High-temperature harsh-environment strain sensors are needed for industrial process monitoring and control, fault detection, structural health monitoring applications, in power plant environments, steel and refractory material manufacturing, aerospace, and defense [1]-[4]. At temperatures above a few hundred degree Celsius and under the harsh-environments encountered in the aforementioned applications, strain sensing poses significant challenges. Among these one can list: (i) the resilience of the sensor itself; (ii) difficulty in attaching the sensor to the part to be monitored due to mismatch between the coefficient of thermal expansion (CTE) of the sensor material and the CTE of the test part; (iii) high-temperature harsh-environment sensor packaging; and (iv) quick variations in environmental conditions (e.g., temperature, pressure, and vibration) [5], [6]. Wired strain gauge, which operates based on gauge resistance as a function of strain, is one of the hightemperature static strain sensor technologies available and reported to operate at temperatures in excess of 1300°C [7]. These sensors are made of high-temperature capable materials that are either welded to the test part by flame spraying technique or attached by epoxying the sensor using a ceramic-based cement. Current limitations in wired strain gauge performance and applications are related to: (i) sensor wiring; (ii) bulky harsh-environment attachment and housing (such as fiberglass insulation or ceramic braiding or a metal jacket); (iii) drift in response due to oxidation; (iv) accuracy and precision [8]. Commercially available optical strain sensors use Fiber Bragg Grating technology (FBG) and target operation up to 1000°C for oil and gas pipeline leakage, structural health, and security monitoring [9]. These sensors can be lightweight and small, but may drift due to changes in fiber doping profile at high-temperature, harsh-environment. In addition, they suffer the interference of deposits in lenses or interfaces, and may require special housing and packaging for access in high-temperature operation conditions, which adds volume to the sensor assembly and complicates instrumentation [10]. Capacitive micro-electro-mechanical-systems (MEMS) static pressure sensors implemented as individual devices or as thin films deposited directly on the part to be monitored have been explored in the temperature range of 600°C to 1100°C [8], [11]. The devices have been fabricated using wide band-gap materials, such as silicon carbide (SiC) for up to 600°C, and high-temperature substrates, such as alumina, for thin film deposition up to 1100°C. MEMS based sensors often require signal conditioning and amplification through embedded integrated circuitry, which translates into increased complexity, size, and weight. The need for battery or some sort of power source often implies the necessity of regular maintenance, which increases cost and is a complication for high-temperature operation of MEMS-based harsh-environment static strain sensors. Overall, a limiting aspe...
