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.
Micro Scratch Testing Iso 20502
Standard Definition
Tribological assessment methodology defines the mechanical performance of thin films and coatings through controlled surface deformation. ISO 20502 establishes the technical protocols for micro scratch testing to determine the critical load at which adhesion failure or cohesive fracture occurs. This international standard details the instrumentation requirements including stylus geometry and loading rates necessary to produce repeatable measurements across different laboratory environments.
Instrument Parameter
Force application relies on a diamond indenter driven across a surface while the normal load increases linearly. Diamond geometry typically follows a Rockwell C specification to maintain consistency in stress concentration at the contact point. Friction force and acoustic emission sensors monitor the progression of surface damage during the entire traverse.
Sudden transitions in the coefficient of friction or spikes in acoustic signal energy provide the data points for identifying failure thresholds.
Metrological Condition
Calibration procedures for the test apparatus require verification of load cell linearity and stylus tip radius. Thermal drift poses a measurement error during extended tests where the instrument chassis expands and influences the depth of the indenter penetration. Humidity levels affect the interfacial bond strength of certain hydrophilic coatings, requiring controlled atmospheric conditions during the evaluation.
Standardized calibration blocks with known hardness values allow operators to confirm that the displacement sensors maintain accuracy relative to the applied normal force.
Material Validation
Empirical data obtained from these tests determine the suitability of protective layers for industrial applications. Coatings with high critical loads resist delamination under shear stress compared to those showing premature failure at low load values. Variation in the failure mode provides evidence of whether the weakness resides in the film cohesion or the interface between the substrate and the deposited material.
Consistent performance under standardized loading conditions confirms that a coating system remains compliant with documented mechanical requirements.