This paper describes a study of a plasma spray process to deposit ceramic coatings with finer structures than those that can be produced by conventional DC plasma spraying. The structures are similar to those produced by plasma-enhanced chemical vapor deposition (PECVD), but with a very fast coating rate and the ability to produce a wide range of thicknesses (1 < e < 100 µm). The process involves injecting a ceramic suspension containing submicronic ceramic particles into a DC plasma jet. The goal is to develop the process so that it can be used for the production of such coatings as yttria stabilized zirconia (YSZ), Ni-YSZ, or perovskite (for solid oxide fuel cells) or YSZ, Al 2 O 3 -YSZ, or Pyrochlore (for thermal barrier applications). The research stage described in this paper is related to the study of the growth of YSZ coatings, to calculations of the dynamic and thermal plasma-particle transfers, linked to the effect on particle flattening of the various spray parameters (arc current intensity, particle collection distance, substrate temperature, plasma gas composition, and so on). It also describes the achievement of a multi layered coating of YSZ and alumina.

Alumina (Al 2 O 3 ) coatings on stainless steel substrates were fabricated by electromagnetically accelerated plasma spraying (EMAPS), which generated a high-velocity pulsed plasma jet of more than 2.5 km/s with a high peak pressure of 1 MPa. High amounts of α-Al 2 O 3 crystalline phase in the raw powder was preserved in the EMAPS coatings where the ratio of α-Al 2 O 3 to γ-Al 2 O 3 was approximately 65:35, which is generally different from the α-Al 2 O 3 -rich coatings formed from α-Al 2 O 3 raw powders using conventional spraying methods, such as plasma spraying, detonation gun spraying, and high-velocity oxygen-fuel (HVOF) spraying. The specific wear rate of the coating formed from 6.6 µm raw powder at a spray distance of 100 mm was 2.3 × 10 -6 mm 3 /Nm, which is comparable to that of sintered bulk α-Al 2 O 3 . Morphological features of the coatings and the crystal structures suggested that the plastic deformation by the high-velocity impact of highly viscous raw powders due to insufficient heating was dominant in the formation of dense coatings with highly structural ratios of α-Al 2 O 3 .

To protect structural and functional materials against influence of the severe environments, various coating technologies can be used. This study considers the use of high velocity oxy-fuel process the coatings available for applications based on deposition using the continuous detonation spray process. The properties of the continuous detonation sprayed WC-NiCr and Cr 3 C 2 -NiCr cermet coating are investigated through the mechanical, corrosion and wear resistance test. The test results are also compared with the properties of the duplex phase stainless steel substrate and the atmospheric plasma sprayed coatings with the same powders. The corrosion resistance was examined by electochemical method, corrosion potential monitering. Furthermore, the erosion wear resistance was examined by the abrasive and cavitation erosion wear test. The experimental results exhibited that the anti-corrosive and anti-erosive properties of the continuous detonation sprayed coatings are superior to that of their plasma sprayed coatings. It was found that the WC-NiCr cermet coating appears to be more effective than the Cr 3 C 2 -NiCr cermet coating in abrasive erosion environment, whereas the Cr 3 C 2 -NiCr cermet coating is more effective in cavitation erosion environment.

In Activated Combustion HVAF process, coatings are formed of powder particles, heated and accelerated by high-velocity jet of air and gaseous fuel combustion products. A distinguished feature of the process is that spray particles are heated below their melting point while accelerated to velocity well above 700 m/s. Such spray scheme appeared beneficial for deposition of cemented carbides, in particular, WC-based composites. Dense, practically non-oxidized neither heat-deteriorated coatings were formed. Spray rates from 1 to 25 kg/hr were achieved without decline of coating quality or deposition efficiency. Specific coating structure resulted in noticeably improved resistance to fatigue at high level of stresses.

HVOF-, arc- and plasma sprayed coatings are widely used for wear protection. Today these type of layers are dominant if thin coatings from 50 up to 500 µm and low heat input into the work piece are required. The main disadvantage of thermally sprayed coatings is the adhesion to the substrate and the early failure when cyclic loaded. In both cases a metallurgical bonding to the substrate can improve the life cycle time. Plasma transferred arc (PTA) welded coatings show a metallurgical bonding to the substrate. The main disadvantages of this coating technology are the dilution of about 5%, the heat input into the substrate and that nowadays all welding positions seem to be impossible to carry out. In this paper the theoretical background for welding thin coatings (less than 500 µm) with a decreased dilution and in all welding positions is given and experimentally proved.

