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.
Alumina Film Deposition
Dielectric Synthesis
Thin ceramic layer formation provides electrical isolation and environmental passivation across metallic substrates exposed to aggressive thermal environments. Reactive sputtering and atomic layer growth form alumina film deposition layers that isolate thin film sensors from conductive base alloys up to operating limits near 1100 degrees Celsius. Film thickness targets range between two and ten micrometres depending on the breakdown voltage requirement.
Sputtered targets run in argon and oxygen mixtures to establish stoichiometric aluminium oxide phases. Microstructure density determines leakage resistance across the base metal boundary.
Insulation Resistance
Megohm values drop exponentially when pinholes or microcracks bridge the dielectric barrier at elevated operational temperatures. Dielectric strength measurements verify that the insulating layer maintains a minimum volume resistivity of ten megaohm centimetres at maximum rated service conditions. Surface roughness on the underlying superalloy must stay below twenty nanometres arithmetic mean deviation to prevent defect propagation through the coating.
Post-deposition annealing in air promotes alpha phase conversion and relieves residual stress. Purity levels govern ionic conduction paths through the ceramic grid.
Coating Defect
Mechanical spallation occurs when intrinsic growth stress combines with differential expansion between the ceramic and the metallic base. High compressive stresses above two gigapascals cause delamination during rapid thermal cycling. Deposition rate fluctuations alter film density and generate columnar grains that allow oxygen diffusion toward the substrate.
Critical adhesion testing uses acoustic emission detection during scratch passes to identify film detachment thresholds.
Metrological Boundary
Sputter chamber pressure drift alters stoichiometry and induces variable dielectric loss factors across consecutive production batches. Qualification protocols measure room-temperature capacitance and high-temperature leakage current to certify electrical isolation integrity before transducer patterning proceeds. Process drift appears as increased phase shift during impedance spectroscopy.
Thickness uniformity must hold within three percent across the active sensing area.