A critical analysis of the scratch test for thermally sprayed coatings

Abstract Thermally sprayed coatings are extensively used in aerospace, energy, automotive, and industrial applications because of their superior wear, corrosion, and high-temperature resistance. However, the reliable evaluation of coating adhesion and cohesive integrity remains a major challenge due to the complex lamellar microstructure, residual stresses, porosity, and heterogeneous failure behaviour inherent to thermal spray processes. Among the available characterization techniques, the scratch test has emerged as a rapid, versatile, and sensitive approach for assessing coating integrity and tribological performance. Despite its widespread use, the interpretation of scratch test results for thermally sprayed coatings is still fragmented, with significant variations in testing methodologies, failure criteria, and critical load determination techniques. This review critically analyses the scratch test specifically for thermally sprayed coatings by systematically correlating scratch response with coating microstructure, process parameters, residual stress, porosity, and failure mechanisms. The work comprehensively discusses the evolution of scratch testing, different scratching methodologies, operational parameters, and advanced critical load evaluation techniques such as optical microscopy, acoustic emission analysis, and multi-axis force measurements. Furthermore, the study consolidates and compares the findings of a wide range of coating systems processed through APS, HVOF, HVAF, D-gun, and related thermal spray techniques. The novelty of this work lies in providing an integrated and application-oriented perspective that bridges the gap between fundamental scratch mechanics and practical coating performance evaluation. Unlike previous studies that focus on isolated coating systems or individual testing approaches, this review establishes a unified understanding of how coating architecture and processing conditions govern scratch-induced damage and failure evolution. In addition, the manuscript identifies current limitations, inconsistencies in interpretation, and the need for standardization in scratch testing methodologies for thermally sprayed coatings. The present work, therefore serves as a comprehensive reference for researchers and industrial practitioners working toward the design, optimization, and reliable performance assessment of advanced thermal spray coatings.