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Duplex surface treatment of moulds for pressure casting of aluminium
Surface and Coatings Technology 97 (1997) 453–464
Duplex surface treatment of moulds for pressure casting of aluminium Jan Walkowicz *, Jerzy Smolik, Krzysztof Miernik, Jan Bujak Institute for Terotechnology, ul. Pułaskiego 6/10, 26-600 Radom, Poland
Abstract The subject of this paper is the technology of the production of composite-nitrided case/TiN coatings. This technology is used for improving the lifetime of tools operating at heavy thermal and mechanical loads and at conditions of intensive abrasive wear. In the case of such tools, the standard methods of improving the surface properties by thermal treatment, thermo-chemical treatment or by deposition of anti-wear monolayers [e.g. TiN, Ti(C, N )] do not lead to the desired increase in tool life. This goal can be achieved only by the application of a duplex surface treatment which involves beside the deposition of mono and multilayers, a thermo-chemical treatment (for example, nitriding), and determining the structure and properties of the tool’s surface layer. As a result of a duplex treatment, a multi-layer composite is obtained, which in terms of anti-wear properties outdoes both diffusion layers obtained by thermo-chemical treatment and multilayers deposited by PAPVD methods. The chemical composition and internal structure of the composite must be selected with regard to its final application. The technology of duplex surface treatment was used to improve the durability of moulds for aluminium injection moulding. © 1997 Elsevier Science S.A. Keywords: Duplex treatment; Continuous process; Discontinuous process; Pressure casting moulds; Ion nitriding; Gas nitriding; Vacuum arc method; Nitrided case; TiN coating
1. Introduction During operation, moulds for pressure casting are subjected to both extreme mechanical stresses induced by the high-pressure flow of the liquid metal and the thermal stresses induced by alternate contact with the cast material in temperatures exceeding 700 °C and the parting compound, the temperature of which approaches 20 °C. Such working conditions are the reason for the quick wear of moulds, which is manifested by intensive erosion of the mould’s surface in narrow cross-sections or even surface cracks in the areas of highest pressures of the flowing working medium [1,2]. Each of these wear forms disqualifies the tool and necessitates its regeneration. To extend the mould life, one must look for solutions among modern surface treatment methods. One of such methods is the production of composites consisting of a diffusion layer created by thermo-chemical treatment of the substrate and an anti-wear layer deposited on the tool surface by plasma-chemical meth-
* Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 20 3 - X
ods. The first attempts of applying such duplex surface treatment were carried out in the 1980s [3–5]. However, intensive work concerning the development of this method for tool life improvement was undertaken only at the beginning of the 1990s [6–12] and is carried out at present in many scientific centres [13–22]. The method of duplex treatment is based on a practically confirmed assumption that the very good properties of thin, antiwear layers of transition metal nitrides deposited by PAPVD methods can be fully utilized only on the condition that the substrate is able to withstand the occurring mechanical loads without much plastic deformation, otherwise the layer will be destroyed due to breakage in cohesive bonds [10–14,16,17]. The nitrided layer structure is of key importance for the final effect of duplex treatment because it significantly determines adhesion of the titanium nitride layer. It was found out that during the initial stage of the TiN layer deposition process by PAPVD methods, i.e. during ion etching, decomposition of the e and e+c∞-‘‘white’’ compound layer [6–8] can occur. This effect, depending on the ion etching conditions, can occur in temperatures significantly lower than 500 °C, which is defined as the stability point of e and c∞ phases. On the other hand, adhesion
Thickness of nitrided layer (mm)
0.08/0.10
0.08/0.09
0.10/0.12
Nitriding process parameters
Atmosphere: 25% N +75% H . 2 2 T=540 °C; p=4.0 mbar; t=8 h Atmosphere: 15% N +85% H . 2 2 T=530 °C; p=2.5 mbar; t=8 h Atmosphere: 100% NH . 3 Flow: 208 m /h; T=540 °C; t=8 h 3
Method of duplex treatment
Continuous process (ion nitriding+TiN layer deposition) Discontinuous process (ion nitriding+TiN layer deposition) Discontinuous process (gas nitriding+TiN layer deposition)
1159
1103
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
TiN deposition parameters
=-150 V bias
=-150 V bias
=-150 V bias
2.0
2.0
2.0
Thickness of TiN coating (mm)
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1103
Hardness of nitrided layer (mHV ) 1
Table 1 The parameters of the processes of nitriding and layer deposition and some properties of created composites
454 60
54
45
Adhesion of TiN coating (critical force) (N )
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Fig. 1. Composite: nitrided case/TiN coating obtained in continuous technological cycle (ion nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
