- Email: [email protected]
Development of a hybrid coating process as an advanced surface modification for cutting tools and moulds
Surface and Coatings Technology 169 – 170 (2003) 45–48
Development of a hybrid coating process as an advanced surface modification for cutting tools and moulds Takayasu Sato*, Kenichi Sugai, Shizuyo Ueda, Kouji Matsunami, Manabu Yasuoka Department of Coating, Nachi-fujikoshi Corp, 176-12 Namerikawa, Toyama 936-0802, Japan
Abstract We have developed new ion plating equipment that has made it possible to perform coating after performing plasma nitriding in the same chamber. First, the nitriding behavior of various steels was investigated using this equipment. A nitrogen diffusion layer thickness of 40 mm and a maximum hardness of 1400 HV50 were obtained without a compound layer on top of the nitrided SKH57 by treating for 60 min at 450 8C in Ar–N2 plasma. Secondly, plasma nitriding and TiN coating were carried out successively, using what is referred to as hybrid coating, in the same chamber, and the change in film characteristics with or without nitriding was clarified. Using nitriding as a pretreatment, the apparent film hardness was increased and adhesion between the film and nitrided substrate was improved compared with an untreated substrate. There was no change in the surface roughness after film deposition due to the nitriding. Finally, it was confirmed by results of a drill test that the performance of TiN coating with plasma nitriding was superior to that of a TiAlN film. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma nitriding; Ion plating; Hybrid coating; Adhesion; Drilling test
1. Introduction Nitriding is a well-known surface hardening process and various nitriding processes have been put to practical use. Ion or plasma nitriding of steels is attractive because a hard layer is rapidly obtained to a substantial depth. Moreover, dry coating such as ion plating is also an important kind of surface modification, and it is also a process that utilizes plasma. Until now, just the use of a single nitriding process or coating has provided sufficient performance. However, the recent demands placed on tools and moulds cannot be satisfied using simple surface modification processes. The application of an additional hard coating process on a nitrided substrate has therefore become more common, but the nitriding and coating processes are carried out separately w1–6x. We then proposed a hybrid process, which combines the initial processing of plasma nitriding with a subsequent coating process in the same chamber. Industrial equipment for implementing this process has been developed. First, the behavior of nitriding various steels using this equipment was examined. The advantages of this new *Corresponding author. Tel.: q81-774-713366; fax: q81-774713316. E-mail address: [email protected] (T. Sato).
process, i.e. the sequential plasma nitriding and ion plating, were investigated. 2. Experimental procedure SS-2-8 type ion plating equipment, available from Nachi-fujikoshi Co, was used. We improved this apparatus to implement plasma nitriding as follows. By installing a new, round, dummy crucible around the main crucible, the generation of a low-pressure, highdensity nitrogen plasma using an electron beam became possible without melting the titanium in the main crucible. This plasma was utilized for nitriding. TiN coating was carried out by focusing the electron beam from the dummy crucible onto the crucible after the nitriding process. This remodeled equipment is shown in Fig. 1. Mirror-like polished SKH57, SKD11 and SKD61 were used as sample materials for the nitriding experiments. The nitriding and coating conditions are shown in Table 1. After surface treatment, the microhardness distribution of the nitrided layer and the TiN films was investigated using Vickers hardness measurements at a load of 50 g. The thickness of the nitrided layer and the film were examined using standard optical metallographic tech-
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 7 3 - 2
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
46
Fig. 2. Vickers hardness profiles on various steels after plasma nitriding. Fig. 1. Apparatus for hybrid coating process.
