7.2 Case study 1: The kitchen knife
A kitchen knife is an everyday item that can be treated by surface engineering to greatly improve its performance. Great demands are made on kitchen knives: they are expected to retain their sharpness without regular sharpening, and they must withstand the corrosive environments that they encounter. Corrosion can arise from the natural acids contained within some foods, or the hot and humid conditions encountered in a dishwasher – including the corrosive nature of dishwashing powders.
Domestic knives are almost all now made from Stainless steel strip. Depending on the knife, the strip can vary in thickness from 0.5 to 3.0 mm, with better quality knives generally being cut from thicker strip. Initially, the strip is supplied as a coil and the basic knife shape is 'blanked out'. The profile and cutting edge of the blade are then ground onto the edge of the strip by passing the knife through a series of grinding wheels. This process is outlined in Figure 52. Finally the knife is polished to give it an aesthetically pleasing appearance and to reduce the possibility of food sticking to the surface.
Once the knife has been produced, a handle is fitted. In some knives, the handle attaches to a short 'tang' (the projecting end of the blade which allows the blade to be held firmly in the handle), and in other knives, the tang runs completely through the handle. A full tang offers a more rigid handle and therefore tends to be used on higher quality knives. The tangs are shown in Figure 53. The handle of the blade can be made from numerous different materials, such as wood, polymer or even stainless steel!
The most important property for a knife is its sharpness: it should be sharp enough to cut vegetables, etc., and it should stay sharp, and not become blunt after only a few uses.
If we want to improve the cutting performance of a knife, it might be better to improve the way in which the sharpness is retained rather than make the blade sharper. Indeed, sharper blades tend to blunt faster, because of the higher stress at the blade edge (a sharper blade carries the cutting load over a smaller area).
So is better sharpness retention possible without resorting to surface engineering? Let's examine a few possible strategies.