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Magnetostrictive Sensor Development Using Thin-Film Deposition, 14-R9610

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Principal Investigator
Clinton J. Thwing

Inclusive Dates:  03/10/06 – 07/10/06

Background - Southwest Research Institute has been working with magnetostrictive sensor (MsS®) technology in the pipeline industry for many years and has been awarded patents for its applications. This application has shown that corrosion detection using guided waves is a useful tool for determining pipeline integrity. This same technique could be as useful in the aerospace industry for detection of defects and monitoring of those same defects. SwRI believes there exists the need to monitor aerospace structures and that MsS technology could provide the tool for such a monitoring process. The need for a sensor to operate at higher frequency without the use of an external magnetic field is required to show that the MsS technology is feasible for health and usage monitoring systems (HUMS). This project developed sensor technology using multilayer thin-film deposition to create sensors in a vacuum chamber in the presence of a magnetic bias field and then heat treat the sensors to create standalone sensors capable of detecting small flaws without the need of an external bias field.

Approach - The Surface Engineering group in the Mechanical and Materials Engineering Division used one of two vacuum deposition tools to conduct a narrowly focused design of experiments to identify the critical factor(s) that influenced magnetorestrictive properties and adhesion to thin metal foils. Multilayers of iron-terbium (FeTb) and iron-cobalt (FeCo) (on the order of 5 micrometers thick) were deposited onto 1- to 5-mil-thick aluminum foil by DC magnetron sputtering to experimentally screen the influences of RF bias, layer thickness, and composition on the output responses of coercivity (Hc) and magnetic saturation (Ms). All thin-film development work was carried out on an AJA International magnetron sputter deposition system, shown in Figure 1. These newly developed thin-film sensors were then tested on a simulated A-10 aircraft specimen in a load cell.

Accomplishments - Adjustments to multilayer thin-film process and subsequent testing of the sensors allowed the researchers to quantify both the process and the sensitivity of the sensors by measurement of the signal-to-noise ratio of each sensor produced. These sensors, shown in Figure 2, were then heat treated in a magnetic field. This process produced the best sensor to date and was validated on a simulated A-10 test article by monitoring crack growth and correlating actual size information to defect size. The test article in Figure 3 shows placement of the sensor (thin film in the exciting coil). This sensor monitored growth of a crack in a 3/8-inch hole approximately 2.75 inches away without the use of an external magnetic biasing field. The data can be seen in Figure 4.

Figure 1. AJA International Magnetron Sputter Deposition System Figure 2. Typical Sensor Sample after Coating
Figure 3. Probe and Sensor Location on A-10 Test Article Figure 4. MsS Data from the A-10 Test Specimen

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