ISO 19674:2017

Overview - ISO 19674:2017 (Stoney formula for ceramic coatings)

ISO 19674:2017 specifies a laboratory method to determine the internal (residual) stress in thin ceramic coatings by applying the Stoney formula to curvature measurements of coated strips or discs. The standard is part of the fine ceramics (advanced ceramics) test-method suite and is intended for thin-film ceramic coatings where the film thickness is small compared with the substrate. The procedure converts a measured radius of curvature into a mean film stress using known substrate properties (Young’s modulus, Poisson’s ratio, thickness) and the measured coating thickness.

Key topics and requirements

• Principle: Uses the Stoney formula to relate measured curvature to mean film stress; assumes elastic deformation and spherical curvature.
• Applicability limits: Valid when coating thickness is << substrate thickness (recommendation: coating < 2% of substrate thickness) and when substrate remains elastic (no plastic deformation).
• Specimen geometry: Strip-shaped specimens are preferred; disc-shaped samples are acceptable. Samples must be uniform in thickness and stress-free before coating (annealing recommended if necessary).
• Measurement needs:
  • Accurate measurement of coating thickness and substrate thickness
  • Known substrate Young’s modulus (E) and Poisson’s ratio (ν)
  • Profile (bowing) measured by optical profilometer, high-magnification optical microscope with stage sensor, contact probe (with minimal load), or other suitable non-distorting techniques
• Surface finish: Side to be coated should have a finish suitable for accurate curvature measurement (minimum: 1200 grit emery).
• Procedure details: Measure initial profile, deposit coating, measure final profile, calculate radius of curvature and apply Stoney formula. Repeat measurements and choose specimen dimensions to maximize curvature without inducing plasticity.
• Thermal effects: Stress measured at a temperature different from deposition will include thermal mismatch components; thermal expansion coefficients are needed to separate intrinsic and thermal stresses.

Applications and users

• Practical uses: residual stress measurement, process control for thermal barrier coatings, wear/tribological coatings (e.g., DLC-like ceramics), optical/reflective surface coatings, lifetime and delamination risk assessment.
• Typical users: coating process engineers, materials scientists, thin-film researchers, failure analysis labs, quality control and R&D teams in ceramics, aerospace, automotive and tooling industries.

Related standards (referenced in ISO 19674)

• ISO 18452 - step height measurement methods (for coating thickness)
• ISO 26423 - crater grinding (coating thickness)
• EN 1071-10 - cross-sectioning (coating thickness)
• (ISO 3274 mentioned in an example relating to contact force effects)

Keywords: ISO 19674:2017, Stoney formula, internal stress, ceramic coatings, residual stress measurement, radius of curvature, thin film stress, fine ceramics.