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CrNx — a hard coating for corrosion and wear resistance
Materials Science and Engineering, A163 (1993) 193-196
193
CrNx -- a hard coating for corrosion and w e a r resistance P. B a l l h a u s e , B. H e n s e l , A. R o s t a n d H . Schiissler Leybold AG, Wilhelm Rohn Strasse 25, 6450 Hanau (Germany)
Abstract Cr/CrN~ films were deposited using plasma booster technology, a modified physical vapour deposition sputter ion plating process. The films showed excellent tribological and corrosion-resistant behaviour for a process temperature of 160-230 °C.
1. Introduction
2. Experimental details
Physical vapour deposition (PVD), especially the sputter ion plating process, allows the deposition of various functional coatings of excellent physical, mechanical and chemical quality. A wide range of layer materials are industrially deposited onto substrate materials such as steels, ceramics or even plastics. Today a multitude of tools and parts made of high speed steels are usually coated with hard surface films based on refractory metals, such as TiN, Ti(C,N) or (Ti,A1) N. To achieve good tribological behaviour of these layers, process temperatures between 400 °C and 500 °C are required. Many substrate materials are very sensitive with regard to temperature and cannot be coated using such a process. In order to produce a similar improvement in surface quality for some of these substrates, an optimized functional CrNx layer system has been developed. For the production of this CrNx layer the conventional PVD sputter machine was modified. The plasma booster arrangement used for the experiments described here is an improvement of the conventional magnetron sputtering technique [1-3]. The consequences of this modification are among others an extended plasma cloud and a higher plasma density near the substrate surface. This arrangement is also suited to coating large and three-dimensional substrates of complex shape with hard coatings. Regarding substrate material properties, the process temperature can be adjusted to between 160 °C and 230 °C. The substrates in these experiments were coated using this plasma booster technology. The layer quality was determined by several analytical methods.
The sputter coating system used in this investigation is a versatile batch coater, Leybold's TriTec 750 system. This machine offers a coating volume of up to 400 mm in diameter and 400 mm in height. Figure 1 shows a schematic cross-section of the plasma booster arrangement. The plasma booster technology is described in detail in refs. 4-6. In brief, the arrangement of two opposing coil boostered magnetron cathodes, which operate in an extended unbalanced mode, is combined with two magnetically enhanced anodes. A rotatable substrate holder is located in the centre of this arrangement. The avoid damaging arcs on the substrate surface, an ASI (arc suppressor interface) is installed between the sputter cathodes and their power supplies. With the aid of this arrangement very intensive plasma densities are available. Ion currents of up to 6 mA cm 2 were measured. The samples used in these experiments were made of 100 Cr 6 bearing steel and high speed cutting steel (S 6-5-2). The surfaces of the samples were mirror polished. All samples were cleaned wet chemically and etched by argon bombardment before starting the coating process. Then a thin chromium interface layer approximately 0.2 /zm thick and the CrNx layer approximately 1.8/zm thick were deposited. For the scanning electron microscopy (SEM) investigations, samples with a film thickness of 3.5 ~m were used. The bias potential during the reactive process was held at - 4 0 V. The anodic potential was 12.5 V. The argon and nitrogen flow was controlled by flowmeters and was identical for all coated samples. The total pressure was measured by a Viscovac manometer. All Cr/CrNx depositions were carried out under permanent rotation (continuous process). The temperature
0921-5093/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
194
P. Ballhause et al. / CrNx hard coatings
inwr~
=,bstrate
B target
I
material
~
cooling syatem
- - ~ ~ement
magnet
vacuum chamber
Fig. 1. Schematic drawing of the plasma booster arrangement.
of the substrates was controlled by thermocouples and an IR pyrometer. Analytical investigations were carried out to determine the hardness, adhesion, friction coefficient for different sliding partners, layer structure and corrosion behaviour of the coated samples.
3. Characterization of the coatings 3.1. Measurements of hardness Vickers hardness measurements were carried out. Regarding film thickness, it should be mentioned that the monitored hardness values are a mixture of the hardness of the film material and of the bulk material [7]. By increasing the Nz flow, the hardness of the CrN~ film was increased as also described in ref. 4. The measured hardness values (HVo.0o2) of the investigated samples were between 1950 kp mm -2 and 2100 kp
(a)
(b)
Fig. 2. Micrographs of scatched Cr/CrN~-coated samples (magnification 100 x ): (a) bulk material 100 Cr 6, layer thickness 2.1 p.m; (b) bulk material S 6-5-2, layer thickness 2.1 p.m.
mn1-2.
