A novel plasma spray process for producing nanostructured coatings, hypersonic plasma particle deposition (HPPD), has been experimentally investigated. In HPPD, vapor phase precursors are injected into a plasma stream generated by a DC arc. The plasma is quenched by supersonic expansion through a nozzle into a vacuum (~ 2 torr) deposition chamber. Ultrafine particles nucleated in the nozzle are accelerated in the hypersonic free jet downstream of the nozzle and inertially deposited onto a substrate. The short transit times between the nozzle and the substrate (< 50 μs) prevent inflight agglomeration, while the high particle deposition velocities result in the formation of a consolidated coating. We have investigated the production of silicon and silicon carbide coatings using SiCl 4 and CH 4 precursors. Silicon deposits analyzed by transmission electron microscopy were found to have nanostructured regions with grain sizes varying from 5-20 nm. Corresponding particle size distributions measured before deposition using an extractive aerosol probe peaked around 15 nm, suggesting negligible grain growth occurred in the samples studied. Silicon carbide particle size distributions measured at various deposition chamber pressures verify that the low residence time characteristic of the HPPD process minimizes in-flight agglomeration.

The performance (particle velocity and velocity distribution) of a typical injector, and the resulting particle spray pattern for metallic (NiCrAlY) and ceramic (ZrO 2 ) particles are examined as a function of carrier gas flow rate and the effect of varying the geometry immediately upstream of the injector. Injector performance is also examined for a 1:1 mixture of ceramic and metallic particles such as is used in the spraying of functionally graded materials. The upstream geometries tested included a 90° "tee," a 90° elbow, and a straight entrance. The elbow geometry was tested in both "up" and "down" orientation to determine the influence of gravity. The upstream geometry can alter the average particle injection velocity by 10-15% influencing both the spray pattern trajectory and width.

Fine (median size 6 μm and 0.3 μm) cobalt spinel (Co 3 O 4 ) powders were processed suspended in a suitable liquid phase. Suspensions exceeding 50 wt.% solid phase content were successfully injected into an inductively coupled plasma. Spheroidized powders with large particle size (up to 80 μm) were prepared, and cobalt oxide coatings were produced by this novel RF-SPS method. The microstructural features of the coatings can be controlled by parameter optimization similarly to plasma spraying of dry powders. Numerous variations of the physical and chemical conditions of the process were performed in an attempt to overcome the main disadvantage of the process, i.e. the decomposition of the spinel phase to CoO. So far, the spinel phase could be reestablished only by a post-treatment of the deposited coatings with atomic oxygen in the RF plasma.

LaMnO 3 powders and coatings have been prepared by reactive suspension plasma spraying (SPS) of MnO 2 powders and LaCl 3 solutions. A 40 kW inductively coupled plasma with an oxygen plasma sheath gas has been used. Water and ethanol have been tested as the liquid phase in the SPS process. High perovskite content (70-90%) has been achieved for both powders and coatings when spraying a suspension of fine MnO 2 powder in a saturated ethanol solution of LaCl3 with a 1:1 molar ratio of La and Mn. Materials obtained by a 1100 °C oven treatment have been used as reference during the study. The reactor pressure was varied from 30 to 80 kPa. Low pressure was found to be necessary to suppress the formation of undesired phases in the powders and coatings obtained. A plasma post treatment of the coatings results in an increase of the perovskite content.

Suspension Plasma Spraying (SPS) is a thermal spray process based on a suspension of fine (<10 μm) or even ultrafine (<100 nm) powders which is axially fed into the induction plasma through an atomization probe. The atomization of the suspension results in microdroplets (20 μm in size). They are flash dried in the plasma, melted and finally can impact a substrate to build a coating or be cooled down and collected as a spheroidized powder. The large industrial potential of this technology results first from the use of fine powder or even sol-gel which is one of the starting step for many ceramic processes, and second from the various side benefits of the liquid phase in the SPS. Indeed, the liquid phase can be simply a carrier for ultrafine powder, or a protection against oxidation in the case of metals, or a protection for health in the case of whiskers, for instance. It can also take a part in chemical reactions when the liquid phase is a solution of chloride, nitrates... or it can be an organic liquid for the synthesis of carbide, where CO is a strong reducer. Furthermore the liquid phase can also release some energy because of its combustion at the very end of the process. It can also change the local atmosphere surrounding the in flight droplets in the plasma where it is possible to use H 2 O 2 as a carrier in order to increase the oxygen partial pressure around sensitive to oxygen decomposition materials. The applications of SPS are in the powder synthesis (in R&D or production), in the spraying of metals, ceramics or composites directly synthesized, or in production of very reactive with air materials. Applications of SPS will be presented for hydroxyapatite (HA) and NiAlMo. Induction plasma SPS coatings and/or powders properties will be discussed as a function of the SPS process variables.

