Thin Film Interlayer Adhesion Stress Analysis for High Temperature Strain Gauges
High temperature thin film gauge survival requires balancing deposition stress and CTE mismatch to hold interlayer shear below critical fracture limits.
Kirkendall Void Formation
Diffusion Imbalance
Atomic migration through a metal lattice proceeds via the exchange of vacancies where the kirkendall void formation occurs when two distinct metals possess unequal intrinsic diffusion rates. Atoms move from the material with a higher diffusion coefficient toward the material with a lower coefficient to reach chemical equilibrium. This displacement of matter leaves behind vacant sites that coalesce into microscopic pores near the original interface.
Material Mechanism
Metal A and metal B join to create an alloy through high temperature annealing. Atoms of the faster species traverse the boundary into the slower lattice structure at a rate exceeding the counterflow of slower atoms. The lattice sustains damage as those vacated spaces cluster to relieve internal strain.
Interface Instability
Microscopic cavities gather along the contact zone as the flux of matter loses symmetry. These pores grow until the connection between materials loses structural integrity or electrical conductivity. Surface analysis reveals these defects often cluster on the side occupied by the faster diffusing element.
Performance Limitation
Reliability testing for solder joints or thin film coatings monitors for these gaps to assess long term stability. The presence of internal porosity degrades mechanical strength and increases the electrical resistance of the junction. Mechanical failure under thermal cycling conditions happens when these clusters propagate into cracks across the bond.