Process Diagnostics and Online Monitoring and Control

Abstract

Whatever maybe the spray process, the user expects that coating thickness and weight tolerances are respected, the reproducibility of coating microstructure and reliability of service properties are at least good and, if possible, excellent. Good quality control of coatings, before (powders… substrate preparation), during, and after the spray process presents many benefits: reduced rework, predictable performance and life linked to coatings, and high reproducibility with narrow variability. This chapter defines first what are coatings repeatability, reliability, and reproducibility and place the process control relatively to the other different errors observed in a production unit. A brief history of the influence of the spray process monitoring on coating quality is presented, with the spray process parameters that should and could be controlled. The high-energy jet characterizations (temperatures, velocities, turbulences, electrodes erosion), developed in laboratories, are presented. Sensors able to work in the harsh environment of spray both and developed since the nineties are then described. They include enthalpy probes, sensors for hot and cold in-flight particles, to measure their trajectories distribution, their steady or transient temperatures, velocities, and diameters either as ensemble (large measurement volume) or local measurements. Measurements of the coating under formation (hot gases flux, temperature, stress development, thickness) are then considered. The use of robots, artificial neural networks (ANN), and fuzzy logic (FL) to monitor and further control coating generation is then discussed. Finally, this chapter presents a few measurements that are used in laboratories and are very important for a better understanding of coatings generation such as particle vaporization, particle flattening and splat formation, and high-energy jet–liquid interaction.

Nomenclature

a :

Sound velocity (m/s)

Bi :

Biot number (Bi = h L _c/κ _p)

c _pw :

The specific heat of the water at constant pressure (J/K kg)

d _p :

Particle size (m)

h :

Heat transfer coefficient (W/m² K)

h _o :

Enthalpy of the gas sample at the probe outlet (J/kg)

h _fg :

Enthalpy of fusion (J/kg)

h _p :

Plasma enthalpy (J/kg)

L :

Spray distance (m)

L _c :

Characteristic length (m)

M :

Mach number (M = v _f/a, where a is the sound velocity)

m ⁰ _wf :

Mass flow rate of the cooling water during sampling flow (kg/s)

nf:

Subscript stands for the non-flow conditions, when no sample is being drawn

p :

Pressure of the flow (Pa)

p ₀ :

Pressure at the stagnation point of the flow (i.e. with a zero velocity) (Pa)

R :

Ideal gas constant (8.32 J/K.mole)

Re :

Reynolds number $\mathrm{Re}=\frac{d_{\mathrm{p}}{v}_{\mathrm{p}}{\rho}_{\mathrm{p}}}{\mu_{\mathrm{p}}}$

T _f :

Gas temperature near the in-flight particle (K)

T _p :

Measured particle surface temperature (K)

T _t :

Transition temperature (K)

v _f :

Flow velocity (m/s)

v _p :

Particle velocity (m/s)

We :

Weber number ($We=\frac{\rho_{\mathrm{p}}{v}_{\mathrm{p}}^2{d}_{\mathrm{p}}}{\sigma_{\mathrm{p}}}$)

Δt _m :

Particle melting time (s)

Δt _fly :

Particle in-flight time (s)

ΔT _f :

Cooling water temperature difference during sample flow (K)

γ :

Ratio of the gas specific heats at constant pressure and volume (γ = c _p/c _v)

ε _λ :

Volumetric emission coefficient of the line at the wavelength λ (W/m³.ster)

κ _p :

Particle thermal conductivity (W/m K)

μ _p :

Liquid particle molecular viscosity (Pa/s)

ρ _f :

Specific mass of the flow (kg/m³)

ρ _p :

Specific mass of the melted particle (kg/m³)

σ _p :

Surface tension (J/m²)