“The design and production of a ceramic watch case is a lengthy and difficult undertaking. IWC’s engineers liaise closely with their suppliers’ own specialists in the search for the best solutions,” says Brunner, in summary. One of the more unusual challenges is that ceramic shrinks by about a third during the sintering process. To ensure that the movement later fits snugly inside the case, the shrinkage has to be factored in as early as the design phase. And there is another unusual feature: unlike metals, whose properties are clearly defined and finalized before machining, ceramics – and thus the finished product – are influenced by the individual stages in the manufacturing process. The use of different sintering methods, together with the chosen grain size and sintering temperature, can lead to end products with significantly different properties from the same basic materials.
In the case of the brown ceramic used for the Pilot’s Watch Chronograph Antoine de Saint-Exupéry, the engineers carried out countless attempts and had to feel their way towards the optimum manufacturing process. During sintering, for example, the silicon nitride must not come into contact with oxygen because it could otherwise oxidize. This meant that the cases had to be baked in an oven at 1800 degrees Celsius using nitrogen to create an inert gas atmosphere. To make the material even denser, it is sintered under overpressure. The sumptuous brown colour was achieved by mixing the silicon nitride with titanium nitride.
This specific combination of raw materials makes the sintered silicon nitride even harder and more shatterproof than most other ceramics. But this, in turn, posed an additional challenge for the subsequent machining of the case. “The fact that the new material was so hard and robust made the milling process enormously hard work and we found ourselves replacing the diamond-tipped tools much more frequently than usual,” reminisces Brunner.