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Hybrid technology hard coating – Electron beam surface hardening
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Surface & Coatings Technology 202 (2007) 804 – 808 www.elsevier.com/locate/surfcoat
Hybrid technology hard coating – Electron beam surface hardening R. Zenker a,b , G. Sacher a,⁎, A. Buchwalder a , J. Liebich c , A. Reiter d , R. Häßler e a
TU Bergakademie Freiberg, Institute of Materials Engineering, Gustav-Zeuner-Str. 5, D-09599 Freiberg, Germany b Zenker-Consult, Johann-Sebastian-Bach-Str. 12, D-09648 Mittweida, Germany c Ionbond Germany GmbH, Gewerbering 10, D-09337 Hohenstein-Ernstthal, Germany d OC Oerlikon Balzers AG, Iramali 18, LI-9496 Balzers, Liechtenstein e Beschichtungstechnik Chemnitz GmbH, Straßburger Str. 3, D-09120 Chemnitz, Germany Available online 2 June 2007
Abstract Hard surface layers often cannot show advantages of their good hardness, strength and wear resistance over relatively soft materials so that an additional thermal treatment of the base materials before or after coating is necessary. Surface treatment technologies with high energy beams {electron beam (EB) or laser beam (LB)} offer a good and modern alternative to the mostly used bulk heat treatment. The energy deposition is precisely focused, so it is possible to exactly limit the heat treatment to highly loaded areas and up to the depth where a transformation (hardening) is necessary. Therefore, the bulk materials are not heated up to critical temperatures. The thermal load of the overall component is minimized and thus distortion can also be avoided. The paper deals with current results of investigations on the combination of PVD hard protective coatings {based on Ti(C)N, TiAlN, CrxNy and DLC} with electron beam surface hardening. Base materials used for these investigations were different steels (unalloyed steel: C45; low alloyed tool steel: 100Cr6; high alloyed tool steel: X155CrVMo12-1). It is not only the sequence of treatments (beam hardening before or after coating) which has a considerable influence on treatment results, but also the parameters of EB surface treatment (energy density distribution, speed of treatment, vacuum). It will be discussed the relations between treatment conditions, process parameters and surface deformation, layer structure and composition, structure and composition gradients, surface properties, properties gradients and transformation behavior of matrix materials. These modern combined technologies open up new fields of industrial application for tools and components subjected to locally high load. © 2007 Elsevier B.V. All rights reserved. Keywords: Hybrid technology; Surface hardening; Electron beam; PVD; Hard coating; Layer microstructure
1. Introduction Close to the surface tools and components often are subjected to complex load conditions. Their performance visà-vis extraneous mechanical factors is mostly determined by the properties of the surface layers [1–4]. With regard to complex load conditions, the properties attainable by single treatments (mechanical, thermal, thermo-chemical and coating methods), in particular, are insufficient in an increasing number of cases. It is possible to meet requirements by combining no less than two single treatments [1–3,5]. It has been shown that in the case of optimized process parameters the advantages of the combined treatment complement each other and the disadvantages of the ⁎ Corresponding author. Tel.: +49 3731 39 2906; fax: +49 3731 39 12906. E-mail address: [email protected] (G. Sacher). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.089
single processes cancel each other out at least partially. In certain cases novel effects arise which are not attainable by any single process [1–5]. Therefore, great importance will be attached to these combined processes (duplex or hybrid processes) [1,4,6,7]. Hard protective coatings (HC) mostly are not able to bring completely to bear their good hardness and wear resistance on relatively soft matrix materials, so that an additional (thermal) treatment is necessary. Hybrid technologies combining hard coatings with previous or subsequent heat treatment are the state of the art in several ways. Matrix materials used for physical vapor deposition (PVD)-coated tools are usually bulk-hardened and -tempered [8] or thermo-chemically treated (widely in use, e.g. [9–15]) prior to layer deposition in order to provide a high strength and hardness of the matrix material and thus a sufficient load support for the layer (combination of bulk heat
R. Zenker et al. / Surface & Coatings Technology 202 (2007) 804–808
treatment + coating). Because of the high temperatures (≈ 1000 °C) of chemical vapor deposition (CVD)-processes in connection with small cooling rates, a subsequent heat treatment is absolutely necessary there [2,16,17]. The disadvantages of these combinations are extended time-temperature cycles of the overall component which result in a negative influence on the hard coating in certain circumstances and in an unsatisfactory overall energy balance. As the investigations relating to subsequent heat treatment of CVD- and PVD-hard-coated steels by induction surface hardening [18–20], and laser beam hardening [21–26,30,31] show, the surface treatment technology is an adequate alternative to the generally used bulk heat treatment. By using an electron beam hardening (EBH) [27–32], as a modern and innovative method of local surface heat treatment in combination with a hard coating, an energy saving and environment-friendly technology comes into application. Energy deposition is precisely defined to focus the heating of the materials on the area and up to the depth where a transformation (hardening) is necessary. Therefore, the bulk material is not heated up to the critical temperature. So this hybrid technology does not cause significant distortion of components. Moreover, it permits to prevent undesirable changes of composition, structure and properties of the hard coating because of a very short interaction time and the process-related treatment in vacuum. The paper deals with current results of investigations relating to the combination of PVD hard coatings {based on Ti(C)N, TiAlN, CrN and DLC} with an EB surface hardening. 