Characterization of surface mechanical properties of H13 steel implanted by plasma immersion ion implantation

Abstract

AISI H13 steel samples were implanted with nitrogen using plasma immersion ion implantation (PIII) at 320 or 380 °C. In addition, PIII was used for simultaneous implantation of carbon and nitrogen into H13 steel at 380 °C. Treated and untreated samples were studied by wear-friction testing, microhardness measurement, atomic force microscopy and glancing angle X-ray diffraction (XRD) diagnosis. Experimental results indicated that the hardness and wear resistance were improved by increasing the PIII treatment temperature, whereas the presence of C ions does the opposite. It is explained by a nitride formation and a lattice expansion between 1.7 and 2.23%.

Introduction

There has been an increasing demand for industrial surface treatments to enhance corrosion resistance, hardness, and to reduce friction coefficient and wear. Besides conventional coating technologies, research and development on surface engineering has focused on plasma nitriding and ion implantation [1]. One promising technology to improve the surface hardness without incurring additional problems as in poor adhesion is plasma immersion ion implantation (PIII), which allows fast and cost-effective modification of three-dimensional tools [2].

In this study, high negative voltage pulses are applied to workpieces immersed in a plasma for PIII. Positive ions from the plasma are accelerated towards the workpieces and bombarded the surface with energies equal to the applied voltage [3]. A typical PIII device is shown in Fig. 1 [4]. For practical purposes, the acceleration voltage is limited approximately to 40 kV (at higher voltages, X-ray emission through secondary electrons would be hazardous). In order to obtain the full ion energy at the target surface, the pressure must be kept sufficiently low (below 0.5 Pa). The high voltage has to be applied in 10 μs duration with repetition frequencies ranging from 10 Hz to 1 kHz to avoid excessive power loads into the target [5].

One of the main reasons to have an interest in PIII is because it can be used as a hybrid treatment together with a diffusion process. When the temperature exceeds 250 °C, diffusion of implanted species (such as nitrogen ions) plays a significant role in certain metals [1]. A nitrogen-strengthened diffusion zone is produced so that the zone extends well beyond the implantation range and gives rise to the increased surface hardness [6].

AISI H13 steel (DIN X40CrMoV51) is a high strength steel used commonly in industry as a raw material for tools for both hot and cold applications. The surfaces of these tools are usually strengthened by salt, gas or plasma nitriding to improve surface characteristics, such as friction or wear. In addition to the environmental concerns of these salt or gas nitriding processes, all of the cited methods result in a compound (or white) layer consisting of brittle iron nitrides. In plasma immersion ion implantation, it is possible to control this layer more easily [7].

Previous studies [1], [7], [8] were concentrated mainly on the study of phase composition, depth profiling, and hardness changing of AISI H13 steels and other steels after nitrogen PIII. In this paper, plasma immersion ion implantation in H13 steel was performed by nitrogen ions at 320 or 380 °C. Besides hardness, phase composition and surface structures, wear resistance and friction coefficient were studied. It is known that carbon plasma treatments can also improve the surface properties. Previous efforts were also devoted to carry out a comparative study of PIII treatment with nitrogen, carbon, and nitrogen and carbon [9], [10], [11], [12], [13]. In this study, nitrogen and carbon plasma-immersion ion implantation (PIII) at 380 °C are presented.