Apple has filed a patent for laser cutting sapphire, a potential cover glass material for the company’s mobile devices. The patent, entitled ‘System and method for laser cutting sapphire using multiple gas media’, details two separate laser cutting stages with different assist gases.
Apple uses sapphire as a cover glass in its Watch products, but not as yet in its iPhones or iPads. The company’s link with sapphire manufacturer GT Advanced ended when GT Advanced declared bankruptcy last year.
Sapphire has a hardness of 9.0 on the Mohs scale, second in hardness only to diamond. It therefore doesn’t scratch or break easily, which makes it an attractive alternative to strengthened glass like Gorilla Glass which is currently used.
However, its hardness properties mean it is difficult to manufacture by traditional methods, which is why non-contact laser cutting is used. In addition, the brittle nature of the material means it can be damaged easily during production.
The process described is a laser fusion cut, whereby the laser beam partially melts the material which is then removed by a gas stream. The fusion cut is designed to reduce the number of defects such as micro-cracks that would weaken the part.
In some cases the inert gas used to produce the fusion cut can cause discolouring of the sapphire, according to the patent. To remove this, the material is irradiated a second time using a different gas medium with a higher percentage oxygen content.
In one example, the sapphire is cut using a laser operating at 1,070nm and a pulse width ranging from 0.2ms to 10ms, although the parameters will vary depending on the cutting operation. The second laser processing step is typically 600W, or in some cases 750W, lower in power than the laser used for cutting.
Ultrafast lasers operating at picosecond or femtosecond pulse durations have also been shown to be effective at producing high quality cuts in brittle materials like sapphire, but these systems are not part of the Apple patent.