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Thermal Spray Coatings Booth Equivalency Unit Plasma and thermally sprayed coatings are utilized for wear resistance, thermal protection, build-up of mis-machined parts, corrosion protection, biomedical implant strengthening, printing press roll laser engraving, textile mill applications, etc. Both commercial industries and the DoD are dependent upon the coatings. In the United States, over 3,000 plasma spray cells or “booths” are currently in operation. The delivered coatings vary in quality of engineering attributes significantly, i.e. hardness, density, thermal transfer properties, wear resistance. There are two major manufacturers of plasma spray equipment in the United States. The equipment being utilized at the plasma spray shops varies from new to over 30 years old. The CTMA Thermal Spray Booth Equivalency project was launched to address a critical need in the plasma spray coatings industry—variances from booth-to-booth or cell-to-cell for parameters which affect the plasma plume, and hence, the resultant coatings. While a multitude of design of experiments over the last approximately 15 years have shown coatings to be sensitive to as little as +2–3% variance for gas flows, amperage, and gun voltage, the cumulative measured variances exceeded 35% across 17 booths in eight facilities in five different states. The measurements were performed by the Battelle Memorial Institute under contract to General Electric Aircraft Engines (GEAE) at plasma spray shops which performed spray coatings work for GEAE. The manufacturers’ stated accuracy of some of the equipment was +15% for each gas, with typically two gases being used for spraying. Another anomaly documented was process indicated voltage: the voltage was measured at the plasma spray console as the cumulative voltage drops across the system—positive and negative cables, the plasma spray gun, connections, etc. As such, if one booth had a long set of older cables (more resistance) and an adjacent booth had a short set of new cables (less resistance), the indicated voltage could be as much as 10 volts different. It was (and still is) common practice to increase hydrogen (or helium) gas flow to achieve a desired console-indicated voltage; as such, one booth could be running hydrogen-lean while an adjacent booth might be running hydrogen-rich, and consequently producing very different coatings. The following year, the results of the differences between booths on coatings was presented. The effects were significant—tensile strengths more than doubled, and hardnesses were significantly effected. When the devices are calibrated yearly, of the various parameters being measured, the most controversial is gas flow. This is due to the sensitivity of the gas flow measuring devices, often mass flow meters, to “drift” or damage during shipping. The gas flow measurement controversy was recognized by Sandia National Laboratory, and in the mid-1990s, a new measurement axiom was proposed: measure the gas flows with two separate flow measurement devices, both +1%, and both with calibrations traceable to the NIST; if they agree, then accuracy will have been verified. NIST published the axiom in 1999. The CTMA Plasma Spray Booth Equivalency project adapted this fundamental axiom of redundant flows, with the anticipation of both selling devices to the plasma spray shops and coatings end-users, and utilizing the devices as a calibration/booth equivalency service to render the spray shops more able to produce consistent high-quality coatings. The initial booth equivalency unit concept was provided by TubalCain Company Inc. of New Braunfels, Texas. TubalCain was tasked with deployment after build, as well as assistance in helping spray shops achieve better coatings from the usage of the devices. The unit consisted of EG&G Smart Flow Turbines, with stated accuracies of +0.1% of reading, and Alicat Scientific Laminar Flow Elements, with stated accuracies of +1% of full scale. Measurement of amperage, voltage, and water flow and temperatures was also provided. This initial unit failed to achieve the axiom of agreement between gas flow measurement devices. Not only did they not agree, but also the individual meters were inconsistent from measurement to measurement. The attempt to read the gas flows was performed on a laptop computer using National Instruments Lab View software. A decision was made after the failure of the first unit to separate the two gas flow measurement devices, and introduce a new flow measurement technology: +1% Precision Rotameters. For the second measuring device, TubalCain contracted with Alicat Scientific to design and build a device encapsulating their meters to measure gas flows and pressures. Amperage and voltage were measured using calibrated Fluke Model 867B Digital Graphing multimeters. In February of 2003, the devices were connected in series on a booth at Chromalloy Turbine Components in San Antonio, Texas. The equivalent gas flow measurement axiom was finally achieved: < 1% difference was measured for all gases. Having established that the axiom of redundant measurements agreeing could be demonstrated, it remained to verify that repeatability of each device was good. To accomplish this, one plasma spray console was measured six separate times over a twenty month period. The Precision Rotameters had near-exact agreement, even though they had accumulated over 25,000 miles of transportation, and had been used on many booths in different facilities, and handled by several people. The Alicat Scientific Laminar Flow Element gas flow device did not exhibit such repeatability. This is possibly due to the need to “tare” the laminar flow elements before each measurement. The robustness of the Precision Rotameter Rack has led TubalCain to recommend its use as the standard for Plasma Spray Booth Equivalency. The primary benefit for the use of the Plasma Spray Booth Equivalency is the avoidance of work stoppages due to unacceptable coatings. TubalCain has been involved in halting several work stoppages over the last decade, using the spray booth equivalency unit to optimize coatings, and transfer those booth parameters to other spray booths. TubalCain has helped Kelly AFB and OC-ALC recover from work stoppages after projected losses of $150,000 due to the stoppages. The Spray Booth Equivalency Unit is an invaluable service for producing optimized coatings on a variety of components. Program Manager: Chuck Ryan, (734)995-4905, chuckr@ncms.org
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