of layers deposited by PAPVD methods is proportional to the substrate temperature during deposition. The mentioned process of decomposition of the compound layer can be initiated even at the substrate temperature of 350 °C [6 ]. It occurs in the case of ion etching in argon atmosphere, which is often used in PAPVD equipment. This effect does not occur in the equipment where ion etching is carried out with the use of titanium ions. In such a system, however, the moment when ion etching proceeds into the stage of TiN deposition on the surface of an element, a thin titanium sub-layer is created. It is
able to activate the ‘‘white layer’’ decomposition if the latter is initiated by a temperature close to the critical one. As a result of the ‘‘white layer’’ decomposition, the so-called ‘‘black layer’’ is created between a nitrided substrate and TiN layer, which causes a complete flaking of the TiN layer from the substrate. It was found out [6,7] that the ‘‘black layer’’ contains mainly a-Fe phase of low hardness and it is created due to retrodiffusion of the nitride generated in the process of decomposition of the phases: e-Fe N and c∞-Fe N. One of the 2–3 4 research trends focused on improvement in adhesion of
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Fig. 2. Composite: nitrided case/TiN coating obtained in discontinuous technological cycle (ion nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
TiN layers to nitrided substrates attempted to deposit TiN layers at such process parameters which would prevent the decomposition of e and c∞ iron nitrides. Such processes proved feasible due to methods which, for example, utilized the hollow cathode [15], but the properties of the composite containing ‘‘the white layer’’ were significantly worse [6,8]. However, the emergence of the ‘‘black layer’’ was not noted while depositing TiN layers on the substrates whose nitrided layer containing the a-Fe(N ) phase, i.e. nitrogen ferrite, exclusively. On the basis of the described research results, we can claim
that the necessary prerequisite for achieving a good adhesion of TiN layer to nitrided substrates and good properties of the composite is the creation of a single-phase structure of the nitrided substrate containing exclusively the a-Fe(N ) phase or removal of the ‘‘white layer’’ of e and c∞ nitrides. The choice of one of these methods determines the organization of the whole process of a duplex treatment. The research on duplex treatment, which is carried out in most European centres dealing with this problem, aims at the production of such composites in one
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Fig. 3. Composite: nitrided case/TiN coating obtained in discontinuous technological cycle (gas nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
continuous technological cycle [7,8,10,15,16,18]. The mentioned cycle is executed in the vacuum chamber of a PAPVD equipment (most often vacuum-arc). It consists of ion nitriding immediately followed by the deposition of a wear-resistant hard layer. Such a method of duplex treatment is expensive because the relatively simple but long operation of thermo-chemical treatment (e.g. nitriding) is carried out by very expensive, special equipment, which cannot be used for hard coating for many hours. Besides, in the case of a continuous prosess of duplex treatment, there is no possibility of choosing
the nitriding method. This limits potential applications of the treatment because some mould elements, complicated in shape, must be nitrided by the conventional gas method.
2. Experimental details The purpose of the work undertaken by the authors of this paper was the development of a duplex treatment
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of moulds for pressure casting of aluminium in three types of production : (1) in the single-stage, continuous process (ion nitriding+TiN layer deposition by the vacuum-arc method ); (2) in the two-stage, discontinuous process (ion nitriding+TiN layer deposition by vacuum-arc method ); (3) in the two-stage, discontinuous process (gas nitriding+TiN layer deposition by vacuum-arc method ). Within the framework of research carried out by the authors while developing a duplex-treatment technology, samples of hot working steel ( WCL grade: 0.4% C; 0.4% Mn; 1.0% Si; 5.0% Cr; 1.3% Mo; 0.3% V ) were treated in these three ways. In the single-stage cycle, the treatment was carried out in the vacuum-arc device MZ 383 manufactured by Metaplas Ionon (Bergisch Gladbach) which, thanks to its additional equipment, enabled the ion nitriding process. In two-stage cycles, the ion nitriding was carried out in the device GZ 40 manufactured by Metaplas Ionon and gas nitriding in the retort furnace PET (Polish Manufacturer). The gas nitrided samples were ground in order to remove the
‘‘white layer’’ of e and c∞ nitrides. Deposition of TiN layers on nitrided substrates was carried out in the vacuum-arc device, MZ 383. The parameters of the processes of nitriding and layer deposition and some properties of created composites are presented in Table 1. The prepared samples were examined in microscopic metallographic investigation and an investigation of chemical and phase composition. Metallographic examinations were conducted with the use of the optical microscope, Neophot 32; diffraction examinations utilized an X-ray diffractometer Philips PW 1830 and a linear qualitative analysis of the chemical composition was conducted with the help of a scanning microscope Hitachi equipped with an X-ray microprobe, Noran. TiN layer thickness measurements were carried out by the calotest method by utilizing the Bernex apparatus.
