Sapphire meets metal: precision welding of endoscope lens
Endoscope Deep into the human body, its end lens is like the extended eye of a doctor. The traditional optical glass It is already weak. Sapphire glass, with its extremely high hardness, excellent chemical inertness and biocompatibility, is an ideal choice for top endoscope lenses. However, firmly and reliably welding this piece of synthetic diamond like material with the metal lens tube is a demanding precision connection challenge.
Metallized coating: “messenger” of sapphire and solder
Sapphire is essentially pure single crystal aluminum oxide (Al ₂ O ∨), and its surface is like smooth ice, which is difficult to be infiltrated and combined by conventional metal solder. If the metal solder is forced to be heated and melted on the sapphire surface, the solder will only shrink into a bead shape and cannot form an effective connection – this is the direct reflection of “insufficient wettability”. The endoscope lens assembly and the metal lens cylinder must form a firm, closed and long-term stable connection structure to isolate body fluid intrusion. Therefore, the first key step of the “endoscopic lens welding metallized sapphire glass process” is metallization.
Precise thin film deposition technology is required to obtain a metal layer with high strength and good bonding with solder on sapphire substrate. Physical vapor deposition (PVD) process is an ideal choice, such as magnetron sputtering: high purity titanium (Ti) or molybdenum manganese (Mo Mn) alloy is used as the backing layer to directly deposit on the surface of sapphire. Titanium atoms can form a stable bond with oxygen atoms on the surface of sapphire lattice, providing a strong “anchoring” force. On top of the titanium layer, a layer of gold (Au) or platinum (Pt) is precisely covered as the anti-oxidation surface and solder wetting layer. The thickness of this composite metal layer is usually at the level of submicron to several micrometers, but it is precisely this layer of precision slicing that is invisible to the naked eye, which has built a solid bridge between sapphire and subsequent solder.
Precision brazing: strength challenge in extremely narrow space
In the welding process, the core challenge is how to make the sum of welding thermal stress and internal stress of sapphire lower than its fracture limit – sapphire is hard but brittle under the premise of ensuring high strength and strict sealing. Technically, triple control must be realized:
- Matching CTE solder design: The expansion coefficient of solder alloy should be between sapphire (about 5-7 x 10 ⁻⁶⁶⁶⁶⁶⁶⁶/K) and mirror tube metal (such as titanium alloy about 8-9 x 10 ⁻⁶⁶⁶/K or 316L stainless steel about 16-18 x 10 ⁻⁶⁶/K). Gold based solders (such as AuSn, AuGe) and some high melting point active solders (Ag based alloys containing Ti) are widely used in this field. They not only have proper expansion performance, but also have excellent fluidity and corrosion resistance.
- Accurate temperature control curve: The vacuum or protective atmosphere (such as high-purity argon) furnace must be used for welding, the heating rate must be strictly controlled (to avoid temperature shock), the brazing temperature must be accurately reached and maintained (for example, AuSn eutectic solder needs to be heated to more than 280 ° C and accurately insulated), and then slowly cooled (annealing to release welding stress).
- Precise structure and interface control: The weld width is usually controlled within a very narrow range (such as 0.05-0.2mm) to reduce the overall thermal stress. During brazing, the molten solder must fully infiltrate the surface of the metallized layer and fill the entire gap capillary, while limiting its contact area with the sapphire to the maximum extent (only at the edge), to prevent sapphire strength from weakening due to diffusion and penetration of metal elements or stress concentration.
This process is like carving the path of flame on the ice: the strong and tough metallurgical combination between the solder and the metallized layer makes the sapphire core and the outer mirror tube build a solid connection that can withstand millions of times of steam disinfection. The interface of precision welding not only carries the external mechanical force, but also is the boundary guarantee of biocompatibility – to prevent the copper and nickel elements in solder from dissolving out to the human side, meeting the core indicators of endoscopic biosafety standards.
Application prospect of sapphire lens in surgery
In minimally invasive interventional surgery, such as complicated biliary tract lithotomy, the conventional endoscope lens may have slight scratches due to tool collision, which may affect the imaging clarity. Sapphire glass, with its excellent abrasion resistance, has always maintained a high optical transmittance, enabling doctors to clearly identify small bile duct branches and stone forms. Its surface finish can be maintained for a long time to avoid image blurring caused by dirt accumulation, and significantly reduce the dependence on the lens during surgery. When the surgical tool moves rapidly in the narrow bile duct, the impact strength of sapphire can better resist the accidental impact of metal instruments, and prevent the lens from breaking and causing secondary injury.
Conclusion: life channel led by precision welding
The precision welding of sapphire glass and metal mirror tube goes beyond the limitation of traditional mechanical fixation and realizes the complementary fusion of material properties on the micrometer scale. Every radiofrequency catheter navigation and every endoscopic tumor resection rely on the sapphire at the core of the lens to stabilize imaging in extreme chemical environments. It is not only the crystallization of industrial technology, but also the cornerstone of minimally invasive surgery to break the limit – providing surgeons with not only a clearer window, but also a security guarantee for in-depth exploration of life organization.
From the perspective of industrial innovation, this mature sapphire welding system is expected to extend to a wider range of precision optics and high-end sensor fields. Its core connection technology makes the endoscope become the first pair of eyes to explore the interior of life. It breaks through not only the barrier of space, but also the scientific domain related to life and health.