Duplex-PACVD coating of surfaces for die casting tools

Surface & Coatings Technology 201 (2007) 5628 – 5632 www.elsevier.com/locate/surfcoat

Duplex-PACVD coating of surfaces for die casting tools K.S. Klimek a,⁎, A. Gebauer-Teichmann b , P. Kaestner a , K.-T. Rie a a

b

Institut für Oberflächentechnik, Technische Universität Braunschweig, Bienroder Weg 53, D-38108 Braunschweig, Germany Volkswagen AG Werk Kassel, Business Unit Gießerei & Bearbeitung, Technologiezentrum, Brieffach 4720, Postfach 1451, 34219 Baunatal, Germany Available online 22 August 2006

Abstract Duplex-PACVD hard coatings are well known for increasing the tool performance in terms of adhesion, wear, fatigue, and corrosion resistance of the steel. The developments made in synthesizing duplex nanostructure and nanocomposite, mono and gradient layers based on borides are described. The aim of the investigation is to optimize the surface capability by plasma process combinations: duplex process and gradient layer. Within this work different types of duplex hard coatings produced by PACVD were investigated in terms of their tribological behavior and were tested in aluminum and magnesium die casting applications. Practical tests have been carried out by automobile producers and part suppliers. All coatings tested on die casting tools showed a significant increase of lifetime and a reduced metal adhesion tendency. The economic efficiency of coated die casting tools could be proved. © 2006 Elsevier B.V. All rights reserved. PACS: 68.55.-a; 81.15.-z Keywords: Duplex Process; Die casting; Multilayer; Gradient layer; Nanostructure; Nanocomposites; Al-alloys

1. Introduction The automotive lightweight construction is a growing sector, because of the ecological necessity to save fuel and to increase the load capacity of vehicles. From that background previous structural parts made of steel were replaced by aluminum or magnesium components (Figs. 1 and 2). One of the economically most significant methods to produce aluminum or magnesium parts in the industrial series production is the die casting process. In the modern automobile industry aluminum/magnesium pressure die casting, aluminum forging and extrusion are frequently applied manufacturing processes for the near end shape production of precision parts. The automobile producer and part supplier are encountered with serious lifetime problems of the tools. The early damage of die casting, extrusion, and cutting tools has negative effects in two folds: a) A quality assurance becomes risky. b) The expenses for standstill of production machines and replacement of the tools are enormous. ⁎ Corresponding author. Tel.: +49 531 391 9415; fax: +49 531 391 9400. E-mail address: [email protected] (K.S. Klimek). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.07.163

The predominant failure mechanisms of die casting tools are thermal fatigue by thermal shock stress, abrasion, adhesion, erosion, hydrogen embrittlement, and corrosion by liquid metal melts. As a consequence of the failure mechanisms the tools show only a limited lifetime. Nowadays parting agents are used to increase the service times of die casting tools. Parting agents are indispensable to protect the tools against corrosion and adhesion of metal melts. The operating principle is the vaporization/decomposition in the moment when they are coming in contact with the metal melt and a gaseous sliding film between die and metal melt is formed. However, the negative side effects are considerable. Parting agents are reported to decrease the lifetime of tools by the thermal shock at the moment of spraying, to increase the cycle times, to decrease the quality of the products by absorption of gases into the solidifying metals, and have to be seen critical under ecological aspects. To overcome the shortcomings of parting agents and improving the lifetime of tools various hard coatings were used [1]. All coating systems have to fulfill some requirements on process and layer. The process temperature has to be below the annealing temperature of the die material and complex geometries of complete die casting tools have to be coated in the

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Table 1 Field of parameters for PN and PACVD

Temperature / °C Voltage / V Duty cycle / μs/μs Pressure / Pa N2 / Vol% TiCl4 / Vol% BCl3 / Vol% Ar / Vol% H2 / Vol% Duration / h

Fig. 1. Die casting products in automobile industry.

process. The layer systems need a compact and dense structure without full length diffusion path for corrosive agents. The conventional CVD requires high deposition temperatures which limits their applications. Using the superposition of a plasma, the coating temperature can be reduced to 500–550 °C [2]. The uniform coating of complicated shapes by means of plasma assisted CVD processes and explicitly by pulsed DC– PACVD is another advantage of this deposition method [3,4]. PACVD hard coatings containing nitrides and borides of titanium are well known for their high hardness, low solubility, minimal difference in coefficient of thermal expansion between substrate and coating, less tendency of adhesion and high corrosion stability against metal melts [5]. Duplex treatments consisting of plasma nitriding and PACVD hard coating have been proven to be successful in improving wear, fatigue and corrosion resistance and the load carrying capability of steel substrates [6,7]. The “Duplex Process” leads to an increased adhesion of the hard coating on substrates compared to a coating without pretreatment [7]. Plasma duplex treatment combines the advantages

Fig. 2. Aluminum space frame technology.