3. Results and discussion The examinations carried out showed that each of the varieties of duplex treatment execution anables creation of composites: nitrided case/TiN coating with optimum
Fig. 4. X-ray diffractogram of the composite obtained in continuous technological cycle (ion nitriding+TiN arc deposition).
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Fig. 5. X-ray diffractogram of the composite obtained in discontinuous technological cycle (ion nitriding+TiN arc deposition).
structure (without ‘‘the white layer’’ of e and c∞ nitrides) and similar properties, which proves that they are equally useful and makes it possible to adequately choose the variety to match the planned application and technical possibilities available. Figs. 1–3 present the structures of the composites obtained on hot working steel, grade WCL (35CrMoV5 according to ISO), which is used most frequently for making moulds for pressure casting of aluminium. They were produced in the processes of duplex treatment in each of the three described varieties. The structures revealed in metallographic examinations and the linear analysis of the nitrogen concentration distribution in the obtained composites confirmed possibility of obtaining similar effects of duplex treatment irrespective of the technological variety applied. XRD spectra of the structures created prove that the conclusions presented (Figs. 4–6) are correct. The same diffraction lines were identified in diffractograms of each examined sample. Three of them are derived from the a-Fe(N ) phase in the substrate and are shifted slightly towards smaller angles due to the increase in lattice parameter and interplanar distances caused by nitriding. Differences in intensity of the mentioned lines for particular samples are probably caused by differences in TiN
layer thicknesses not revealed by the calotest measurements. The remaining two lines belong to TiN — lines (111) and (222). Stronger adhesion of the TiN layer obtained in the discontinuous process with ion nitriding as compared with the continuous one is, in our opinion, caused by cooling of the substrate to ambient temperature after nitriding, which enables longer ion etching of the nitrided layer prior to layer deposition. In the single-stage process, the substrate was cooled after nitriding to 350 °C only. A very good adhesion of the layer obtained in the discontinuous process with gas nitriding should be ascribed to intermediate operation of grinding of the nitrided substrate.
4. Application The developed two-stage duplex treatment process with conventional gas nitriding has already been implemented in practice. Fig. 7 presents the moulds for pressure casting of the pneumatic valve body which was made and duplex treated in our Institute for Terotechnology. The two-stage, process of composite manufacture, developed in the Institute for
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Fig. 6. X-ray diffractogram of the composite obtained in discontinuous technological cycle (gas nitriding+TiN arc deposition).
Fig. 7. Duplex treated mould for pressure casting of aluminium manufactured and used in the Institute for Terotechnology.
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Terotechnology, is characterized by the mechanical removal of the e and e+c∞ compound layer after gas nitriding. After that, polishing of the mould forming surfaces and chemical cleaning are carried out. A schematic diagram of discontinuous duplex surface treatment is presented in Fig. 8. The structure of the surface layer of the treated element after the following steps of the process is shown in Fig. 9. Prior to deposition of a TiN layer, the surface of the mould is cleaned by ion sputtering and heated in the titanic metallic plasma of arc discharge. Ionic cleaning is executed in a pulsed discharge. Pulse duration and repetition are selected in such a way that the temperature of the surface of the mould does not exceed 350 °C. The duplex treatment execution in the two-stage, discontinuous process, irrespective of the applied method of nitriding, enables, in some cases, the utilization of the experience and equipment of an industrial customer in the field of thermochemical treatment. It decreases the costs significantly because the process of nitriding and further preparation of the surface must be only adjusted to the requirements
461
of the second stage of the treatment, i.e. TiN coating deposition. This type of treatment was implemented in ´ the Zelmer Plant in Rzeszow. Figs. 10 and 11 show some examples of moulds which underwent the duplex treatment by this method. Fig. 12 shows the increase in the selected mould element’s life. A very important additional effect of the treatment was complete elimination of faulty elements due to sticking of aluminium to the mould surface.