3.2. Adhesion To determine the adhesion of the Cr/CrNx films, scratch tests with additional acoustical detection and Rockwell C indentations were carried out. The values of the critical load Lc, a criterion for the quality of adhesion, were in the range 30-50 N. In Fig. 2 details of scratch tracks are shown on a CrNx coated 100 Cr 6 sample (a) and an S 6-5-2-sample (b). Microcracks are visible in the track, but peeling or cracking of the surface film was not observed. Rockwell C indentation led to similar results. In spite of the displacement of the bulk materials during penetration of the indenter, which caused concentric cracks around the penetration crater, no peeling or flaking of the layer material was observed.
3.3. Friction tests The friction behaviour of the coated samples was determined using a ball and disc machine (tribometer). Cemented carbide and silicon nitride were chosen as ball materials. The humidity during the sliding tests was approximately 35%. The applied load was 5 N and the length of sliding track was 50 m. The relative velocity of rotation was adjusted to 0.6 m rain-1. The friction coefficient of the sliding partners CrNx/ cemented carbide was determined between 0.2 and 0.3. For the CrNx/Si3N4 system, the friction coefficient was 0.3. In Fig. 3, tested sample surfaces of the tribological systems CrNx/100 Cr 6 (a) and the CrNx/cemented carbide system (b) are shown. In all cases no failure
P. Ballhause et aL / CrN, hard coatings
195
(a) (b) Fig. 3. Sliding tracks (sliding distance 50 m, load 5 N) of: (a) CrN+/100 Cr 6, /~= 0.3, and (b) CrNJcemented carbide, p~=0.2. Fig. 5. SEM image of a broken Cr/CrNx layer, thickness 3.6/.tm. with samples coated using the plasma booster enhanced sputter technology. 3.5. Structure and chemical composition
Cr
(rN× (10 %ccm}
CrN× (25 seem)
(rN x (25 !,
CrNx (25 s(:cm)
CrN× (25 scorn)
2,! Hm
?,I iml
3,5 ~ir,
(b) Fig. 4. NaCl-salt spray test of Cr/CrNx-coated samples (bulk material 100 Cr 6): (a) without plasma booster (24 h); (b) with plasma booster (48 h)
of the protecting hard coating layer was detected during the tests. 3. 4. Corrosion resistance
One set of samples (100 Cr 6) coated using the conventional magnetron sputtering technique and another set coated using the plasma booster technique were subjected to a salt spraying corrosion test. Figure 4 shows the CrNx-coated samples after the corrosion test. The test duration for set (a) was 24 h, while set (b) was tested for 48 h. The layer thickness of all investigated samples was 2.1 /~m. A qualitative inspection shows that corrosive attack can be reduced
For preliminary investigation of the layer structure, SEM images were taken from broken samples. X-ray diffraction experiments will follow. The SEM image in Fig. 5 shows a Cr/CrNx layer of 3.5 tzm thickness. A smooth surface, high density of the layer material and a nearly amorphous structure are visible. These layer properties are caused by the intensive ion bombardment of the plasma booster technique during the deposition process. The intensive ion bombardment produces a high mobility of surface atoms which is necessary to obtain a smooth and highly dense surface layer [8]. Secondary neutral particle mass spectroscopy (SNMS) investigations delivered additional information on the homogeneity and chemical composition of the Cr/CrNx layer. Figure 6 shows a SNMS depth profile, taken from a Cr/CrNx-coated silicon waver. The total layer thickness was approximately 1 /zm. Broadening of the interface is also caused by intensive ion bombardment during the chromium deposition. The chromium interface layer is mixed with the bulk material. This leads to the excellent adhesion of the Cr/CrNx layer on the substrate surface. The stoichiometric ratio of chromium to nitrogen in the deposited CrNx layer is constant and evaluated as x = 0.45.
