- Email: [email protected]
Characterization of hard chromium nitride coatings deposited by cathodic arc vapor deposition
Surface and Coatings Technology 86-87 (1996) 788-796
Characterization
of hard chromium nitride coatings deposited by cathodic arc vapor deposition
W.K. Grant a,*, C. Loomis b, J.J. Moore a, D.L. Olson a, B. Mishra a, A.J. Perry ’ a Advanced Coatings and Surface Engineering Laboratory (ACSEL),
Colorado School of Mines, Golden, CO 80401, USA b Coors Brewing Co., Golden, CO 80401, USA ’ ISM Technologies, San Diego, CA, USA
Abstract Cr-N coatings, of various stoichiometries, were deposited on 304 stainless steel and M4 tool steel by planar cathodic arc vapor deposition (CAVD). This investigation concentrated on the effects of the CAVD process parameters and the resulting mechanical properties. To achieve this, several properties of the deposited films were studied; hardness, adhesion, coefficients of friction, residual stress, texture and stoichiometry. Hardness and adhesion were measured by low load Vicker’s indentation and diamond scratch testing respectively. The adhesion exhibited an inverse relationship with respect to the hardness of the Cr-N coatings. Hardness values averaged 17.7 GPa while maximum adhesion values of the Cr-N coatings exceeded 85 N using a 0.2 mm diamond stylus. Typical values for the coefficients of friction remained lower than 0.10 (diamond against Cr-N). Residual stress was measured by glancing incidence X-ray diffraction (GIXRD). Residual stress measurements indicated a transition from compressive to tensile stresses for midrange levels of both nitrogen partial pressure (e.g., 3.33 Pa) and substrate bias (e.g., - 150 V). The stoichiometry of the films (N:Cr) ranged from 0.7 to 1 for nitrogen partial pressures of 2.00 and 5.32 Pa, respectively. Keywords:
Chromium nitride; Cathodic arc
1. Introduction For several years, nitride based coatings have offered a way of improving performance and extending the life of high speed steel tooling [l-5]. Included in this category are chromium nitride coatings which have been shown to have superior impact resistance compared to other physical vapor deposited (PVD) coatings [ 1,6]. In addition, cathodic arc vapor deposition (CAVD) technology has provided a method for depositing an extremely dense, well adhered coating. It may be predicted that, in order to achieve a reliable wear barrier in impact situations, a coating with high hardness and adhesion and a low coefficient of friction would need to be deposited. Therefore, the ability to measure and understand the relationship between these properties is essential. As will be discussed, hard, brittle films were more likely to fail under a lower load than were the softer, tougher films. In optimizing deposition parameters, numerous factors need to be examined * Corresponding author. 6330 Gunpark Drive, Boulder, USA. Tel.: + 1 303 5819400; fax: + 1 303 5819700.
CO 80301,
0257-8972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PZZ SO257-8972(96)03071-X
including residual present ducting of Cr-N process
hardness, adhesion, coefficients of friction, stress, texture and stoichiometry. Thus, the study was conceived with the intention of cona comprehensive investigation of the properties coatings with respect to the CAVD coating parameters.
2. Experimental
procedure
Cr-N coatings were deposited by cathodic arc vapor deposition (CAVD) using a 127 mm (5”) circular planar cathode source. The source to substrate distance was kept constant at 200 mm. A schematic representation of the CAVD apparatus is shown in Fig. 1. The substrate materials in this investigation were M4 tool steel and 304 stainless steel. The dimensions of the M4 tool steel and 304 stainless steel substrates were 4 x 2 x 1 and 7.5 x 2.5 x 0.2 cm, respectively. The substrates were clamped to a steel plate with dimensions 10 x 8 x 0.4 cm. The substrate holder was placed on a shielded alumina plate to isolate it and the substrates
W.K. Grant et al.ISurface and Coatings Technology 86-87 (1996) 788-796 1. Catho&(source) 2. Allodc(&oulld) 3. vacllumch;lmbar 4. St&x 5. DCSlIbstrrteBiCiSSUpp~ 6. Arc Power Supply I. SuMrateHolder
Fig. 1. Schematic deposition.
representation
of CAVD
chamber
used for Cr-N
from the chamber. Actual substrate temperature was measured in a similar system under similar conditions with a type J thermocouple. Both sets of substrates were polished to an average roughness of 0.1 urn. The substrate materials underwent identical pretreatment prior to deposition, The cleaning procedures consisted of a 20 min ultrasonic acetone bath, a 20 min heated ultrasonic isopropyl alcohol bath and a 1 min nitrogen gas dry. Finally, a 10 min plasma etch in vacuum was performed using a -700 V bias (ion current density = 3.2 mA cm-‘) at a pressure of 5.98 Pa argon. A vacuum of 2.7 x 1O-6 Pa or better was achieved prior to plasma cleaning and deposition. Deposition followed directly after the d.c. plasma cleaning process. In these sets of experiments, nitrogen partial pressure, substrate bias and arc current were varied in order to study their effects on hardness, adhesion, coefficients of friction, texture, residual stress and stoichiometry. Coating thickness varied between 1.0 and 1.5 urn for a 20 min deposition. Variations in the coating thickness were attributed to changes in the coating process conditions (substrate bias, arc current and nitrogen partial pressure). Substrate temperature was not held constant during deposition and again varied with respect to the arc deposition process parameters. Microhardness was determined by Vickers indentation with a 25 g load. Each test was repeated five to seven times to ensure statistical accuracy. The reproducibility of the Vickers hardness values is reported to be + 1.5 GPa [7]. Other variations of hardness data were attributed to irregularities in the surface condition and composition of the coating (i.e., various Cr-N phases). Adhesion and friction measurements were made with a Revetest 200 single pass diamond scratch tester at
189
