The generation of internal stress in sapphire super-hard glass primarily arises from three sources: thermal stress, phase transformation stress, and mechanical stress. Thermal stress results from temperature gradients within the material during the cooling process. Phase transformation stress is induced by volume changes associated with alterations in crystal structure. Mechanical stress originates from external forces applied during manufacturing and processing stages. If these internal stresses are not adequately relieved, they may compromise the mechanical strength of sapphire glass, degrade its optical performance, and potentially lead to cracking during service.

In industrial production, the following are the most common methods used to eliminate internal stress.

1. Annealing treatment

Annealing is a traditional and effective method for eliminating internal stress. The process is carried out by heating sapphire to a precise temperature, which is then maintained for a predetermined period, thereby releasing the inherent stress in the material. Depending on the particular process specifications, annealing can be categorized in the following manner.

(1) Full annealing: The sapphire glass should be heated to above its recrystallization temperature, then maintained at that temperature before being cooled slowly.

(2) Isothermal annealing: Holding sapphire glass at a specific temperature for a long time.

(3) Stepped annealing: Annealing of sapphire glass is carried out in multiple stages, with each stage being performed at a specific temperature.

The annealing temperature of sapphire super-hard glass typically ranges from 1200 to 1500 °C, and the holding time is contingent on the thickness of the glass, generally requiring 1 to 2 hours for each millimetre of thickness. The cooling rate has a significant impact on the final stress relief effect and is usually controlled at 10-50℃ per hour.

2. Hot isostatic pressing treatment

Hot isostatic pressing (HIP) is an advanced technology for internal stress relief. Treating materials under high-temperature and high-pressure conditions is an effective method of eliminating internal defects and stresses. Typically, HIP treatment uses argon gas as the pressure medium, with a processing temperature ranging from 1000 to 1400°C and a pressure of up to 100 to 200MPa. The key benefit of this method is that it can effectively eliminate pores and microcracks in the material, enhancing its density and mechanical properties.

3. Chemical etching method

Chemical etching involves subjecting the surface of sapphire to a precise etching solution to remove areas where surface stress is concentrated. Common etching solutions include mixed solutions of phosphoric acid, sulfuric acid and hydrofluoric acid. This method is especially effective in eliminating surface stress generated during processing. However, it is crucial to strictly control the etching time and temperature to prevent any impact on the dimensional accuracy of the product.

4. Laser treatment

Laser annealing is an emerging internal stress relief technology. The product achieves precisely controlled stress relief by locally heating the material with a laser beam. This method offers distinct advantages, including energy concentration, a small heat-affected zone and strong controllability, making it particularly well-suited for products with complex shapes or local stress concentration. The commonly used laser parameters are power 100-500W and scanning speed 0.1-1m/s.

5. Ultrasonic-assisted processing

Ultrasonic vibration has been shown to promote the diffusion and rearrangement of atoms within materials, thereby accelerating the stress release process. This method is typically used in conjunction with other heat treatment processes, which can significantly reduce processing time and enhance efficiency. The ultrasonic frequency is typically selected within the range of 20-40 kHz, with a power density of 1-5 W/cm².

In actual production, the selection of internal stress relief processes needs to take into account multiple factors:

(1) Size and shape of sapphire glass

In the event of large, complex products, it may be necessary to implement a segmentation process or utilize specialized equipment. 

(2) Material properties

The stress distribution characteristics of sapphire crystals may vary depending on the growth method utilized. 

(3) Application requirements

Optical-grade sapphire glass requires additional stress relief measures to ensure optimal durability and performance.

(4) Cost-effectiveness

It is essential that sapphire products achieve a balance between the effect and the cost of production.

The internal stress relief in sapphire super-hard glass constitutes a systematic process that requires the selection of an appropriate combination of manufacturing techniques tailored to specific application requirements. Through optimization of processing parameters and integration of advanced technologies, both stress relief performance and production efficiency can be progressively enhanced, thereby ensuring reliable support for the widespread application of sapphire glass.