In a DC plasma spray torch, the plasma-forming gas is the most intensively heated and accelerated at the cathode arc attachment due to the very high electric current density at this location. A proper prediction of the cathode arc attachment is, therefore, essential for understanding the plasma jet formation and cathode operation. However, numerical studies of the cathode arc attachment mostly deal with transferred arcs or conventional plasma torches with tapered cathodes. In this study, a 3-D time-dependent and two-temperature model of electric arc combined with a cathode sheath model is applied to the commercial cascaded-anode plasma torch SinplexPro. The model is used to investigate the effect of the cathode sheath model and bidirectional cathode-plasma coupling on the predicted cathode arc attachment and plasma flow. The model of the plasma-cathode interface takes into account the non-equilibrium spacecharge sheath to establish the thermal and electric current balance at the interface. The radial profiles of cathode sheath parameters (voltage drop, electron temperature at the interface, Schottky reduction of the work function) were computed on the surface of the cathode tip and used at the cathode-plasma interface in the model of plasma torch operation. The latter is developed in the open-source CFD software Code_Saturne. It makes it possible to calculate the flow fields inside and outside the plasma torch as well as the enthalpy and electromagnetic fields in the gas phase and electrodes. This study shows that the cathode sheath model results in a higher constriction of the cathode arc attachment, more plausible cathode surface temperature distribution, more reliable prediction of the torch voltage, cooling loss, and more consistent thermal balance in the torch.

An HVOF extension gun for spraying internal diameters down to 5” (127 mm) has been developed and evaluated. The spray gun utilizes vortex stabilization of the flame which permits operation with a minimal length combustion chamber. Small spray distances and subsequent reduced particle dwell times present a challenge for heating and accelerating powders to achieve typical HVOF coating benchmark properties such as hardness, density and residual stresses. Initial coating characteristics and process/system capabilities are discussed.

A high efficiency single electrode (HESE) version of the popular TriplexPro Plasma Gun (Sulzer Metco, Westbury, NY) has been developed and evaluated. A single electrode gun has the advantage that in most cases it can be directly or simply retrofitted to any existing conventional plasma spray system. Three electrode cascade guns like the TriplexPro platform require a unique power supply and control configuration. The design of newly developed single electrode plasma gun was based upon the TriplexPro platform and retains many of the features that contribute to the guns high efficiency characteristics, in particular the cascaded arc configuration. A study was conducted to compare the performance of the newly developed HESE gun against a conventional single electrode design. Factors examined include: voltage stability; in flight particle characterization; coating properties for selected materials and spray spot size.

This paper focuses on the use of hydrogen and nitrogen as secondary gases for atmospheric plasma spray using the TriplexPro-210 gun platform. The paper includes process mapping of particle state in addition to measurements of actual stress states within the coating during coating application. The feedstock powders used for this investigation include yttria stabilized zirconia, chromium oxide, nickel chromium aluminum and nickel aluminum. In addition, the paper discusses differences in application costs.

Triplex is a second generation plasma gun technology that offers multiple benefits in term of rates of application and deposit efficiencies. This paper focuses on Triplex technology as it relates to daily operational aspects of a typical thermal spray facility. Today, Triplex is the only plasma technology that features "fixed" parameter operation for extended run times which has significant impact at multiple levels within the spray shop. Data presented will compare Triplex to first generation plasma technology with regards to quality, training, simplification, and process repeatability.

The coating stresses induced by thermal spray using a High Velocity Gas Fuel (HVOF) and Liquid Fuel (HVLF) gun and a High Velocity Plasma (HVP) gun with the high velocity nozzle are compared using a curvature based in-situ coating stress analysis approach that measures the deflection of a beam while a coating is applied to it. This novel diagnostic tool provides new insights into the internal stresses generated in a coating system during the actual application of the coating. Coatings were sprayed with three process guns and the same material feed stock that result in similar coating structures and properties. HVOF, HVLF and HVP processes induce similar particle energy states at high velocity regimes as measured with particle diagnostic tools during spraying but due to the differences in particle history are expected to result in different coating stresses. In some cases the actual measured stress conditions using the in-situ coating stress method were dramatically different. Analysis is presented to explain the reason for these surprising results. The understanding of these differences will lead to an improved methodology for mapping coating processes from one another along with a more in depth understanding of coating stresses buildup.