PN

TiBN

TiB2

Grad.

400–550 500–600 0.75–0.88 300–600 10–80 – – 0–20 20–90 8–30

530–590 550–700 0.5 70–150 11–33 0.5 1–10 16–21 43–57 2

530 450–500 0.75 80–200 – 1–2 5–13 7–9 77–84 2–3

500–590 450–700 0.5–0.9 70–250 11–33 0.5–2.5 1–15 0–21 20–90 2–30

of both processes, plasma nitriding and PACVD. The compound and diffusion layer created during the plasma diffusion treatment offers a mechanical support to the hard coating, so that the danger of the egg-shell effect on soft substrates is reduced and the adhesion of the hard coating to the tool steel is optimized. Another advantage is the possibility to combine both processes to a single continuous process in order to reduce process time and costs. 2. Experimental The plasma diffusion treatment (PN) and the PACVD coating were carried out in DC-Pulse-PACVD reactors described in the literature [8,9]. The duplex coatings were performed on steel substrates (X38CrMoV5-1) in two steps. Plasma nitriding (PN) was followed by the deposition of boron and titanium containing nanostructure and nanocomposite mono and gradient (Ti(B; N); TiB2, TiN–Ti(B; N)–TiB2-grad) layer systems. The duplex process was performed in a continuous process, plasma nitriding and hard coating in one furnace directly after one another, or in a discontinuous process, plasma nitriding and hard coating in different reactors. The TiN–Ti(B; N)–TiB2-gradient layer was made by a continuous change of the BCl3 and N2 gas flow rates over a period of 2–3 h using computer controlled mass flow controllers

Fig. 3. View into a PACVD reactor during duplex process.

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Fig. 5. Critical load of duplex layer systems. Fig. 4. Metallographic cross section of a duplex TiN–Ti(B; N)–TiB2-gradient layer.

(continuous increase of the BCl3 flow rate and a continuous decrease of N2 gas flow rate). TiCl4 was served as a liquid metal precursor. The process gas was consisting of a mixture of Ar, H2, N2, and BCl3. The process development, the influence of the deposition parameters temperature, pressure, voltage, duty cycle, and gas composition were studied. Table 1 summarizes the field of parameters. The grain size was determined by TEM. Model test specimens were investigated in metallographical and mechanical technological studies. The critical load was determined in a scratch test by an optical evaluation of the scratch channels. Duplex layers with optimized process parameters were deposited on aluminum/magnesium die casting tools. The tools were tested in field tests in an automobile industry and part suppliers.

After plasma nitriding the nitrogen is concentrated in the surface of the substrates and a continuous hardness gradient between the surface and the bulk is created [13]. A typical metallographic cross section of a duplex treated steel sample with a gradient TiN–Ti(B; N)–TiB2 hard coating can be seen in Fig. 4. Table 2 shows the properties of the duplex hard coatings applied to the aluminum and magnesium die casting process. For the optimization of the duplex process compound layers and single diffusion layers were coated with the boron containing hard coatings Ti(B; N), TiB2 and TiN–Ti(B; N)–TiB2-gradient. The thickness of all deposited films was about 3 μm and the Vickers hardness was between 2800–3000 HV for duplex Ti (B; N), 3150–4000 HV for duplex TiB2, and 4000–4900 HV for duplex TiN–Ti(B; N)–TiB2-gradient films. The gradient layer with TiB2 upper-layer is harder than the TiB2 mono layer on a nitrided under-layer because of the more effective supporting under-layer.

3. Results and discussion Fig. 3 shows a view into the chamber of a PACVD plant with die casting pins during the duplex process. Several parameter studies concerning layer properties of PACVD hard coatings containing nitrides and borides of titanium are described in the literature [7,10–12].