niques and calotest methods, respectively. XRD analysis was applied for phase identification. The adhesion between the film and substrate was examined by measuring the critical load using a scratch tester. The surface roughness of the nitrided substrate and film were determined using a contact probe tester. The conditions of the drill test for confirming the advantage of the nitriding prior to TiN deposition are listed in Table 2 3. Results and discussion 3.1. Plasma nitriding The microhardness distribution profiles of various steels after nitriding are shown in Fig. 2. The maximum hardness of the surface was dependent on the hardness of the base metal. In the case of SKH57, it was increased by more than 1400 HV with nitriding. The maximum hardness of SKD61 and SKD11 was 1200 and 1300 HV, respectively. The hardness gradually decreased
inwards from the surface, owing to the diffusion of nitrogen into the material. A photograph of a crosssection of SKH57 after nitriding is shown in Fig. 3. It is observed that the nitriding layer reached over 40 mm from the surface. Fig. 4 shows the XRD patterns of SKH57 before and after nitriding. It is clear that a compound layer has not been formed near the top surface, since no new peaks are detected after nitriding. We also confirmed, as shown in Fig. 3, that a compound layer was not observed at the interface between the TiN film and the nitrided substrate. Nitrogen seems to form a solid solution in the steel matrix, because the halfwidth of the diffraction peak has increased (Fig. 4) and the diffusion layer is blackened by the corrosion test (Fig. 3). This solid solution causes the increase in hardness. The nitriding rate for this process is faster than usual ion nitriding and radical nitriding. Nitriding using plasma is generally carried out in a pressure range from several 100 Pa to 1000s of Pa. On the other hand, low-
Table 1 Conditions for plasma nitriding and coating
Plasma power (kW) Substrate bias voltage (V) Reaction gas Pressure (Pa) Temperature (8C) Treatment time (min)
Plasma nitriding
Coating
10 150 Ar–N2 1.0 450 0–60
10 100 Ar–N2 0.3 450 60
Table 2 Conditions for the drilling test Drill Speed (rpm) Feed (mmymin) Work material Depth (mm) Coolant
w 6 (SKH57) 957 153 SCM440 20 (through hole) Emulsion
Fig. 3. Photograph of cross-section of nitrided SKH57.
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
47
Fig. 4. XRD patterns of SKH57 before and after nitriding. Table 3 Changes in the characteristics of the film with or without nitriding
TiNqplasma nitriding TiN
Thickness (mm)
Film hardness (HV50)
Critical load (N)
Surface roughness (Rmax) (nm)
2.9 2.8
2194 1648
86 66
0.22 0.26
pressure, highly ionized nitrogen plasma is generated by the electron beam at 1 Pa. Therefore, although the density of nitrogen on the whole in the chamber is lower, many active particles could still exist, which would enhance nitriding. This is because a compound
Fig. 5. Scratch marks on TiN films with and without nitriding.
layer, which inhibits the diffusion of nitrogen, is not produced in the nitrided layer under these conditions. 3.2. Hybrid coating Changes in the characteristics of the films deposited on the substrate with or without nitriding are listed in Table 3. The coating conditions were the same, except for the nitriding, and the film thickness was therefore almost the same within the range of measurement error. The film deposited on the substrate after nitriding is harder than that of the untreated substrate. This is as a result of the substrate being hardened by the nitriding, and is also a consequence of the increase in hardness of the substrate being added to the film hardness. Furthermore, the critical load of the scratch test is increased by nitriding. Flaky cracks are observed in the scratch marks of the film when larger loads are deposited on the nitrided substrate (Fig. 5). Nitriding causes the base metal hardness to increase, lowering the stress along the base material interface as the hardness difference between the film and the base material narrows. As the base metal hardness gradually approaches that of the film on plasma nitriding, a kind of gradient functional material is formed. The film is mechanically supported on the nitrided layer, so the adhesion between the film and substrate is improved. The surface roughness of the film on the substrate with and without nitriding was also evaluated using a surface roughness contact tester. Hardly any change in surface roughness of the deposited
48
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
In general, moisture and dust in the atmosphere may be absorbed into the surface when the vacuum is alleviated and the chamber is changed to enable coating after nitriding. However, this problem of exposure to the atmosphere is removed with a hybrid coating because the processes are sequentially carried out in the same chamber. Therefore, the hybrid coating is qualitatively stabilized since the entire process is carried out in a clean environment and run times are reduced, in comparison with other processes that use different chambers for nitriding and coating. Moreover, it was not necessary to polish the material, since a compound layer, which provides a negative influence, was not formed on the surface after nitriding. Thus, a cost reduction of approximately 15% is possible using this hybrid coating. 4. Conclusions Fig. 6. Results of drilling tests on various surface modifications.