4. Summary and conclusion The plasma booster technology, a modified PVD sputter ion plating process, allowed the deposition of Cr/CrNx films with excellent tribological and corrosionresistant behaviour at process temperatures of 160-230 °C. Smooth surface layers with a very dense structure
196
P. Ballhause et al. / CrNx hard coatings Sample
CrN
8
SNMS -
Depth P r o f i l e
L J Cn
intensity
[cpe] ltn
Si
8O
70" 60" 50"
40-
N
30" 20" iO" 400 v
B00
i
0
t600
::'000
D ~s
Fig. 6. SNMS depth profile of a Cr/CrNx-coated silicon waver (layer thickness approximately 1 /zm).
could be produced in a reproducible manner by the aid of a completely computer-controlled PVD sputter coater. The adhesion of the layers was investigated by scratch testing and Rockwell C indentations. The critical load was higher than 30 N. The evaluated friction coefficient of CrNx against cemented carbide reached values of 0.2 and 0.3 against silicon nitride. At present, the experience and results of these investigations are being transferred to practical applications. Industrial investigations have already been carried out. For example, Cr/CrNx deposited onto sliding tubes of motorbikes can successfully replace electroplated hard chromium. During the practical tests, reduced wear rates and good corrosion resistance could be observed, while the splip-stick effect between hard chromium and the plastic sealings was reduced remarkably. Further applications, especially CrNx-coated engine components and machine parts are being developed under industrial conditions. The first results promise that this recently developed Cr/CrNx film rep-
resents a serious competitor for conventional protective coatings.
References 1 W. D. Mfinz, D. Hofmann and K. Hartig, Thin Solid Films, 96 (1982) 79. 2 D. Hofmann, Proc. Int. Conf. on Technische Hochschule Darmstadt, 1983, T H Darmstadt, Darmstadt, 1983, p. 323. 3 D. Hofmann, S. BeiBwenger and A. Feuerstein, Surf. Coat. Technol., 49 (1991) 330-335. 4 D. Hofmann, A. Rost and H. SchfiBler, Mater. Sci. Eng., A139 (1991) 290-293. 5 D. Hofmann, H. Schussler and A. Feuerstein, German Patent D E 4038 497 C1, 1990. 6 D. Hofmann, P. Ballhause, A. Feuerstein and J. Snyder, 35th Annu. Tech. Conf. Proc. of SVC, Baltimore, AID, March 1992, to be published, DGM Oberusel, 1987, Vol. 7. 7 E. Matthaei, Hiirteprufung mit kleinen Auflagekriiften, Vol. 7, DGM, 1987. 8 J. E. Sundgren, Proc. Decorative Hard coating Syrup., Leybold reprint No. 11-$31.02, 1989 (Leybold, Hanan, Germany).
193
CrNx -- a hard coating for corrosion and w e a r resistance P. B a l l h a u s e , B. H e n s e l , A. R o s t a n d H . Schiissler Leybold AG, Wilhelm Rohn Strasse 25, 6450 Hanau (Germany)
Abstract Cr/CrN~ films were deposited using plasma booster technology, a modified physical vapour deposition sputter ion plating process. The films showed excellent tribological and corrosion-resistant behaviour for a process temperature of 160-230 °C.
1. Introduction
2. Experimental details
Physical vapour deposition (PVD), especially the sputter ion plating process, allows the deposition of various functional coatings of excellent physical, mechanical and chemical quality. A wide range of layer materials are industrially deposited onto substrate materials such as steels, ceramics or even plastics. Today a multitude of tools and parts made of high speed steels are usually coated with hard surface films based on refractory metals, such as TiN, Ti(C,N) or (Ti,A1) N. To achieve good tribological behaviour of these layers, process temperatures between 400 °C and 500 °C are required. Many substrate materials are very sensitive with regard to temperature and cannot be coated using such a process. In order to produce a similar improvement in surface quality for some of these substrates, an optimized functional CrNx layer system has been developed. For the production of this CrNx layer the conventional PVD sputter machine was modified. The plasma booster arrangement used for the experiments described here is an improvement of the conventional magnetron sputtering technique [1-3]. The consequences of this modification are among others an extended plasma cloud and a higher plasma density near the substrate surface. This arrangement is also suited to coating large and three-dimensional substrates of complex shape with hard coatings. Regarding substrate material properties, the process temperature can be adjusted to between 160 °C and 230 °C. The substrates in these experiments were coated using this plasma booster technology. The layer quality was determined by several analytical methods.