Southwest Research Institute in San Antonio, TX. The apparatus had a 0.2 mm conical diamond stylus. The load was increased linearly from 0 to 150 N while traversing the sample at a rate of 10 mm min-‘. The load rate was set at 100 N min-‘. The samples were placed on a goniometer which allowed visual inspection of the scratch through the entire loading range with an optical microscope mounted to the apparatus fixed with two objective lenses (i.e., 200 x and 500 x). Failure loads were determined following similar guidelines established by previous investigators [ 8,9]. Reproducibility of the failure loads was determined to be 15 N. Friction measurements were determined during the scratch test using the Revetest apparatus. Coefficients of friction were calculated from the normal and tangential loads recorded throughout the scratch test. The coefficients of friction were correlated with both the material in contact with the stylus (e.g., film or substrate) and the mechanical properties of the films (e.g., hardness). In addition to the visual inspection of the scratch test, the coefficient of friction also aided in determining the failure load of the Cr-N coatings. Auger electron spectroscopy (AES) was used to determine the N:Cr ratio in the coatings. Stoichiometries were determined from these ratios and were correlated to phase formation in the films. Glancing incidence X-ray diffraction (GIXRD) with parallel beam optics was used to study phase formation, texture and residual stress. Tests were conducted at Martin-Lockheed in Denver, CO. Diffraction patterns were acquired with Cu Kcx radiation at an accelerating voltage of 50 kV. Scan ranges were typically set from 10 to 145” 28 with a step size of 0.05”. The incident angle, y, was kept constant at 2”. In all cases, AISI Si powder standard was used to identify peak shifts occurring due to surface abnormalities. The residual stresses present in the films were determined from the plots of the lattice parameter versus sin2y, [ 10-121. The slope of this plot can be related to the stress in the film using the relationship: m = ba,( 1 + v)/E
where m is the slope of the line, g is the residual stress in the film, a, is the equilibrium lattice parameter, v is the Poisson’s ratio and E is the Young’s modulus of the film. A value of 280 GPa was used for the Young’s Modulus as determined by Sue et al. [ 131
3. Results 3.1. Stoichiometry Of particular interest with the Cr-N coating system in this study was the stoichiometry of the Cr-N system and its dependence on CAVD deposition parameters
WK. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
790
and, ultimately, how these affected the properties of the coatings. The effects of nitrogen partial pressure on the N:Cr ratio in the films deposited on 304 stainless steel is shown in Fig. 2. As expected, an increasing N:Cr ratio was observed in the films deposited at the higher nitrogen partial pressures. Note, however, that a one to one ratio of nitrogen to chromium is not reached until nitrogen partial pressure of 5.32 Pa. The ratio of nitrogen to chromium in this investigation ranged from 0.7 at a nitrogen pressure of 2.00 Pa to 1.Oat a nitrogen pressure of 5.32 Pa. 3.2. Texture, residual stress and phase formation X-ray diffraction patterns were used to study orientation and phase formation in the films. All of the films deposited had a preferred orientation of either the ( 111) or (200) planes. Transition from the (200) orientation to the (111) orientation occurred when the substrate bias was increased from - 50 to -250 V as shown in Fig. 3(a). In addition, the texture of the films was also influenced by the nitrogen partial pressure as shown in Fig. 3(b). The ratio of the (200) planes to (111) planes increased as the nitrogen partial pressure increased. The results from the residual stress analysis indicated a transition from compressive stress to tensile stress in the Cr-N films as a function of substrate bias, as shown in Fig. 4. Examination of the data presented in Fig. 4, shows that the residual stress in the film changes from
compressive to tensile when using a substrate bias of between - 150 and -200 V. A further increase in the bias to - 250 V returns the stress to a compressive value. A similar trend is observed with respect to the effect of nitrogen partial pressure as shown in Fig. 5. In this respect, tensile stresses were measured for Cr-N films deposited with a nitrogen partial pressure of between approximately 2.39-3.06 Pa. 3.3. Microhardness Microhardness values recorded for the Cr-N coatings on M4 tool steel are shown in Fig. 6. The hardness is seen to be dependent on both substrate bias and arc current. The two series of data illustrate the dependence of film hardness on arc current when varied between 125 and 60 A. As is seen, both series of data yield similar trends, however, the higher arc current coatings produced somewhat harder coatings with a maximum hardness of 22.0 GPa. The error bars in the figure represent the standard deviation from the measurements. Hardness was also influenced slightly by the nitrogen partial pressure (nitrogen content in the film) used during the CAVD process, as indicated in Fig. 7. The average hardness remained relatively constant for the films deposited with the higher arc current above a nitrogen partial pressure of 2.93 Pa. The hardness of the coatings deposited at the higher arc current did indicate slightly higher values of hardness averaging 17.2 GPa at
1
0.2
0
t
!0
I
2
3 Nitrogen
Fig. 2. Atomic
ratios of Cr-N
coatings
Pressure
on 304 stainless
4
5
6
(Pa)
steel for varying
nitrogen
partial
pressures.
W.K. Grant
et
al./Surface and Coatings Technology86-87 (1996) 788-796
CrN(lll)
791
Substrate Bias
-150 Volts B
a
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Two Theta (a)
CrN (200)
N, Pressure
1
CrN (111)
’
CrN (220) CrN (311)
CrN (400) 4.00 Pa
3.33 Pa
2.66 Pa
2.00 Pa
10
20
30
40
50
70
60
80
90
100
110
120
130
140
Two Theta @) Fig. 3. X-ray diffraction patterns of Cr-N films (a) with respect to varying levels of substrate bias (constant P=4.0 Pa) showing preferred orientation of the (111) planes at high bias levels (e.g., - 250 V), and (b) with respect to varying levels of nitrogen partial pressures (constant substrate bias = - 100 V) showing preferred orientation of the (200) planes for all the pressures tested.