In an emerging thermal spray process a coating is formed by exposing a metallic or dielectric substrate to a high-velocity jet of solid-phase particles, which have been accelerated by a supersonic gas jet at a temperature much lower than the melting or softening temperature of the particle material. This is known as "Cold Gas-Dynamic Spray" (CGDS) method. Using this method, 2618 Al substrates were coated with nickel-aluminum bronze powders (~100 and ~400 mesh) in an effort to obtain improved wear resistance. The coatings have been examined for their microstructure, hardness, and bond strength. Triple lug shear tests performed on coated panels provided quantitative measurement of the coating/substrate interfacial shear strength. The steady state wear rates were determined using the pin-on-rotating ring test at a pressure of 690 kPa and a sliding velocity of 9 m/s. The wear resistance of the nickel-aluminum bronze coatings is discussed in conjunction with scanning electron microscopy (SEM) examination of the wear tracks and metallography of the polished transverse cross-sections. Though the coatings are not completely free from porosity, they exhibit high interfacial shear strength and wear resistance due to the low-temperature, ballistic impingement of the powders in the cold gas-dynamic spray method. The ~400 mesh powder coating shows higher interfacial shear strength and wear resistance in comparison with the ~100 mesh powder coating.

At the present, components which require both nitriding and locally a thermal sprayed coating or nitrided components which should l)e reworked are usually nitrided before spraying and the area to be coated is masked during nitriding or is prepared before spraying by locally removing the nitrided layer by grinding. Seen technically, advantages are to be expected if the nitriding process can be carried out after spraying. Moreover a post-nitriding of thermal sprayed coatings is of interest for improving coating characteristics, mainly wear resistance. Understanding the behaviour of sprayed coatings during nitriding in comparison to bulk materials will help to understand generally the behaviour of such coatings in gas atmospheres at increased temperatures. The objectives of the project are the investigation of the interaction between thermal spraying and nitriding, and the optimisation of both processes to achieve improved bonding, wear and corrosion characteristics respectively to get nitriding of the substrate through the coating without spalling or cracking. Furthermore the behaviour and structural changes of different coatings at increased temperatures are determined. The metallographic, X-ray, wear and corrosion results of the resulting compound coatings and parts are presented. Possible new applications are discussed. The project is funded by the German Research Ministry.

The Electromagnetic Powder Deposition (EPD) process converts pulsed electrical energy into kinetic and thermal energy to accelerate and heat powder material to conditions suitable for bonding. A high pressure plasma armature is electromagnetically accelerated using a railgun. A supersonic pressure wave is created when the armature accelerates through and "snowplows" the ambient gas ahead of it. The gas column is heated, compressed, and accelerated to the entrainment section of the gun, where some of the thermal and kinetic energy is transferred to an injected stream of powder material. The acceleration burst is repeated rapidly to supply the required deposition rate and to achieve steady thermal conditions. Development of a starter plasma which is reliable at ambient pressure was a major programmatic task. Generation of a low pressure linear arc required to form a planar armature during the pulsed event was investigated. Several geometries (point-to-point breakdown, rail-to-rail breakdown, and confined glow discharge) were explored using different voltage sources (dc, 60 Hz ac, 150 MHz rf, pulsed). Satisfactory operation of the confined glow discharge approach at atmospheric pressure was achieved using rf excitation. Results of testing under the various scenarios are presented and critiqued.

The electromagnetic powder deposition (EPD) system employs high velocity gas flow to accelerate powder material to conditions required for high strength plating. The gas flow, however, is not continuous; rather it consists of bursts generated by an electromagnetic railgun and pulsed power system. Each gas burst is created by a high pressure plasma arc which fills a transverse section of the gun. This current carrying arc is driven by the railgun Lorentz force (magnetic pressure) and acts much like a piston, which via a snowplow process accelerates and compresses an ambient gas column to the flow speed required to accelerate powder particles. Analysis of the total system was carried out to provide scaling relations which give guidance in design of the system. Plating considerations define a desired powder velocity; this combined with the choice of working gas and ambient pressure determines the velocity and duration of each gas burst. Selection of gun geometry completes the definition of the pulsed power system requirements. An outline of the analysis is presented along with the physical models used.