Development of coatings using the TriplexPro 200 plasma gun has provided an ideal means for implementing process maps due to the large operating window in terms of particle velocity and particle temperature, as well as the flexibility to use multiple plasma gasses to tailor the coating process. Process mapping enables tracking of coating characteristics, such as hardness, and relating those characteristics to the conditions of the particle that are induced upon the particle by the process parameters. Work performed to date has provided new insights into conditions of the powder particle that result in specific characteristics in the coating. An example is the ability to determine the critical particle energy state that affects coating stress. This work affords an understanding of general theory behind coating characteristics that result from the conditions of the particle. This paper describes the parameter impact in controlling coating stresses and determining optimum particle conditions to produce a desired, or set of desired, coating characteristics.

Process mapping is an ideal method for tracking coating characteristics in the thermal spray process. With the increased utilization of in-flight particle diagnostic tools in recent years it is now possible to quickly and effectively characterize inflight powder particle properties. With industries' increasing understanding of the relationship of these properties and coating characteristics, it is now possible to rapidly understand the implications of in-process changes with respect to coating performance. This paper is an exploratory exercise that describes the utilization of process mapping of in-flight particle velocity and temperature characteristics to optimize tungsten carbide (WC) coatings sprayed with a High Velocity Plasma torch (HVP). Key performance factors of WC coatings include high inherent hardness, low porosity and neutral to compressive stress conditions. The combination of these factors all contribute to the coatings' overall success in it's intended application and elude to its toughness, wear resistance, corrosion resistance and general ability to protect the required components. Presently, the High Velocity Oxygen Fuel (HVOF) and High Velocity Liquid Fuel (HVLF) combustion processes are the favored method of applying dependable and commercially viable WC coatings that meet all of these criteria.

Various methods of calculating coating costs and process performance have been used throughout the development of thermal spray processes; each tending to be unique to each process or coating method. Comparison of the effective performance of each process, in relation to each other, is hampered and difficult to compare on an equal basis. A more generic and global method is presented, based on deriving a unified process efficiency formula that takes into account all energy inputs and energy outputs of a process in the same energy units. Applying the coating process specifics such as deposit efficiency can then be used to determine a unitized process cost in terms of energy required and subsequently coating cost. This method permits direct comparison of process efficiency for each process and specific coating conditions, promoting the advancement of more efficient and controlled thermal spray processes. Example of the results are the surprisingly low process efficiencies, less than 5%, for processes that use higher energy levels, and the highest efficiency recorded by arc wire at nearly 30%.

The use of computational fluid dynamics (CFD) to model the operation of thermal spray processes has gained interest in the thermal spray community, able to provide an understanding as to how a process functions, and better yet how to make a process work better. Advancements to the science of modeling now permits the ability to create a comprehensive model of a plasma gun that not only simulates the dynamics of the gas but also the mechanics of arcs (plasma), thermodynamics, and entrained particulates to form a nearly complete model of a working thermal spray process. Work presented includes the methods and procedures used to validate the model to a Sulzer Metco TriplexPro 200 plasma gun and exploration of the operating regime to give an in depth and insightful look into the physics behind the operation of a triple arc cascaded plasma gun.

Utilization of a comprehensive validated computer model of a thermal spray process enables an ability to improve, optimize, and fine tune the performance of that thermal spray process. A validated model of the Sulzer Metco TriplexPro 200 plasma gun has been used to improve the performance of the actual gun in terms of enhancing gas flow dynamics, thermal management, and overall performance in terms of a robust design. Internal changes to the gun geometry using the model have extended the life of the hardware beyond any current plasma gun. In addition the model has permitted the investigation of the fundamental operation of the gun, specific to the behavior and path of the arcs, as well as the ability to operate the plasma gun, under simulation, in operating regimes that currently cannot be supported by the physical hardware. The model has been run at gas pressures above 14 bar and/or voltages above 300V that currently cannot be obtained with the physical hardware due to equipment limitations to evaluate the potential to extend the operating window of the Sulzer Metco TriplexPro 200 gun beyond current levels in terms of particle velocity and temperature. The end result is an improved process tool for applying thermal spray coatings from high temperature ceramics to relatively colder and faster carbides and alloys.