Evaluation of the Effect of Measurement Area on Laser Flash Characterisation of Partially Debonded Artefacts

Abstract

Delamination in the interlayers of multi-layer systems can cause a degradation in functionality, stability and life span of such systems. This is even more pertinent for systems at high temperatures. Laser flash analysis (LFA) has long been used for thermophysical properties measurements at high temperatures. In the European Metrology Programme for Innovation and Research (EMPIR) funded JRP 17IND11 Hi-TRACE project, the National Physical Laboratory (NPL) is developing multi-layer reference artefacts, including both fully bonded and partially de-bonded systems for validating thermal characterisation of multi-layer systems at temperatures from room temperature to above 1000 °C using LFA. The sensitivity studies carried out before creating the partially debonded artefacts have shown that the measurement area of the detector is one the important parameters that affect the accuracy of the thermal characterisation using LFA. However, this parameter is unknown to users of the early version of the Netzsch LFA 427, as it has negligible effect on radially homogenous samples which LFA is typically used for. This paper details the effect of measurement area on thermal characterisation of artefacts with partial debonding in the interface using LFA. For a bi-layer IG-210 grade graphite-hafnium foil system with a 6 mm diameter defect at the centre of the foil interface, variation in the measurement area can change the heat pulse transmission half-rise time by up to ~ 40%. Three methods to determine the system measurement area are discussed. The physical measurement diameter for the LFA 427 at NPL was estimated to be 7.7 mm ± 0.3 mm, independent of temperature. Within the uncertainty range of the measurement diameter, the thermal conductance of the defect region and the average heat transfer coefficient (HTC) can vary by ± 36% and ± 3.5% respectively. It is important to remember that this change is for a defect of 6 mm diameter which is a significant portion of the measurement area. For much smaller defects a much smaller change can be expected.