Silicon has changed the hierarchy of materials in watchmaking more than any other innovation in the last fifty years. It is not a metal: it is a crystalline semiconductor, the same foundation as semiconductors, and this industrial origin explains why silicon components are produced using MEMS technology — Micro-Electro-Mechanical Systems — the same processes used for chips. At the bench, silicon parts are immediately recognizable: slightly iridescent surface, color varying from anthracite to violet depending on thickness, sharp edges that would not exist with traditional machining.
The technical advantages are four and they compound. Silicon is about one-third lighter than steel, which reduces inertia in oscillating components and lowers the energy consumption of the escapement. It is diamagnetic: magnetic fields do not disturb it, unlike steel which magnetizes and alters the balance wheel frequency. It does not corrode and requires no lubrication, or only minimal quantities, which extends service intervals. Finally, the coefficient of thermal expansion is low and stable: the hairspring maintains its elastic properties even with temperature variations that would render a traditional movement inaccurate.
The fields of application are three. The hairspring — called Silinvar by Patek Philippe, Syloxi by Rolex in caliber 2232 and in the 1908 and Land-Dweller movements — is the component where the thermal and magnetic benefits weigh most heavily. The escapement — escape wheel and pallet fork — is the point where silicon reduces friction in the most measurable way: Rolex in caliber 7135 with the Dynapulse escapement claims an efficiency gain of around 30% compared to conventional Swiss escapement, achieved also thanks to an impulse mechanism via intermediate pinion. Ulysse Nardin added a diamond-like surface treatment with DIAMonSIL technology, further increasing hardness. The third field is rarer: Girard-Perregaux in the Neo Constant Escapement has derived from a silicon wafer of only 14 microns a constant-force device integrated into the pallet fork itself, exploiting the buckling phenomenon — the wafer flexes and returns to its original shape releasing uniform energy throughout the power reserve duration.
The main limitation of silicon is fragility to transverse impacts: it is hard but not tough, it chips rather than deforms. For this reason some manufacturers pair silicon components with titanium or non-magnetic alloys like Nivagauss for parts subject to more violent mechanical stresses. I am not aware of documented cases of artisanal laboratory repair of silicon MEMS components: they are replaced, not repaired.