2. Experimentals Base materials used for these investigations are several steels differing in carbon and chromium content (Table 1), which usually are applied in the hardened and tempered (H + T) pre-heattreated state. However, some applications would be conceivable where a previous bulk heat treatment could be completely avoided and only a treatment of the (highest) loaded areas of the tool or component is necessary. Therefore, the normalized (N) or annealed (A) pre-heat-treated state was also investigated. The specimens with cylindrical geometry (Ø 40 mm × 15 mm) were ground up and polished to a roughness of Rz b 1 μm before coating. EBH was carried out on an electron beam facility (type S20) with an acceleration voltage (Ua) of 60 kVand an output power of 12 kW. The optimum working distance was 500 mm. For energy transfer, high speed beam deflection was used. The principle of this technique is that the EB – because of its good deflectibility – Table 1 Chemical composition of base materials Base material
wt.% Fe
C
Cr
Mn
P
S
Si
Mo
V
C45 98,34 0.54 0,11 0.69 0.013 0.022 0,30 n.n. n.n. 100Cr6 97.03 1.05 1.24 0.44 0.007 0.004 0.23 n.n. n.n. X155CrVMo12-1 83.79 1.54 12.70 0.31 0.019 0.002 n.n. 0.68 0.81
805
Table 2 Application of energy during EBH of hybrid technology EBH + HC and HC + EBH Base material
Pretreatment
Energy input per unit area (eF) [Ws/cm2] EBH1
Hybrid technology EBH + HC C45 N 100Cr6 A X155CrVMo12-1 A Hybrid technology HC + EBH C45 N H+T 100Cr6 A H+T X155CrVMo12-1 A H+T
EBH2
EBH3
550 900 1200
1400 2400 2400
2100 – –
480 370–570 780 450–500 1000 400–520
1200 1200 2050 970 2050 880
1790 1790 – 1750 2400 1400–1500
eF ¼ vUxawIbf [Ws cm− 2]. Ua - acceleration voltage [V], Ib - beam current [A], wf - field width [cm], vx feed rate [cms− 1].
moves with high frequencies (up to 100 kHz) within an energy transfer field so that an isothermal energy distribution is guaranteed there. This results in a track-type hardening with a track width (wf) of 20 mm, whereas the neighbored areas remain nearly unaffected. The acceleration voltage and the width of the energy transfer field were constant. Beam currency (Ib) and feed rate (vx) were changed in combination and optimized in such a manner that the temperature near the surface remained just below the melting point of the lowest melting phase of the steel: – vx: 0.5–2.5 cm s− 1 – Ib: 25–50 mA. A summary of the applied energy input per unit area (eF) is given in Table 2. Transformation depths of 0.2–1.0 mm have been achieved by using these parameters. The specimens have been coated either before or after surface heat treatment. The PVD hard coatings {Ti(C)N, TiAlN and CrN} are commercially produced. Depending on the producer they differ in composition (monolayer, gradient layer, multilayer) and layer thickness(1–4 μm). Because of different coating parameters (time and temperature) different effects on the properties of the layer-matrix-compound can be expected. Furthermore, diamond-like carbon coatings (DLC) have been deposited by plasma activated CVD (PACVD)-process. The results have been characterized by the following methods of investigation: – microscopic investigations (optical microscope, scanning electron microscope) of the surface as well as the metallographic cross sections and fractured surfaces – surface deformation measurements by laser-optical measuring system – glow discharge optical emission spectroscopy (GDOES) in order to create concentration-depth-profiles of the layermatrix-compound
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– measurements of compound hardness (HV 1) and hardnessdepth-profiles (HV 0.3) as well as instrumented indentation measurements of layer hardness (Martens hardness) – scratch tests with continuously increasing loads up to 100 N with subsequent evaluation of critical loads and failure modes by optical microscopy. 