Tribology of GA and GI Steel Sheets in Forming: Coating-Failure Mechanisms, Evaluation Methods, and Governing Factors

This review provides a critical overview of wear and coating failure in galvanized (GI) and galvannealed (GA) steel-sheet forming. Galvanized (GI) and galvannealed (GA) steel sheets are extensively used in automotive sheet-metal forming because they combine corrosion protection with acceptable formability. However, their zinc-based coatings are highly susceptible to friction- and deformation-induced damage during forming operations, which can lead to coating degradation, surface deterioration, galling, unstable production, and accelerated tool wear. Owing to substantial differences in coating microstructure and mechanical response, GI and GA sheets exhibit distinct tribological behaviors and failure modes under comparable forming conditions. GI coatings, characterized by a softer Zn-rich outer layer, are more prone to smearing, adhesion, and galling, whereas GA coatings, composed of brittle Fe-Zn intermetallic phases, are more susceptible to cracking, flaking, powdering, and debris-induced abrasion. This review provides a critical overview of wear and coating failure in GI and GA steel-sheet forming. First, the intrinsic characteristics of GI and GA coatings are compared in terms of microstructure, surface morphology, and mechanical and tribological properties. Second, widely used laboratory methods for friction and wear evaluation are reviewed, including strip-drawing, twist compression, draw-bead, tensile strip, deep-drawing, and U-channel forming tests, with emphasis on the contact conditions and failure mechanisms they can reveal. Third, the dominant damage mechanisms are discussed, including adhesive wear, abrasive wear, coating fracture, material transfer, and galling. Finally, the effects of tool properties, lubrication, forming parameters, and finite-element simulation on coating failure are critically assessed. The available evidence indicates that coating failure in GI and GA sheets is governed by the coupled interaction among coating microstructure, contact pressure, tool-surface condition, lubrication state, and deformation path. However, current studies remain fragmented, and an integrated understanding of coating-damage evolution under industrially relevant forming conditions is still lacking. Future research should therefore focus on coating-specific tribological models, in situ and multiscale characterization, and coupled experimental-numerical strategies for process optimization while preserving coating integrity.