5. Conclusions On the basis of the research and production tests conducted, the authors of this paper believe that obtaining a composite of the type nitrided case/TiN coating, which has improved anti-wear properties in comparison with those of the nitrided layers, is possible in both continuous and two-stage, discontinuous technological cycles. Both methods mentioned have different charac-
Fig. 8. Schematic diagram of main steps in discontinuous duplex surface treatment.
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layer on the surface of parts having complicated shapes. This limits its application significantly. Considering the fact that its accomplishment requires expensive special equipment and that it is a time-consuming process, it must be noted that obtaining a composite of this type coating in a continuous process is one of the most expensive technologies of surface treatment. A two-stage cycle enables the selection of the nitriding method (gas or ion nitriding), which is important in the case of components having complicated shapes, as well as close technological co-operation with industrial customer including the use of customer’s nitriding equipment, which reduces duplex treatment costs significantly. On the other hand, if gas nitriding is applied, additional mechanical treatment is required. Yet, in both cases, the correct selection of technological parameters enables the obtaining of a nitrided layer having a structure and depth correct for the given material as well as a uniform TiN layer of proper adhesion to the substrate.
6. Summary Fig. 9. Structure of the surface layer after the following steps of discontinuous technological cycle (gas nitriding+TiN arc deposition): (a) gas nitriding; (b) removal of the ‘‘white layer’’; (c) chemical cleaning; and (d ) TiN deposition.
teristics. A continuous cycle requires ion nitriding which frequently inhibits the obtaining of a uniform nitrided
The paper presents the method of duplex surface treatment consisting of nitriding and TiN coating deposition. The method is used for moulds for pressure casting of aluminium. Three technological varieties of executing a duplex treatment are presented. Examinations of the structure and properties of the composite (nitrided case/TiN coating) manufactured by the three methods
´ Fig. 10. Duplex treated mould for pressure casting of aluminium manufactured and used in ‘‘Zelmer’’ Rzeszow.
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´ Fig. 11. Duplex treated mould for pressure casting of aluminium manufactured and used in ‘‘Zelmer’’ Rzeszow.
discussed proved that they are comparable in practice. The developed method of two-stage, discontinuous duplex treatment was used in practice bringing about a significant increase in mould life and improvement in the quality of manufactured elements.
´ Rzeszow and Metaplas Ionon for help in the experiments.
References Acknowledgement The work is a part of the research project no. 7T08B 003 95C/2574 supported by the Research Committee of Poland. The authors wish to thank companies Zelmer
Fig. 12. Increase in life of duplex treated different elements of moulds in comparison to gas nitrided specimens.
¨ [1] O. Knotek, F. Loffler, B. Bosserhoff, Surf. Coat. Technol. 62 (1993) 630. [2] R. Shivpuri, Y.-L. Chu, K. Venkatesan, J.R. Conrad, K. Sridharan, M. Shamim, R.P. Fetherson, Wear 192 (1996) 49. [3] B.M. Kramer, Thin Solid Films 108 (1983) 117. [4] A.S. Korhonen, E. Sirva, M. Sulonen, Thin Solid Films 107 (1983) 387. [5] A. Matthews, V. Murawa, Chartered Mech. Engng 10 (1985) 33. [6 ] Y. Sun, T. Bell, Trans. Inst. Metal Finishing 70 (1992) 38. [7] N. Dingremont, A. Pianelli, E. Bergmann, H. Michel, Surf. Coat. Technol. 61 (1993) 187. [8] J. D’Haen, C. Quaeyhaegens, L.M. Stals, M. Van Stappen, Surf. Coat. Technol. 61 (1993) 194. ´ ´ ¨ [9] T. Gredic, M. Zlatanovic, W.-D. Munz, Surf.Coat. Technol. 61 (1993) 338. [10] M. Van Stappen, M. Kerkhofs, C. Quaeyhaegens, L. Stals, Surf. Coat. Technol. 62 (1993) 655. ´ ´ ´ ˇ [11] T. Gredic, M. Zlatanovic, N. Popovic, Z. Bogdanov, Thin Solid Films 228 (1993) 261. ´ ´ ´ ´ [12] M. Zlatanovic, T. Gredic, A. Kunosic, N. Backovic, N. Whittle, Surf. Coat. Technol. 63 (1993) 35. [13] H.H. Huang, J.L. He, M.H. Hon, Surf. Coat. Technol. 64 (1994) 41. ´ ´ [14] M. Zlatanovic, D. Kakas, L.J. Mazibrada, A. Kunosic, W.-D. ¨ Munz, Surf.Coat. Technol. 64 (1994) 173. [15] B. Buecken, G. Leonhardt, R. Wilberg, K. Hoeck, H.J. Spies, Surf. Coat. Technol. 68/69 (1994) 244. [16 ] N. Dingremont, E. Bergmann, P. Collignon, Surf. Coat. Technol. 72 (1995) 157.