3. Results and discussion 3.1. Surface hardening before hard coating In case of complex load conditions a functional sharing between surface and core materials properties is often required. High bulk toughness, e.g. is as important for the application as high surface hardness and wear resistance are. A toughness increasing bulk heat treatment of the overall tool or component is followed by a selected EBH of highest loaded areas. Thus, it is possible to meet the requirements with regard to an optimized load support for the subsequent deposition of the hard, wear resistant layer. Fig. 1 shows the results of surface hardness measurements (compound hardness of base materials and layer) of preliminary EB hardened and DLC-coated specimens. Because of high cooling rates during EBH, the solid state martensitic transformation of the investigated steels results in a very fine-grained martensite so that the hardness of the surface hardened layer is even higher than the hardness of the bulkhardened and -tempered base materials. During subsequent low temperature PACVD layer deposition the hardness of the substrate is not significantly influenced. Therefore, surface hardness and load support are increased. The latter effect is also confirmed by results of scratch test (Fig. 1). It stands to reason that the sequence of combined surface heat treatment EBH + HC is only useful if the layer deposition is carried out at temperatures lower than the annealing temperature of the base materials. The level of the processing temperature in relation to the tempering stability of the bulk materials determines the success of this treatment combination. The better the tempering stability of the steel, the smaller the hardness reduction in the previously produced EBH layer as a result of the hard coating process. In some cases of application it might be possible to substitute a bulk heat treatment of the overall component by a selective
Fig. 1. Influence of hybrid technology EBH + DLC on surface hardness HV 1 and critical loads (standardized values) in scratch test of hardened and tempered (H + T) base materials (100…100Cr6; 155…X155CrVMo12-1).
Fig. 2. Influence of hybrid technology EBH + TiN on surface hardness HV 1 and critical loads (standardized values) in scratch test of annealed (A) base materials (100…100Cr6; 155…X155CrVMo12-1).
surface heat treatment of the highest loaded areas only. As shown in Fig. 2, surface hardness and critical loads of no preliminary bulk heat-treated base materials are increased noticeably by local EBH before PVD coating. 3.2. Surface hardening after hard coating A subsequent EB heat treatment has no significant influences on the visual appearance of the coatings on the combined treated specimens. Because the EB treatment is carried out in vacuum, the hybrid treated specimens of the sequence HC + EBH keep their typical colouring after EBH if optimized EB parameters are used. An important aspect in connection with a subsequent heat treatment of hard-coated specimens is an unavoidable surface deformation as a result of volume change during martensitic transformation [24,33]. The dimension of the surface deformation (shape of profile and maximum extend) depends on the chemical composition of the base material and thus the
Fig. 3. Surface profiles across the hard-coated and subsequently EB hardened track {steel: 100Cr6 (hardened and tempered), parameter set EBH1}.
R. Zenker et al. / Surface & Coatings Technology 202 (2007) 804–808
807
Fig. 6. Improvement of surface hardness HV 1 and critical load (standardized values) of TiAlN coatings on C45 (hardened and tempered) base material by subsequent EB hardening.
Fig. 4. Failure because of crack initiation during subsequent EBH.
transformation products as a result of beam hardening as well as on the pre-heat-treated state of the bulk material {e.g. hardened and tempered (H + T) or annealed (A)} [33]. Furthermore, the energy deposition during EBH is of great importance. The surface deformation of the base material perpendicular to the EBH track (Fig. 3) neither depends on the structure of the hard coating nor on coating thickness. The roughness Ra of the coated surfaces itself is not significantly influenced by a subsequent EBH. The maximum amount of energy input and therefore the achievable hardening depth is predominantly limited by the first occurrence of fine cracks parallel to the direction of feed (Fig. 4). The tendency to formation of cracks depends on the structure and chemical composition of the layer. The resistance to crack
formation increased by the way of CrN → TiN → TiCN → TiAlN. Furthermore, it stands out that the layer thickness is also important. Because of short interaction time of EB (b1…5 s), an influence of subsequent EB treatment on the chemical composition of the hard coating could not be found (see concentration depth profiles of nitrogen and carbon display in Fig. 5). In agreement with prior investigations [21–24,26–32] the present results confirm the possibility of creating a martensitic transformed layer beneath a hard-coated layer by a subsequent beam heat treatment without any (significant) changes in the coatings. The attainable transformation depth depends on the chemical composition and the pre-heat-treated state of the base material and on the beam hardening conditions. Contrarily to LBH, at which the surface quality in relation to the light absorption coefficient of the hard coating has a strong influence on transformation depth [30,31], no differences of the hardnessdepth-profiles for differently hard-coated and subsequently EB hardened specimens occur. EBH after hard coating significantly increases the critical load Lc in scratch test. The attainable improvement depends on the chemical composition of the matrix material. The critical loads increase with a rising alloying content, but as it is known, the transformation behavior of the matrix material is also important. Furthermore, the higher the surface hardness as indication of increasing supporting effect of the matrix onto the hard-coated layer, the higher the measured critical loads in the scratch test (Fig. 6). 4. Conclusions
Fig. 5. Concentration depth profiles of nitrogen and carbon of subsequently EB hardened TiAlN coating (steel 100Cr6 hardened and tempered).