a pressure of 4.00 Pa. As expected the films deposited with no nitrogen exhibited the lowest hardness with averages near 11.8 GPa. 3.4. Adhesion Adhesion measurements of the Cr-N coatings deposited on M4 tool steel are shown in Fig. 8 as a function of substrate bias. Although factors such as phase formation and residual stress contribute to the adhesion load of the Cr-N coatings, an inverse trend between film adhesion and film hardness with substrate bias (compare Fig. 6 and Fig. 8) is apparent. A peak average adhesion, L,, of 80 N was measured at a substrate bias of about -150v. Adhesion values with respect to variations in nitrogen pressure are shown in Fig. 9. The adhesion values of the
films deposited with no nitrogen (i.e., Cr films) exhibit the highest adhesion with values averaging around 135 N. As nitrogen is incorporated into the coatings the adhesion values drop. A minimum adhesion value of 25 N is measured for films deposited at a nitrogen pressure of 2.93 Pa. Relatively high adhesion values were measured for films deposited at 2.00 and 5.32 Pa nitrogen partial pressures with critical loads around 85 N. The error bars in the figures represent an error of +5 N determined for the scratch adhesion tests. 3.5. Coeficients
offriction
Coefficients of friction were recorded and calculated during scratch testing from the measurements of the normal and tangential loads on the stylus. A summary of these data is given in Table 1 and Table 2 for varia-
WK. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
192 3
I
l
2
1
l
1 0 50
s 5
100
-1
-1
200
150
250
l
t tj -2 2 E -3 a p! -4 l
+
-5
I
l
Substrate Bias (-Volts) Fig. 4. Residual stress measurements of Cr-N coatings were held constant at 75 A and 30 mTorr, respectively.
on M4 tool steel as a function
of substrate
bias. Arc current
and nitrogen
partial
pressure
-1t -1.2 1 Nitrogen Pressure (Pa) Fig. 5. Residual stress measurements of Cr-N coatings were held constant at 75 A and - 125 V, respectively.
on M4 tool steel as a function
of nitrogen
partial
pressure.
Arc current
and substrate
bias
W.K Grant et al./Surface
and Coatings Technology 86-87 (1996) 788-796
193
25
1 + 60 Amperes -...
n
125 Amperes
_.____t__--+
1
0
0
50
100
200
150
250
300
Substrate Bias (-Volts) Fig. 6. Microhardness
of Cr-N
films as a function
of the substrate
bias on M4 tool steel. The nitrogen
partial
pressure
was kept constant
at 4.0 Pa.
15
-i
0, 0
Fig. 7. Microhardness
of Cr-N
1
films as a function
2
of the nitrogen
3 Nitrogen Pressure (Pa) partial
in substrate bias and nitrogen partial pressure respectively. The numbers reported in these tables indicate the coefficient of friction measured with a diamond tions
pressure
4
5
on M4 tool steel. The substrate
6
bias was kept constant
at - 125 V.
stylus against the Cr-N films at a 20 N load. If, however, the film delaminated from the substrate prior to a 20 N load the term “No Film” appears. The values for the
WX Grantetal/Suiurface andCoatings Technology 86-87(1996)788-796
794
+Arc
Current = 60 amperes
- . * - . Arc Current = 125 amperes
OL 0
50
100
150
300
250
200
Substrate Bias (-Volts)
Fig. 8. Critical load measurements with respect to varying substrate bias levels of Cr-N films on M4 tool steel. Nitrogen partial pressure was kept constant at 30 mTorr.
160
-._
+Arc
Current = 60 Amperes
- I - Arc Current = 125 Amperes
120
z^
100
2
c- 80
2 t f
60 I
40
20
0
I I
I 0
1
2
3
4
5
6
Nitrogen Pressure (Pa)
Fig. 9. Critical load measurements with respect to varying nitrogen partial pressures of Cr-N films on M4 tool steel. Substrate bias levels were held constant at - 125 V.
W.K. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
Table 1 Coefficients of friction for Cr-N films on M4 tool steel as a function of substrate bias and arc current Substrate bias (-V)
Arc current (A)
Coefficient of friction (20 N)
10 70 150 200 300 10 70 125 200 300
60 60 60 60 60 125 125 125 125 125
0.068 No Film 0.055 0.090 0.078 0.052 0.060 0.088 0.052 0.102
Films were deposited at a constant nitrogen partial pressure of 4.0 Pa. “No Film” indicates coating failure prior to a 20 N load.
Table 2 Coefficients of friction for Cr-N films on M4 tool steel as a function of nitrogen partial pressure and arc current Nitrogen pressure (Pa)
Arc current (A)
Coefficient of friction (20N)
0 2.0 3.0 5.3 0 2.0 4.0 5.3
60 60 60 60 125 125 125 125
0.112 0.068 0.085 0.087 0.128 0.083 0.087 0.075
Substrate bias levels were kept constant at - 125 V.
coefficients of friction indicated numbers between approximately 0.05 and 0.13 depending on the deposition process conditions. 3.6. Substrate temperature The substrate temperature was not measured during the actual deposition but was measured in a similar chamber under the same operating conditions. As expected, the temperature of the substrate was a function of the variations in the process conditions. For all deposition conditions, though, the substrate temperature never exceeded 300°C and, therefore, can be considered a low temperature deposition. The substrate bias and arc current were the main contributors to changes in the substrate temperatures. Maximum temperatures of 275°C were reached for a substrate bias of -250 V and 125 A. As expected, the substrate temperature decreased as the substrate bias and arc current were reduced. Substrate temperature at a substrate bias of -50 V and an arc current of 60 A was 140°C.
