A brief feasibility study was performed to produce thermal spray coatings using gas atomized powders of Cu47Ti34-xZr11Ni8Six, where x=0 and 1. These alloys have previously been shown to be capable of forming metallic glasses having thick (1-2 cm) cross sections because they can be cooled from the melt at relatively low cooling rates (e.g., 100-102Ks-1). The properties of these metallic glasses include high strength, high elasticity and high fracture toughness. Amorphous plasma arc sprayed coatings were produced which were close in composition to the starting powders, and exhibited comparable glass transition and crystallization behavior. The amorphous structure of the as-sprayed coatings was used as a source for forming a range of partially devitrified and fully crystallized structures. The average hardness of the coatings increased from around 6 GPa to near 10 GPa as the degree of crystallization increased.

The plasma torch is one of the most important equipment during air plasma spraying, is used to produce the plasma arc in the nozzle for the acceleration and heating of particles. The voltage-current characteristic of the plasma arc is very important to the fusion of particles. The volt-ampere characteristic is affected by many factors, such as gas species, gas flow rate, nozzle geometry and the type of gas injection. So the factors influencing the voltage-current characteristics in high velocity plasma spraying were studied. As the results showed, the arc voltage is increased with the increase of the primary gas flow rate in some range. The secondary gas flow rate has an important effect on the electric arc voltage. The influence of nitrogen on the arc voltage is more than argon when they are chose as the primary gas in plasma spraying. The arc voltage is decreased with the increase of the nozzle’s compression angle and the decrease of the nozzle’s aperture, when the length of the nozzle is unvarying.

The process properties for DC plasma spraying are affected by the arc as the source of energy for plasma generation. For instance, the position of the anode attachment point, the arc movement and the arc formation are significantly influencing process stability, reproducibility and coating quality. Following a qualitative and objective assessment of the complete arc movement and plasma generation is leading to an improved process characterization. Therefore, a preventive arc-based analytical method for DC plasma spraying is developed. Based on a representative data volume and realized by an automatic analysis of high-velocity recordings of the arc dynamics and correspondent arc voltage measurements, the evaluations are carried out. A developed software algorithm automatically detects for all images of the video sequence (at least 3200 images) the anode center axis, the arc orientation and the position of anode attachment point on the anode surface referring to the anode center, with simultaneous compensation of aberrations. This allows an objective assessment of the complete arc movement. In further investigations, the detection limit of the developed measuring system is determined and the effects on the arc behavior and the coating process could be quantified. Thus, the developed automated analysis of the arc dynamics in the DC plasma generator corresponds to an arc-based process characterization in DC plasma spraying process with relevance to developers (e.g. new anode nozzle designs) and end users (process control).

Thermal spray technology offers the advantage of producing thin-walled parts. Especially thermal spraying of NiTi shape memory foils demands highest control of atmospheric conditions due to their affinity to e.g. oxygen, hydrogen and nitrogen at high temperatures. This article describes two inert thermal spray techniques in order to produce free-standing shape memory foils, namely low pressure wire arc (LPWAS) and vacuum plasma spraying (VPS). The advantage of low pressure wire arc spraying is the ability to use NiTi wire from commercial suppliers without the detour of powder production, which may result in additional undesired reactions and phases. The benefit of vacuum plasma spraying is the higher kinetic energy of the particles due to higher plasma gas amount and speed. Within the scope of this paper, coatings produced with both methods (LPWAS and VPS) are discussed with regard to their coating microstructure and physical / chemical properties, transformation temperatures (DSC), porosity (%) and their shape memory effect.

Plasma spray coating has achieved outstanding technological and commercial progress. However the underlying fundamentals still require a better understanding to overcome some limitations coming from, in particular, the instabilities of the arc and the strong erosion of the electrodes. In this paper we present experimental investigations of the fluctuating behavior of a Sulzer Metco F4 gun operated at atmosphere. The temporal evolution of the torch voltage and current, and of the plasma jet emission have been measured, hi addition, an optical fiber inserted inside the gun allows to measure fluctuations of the arc emission directly. Depending on the external parameters, different modes of operation have been identified. In the "restrike" mode which prevails for spraying-relevant operation conditions, detailed analysis of the voltage signals and corresponding arc and jet light emission reveals different categories of voltage drops corresponding to arc interruptions or reconnections. Spectral analysis of the different fluctuation signals shows clearly-defined peaks in the frequency range 3-30 kHz which are attributed to the arc motion and restrike inside the torch. The dependence of these peaks on operation conditions in terms of gas flows and composition, and gas injection geometry is presented. In addition a study of the effect of electrode aging on the torch fluctuations is reported.

