The generation of internal stress in sapphire superhard glass mainly comes from three aspects: thermal stress, phase transition stress, and mechanical stress. Thermal stress is caused by temperature gradients during the cooling process of materials; Phase transition stress is a volume change caused by changes in crystal structure; Mechanical stress comes from external forces during the machining process. If these internal stresses are not eliminated in a timely manner, they can lead to a decrease in the mechanical strength and optical performance of sapphire glass, and even fracture during use. In industrial production, commonly used internal stress relief methods mainly include the following:

1. Annealing treatment

Annealing is a traditional and effective internal stress relief method, which involves heating sapphire to a certain temperature and holding it for a sufficient period of time to release the internal stress of the material. According to different process requirements, annealing can be divided into:

① Complete annealing: Heat the material above the recrystallization temperature, hold it for a while, and then slowly cool it down

② Isothermal annealing: holding at a specific temperature for a long time

③ Segmented annealing: using multiple temperature stages for annealing

The annealing temperature of sapphire superhard glass is usually between 1200-1500 ℃, and the holding time depends on the thickness of the workpiece, generally maintaining 1-2 hours per millimeter thickness. The cooling rate has a significant impact on the final stress relief effect, usually controlled between 10-50 ℃/hour.

2. Hot isostatic pressing treatment

Hot isostatic pressing (HIP) is an internal stress relief technique that effectively eliminates internal defects and stresses by treating materials in high-temperature and high-pressure environments. HIP treatment usually uses argon gas as the pressure medium, with a treatment temperature of 1000-1400 ℃ and a pressure of up to 100-200MPa. The advantage of this method is that it can simultaneously eliminate pores and microcracks inside the material, improving its density and mechanical properties.

3. Chemical etching method

Chemical etching is the process of treating the surface of sapphire with a specific corrosive solution to remove areas of surface stress concentration. The commonly used etching solution includes a mixed solution of phosphoric acid, sulfuric acid, and hydrofluoric acid. This method is particularly suitable for eliminating surface stresses generated during processing, but strict control of etching time and temperature is required to avoid affecting the dimensional accuracy of the workpiece.

4. Laser processing

Laser annealing is an emerging internal stress relief technology that achieves accurate stress relief by locally heating the material with a laser beam. This method has the advantages of energy concentration, small heat affected zone, and strong controllability, and is particularly suitable for processing workpieces with complex shapes or local stress concentration. The commonly used laser parameters are a power of 100-500W and a scanning speed of 0.1-1m/s.

5. Ultrasonic assisted processing

Ultrasonic vibration can promote the diffusion and rearrangement of atoms inside materials, thereby accelerating the stress release process. This method is usually used in conjunction with other heat treatment processes, which can significantly shorten processing time and improve efficiency. The ultrasonic frequency is generally selected between 20-40kHz, with a power density of 1-5W/cm ².

In actual production, the selection of internal stress relief process needs to consider multiple factors:

① Workpiece size and shape: Large and complex workpieces may require segmented processing or special fixtures

② Material properties: Sapphire crystals grown by different methods may have different stress distribution characteristics

③ Application requirements: Optical grade sapphire requires more thorough stress relief

④ Cost effectiveness: Balancing processing efficiency and production costs

The internal stress relief of sapphire superhard glass is a systematic engineering that requires the selection of appropriate process combinations based on specific application requirements. By optimizing processing parameters and introducing new technologies, the stress relief effect and production efficiency can be continuously improved, providing reliable guarantees for the widespread application of sapphire glass.