SERS analysis of lubricant additives in the context of technical cleanliness in automotive production

Filmic contaminations (FCs) from lubricants and other process fluids are an increasingly urgent problem in manufacturing of high-voltage components due to their potential of jeopardizing production steps and, therefore, leading to possible quality issues. In this study, surface-enhanced Raman scattering (SERS) is introduced as a promising candidate for the at-line chemical analysis of FCs with low preparation effort and fast measurements. Simple SERS substrates produced via magnetron sputtering of gold onto aluminum surfaces were tested to successfully analyze model compounds of common lubricant additives 1H-benzotriazole (BTA) and 1,3,4-thiadiazole-2,5-dithiol (DMTD). The determined limit of detection (LOD) for the substrates resulted in a value of 6 × 10^-7 mol/L for BTA and 9 × 10^-6 mol/L for DMTD, respectively. Additionally, their shelf-life was determined to assess long-term functionality. It was shown that the substrates could preserve 70% of their functionality after a month of storage under ambient conditions. Afterwards, the method was used for the analysis of three lubricants and it was found that the substrates allowed the identification of BTA additives despite the hydrocarbon matrix, as well as the differentiation of the three lubricants via the used BTA derivative and other additive components. Finally, an extraction method was introduced to transfer an artificial FC from the contaminated surface onto the prepared SERS surfaces and successfully perform chemical analysis of its corrosion-inhibiting additive.