Surface Preparation

Abstract

Thermal spray coating starts with proper surface preparation, which is an essential and critical operation on which the quality of the coating and its adhesion to the substrate strongly depend. The coating material and the nature of the substrate are the major factors in determining what kind of surface preparation is necessary to achieve optimal bonding. Substrate surface preparations comprise essentially of; machining of the part to its final dimensions, cleaning the surface, masking to prevent deposit formation on areas that are not to be coated, surface roughening followed by final cleaning. It is also important to keep in mind that the coating must never end abruptly at the part extremity and that surface preparation should be done as close as possible to the timing of the spraying operation to avoid the deterioration of the quality of the prepared surface. In this chapter, following a brief review of the surface preparation, attention is given to different technologies currently used for the cleaning and surface roughening steps, including grit blasting, high pressure water roughening, and laser treatment.

Nomenclature

Units are indicated in parentheses; when no units are indicated, the parameter is dimensionless.

1.1 Latin Alphabet

AA:

Arithmetic average (μm), Eq.14.1

c _m :

Sound velocity in the substrate (m/s)

c _s :

Shock speed in the substrate (m/s)

c _w :

Sound velocity in water (m/s)

d :

Blasting distance (m)

d _b :

Blasting distance (m)

d _o :

Sapphire orifice internal diameter for water jet (m)

d ₅₀ :

Particle diameter below which 50 wt.% of the particles are found

Ds :

Sample support diameter (m)

E _j :

Water jet energy (J)

h _i :

Peak height (m)

Ku :

Kurtosis, or third moment

L _c :

Distance to the beginning of the droplet zone (m)

l _n :

Assessment length (m), Fig. 14.5

l _t :

Transverse length (m), Fig. 14.5

L _T :

Length of the profiled section with the water jet (m)

${\dot{m}}_g$ :

Air mass flow rate (kg/s)

${\dot{m}}_w$ :

Water jet mass flow rate (kg/s)

m _p :

Particle mass (kg)

${\dot{m}}_p$ :

Grit particle flow rate (kg/s)

N :

Number of measurements (−)

p :

Blasting pressure (MPa)

P:

Number of passes

r _t :

Radius of the target area hit by blasting particles (m)

R _a :

Average roughness (μm), Eq. 14.2

R _sm :

Arithmetic mean of the widths of the profile (μm) Eq. 14.6

R _t :

Distance between the highest peak and the deepest undercut (μm)

R _z :

Root-mean square roughness (μm) Eq. 14.3

R _Δq :

Root mean square value (μm) Eq. 14.4

S _k :

Skewness parameter , Eq. 14.5

S _n :

Blasting nozzle internal cross section area (m²)

t _b :

Blasting time (s), Eq. 14.7

t _E :

Local exposure time (t_E = L_T/v_t) to the water jet (s)

t _EC :

Critical exposure time to a water jet (s)

t _Es :

Time to saturation point (s) Fig. 14.36

v _g :

Air velocity (m/s)

v _j :

Water jet velocity (m/s)

v _m :

Displacement velocity of the blasting nozzle (m/s)

v _p :

Particle velocity (m/s)

v _t :

Transverse velocity of the water jet/substrate (m/s)

x :

Sampling length (m)

z :

Surface height (μm)

1.2 Greek Alphabet

δ _uc :

Characteristic dimension of the undercut (m)

ρ _g :

Air specific mass (kg/m³)

ρ _m :

Specific mass of the substrate (kg/m³)

ρ _p :

Specific mass of the particle (kg/m³)

ρ _w :

Specific mass of water (kg/m³)

σ _p :

Surface tension liquid drop

φ :

Water jet nozzle efficiency parameter (−)

φ(x):

Distribution function (−)