Sapphire glass is renowned for its scratch-resistant and durable properties, making it a popular choice for a wide range of applications, including smartphones, luxury watches, barcode scanners, industrial cameras, and optical instruments. However, it is important to note that there is a clear boundary to durability. While sapphires are well-regarded for their hardness and chemical stability, there are some potential areas for improvement in terms of weaknesses.

1. Why is sapphire glass considered durable

Synthetic sapphire is a single crystal form of aluminium oxide, with a Mohs hardness rating of approximately 9, placing it second only to diamond among common transparent materials. Its high level of hardness makes it extremely scratch-resistant, able to withstand scratches from objects such as metal keys, sand grains, steel tools, daily wear and tear, industrial dust, etc. Sapphire, when compared with ordinary glass or chemically tempered glass, is able to maintain optical clarity for a longer time in harsh environments.

2. The true boundary: Hardness ≠ toughness

A common misconception is that “the harder” means “indestructible”. However, in materials engineering, hardness and toughness are two distinct properties. Sapphire is renowned for its extreme hardness, though it is also considered to be relatively brittle. Sapphire glass is highly scratch-resistant, but its ability to absorb impact energy is relatively weak. It is therefore prone to cracking, splitting or breaking under the action of concentrated force. For instance, sapphire glass retains its quality even after years of use, but if its edges strike metal at a sharp angle, cracks may occur instantly.

3. Impact resistance limit

Sapphire is generally considered to have a higher surface hardness than chemically strengthened aluminosilicate glass and laminated optical glass, but a lower fracture toughness. Consequently, many smartphone manufacturers opt to utilize sapphire for small protective windows, such as camera lens covers and fingerprint sensor windows, rather than for the entire display screen.

4. Thermal performance and environmental limitations

Sapphire performs well in high temperatures, ultraviolet radiation, corrosive chemicals and vacuum environments, and is more stable than plastic and most glasses. However, it still has some durability limitations.

(1) Sudden thermal shock may cause cracks

(2) Mechanical installation stress can induce fracture point

(3) Repeated vibration will cause microcracks to expand

(4) Poor adhesion of the coating will reduce optical performance over time

5. The trade-off of optical performance

Sapphire also has some optical deficiencies

(1) The refractive index is higher than that of standard optical glass

(2) The reflectivity is higher without an anti-reflection coating

(3) The processing and polishing are very difficult

(4) High production cost

This suggests that pursuing high durability may necessitate the acceptance of costs, including increased glare, more stringent manufacturing tolerances and rising coating costs.

The true durability limit of sapphire glass does not lie in its scratch resistance, but in its fracture mechanical properties. It is one of the transparent materials with high wear resistance, scratch resistance and chemical corrosion resistance, but it is not immune to edge impact, concentrated stress, brittle fracture and thermal shock. Its true advantage lies in maintaining surface integrity for a long time in harsh environments, rather than having unlimited mechanical toughness.