The combination of the surface technologies of hard coating with EBH prior or subsequent to the layer deposition opens up new structure/property relations of the layer-matrix-compounds. A martensitically transformed layer beneath the hard coating is produced which results in a significant improvement of the base material's load support for the hard coatings. Therefore high surface hardness and high critical loads measured by scratch tests are obtained. Additionally the properties gradient is improved. Negative influences on the chemical composition and/or structure and mechanical properties of the hard coatings were not found.
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The sequence of treatment has a considerable influence on treatment results. The combination EBH before HC is only useful if a pre-treatment of the whole bulk material can be avoided that way. Due to the processing temperature of the subsequent PVD process the hardness of the EBH layer beneath the hard coating can be influenced negatively. By using the EBH after HC the hardness of the EBH layer is higher than in case of bulk hardening. Possible applications for combined HC + EBH technologies are cold forming tools with locally high loaded areas (e.g. tools for deep drawing, cold extrusion, hydroforming or solid forming) and components subjected to local high load in the form of high pressure intensity combined with sliding wear such as automotive components. Acknowledgements The research work was supported by Stiftung Industrieforschung Köln (project S699). The authors wish to thank their project partners pro-beam Anlagen GmbH and Laservorm GmbH for their helpful support and intensive cooperation. References [1] R. Zenker, Neue Hütte 28 (10) (1983) 379. [2] R. Zenker, Beitrag zur Entwicklung neuer Wärmebehandlungstechnologien in Verbindung mit hohen Erwärmungsgeschwindigkeiten. Dissertation B, TU Bergakademie Freiberg, 1986 [3] R. Zenker, U. Zenker, Surf. Eng. 5 (1) (1989) 45. [4] O. Keßler, Kombinationsverfahren zur Randschichtbehandlung von Stählen – Stoffeigenschaftsändernde und Beschichtungsverfahren, Shaker-Verlag, Aachen, 2003. [5] O. Keßler, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 108–109 (1998) 211. [6] T. Bell, Surf. Eng. 6 (1) (1990) 31. [7] A. Matthews, A. Leyland, Surf. Coat. Technol. 71 (1995) 88. [8] D.S. Rickerby, A. Matthews, Advanced Surface Coatings: A Handbook of Surface Engineering, Blackie & Son Ltd., Glasgow, 1991. [9] H.J. Spies, B. Larisch, K. Höck, Surf. Eng. 11 (4) (1995) 319.
[10] H.J. Spies, K. Höck, E. Broszeit, B. Matthes, W. Herr, Surf. Coat. Technol. 60 (1993) 441. [11] A. Leyland, S. Fancey, A. Matthews, Surf. Eng. 7 (3) (1991) 207. [12] N. Dingremont, E. Bergmann, P. Collingnon, Surf. Coat. Technol. 72 (1995) 157. [13] N. Dingremont, E. Bergmann, P. Collingnon, Surf. Coat. Technol. 72 (1995) 163. [14] E.H. Sirvio, M. Sulonen, H. Sundquist, Thin Solid Films 96 (1982) 93. [15] H.J. Spies, Stahl Eisen 117 (6) (1997) 45. [16] O. Keßler, F. Hoffmann, P. Mayr, Harterei-Tech. Mitt. 49 (1) (1994) 48. [17] O. Keßler, F. Hoffmann, P. Mayr, Härterei-Tech. Mitt. 49 (3) (1994) 191. [18] K. Pantleon, O. Keßler, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 120121 (1999) 495. [19] K. Pantleon, O. Keßler, F. Hoffmann, P. Mayr, Härterei-Tech. Mitt. 55 (5) (2000) 304. [20] K. Pantleon, O. Keßler, F. Hoffmann, P. Mayr, Härterei-Tech. Mitt. 54 (3) (1999) 150. [21] H. Haferkamp, M. Goede, K. Lindemann, Technica 49 (20) (2000) 38. [22] H. Haferkamp, A. Ostendorf, M. Goede, K. Lindemann, WT, Werkstattstech. 91 (7) (2001) 418. [23] M. Heidkamp, O. Keßler, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 188189 (2004) 294. [24] M. Heidkamp, O. Keßler, F. Hoffmann, P. Mayr, H.W. Zoch, HärtereiTech. Mitt. 60 (2) (2005) 71. [25] N. Sotirov, O. Keßler, F. Hoffmann, H.W. Zoch, Härterei-Tech. Mitt. 61 (3) (2006) 160. [26] O. Keßler, Surf. Coat. Technol. 