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[17] N. Dingremont, E. Bergmann, P. Collignon, H. Michel, Surf. Coat. Technol. 72 (1995) 163. ´ [18] J. Michalski, E. Łunarska, T. Wierzchon, S. AlGhanem, Surf. Coat. Technol. 72 (1995) 189. [19] S.-Ch. Lee, W.-Y. Ho, W.-L. Pao, Surf. Coat. Technol. 73 (1995) 34.
[20] E.I. Meletis, A. Erdemir, G.R. Fenske, Surf. Coat. Technol. 73 (1995) 39. [21] C. Quaeyhaegens, M. Kerkhofs, L.M. Stals, M. Van Stappen, Surf. Coat. Technol. 80 (1996) 181. ¨ [22] K. Hock, H.J. Spies, B. Larisch, G. Leonhardt, B. Buecken, Surf. Coat. Technol. 88 (1996) 44.
Duplex surface treatment of moulds for pressure casting of aluminium Jan Walkowicz *, Jerzy Smolik, Krzysztof Miernik, Jan Bujak Institute for Terotechnology, ul. Pułaskiego 6/10, 26-600 Radom, Poland
Abstract The subject of this paper is the technology of the production of composite-nitrided case/TiN coatings. This technology is used for improving the lifetime of tools operating at heavy thermal and mechanical loads and at conditions of intensive abrasive wear. In the case of such tools, the standard methods of improving the surface properties by thermal treatment, thermo-chemical treatment or by deposition of anti-wear monolayers [e.g. TiN, Ti(C, N )] do not lead to the desired increase in tool life. This goal can be achieved only by the application of a duplex surface treatment which involves beside the deposition of mono and multilayers, a thermo-chemical treatment (for example, nitriding), and determining the structure and properties of the tool’s surface layer. As a result of a duplex treatment, a multi-layer composite is obtained, which in terms of anti-wear properties outdoes both diffusion layers obtained by thermo-chemical treatment and multilayers deposited by PAPVD methods. The chemical composition and internal structure of the composite must be selected with regard to its final application. The technology of duplex surface treatment was used to improve the durability of moulds for aluminium injection moulding. © 1997 Elsevier Science S.A. Keywords: Duplex treatment; Continuous process; Discontinuous process; Pressure casting moulds; Ion nitriding; Gas nitriding; Vacuum arc method; Nitrided case; TiN coating
1. Introduction During operation, moulds for pressure casting are subjected to both extreme mechanical stresses induced by the high-pressure flow of the liquid metal and the thermal stresses induced by alternate contact with the cast material in temperatures exceeding 700 °C and the parting compound, the temperature of which approaches 20 °C. Such working conditions are the reason for the quick wear of moulds, which is manifested by intensive erosion of the mould’s surface in narrow cross-sections or even surface cracks in the areas of highest pressures of the flowing working medium [1,2]. Each of these wear forms disqualifies the tool and necessitates its regeneration. To extend the mould life, one must look for solutions among modern surface treatment methods. One of such methods is the production of composites consisting of a diffusion layer created by thermo-chemical treatment of the substrate and an anti-wear layer deposited on the tool surface by plasma-chemical meth-
* Corresponding author. 0257-8972/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 7 ) 0 0 20 3 - X
ods. The first attempts of applying such duplex surface treatment were carried out in the 1980s [3–5]. However, intensive work concerning the development of this method for tool life improvement was undertaken only at the beginning of the 1990s [6–12] and is carried out at present in many scientific centres [13–22]. The method of duplex treatment is based on a practically confirmed assumption that the very good properties of thin, antiwear layers of transition metal nitrides deposited by PAPVD methods can be fully utilized only on the condition that the substrate is able to withstand the occurring mechanical loads without much plastic deformation, otherwise the layer will be destroyed due to breakage in cohesive bonds [10–14,16,17]. The nitrided layer structure is of key importance for the final effect of duplex treatment because it significantly determines adhesion of the titanium nitride layer. It was found out that during the initial stage of the TiN layer deposition process by PAPVD methods, i.e. during ion etching, decomposition of the e and e+c∞-‘‘white’’ compound layer [6–8] can occur. This effect, depending on the ion etching conditions, can occur in temperatures significantly lower than 500 °C, which is defined as the stability point of e and c∞ phases. On the other hand, adhesion
Thickness of nitrided layer (mm)
0.08/0.10
0.08/0.09
0.10/0.12
Nitriding process parameters