film was observed with or without biasing on the nitriding. It is suggested that luster processing may be used for this plasma nitriding. 3.3. Application of hybrid coating Results of the drilling test for drills treated with various surface modifications are shown in Fig. 6. It is possible to increase tool lifetime by applying a hybrid coating to the drill. With ion plating using a crucible, the synthesis of an alloy thin film such as TiAlN is difficult compared with arc and sputtering processing. However, a TiN-coated drill that has been subjected to appropriate nitriding exhibits improved performance, i.e. a lifetime approximately two-fold greater than a normal TiN drill, and is superior to a TiAlN-coated drill. On the other hand, nitriding over a long period makes the performance worse than when a TiN coat is provided without nitriding. This may be caused by chipping if the drill bit has become too hard as a result of nitriding, or alternatively, the drill edge may be annealed by exposure to plasma so that energized particles may be accelerated to the substrate due to the bias voltage. In addition, the hybrid coating process was applied to a metal mould for stainless steel processing. Although the TiN coating without nitriding exhibited insufficient performance that was little different to the case without treatment, the lifetime of the mould was prolonged several-fold as a result of the hybrid coating.
We developed a hybrid coating process and associated apparatus that sequentially carries out plasma nitriding and coating in the same chamber as a hybrid coating process. It was demonstrated that the plasma nitriding of various steels was possible using this equipment, and the advantages of the hybrid coating process were described. The results are summarized as follows: 1. The nitrided layer reached a depth of 40 mm from the surface and the maximum hardness of SKH57 was 1400 HV50 after nitriding for 60 min at 450 8C without forming a compound layer. 2. When compared to a film on a non-treated substrate, the adhesion between the hybrid TiN film and the nitrided substrate was improved. 3. TiN-coated drills subjected to appropriate nitriding exhibited improved performance and may be superior to a TiAlN drill. References w1x M. Zlatanovic, Surf. Coat. Technol. 48 (1991) 19. w2x A. Leyland, D.B. Lewis, P.R. Stevenson, A. Matthews, Surf. Coat. Technol. 62 (1993) 608. w3x P.R. Stevenson, M.A. Parkin, A. Leyland, A. Matthews, Surf. Coat. Technol. 63 (1993) 135. w4x H.J. Spies, B. Larisch, K. Hock, E. Broszeit, H.J. Schroder, Surf. Coat. Technol. 74y75 (1995) 178. w5x W. Liang, G. Yuzhou, X. Bin, Surf. Coat. Technol. 131 (2000) 452. w6x A.S. Korhonen, E.H. Sirvio, M.S. Sulonen, Thin Solid Films 107 (1983) 387.
Development of a hybrid coating process as an advanced surface modification for cutting tools and moulds Takayasu Sato*, Kenichi Sugai, Shizuyo Ueda, Kouji Matsunami, Manabu Yasuoka Department of Coating, Nachi-fujikoshi Corp, 176-12 Namerikawa, Toyama 936-0802, Japan
Abstract We have developed new ion plating equipment that has made it possible to perform coating after performing plasma nitriding in the same chamber. First, the nitriding behavior of various steels was investigated using this equipment. A nitrogen diffusion layer thickness of 40 mm and a maximum hardness of 1400 HV50 were obtained without a compound layer on top of the nitrided SKH57 by treating for 60 min at 450 8C in Ar–N2 plasma. Secondly, plasma nitriding and TiN coating were carried out successively, using what is referred to as hybrid coating, in the same chamber, and the change in film characteristics with or without nitriding was clarified. Using nitriding as a pretreatment, the apparent film hardness was increased and adhesion between the film and nitrided substrate was improved compared with an untreated substrate. There was no change in the surface roughness after film deposition due to the nitriding. Finally, it was confirmed by results of a drill test that the performance of TiN coating with plasma nitriding was superior to that of a TiAlN film. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma nitriding; Ion plating; Hybrid coating; Adhesion; Drilling test
1. Introduction Nitriding is a well-known surface hardening process and various nitriding processes have been put to practical use. Ion or plasma nitriding of steels is attractive because a hard layer is rapidly obtained to a substantial depth. Moreover, dry coating such as ion plating is also an important kind of surface modification, and it is also a process that utilizes plasma. Until now, just the use of a single nitriding process or coating has provided sufficient performance. However, the recent demands placed on tools and moulds cannot be satisfied using simple surface modification processes. The application of an additional hard coating process on a nitrided substrate has therefore become more common, but the nitriding and coating processes are carried out separately w1–6x. We then proposed a hybrid process, which combines the initial processing of plasma nitriding with a subsequent coating process in the same chamber. Industrial equipment for implementing this process has been developed. First, the behavior of nitriding various steels using this equipment was examined. The advantages of this new *Corresponding author. Tel.: q81-774-713366; fax: q81-774713316. E-mail address: [email protected] (T. Sato).
