Abstract
D.C. plasma jets temperature and velocity distributions as well as the arc root fluctuations at the anode were studied for Ar-H2 (25 vol%) plasma forming gases. The parameters were the arc current up to 700 A, the total gas flow rate up to 100 slm, and the nozzle diameter which was varied from 6 to 10 mm. The trajectories of partially stabilized zirconia particles into the jet were studied by a 2D laser imaging technique and two fast (100 ns) two color pyrometers. The results have revealed the difficulty to inject small particles into the plasma flow since most were found to by-pass the jet rather than penetrate it. The results also show the broad trajectory distribution within the jet and the influence of the arc root fluctuations on the mean particle trajectory distribution within the jet. Beside the measurements of the particle surface temperature and velocity distributions in flight, the particle flattening and the cooling of the resulting splats were studied statistically for single particles all over the spray cone. Such studies have emphasized the drastic influence of the substrates or previously deposited layers temperature on the contact between them and the splats. At 200–300°C this contact is excellent (cooling rates of the order of 100 K/μs for 1 μm thick splats) and it results in a columnar growth within the splats and the layered splats of a bead (up to 500 layered splats). This growth can be observed through passes provided the bead surface temperature has not cooled too much (a few tens of K) before the next bead covers it. A/C values up to 60 MPa were achieved with PSZ coatings. The effect of impact velocity of the particles, of substrate preheating temperature, of relative movments torch to substrate, of substrate oxidation on A/C values and splat formation were also studied.
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Abbreviations
- a:
-
thermal diffusivity (m2/s)
- cp :
-
specific heat at constant pressure (J/kg.K)
- D:
-
splat diameter (μm)
- d:
-
nozzle internal diameter (mm)
- di :
-
internal injector diameter (mm)
- dp :
-
particle diameter (μm)
- E:
-
Young's modulus (GPa)
- f:
-
restrike mode frequency (Hz)
- I:
-
arc current (A)
- L:
-
latent heat of fusion (J/kg)
- mcg :
-
carrier gas flow rate (slm)
- mg :
-
plasma forming gas flow rate (slm or kg/s)
- mp :
-
powder mass flow rate (kg/h)
- q ow :
-
losses in torch cooling water (W)
- r:
-
radial distance from the torch axis (mm)
- Rth :
-
thermal contact resistance (m2.K/W)
- Tf :
-
coating surface temperature (°C)
- Ti :
-
substrate surface temperature after preheating (°C)
- Ts :
-
particle surface temperature (K)
- V:
-
arc voltage (V)
- vg :
-
plasma gas velocity (m/s)
- vo :
-
axial gas velocity at the torch nozzle exit (m/s)
- vp :
-
particle velocity (m/s)
- vr :
-
cylindrical substrate rotation velocity (rpm)
- vt :
-
torch translation velocity (m/s)
- zsp :
-
stand off distance (mm)
- α:
-
linear expansion coefficient (m/m.K)
- δ:
-
maximum thickness of one bead (μm)
- δp :
-
pass thickness (μm)
- δs :
-
splat thickness (μm)
- Δv:
-
vg - vp (m/s)
- η:
-
thermal efficiency of the torch [ ϱ = (V .I. - q ow )/VI ] (%)
- κ:
-
thermal conductivity (W/m.K)
- \(\bar \kappa \) :
-
mean integrated thermal conductivity (W/m.K)
- κp :
-
particle thermal conductivity (W/m.K)
- ν:
-
Poisson's coefficient (−)
- ϱ:
-
specific mass (kg/m3)
- ζ:
-
flattening degree (−) mean integrated thermal conductivity
- A/C:
-
adhesion/cohesion
- FC:
-
fused and crushed
- PSZ:
-
partially stabilized zirconia (ZrO2 + 8 wt% Y2O3)
- RC:
-
average cooling rate of a splat (K/μs)
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Fauchais, P., Vardelle, M., Vardelle, A. et al. Parameters controlling the generation and properties of plasma sprayed zirconia coatings. Plasma Chem Plasma Process 16 (Suppl 1), S99–S125 (1995). https://doi.org/10.1007/BF01512630
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DOI: https://doi.org/10.1007/BF01512630