet from Tafa). To spray a Zirconia coating, high powers are necessary such as 200 kW providing for the substrate an important increase in the surface temperature. Furthermore, the good quality of a thermal spray coating results in the low surface roughness, a satisfactory microstructure cohesion, a minimal distortion of the part when spraying on large sheets with a thick thermal barrier coating. Cooling with compressed air is widely used but other solutions are studied concurrently; CO 2 , nitrogen, and air; these gas are tested in different conditions of pressure and flow. Some improvements in terms of temperature in the coating and roughness have been found.

Advanced spraying technologies as High Pressure-HVOF Spraying or Cold Gas Spraying as well as new concepts for the next generation of plasma sprayed TBC's demanding spray powders with strongly decreased powder grain sizes (< 15µm or even less). Although, with decreasing powder grain size the flowability of the powder is also decreased, causing finally problems in the transport of powder. Within this paper a new powder feeder design will be presented, that makes the precise feeding of ultrafine powders possible, even through feeding lines several meters in length. All known powder feeders for thermal spraying use the pneumatic convey for powder transport through the feeding line. In opposite, the new developed powder feeder is using the dense phase convey of powders. The powder transport can be compared with the transport of a liquid in an hydraulic system. The new powder feeder shows no restrictions in the flowability of a powder. The feeding of normal spray powders (carbides, oxides, metals) as well as submicron powders were successfully tested. Feeding rates from several grams per hour to hundred kilogram and more per hour can be realised, depending on the design of the system. The transport through feeding lines longer than 10 meters also causes no difficulties. The system is self-cleaning and there are no mechanical parts in contact with the powder.

Arc spraying has always been the most cost-effective way of thermal spraying metal alloys but oxide content and degradation of the alloy by loss of particular alloying elements have limited quality of the coatings. In this paper a process is described which greatly reduces degradation and improves coating density and oxide content. Corrosion behaviour both in aqueous and high temperature environments is markedly improved. For aqueous applications coatings of Inconel 625 were tested in a potentio-dynamic cell and by salt spray testing to evaluate both the inherent properties and the permeability of the coating and significantly improved behaviour was found in both cases. For high temperature corrosion, samples of FeCrAl were tested in air and in a sulphidising environment with and without thermal cycling. Coatings of NiCrAl and NiCrTi were also examined. The coating types and test regimes were aimed at specific practical applications such as fireside corrosion in boilers and waste incinerators, hot oxidation of flare stack burners etc. In the aqueous situation valves and process vessels are being examined as candidate applications for Inconel 625 coatings.

Excepted in a few cases where metallurgical bonding occurs between deposit and substrate, thermal spray deposit adhesion generally results from a mechanical anchoring. In this case, the very first impinging particles forming the first deposited layer spread and solidify into and around the cavities of the grit-blasted surface. A palliative process to degreasing and grit-blasting prior to thermal spraying is simultaneous laser ablation; i.e., the PROTAL process. In such a case, little topographic change results from the laser-matter interaction: deposit adhesion does not derive anymore mainly from mechanical anchoring but from other types of bonding such as chemical bonding. This paper aims to clarify the bonding mechanisms of thermal spray coatings manufactured implementing the PROTAL process. The case of metallic coatings deposited on metallic substrates is especially discussed. At first, laser ablation effects on various metallic substrates are presented, from the topographic and energetic points of view. Then, the induced effects on impinged particle morphologies are discussed. The results are correlated to thick deposit adhesion.

In the aerospace field as well as in the stationary gas turbine field, thermal sprayed coatings are used to improve the surface properties of Nickel-super-alloys materials. Coatings are commonly used as bond coat and antioxidation materials (mainly MCrAlY alloys) and as thermal barrier coatings (mainly Yttria partially stabilized Zirconia) In the present study, our purpose was to assess the properties of thermally sprayed bond coat CoNiCrAlY comparing the performance of three different techniques: Vacuum Plasma Spray (VPS), High Velocity Oxygen Flame (HVOF) and Axial Plasma Spray (AxPS). The quality of the deposited films has been assessed and compared from the point of view of structural (porosity, oxide concentration, unmelted particles presence) and mechanical characteristics (hardness, adhesion). Furthermore, a study of the surface composition and morphology has been carried out. Specific efficiency trial has been carried out to compare the efficiency of the three examined technologies. We observed that the highest quality films are obtained by VPS, but that also HVOF and AxPS sprayed films have interesting properties which can make their use interesting for some applications in view of the lower cost of HVOF and AxPS.