795
4. Discussion The data from the auger spectroscopy (Fig. 2) indicates that a stoichiometric CrN film is not deposited until a nitrogen partial pressure of 5.32 Pa is reached. Nitrogen partial pressures below 5.32 Pa yielded a nitrogen to chromium ratio of less than one. Surprisingly, the X-ray diffraction data indicate that the CrN phase is the dominant phase in all the films deposited at a nitrogen partial pressure of 2.00 Pa or greater. It is unusual that the major phase in all the coating tested was the CrN phase and little if any of the Cr,N phase was discernible noting the small range of stoichiometry where CrN is a stable phase. The fact that little Cr,N was observed from the diffraction patterns could possibly be attributed to peak shifts due to lattice strain in the coatings where further peak deconvolution would be necessary to separate the peaks. The apparent phases present (according to the AES data) in the Cr-N films, their orientation and the corresponding mechanical properties of the coatings are summarized in Table 3. The transition in the residual stress values from compressive to tensile shown in Figs. 4 and 5 is an issue of some interest. One should note that the compressive and tensile residual stress values reported in Fig. 5 are relatively low compared to other stress values reported for coatings deposited by cathodic arc methods [ 14,151. The presence of tensile stress in thin films has been attributed to void formation in the films due to the inability of the structure to relieve the strain in the direction of the voids [ 161. This trend, with respect to the nitrogen partial pressure, has been observed previously [ 16,171. The hardnesses of the films, shown in Fig. 6 and Fig. 7 vary between 11.8 and 22.6 GPa depending on the process conditions. As expected, due to the different bonding, crystallographic structure and slip systems between Cr and CrN, the Cr films (i.e., no nitrogen) exhibited the lowest hardness values. As nitrogen is incorporated in the film, the hardness increases up to a maximum located at 2.93 Pa. The ratio of nitrogen to chromium at this level is approximately 0.8 and, therefore, these films most likely contain some combination of CrN and Cr,N. The hardness with respect to variations in substrate Table 3 Phase formation of the CAVD Cr-N corresponding mechanical properties
on M4 tool steel and the
Film
Nitrogen pressure (Pa)
Hardness @Pa)
Adhesion (N)
Cr CrN(lll)+Cr,N CrN(200) + Cr,N CrN(200)
0 2.0 3.0 5.3
12.3 13.7 16.2 13.2
150 80 25 92
Mechanical properties reported are for films deposited at - 150 V substrate bias and 60 ampere arc current.
796
W.K. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
(1) Stoichiometries of the films (N:Cr) varied between 0.7 and 1.0 depending on the nitrogen partial pressure which was varied between 2.0 and 5.32 Pa. (2) The preferred orientation of the CrN present in the films changed from (200) to (111) on increasing substrate bias. The preferred orientation with respect to changes in the nitrogen partial pressure remained (200) for the ranges tested. An increase of the ratio of (111) to (200) planes was observed for lower nitrogen partial pressures. (3) Tensile stresses were present in Cr-N films deposited at intermediate substrate biases (e.g., - 1 5 0 to - 2 2 0 V) and intermediate nitrogen partial pressures (e.g., 2.66 Pa). (4) Hard, brittle Cr-N films exhibited earlier failure compared with the softer films in the scratch/ adhesion test. An inverse relationship between film hardness and adhesion was observed for the CAVD deposited Cr-N films. Acknowledgement
Fig. 10. Micrographs of failure mechanisms on M4 tool steel by (a) delamination at the side of the wear track and (b) ploughing in the center of the wear track; (a) and (b) represent a hard and soft coating, respectively.
The authors wish to thank Cheryl Blanchard of Southwest Research Institute (San Antonio, TX), Daniel Geist of Martin Lockheed Corp. (Denver, CO), Jerry Wright of Crucible Material Corp. (Syracuse, NY) and Bill Goldsworth of Integrated Vacuum Technologies (Golden, CO). All of these individuals provided invaluable support to this particular study and the overall research program. References
bias (Fig. 6) exhibits an unusual trend and is not well understood why this type of trend is observed. It is apparent, however, that the adhesion of the coating exhibits the exact opposite trend as shown in Fig. 8. The same relationship between hardness and adhesion is seen comparing Figs. 7 and 9. Thus the hardness of the coatings appears to be playing a significant role in the adhesion of the coatings to the substrate. The point of failure for both a hard coating and soft coating are shown in Fig. 10(a) and (b), respectively. In Fig. 10(a), cracking and spalling at the side of the wear track is apparent. This was typical of the failure modes occurring in the harder (> 15 GPa) Cr-N films. Fig. 10(b) illustrates delamination in the center of the wear track where the diamond stylus pierced through the coating to the substrate. This type of failure occurred in the softer (< 15 GPa) Cr-N coatings deposited on M4 tool steel.
5. Conclusions
The following general conclusions can be made from the current investigation:
[1] O. Knotek, B. Bosserhoff, A. Schrey, T. Leyendecker, O. Lemmer and S. Esser, Surf Coat. Technol., 54/55 (1992) 102-107. [2] F.W. Loftier, Surf Coat. Technol., 68/69 (1994) 729 740. [3] M. Kerkhofs, M. Van Stappen, M. D'Olieslaeger, C. Quaeyhaegens and L.M. Stals, Surf Coat. Technol., 68/69 (1994) 741-746. [4] D.E. Peebles and L.E. Pope, Thin Solid Films, 171 (1989) 171-181. [5] Y. Chiba, T. Omura and H. Ichimura, J. Mater Res., 8(5) (1993) 1109-1115. [6] J.A. Sue, A.J. Perry and J. Vetter, Surf Coat. Technol., 68/69 (1994) 126-130. [7] A.A. Ivan'ko, Handbook of Hardness Data, US Department of Commerce, Israel Program for Scientific Translation, 1971. [8] A.J. Perry, Thin Solid Films, 78 (1981) 77-93. [9] K.J. Bhansal and T.Z. Katamis, Wear, 141 (1990) 59-71. [10] R. Feder and B.S. Berry, J. Appl. Cryst., 3 (1970) 372. [11] E.L. Haase, Thin Solid Films, 124 (1985) 283. [12] D.E. Geist, A.J. Perry, J.R. Treglio, V. Valvoda and D. Rafaja, Rigaku J., 11 (1994) 9-29. [13] J.A. Sue, AJ. Perry and J. Vetter, Surf Coat. Technol., 68/69 (1994) 126. [14] R. Gahlin, M. Bromark, P. Hedenqvist, S. Hogmark and G. Hakansson, Surf Coat. Technol., 76/77 (1995) 174-180. [15] A. Ehrlich, M. Kuhn, F. Ricter and W. Hoyer, Surf Coat. Technol., 76/77 (1995) 280-286. [16] J.A. Thornton, Annual Review of Materials Science, Annual Reviews Inc., Palo Alto, CA, 1977. [17] B. Navinsek, P. Panjan and A. Cvelbar, Surf Coat. Teehnol., 74/75 (1995) 155 161.
