PVD coating’ on the durability of tools for hot working

PVD coating’ on the durability of tools for hot working

Surface and Coatings Technology 125 (2000) 134–140 www.elsevier.nl/locate/surfcoat Influence of the structure of the composite: ‘nitrided layer/PVD c...

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Surface and Coatings Technology 125 (2000) 134–140 www.elsevier.nl/locate/surfcoat

Influence of the structure of the composite: ‘nitrided layer/PVD coating’ on the durability of tools for hot working Jerzy Smolik a, *, Jan Walkowicz a, Jan Tacikowski b a Institute for Terotechnology, Pulłaskiego 6/10, 26-600 Radom, Poland b Institute of Precision Mechanics, Duchnicka 3, 00-967 Warsaw, Poland Accepted 30 June 1999

Abstract The paper presents research results of the influence of the ‘nitrided layer/PVD coating’ composite on the durability of tools for hot plastic working. Four structures of the composite differing in the PVD coating material were investigated. They were: TiN, CrN, (Ti,Cr)N and Ti(C,N ). The composites investigated were created by means of the surface ‘duplex’ treatment method in a two stage separable cycle (the nitriding process and the PVD coating deposition were carried out with different devices). The nitriding process was executed with the use of the regulated gas nitriding method, whereas the PVD coating was executed by means of the arc-vacuum method. The tools tested were forge dies made of ISO steel 35CrMoV5 (0.4%C, 0.4%Mn, 1.0%Si, 5.0%Cr, 1.3%Mo, 0.3%V ) designed for the plastic working of automotive half-shafts. The paper presents the results of maintenance investigations, executed under manufacturing conditions, obtained for tools used for hot forging which were covered with different composites. The investigations proved that the best durability was achieved for tools covered with the composite ‘nitrided layer/CrN coating’, for which the increase in durability was almost 90%. The smallest durability was noted for tools covered with the composite ‘nitrided layer/TiN coating’. The results obtained proved that a proper choice of the composite ‘nitrided layer/PVD coating’ structure may increase the durability of tools considerably for hot plastic working. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Composite layer; Duplex treatment; Physical vapor deposition coating

1. Introduction Large cyclic mechanical loads and thermal shocks caused by the contact of a tool with hot treated material are characteristic features of hot plastic working. Apart from the abrasive wear dominant in the wearing process of cutting tools, in the case of tools used for hot plastic working it also causes plastic strain and thermal fatigue of the tool material. The processes of gas and ion nitriding are commonly used to give good antiwear properties to tools for hot working. The rapid development of plasmo-chemical technologies to modify the antiwear properties of cutting tools prompted trials in using them for plastic working tools to be undertaken. The use of PAPVD methods to coat the working surfaces of cutting tools with thin antiwear coatings promotes a considerable increase in their durability. However, this effect has not been confirmed for hot working tools. Research works performed in the 1990s concerning the * Corresponding author. Tel.: +48-48-43884; fax: +48-48-44760. E-mail address: [email protected] (J. Smolik)

use of the PAPVD technology in the treatment of tools for hot working have resulted in elaboration of the ‘duplex’ surface treatment technology [1–3] assisting in creation of the composite ‘nitrided layer/PAPVD coating’. On the basis of research results described in the literature [4–8] it was stated that the nitrided layer structure created significantly influences the composite properties. It also determines quality of the PAPVD coating adhesion to the nitrided substrate. It turns out that appearance of only the Fe (N ) phase on the nitrided a surface is essential for achieving a good adhesion of the PAPVD coating to the substrate. In the literature, it has been proved that a layer of iron nitrides e-Fe N and 2–3 c∞-Fe N, in addition to a thin layer of titanium [9,10], 4 often occurring directly on the nitrided element surface under the PAPVD coating, have a harmful influence on the PAPVD coating adhesion in the composite’s ‘nitrided layer/PAPVD coating’. However, the majority of experiments were carried out concerning the TiN coating. According to this paper’s authors, the PAPVD coating adhesion to the nitrided substrate, as well as the

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J. Smolik et al. / Surface and Coatings Technology 125 (2000) 134–140 Table 1 ‘Duplex’ treatment parameters Nitriding Temperature (°C ) 450 510 Surface mechanical treatment Type of treatment Shot blasting Shot blasting Polishing PVD coating deposition Coating TiN CrN ( Ti,Cr)N Ti(C,N )