Hypoeutectic AlSi engine blocks of modern passenger cars are generally equipped with cast iron liners in order to provide cylinder running surfaces that meet the tribological requirements. A very promising alternative to the use of cylinder liners lies within the application of thermally sprayed coatings onto the walls of cylinder bores as friction partners for the piston rings. This work describes the development of a novel iron based wire feedstock as well as its application by the Plasma Transferred Wire Arc internal diameter coating system. The material developed within the frame of this work leads to partially amorphous coatings with embedded nanoscale precipitations if processed by thermal spraying. The coatings were applied onto the inner diameters of test liners made of Aluminium EN AW 6060 and onto cylinder bore walls of in-line 4 cylinder engines. All substrates were mechanically roughened in order to obtain high bond strengths of the sprayed coatings. The coatings microstructure was analysed by light optical microscopy, hardness measuring by transmission electron microscopy. Furthermore the oil storage capacities of the honed surfaces were determined.

In magneto-plasma-dynamic (MPD) arc jet generators, plasma is accelerated by electromagnetic body forces. Silicon nitride reactive spraying was carried out using an MPD arc jet generator with crystal silicon rods and nitrogen gas. Because higher-velocity, higher-temperature and higher-density and larger-area plasmas are produced with the MPD arc jet generator than those with conventional thermal plasma torches, nitriding of silicon can be enhanced. A dense and uniform β-Si 3 N 4 coating 30 µm thick was formed after 200 shots at a repetitive frequency of 0.03 Hz with a discharge current of 9 kA and a substrate temperature of 700 °C. The Vickers hardness reached about 1300. Furthermore, silicon carbide and aluminum nitride sprayings were conducted with the same spraying system. Surface modification is under study with lots of chemically reactive gases. All results showed that the MPD arc jet generator had high potential for spraying and surface modification.

This study evaluates the erosion-corrosion performance of thermal spray hardcoats on bronze-coated gray cast iron. In the experiments, gray cast iron plates are coated with a bronze powder by PTA welding and the coatings are characterized based on microstructure and corrosion and wear testing. The bronze coatings provide good corrosion protection, but are shown to be susceptible to cavitation and erosion wear. To compensate, thermal spray hardcoats, including atmospheric plasma sprayed Al 2 O 3 and Cr 2 O 3 and HVOF sprayed WC-Co, were applied over bronze-coated cast iron and corrosion and wear tests were performed. It is shown that the thermal spray hardcoats greatly improve wear resistance, but despite their interconnected porosities, do not affect the corrosion performance of the underlying bronze.

Both bonding strength of coating to substrate in low pressure plasma spraying and the effect of reverse transferred arc treating before spraying are studied in this paper. It is difficult to obtain the bonding strength precisely in low pressure plasma spraying by standard testing methods, such as, ASTM Standard C633-79 and JIS H8666-80. Therefore, for the bonding strength test rather than using a conventional adhesive, we believe a vacuum brazing process using Ag-Cu-In-Ti active filler metal at 1023 K should be used. We have also confirmed the practicality of this step. By the above test method, it has been proven that the bonding strength of low pressure plasma sprayed coating is over 100 MPa. Also, that reverse transferred arc treating after blasting enhances the bonding strength of low pressure plasma sprayed coating. It is also believed that the projections formed on the substrate surface by reverse transferred arc treating are buried into the coating and perform the pile effect.

This paper discusses the double-arc phenomenon that occurs in plasma arc spraying and its prevention through the use of a bi-anode torch. It also presents the results of a study that compares voltage-current characteristics of the arc for different plasma gases and arc root attachment points. It is shown that the arc has different electrical characteristics when changing from the first to the second anode, which is explained using a simplified arc model.

This paper describes the development of a numerical model and explains how it is used to investigate arc-cathode interactions in a plasma arc torch. The model is based on magnetohydrodynamic (MHD) theory and couples Navier-Stokes equations for a nonisothermal fluid with Maxwell’s equations for electromagnetic fields. The equations account for the internal geometry of the torch as well as arc current and gas type and flow rate. They are solved using CFD code and relevant boundary conditions and are shown to provide insight on arc dynamics and the effect of cathode shape on arc behavior.

Anode erosion is a common concern in dc plasma spray torches. It depends largely on the heat flux brought by the arc and the dimensions, residence time, and mode of the arc attachment to a given location on the anode wall. This paper compares anode arc attachment modes predicted by LTE (local thermodynamic equilibrium) and 2-T (two-temperature) arc models that include the electrodes in the computational domain. The analysis is based on a commercial cascaded-anode plasma torch operated at high current (500 A) and low gas flow rate (60 NLPM of argon). It shows that the LTE model predicted a constricted anode arc attachment that moves on the anode ring while the 2-T model predicted a diffuse and steady arc attachment. The comparison between the predicted and measured arc voltage indicated that the 2-T prediction is closer to the actual voltage. A post-mortem observation of a new anode ring on a plasma torch operated under the same conditions confirmed the diffuse arc attachment predicted by the 2-T arc model.

