Technical report | Development of a Thermal Wave Interferometry System for Thin-Film Characterisation

Abstract

This report describes the development of a thermal wave interferometry system for the characterisation of film thickness. The system furnishes a measurement of the relative phase between a modulated heat flux applied to a thin film by a diode laser and the oscillatory temperature response of the film. The relative phase is then converted to a thickness estimate by means of an inverse solution of the heat diffusion equation. A validation of the facility was performed on an experimental test coupon prepared with paint layers of varying thickness. Estimates of the paint thickness profile furnished by the facility were compared to and corroborated by independent measurements obtained using an eddy current probe and a surface profilometer. The system is shown to resolve variations in paint thickness of less than 1.5 μm.

Executive Summary

This report describes the development of a Thermal Wave Interferometry (TWI) system for the characterisation of film thickness. The capability addresses the need for a method to assess bondline uniformity for the compact multi-parameter load evaluation (CMPLE ) device, a low-cost surface-mounted sensor package with an in-flight strain measurement capability. The device was conceived and developed by the Defence Science and Technology Organisation (DSTO) and successfully tested in-flight on the tail plane of an Australian Defence Force (ADF) Caribou aircraft. However, in validation testing of the device it was revealed that accurate strain readings require that the device be attached with an adhesive bondline that is relatively thin and of uniform thickness. In practice, this can be difficult to achieve even under nominally ideal experimental conditions, raising the need for a reliable method of verifying the thickness and uniformity of the bondline. As an acutely sensitive method of measuring film thickness, thermal wave interferometry has the potential to provide an effective in situ capability for adhesive bondline metrology. As a first step toward investigating that possibility, a laser-based TWI facility was developed and experimentally validated. The system furnishes a measurement of the relative phase between a modulated heat flux applied by a diode laser and the oscillatory temperature response of the object under inspection. The phase measurement is converted to a thickness estimate by means of an inverse solution of the heat diffusion equation. A validation of the facility was performed on an experimental test coupon prepared with paint layers of varying thickness. Estimates of the paint thickness profile furnished by the facility were compared to and corroborated by independent measurements obtained using an eddy current probe and a surface profilometer. The system was shown to resolve variations in paint thickness of less than 1.5 μm. Although the main purpose of the facility is to enable the development of an in situ TWI capability for bondline metrology, the facility also offers the potential to assist in the evaluation of coating systems used in military platforms for corrosion prevention and thermal barrier applications.