The purpose of this work is to optimise the chemical composition of perovskite coatings prepared by injecting a suspension of submicrometric LaMnO 3 perovskite particles (d 50 =~ 1 µm) in a direct current (d.c.) plasma jet. The perovskite powder composition, the particle size and the plasma parameters were modified in order to diminish the manganese evaporation. The process consists in mechanically injecting a well dispersed stable suspension of submicrometric perovskite particles in a dc plasma jet. In the process, large suspension droplets (~300 µm) are sheared into tiny ones (a few µm) by the plasma jet flow. Then the solvent is evaporated and the particles melt resulting in perovskite droplets of about 1 µm impacting on the substrate, the coating resulting from their layering. Such coatings are to be used as cathodes for the SOFCs (Solid Oxide Fuel Cells). Best results were obtained by injecting a stable suspension containing a 10 mol% MnO 2 doped perovskite powder with 3 µm particle size in an Ar plasma forming gas and 300 A of current intensity.

The innovative suspension plasma spraying (SPS) technique, in which the carrier gas used to inject particles (10…100 µm) into the plasma jet is replaced by a liquid feedstock, is currently under development procuring denser ceramic coatings due to the use of submicron particles. The suspension properties, as well as the most relevant injection parameters - injection angle and liquid velocity - are adjusted to improve the coating quality at acceptable deposition rates. In addition, the plasma jet instabilities are studied and correlated to the coating properties. In the present work, a feasibility study is conducted addressing the key factors influencing the coating morphological properties such as the porosity, cracks, molten-fraction and amount of over-spray. The experimental setup is adapted to different DC plasma torch architectures (Sulzer-Metco F4 and Triplex) operating under atmospheric conditions with the aim of correlating the plasma jet qualities to the properties of the feedstock under consideration. The metallographic probes and fractured surfaces of the resulting yttria stabilized zirconia (YSZ, 5 wt. % Y 2 O 3 ) coatings are analyzed by means of light microscopy and scanning electron microscopy (SEM).

Conventional equipments used for low pressure plasma spraying are designed for a chamber pressure of a few tens of mbar. During the past three years, an increasing interest for thermal spray under very low pressure conditions (as low as 1 mbar) was encountered but no significant advances concerning the spray gun design was proposed. However, it is clear that actual tools do not allow providing uniform plasma jets and that the produced jets present successive expansions and constrictions due to a poor adaptation of the nozzle exit pressure regarding to the chamber one. In the present paper, the use of a nozzle extension is proposed. The holding nozzle is a lengthened F4 type one and the diverging part of the nozzle is formed by a fastened external extension. The design of the nozzle extension is a bell-contoured Laval type one and its dimensions (exit diameter and length) are much higher than for conventional diverging nozzles. An improvement in the jet homogeneity and uniformity is expected.

This paper is described the experimental results, on the simple method for measure the accuracy of plasma gun autonomous control system with plural laser range sensors for thermal spraying robot. In this robot, plasma gun position and orientation under spraying condition are measured with two laser range finders. Laser wavelength is about 600 nm, visible. Measured data are fed to microcomputer system in order to generate the control signal for the robot movement. The plasma gun installed on thermal spraying robot is maintained preset distance and perpendicularity within preset angle to the work piece surface and kept constant feed rate during thermal splaying operation. The processing feed rate on the surface has influence important for the quality of the processed surface. This simple measuring method is very useful for thermal splaying process.

Providing for the DZ400 arc spray system, the new design high-velocity arc spray gun with three different nozzles is developed to produce the coatings with the structure of superfine and low porosity. This system can be used to spray the three normal wires of 4Crl3, FeCrAl and 7Crl3 (flux cored wires). With the SEM to analysis shape and particles size sprayed by the nozzles by different parameters, as well as with the S-3500N SEM (made by HITACHI) and the INCA ESA (made by OXFORD) identify the content of the oxides, porosity and thickness of the coatings. The result shows that the porosity in the coatings of solid wire is less than 2%, of the flux-cored wires is less than 4%, and the coating sprayed by the nozzle with the secondary supplementary airflow is typically shown to be distributed in the form of high-density lamellar splat structure and the average lamellar thickness is around 5 μm.

