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Online Characterization Of Particles During Thermal / Cold Spraying
  Thermal/Cold spray processes (Plasma, Flame, HVOF, Wire-arc & ColdSpray) are used for several decades now in order to apply functional coatings on various substrates/parts.  A few examples of such coatings are:
  • Thermal Barrier Coatings (TBCs) for combustion chambers (aircraft & and based turbines);
  • Wear Resistant Coatings for tools or landing gears
  • Biocompatible coatings for human body implants;
  In the case of thermal spray, a material initially under the form of solid powder or wire is fed to the spray torch, melted & accelerated toward the parts to be coated.  Before impinging on the substrate, the material is under the form of liquid droplet/particles the size of which can range from 1 to 150 microns depending on the application.  The particles typically travel at velocities ranging from 50 to 1200 m/s depending on the type of process/spray conditions being used.  Finally, the surface temperatures reached by those particles are typically between 1200 & 3500 Celsius.

(HVOF Spraying)                                                                                  (Plasma Spraying)

Pictures courtesy of NRC-IMI (Boucherville, Qc, Canada)
  In the case of ColdSpray, solid metallic (and more recently cermet) particles preheated between 100 & 800 Celsius are fed to a very high-speed gas nozzle (He, N2 or air) that will accelerate them to velocities between 400 & 1200 m/s.  Upon impingement on the part, the metallic particles that have reached or surpassed their so-called “critical velocity” will plastically deform & stick to the part.
  For both Thermal & Cold Spray processes, there are several parameters that will directly affect reproducibility of coatings.  A few examples of such parameters are:
  • Plasma current, primary & secondary gas flows
  • Oxy-fuel ratio (HVOF)
  • Wire spool radius (wire-arc)
  • Oxidation during spraying
  • Cooling water temperature
  • Gas purity
  • Carrier gas flow
  • Substrate temperature
  • Quality of feedstock materials as well as initial temperature
  • Part preparation
  • And more!
  Around the mid 80’s, the thermal spray community recognized that in order to bring thermal spray processes to the next level, there was a need for sensors able to provide real-time, in-flight particles information just before impingement on the part. Various researchers around the world started to develop such sensors. Some (non-exhaustive list) of the most well known pioneers in that field are:
  • Professor Pierre Fauchais & Michel Vardelle (University of Limoges, France)
  • Dr. Jim Finck (US National Labs network)
  • Dr. Christian Moreau (NRC-IMI, Canada)
  • Professor Maher Boulos (University of Sherbrooke, Canada)
  • Professor Joachim Heberlein (University of Minnesota, USA)
  • Professor Klaus Landes (University of Munich, Germany)
  In the case of thermal spray, the particles are very hot so they emit IR radiation.  That radiation is used to actually “see” the particles & in turn characterize them (temperature, velocity, size, flux, trajectory).  VELOCITY is usually the result of a time-of-flight measurement, whereas TEMPERATURE & SIZE are normally calculated using the two-color pyrometry technique & assuming that:
  • The sprayed material behaves as a gray body
  • The melted particles are spherical
  In the case of ColdSpray however, particles are too cold (below 800 Celsius) to be detectable by conventional pyrometric techniques.  Therefore, the approach consists of shining light on them and looking at the scattered light.  In other words: we work with light reflection rather than light emission.  VELOCITY, SIZE & FLUX can then be measured, but not temperature since the illumination light directly influences the pyrometric measurement.
  In 1994, TECNAR has been the pioneer company in offering the first commercially available thermal spray sensor (DPV-2000).  Ever since, TECNAR has remained THE leader in that field with more than 450x sensors (DPV-2000/eVOLUTION, ColdSprayMeter, Accuraspray G3 & G3C, PlumeSpector, ShotMeter, SprayView) out in the field all around the world.
  To know more about this system or to receive a quotation, please do not hesitate to contact us at: sales@tecnar.com
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