Citation: Królicka, A.; Maj, A.; Lój, G. Application of Laser-Induced Breakdown Spectroscopy for Depth Profiling of Multilayer and Graded Materials. Materials 2023, 16, 6641. https://doi.org/10.3390/ma16206641 Academic Editor: Hansang Kwon Received: 1 September 2023 Revised: 29 September 2023 Accepted: 7 October 2023 Published: 11 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Application of Laser-Induced Breakdown Spectroscopy for Depth Profiling of Multilayer and Graded Materials Agnieszka Królicka * , Anna Maj and Grzegorz Lój Department of Building Materials Technology, Faculty of Materials Science and Ceramics, AGH University of Krakow, Mickiewicza 30, 30-059 Krakow, Poland; amaj@agh.edu.pl (A.M.); gloj@agh.edu.pl (G.L.) * Correspondence: krolicka@agh.edu.pl Abstract: Laser-induced breakdown spectroscopy (LIBS) has emerged as a powerful analytical method for the elemental mapping and depth profiling of many materials. This review offers insight into the contemporary applications of LIBS for the depth profiling of materials whose elemental composition changes either abruptly (multilayered materials) or continuously (functionally graded or corroded materials). The spectrum of materials is discussed, spanning from laboratory-synthesized model materials to real-world products including materials for fusion reactors, photovoltaic cells, ceramic and galvanic coatings, lithium batteries, historical and archaeological artifacts, and polymeric materials. The nuances of ablation conditions and the resulting crater morphologies, which are instrumental in depth-related studies, are discussed in detail. The challenges of calibration and quantitative profiling using LIBS are also addressed. Finally, the possible directions of the evolution of LIBS applications are commented on. Keywords: laser-induced breakdown spectroscopy; multilayer material; functionally graded materials; elemental depth profiling 1. Introduction Today’s advanced materials have evolved beyond simple single-component systems, embracing intricate multi-elemental compositions to achieve unprecedented properties. The pursuit of materials that can address the diverse and rigorous demands of contemporary applications has urged the development of a new generation of materials. These are not the monolithic single component materials of the past, as can be easily noticed by tracking how materials used in industries such as aviation have evolved over the years [1]. Instead, the focus has been on complex multicomponent systems that amalgamate the best attributes of their components [2]. Three categories stand out due to their innovative nature and adaptability: composites [3], multilayer materials [4], and functionally graded materials (FGMs) [5,6]. While all three categories benefit from the advantages of blending different materials, they differ in structural organization and how these materials come together (Scheme 1). The primary distinguishing feature among them is the method in which the components are merged and distributed: (i) in composites, distinct phases are combined yet remain separate, ensuring even distribution within the material (Scheme 1a); (ii) multilayered materials consist of distinct layers stacked sequentially (Scheme 1b); and (iii) in FGMs, a smooth and continuous transition in composition and/or properties is observed across the material (Scheme 1c). Some examples of multilayer and diffusion-controlled materials are presented in Figures 1 and 2. As the need for materials with customized properties grows, especially in high- performance applications such as nuclear reactors, electronics, and energy production and storage devices, the study and development of multilayered materials are expected to Materials 2023, 16, 6641. https://doi.org/10.3390/ma16206641 https://www.mdpi.com/journal/materials