Barrier Function
An insulating layer of aluminum oxide deposited on a metallic substrate prevents chemical diffusion and electrical leakage at elevated temperatures. In high temperature sensor assemblies, the alumina interlayer separates the active strain gauge grid from the conductive metallic base. This layer prevents the migration of metallic ions from the substrate into the sensor, which would otherwise degrade the electrical resistance and insulation performance above five hundred degrees Celsius.
Chemical passivation of the underlying metal occurs during the initial deposition phase, creating a stable chemical junction that restricts oxygen transport.
Deposition Sequence
The physical positioning of this oxide barrier occurs directly between the metal substrate and the secondary dielectric or sensing film. Magnetron sputtering is used to apply the alumina interlayer in a vacuum chamber, creating a dense, amorphous structure that resists cracking. Post deposition heat treatment transitions this amorphous structure to a stable crystalline phase, which increases the density and electrical resistance of the material.
Thermal Compatibility
Thermal mismatch between the oxide and the substrate is managed by controlling the rate of temperature change during operation. Because the thermal expansion coefficient of alumina is lower than that of superalloys, thermal stress accumulates during heating cycles. The thickness of the layer is limited to under two micrometers to prevent delamination caused by these thermal stresses.
Adhesion Performance
Mechanical reliability is evaluated through specialized scratch testing and interface examination. Thermal cycling reveals the durability of the boundary, with failures typically occurring due to cohesive fracture within the oxide rather than adhesive failure at the metal interface. Substrate preparation, including surface polishing and plasma etching, determines the strength of the bond.
