ISO 3868:1976

Overview

ISO 3868:1976 defines a precise measurement method for the thickness of thin metallic and other non-organic coatings using Fizeau multiple-beam interferometry. This international standard targets coatings up to 2 micrometers thick, emphasizing highly reflective surfaces. It excludes vitreous enamel coatings from its scope. The method relies on optical interferometry, measuring the vertical displacement at a step formed between coating and substrate to determine thickness with high accuracy. Key components include monochromatic light, a Fizeau plate, and a filar micrometer eyepiece.

Key Topics

• Measurement Principle
  The method creates a step by dissolving or masking part of the coating, then uses an interferometer to observe fringe patterns formed by reflections between the specimen surface and a reference plate. Changes in fringe spacing and offsets correlate directly to coating thickness.

• Equipment Requirements
  Essential equipment includes a monochromatic light source, optics to direct light through a specially coated Fizeau plate, and a low-power microscope with an adjustable filar micrometer eyepiece to analyze interference fringes.

• Factors Affecting Accuracy
  Accuracy depends on several critical factors:

  • Reflective overcoat application to minimize phase shift errors
  • Proper step formation with minimal abruptness to follow fringes easily
  • Surface flatness and smoothness to ensure sharp and well-defined fringes
  • Cleanliness to avoid contamination artifacts
  • Calibration with alternative methods for verification

• Step Preparation Procedures
  Step edges can be created either by masking before plating or by dissolving the coating after deposition. The step must be well-defined and slope gently to allow clear fringe tracking with minimal disturbance.

• Measurement Procedure
  The procedure involves adjusting specimen position, Fizeau plate tilt, focus, and lighting conditions to obtain a clear fringe line pattern. Thickness is calculated from fringe displacement using the wavelength of incident light and observed fringe shifts.

• Measuring Precision
  The method ensures highly precise thickness measurements, with an accuracy of ±0.001 µm for coatings between 0.002 and 0.01 µm, and ±0.003 µm for thicker coatings up to 2 µm.

Applications

• Thin Film Thickness Measurement
  Ideal for characterizing metallic and other non-organic coating thicknesses in industries where coatings are extremely thin and reflective.

• Quality Control in Plating Processes
  Enables precise thickness verification for electroplated or vacuum-deposited metallic coatings to ensure compliance with design requirements.

• Surface Engineering and Material Science
  Used in research and development to analyze coating uniformity and deposition processes on substrates.

• Calibration and Instrument Validation
  Acts as a standard method to validate optical thickness measurement instruments by providing a traceable reference value.

Related Standards

• ISO/TC 107 - Metallic and Other Non-organic Coatings
  ISO 3868 is part of a series of standards managed by ISO Technical Committee 107 focusing on coating properties and measurement techniques.

• Interferometry Standards
  Complementary standards covering other interferometric methods such as white-light and two-beam interferometry provide alternative means of coating thickness measurement.

• Surface Roughness and Preparation Standards
  Standards that define surface preparation techniques support consistent measurement outcomes according to ISO 3868.

Key Keywords: ISO 3868, Fizeau multiple-beam interferometry, coating thickness measurement, metallic coatings, non-organic coatings, thin film measurement, optical interferometry, filar micrometer, reflective coatings, step preparation, measurement accuracy, surface flatness, international coating standards.