Due to the ubiquity of smartphones, sapphire, a high-end material, is extensively utilized in the fabrication of mobile phone mirrors (i.e. screen protection glass). Sapphire is renowned for its outstanding hardness and wear resistance. However, many users have found that sapphire mirrors, which are claimed to be “scratch-free”, are susceptible to cracking during use. This article will conduct an in-depth analysis of the causes of cracks in sapphire mobile phone mirrors from three perspectives: material properties, processing technology and usage environment.

Firstly, in terms of material properties, the chemical composition of sapphire is aluminium oxide (Al2O3), and its Mohs hardness is as high as 9, second only to diamond. In theory, with the exception of diamonds and certain special abrasives, it is very difficult for everyday items to leave scratches on its surface. However, high hardness does not necessarily guarantee good toughness. Conversely, sapphire is a typical hard and brittle material with very low fracture toughness. Sapphire is susceptible to fracture when subjected to impact or bending stress, due to the rapid expansion of tiny internal defects. In the event of accidental dropping or impact, the sapphire lens has a limited capacity to absorb shock, which can result in cracks forming at stress concentration points, or even complete shattering of the lens.

In addition, the processing procedure may also introduce potential defects. Sapphire is a material of choice for mobile phone mirror surfaces due to its thin plate form and extremely high precision requirements, and it is typically manufactured using multi-wire cutting technology. This technology utilizes the high-speed reciprocating motion of metal wires to drive abrasive materials to grind sapphire crystals. However, due to the linear contact between the metal wire and the workpiece, there is high cutting resistance and low efficiency, with the potential to form microcracks and residual stress on the crystal surface. Although the subsequent grinding and polishing processes can remove some of the damaged layers, if the process control is not proper, microscopic defects that are difficult to detect with the naked eye will still be left. These defects will become stress concentration points in future use and induce crack propagation under the action of minor external forces.

Furthermore, improper usage in the specified environment is directly responsible for the observed cracking. Although sapphire is scratch-resistant, sharp objects (such as gravel and diamond particles) can still scratch its surface under sufficient pressure and speed. When the force-bearing surface of the contact point is actively small, the pressure increases significantly, which is prone to causing local damage. It is particularly evident at a force angle of approximately 45 degrees that both the depth and width of surface scratches attain relatively large values. Unlike ordinary glass, scratches on sapphire cannot be removed by simple grinding or polishing because of its extremely high hardness. Ordinary polishing powder is ineffective for this purpose, and the entire screen surface must be replaced. Sudden temperature changes have been shown to increase the risk of cracking. Sapphire is not an efficient conductor of heat. If a mobile phone comes into contact with a low-temperature object in a high-temperature environment, the thermal stress caused by the temperature difference between the inner and outer layers of the sapphire surface may lead to the initiation of cracks.

To summarize, the main cause of cracks in sapphire mobile phone screens is the inherent hardness and fragility of the material, along with the micro-defects introduced during processing. These are compounded by the inevitable impacts, scratches and temperature changes that occur during use. When using the device, users should avoid dropping and colliding, keep away from hard particles such as sand grains, and pay close attention to environmental factors such as temperature.