The sputter coating system used in this investigation is a versatile batch coater, Leybold's TriTec 750 system. This machine offers a coating volume of up to 400 mm in diameter and 400 mm in height. Figure 1 shows a schematic cross-section of the plasma booster arrangement. The plasma booster technology is described in detail in refs. 4-6. In brief, the arrangement of two opposing coil boostered magnetron cathodes, which operate in an extended unbalanced mode, is combined with two magnetically enhanced anodes. A rotatable substrate holder is located in the centre of this arrangement. The avoid damaging arcs on the substrate surface, an ASI (arc suppressor interface) is installed between the sputter cathodes and their power supplies. With the aid of this arrangement very intensive plasma densities are available. Ion currents of up to 6 mA cm 2 were measured. The samples used in these experiments were made of 100 Cr 6 bearing steel and high speed cutting steel (S 6-5-2). The surfaces of the samples were mirror polished. All samples were cleaned wet chemically and etched by argon bombardment before starting the coating process. Then a thin chromium interface layer approximately 0.2 /zm thick and the CrNx layer approximately 1.8/zm thick were deposited. For the scanning electron microscopy (SEM) investigations, samples with a film thickness of 3.5 ~m were used. The bias potential during the reactive process was held at - 4 0 V. The anodic potential was 12.5 V. The argon and nitrogen flow was controlled by flowmeters and was identical for all coated samples. The total pressure was measured by a Viscovac manometer. All Cr/CrNx depositions were carried out under permanent rotation (continuous process). The temperature
0921-5093/93/$6.00
© 1993- Elsevier Sequoia. All rights reserved
194
P. Ballhause et al. / CrNx hard coatings
inwr~
=,bstrate
B target
I
material
~
cooling syatem
- - ~ ~ement
magnet
vacuum chamber
Fig. 1. Schematic drawing of the plasma booster arrangement.
of the substrates was controlled by thermocouples and an IR pyrometer. Analytical investigations were carried out to determine the hardness, adhesion, friction coefficient for different sliding partners, layer structure and corrosion behaviour of the coated samples.
3. Characterization of the coatings 3.1. Measurements of hardness Vickers hardness measurements were carried out. Regarding film thickness, it should be mentioned that the monitored hardness values are a mixture of the hardness of the film material and of the bulk material [7]. By increasing the Nz flow, the hardness of the CrN~ film was increased as also described in ref. 4. The measured hardness values (HVo.0o2) of the investigated samples were between 1950 kp mm -2 and 2100 kp
(a)
(b)
Fig. 2. Micrographs of scatched Cr/CrN~-coated samples (magnification 100 x ): (a) bulk material 100 Cr 6, layer thickness 2.1 p.m; (b) bulk material S 6-5-2, layer thickness 2.1 p.m.
mn1-2.
3.2. Adhesion To determine the adhesion of the Cr/CrNx films, scratch tests with additional acoustical detection and Rockwell C indentations were carried out. The values of the critical load Lc, a criterion for the quality of adhesion, were in the range 30-50 N. In Fig. 2 details of scratch tracks are shown on a CrNx coated 100 Cr 6 sample (a) and an S 6-5-2-sample (b). Microcracks are visible in the track, but peeling or cracking of the surface film was not observed. Rockwell C indentation led to similar results. In spite of the displacement of the bulk materials during penetration of the indenter, which caused concentric cracks around the penetration crater, no peeling or flaking of the layer material was observed.
3.3. Friction tests The friction behaviour of the coated samples was determined using a ball and disc machine (tribometer). Cemented carbide and silicon nitride were chosen as ball materials. The humidity during the sliding tests was approximately 35%. The applied load was 5 N and the length of sliding track was 50 m. The relative velocity of rotation was adjusted to 0.6 m rain-1. The friction coefficient of the sliding partners CrNx/ cemented carbide was determined between 0.2 and 0.3. For the CrNx/Si3N4 system, the friction coefficient was 0.3. In Fig. 3, tested sample surfaces of the tribological systems CrNx/100 Cr 6 (a) and the CrNx/cemented carbide system (b) are shown. In all cases no failure
P. Ballhause et aL / CrN, hard coatings
195
(a) (b) Fig. 3. Sliding tracks (sliding distance 50 m, load 5 N) of: (a) CrN+/100 Cr 6, /~= 0.3, and (b) CrNJcemented carbide, p~=0.2. Fig. 5. SEM image of a broken Cr/CrNx layer, thickness 3.6/.tm. with samples coated using the plasma booster enhanced sputter technology. 3.5. Structure and chemical composition