Characterization
of hard chromium nitride coatings deposited by cathodic arc vapor deposition
W.K. Grant a,*, C. Loomis b, J.J. Moore a, D.L. Olson a, B. Mishra a, A.J. Perry ’ a Advanced Coatings and Surface Engineering Laboratory (ACSEL),
Colorado School of Mines, Golden, CO 80401, USA b Coors Brewing Co., Golden, CO 80401, USA ’ ISM Technologies, San Diego, CA, USA
Abstract Cr-N coatings, of various stoichiometries, were deposited on 304 stainless steel and M4 tool steel by planar cathodic arc vapor deposition (CAVD). This investigation concentrated on the effects of the CAVD process parameters and the resulting mechanical properties. To achieve this, several properties of the deposited films were studied; hardness, adhesion, coefficients of friction, residual stress, texture and stoichiometry. Hardness and adhesion were measured by low load Vicker’s indentation and diamond scratch testing respectively. The adhesion exhibited an inverse relationship with respect to the hardness of the Cr-N coatings. Hardness values averaged 17.7 GPa while maximum adhesion values of the Cr-N coatings exceeded 85 N using a 0.2 mm diamond stylus. Typical values for the coefficients of friction remained lower than 0.10 (diamond against Cr-N). Residual stress was measured by glancing incidence X-ray diffraction (GIXRD). Residual stress measurements indicated a transition from compressive to tensile stresses for midrange levels of both nitrogen partial pressure (e.g., 3.33 Pa) and substrate bias (e.g., - 150 V). The stoichiometry of the films (N:Cr) ranged from 0.7 to 1 for nitrogen partial pressures of 2.00 and 5.32 Pa, respectively. Keywords:
Chromium nitride; Cathodic arc
1. Introduction For several years, nitride based coatings have offered a way of improving performance and extending the life of high speed steel tooling [l-5]. Included in this category are chromium nitride coatings which have been shown to have superior impact resistance compared to other physical vapor deposited (PVD) coatings [ 1,6]. In addition, cathodic arc vapor deposition (CAVD) technology has provided a method for depositing an extremely dense, well adhered coating. It may be predicted that, in order to achieve a reliable wear barrier in impact situations, a coating with high hardness and adhesion and a low coefficient of friction would need to be deposited. Therefore, the ability to measure and understand the relationship between these properties is essential. As will be discussed, hard, brittle films were more likely to fail under a lower load than were the softer, tougher films. In optimizing deposition parameters, numerous factors need to be examined * Corresponding author. 6330 Gunpark Drive, Boulder, USA. Tel.: + 1 303 5819400; fax: + 1 303 5819700.
CO 80301,
0257-8972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PZZ SO257-8972(96)03071-X
including residual present ducting of Cr-N process
hardness, adhesion, coefficients of friction, stress, texture and stoichiometry. Thus, the study was conceived with the intention of cona comprehensive investigation of the properties coatings with respect to the CAVD coating parameters.
2. Experimental
procedure
Cr-N coatings were deposited by cathodic arc vapor deposition (CAVD) using a 127 mm (5”) circular planar cathode source. The source to substrate distance was kept constant at 200 mm. A schematic representation of the CAVD apparatus is shown in Fig. 1. The substrate materials in this investigation were M4 tool steel and 304 stainless steel. The dimensions of the M4 tool steel and 304 stainless steel substrates were 4 x 2 x 1 and 7.5 x 2.5 x 0.2 cm, respectively. The substrates were clamped to a steel plate with dimensions 10 x 8 x 0.4 cm. The substrate holder was placed on a shielded alumina plate to isolate it and the substrates
W.K. Grant et al.ISurface and Coatings Technology 86-87 (1996) 788-796 1. Catho&(source) 2. Allodc(&oulld) 3. vacllumch;lmbar 4. St&x 5. DCSlIbstrrteBiCiSSUpp~ 6. Arc Power Supply I. SuMrateHolder
Fig. 1. Schematic deposition.
