Sapphire watch mirrorAs one of the core components of a wristwatch, its hardness and scratch resistance make it a standard for the brand. However, the processing difficulty of this superhard material far exceeds that of ordinary glass or mineral crystals, especially in the polishing process, which requires precise technology and strict process control. This article will analyze the entire polishing process of sapphire watch mirrors from blank to mirror surface, revealing this "hard hitting" technical game.

1、 Raw material characteristics and pretreatment

The sapphire crystal mirror is made of alpha alumina single crystal, and its crystal structure determines its anisotropic characteristics - the hardness difference between different crystal orientations can reach up to 20%. During the cutting stage, the crystal orientation needs to be determined using an X-ray diffractometer to ensure that the cutting surface is parallel to the C-axis. After being cut by a diamond wire saw, a micro crack layer with a depth of about 5-10 μ m will form on the surface of the blank, which needs to be eliminated by rough grinding. Diamond abrasive with a particle size of 15-20 μ m is used for one grinding process. During this stage, the pressure should be controlled at 0.2-0.3MPa and the speed should be maintained at 800rpm to effectively remove the damaged layer without causing new cracks.

2、 Step polishing

The polishing process generally adopts a three-level progressive system:

1. Diamond micro powder rough polishing: Use 2-5 μ m single crystal diamond suspension and operate on a cast iron polishing disc at a speed of 1200rpm. Adding alkaline lubricant with pH 8.5 can reduce surface roughness, as alkaline environment can promote hydrolysis softening of alumina surface. At this stage, it is necessary to maintain a constant temperature of 22 ± 1 ℃. Temperature fluctuations can cause the "orange peel effect" caused by differences in crystal expansion coefficients.

2. Silicon dioxide polishing: Use colloidal silicon dioxide with a particle size of 0.5-1 μ m, combined with polyurethane polishing pads.

3. Ion beam ultimate polishing: This non-contact polishing can control the surface roughness within 0.5nm.

3、 The Devil's Detail of Process Control

1. The golden ratio of pressure to rotational speed: When the product of specific pressure (unit area pressure) and linear velocity is maintained at 2.5 × 10 ^ 6 N · m/s · m ², the desired material removal rate and surface quality can be obtained. Exceeding this threshold will result in sub surface damage, while insufficient will prolong working hours.

2. The lifeline of cleanliness: The number of particles per cubic foot of air must be controlled within Class 100 (ISO 5), and the cleanroom adopts a three-stage filtration system, including ULPA ultra high efficiency filters.

3. Real time monitoring technology: The laser confocal microscope online detection system can achieve a resolution of 0.1nm, and with the help of machine learning algorithms, polishing parameters can be dynamically adjusted.

4、 Special processing techniques

1. Double sided curved polishing: For dome shaped sapphire mirrors, flexible magnetorheological polishing technology is required.

2. Surface activation before anti reflection coating: Nano scale pits are generated by plasma bombardment to increase coating adhesion.

3. The art of edge chamfering: 45 ° beveled edge polishing requires a special fixture to be used in conjunction with a diamond fiber pen, applying a micro pressure of 0.05N.

The polishing process of sapphire crystal mirrors is a combination of mechanical precision machining and materials science. With the maturity of new technologies such as ion beam polishing and plasma assisted machining, atomic level smooth surfaces may appear. But no matter how technology evolves, the pursuit of craftsmanship remains the essence of watchmaking art.