One of the criteria in selecting wire guides is the hardness value. Ultra hard materials such as high alumina and tungsten carbide are preferred over plastics or steel. Figure 1 shows the Vickers hardness of the ultra hard materials that we use and their fracture toughness:
Another criterion in selecting wire guides is the surface finish, which attributes to the guides’ coefficient of friction against the wire.
Scientific means are employed to ascertain the surface finish in unambiguous numbers. One such instrument is the diamond probe profilometer, which gives readings in microns or micro inches. One micron is equal to 40 micro inches. A lower profilometer reading generally indicates a smoother surface finish. An interesting yet little known fact is that higher hardness is always associated with a finer surface finish, while mono-crystalline (that is, a single crystal) materials like ruby and sapphire finish better compared with polycrystalline materials of the same hardness.
In recent years Fracture Toughness, a measure of the resistance of a material to the propagation of a crack, has had an increasing involvement in the interpretation of wear phenomena.
It can be seen in Figure 1 that transformation toughened Zirconia (TTZ) and tungsten carbide offer higher fracture toughness compared with sapphire and alumina s.
Over the years, extensive tribologial tests have been performed on the ultra hard materials that we choose for wire guides.
Synthetic Sapphire from one single source* and of the same lot is chosen as the control and subject to an identical set of testing parameters. By dividing the unit weight loss of the material under the test by that of the synthetic sapphire, a specific wear rate is obtained.
It can be seen that among the materials tested, TTZ Zirconia, of an inferior hardness but of a higher fractural toughness, outwears synthetic Sapphire. This is also true in the case of tungsten wire. All the polycrystalline Alumina s of an inferior hardness and adjacent fracture toughness perform less satisfactorily. The spread in wear resistance among the alumina s is attributed to the percentage of the alumina content; the compacting pressure of the green body; and the mesh distribution (of the powder). It can be postulated at this writing that hardness and toughness should be considered hand in hand when selecting ultra hard materials for wire guides