Degradation Mechanism
Material failure occurs when atmospheric gases penetrate a substrate at elevated thermal levels. High temperature oxygen ingress typically results in the internal oxidation of metallic alloys used in sensor housings. This process changes the local chemistry of the grain boundaries and creates brittle phases.
Penetration Depth
Measurement of the oxide layer thickness provides data on the severity of the exposure. A scanning electron microscope identifies the boundary where high temperature oxygen ingress stops and the pristine bulk material begins. If the depth exceeds the design tolerance of 50 micrometres, the structural integrity of the component is compromised.
Diffusion Constraint
Rate of gas transport through the material lattice determines the speed of the failure. Factors such as temperature and partial pressure drive the high temperature oxygen ingress across the surface interface. High partial pressures accelerate the movement of ions into the metal.
Lowering the operating temperature reduces the kinetic energy available for this diffusion.
Protective Barrier
Coating technologies provide a method to block the pathway of reactive molecules. Ceramic layers often mitigate high temperature oxygen ingress by acting as a physical seal. These barriers are qualified by cycling them through extreme temperature swings to ensure no cracks develop.
Reliability depends on the adhesion between the shield and the underlying part. Testing protocols involve long duration heat exposure followed by cross sectional analysis to verify that the substrate remained protected during the entire cycle.