representation
of CAVD
chamber
used for Cr-N
from the chamber. Actual substrate temperature was measured in a similar system under similar conditions with a type J thermocouple. Both sets of substrates were polished to an average roughness of 0.1 urn. The substrate materials underwent identical pretreatment prior to deposition, The cleaning procedures consisted of a 20 min ultrasonic acetone bath, a 20 min heated ultrasonic isopropyl alcohol bath and a 1 min nitrogen gas dry. Finally, a 10 min plasma etch in vacuum was performed using a -700 V bias (ion current density = 3.2 mA cm-‘) at a pressure of 5.98 Pa argon. A vacuum of 2.7 x 1O-6 Pa or better was achieved prior to plasma cleaning and deposition. Deposition followed directly after the d.c. plasma cleaning process. In these sets of experiments, nitrogen partial pressure, substrate bias and arc current were varied in order to study their effects on hardness, adhesion, coefficients of friction, texture, residual stress and stoichiometry. Coating thickness varied between 1.0 and 1.5 urn for a 20 min deposition. Variations in the coating thickness were attributed to changes in the coating process conditions (substrate bias, arc current and nitrogen partial pressure). Substrate temperature was not held constant during deposition and again varied with respect to the arc deposition process parameters. Microhardness was determined by Vickers indentation with a 25 g load. Each test was repeated five to seven times to ensure statistical accuracy. The reproducibility of the Vickers hardness values is reported to be + 1.5 GPa [7]. Other variations of hardness data were attributed to irregularities in the surface condition and composition of the coating (i.e., various Cr-N phases). Adhesion and friction measurements were made with a Revetest 200 single pass diamond scratch tester at
189
Southwest Research Institute in San Antonio, TX. The apparatus had a 0.2 mm conical diamond stylus. The load was increased linearly from 0 to 150 N while traversing the sample at a rate of 10 mm min-‘. The load rate was set at 100 N min-‘. The samples were placed on a goniometer which allowed visual inspection of the scratch through the entire loading range with an optical microscope mounted to the apparatus fixed with two objective lenses (i.e., 200 x and 500 x). Failure loads were determined following similar guidelines established by previous investigators [ 8,9]. Reproducibility of the failure loads was determined to be 15 N. Friction measurements were determined during the scratch test using the Revetest apparatus. Coefficients of friction were calculated from the normal and tangential loads recorded throughout the scratch test. The coefficients of friction were correlated with both the material in contact with the stylus (e.g., film or substrate) and the mechanical properties of the films (e.g., hardness). In addition to the visual inspection of the scratch test, the coefficient of friction also aided in determining the failure load of the Cr-N coatings. Auger electron spectroscopy (AES) was used to determine the N:Cr ratio in the coatings. Stoichiometries were determined from these ratios and were correlated to phase formation in the films. Glancing incidence X-ray diffraction (GIXRD) with parallel beam optics was used to study phase formation, texture and residual stress. Tests were conducted at Martin-Lockheed in Denver, CO. Diffraction patterns were acquired with Cu Kcx radiation at an accelerating voltage of 50 kV. Scan ranges were typically set from 10 to 145” 28 with a step size of 0.05”. The incident angle, y, was kept constant at 2”. In all cases, AISI Si powder standard was used to identify peak shifts occurring due to surface abnormalities. The residual stresses present in the films were determined from the plots of the lattice parameter versus sin2y, [ 10-121. The slope of this plot can be related to the stress in the film using the relationship: m = ba,( 1 + v)/E
where m is the slope of the line, g is the residual stress in the film, a, is the equilibrium lattice parameter, v is the Poisson’s ratio and E is the Young’s modulus of the film. A value of 280 GPa was used for the Young’s Modulus as determined by Sue et al. [ 131
3. Results 3.1. Stoichiometry Of particular interest with the Cr-N coating system in this study was the stoichiometry of the Cr-N system and its dependence on CAVD deposition parameters
WK. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
790
and, ultimately, how these affected the properties of the coatings. The effects of nitrogen partial pressure on the N:Cr ratio in the films deposited on 304 stainless steel is shown in Fig. 2. As expected, an increasing N:Cr ratio was observed in the films deposited at the higher nitrogen partial pressures. Note, however, that a one to one ratio of nitrogen to chromium is not reached until nitrogen partial pressure of 5.32 Pa. The ratio of nitrogen to chromium in this investigation ranged from 0.7 at a nitrogen pressure of 2.00 Pa to 1.Oat a nitrogen pressure of 5.32 Pa. 3.2. Texture, residual stress and phase formation X-ray diffraction patterns were used to study orientation and phase formation in the films. All of the films deposited had a preferred orientation of either the ( 111) or (200) planes. Transition from the (200) orientation to the (111) orientation occurred when the substrate bias was increased from - 50 to -250 V as shown in Fig. 3(a). In addition, the texture of the films was also influenced by the nitrogen partial pressure as shown in Fig. 3(b). The ratio of the (200) planes to (111) planes increased as the nitrogen partial pressure increased. The results from the residual stress analysis indicated a transition from compressive stress to tensile stress in the Cr-N films as a function of substrate bias, as shown in Fig. 4. Examination of the data presented in Fig. 4, shows that the residual stress in the film changes from
compressive to tensile when using a substrate bias of between - 150 and -200 V. A further increase in the bias to - 250 V returns the stress to a compressive value. A similar trend is observed with respect to the effect of nitrogen partial pressure as shown in Fig. 5. In this respect, tensile stresses were measured for Cr-N films deposited with a nitrogen partial pressure of between approximately 2.39-3.06 Pa. 3.3. Microhardness Microhardness values recorded for the Cr-N coatings on M4 tool steel are shown in Fig. 6. The hardness is seen to be dependent on both substrate bias and arc current. The two series of data illustrate the dependence of film hardness on arc current when varied between 125 and 60 A. As is seen, both series of data yield similar trends, however, the higher arc current coatings produced somewhat harder coatings with a maximum hardness of 22.0 GPa. The error bars in the figure represent the standard deviation from the measurements. Hardness was also influenced slightly by the nitrogen partial pressure (nitrogen content in the film) used during the CAVD process, as indicated in Fig. 7. The average hardness remained relatively constant for the films deposited with the higher arc current above a nitrogen partial pressure of 2.93 Pa. The hardness of the coatings deposited at the higher arc current did indicate slightly higher values of hardness averaging 17.2 GPa at
1
0.2
0
t
!0
I
2
3 Nitrogen
Fig. 2. Atomic
ratios of Cr-N
coatings
Pressure
on 304 stainless
4
5
6
(Pa)
steel for varying
nitrogen
partial
pressures.
W.K. Grant
et
al./Surface and Coatings Technology86-87 (1996) 788-796
CrN(lll)
791
Substrate Bias
-150 Volts B
a
10
20
30
40
50
60
70
80
90
100
110
120
130
140
Two Theta (a)
CrN (200)
N, Pressure
1
CrN (111)
’
CrN (220) CrN (311)
CrN (400) 4.00 Pa
3.33 Pa
2.66 Pa
2.00 Pa
10
20
30
40
50
70
60
80
90
100
110
120
130
140
Two Theta @) Fig. 3. X-ray diffraction patterns of Cr-N films (a) with respect to varying levels of substrate bias (constant P=4.0 Pa) showing preferred orientation of the (111) planes at high bias levels (e.g., - 250 V), and (b) with respect to varying levels of nitrogen partial pressures (constant substrate bias = - 100 V) showing preferred orientation of the (200) planes for all the pressures tested.