201 (2006) 4046. [27] S. Friedrich, H.J. Spies, D. Rathjen, in: H. Oettel (Ed.), Freiberger Forschungshefte: Randschichttechnik – Erzeugung und Anwendung tribologisch beanspruchter Schichtverbunde, TU Bergakademie Freiberg, Freiberg, 1999, p. 131, B297. [28] S. Friedrich, H.J. Spies, in: F. Sepold, F. Wagner, J. Tobolski (Eds.), Strahltechnik, vol. 18, BIAS Verlag Bremen, 2002, p. 265, Kurzzeitmetallurgie. [29] H.J. Spies, S. Friedrich, A. Buchwalder, Mater.Wiss. Werkst.Tech. 34 (2003) 128. [30] G. Sacher, A. Buchwalder, R. Zenker, Journeé Européenes – Traitement thermique pour durissement superficial (A3TS, Strasbourg), in preparation: for Traitement Thermique [31] R. Zenker, A. Buchwalder, G. Sacher, 10th PSE (EFDS, GarmischPartenkirchen), in preparation for Heat Treatment and Surface Engineering. [32] R. Zenker, H.-J. Spies, A. Buchwalder, G. Sacher, 15th IFHTSE (ASMET, Wien), in preparation for Heat Treatment and Surface Engineering. [33] R. Zenker, Härterei-Tech. Mitt. 45 (4) (1990) 230.
Surface & Coatings Technology 202 (2007) 804 – 808 www.elsevier.com/locate/surfcoat
Hybrid technology hard coating – Electron beam surface hardening R. Zenker a,b , G. Sacher a,⁎, A. Buchwalder a , J. Liebich c , A. Reiter d , R. Häßler e a
TU Bergakademie Freiberg, Institute of Materials Engineering, Gustav-Zeuner-Str. 5, D-09599 Freiberg, Germany b Zenker-Consult, Johann-Sebastian-Bach-Str. 12, D-09648 Mittweida, Germany c Ionbond Germany GmbH, Gewerbering 10, D-09337 Hohenstein-Ernstthal, Germany d OC Oerlikon Balzers AG, Iramali 18, LI-9496 Balzers, Liechtenstein e Beschichtungstechnik Chemnitz GmbH, Straßburger Str. 3, D-09120 Chemnitz, Germany Available online 2 June 2007
Abstract Hard surface layers often cannot show advantages of their good hardness, strength and wear resistance over relatively soft materials so that an additional thermal treatment of the base materials before or after coating is necessary. Surface treatment technologies with high energy beams {electron beam (EB) or laser beam (LB)} offer a good and modern alternative to the mostly used bulk heat treatment. The energy deposition is precisely focused, so it is possible to exactly limit the heat treatment to highly loaded areas and up to the depth where a transformation (hardening) is necessary. Therefore, the bulk materials are not heated up to critical temperatures. The thermal load of the overall component is minimized and thus distortion can also be avoided. The paper deals with current results of investigations on the combination of PVD hard protective coatings {based on Ti(C)N, TiAlN, CrxNy and DLC} with electron beam surface hardening. Base materials used for these investigations were different steels (unalloyed steel: C45; low alloyed tool steel: 100Cr6; high alloyed tool steel: X155CrVMo12-1). It is not only the sequence of treatments (beam hardening before or after coating) which has a considerable influence on treatment results, but also the parameters of EB surface treatment (energy density distribution, speed of treatment, vacuum). It will be discussed the relations between treatment conditions, process parameters and surface deformation, layer structure and composition, structure and composition gradients, surface properties, properties gradients and transformation behavior of matrix materials. These modern combined technologies open up new fields of industrial application for tools and components subjected to locally high load. © 2007 Elsevier B.V. All rights reserved. Keywords: Hybrid technology; Surface hardening; Electron beam; PVD; Hard coating; Layer microstructure
1. Introduction Close to the surface tools and components often are subjected to complex load conditions. Their performance visà-vis extraneous mechanical factors is mostly determined by the properties of the surface layers [1–4]. With regard to complex load conditions, the properties attainable by single treatments (mechanical, thermal, thermo-chemical and coating methods), in particular, are insufficient in an increasing number of cases. It is possible to meet requirements by combining no less than two single treatments [1–3,5]. It has been shown that in the case of optimized process parameters the advantages of the combined treatment complement each other and the disadvantages of the ⁎ Corresponding author. Tel.: +49 3731 39 2906; fax: +49 3731 39 12906. E-mail address: [email protected] (G. Sacher). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.05.089
