CNC tools - proper insert grade selection

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A shoe must be comfortable on the feet, made of material that is soft and that molds itself to the shape of the foot. Leather has this quality, so we make shoes out of leather. But this same quality of softness causes problems on the sole, which wears out fast. We want the body of the shoe to be soft, but the sole to be hard. So, to get a hard sole that withstands wear and tear and has a long life, we stick a sheet of hard rubber on the sole. We are combining the comfort of the leather with the long life of the hard rubber. We have a similar problem with inserts, and a similar solution is used. We want the cutting edge to be hard so it can cut faster and have a longer life.
Unfortunately, the harder a cutting tool material is, the more brittle it is and the easier it breaks. We need two opposite qualities, toughness and hardness. We want the body of the insert to be tough, but the cutting edge surface to be hard, with high wear resistance. To improve the wear resistance and life of an insert, we just coat it with hard but brittle materials like Aluminum Oxide, Titanium Nitride, or Titanium Carbonitride. This picture (pic. with shoes, milling insert - plain and coated carbide) shows a plain Tungsten Carbide insert, the grey one on the left, and a coated insert, the gold one on the right. An insert may have multiple coatings, of different materials. The cross section in this picture shows 3 different coatings, each between 2 microns and 10 microns thick.