Atmosphere: 25% N +75% H . 2 2 T=540 °C; p=4.0 mbar; t=8 h Atmosphere: 15% N +85% H . 2 2 T=530 °C; p=2.5 mbar; t=8 h Atmosphere: 100% NH . 3 Flow: 208 m /h; T=540 °C; t=8 h 3
Method of duplex treatment
Continuous process (ion nitriding+TiN layer deposition) Discontinuous process (ion nitriding+TiN layer deposition) Discontinuous process (gas nitriding+TiN layer deposition)
1159
1103
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
Atmosphere: 100% N . 2 T=350 °C; p=1.2×10-2 mbar; U
TiN deposition parameters
=-150 V bias
=-150 V bias
=-150 V bias
2.0
2.0
2.0
Thickness of TiN coating (mm)
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Hardness of nitrided layer (mHV ) 1
Table 1 The parameters of the processes of nitriding and layer deposition and some properties of created composites
454 60
54
45
Adhesion of TiN coating (critical force) (N )
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Fig. 1. Composite: nitrided case/TiN coating obtained in continuous technological cycle (ion nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
of layers deposited by PAPVD methods is proportional to the substrate temperature during deposition. The mentioned process of decomposition of the compound layer can be initiated even at the substrate temperature of 350 °C [6 ]. It occurs in the case of ion etching in argon atmosphere, which is often used in PAPVD equipment. This effect does not occur in the equipment where ion etching is carried out with the use of titanium ions. In such a system, however, the moment when ion etching proceeds into the stage of TiN deposition on the surface of an element, a thin titanium sub-layer is created. It is
able to activate the ‘‘white layer’’ decomposition if the latter is initiated by a temperature close to the critical one. As a result of the ‘‘white layer’’ decomposition, the so-called ‘‘black layer’’ is created between a nitrided substrate and TiN layer, which causes a complete flaking of the TiN layer from the substrate. It was found out [6,7] that the ‘‘black layer’’ contains mainly a-Fe phase of low hardness and it is created due to retrodiffusion of the nitride generated in the process of decomposition of the phases: e-Fe N and c∞-Fe N. One of the 2–3 4 research trends focused on improvement in adhesion of
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Fig. 2. Composite: nitrided case/TiN coating obtained in discontinuous technological cycle (ion nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
TiN layers to nitrided substrates attempted to deposit TiN layers at such process parameters which would prevent the decomposition of e and c∞ iron nitrides. Such processes proved feasible due to methods which, for example, utilized the hollow cathode [15], but the properties of the composite containing ‘‘the white layer’’ were significantly worse [6,8]. However, the emergence of the ‘‘black layer’’ was not noted while depositing TiN layers on the substrates whose nitrided layer containing the a-Fe(N ) phase, i.e. nitrogen ferrite, exclusively. On the basis of the described research results, we can claim
that the necessary prerequisite for achieving a good adhesion of TiN layer to nitrided substrates and good properties of the composite is the creation of a single-phase structure of the nitrided substrate containing exclusively the a-Fe(N ) phase or removal of the ‘‘white layer’’ of e and c∞ nitrides. The choice of one of these methods determines the organization of the whole process of a duplex treatment. The research on duplex treatment, which is carried out in most European centres dealing with this problem, aims at the production of such composites in one
J. Walkowicz et al. / Surface and Coatings Technology 97 (1997) 453–464
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Fig. 3. Composite: nitrided case/TiN coating obtained in discontinuous technological cycle (gas nitriding+TiN arc deposition): (a) qualitative analysis of nitrogen; (b) structure of the composite.
continuous technological cycle [7,8,10,15,16,18]. The mentioned cycle is executed in the vacuum chamber of a PAPVD equipment (most often vacuum-arc). It consists of ion nitriding immediately followed by the deposition of a wear-resistant hard layer. Such a method of duplex treatment is expensive because the relatively simple but long operation of thermo-chemical treatment (e.g. nitriding) is carried out by very expensive, special equipment, which cannot be used for hard coating for many hours. Besides, in the case of a continuous prosess of duplex treatment, there is no possibility of choosing
the nitriding method. This limits potential applications of the treatment because some mould elements, complicated in shape, must be nitrided by the conventional gas method.
