**6. Wear mechanism formed during surface polishing applications**

This mechanism includes abrading and polishing mechanisms on grinding heads that are used for leveling and polishing of surfaces of different materials.

Although some of grinding mills are used in the literature, there are some significant differences between the grinding mechanisms of grinding mills. Abrading that is made with the help of abrasive grains on the surface of grinding heads is in fact quite similar with the abrading on sandpaper. Abrasive grains on the abrading product makes cutting, rubbing or ploughing as of their position. According to the application of abrading operation, complication of the mechanism is somewhere in between the mechanisms that are produced in sandpaper and grinding mill applications.

If grinding heads is turning around vertical axis on material surface, grains on it will make cutting, breaking through or friction during abrading. If there is the linear movement besides turning, the situation will be more complicated. Abrasive grain will be able to make these three moves during operation in different times.

In figure 18, the model that is developed by Lawn and Swain (1975) about crack movement on material surface and material removal. At the first contact point between grinding and surface, because of the applied loads high stress occurs. If the tip of the abrasive grain is perfectly sharp (namely, if radius of curvature is 0) stresses at this point will be infinite. These dense stresses relax with permanent changes in the shape and changes in density.

When applied loads reach a critical value, the middle crack shown with M start to increase because of tensile stress occur on vertical plane. In parallel with the decrease of load, middle fracture filling and when it reduced more, lateral cracks shown with L occur. These cracks are formed as there are residue elastic stresses after relaxation in contact points. Crack reaches surface with the removal of complete load and it cause wear with the breaking of material from surface.

According to Chandrasekar and Farris (1997), a few mechanisms are dominant in removing material from brittle surfaces. These are brittle break that is formed according to crack systems that is parallel and vertical to the surface and ductile cutting in the shape of chip similar to slim ribbon. The process that will occur depends on the load on abrasive grain, location and velocity of slip. Abrading process cause destruction in places close to the surface in the shape of small scaled crack, residual stress and permanent change in shape.

Theories on Rock Cutting, Grinding and Polishing Mechanisms 203

**Figure 19.** Slipping of grinding indenter on brittle surface and schematic demonstration of fracture

**Figure 20.** Cracks on the surface formed during abrasion (Regiani et al., 2000)

Here, it is thought that one single abrasive grain slipped over the surface and groove. Normal load is very low and groove following a permanent change in the shape without breaking. It is assumed that middle cracks are vertical to the surface and the depth is in direct proportion to the size of normal power applied on abrasive grain. Middle crack starts to unite with lateral cracks in parallel with the increase in normal power. At high loads, lateral cracks are broken and cause material wear. Again at high loads, scratches break along the middle crack and material wear occurs. A similar model is developed by Regiani et al.

after the process

(2000) (Figure 20).

**Figure 18.** Formation of crack on brittle material (Lawn and Swain, 1975)

Material wear observed in the surfaces analyzed under electron microscope are in these manners; breaking of pieces by breaking of lateral cracks in parallel with the surface, big cracks resulting from breaking of grains on the surface, breakages resulting from uniting of other cracks and radial cracks and cutting movement that produce chip like metals. Formation of mechanism is proportionate to the load on abrasive grain. If load affecting abrasive grain is little, plastic micro cutting or escalloping mechanism is dominant. Surface that is formed with this process is very remarkable with it smoothness. Plastic micro-cutting movements cause creation of chip. If big loads affect discs, brittle cracks are formed on the surface. The most common types of material loss that is caused by brittle cracks are –as mentioned before- lateral crack breakings, breaking of grains on the surface and breaking of pieces from the surface in the shape of spalling. In order to understand the mechanism better, the model developed by Chandrasekar and Farris (1987) is given in Figure 19.

systems that is parallel and vertical to the surface and ductile cutting in the shape of chip similar to slim ribbon. The process that will occur depends on the load on abrasive grain, location and velocity of slip. Abrading process cause destruction in places close to the surface in the shape of small scaled crack, residual stress and permanent change in shape.

**Figure 18.** Formation of crack on brittle material (Lawn and Swain, 1975)

Material wear observed in the surfaces analyzed under electron microscope are in these manners; breaking of pieces by breaking of lateral cracks in parallel with the surface, big cracks resulting from breaking of grains on the surface, breakages resulting from uniting of other cracks and radial cracks and cutting movement that produce chip like metals. Formation of mechanism is proportionate to the load on abrasive grain. If load affecting abrasive grain is little, plastic micro cutting or escalloping mechanism is dominant. Surface that is formed with this process is very remarkable with it smoothness. Plastic micro-cutting movements cause creation of chip. If big loads affect discs, brittle cracks are formed on the surface. The most common types of material loss that is caused by brittle cracks are –as mentioned before- lateral crack breakings, breaking of grains on the surface and breaking of pieces from the surface in the shape of spalling. In order to understand the mechanism

better, the model developed by Chandrasekar and Farris (1987) is given in Figure 19.

**Figure 19.** Slipping of grinding indenter on brittle surface and schematic demonstration of fracture after the process

Here, it is thought that one single abrasive grain slipped over the surface and groove. Normal load is very low and groove following a permanent change in the shape without breaking. It is assumed that middle cracks are vertical to the surface and the depth is in direct proportion to the size of normal power applied on abrasive grain. Middle crack starts to unite with lateral cracks in parallel with the increase in normal power. At high loads, lateral cracks are broken and cause material wear. Again at high loads, scratches break along the middle crack and material wear occurs. A similar model is developed by Regiani et al. (2000) (Figure 20).

**Figure 20.** Cracks on the surface formed during abrasion (Regiani et al., 2000)

According to Regiani et al. (2000), basic mechanisms in material wear are; grains breaking, smashing, formation of ductile chip, and spalling. Wear of these types of materials are affected from various variables. Viscosity of used liquid, applied force on the disc, type of the disc and small scaled form of the material that is abraded are the most important of these. Small scaled form of the material has a significant effect on the development of crack that is formed as a result of abrading.

Theories on Rock Cutting, Grinding and Polishing Mechanisms 205

**Figure 21.** Chip formation model (Samuels, 1971).

**Figure 22.** Discontinuous chip creation on brittle material surface (Boothroyd, 1975)

According to Boathroyd (1975), contact angle between abrasive grain and surface is very significant in the process of abrading in order to determine chip formation. on the other

As a result, there is a limiting contact angle for abrasive grains and grinding tip cuts a chip on this angle while it grooves under it. When abraded surfaces are analyzed, it was seen that

hand, deformation distribution of chip area is also affected from this contact angle.

Regiani et al. (2000) stated that small scaled formation whose shape and crystal lengths of crystals that form the material are more enduring to wear than homogenous small scaled formation whose shape and whose crystal grains' lengths are similar. Again, according to Regiani et al. (2000), intrusion, gaps and crystal grain limits behave like borders for crack progress at each type of abrading process. Used liquid, size and type of disc have the secondary importance in removing material.

As a result, volumetric wear according to different operations haven't been revealed yet. Complete understanding of wear will enable the development of productive abrading processes that will create smooth surfaces.
