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Laboratory and field analysis on the tribological behavior of coated anduncoated forming toolsM.A.R.S. Mendes 1 , R.M. Souza 1 , P.K. Vencovsky 2 , Y. Berthier 31 Surface Phenomena Laboratory, Department of Mechanical Engineering, Polytechnic School of theUniversity of São Paulo – Av. Prof. Mello Moraes, 2231, 05508-900 São Paulo, SP, BrazilURL: www.poli.usp.bre-mail: roberto.souza@poli.usp.br; marcorsm@usp.br2 Bodycote Brasimet – Av. Nações Unidas, 21476, 04795-912 São Paulo, SP, BrazilURL: www.brasimet.com.bre-mail:Paulo.vencovsky@bodycote.com3 INSA de Lyon, LaMCoS – 20 avenue Einstein, 69621 Villeurbanne, FranceURL: lamcos.insa-lyon.fre-mail: yves.berthier@insa-lyon.frABSTRACT: Frequently, the life of the tools used in sheet metal forming operations is determined by aphenomenon known as galling, which originates from the adhesion of the sheet to the forming tool surface.The application of coating architectures composed by single or multiple layers of Physical Vapor Deposition(PVD) films, such as TiN, TiCN, CrN, TiCNAl, may significantly reduce the chemical interaction in thecontact, up to the point that no significant adhesion may be observed for an extended number of formingoperations. Usually, the evaluation of the behavior of different thin film architectures is conducted usingtribometers that may or may not reproduce the conditions found in industrial practice. This work presents atribological analysis of coated and uncoated surfaces of tools used in industrial sheet metal formingoperations and discusses the capability of laboratory tests in reproducing the situations found in practice.Key words: Galling, PVD coatings, Laboratory tests, Field tests1 INTRODUCTIONFrequently, the life of the tools used in sheet metalforming operations is determined by a phenomenonknown as galling. Galling is described in the ASTMG40 Standard [1] as “a form of surface damagearising between sliding solids, distinguished bymacroscopic, usually localized, roughening andcreation of protrusions above the original surface; itoften includes plastic flow or material transfer orboth”. In spite of the significant number ofpublications on this phenomenon, some attention isstill devoted towards its effective occurrence [2]and, more importantly, towards the development oftests able to reproduce galling in laboratory [2-10].One important reference regarding these tests is theASTM G98 method [3], according to which athreshold pressure for galling is calculated based onthe visual inspection of the presence of galling at thesurfaces of a button and/or a block that were pressedagainst and manually rotated with respect to eachother at increasing loads. Despite the popularity ofthe ASTM G98 test method, many works criticizethe applicability of such threshold pressure value fordesign purposes and suggest modifications to theoriginal standard method. These changes intend toovercome some of the method limitations, such asthe heterogeneity of contact pressure distribution [4]or the fact that the velocity is zero at the centre ofthe rotating button [2,4]. Other questionings on theASTM G98 test method include the nonconsideration of the statistical nature of galling[4,5]; the absence of constant speed during manualrotation [6] and even cost issues associated with thenecessity of a large number of specimens or theavailability of an equipment capable of applyinglarge normal loads [7].Some of the alternatives for laboratory testing ofgalling include a modification in the geometry of thespecimens, such as the shape of initial contact,which is considered as a line [8] or a point [7,9],rather than an area (ASTM standard). In these tests,the analysis of the occurrence of galling may remainbased on the visual inspection of contact surfaces [8]or, as in the case of two crossed cylinders that slideagainst each other, may be based on the frictioncoefficient calculated during the experiments [7,11].