process, i.e. the sequential plasma nitriding and ion plating, were investigated. 2. Experimental procedure SS-2-8 type ion plating equipment, available from Nachi-fujikoshi Co, was used. We improved this apparatus to implement plasma nitriding as follows. By installing a new, round, dummy crucible around the main crucible, the generation of a low-pressure, highdensity nitrogen plasma using an electron beam became possible without melting the titanium in the main crucible. This plasma was utilized for nitriding. TiN coating was carried out by focusing the electron beam from the dummy crucible onto the crucible after the nitriding process. This remodeled equipment is shown in Fig. 1. Mirror-like polished SKH57, SKD11 and SKD61 were used as sample materials for the nitriding experiments. The nitriding and coating conditions are shown in Table 1. After surface treatment, the microhardness distribution of the nitrided layer and the TiN films was investigated using Vickers hardness measurements at a load of 50 g. The thickness of the nitrided layer and the film were examined using standard optical metallographic tech-
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 0 7 3 - 2
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
46
Fig. 2. Vickers hardness profiles on various steels after plasma nitriding. Fig. 1. Apparatus for hybrid coating process.
niques and calotest methods, respectively. XRD analysis was applied for phase identification. The adhesion between the film and substrate was examined by measuring the critical load using a scratch tester. The surface roughness of the nitrided substrate and film were determined using a contact probe tester. The conditions of the drill test for confirming the advantage of the nitriding prior to TiN deposition are listed in Table 2 3. Results and discussion 3.1. Plasma nitriding The microhardness distribution profiles of various steels after nitriding are shown in Fig. 2. The maximum hardness of the surface was dependent on the hardness of the base metal. In the case of SKH57, it was increased by more than 1400 HV with nitriding. The maximum hardness of SKD61 and SKD11 was 1200 and 1300 HV, respectively. The hardness gradually decreased
inwards from the surface, owing to the diffusion of nitrogen into the material. A photograph of a crosssection of SKH57 after nitriding is shown in Fig. 3. It is observed that the nitriding layer reached over 40 mm from the surface. Fig. 4 shows the XRD patterns of SKH57 before and after nitriding. It is clear that a compound layer has not been formed near the top surface, since no new peaks are detected after nitriding. We also confirmed, as shown in Fig. 3, that a compound layer was not observed at the interface between the TiN film and the nitrided substrate. Nitrogen seems to form a solid solution in the steel matrix, because the halfwidth of the diffraction peak has increased (Fig. 4) and the diffusion layer is blackened by the corrosion test (Fig. 3). This solid solution causes the increase in hardness. The nitriding rate for this process is faster than usual ion nitriding and radical nitriding. Nitriding using plasma is generally carried out in a pressure range from several 100 Pa to 1000s of Pa. On the other hand, low-
Table 1 Conditions for plasma nitriding and coating
Plasma power (kW) Substrate bias voltage (V) Reaction gas Pressure (Pa) Temperature (8C) Treatment time (min)
Plasma nitriding
Coating
10 150 Ar–N2 1.0 450 0–60
10 100 Ar–N2 0.3 450 60
Table 2 Conditions for the drilling test Drill Speed (rpm) Feed (mmymin) Work material Depth (mm) Coolant
w 6 (SKH57) 957 153 SCM440 20 (through hole) Emulsion
Fig. 3. Photograph of cross-section of nitrided SKH57.
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
47
Fig. 4. XRD patterns of SKH57 before and after nitriding. Table 3 Changes in the characteristics of the film with or without nitriding
TiNqplasma nitriding TiN
Thickness (mm)
Film hardness (HV50)
Critical load (N)
Surface roughness (Rmax) (nm)
2.9 2.8
2194 1648
86 66
0.22 0.26
pressure, highly ionized nitrogen plasma is generated by the electron beam at 1 Pa. Therefore, although the density of nitrogen on the whole in the chamber is lower, many active particles could still exist, which would enhance nitriding. This is because a compound
Fig. 5. Scratch marks on TiN films with and without nitriding.