single processes cancel each other out at least partially. In certain cases novel effects arise which are not attainable by any single process [1–5]. Therefore, great importance will be attached to these combined processes (duplex or hybrid processes) [1,4,6,7]. Hard protective coatings (HC) mostly are not able to bring completely to bear their good hardness and wear resistance on relatively soft matrix materials, so that an additional (thermal) treatment is necessary. Hybrid technologies combining hard coatings with previous or subsequent heat treatment are the state of the art in several ways. Matrix materials used for physical vapor deposition (PVD)-coated tools are usually bulk-hardened and -tempered [8] or thermo-chemically treated (widely in use, e.g. [9–15]) prior to layer deposition in order to provide a high strength and hardness of the matrix material and thus a sufficient load support for the layer (combination of bulk heat
R. Zenker et al. / Surface & Coatings Technology 202 (2007) 804–808
treatment + coating). Because of the high temperatures (≈ 1000 °C) of chemical vapor deposition (CVD)-processes in connection with small cooling rates, a subsequent heat treatment is absolutely necessary there [2,16,17]. The disadvantages of these combinations are extended time-temperature cycles of the overall component which result in a negative influence on the hard coating in certain circumstances and in an unsatisfactory overall energy balance. As the investigations relating to subsequent heat treatment of CVD- and PVD-hard-coated steels by induction surface hardening [18–20], and laser beam hardening [21–26,30,31] show, the surface treatment technology is an adequate alternative to the generally used bulk heat treatment. By using an electron beam hardening (EBH) [27–32], as a modern and innovative method of local surface heat treatment in combination with a hard coating, an energy saving and environment-friendly technology comes into application. Energy deposition is precisely defined to focus the heating of the materials on the area and up to the depth where a transformation (hardening) is necessary. Therefore, the bulk material is not heated up to the critical temperature. So this hybrid technology does not cause significant distortion of components. Moreover, it permits to prevent undesirable changes of composition, structure and properties of the hard coating because of a very short interaction time and the process-related treatment in vacuum. The paper deals with current results of investigations relating to the combination of PVD hard coatings {based on Ti(C)N, TiAlN, CrN and DLC} with an EB surface hardening. 2. Experimentals Base materials used for these investigations are several steels differing in carbon and chromium content (Table 1), which usually are applied in the hardened and tempered (H + T) pre-heattreated state. However, some applications would be conceivable where a previous bulk heat treatment could be completely avoided and only a treatment of the (highest) loaded areas of the tool or component is necessary. Therefore, the normalized (N) or annealed (A) pre-heat-treated state was also investigated. The specimens with cylindrical geometry (Ø 40 mm × 15 mm) were ground up and polished to a roughness of Rz b 1 μm before coating. EBH was carried out on an electron beam facility (type S20) with an acceleration voltage (Ua) of 60 kVand an output power of 12 kW. The optimum working distance was 500 mm. For energy transfer, high speed beam deflection was used. The principle of this technique is that the EB – because of its good deflectibility – Table 1 Chemical composition of base materials Base material
wt.% Fe
C
Cr
Mn
P
S
Si
Mo
V
C45 98,34 0.54 0,11 0.69 0.013 0.022 0,30 n.n. n.n. 100Cr6 97.03 1.05 1.24 0.44 0.007 0.004 0.23 n.n. n.n. X155CrVMo12-1 83.79 1.54 12.70 0.31 0.019 0.002 n.n. 0.68 0.81
805
Table 2 Application of energy during EBH of hybrid technology EBH + HC and HC + EBH Base material
Pretreatment
Energy input per unit area (eF) [Ws/cm2] EBH1
Hybrid technology EBH + HC C45 N 100Cr6 A X155CrVMo12-1 A Hybrid technology HC + EBH C45 N H+T 100Cr6 A H+T X155CrVMo12-1 A H+T
EBH2
EBH3
550 900 1200
1400 2400 2400
2100 – –
480 370–570 780 450–500 1000 400–520
1200 1200 2050 970 2050 880
1790 1790 – 1750 2400 1400–1500
eF ¼ vUxawIbf [Ws cm− 2]. Ua - acceleration voltage [V], Ib - beam current [A], wf - field width [cm], vx feed rate [cms− 1].
