High Temperature Thin Film Strain Sensor Calibration and Mounting
High temperature thin film strain sensor accuracy depends on rigorous apparent strain polynomial correction, dielectric barrier integrity, and drift mitigation.
Thermal Oxidation Kinetics
Reaction Mechanism
High-temperature reaction rates govern the chemical degradation and protective oxide layer growth on metallic and ceramic sensor materials exposed to oxidizing atmospheres. Quantifying thermal oxidation kinetics provides mathematical models to predict structural consumption, scale thickness growth and functional lifetime under thermal loading. Oxidation reactions typically follow linear, parabolic or logarithmic rate laws depending on temperature, oxygen partial pressure and alloy composition.
Parabolic kinetics describe diffusion-limited growth of protective oxide scales such as chromia and alumina on superalloys. Determining oxidation rate constants requires thermogravimetric analysis and continuous mass gain measurements.
Scale Formation
Initial oxygen adsorption transitions rapidly to continuous nucleation of oxide grains across the exposed metallic surface. Scale growth rates decrease over time when dense, adherent oxide films establish diffusion barriers against inward oxygen and outward metal ion transport. Parabolic rate constants quantify mass gain per unit surface area squared as a function of exposure time.
Temperature dependence of the reaction rate constant follows the classic Arrhenius equation governed by the activation energy of ionic diffusion. Non-protective porous oxide scales follow linear reaction kinetics, leading to rapid material consumption.
Sensor Degradation
Oxidation of thin film sensing elements reduces the effective cross-sectional area of conductive grids, causing progressive baseline resistance drift. Selective depletion of reactive elements alters alloy stoichiometry and shifts both the gauge factor and the resistance thermal coefficient. Oxide scale cracking under thermal cycling creates pathways for accelerated localized oxidation attack.
Substrate oxidation under dielectric isolation layers generates volume expansion stresses that promote coating spallation. Protective encapsulation films must provide low oxygen diffusion coefficients to shield underlying functional sensing layers.
Kinetic Boundary
Linear-to-parabolic transitions occur during early oxidation stages, complicating short-term life prediction models. High gas velocities and erosive particles strip protective oxide scales, causing continuous linear oxidation and rapid sensor failure. Reaction kinetics accelerate dramatically if operating temperatures exceed the phase stability limits of the protective oxide layer.
Trace atmospheric contaminants, including sulfur and water vapor, alter scale microstructure and increase transport kinetics. Extrapolating isothermal oxidation models to dynamic thermal cycling conditions requires empirical scaling factors to account for scale cracking.