layer, which inhibits the diffusion of nitrogen, is not produced in the nitrided layer under these conditions. 3.2. Hybrid coating Changes in the characteristics of the films deposited on the substrate with or without nitriding are listed in Table 3. The coating conditions were the same, except for the nitriding, and the film thickness was therefore almost the same within the range of measurement error. The film deposited on the substrate after nitriding is harder than that of the untreated substrate. This is as a result of the substrate being hardened by the nitriding, and is also a consequence of the increase in hardness of the substrate being added to the film hardness. Furthermore, the critical load of the scratch test is increased by nitriding. Flaky cracks are observed in the scratch marks of the film when larger loads are deposited on the nitrided substrate (Fig. 5). Nitriding causes the base metal hardness to increase, lowering the stress along the base material interface as the hardness difference between the film and the base material narrows. As the base metal hardness gradually approaches that of the film on plasma nitriding, a kind of gradient functional material is formed. The film is mechanically supported on the nitrided layer, so the adhesion between the film and substrate is improved. The surface roughness of the film on the substrate with and without nitriding was also evaluated using a surface roughness contact tester. Hardly any change in surface roughness of the deposited
48
T. Sato et al. / Surface and Coatings Technology 169 – 170 (2003) 45–48
In general, moisture and dust in the atmosphere may be absorbed into the surface when the vacuum is alleviated and the chamber is changed to enable coating after nitriding. However, this problem of exposure to the atmosphere is removed with a hybrid coating because the processes are sequentially carried out in the same chamber. Therefore, the hybrid coating is qualitatively stabilized since the entire process is carried out in a clean environment and run times are reduced, in comparison with other processes that use different chambers for nitriding and coating. Moreover, it was not necessary to polish the material, since a compound layer, which provides a negative influence, was not formed on the surface after nitriding. Thus, a cost reduction of approximately 15% is possible using this hybrid coating. 4. Conclusions Fig. 6. Results of drilling tests on various surface modifications.
film was observed with or without biasing on the nitriding. It is suggested that luster processing may be used for this plasma nitriding. 3.3. Application of hybrid coating Results of the drilling test for drills treated with various surface modifications are shown in Fig. 6. It is possible to increase tool lifetime by applying a hybrid coating to the drill. With ion plating using a crucible, the synthesis of an alloy thin film such as TiAlN is difficult compared with arc and sputtering processing. However, a TiN-coated drill that has been subjected to appropriate nitriding exhibits improved performance, i.e. a lifetime approximately two-fold greater than a normal TiN drill, and is superior to a TiAlN-coated drill. On the other hand, nitriding over a long period makes the performance worse than when a TiN coat is provided without nitriding. This may be caused by chipping if the drill bit has become too hard as a result of nitriding, or alternatively, the drill edge may be annealed by exposure to plasma so that energized particles may be accelerated to the substrate due to the bias voltage. In addition, the hybrid coating process was applied to a metal mould for stainless steel processing. Although the TiN coating without nitriding exhibited insufficient performance that was little different to the case without treatment, the lifetime of the mould was prolonged several-fold as a result of the hybrid coating.
We developed a hybrid coating process and associated apparatus that sequentially carries out plasma nitriding and coating in the same chamber as a hybrid coating process. It was demonstrated that the plasma nitriding of various steels was possible using this equipment, and the advantages of the hybrid coating process were described. The results are summarized as follows: 1. The nitrided layer reached a depth of 40 mm from the surface and the maximum hardness of SKH57 was 1400 HV50 after nitriding for 60 min at 450 8C without forming a compound layer. 2. When compared to a film on a non-treated substrate, the adhesion between the hybrid TiN film and the nitrided substrate was improved. 3. TiN-coated drills subjected to appropriate nitriding exhibited improved performance and may be superior to a TiAlN drill. References w1x M. Zlatanovic, Surf. Coat. Technol. 48 (1991) 19. w2x A. Leyland, D.B. Lewis, P.R. Stevenson, A. Matthews, Surf. Coat. Technol. 62 (1993) 608. w3x P.R. Stevenson, M.A. Parkin, A. Leyland, A. Matthews, Surf. Coat. Technol. 63 (1993) 135. w4x H.J. Spies, B. Larisch, K. Hock, E. Broszeit, H.J. Schroder, Surf. Coat. Technol. 74y75 (1995) 178. w5x W. Liang, G. Yuzhou, X. Bin, Surf. Coat. Technol. 131 (2000) 452. w6x A.S. Korhonen, E.H. Sirvio, M.S. Sulonen, Thin Solid Films 107 (1983) 387.