a pressure of 4.00 Pa. As expected the films deposited with no nitrogen exhibited the lowest hardness with averages near 11.8 GPa. 3.4. Adhesion Adhesion measurements of the Cr-N coatings deposited on M4 tool steel are shown in Fig. 8 as a function of substrate bias. Although factors such as phase formation and residual stress contribute to the adhesion load of the Cr-N coatings, an inverse trend between film adhesion and film hardness with substrate bias (compare Fig. 6 and Fig. 8) is apparent. A peak average adhesion, L,, of 80 N was measured at a substrate bias of about -150v. Adhesion values with respect to variations in nitrogen pressure are shown in Fig. 9. The adhesion values of the
films deposited with no nitrogen (i.e., Cr films) exhibit the highest adhesion with values averaging around 135 N. As nitrogen is incorporated into the coatings the adhesion values drop. A minimum adhesion value of 25 N is measured for films deposited at a nitrogen pressure of 2.93 Pa. Relatively high adhesion values were measured for films deposited at 2.00 and 5.32 Pa nitrogen partial pressures with critical loads around 85 N. The error bars in the figures represent an error of +5 N determined for the scratch adhesion tests. 3.5. Coeficients
offriction
Coefficients of friction were recorded and calculated during scratch testing from the measurements of the normal and tangential loads on the stylus. A summary of these data is given in Table 1 and Table 2 for varia-
WK. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
192 3
I
l
2
1
l
1 0 50
s 5
100
-1
-1
200
150
250
l
t tj -2 2 E -3 a p! -4 l
+
-5
I
l
Substrate Bias (-Volts) Fig. 4. Residual stress measurements of Cr-N coatings were held constant at 75 A and 30 mTorr, respectively.
on M4 tool steel as a function
of substrate
bias. Arc current
and nitrogen
partial
pressure
-1t -1.2 1 Nitrogen Pressure (Pa) Fig. 5. Residual stress measurements of Cr-N coatings were held constant at 75 A and - 125 V, respectively.
on M4 tool steel as a function
of nitrogen
partial
pressure.
Arc current
and substrate
bias
W.K Grant et al./Surface
and Coatings Technology 86-87 (1996) 788-796
193
25
1 + 60 Amperes -...
n
125 Amperes
_.____t__--+
1
0
0
50
100
200
150
250
300
Substrate Bias (-Volts) Fig. 6. Microhardness
of Cr-N
films as a function
of the substrate
bias on M4 tool steel. The nitrogen
partial
pressure
was kept constant
at 4.0 Pa.
15
-i
0, 0
Fig. 7. Microhardness
of Cr-N
1
films as a function
2
of the nitrogen
3 Nitrogen Pressure (Pa) partial
in substrate bias and nitrogen partial pressure respectively. The numbers reported in these tables indicate the coefficient of friction measured with a diamond tions
pressure
4
5
on M4 tool steel. The substrate
6
bias was kept constant
at - 125 V.
stylus against the Cr-N films at a 20 N load. If, however, the film delaminated from the substrate prior to a 20 N load the term “No Film” appears. The values for the
WX Grantetal/Suiurface andCoatings Technology 86-87(1996)788-796
794
+Arc
Current = 60 amperes
- . * - . Arc Current = 125 amperes
OL 0
50
100
150
300
250
200
Substrate Bias (-Volts)
Fig. 8. Critical load measurements with respect to varying substrate bias levels of Cr-N films on M4 tool steel. Nitrogen partial pressure was kept constant at 30 mTorr.
160
-._
+Arc
Current = 60 Amperes
- I - Arc Current = 125 Amperes
120
z^
100
2
c- 80
2 t f
60 I
40
20
0
I I
I 0
1
2
3
4
5
6
Nitrogen Pressure (Pa)
Fig. 9. Critical load measurements with respect to varying nitrogen partial pressures of Cr-N films on M4 tool steel. Substrate bias levels were held constant at - 125 V.
W.K. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
Table 1 Coefficients of friction for Cr-N films on M4 tool steel as a function of substrate bias and arc current Substrate bias (-V)
Arc current (A)
Coefficient of friction (20 N)
10 70 150 200 300 10 70 125 200 300
60 60 60 60 60 125 125 125 125 125
0.068 No Film 0.055 0.090 0.078 0.052 0.060 0.088 0.052 0.102
Films were deposited at a constant nitrogen partial pressure of 4.0 Pa. “No Film” indicates coating failure prior to a 20 N load.
Table 2 Coefficients of friction for Cr-N films on M4 tool steel as a function of nitrogen partial pressure and arc current Nitrogen pressure (Pa)
Arc current (A)
Coefficient of friction (20N)
0 2.0 3.0 5.3 0 2.0 4.0 5.3
60 60 60 60 125 125 125 125
0.112 0.068 0.085 0.087 0.128 0.083 0.087 0.075
Substrate bias levels were kept constant at - 125 V.
coefficients of friction indicated numbers between approximately 0.05 and 0.13 depending on the deposition process conditions. 3.6. Substrate temperature The substrate temperature was not measured during the actual deposition but was measured in a similar chamber under the same operating conditions. As expected, the temperature of the substrate was a function of the variations in the process conditions. For all deposition conditions, though, the substrate temperature never exceeded 300°C and, therefore, can be considered a low temperature deposition. The substrate bias and arc current were the main contributors to changes in the substrate temperatures. Maximum temperatures of 275°C were reached for a substrate bias of -250 V and 125 A. As expected, the substrate temperature decreased as the substrate bias and arc current were reduced. Substrate temperature at a substrate bias of -50 V and an arc current of 60 A was 140°C.