Plasma spheroidization utilizing a plasma spray gun has been demonstrated for water-atomized stainless steel SUS316L powder. The angular particles were successfully spheroidized, and the D50 size of particles were decreased from 35 μm to 24 μm with 9 kW of processing power and to 15 μm with 17 kW of processing power. It was found that the high processing power of 17 kW generates a significant number of fine particles with the size of under 1 μm. By contrast, the powder formed on low processing power of 9 kW has better flow-ability and low cohesiveness, suggesting that an appropriate processing power exists to form the spherical powder suited for additive manufacturing

Plasma arc sprayed coatings are largely developed in Perfect Circle Europe because of the wide range of coating materials which ensure numerous tribological properties. Spraying parameters are of great importance in improving the coating structure and therefore the coating wear, adhesive, cohesive and corrosive resistances. The utilized plasma gas is first studied; designed experiments are then achieved to optimize the all spraying parameters. More than the spraying parameters, the powder formulation has a major influence on the coating tribological properties. Several powders are studied with the aim to reduce the ring-liner system wear. Tests on idealized samples coupled with engine tests help us to characterize the tribological behaviour of some molybdenum based coatings.

A study was performed to examine the effects of starting powder composition, substrate thermal conductivity, and substrate temperature on the composition and structure of individual Al-Cu-Fe splats formed during thermal spraying. The fraction of quasicrystalline phase which formed was found to depend on the chemistry and solidification history of the splats. Due to evaporative loss of Al during spraying, an initial powder composition higher in Al produced splats closer to the desired composition, which yielded more of the quasicrystalline phase. Deposition onto lower thermal conductivity surfaces resulted in an increase in the quasicrystalline phase, as did solidification onto higher temperature substrates.

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.

Three-dimensional, transient simulations of the plasma flow inside different plasma spray torches have been performed using a local thermodynamic equilibrium model solved by a multiscale finite element method. The model describes the dynamics of the arc without any further assumption on the reattachment process except for the use of an artificially high electrical conductivity near the electrodes. Simulations of an F4-MB torch from Sulzer-Metco and two configurations of the SG-100 torch from Praxair are presented. The simulations show that, when straight or swirl injection is used, the arc is dragged by the flow and then jumps to form a new attachment, preferably at the opposite side of the original attachment, as has been observed experimentally. Although the predicted reattachment frequencies are at present higher than the experimental ones, the model is suitable as a design tool.

Different post treatment methods such as heat treatment, mechanical processing, sealing, etc. are known to be capable to improve microstructure and exploitation properties of thermal spray coatings. In this work a plasma electrolytic oxidation of aluminium coatings obtained by arc spraying on aluminium and carbon steel substrates is carried out. Microstructure and properties of oxidised layers formed on sprayed coating as well as on bulk material are investigated. Oxidation is performed in electrolyte containing KOH and liquid glass under different process parameters. It is shown that thick uniform oxidised layers can be formed on arc sprayed aluminium coatings as well as on solid material. Distribution of alloying elements and phase composition of obtained layers are investigated. A significant improvement of wear resistance of treated layers in two types of abrasive wear conditions is observed.

This work deals with a 3-D time-dependent modeling of the arc behavior in a plasma spray torch. The mathematical model is based on the simultaneous solutions of the conservation equations of mass, momentum, energy, electric current and the electromagnetism equations. It makes it possible to predict the motion of the anode attachment root on the anode surface under the combined effect of hydrodynamic and magnetic force and, the time-evolution of arc voltage and gas fields in the nozzle. The calculations show that the latter exhibit significant three-dimensional characteristics. The projected arc behavior and voltage fluctuations agree rather well with experimental observations.

Many studies have investigated methods to reduce cavitation damage in hydraulic turbines and reduce residual stresses after coating deposition. In this work, a cobalt stainless steel was applied by arc thermal spraying. The influence of air pressure deposition and plasma remelting on the microstructure, oxide volume fraction, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties of the AS895HY cobalt stainless steel coatings were investigated by x-ray diffraction, optical microscopy, scanning electron microscopy, microhardness testing, and ultrasonic cavitation testing (ASTM G32-93). The increase in air pressure, 280 to 550 kPa, modified the oxide fraction from 10.9±1.8% to 24.1±2.8% in the samples. The mass loss results in the cavitation tests were 13.8, 19.2, and 15.0 mg/h for the samples with 280, 410, and 550 kPa, respectively. The remelting of the sprayed coatings eliminated the oxides and porosity. Austenite formation was observed in the two remelted layers with decreases in microhardness; for the first layer, this occurred because of the AWS309L substrate dilution. The PTA remelting reduces the mass loss rate to 0.497 mg/h, with 8.02 hours incubation period. Phase transformations were observed in the remelted coating, but not verified in the arc thermal sprayed coatings.