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

The paper discusses the influence of the ion etching process on the nitride d case structure created with the use of the gas nitriding method in hot wo rking steel. The nitrided case structure is of key importance for the final effect of duplex treatment, consisting of the nitriding process and subseq uent 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 epsil on and epsilon+gamma'- 'white' compound layer can occur, which results in b ackward diffusion of nitrogen and destruction of the created nitrided case. The critical temperature for the decomposition process is 500 degrees C, a lthough in some conditions, it can start even at 350 degrees C. The purpose of the investigation described in the paper was to examine changes in the nitrided case structure tin the hot working steel ISO 35CrMoV5) caused by t he etching process executed in different experimental conditions(1: differe nt pressures: 2 x 10(-5) mbar, 1 x 10(-3) mbar, 1 x 10(-2) mbar; 2. differe nt working atmospheres: titanium plasma and mixed titanium-argon plasma; 3: two types of substrate polarization: U-bias=900 V/d.c., and pulsed U-bias =900 V, f= 5 kHz, Delta =90%. The experiments were carried out in the vacuu m are deposition equipment MZ383 manufactured by Metaplas Ionon (Germany). Prepared samples were examined, before and after ion etching, in the micros copic metallographic investigation using the optical microscope Neophot32 a nd in the investigation of the phase composition using the X-ray diffractom eter Philips PW1830. The influence of the ion etching process conditions on the nitrided layers structure and on the intensity of compound zone decomp osition was characterized on the basis of results obtained from experimenta l investigations. (C) 1999 Elsevier Science S.A. All rights reserved.