The best Hardened and Tempered steel used for scissors has a hardness of approximately 55 Rockwell C. This is equivalent to HV 600 or thereabouts on the Vickers scale, usually used for hard materials. The compressive yield strength for this type of steel is not likely to be higher than 1,500 MPa.
Zirconia ceramic has a Vickers hardness of 1250, and a compressive strength of over 2000 MPa. The hard, strong edges on the ceramic blade cannot deform by plastic flow as may occur on the steel blade - its well known that ceramics cannot deform plastically at normal temperatures - and so the fibres are penetrated and cleanly sheared over a long service life.
The Zirconia ceramic blades are made and ground near Stuttgart by a company who also make machine scissor blades from the same zirconia ceramic material for automatic splicers and textile cutters. The ultra-fine grain material retains its edge particularly well, since the Young's Modulus is quite low and similar to that of steel, so that a degree of elasticity is built into the cutting system.
Tricks of the scissor-smith
The blades on a steel scissor are hollow ground on their contacting faces, and convex ground on the knife edges. The blades are then assembled and skilfully twisted and bent until the correct 'set' is achieved, giving the advancing cutting point the correct preload as the crossover point moves along the length of the blade.
The disadvantage of using Yttria-Zirconia Ceramic material for scissor blades is that it is (a) very expensive and (b) extremely difficult to grind, being one of the most wear resistant materials in the world. However, using special diamond wheels, it is possible to get a really good corner on the material which is both chip-free and very smooth. As explained above, ceramic blades cannot be bent to shape, so how was the ceramic scissorsmith to solve the problem of the 'set'?
The Oriental solution
The first scissors with ceramic blades were made in Japan, and had blades which were ground flat on the faces and straight on the knife edges - cheap to do using standard grinding techniques. Many thousands are still made this way today, but they all have the same inferior performance because the 'set' cannot be made progressive or precise as with a traditional steel scissor, while the straight, as opposed to convex, blade meant that the shearing angle diminished rapidly as the cut reached the tip area of the blade. For most of our competitors ceramic products, this is still the case. The pre-loading of the blades is done by inserting a bump or cam at the far side of the blade pivot, which forces the cutting edges together.
The European precision-ground product If you need a bigger ceramic-bladed scissor, or one for a machine application, please contact us for alternative designs.
This imprecision was not at all to the liking of our German scissor producer, and so a special grinding machine was purchased at great expense to produce the hollow grind, the twist and the set all by contour grinding the solid ceramic material. As a result, the action, precision, cutting quality and longevity of Peiniger's ceramic bladed scissors is second to none.
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If you need a bigger ceramic-bladed scissor, or one for a machine application, please contact us for alternative designs.