Laser cladding is a very promising technology from a metallurgical point of view. The benefits are low heat input into the substrate, high cooling rates resulting in a fine microstructure of the coating and an excellent metallurgical bonding to the substrate. The main disadvantage of laser cladding, however, is poor process efficiency, which causes high processing costs. In order to gain interest on the industrial level an optimisation of the energy management, for example reducing the energy loss through conduction, is necessary. Improved process efficiency and reduced processing time can be realized by a hybrid technology of coupling laser with a plasma arc. The feed can be coaxial to the plasma gun in form of powder. The main advantage of this process is that the processing area as well as the powder is preheated and activated by a transferred arc. The laser power is mainly used for heating up the feedstock material till melting temperature. The experimental results prove the theoretical considerations. Compared to laser cladding an increase in cladding speed and a lower energy input are reached. Further the efficiency towards material can be nearly doubled, reducing stock costs. At last the heat-affected zone is reduced, which offers the possibility to process crack sensitivity materials.

The shielding controlled plasma spraying process is investigated to improve corrosion resistance of the metal surface. In this process, a shielding nozzle that covers just the spraying area is attached in front of the tip of a commercial plasma spray gun nozzle, and the environment surrounding the plasma jet is controlled by nitrogen flow. As the oxygen concentration in the shielding nozzle is maintained as low as 0.5%, the metal oxide contents in volume of CoNiCrAlY coating and the porosity of the coating reduced to 0.2% and 0.3% respectively under optimal spray particle size. The corrosion potential of CoNiCrAlY coating sprayed by this process in an acid solution including chloride ions is staying about -150 mV for 1000 hours, and no rust is observed during this test. On the other hand, that of the coating sprayed by atmospheric plasma spraying process changes from about -300 mV to about -500mV for 1000 hours, and the rust comes to the surface of the coating after 10 hours. Therefore the developed shielding controlled plasma spraying process is concluded to improve the corrosion of the metal.

An optimised de-Laval-type-contour for the Atmospheric Plasma Spraying (APS) with respect to the thermal and deposition efficiency as well as to the arc voltage fluctuations and the sound level of the plasma torch is presented. Investigations of the plasma gas dynamics and two different feedstock materials (Al 2 O 3 (fused and crashed) and Cr 2 O 3 (agglomerated and sintered)) have been done to promote the industrial utilization of convergent-divergent nozzles for the APS.

Aluminum nitride (AlN) is one of the attractive ceramics applicable to the surface modification because of its excellent properties in chemical stability and thermal conductivity. In this research AlN coating was fabricated by reactive RF (Radio Frequency) plasma spray process, a kind of thermal spraying techniques. Reactive plasma spraying, in which metal element reacts with surrounding active species in plasma, has been considered to be an useful process for the formation of non-oxide ceramics thick coatings. By increasing nitrogen content in plasma gas, AlN coating without pure Al part was attained while the coating microstructure was heterogeneous, brittle and quite porous. By decreasing nitrogen content in plasma gas, on the other hand, Al/AlN composite coating with more homogeneous, less porous microstructure could be attained. Changing nitrogen fraction in plasma gas may be effective for controlling AlN content in Al/AlN composite coating. Nitriding process of aluminum in reactive RF plasma spraying was also investigated in this study. It could be considered that nitridation process of Al was occured during the particle flight in plasma or after the particle deposited onto the substrate. Nitriding reaction process of Al in the reactive plasma spray process was verified in the study.

In magneto-plasma-dynamic (MPD) arcjet generators, plasma is accelerated by electromagnetic body forces. The MPD arcjet generator can produce higher-velocity, higher-temperature, higher-density and larger-area plasmas than those of conventional thermal plasma torches. Two types of MPD arcjet generator were developed for applications to ceramic spray coatings. One generator was installed with a cathode covered with mullite or zirconia ceramics and the other with a titanium cathode. The former was operated with Ar for mullite or zirconia coating by an ablation process of the cathode cover and the latter with N 2 for titanium nitride coating by a reactive process between ablated titanium particles and nitrogen plasma. Silicon nitride reactive spraying was also carried out. The MPD spray process could successfully form dense, uniform and hard ceramic coatings. In titanium nitride reactive spraying, plasma diagnostic measurement and flowfield analysis were conducted. A large amount of N and N + was expected to be exhausted with a high velocity from the MPD generator. Both the electron temperature and the electron number density were kept high at a substrate position compared with those for conventional low-pressure thermal sprayings. A chemically active plasma with excited particles of N + , Ti, Ti + and Ti 2+ was considered to contribute to better titanium nitride coatings. All coating characteristics showed that the MPD arcjet generators had high potentials for ceramic spray coatings.

