Investigation of the influence of ion etching parameters on the structure of nitrided case in hot working steel

Abstract

The paper discusses the influence of the ion etching process on the nitrided case structure created with the use of the gas nitriding method in hot working steel. The nitrided case structure is of key importance for the final effect of duplex treatment, consisting of the nitriding process and subsequent deposition of a hard coating, because it significantly determines the adhesion of PVD coating. It was found out that during the initial stage of the coating deposition, i.e. during ion etching, decomposition of the Ë and Ë+γ′- ‘white’ compound layer can occur, which results in backward diffusion of nitrogen and destruction of the created nitrided case. The critical temperature for the decomposition process is 500°C, although in some conditions, it can start even at 350°C. The purpose of the investigation described in the paper was to examine changes in the nitrided case structure (in the hot working steel ISO 35CrMoV5) caused by the etching process executed in different experimental conditions (1: different pressures: 2×10−5 mbar, 1×10−3 mbar, 1×10−2 mbar; 2: different working atmospheres: titanium plasma and mixed titanium–argon plasma; 3: two types of substrate polarization: Ubias=900 V/d.c., and pulsed Ubias=900 V, f=5 kHz, Δ=90%. The experiments were carried out in the vacuum arc deposition equipment MZ383 manufactured by Metaplas Ionon (Germany). Prepared samples were examined, before and after ion etching, in the microscopic metallographic investigation using the optical microscope Neophot32 and in the investigation of the phase composition using the X-ray diffractometer Philips PW1830. The influence of the ion etching process conditions on the nitrided layers structure and on the intensity of compound zone decomposition was characterized on the basis of results obtained from experimental investigations.