Atmosphere NH 3 NH 3

Flow ( l/min) 150 80

Time (h) 0.5 7

Abrasive material Elektrokorund 150 Interminglas 40–70 Paste: Cr O 2 3

Carrier (MPa) Air: 0.6 Air: 0.6 Felt target

Time (min) 6 4 Till attainment of a uniform polish

Atmosphere 100% N 2 100% N 2 100% N 2 25%C H +75% N 2 2 2

Pressure ×10−2 mbar) 1.2 2.0 2.0 0.8

Temperature (°C ) 400 400 400 400

Ubias ( V ) −150 −150 −150 −200

Current (A) 80 80 80 80

Table 2 PVD coating parameters Coating

Thickness (mm)

Ra (mm)

Chemical composition

Phase structure

HV0.05

Young’s modulus (GPa)

TiN CrN ( Ti,Cr)N Ti(C,N )

3.0 4.3 3.8 2.8

0.35 0.15 0.23 0.39

70%Ti, 30%N 78%Cr, 22%N 33%Ti, 39%Cr, 28%N 58%Ti, 15%C, 27%N

TiN-fcc CrN-fcc, Cr N-hex 2 TiN-fcc, CrN-fcc Ti(C,N )-fcc

2200 2410 2250 2800

421 314 384 508

resistance to thermal fatigue of the whole composite, is significantly determined, not only by the nitrided layer structure, but also by the chemical composition and the structure of the PAPVD coating material. This paper presents research results on the influence of different materials in PAPVD coatings, produced by the reactive arc-vacuum sputtering method, on the maintenance properties of the composite’s ‘nitrided layer/PAPVD coating’.

2. Experimental 2.1. Preparation of composites The composites investigated were created by means of the ‘duplex’ surface treatment method in a two stage separable cycle (the nitriding process and the PVD coating deposition were carried out with different devices). The nitriding process was carried out by means

Fig. 1. The structure of the composite ‘nitrided layer/TiN coating’ at successive stages of its creation: (a) nitrided layer created in Stage 1; (b) nitrided layer after mechanical surface treatment; (c) the composite ‘nitrided layer/TiN coating’.

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Fig. 2. Tools for hot-forging designed for maintenance investigations.

of regulated gas nitriding and was the first stage of ‘duplex’ treatment. The second stage was to produce the PVD coating by means of the arc-vacuum method using an MZ383 arc-vacuum device (Metaplas Ionon GmbH ). Mechanical surface treatment aimed at removing the superficial zone of e-Fe N and c∞-Fe N iron nitrides 2–3 4 was carried between the two main stages of the ‘duplex’ treatment. The parameters of the individual stages of the surface ‘duplex’ treatment are presented in Table 1. 2.2. Characterization of the composites The nitrided layer created in the Stage 1 of ‘duplex’ treatment contained a diffusion layer Fe (N ) of thicka ness 0.085 mm and a zone of iron nitrides e-Fe N and 2–3 c∞-Fe N of thickness 5.5 mm. As a result of the mechan4 ical surface treatment executed after the nitriding stage, a nitrided layer of the thickness 0.080 mm and surface hardness HV1=1265 was created. The high value of the surface hardness is the result of the residual amount of iron nitrides c∞ left on the surface after the mechanical surface treatment. The metallographic investigations carried out after the mechanical surface treatment revealed that only the Fe (N ) structure and the c∞ a structure issued at grain boundaries in the cooling process. The parameters of four different PVD coatings are presented in Table 2. The chemical composition of the investigated coatings was determined by the EDS method using an X-ray microanalyser (Noran Instruments) installed on a scanning microscope (Hitachi-S2460N ). Values of Young’s modulus for the materials of particular layers in the composites investigated were measured by means of the indentation test

Fig. 3. Results of maintenance investigations.

method. To achieve this, the nano-hardness tester made by the Swiss firm Centre Suisse d’Electronique et de Microtechnique S.A. was used. Measurements were carried out with a Vicker’s indentor in a single cycle without stopping using the following parameters: F=10 mN, dF/dt=20 mN/min. To eliminate the influence of the substrate material on the measurement of Young’s modulus of the layer material, the range of the indentor’s penetration depth was limited to g≤0.1d; (d=layer thickness). X-ray investigations to determine the coating phase composition were carried out using the Philips Analytical PW1840 diffractometer with a cobalt anode. The structure of the composite ‘nitrided layer/TiN coating’ in successive stages of creation is presented in Fig. 1.