2. Experimental details The purpose of the work undertaken by the authors of this paper was the development of a duplex treatment
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of moulds for pressure casting of aluminium in three types of production : (1) in the single-stage, continuous process (ion nitriding+TiN layer deposition by the vacuum-arc method ); (2) in the two-stage, discontinuous process (ion nitriding+TiN layer deposition by vacuum-arc method ); (3) in the two-stage, discontinuous process (gas nitriding+TiN layer deposition by vacuum-arc method ). Within the framework of research carried out by the authors while developing a duplex-treatment technology, samples of hot working steel ( WCL grade: 0.4% C; 0.4% Mn; 1.0% Si; 5.0% Cr; 1.3% Mo; 0.3% V ) were treated in these three ways. In the single-stage cycle, the treatment was carried out in the vacuum-arc device MZ 383 manufactured by Metaplas Ionon (Bergisch Gladbach) which, thanks to its additional equipment, enabled the ion nitriding process. In two-stage cycles, the ion nitriding was carried out in the device GZ 40 manufactured by Metaplas Ionon and gas nitriding in the retort furnace PET (Polish Manufacturer). The gas nitrided samples were ground in order to remove the
‘‘white layer’’ of e and c∞ nitrides. Deposition of TiN layers on nitrided substrates was carried out in the vacuum-arc device, MZ 383. The parameters of the processes of nitriding and layer deposition and some properties of created composites are presented in Table 1. The prepared samples were examined in microscopic metallographic investigation and an investigation of chemical and phase composition. Metallographic examinations were conducted with the use of the optical microscope, Neophot 32; diffraction examinations utilized an X-ray diffractometer Philips PW 1830 and a linear qualitative analysis of the chemical composition was conducted with the help of a scanning microscope Hitachi equipped with an X-ray microprobe, Noran. TiN layer thickness measurements were carried out by the calotest method by utilizing the Bernex apparatus.
3. Results and discussion The examinations carried out showed that each of the varieties of duplex treatment execution anables creation of composites: nitrided case/TiN coating with optimum
Fig. 4. X-ray diffractogram of the composite obtained in continuous technological cycle (ion nitriding+TiN arc deposition).
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Fig. 5. X-ray diffractogram of the composite obtained in discontinuous technological cycle (ion nitriding+TiN arc deposition).
structure (without ‘‘the white layer’’ of e and c∞ nitrides) and similar properties, which proves that they are equally useful and makes it possible to adequately choose the variety to match the planned application and technical possibilities available. Figs. 1–3 present the structures of the composites obtained on hot working steel, grade WCL (35CrMoV5 according to ISO), which is used most frequently for making moulds for pressure casting of aluminium. They were produced in the processes of duplex treatment in each of the three described varieties. The structures revealed in metallographic examinations and the linear analysis of the nitrogen concentration distribution in the obtained composites confirmed possibility of obtaining similar effects of duplex treatment irrespective of the technological variety applied. XRD spectra of the structures created prove that the conclusions presented (Figs. 4–6) are correct. The same diffraction lines were identified in diffractograms of each examined sample. Three of them are derived from the a-Fe(N ) phase in the substrate and are shifted slightly towards smaller angles due to the increase in lattice parameter and interplanar distances caused by nitriding. Differences in intensity of the mentioned lines for particular samples are probably caused by differences in TiN
layer thicknesses not revealed by the calotest measurements. The remaining two lines belong to TiN — lines (111) and (222). Stronger adhesion of the TiN layer obtained in the discontinuous process with ion nitriding as compared with the continuous one is, in our opinion, caused by cooling of the substrate to ambient temperature after nitriding, which enables longer ion etching of the nitrided layer prior to layer deposition. In the single-stage process, the substrate was cooled after nitriding to 350 °C only. A very good adhesion of the layer obtained in the discontinuous process with gas nitriding should be ascribed to intermediate operation of grinding of the nitrided substrate.