Cr
(rN× (10 %ccm}
CrN× (25 seem)
(rN x (25 !,
CrNx (25 s(:cm)
CrN× (25 scorn)
2,! Hm
?,I iml
3,5 ~ir,
(b) Fig. 4. NaCl-salt spray test of Cr/CrNx-coated samples (bulk material 100 Cr 6): (a) without plasma booster (24 h); (b) with plasma booster (48 h)
of the protecting hard coating layer was detected during the tests. 3. 4. Corrosion resistance
One set of samples (100 Cr 6) coated using the conventional magnetron sputtering technique and another set coated using the plasma booster technique were subjected to a salt spraying corrosion test. Figure 4 shows the CrNx-coated samples after the corrosion test. The test duration for set (a) was 24 h, while set (b) was tested for 48 h. The layer thickness of all investigated samples was 2.1 /~m. A qualitative inspection shows that corrosive attack can be reduced
For preliminary investigation of the layer structure, SEM images were taken from broken samples. X-ray diffraction experiments will follow. The SEM image in Fig. 5 shows a Cr/CrNx layer of 3.5 tzm thickness. A smooth surface, high density of the layer material and a nearly amorphous structure are visible. These layer properties are caused by the intensive ion bombardment of the plasma booster technique during the deposition process. The intensive ion bombardment produces a high mobility of surface atoms which is necessary to obtain a smooth and highly dense surface layer [8]. Secondary neutral particle mass spectroscopy (SNMS) investigations delivered additional information on the homogeneity and chemical composition of the Cr/CrNx layer. Figure 6 shows a SNMS depth profile, taken from a Cr/CrNx-coated silicon waver. The total layer thickness was approximately 1 /zm. Broadening of the interface is also caused by intensive ion bombardment during the chromium deposition. The chromium interface layer is mixed with the bulk material. This leads to the excellent adhesion of the Cr/CrNx layer on the substrate surface. The stoichiometric ratio of chromium to nitrogen in the deposited CrNx layer is constant and evaluated as x = 0.45.
4. Summary and conclusion The plasma booster technology, a modified PVD sputter ion plating process, allowed the deposition of Cr/CrNx films with excellent tribological and corrosionresistant behaviour at process temperatures of 160-230 °C. Smooth surface layers with a very dense structure
196
P. Ballhause et al. / CrNx hard coatings Sample
CrN
8
SNMS -
Depth P r o f i l e
L J Cn
intensity
[cpe] ltn
Si
8O
70" 60" 50"
40-
N
30" 20" iO" 400 v
B00
i
0
t600
::'000
D ~s
Fig. 6. SNMS depth profile of a Cr/CrNx-coated silicon waver (layer thickness approximately 1 /zm).
could be produced in a reproducible manner by the aid of a completely computer-controlled PVD sputter coater. The adhesion of the layers was investigated by scratch testing and Rockwell C indentations. The critical load was higher than 30 N. The evaluated friction coefficient of CrNx against cemented carbide reached values of 0.2 and 0.3 against silicon nitride. At present, the experience and results of these investigations are being transferred to practical applications. Industrial investigations have already been carried out. For example, Cr/CrNx deposited onto sliding tubes of motorbikes can successfully replace electroplated hard chromium. During the practical tests, reduced wear rates and good corrosion resistance could be observed, while the splip-stick effect between hard chromium and the plastic sealings was reduced remarkably. Further applications, especially CrNx-coated engine components and machine parts are being developed under industrial conditions. The first results promise that this recently developed Cr/CrNx film rep-
resents a serious competitor for conventional protective coatings.
References 1 W. D. Mfinz, D. Hofmann and K. Hartig, Thin Solid Films, 96 (1982) 79. 2 D. Hofmann, Proc. Int. Conf. on Technische Hochschule Darmstadt, 1983, T H Darmstadt, Darmstadt, 1983, p. 323. 3 D. Hofmann, S. BeiBwenger and A. Feuerstein, Surf. Coat. Technol., 49 (1991) 330-335. 4 D. Hofmann, A. Rost and H. SchfiBler, Mater. Sci. Eng., A139 (1991) 290-293. 5 D. Hofmann, H. Schussler and A. Feuerstein, German Patent D E 4038 497 C1, 1990. 6 D. Hofmann, P. Ballhause, A. Feuerstein and J. Snyder, 35th Annu. Tech. Conf. Proc. of SVC, Baltimore, AID, March 1992, to be published, DGM Oberusel, 1987, Vol. 7. 7 E. Matthaei, Hiirteprufung mit kleinen Auflagekriiften, Vol. 7, DGM, 1987. 8 J. E. Sundgren, Proc. Decorative Hard coating Syrup., Leybold reprint No. 11-$31.02, 1989 (Leybold, Hanan, Germany).