Existing state of the art thermal spray processes (HVOF, D-Gun, Plasma Spraying) are limited to powder velocities of about 1 km/sec because they rely on the thermodynamic expansion of gases. A new thermal spray process using electromagnetic forces can accelerate powder particles to a final velocity of up to 2 km/sec. At this velocity powder particles have sufficient kinetic energy to melt their own mass and an equivalent substrate mass on impact. The process is based on railgun technology developed by the Department of Defense. A railgun is filled with argon gas and a high energy electrical pulse, provided by a capacitor bank, drives the gas down the railgun to a final velocity of up to 4 km/sec. This gas passes over a powder cloud and accelerates the powder through drag forces. The electrical and powder discharge frequency can be adjusted so that the deposition rate and thermal input to the substrate can be controlled.

This paper describes the diagnostic tools used in the development of a new electromagnetic powder deposition system. The instrumentation, interpretation of data, and subsequent decisions regarding the direction of system development are discussed. Important system parameters, their impact on system performance, and techniques to measure them are presented. The electromagnetic powder deposition system is based on railgun technology developed by the Department of Defense. The system drives an ionized plasma sheet down the length of a railgun, reaching a final plasma velocity of 4 km/sec. The high velocity plasma, in turn, snowplows [1] a shock compressed gas column in front of it. This gas column sweeps through a powder cloud and accelerates it by viscous drag to a final velocity of 2 km/sec. Important system parameters include particle velocity, gas velocity, gas column pressure, and plasma propagation and velocity. Diagnostic tools include pressure transducers, a high speed digital framing camera, fiber optics and magnetic probes.

Arc voltage fluctuations of a plasma spray torch are primarily an indication of the movement of the arc attachment inside the anode nozzle. These fluctuation have been shown to influence the deposition process. In order to detect changes in the operating conditions which affect coating quality, a method has been developed for on-line analysis of these fluctuations. Voltage fluctuation have been recorded together with light emission fluctuations and with acoustic emissions from the plasma jet and analyzed on-line using a workstation operating with the LabView environment. Anodes with different wear characteristics have been examined in this study. A clear correlation has been found between the changes in the dominant frequencies of all three signals and the conditions of the torch anode and the coating properties. Appearance of a group of frequency peaks in the 2 to 5 kHz range indicates a more unstable plasma jet and is correlated with anode erosion and increased coating porosity. The results of this study provide us with a convenient method to detect coating deterioration due to anode erosion.

Presented in this paper are the results of preliminary experiments carried out for monitoring of plasma nozzle wear. The experiment system consists of two microphones, one accelerometer and voltage measurement. In the experiment, three nozzles were tested, new, used and worn. The test results show clearly the difference in acoustic characteristics with wear in the nozzle. Some observations obtained from the experiment include: 1) the microphone mounted at 45° may be the best choice for nozzle wear monitoring; 2) voltage signal could be used with the 45° microphone as an indicator for severe nozzle wear; 3) vibration signal is not as sensitive to nozzle wear as 45° microphone and voltage signal; and 4) the 90° microphone is insensitive to nozzle wear.

A study is carried out of the spheroidization of ceramic and metallic powders using induction plasma technology. The process is based on the central injection of the powder in the plasma discharge followed by the in-flight cooling and solidification of the molten droplets prior to their collection at the bottom of a stainless-steel water cooled chamber. The degree of spheroidization is evaluated using image analysis. The results are correlated as a function of the powder feed rate, the plasma operating conditions and the thermophysical properties of the powders treated. The model's fit to the obtained experimental data is very good. The results show that the technology can be successfully used for the spheroidization and densification of a wide range of materials.

The plasma transferred arc process continues to be the coating method of choice for the application of cobalt base alloys onto valve and valve trim. Although new applications have been developed over the years, the process remains largely associated with the application of high performance, highly alloyed powders for relatively small parts or small areas of large parts. The use of the plasma transferred arc process for large volume application has been limited by the robustness and performance characteristics of the equipment and the use of cobalt. A new plasma transferred arc system (power source, torch and process controller) has been developed which allows the application of powder metal alloys at deposition rates of up to 40 pounds per hour. In addition, there has been a development of new non-cobalt powder alloys with excellent mixed corrosion and wear resistance properties. These capabilities have rendered the process technically and economically viable for large and demanding applications in the mining, power utility and steel industries. The new PTA system and the recent developments in powder alloys will be discussed. Reference will be made to specific applications in target industries.