795
4. Discussion The data from the auger spectroscopy (Fig. 2) indicates that a stoichiometric CrN film is not deposited until a nitrogen partial pressure of 5.32 Pa is reached. Nitrogen partial pressures below 5.32 Pa yielded a nitrogen to chromium ratio of less than one. Surprisingly, the X-ray diffraction data indicate that the CrN phase is the dominant phase in all the films deposited at a nitrogen partial pressure of 2.00 Pa or greater. It is unusual that the major phase in all the coating tested was the CrN phase and little if any of the Cr,N phase was discernible noting the small range of stoichiometry where CrN is a stable phase. The fact that little Cr,N was observed from the diffraction patterns could possibly be attributed to peak shifts due to lattice strain in the coatings where further peak deconvolution would be necessary to separate the peaks. The apparent phases present (according to the AES data) in the Cr-N films, their orientation and the corresponding mechanical properties of the coatings are summarized in Table 3. The transition in the residual stress values from compressive to tensile shown in Figs. 4 and 5 is an issue of some interest. One should note that the compressive and tensile residual stress values reported in Fig. 5 are relatively low compared to other stress values reported for coatings deposited by cathodic arc methods [ 14,151. The presence of tensile stress in thin films has been attributed to void formation in the films due to the inability of the structure to relieve the strain in the direction of the voids [ 161. This trend, with respect to the nitrogen partial pressure, has been observed previously [ 16,171. The hardnesses of the films, shown in Fig. 6 and Fig. 7 vary between 11.8 and 22.6 GPa depending on the process conditions. As expected, due to the different bonding, crystallographic structure and slip systems between Cr and CrN, the Cr films (i.e., no nitrogen) exhibited the lowest hardness values. As nitrogen is incorporated in the film, the hardness increases up to a maximum located at 2.93 Pa. The ratio of nitrogen to chromium at this level is approximately 0.8 and, therefore, these films most likely contain some combination of CrN and Cr,N. The hardness with respect to variations in substrate Table 3 Phase formation of the CAVD Cr-N corresponding mechanical properties
on M4 tool steel and the
Film
Nitrogen pressure (Pa)
Hardness @Pa)
Adhesion (N)
Cr CrN(lll)+Cr,N CrN(200) + Cr,N CrN(200)
0 2.0 3.0 5.3
12.3 13.7 16.2 13.2
150 80 25 92
Mechanical properties reported are for films deposited at - 150 V substrate bias and 60 ampere arc current.
796
W.K. Grant et al./Surface and Coatings Technology 86-87 (1996) 788-796
(1) Stoichiometries of the films (N:Cr) varied between 0.7 and 1.0 depending on the nitrogen partial pressure which was varied between 2.0 and 5.32 Pa. (2) The preferred orientation of the CrN present in the films changed from (200) to (111) on increasing substrate bias. The preferred orientation with respect to changes in the nitrogen partial pressure remained (200) for the ranges tested. An increase of the ratio of (111) to (200) planes was observed for lower nitrogen partial pressures. (3) Tensile stresses were present in Cr-N films deposited at intermediate substrate biases (e.g., - 1 5 0 to - 2 2 0 V) and intermediate nitrogen partial pressures (e.g., 2.66 Pa). (4) Hard, brittle Cr-N films exhibited earlier failure compared with the softer films in the scratch/ adhesion test. An inverse relationship between film hardness and adhesion was observed for the CAVD deposited Cr-N films. Acknowledgement
Fig. 10. Micrographs of failure mechanisms on M4 tool steel by (a) delamination at the side of the wear track and (b) ploughing in the center of the wear track; (a) and (b) represent a hard and soft coating, respectively.
The authors wish to thank Cheryl Blanchard of Southwest Research Institute (San Antonio, TX), Daniel Geist of Martin Lockheed Corp. (Denver, CO), Jerry Wright of Crucible Material Corp. (Syracuse, NY) and Bill Goldsworth of Integrated Vacuum Technologies (Golden, CO). All of these individuals provided invaluable support to this particular study and the overall research program. References
bias (Fig. 6) exhibits an unusual trend and is not well understood why this type of trend is observed. It is apparent, however, that the adhesion of the coating exhibits the exact opposite trend as shown in Fig. 8. The same relationship between hardness and adhesion is seen comparing Figs. 7 and 9. Thus the hardness of the coatings appears to be playing a significant role in the adhesion of the coatings to the substrate. The point of failure for both a hard coating and soft coating are shown in Fig. 10(a) and (b), respectively. In Fig. 10(a), cracking and spalling at the side of the wear track is apparent. This was typical of the failure modes occurring in the harder (> 15 GPa) Cr-N films. Fig. 10(b) illustrates delamination in the center of the wear track where the diamond stylus pierced through the coating to the substrate. This type of failure occurred in the softer (< 15 GPa) Cr-N coatings deposited on M4 tool steel.
5. Conclusions
The following general conclusions can be made from the current investigation:
[1] O. Knotek, B. Bosserhoff, A. Schrey, T. Leyendecker, O. Lemmer and S. Esser, Surf Coat. Technol., 54/55 (1992) 102-107. [2] F.W. Loftier, Surf Coat. Technol., 68/69 (1994) 729 740. [3] M. Kerkhofs, M. Van Stappen, M. D'Olieslaeger, C. Quaeyhaegens and L.M. Stals, Surf Coat. Technol., 68/69 (1994) 741-746. [4] D.E. Peebles and L.E. Pope, Thin Solid Films, 171 (1989) 171-181. [5] Y. Chiba, T. Omura and H. Ichimura, J. Mater Res., 8(5) (1993) 1109-1115. [6] J.A. Sue, A.J. Perry and J. Vetter, Surf Coat. Technol., 68/69 (1994) 126-130. [7] A.A. Ivan'ko, Handbook of Hardness Data, US Department of Commerce, Israel Program for Scientific Translation, 1971. [8] A.J. Perry, Thin Solid Films, 78 (1981) 77-93. [9] K.J. Bhansal and T.Z. Katamis, Wear, 141 (1990) 59-71. [10] R. Feder and B.S. Berry, J. Appl. Cryst., 3 (1970) 372. [11] E.L. Haase, Thin Solid Films, 124 (1985) 283. [12] D.E. Geist, A.J. Perry, J.R. Treglio, V. Valvoda and D. Rafaja, Rigaku J., 11 (1994) 9-29. [13] J.A. Sue, AJ. Perry and J. Vetter, Surf Coat. Technol., 68/69 (1994) 126. [14] R. Gahlin, M. