Daniel C. Harris

Erosion and Erosion Protection

One of the greatest challenges in the use of infrared windows and domes is to protect them from damage by particle impact. Collisions with raindrops are a problem at the velocities of airplanes and missiles. Figure 7.1 shows the damage to a missile dome after traversing a rainfield on the rocket sled in Fig. 7.2. Bug impacts at high speed can also damage infrared windows (Fig. 7.3) Blowing sand is harmful in high or low speed collisions. This chapter describes the effects of rain and sand impact, discusses laboratory erosion tests, and reviews work on coatings used to improve window durability.

The Heat of the Night and the Dust of the Battlefield

All objects above absolute zero temperature emit infrared radiation. This radiation can be used to measure the temperature of an object in a laboratory or factory, or can be used to observe military targets on a battlefield. âx80x9cHeat-seekingâx80x9d missiles use infrared radiation from the hot exhaust of a target aircraft to guide themselves to their prey.nnThe nose of the infrared-guided missile in Fig. 0.1 is a hemispheric, infrared-transparent dome made of magnesium fluoride. The ceramic dome protects a delicate, hermetically sealed infrared seeker from the harsh environment of high speed missile flight. The dome must withstand rapid aerothermal heating when the missile is launched and must resist long term erosion from raindrops and dust encountered during captive carry of the missile beneath the wing of an aircraft.nnThis book discusses optical, mechanical and thermal properties of infrared window and dome materials. It describes fabrication techniques and coatings required to enhance optical transmission and mechanical durability and to reject undesired radiation. We will emphasize the few rugged materials meant for external use in demanding environments, but will also mention important materials that can only be used in benign situations.