moves with high frequencies (up to 100 kHz) within an energy transfer field so that an isothermal energy distribution is guaranteed there. This results in a track-type hardening with a track width (wf) of 20 mm, whereas the neighbored areas remain nearly unaffected. The acceleration voltage and the width of the energy transfer field were constant. Beam currency (Ib) and feed rate (vx) were changed in combination and optimized in such a manner that the temperature near the surface remained just below the melting point of the lowest melting phase of the steel: – vx: 0.5–2.5 cm s− 1 – Ib: 25–50 mA. A summary of the applied energy input per unit area (eF) is given in Table 2. Transformation depths of 0.2–1.0 mm have been achieved by using these parameters. The specimens have been coated either before or after surface heat treatment. The PVD hard coatings {Ti(C)N, TiAlN and CrN} are commercially produced. Depending on the producer they differ in composition (monolayer, gradient layer, multilayer) and layer thickness(1–4 μm). Because of different coating parameters (time and temperature) different effects on the properties of the layer-matrix-compound can be expected. Furthermore, diamond-like carbon coatings (DLC) have been deposited by plasma activated CVD (PACVD)-process. The results have been characterized by the following methods of investigation: – microscopic investigations (optical microscope, scanning electron microscope) of the surface as well as the metallographic cross sections and fractured surfaces – surface deformation measurements by laser-optical measuring system – glow discharge optical emission spectroscopy (GDOES) in order to create concentration-depth-profiles of the layermatrix-compound
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– measurements of compound hardness (HV 1) and hardnessdepth-profiles (HV 0.3) as well as instrumented indentation measurements of layer hardness (Martens hardness) – scratch tests with continuously increasing loads up to 100 N with subsequent evaluation of critical loads and failure modes by optical microscopy. 3. Results and discussion 3.1. Surface hardening before hard coating In case of complex load conditions a functional sharing between surface and core materials properties is often required. High bulk toughness, e.g. is as important for the application as high surface hardness and wear resistance are. A toughness increasing bulk heat treatment of the overall tool or component is followed by a selected EBH of highest loaded areas. Thus, it is possible to meet the requirements with regard to an optimized load support for the subsequent deposition of the hard, wear resistant layer. Fig. 1 shows the results of surface hardness measurements (compound hardness of base materials and layer) of preliminary EB hardened and DLC-coated specimens. Because of high cooling rates during EBH, the solid state martensitic transformation of the investigated steels results in a very fine-grained martensite so that the hardness of the surface hardened layer is even higher than the hardness of the bulkhardened and -tempered base materials. During subsequent low temperature PACVD layer deposition the hardness of the substrate is not significantly influenced. Therefore, surface hardness and load support are increased. The latter effect is also confirmed by results of scratch test (Fig. 1). It stands to reason that the sequence of combined surface heat treatment EBH + HC is only useful if the layer deposition is carried out at temperatures lower than the annealing temperature of the base materials. The level of the processing temperature in relation to the tempering stability of the bulk materials determines the success of this treatment combination. The better the tempering stability of the steel, the smaller the hardness reduction in the previously produced EBH layer as a result of the hard coating process. In some cases of application it might be possible to substitute a bulk heat treatment of the overall component by a selective
Fig. 1. Influence of hybrid technology EBH + DLC on surface hardness HV 1 and critical loads (standardized values) in scratch test of hardened and tempered (H + T) base materials (100…100Cr6; 155…X155CrVMo12-1).
Fig. 2. Influence of hybrid technology EBH + TiN on surface hardness HV 1 and critical loads (standardized values) in scratch test of annealed (A) base materials (100…100Cr6; 155…X155CrVMo12-1).
surface heat treatment of the highest loaded areas only. As shown in Fig. 2, surface hardness and critical loads of no preliminary bulk heat-treated base materials are increased noticeably by local EBH before PVD coating. 3.2. Surface hardening after hard coating A subsequent EB heat treatment has no significant influences on the visual appearance of the coatings on the combined treated specimens. Because the EB treatment is carried out in vacuum, the hybrid treated specimens of the sequence HC + EBH keep their typical colouring after EBH if optimized EB parameters are used. An important aspect in connection with a subsequent heat treatment of hard-coated specimens is an unavoidable surface deformation as a result of volume change during martensitic transformation [24,33]. The dimension of the surface deformation (shape of profile and maximum extend) depends on the chemical composition of the base material and thus the
Fig. 3. Surface profiles across the hard-coated and subsequently EB hardened track {steel: 100Cr6 (hardened and tempered), parameter set EBH1}.
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Fig. 6. Improvement of surface hardness HV 1 and critical load (standardized values) of TiAlN coatings on C45 (hardened and tempered) base material by subsequent EB hardening.
