Surface Preparation

Abstract

Thermal spraying begins with proper surface preparation, which is absolutely essential. Steps must be undertaken correctly in order for the coating to perform the design expectation because coating adhesion quality is directly related to the cleanliness, the roughness and sometimes the proper machining for optimal coating performance. The coating material and the nature of the substrate are the major factors in determining what kind of surface preparation is necessary to achieve a resistant bonding. It is also important to keep in mind that the coating must never end abruptly at the part extremity. The different surface preparations comprise: machining, cleaning by various means, and masking preventing deposit formation on areas where it is not wanted. The surface roughening, characterized by different measurements, can be achieved by three main means: grit blasting, high pressure water roughening and laser treatment, which are described in details in this chapter. This description comprised the equipment, the main parameters and their action on surface roughness, with, for grit blasting the influence of the grit used (material and size).

Nomenclature

a :

Radius of the area hit by blasting particles (m)

AA:

Arithmetic average $\mathrm{AA}=\frac{h_1+{h}_2+\dots {h}_N}{N}$

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 _o :

Sapphire orifice internal diameter for water jet (m)

E _j :

Water jet energy (J)

l _n :

Assessment length (m)

l _t :

Transverse length (m)

L _T :

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

m ^∘_g :

Air mass flow rate (kg/s)

m ^∘_w :

Water jet mass flow rate (kg/s)

m ^∘_p :

Grit particle flow rate (kg/s)

p :

Blasting pressure (MPa)

Ra:

Average roughness (μm) $\mathrm{Ra}=\frac{1}{\mathcal{l}}{\displaystyle {\int}_0^{\mathcal{l}}z(x)}\; dx$

R _q :

Root mean square roughness (μm) ${R}_q=\sqrt{\frac{1}{\mathcal{l}}{\displaystyle \underset{\mathcal{l}}{\int }z{(x)}^2\mathrm{d}x}}=\upsigma$

R _t :

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

R _sm :

Arithmetic mean of the widths of the profile (μm)

R _Δq :

Root mean square value ${R}_{\varDelta q}=\sqrt{\frac{1}{\mathcal{l}}{\displaystyle {\int}_0^{\mathcal{l}}{\left(\frac{\mathrm{d}z(x)}{\mathrm{d}x}\right)}^2}\mathrm{d}x}$

S _n :

Blasting nozzle internal surface (m²)

S _k :

Skewness parameter ${S}_k=\frac{1}{\sigma^3}{\displaystyle {\int}_{-\infty}^{+\infty }{\left(z-m\right)}^3}\varphi (x)\mathrm{d}x$.

t _b :

Blasting time (s)

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)

v _g :

Air velocity (m/s)

v _j :

Water jet velocity (m/s)

v _p :

Particle velocity (m/s)

v _t :

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

φ :

Water jet nozzle efficiency parameter (-)

ρ _g :

Air specific mass (kg/m³)

ρ _m :

Specific mass of the substrate (kg/m³)

ρ _w :

Specific mass of water (kg/m³)