After months of trial and error, working with the same specialist supplier that makes IWC’s existing ceramic cases, a ratio was finally achieved that found the “optimum balance between mechanical properties and brightness.”
But that wasn’t the only hurdle to overcome. Standard oxide ceramics are sintered in normal oxygen-rich atmospheres, but it turned out that Superluminova degrades under those conditions. It had to be fired in a special inert gas mixture. “This was really challenging, because that’s not the standard way to sinter zirconia,” Brunner says. “I think that was the most difficult part.”
After figuring out how to craft the case and ensure a smooth, even shine, the IWC team had to ensure that the final product met the durability standards expected of a regular ceramic watch, known for its scratch resistance and surface hardness.
Other watch brands, such as Bell & Ross and Zenith, have infused Super-LumiNova into fiberglass, quartz, and carbon polymers to create fully or partially luminous watch cases, but no brand has attempted it with ceramic until now. “Using ceramic adds a whole other dimension to scratch resistance and durability,” Brunner asserts.
“When you mix zirconia with Superluminova, as material scientists we call it a mixture of structural and functional ceramics. The structural ceramic gives the material its physical stability, and the functional ceramic gives it a specific function, in this case luminosity,” Brunner says. “When you mix the two, you probably lose some mechanical stability, because if it was 100 percent zirconia you’d have full strength.”
“In terms of hardness, measured by Young’s modulus, we found it to be at the same level, so the resistance to scratches is exactly the same as with standard ceramic. The fracture toughness has dropped a little bit – if you drop 100 of them, there is a higher risk that some of them will break if you introduce Superluminova. But surprisingly the drop is not that dramatic. We did all the (usual) tests in-house, we did shock tests from 25G to 5,000G, and it passed all the tests.”
The duration of the luminescence of a Ceralume watch is directly dependent on the amount of Super-LumiNova used. Despite being inlaid with ceramic, it is still able to absorb a significant amount of energy from sunlight.
The case, on the other hand, is not damaged by UV rays. “It’s a matter of mass,” Brunner asserts. “In a normal watch, you might have Super-LumiNova on some of the hands or indices, but it’s very small amounts, a few grams. Here it’s the whole case, and then there’s more Super-LumiNova on the strap.”
Brunner says the strap is relatively easy to design, without the fear of sintering, shrinking or fading. “Of course, one of the key aspects is to get it perfectly homogenous, which is always difficult when you’re mixing, for example, plastic or resin with hard particles,” he says.
