Laser systems depend on the precise alignment of mirrors and prisms to deliver high-energy light beams along desired paths. To mount these optical components to a delivery platform, designers find that adhesives can provide a particularly attractive solution. Bonded mountings result in simple mechanical designs that are light and inexpensive, offering reduced interface complexity and ultimately leading to more compact systems with fewer parts. If adhesive bonds fail, however, the optical system can quickly lose alignment and even experience catastrophic failure. Loss of laser accuracy and even overall functionality is unacceptable in any production laser system, but in military systems, ineffective bonding of optomechanical components can result in mission failure or worse. The challenge of ensuring bond stability becomes particularly difficult in military laser systems required to maintain reliable operation despite harsh conditions. In airborne laser systems, the challenge is further exacerbated by cold temperatures encountered at altitude and by thermal shock associated with rapid altitude changes.
Key Parameters and Requirements
Bonding agents can simplify design of precision optomechanical systems such as military laser systems but these agents nevertheless face significant challenges. Adhesives intended for these systems must exhibit low outgassing to avoid interference with optics and must maintain stability under extremes of thermal and mechanical stress such as those specified in MIL-STD-810F. Designed to provide testing guidelines for equipment intended for harsh environments, MIL-STD-810F describes test conditions including recommended levels of thermal and mechanical stress. In tests performed at these levels, however, engineers discovered adhesive failure in a laser system developed by ASELSAN A.Ş., a Turkish Armed Forces Foundation company. To examine stress conditions and find suitable solutions, a researcher undertook a methodical examination of several adhesives when subjected to mechanical shock, vibration, and thermal shock at stress levels specified in MIL-STD-810F.
In this study, the researcher conducted multiple experiments designed to reveal the most stable configurations for five different optomechanical mounts. For each mount configuration, the researcher examined the bonding properties of five different adhesives including Master Bond EP21TDC-2LO. For this study, the researcher employed two different shakers and a furnace to subject the 25 different test samples to extremes of mechanical shock, vibration, and thermal shock at MIL-STD-810F levels. To quantify the effects, the researcher used a precision autocollimator to measure any deflections in the optical system arising from adhesive instability under each stress condition.
For testing response to mechanical shock, the researcher subjected each mount to MIL-STD-810F dynamic loads including 20G for 15-23ms (functional test for flight equipment), 20G at 45 Hz (acceleration), and 40G for 6ms (crash hazard test for flight equipment). For testing response to mechanical vibration, the researcher applied a random vibration profile that exposed the mounts to fluctuations in vibration load and frequency at an overall level of 14 Grms. For testing response to thermal shock, the researcher repeatedly exposed the bonded mounts to the types of conditions encountered in airborne platforms, including a rise in temperature from 70°C from -40°C in 5 minutes (such as in an aircraft ascending from sea level to 30,000 ft) as well as sustained temperatures of 70°C and -40°C (aircraft parked in a hot hangar and aircraft in a sustained cruise at altitude).
In the face of these extreme conditions, Master Bond EP21TDC-2LO demonstrated continued stability. Although the five mount configurations demonstrated different levels of effectiveness in their design, mounts bonded with Master Bond EP21TDC-2LO showed little or even no effect at these demanding levels of mechanical shock, vibration and thermal shock.
Military laser systems designed for airborne platforms present particularly harsh demands for stability of adhesives used to mount optomechanical components. As demonstrated in this detailed examination of adhesive properties, Master Bond EP21TDC-2LO is an effective solution for bonding materials exposed to the harshest operating environments.