4. Application The developed two-stage duplex treatment process with conventional gas nitriding has already been implemented in practice. Fig. 7 presents the moulds for pressure casting of the pneumatic valve body which was made and duplex treated in our Institute for Terotechnology. The two-stage, process of composite manufacture, developed in the Institute for
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Fig. 6. X-ray diffractogram of the composite obtained in discontinuous technological cycle (gas nitriding+TiN arc deposition).
Fig. 7. Duplex treated mould for pressure casting of aluminium manufactured and used in the Institute for Terotechnology.
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Terotechnology, is characterized by the mechanical removal of the e and e+c∞ compound layer after gas nitriding. After that, polishing of the mould forming surfaces and chemical cleaning are carried out. A schematic diagram of discontinuous duplex surface treatment is presented in Fig. 8. The structure of the surface layer of the treated element after the following steps of the process is shown in Fig. 9. Prior to deposition of a TiN layer, the surface of the mould is cleaned by ion sputtering and heated in the titanic metallic plasma of arc discharge. Ionic cleaning is executed in a pulsed discharge. Pulse duration and repetition are selected in such a way that the temperature of the surface of the mould does not exceed 350 °C. The duplex treatment execution in the two-stage, discontinuous process, irrespective of the applied method of nitriding, enables, in some cases, the utilization of the experience and equipment of an industrial customer in the field of thermochemical treatment. It decreases the costs significantly because the process of nitriding and further preparation of the surface must be only adjusted to the requirements
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of the second stage of the treatment, i.e. TiN coating deposition. This type of treatment was implemented in ´ the Zelmer Plant in Rzeszow. Figs. 10 and 11 show some examples of moulds which underwent the duplex treatment by this method. Fig. 12 shows the increase in the selected mould element’s life. A very important additional effect of the treatment was complete elimination of faulty elements due to sticking of aluminium to the mould surface.
5. Conclusions On the basis of the research and production tests conducted, the authors of this paper believe that obtaining a composite of the type nitrided case/TiN coating, which has improved anti-wear properties in comparison with those of the nitrided layers, is possible in both continuous and two-stage, discontinuous technological cycles. Both methods mentioned have different charac-
Fig. 8. Schematic diagram of main steps in discontinuous duplex surface treatment.
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layer on the surface of parts having complicated shapes. This limits its application significantly. Considering the fact that its accomplishment requires expensive special equipment and that it is a time-consuming process, it must be noted that obtaining a composite of this type coating in a continuous process is one of the most expensive technologies of surface treatment. A two-stage cycle enables the selection of the nitriding method (gas or ion nitriding), which is important in the case of components having complicated shapes, as well as close technological co-operation with industrial customer including the use of customer’s nitriding equipment, which reduces duplex treatment costs significantly. On the other hand, if gas nitriding is applied, additional mechanical treatment is required. Yet, in both cases, the correct selection of technological parameters enables the obtaining of a nitrided layer having a structure and depth correct for the given material as well as a uniform TiN layer of proper adhesion to the substrate.
6. Summary Fig. 9. Structure of the surface layer after the following steps of discontinuous technological cycle (gas nitriding+TiN arc deposition): (a) gas nitriding; (b) removal of the ‘‘white layer’’; (c) chemical cleaning; and (d ) TiN deposition.
teristics. A continuous cycle requires ion nitriding which frequently inhibits the obtaining of a uniform nitrided
The paper presents the method of duplex surface treatment consisting of nitriding and TiN coating deposition. The method is used for moulds for pressure casting of aluminium. Three technological varieties of executing a duplex treatment are presented. Examinations of the structure and properties of the composite (nitrided case/TiN coating) manufactured by the three methods
´ Fig. 10. Duplex treated mould for pressure casting of aluminium manufactured and used in ‘‘Zelmer’’ Rzeszow.
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´ Fig. 11. Duplex treated mould for pressure casting of aluminium manufactured and used in ‘‘Zelmer’’ Rzeszow.
discussed proved that they are comparable in practice. The developed method of two-stage, discontinuous duplex treatment was used in practice bringing about a significant increase in mould life and improvement in the quality of manufactured elements.
´ Rzeszow and Metaplas Ionon for help in the experiments.
References Acknowledgement The work is a part of the research project no. 7T08B 003 95C/2574 supported by the Research Committee of Poland. The authors wish to thank companies Zelmer
Fig. 12. Increase in life of duplex treated different elements of moulds in comparison to gas nitrided specimens.
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