Conventional Coating Formation

Abstract

Thermal spray processes are relatively mature technologies widely used in industry. They mostly involve the introduction of either, particles (in the tens of micrometers size range) into the high-energy gas stream where they are, except for cold spray, accelerated and heated over or below their meting point, or wires, cored wires, rods, cords, which have their tip melted and atomized. The thermal and kinetic energy content of the ductile particles or droplets impinging on the substrate can widely vary with the process used. Moreover, for metals or alloys or composites sprayed in air, high process temperatures tend to increase the in-flight particle oxidation, increasing the oxide content embedded into the coating. At last the coating is formed by ductile particles or droplets flattening to form splats, which layering forms the coating. Thus the coating formation depends also strongly on substrate surface composition, microstructure, roughness and pollution. This chapter starts with the physical and chemical description of substrates with the drastic influence of the oxide layer and the mean to get rid of adsorbates and condensates. Then the impact of a single ductile particle (metal, alloy, cermet, ceramic, polymer) or a droplet is considered first on a smooth surface and then on a rough one. The way parameters characterizing flattening (Reynolds, Weber, Sommerfeld numbers), must be calculated to fit with experiments is discussed, as well as the impact direction. Coating formation is discussed from splats layering with the formation of beads and passes and the importance and means, such as robots and cooling devices, to control the coating temperature during its formation. As pointed out in previous chapters the influence of powder or wire…manufacturing process on coating properties is discussed. The different residual stresses formed during spraying are presented with their influence on coating adhesion-cohesion. Chapters ends-up with coatings finishing and the different post-treatments.

Nomenclature

a :

Ratio of flattening velocity v _f to droplet impact velocity v _p

a _i :

Thermal diffusivity (i = s for solid phase, i = l for liquid phase) (m²/s)

A/C :

Adhesion/cohesion of a coating (MPa)

b :

Splat thickness (m)

Bi :

Biot number (Bi = d_p./(R_th ⋅ κ_d)

c :

Sound velocity (m/s)

c _l :

Sound velocity within the liquid droplet (m/s)

c _pi :

Specific heat at constant pressure (i = s solid phase, i = l liquid phase) (J/kg.K)

d :

Distance in the y direction between successive beads sprayed in x direction (m)

d _d :

Thermal diffusion distance (m)

d _p :

Impacting droplet diameter (m)

D :

Equivalent splat diameter (m)

e _f :

Thermal effusivity e _f = (ρ _p ⋅ c _pp ⋅ κ _p)^1/2 (J/m² K s^0.5)

E _c :

Young’s modulus (GPa)

EF:

Shape factor; ratio of the major to the minor axis of an elliptical splat

G :

Strain energy release rate (J/m²)

h :

Convective heat transfer coefficient (W/m² K)

h′:

Coating thickness (m)

h _b :

Bead height (μm)

h _c :

Convective heat transfer coefficient (θ = 0) (W/m² K)

H :

Substrate thickness (m)

K :

Sommerfeld parameter at impact K = We ^1/2Re^1/4(−)

K _f :

Flattening splashing parameter

K _ic :

Critical interfacial stress-intensity factor (Pa m^1/2)

K _fc :

Critical value of K _f

K′:

Beam curvature (m⁻¹)

L _p :

Latent heat of solidification (J/kg)

m _p :

The particle mass (kg)

m ^°_p :

The powder mass flow rate (kg/s)

M :

Bending moment of a beam (N.m)

Ma :

Impact Mach number; Ma = v _p/c _l

p _h :

Water hammer pressure; p _h = ρ ₁ ⋅ c _l ⋅ v _p(Pa)

p _t :

Transition pressure (Pa)

P :

Splat perimeter (m)

P′:

Force (N)

Pe :

Peclet number (Pe = v _p ⋅ d _p/a _p)

r :

Radius (m)

Ra:

Average rougness (μm)

Re :

Particle Reynolds number at impact (Re = v _p ⋅ d _p ⋅ ρ _p/µ _p)

Re_N :

Reynolds number of the particle at its the normal impact velocity v _N, (-)

R _t :

Roughness: distance between the highest peak and the deepest undercut (μm)

R _th :

Thermal contact resistance (K m²/W)

s :

Thickness of droplet's solidified layer (m)

s*:

Dimensionless solid layer thickness (s * = s/d _p)

S :

Splat surface (m²)

SF:

Splat shape factor; SF = P/4 ⋅ π ⋅ S(m^− 1)

Ste _l :

Stephan number for the liquid phase (Ste _l = c _p ⋅ (T _p − T _m)/L _p)

Ste _s :

Stephan number for the solid phase (Ste _s = c _ps(T _m − T _o)/L _p)

t :

Process time (s)

t _c :

Wave propagation time (t _c = d _p ⋅ v _p/4c ²₁ ) (s)

t _i :

Induction time in cold spray process (or time of deposition delay) (s)

t _ps :

Preheating time (s)

T _m :

Melting temperature (K)

T _mean :

Coating temperature (K)

T _p :

Droplet temperature (K)

T _ps :

Preheating temperature (K)

T _s :

Substrate temperature (K)

T _t :

Transition temperature (K)

v _f :

Maximum flattening velocity (m/s)

v _N :

Normal component of the particle impact velocity (v _N = v _p ⋅ cosφ) (m/s)

v _p :

Particle impact velocity (m/s)

v _r :

The relative velocity spray torch-substrate (m/s)

v _s :

Solidification velocity (m/s)

_Vm :

Heating rate (K/s)

w _b :

Bead width at mid-height (μm)

We :

Weber number at impact (We = ρ _p ⋅ v ²_p  ⋅ d _p/σ _p)

x :

Characteristic system dimension (m)

α _c t :

Expansion coefficient of the splat (K⁻¹)

Δε :

Misfit strain

ε :

Strain

φ :

Angle between particle trajectory and an axis normal to the substrate (°)

η :

Dimensionless radius (η = 2 ⋅ r/d_p)

η _d :

Deposition efficiency

θ :

Contact or wetting angle (°)

K :

Thermal conductivity (W/m K)

μ :

Viscosity of molten particle (Pa s)

μ _o :

Viscosity of molten particle at its melting temperature (Pa.s)

ν :

Kinematic viscosity; ν = µ/ρ (m²/s)

ν′:

Poisson’s ratio

σ :

Stress (Pa)

σ _p :

Surface tension (J/m²) or (N/m)

σ _q :

Quenching stress after relaxation phenomena (MPa)

σ ^∘_q :

Quenching stress (MPa)

Ξ :

Flexural beam stiffness (Pa m⁴)