Next-gen watch balance springs are grown, not drawn
The watch spring, it turns out, is an excellent test bed for materials research. Scientists at the Swiss Federal Laboratories for Materials Science and Technology (Empa) have developed a technique for growing these tiny springs to specifications using photo-etching and electrochemical depositing in cold, aqueous saline solutions instead of conventional drawing and winding techniques.
Balance springs are the indispensable heart of every mechanical clock or watch, and up until now creating one was one of the most difficult parts of the watchmaker's jobs. Balance springs need to be fashioned out of precisely controlled alloys such as Nivarox, drawn to a thickness measured in hundredths of a millimeter, and then wound and tempered to the exact balance of hardness, ductility, and elasticity while compensating for variables such as temperature, humidity, and magnetic fields.
According to Empa, this makes balance springs a very interesting subject for pure materials research because many substances change their properties during fabrication with ductile materials becoming harder and others, like ceramics, starting out brittle, but becoming more flexible.
To study this, an Empa team looked at an unorthodox way of making springs. Instead of drawing wire, they took silicon wafers and coated them with gold and a thin layer of light sensitive paint. This allowed the researchers to etch the shape of a spring into the wafer. The spring was then dipped into a galvanic bath containing a salt solution of a metallic alloy. With the spring acting as the cathode, an electric current was passed through the bath and the metal deposited itself on the spring's gold base.
This may sound simple but, according to Empa, a lot depended on keeping the bath just right.
"We need a good swirl in the galvanic bath, the right temperature, some organic additives and a current at just the right strength and – if it's alternating current – in the right form," says Laetitia Philippe, who oversees the production of the springs.
Once the spring is built up, they are dissolved out of their mold and examined under a microscope. If is has filled properly, it's polished to the desired thickness and checked again using X-ray fluorescence analysis before removing the paint with oxygen plasma and the silicon using a strong alkaline solution. The final spring is washed to smooth it out and then is sent to the watch lab for prototype production.
Empa emphasizes that the purpose of this is not simply to find a new way to make watch balance springs, but to gain a better understanding of materials that go into them and how they behave.
"The prerequisite for any examination, however, is that we are able to produce the objects we are interested in based on defined criteria," says Empa researcher Johann Michler. "Some process steps are closely intertwined," says Michler. "If we change one parameter, such as the geometry of the electroplating molds or the composition of the alloy, we usually have to adjust the preceding and subsequent steps, too. We want to understand these connections and the effects of miniaturization in every aspect."
Empa says that now that the first pilot tests have been completed, the next step will be to produce a steady supply of the springs for a major Swiss watchmaker to see how well they hold up inside of prototype watch mechanisms. In the meantime, the electroplated springs can be seen on display at the Laboratory for Mechanics of Materials and Nanostructures at the Empa campus in Thun, Switzerland.