Femtosecond laser-contoured micro-strain gages

Abstract

A new concept for foil based micro strain gages (μSG) with sensor elements smaller than 1 mm2 and made possible by a laser contouring process will be presented. In contrast to photolithographic structuring, strain gages structured by laser ablation can be precisely tailored at their measuring destination after the strain sensor blank has been applied. Sensors for measuring very spatially confined elongations in particular, require a precisely positioned and small grid. Accurate mechanical positioning of the measuring grid at the micrometer scale can be challenging due to the much larger, bulky, foil substrate. In cases where a blank is positioned prior to grid patterning by laser ablation, it is even possible to write the measuring grid on smoothly curved, three-dimensional surfaces. A “contour cutting” method is used to minimize grid dimensions and reduce laser processing time, which enables the realization of micrometer-sized grids in a fast, flexible and maskless process. This will extend the range of possible applications for this sensor type (e.g. to include use in very confined spaces). This paper will present the development of a suitable process for the gentle, selective ablation of nickel‑chromium on polyimide substrates by means of ultrashort-pulse lasers. The best results were achieved at a laser fluence of ϕ= 0,43 J/cm2 and a scan speed of 2000 mm/s. Furthermore, the thermal stability of the sensor layer will be investigated and sensors will be characterized in bending tests.