P. Pallav, PhD

Introduction

The term erosion is applied to wear processes in which a fluid (water, air, etc.) flows at a surface.

Most of the glossary in tribology has been established during the sixties of the previous century. In the dental field, wear became interesting only some twenty years later. What does it signify then that in dental literature the erosion caused by chewing food is often called abrasion, a term, which refers to a completely different wear mechanism? 

Clean and chemically inert fluids generally don't cause any wear under oral flowing conditions.
In the oral environment erosion is caused by specific components and properties of the fluid c.q. the food, which may contain abrasive particles. Material loss strongly depends on physical properties of the particles like strength and hardness, on the shear stress and pressure which result from the motion of the particles and on properties of the fluid such as viscosity, affinity to the surface and surface tension.

Erosive Activity

As far as we can tell the erosive wear rate is proportional to a parameter (p x v') which we named Erosive Activity (Shearing Action).

This is in fact a fluid mechanical adaptation of general wear equation. The abundant deformations of the food during chewing require the use of fluid mechanical principles if some assessment of erosive wear is to be made.
Application of the general wear equation, (p x v) presents a problem, because normally in fluid mechanics it is assumed that the velocity of the infinitely thin layer of a fluid at a surface is equal to zero. So no wear at all would be predicted!

Obviously abrasive particles move over the surface  with a velocity greater than zero. The particle in the figure will move with the average velocity of the fluid in the layer marked d, which will be half the velocity, which occurs at a distance d from the surface.

So it may be concluded that the velocity of particles at the surface is proportional to the velocity gradient.
If the Erosive Activity (EA) is defined as the product of pressure and velocity gradient, the general wear equation for erosion can be rewritten as

The constant k depends on many properties of the fluid, the particles and the surface material.

Detail

As the particles roll over the surface, the velocity at the top is less than d x v', because the fluid slows down the downward motion at the front and the upward at the rear.

The amount of friction with the fluid, the amount of 'grip' the fluid has on the particle, would be proportional to its outside area, d².
Therefore, the average micro amount of material loss, caused by a contact with an abrasive particle might be estimated to be proportional to v_p·d², or v'·d³.

Because the volume of a particle is proportional to d³ as well, the wear rate is theoretically independent of the particle size and varies only with the volume percentage of abrasive particles.