Wear mechanisms of physical and chemical vapour deposited ceramic coatings used in metal cunlng applications

A series of turning and drilling tests has been carriedout on tool materials coated with titanium nitride usingarc or electron beam evaporation source physicalvapour deposition (PVD) methods. Cemented carbidesubstrates were used for the turning tests while highspeed steel (HSS) substrates were used for the drilltests. Emphasis has been placed on elucidating themechanisms of wear of the coatings, using microscopy.Some comparisons were made with chemical vapourdeposited (CVD) coatings for the turning experiments.When turning plain carbon steel at 300 m min⁻¹ and 0·25 mm rev⁻¹ feed, rake face wear mechanisms involvea combination of discrete plastic flow and dissolutiondiffusionwear; these are identical to the wear mechanismsof CVD coatings when used under the sameconditions. However, the arc source PVD coatingsexhibited relatively poor adherence. Drilling tests werecarried out on austenitic stainless steel at 19·9 m min⁻¹ and 0·08 mm rev⁻¹. Coating failure was dominated byplastic shearing of the HSS substrate. The measuredflank ‘wear land’ varied widely for titanium nitridecoated and uncoated HSS drills – this may be relatedto variation in the high temperature shear strength ofthe HSS drills. Coating loss was sporadic in natureand there was some evidence that fracture of the PVDtitanium nitride coatings took place during metalcutting.