Low running costs, high spray rates and efficiency make electric arc spraying a good tool for coating large areas with high production rates. The main applications are in the field of corrosion and wear protection of large structures, e. g. parts for bridges or offshore industry. New applications are expected for high quality coatings produced by cored wires. Disadvantages of the arc spraying process are that only electrically conductive wires can be processed and the lower particle velocity in comparison to other thermal spray processes like HVOF or APS. Depending on the process parameters the oxidation of particles has a negative effect on the mechanical and the electrochemical properties of the coating, too. In this paper some investigations with new and flexible power supply systems for arc spraying are presented. The particle size and the morphology of the coating can be optimised, due to the possibility of changing the current generator characteristic and modulating the power by pulsing up to 500 Hz. The oxidation of particles can be reduced by a lower heat input based on lower spray voltage. For a higher quality of the coating microstructure investigations with dynamic generators were performed for Zn, Al, ZnAl, 110MnCrTi8 and Al. An enhancement of the process stability was achieved, too.

A microplasma spraying torch with a hollow cathode electrode is designed to melt completely the refractory materials and deposit coatings at plasma power level up to several kilowatts. The designed torch permits spray material to be fed into plasma arc jet through axial powder injection. In the present study, molybdenum is used as a typical refractory spray material. The effects of the main processing parameters including plasma arc power, plasma gas flow and spray distance on the particle velocity during spraying, and the microstructure and properties of the coatings are investigated. The microstructure of coating is characterized with optical microscopy and scanning electron microscopy. The properties of the coating are characterized by microhardness and abrasive wear tests. The particle velocity during in-flight is carried out using a particle velocity/temperature measurement system based on thermal radiation. The comparison of the microstructure and property of micro-plasma sprayed Mo coatings with those of the coating deposited by the conventional plasma spraying operated at a power of 42 kW is performed. The results show that the abrasive wear loss of the Mo coatings deposited by the micro-plasma spray torch is comparable to that of the coating deposited by the conventional plasma spraying disregarding the one order difference in the plasma operating power.

Interactions between multiple splats landing on a substrate was studied experimentally by photographing deformations of droplets as they land and freeze on the substrate, or previously solidified splats. Uniform-size molten tin droplets (550 µm diameter) were produced using a drop-on-demand generator. To achieve high impact velocities the stainless steel coupons used as substrates were mounted on the rim of a rotating flywheel and heated using cartridge heaters to vary substrate temperature. To hit a falling droplet with the substrate and photograph its impact, a timing circuit was used to synchronize the ejection of a droplet, triggering of the camera and a flash to provide illumination. The substrate temperature and substrate roughness significantly affected splat impact dynamics. Droplets hitting a smooth cold substrate splashed extensively whereas those hitting a hot substrate spread in the form of a smooth disc. The final splat shapes were dependent on the offset distance between the impacting droplet and the previously solidified splat. The size of fingers around the splat edge increased with the offset distance. Large pieces of metal detached from the droplet rim when the droplet hit a rough substrate whereas droplets hitting previously solidified splats splashed in a star-like shape with extremely long fingers.

The phase transformation and reaction of ZrO 2 -CaO- ZrSiO 4 and ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings with manganese oxide at 1273 K were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). SiO 2 phase formed in the coatings, which was from the ZrSiO 4 decomposed and easy react with manganese oxide or CaO. SiO 2 has precedence over react with CaO than manganese oxides for ZrO 2 -CaO-ZrSiO 4 coatings, and which result in to promote t-m phase transformation. On the contrary, the reaction between SiO 2 and MnO is primary for the ZrO 2 -Y 2 O 3 -ZrSiO 4 coatings and result in the damage or exfoliation on the surface of the coatings.

Northwest Mettech Corporation has recently developed a new high power torch for spraying internal surfaces, the Axial III ID. The Axial III ID has been successfully operating at 135 kW while spraying inside a drum with an inner diameter of 34 cm. The three-electrode axial injection configuration of the new torch is based on the current Axial III torch. Various coatings were sprayed inside a drum while monitoring the torch temperature. The results of these experiments are presented in this paper.