Table 2 Properties of duplex hard coatings Layer system

Duplex-Ti(B; N)

Duplex-TiB2

Process Pretreatment Grain size Layer thickness Hardness (HV) Scratch test

PACVD Plasma nitriding 4–16 nm 3 μm 2800–3000 30 N

PACVD Plasma nitriding b2 nm 3 μm 3200–4000 55 N

Duplex-TiN–Ti (B; N)–TiB2-gradient PACVD Plasma nitriding 3 μm 4000–4900 120 N

Fig. 6. PACVD pilot plant charged with die casting pins.

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Fig. 7. Aluminum die casting tools for cylinder blocks.

Optimal adjustment between the diffusion treatment and the type of coating is one of the main issues of the development of a duplex treatment. The adhesion of the tested duplex films evaluated by scratch test [14] is shown in Fig. 5. The highest critical load was obtained for substrates with duplex TiN–Ti (B; N)–TiB2-gradient layer systems deposited in a continuous process (LCr = 120 N), followed by films made in a discontinuous process (LCr = 100 N). Hard coatings on substrates with the Fe2–3N phase and the mixed phases showed a significant decrease of adhesion compared to Fe4N compound layers or single diffusion layers. A project in the field of aluminum extrusion provided similar results [15]. Investigations of Walkowicz et al. and Pellizzari et al. approved a better adhesion of TiN and Ti(C, N), CrN and ZrN layers, respectively, using plasma nitriding pretreatment, especially the Fe4N phase [16,17]. All investigations showed an increase of adhesion strength of the hard coatings to the substrate using the duplex process, especially on only diffusion layer or the Fe4N phase, respectively. But duplex coatings with compound layers are only suitable for production processes without impact load, such as die casting, because of the brittleness of compound layers. Processes like forging demand a duplex process with an only diffusion layer. Hard coatings on the basis of titanium are successfully applied as wear protection in various applications [1]. This study focuses on thin films of the system Ti–B–N with nanostructure and nanocomposites. All deposited hard coatings have a high hardness, a good adhesion and provide a high wear protection. Fig. 6 shows the charging of the PACVD pilot plant with pins for aluminum die casting.

Ti(B; N), TiB2 and TiN–Ti(B; N)–TiB2-gradient hard coatings with plasma diffusion pretreatment (PN), especially on Fe4N phases and on only diffusion layers were selected to apply for aluminium/magnesium die casting (Figs. 7 and 8). The field tests with die casting pins (Fig. 6) at Volkswagen Kassel coated with the described layer systems showed a significant increase of tool lifetime about 350–500% (Fig. 9). A waterjacket core (Fig. 7) treated with duplex Ti(B, N) mono layer system extended the operating time about 180% compared to the average tool lifetime. Another aspect is a notable reduction of sticking of metal melts using coatings based on titanium borides and nitrides. In view of the coating costs, tool costs and the extended lifetime of treated tools, it was found that coating of die casting tools is significantly profitable for the users. Considering not only the lifetime extension of tools but also the reduced expenses for standstill of the production machines and tool replacement in accordance with higher productivity, improvement of die casting tools with the described PACVD duplex hard coatings is always economically advantageous.

Fig. 8. Magnesium die casting tools.

Fig. 9. Average life time of aluminum die casting pins.

4. Conclusion The application of the duplex process leads to an increased adhesion of Ti(B; N), TiB2 and gradient hard coatings. As shown on the example of plasma nitriding the diffusion treatment considerably enhances the critical load so that the adhesion of the coating to the compound layer is higher than to the untreated surface. The load bearing capacity of the diffusion treated substrate is indispensable for hard coatings on a soft matrix in order to prevent “egg-shell” effects. Knoop hardness measurements show that the resistance to plastic deformation during impact

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loading of the coating/diffusion-treated-zone/bulk system increases with increasing case depth. A combination of both processes, plasma diffusion treatment (PDT) and PACVD hard coating, increases the wear resistance and lifetime of die casting tools by many times and the cost for coating could be easily covered. Another beneficial aspect is the reduction of parting agents during the die casting process, which results in an increased product quality and an enhanced job quality. All field test results show that a good surface quality of the products and an extended lifetime of the tools are obtained by the use of the dies coated with duplex process, while reducing parting agents. Acknowledgement The authors wish to thank the Bundesministerium für Bildung und Forschung (BMBF) for the financial support of this research (FKZ: 03X2504). The field tests were carried out at Volkswagen Kassel, and Hettich Promodul, Frankenberg. References [1] H.O. Pierson, Mater. Manuf. Process. 8 (1993) 519. [2] W. Eskilsen, C. Mathiasen, M. Foss, Surf. Coat. Technol. 120 (2000) 16.

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