Conventional corrosion protection of steel structures has usually involved the application and reapplication of lead-based paint (LBP), a material now known to be highly toxic and likely to find its way into the environment. LBP is no longer used in the field, but repair crews, nearby communities, and the environment may be exposed to unacceptably high levels of lead as the substrates of older structures are prepared for repainting during routine M&R operations. Conventional dust-containment enclosures used onsite during surface preparation (abrasive blasting) are often inadequate. The most effective containment technologies, on the other hand, tend to be expensive and cumbersome. All of these factors make surface preparation and recoating slow, technically difficult, physically demanding, and hazardous to the worker and the environment. Automated technologies have the potential to address all aspects of these interrelated infrastructure M&R problems. An example of such a technology is the Automated Thermal Spray System (ATSS). The ATSS utilizes a triaxial array of linear motion actuators to form a robot capable of performing preprogrammed sequences. The demonstration proved that the ATSS can successfully remove deteriorated lead-based paint from a steel bridge and then apply a protective coating to the exposed surface.

Reproducibility is a current challenge for the thermal spray industry. Reproducibility associated problems represent a large cost every year not only in terms of rejections and rework, but also in costs for destructive testing and decreased production flow. Thermal spray coatings are moving in the direction of being considered only as a "band aid" to becoming a design element. One of the prerequisites for such a development is an increase in reproducibility for thermal spray coatings. The purpose of this paper is to outline a vision aiming in the direction of a future "ultimate spray booth", where thermal spraying is as reproducible and reliable as machining, grinding or other production processes. A way to increase reproducibility and reliability in the future spray shop involves utilising major parts of IT - technology. This also includes active co-operation design-production in the pre-spray process. This paper will deal with areas such as: operation drawings and lists through multimedia techniques, education programs for operators and designers through multimedia techniques, CAD/CAM, Off-line programming and simulation, On-line diagnostics of flame (particle diagnostics) and coating (temperature & Acoustic emission measurements), on-line Statistical Process Control and Knowledge Based System techniques.

A chromium alloy as used for the metallic bipolar plate of a solid oxide fuel cell was processed by RF-plasma spraying to dense free-standing parts. The plasma spray parameters were successfully adapted for two different types of powder. The layer properties, particularly the porosity and the splat shape were investigated in dependence on the spray angles. All the coatings produced with off-normal spray angles show higher porosity increasing from spray angles of 60° to 30°. The splat orientation changes from parallel to the inclined surfaces to almost perpendicular to the plasma jet axis with shallower angles.

A P.S.Z. coating elaborated by r.f. plasma spraying was studied and compared to industrial arc plasma-sprayed P.S.Z. coatings to evaluate the quality of the corresponding thermal barrier coating system for gas turbine applications. One commercial ZrO 2 - 8% Y 2 O 3 powder was sprayed with two industrial d.c. torches (7MB and F4) and one r.f. plasma torch (Tekna PL50). Physical properties such as density, porosity and thermal diffusivity were measured on the three types of P.S.Z. coating. The microstructure and quantitative phase analysis were respectively investigated by S.E.M. and X-Ray diffraction. The burner rig tests on the T.B.C. systems showed that the thermal shocks resistance on the r.f. coating was at least the same as the others. Induction plasma spraying gave a high deposit efficiency (around 80%) and a P.S.Z. coating with very interesting thermal properties. All these facts demonstrate that r.f plasma spraying can be a competitive process to produce high quality thermal barrier coatings.

This study focuses on RF-Plasma Technology (RFPT) to enhance the physical properties of powdered materials (metals, carbides, ceramics, inter-metallics, and mixtures). The efficiency and flexibility of RFPT allows for the economically viable production of powders with a high degree of densification, spheroidization and purity. RFPT is based on the used of RF inductive power used to create a plasma at atmospheric pressure. A complex model based on theoretical calculations and empirical data has been developed to describe changes in particles exposed to the plasma stream for a variety of process parameters. A specific combination of parameters is referred to as a scheme. Advanced schemes have been developed to increase the coefficient of heat transfer from the plasma stream to particles by up to 35%. RFPT is especially useful for the production of powders that have been difficult or impossible to create with traditional methods. Some of the materials processed include: ZrO 2 , W 2 C, WC and WC-Co combinations. Particle size distribution (PSD) can be tightly controlled, and can vary from 5 to 600 microns.