Definitions and criteria

Reminder
The things are often confused between the property degree and the roughness of a surface (see heading applications). It is important to remind these two notions are different and totally independent one from the other.
- property is a chemical feature.
- roughness is a physical feature.
Roughness can be considered as a supplementary element of care degree (property state) or as the essential element of control according to the nature of the later treatments or coatings. For example: paint, nickel-plating, chromium-plating, shot-peening, etc. In any case, it can naturally be appreciated for qualities or care degrees only whose covering rate is at least 95%.

Definitions
Roughness is the result of the micro geometric modification of a surface caused by the intense bombarding of this surface by projectiles (see heading definitions) It will appear thus:
- rough patches called "Peaks"
- cavities called "Hollows"
In a section perpendicular to the surface treated, the peaks and hollows are divided equally on either side of a "central line" (algebraic mean) also called "average line". The "envelope line" is the one which links the peaks tops and the hollows bottoms. The enveloped line takes into account all of the lengths created by the succession of peaks and hollows. It is important because it generates the increased surface whose adhesion will be function.

Main requirements
- increase the contact area between the substratum and the future coating because the adhesion is directly linked to the micro geometric area of the surface.
- create as many rough patches as possible on which the applied material will be fixed.
- avoid the deep hollows with a small opening that the future coating will not reach. Covered gaps could constitute corrosion starts in addition to the adhesion flaws.
- ban the "peaks" with an abnormal height whose tops might exceed or decrease the punctual thickness of a thin coating.

The accepted criteria
A great number of criteria are defined in the standardization in order to characterize a roughness. In practice, in a profile and a considered length, three of them only are taken into account:
- "Ra": average deviation. This is the arithmetic mean of the deviations absolute values between the peaks and the hollows. "Ra" measures the distance between this mean and the "central line". It is generally admitted that this notion summarizes approximately the different parameters involved in roughness.
- "Rt": total deviation. This is the most important difference in height between the highest top of a peak and the lowest bottom of a hollow.
- "Rz": regularity. This is the average of the "Rt" roughness total deviations observed in 5 lengths.

Results and controls

Reminder
The quality of an impact treatment depends on a set of parameters (see heading definitions). It ensues from this that roughness is directly linked to these parameters and above all to the projectile shape. According to whether it is spherical or angular, the profile of the roughness obtained will be different. It will give varied aspects to the surface treated.

The criteria choice
For an anticorrosion coating for example, the profile of the roughness to obtain will have to be defined according to 3 criteria:
- the nature of the material to project,
- the projection system considered, or the application process,
- the expected thickness for the coating applied (the Rt value cannot be higher to the half of thickness). Roughness will be defined thus by the 3 parameters "Ra" - "Rt" - "Rz" to which will be added the notion of profile linked to the projectile, "angular" or "spherical". The projection process, the material to project and the impact power will be determined according to the criteria of the roughness to obtain.

Operations controls
In order to supervise the quality consistency, it is possible to make periodic controls through the introduction, in the charge or the circuit of the parts to treat, of comparison specimens for whom the aspect, the roughness and the bending out of shape will be checked according to numerous marks (see heading controls)
These controls are not imposed necessarily but are necessary for a good follow-up of the operations.

Important remarks
After the impacts treatment and before the anticorrosion coating, a surface will be qualified by its care degree (property level) and its roughness (adhesion guarantee).
The implementation time of the preparation treatment and the surface coating must be the shortest possible.
The surfaces treated by impacts are particularly sensible and reactive to the atmospheric conditions. In order to avoid condensation, one will try to do the coating on a substratum whose temperature is at least superior of 3°C in relation to the dew point.