INTERFACE STUDY OF PHYSICAL VAPOR-DEPOSITION TIN COATINGS ON PLASMA-NITRIDED STEELS

Abstract

To improve the surface hardness of steel, the substrate can be plasma nitrided prior to the deposition of a TiN layer. The success of this combined treatment depends very strongly on the intermediate thermal treatments carried out between plasma nitriding and TiN deposition. In this paper, some additional steps carried out after plasma nitriding and prior to TiN deposition, have been investigated to reveal the effect of the resulting interface on the adhesion of the coating. These combined treatments are carried out on austenitic stainless steel AISI 304 and on cold work tool steel 1.2379. If the plasma-nitrided steel surface is only coated with a thin Ti intermediate layer before the deposition of the TiN layer 1.5 mum thick-as it is usually the case for a standard TiN coating-flaking of the TiN coating is observed with AISI 304, even if no external load is applied. For the cold work tool steel, the TiN coating flakes when a small external load is applied in a scratch adhesion test. However, if the nitrided specimen is first cooled down, subsequently plasma heated and then sputter etched before the deposition of the Ti-TiN layer, no flaking is observed for both steel types. X-Ray diffraction (XRD) measurements and a transmission electron microscopy (TEM) study are carried out to reveal the interfacial phases which could cause this different behaviour. Scratch tests are carried out to correlate the adhesion of the TiN coating for different intermediate treatments with the results of the XRD and TEM studies.