A metal surface has several layers:

Substrate — the bulk metal (structure set by composition + processing).
Work-hardened layer — plastically deformed during processing; thicker with dull tools / poor cutting.
Oxide layer — forms unless kept inert or a noble metal. Copper: bright when fresh → Cu₂O → CuO (dull). Stainless: chromium-oxide passivation. The oxide is usually harder than the base metal:

Metal	Oxide-to-base hardness ratio
Tin	90
Aluminum	70
Lead	20
Nickel	2
Copper	1.6
Molybdenum	0.3

On top: absorbed gas + moisture, then contaminants (dirt, dust, grease).

Surface Defects

Cracks (sharp separations), craters (shallow depressions), heat-affected zone (thermal cycling, no melting), inclusions (nonmetallic), intergranular attack (grain-boundary corrosion), residual stresses (non-uniform deformation/temperature).

Surface Texture

Flaws — random irregularities (scratches, holes).
Lay — direction of the predominant surface pattern.
Roughness — fine irregular deviations (smaller scale than waviness).
Waviness — broader deviations from flatness (like water waves).

Surface Roughness

Arithmetic mean ( $a, b, c, dots$ = absolute ordinate values, $n$ = number of readings; the datum line makes the areas above and below equal):

$R_a = \frac{a + b + c + d + \cdots}{n}$

Root-mean-square average:

$R_q = \sqrt{\frac{a^2 + b^2 + c^2 + \cdots}{n}}$

Units: µm (micrometer/micron) — note that 1000 µm = 1 mm. Also the maximum roughness height $R_t$ = height between the deepest valley and the highest peak.

Measuring Roughness

Profilometers drag a diamond stylus along the surface; the travel distance = cutoff.
The finite stylus tip radius makes the trace a bit smoother than reality.
Traces use an exaggerated vertical scale.
Roughness can also be observed with an optical or scanning electron microscope.

Roughness in Engineering Practice

Why requirements differ so widely:

Precision — bearings and gages need smooth surfaces.
Friction, wear, lubrication.
Rougher surface → shorter fatigue life.
Rougher surface → higher electrical resistance.
Rougher surface → more entrapped corrosive media.
Appearance and cost.
Subsequent painting / coating.

Typical roughness values:

Component	$R_a$
Clutch-disk faces	3.2 µm
Brake drums	1.6 µm
Crankshaft bearings	0.32 µm
Bearing balls	0.025 µm