Fig. 4. Failure because of crack initiation during subsequent EBH.
transformation products as a result of beam hardening as well as on the pre-heat-treated state of the bulk material {e.g. hardened and tempered (H + T) or annealed (A)} [33]. Furthermore, the energy deposition during EBH is of great importance. The surface deformation of the base material perpendicular to the EBH track (Fig. 3) neither depends on the structure of the hard coating nor on coating thickness. The roughness Ra of the coated surfaces itself is not significantly influenced by a subsequent EBH. The maximum amount of energy input and therefore the achievable hardening depth is predominantly limited by the first occurrence of fine cracks parallel to the direction of feed (Fig. 4). The tendency to formation of cracks depends on the structure and chemical composition of the layer. The resistance to crack
formation increased by the way of CrN → TiN → TiCN → TiAlN. Furthermore, it stands out that the layer thickness is also important. Because of short interaction time of EB (b1…5 s), an influence of subsequent EB treatment on the chemical composition of the hard coating could not be found (see concentration depth profiles of nitrogen and carbon display in Fig. 5). In agreement with prior investigations [21–24,26–32] the present results confirm the possibility of creating a martensitic transformed layer beneath a hard-coated layer by a subsequent beam heat treatment without any (significant) changes in the coatings. The attainable transformation depth depends on the chemical composition and the pre-heat-treated state of the base material and on the beam hardening conditions. Contrarily to LBH, at which the surface quality in relation to the light absorption coefficient of the hard coating has a strong influence on transformation depth [30,31], no differences of the hardnessdepth-profiles for differently hard-coated and subsequently EB hardened specimens occur. EBH after hard coating significantly increases the critical load Lc in scratch test. The attainable improvement depends on the chemical composition of the matrix material. The critical loads increase with a rising alloying content, but as it is known, the transformation behavior of the matrix material is also important. Furthermore, the higher the surface hardness as indication of increasing supporting effect of the matrix onto the hard-coated layer, the higher the measured critical loads in the scratch test (Fig. 6). 4. Conclusions
Fig. 5. Concentration depth profiles of nitrogen and carbon of subsequently EB hardened TiAlN coating (steel 100Cr6 hardened and tempered).
The combination of the surface technologies of hard coating with EBH prior or subsequent to the layer deposition opens up new structure/property relations of the layer-matrix-compounds. A martensitically transformed layer beneath the hard coating is produced which results in a significant improvement of the base material's load support for the hard coatings. Therefore high surface hardness and high critical loads measured by scratch tests are obtained. Additionally the properties gradient is improved. Negative influences on the chemical composition and/or structure and mechanical properties of the hard coatings were not found.
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The sequence of treatment has a considerable influence on treatment results. The combination EBH before HC is only useful if a pre-treatment of the whole bulk material can be avoided that way. Due to the processing temperature of the subsequent PVD process the hardness of the EBH layer beneath the hard coating can be influenced negatively. By using the EBH after HC the hardness of the EBH layer is higher than in case of bulk hardening. Possible applications for combined HC + EBH technologies are cold forming tools with locally high loaded areas (e.g. tools for deep drawing, cold extrusion, hydroforming or solid forming) and components subjected to local high load in the form of high pressure intensity combined with sliding wear such as automotive components. Acknowledgements The research work was supported by Stiftung Industrieforschung Köln (project S699). The authors wish to thank their project partners pro-beam Anlagen GmbH and Laservorm GmbH for their helpful support and intensive cooperation. References [1] R. Zenker, Neue Hütte 28 (10) (1983) 379. [2] R. Zenker, Beitrag zur Entwicklung neuer Wärmebehandlungstechnologien in Verbindung mit hohen Erwärmungsgeschwindigkeiten. Dissertation B, TU Bergakademie Freiberg, 1986 [3] R. Zenker, U. Zenker, Surf. Eng. 5 (1) (1989) 45. [4] O. Keßler, Kombinationsverfahren zur Randschichtbehandlung von Stählen – Stoffeigenschaftsändernde und Beschichtungsverfahren, Shaker-Verlag, Aachen, 2003. [5] O. Keßler, F. Hoffmann, P. Mayr, Surf. Coat. Technol. 108–109 (1998) 211. [6] T. Bell, Surf. Eng. 6 (1) (1990) 31. [7] A. Matthews, A. Leyland, Surf. Coat. Technol. 71 (1995) 88. [8] D.S. Rickerby, A. Matthews, Advanced Surface Coatings: A Handbook of Surface Engineering, Blackie & Son Ltd., Glasgow, 1991. [9] H.J. Spies, B. Larisch, K. Höck, Surf. Eng. 11 (4) (1995) 319.
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