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Fig. 4. Results of microscope observations and roughness measurements for the working surfaces of the die covered with the composite ‘nitrided layer/TiN coating’.

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Fig. 5. Results of microscope observations and roughness measurements for the working surfaces of the die covered with the composite ‘nitrided layer/CrN coating’.

J. Smolik et al. / Surface and Coatings Technology 125 (2000) 134–140

2.3. Antiwear performance test Four sets of hot-forging tools made of ISO steel 35CrMoV5 (0.4%C, 0.4%Mn, 1.0%Si, 5.0%Cr, 1.3%Mo, 0.3%V ) which is used for production of automotive half-shafts (constructed from material 25CrMo4) were investigated. Tools selected for investigations ( Fig. 2) belonged to a group of tools working in long-production series. The shape of required forgings meant that the dies and punches used were characterised by a complex shape in the working surfaces which are used in the forging process. It also meant that during work with large loads they had to ensure a considerable degree of plastic strain on the forged material. Maintenance tests on tools covered with the composites examined were carried at the forging hammer, type VES1600 ( Vesterman and Cleaver) with to the following parameters: the die’s temperature before forging was in the range 250–300°C; the forged material’s temperature was 1150°C; and the lubricant was Delta 31 at concentration 1:7

3. Results The results of the maintenance tests showing the number of forgings made by respective sets of tools are presented in Fig. 3. Microscope analysis of the intensity of the investigated forge dies destruction, because of their shape (centrally situated port), required a proper preparation of the samples to ensure access to the frontal surface of the die as well as to the port surfaces. The way the samples were prepared also enabled us to measure roughness in three different places of the die working areas — the frontal surface; and the port surface at two depths from the die front. The analysis of the dies’ surfaces wear was carried out on tools covered with two composites — ‘nitrided layer/TiN coating’ and ‘nitrided layer/CrN coating’ i.e. for those which in the maintenance test made the smallest and the biggest number of forgings. The way in which the investigated samples’ preparation and the results of the analysis are presented in Figs. 4 and 5. Microscope observations revealed that longitudinal grooves created as a result of the abrasive wear process and a grid of cracks — a result of fatigue processes — are the main forms of the analysed surfaces’ destruction. The abrasive wear is predominant in the die frontal part and in the port upper part, i.e. in places where the forging process is accompanied by the biggest pressures of the treated material on the tool surface. In the lower part of ports, the abrasive wear is minimal. However, a dense grid of cracks is visible there, which points to the predominance of fatigue processes. Conclusions from the microscope observation also seem to be confirmed

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by the results of the substrates’ roughness measurements. As can be seen in Figs. 4 and 5 the substrates’ roughness indexes Ra, Rz, Rt for the dies’ frontal surfaces with TiN and CrN coatings are similar. It points to a similar intensity of destruction of this part of both dies. So, it should be true that resistance of the investigated composites to mechanical loads, a dominant destruction factor in this part of the tools, is comparable. On the other hand, the further from the die frontal area, the smaller the abrasive wear intensity and the larger intensity of fatigue cracks. The crack intensity is smaller in the case of a die with a CrN layer than in the case of one with a TiN layer, which is proved by the roughness measurement . Thus, this points to a considerably bigger resistance to thermal shock of dies with the composite ‘nitrided layer/CrN coating’ than those with the composite ‘nitrided layer/TiN coating’.

4. Conclusions These results enable us to formulate the following conclusions: 1. use of ‘nitrided layer/PVD coating’ composites offers the potential to increase the durability of tools for hot plastic working; 2. correct choice of the PVD coating material significantly influences the durability of the ‘nitrided layer/PVD coating’ composite in hot plastic working; and 3. the destruction intensity of investigated tools for hot plastic working which were covered with various composites ‘nitrided layer/PVD coating’ was different in different parts of the tool. The degree of frontal area destruction in dies covered with the composites ‘nitrided layer/TiN coating’ and ‘nitrided layer/CrN coating’ was comparable, which testifies to their similar resistance to mechanical loads. Smaller wear of the ports’ surfaces in forging tools covered with the ‘nitrided layer/CrN coating’ composite testifies to their significantly bigger resistance to thermal shocks. Acknowledgement The work is a part of the research project European Concerted Action COST 516.

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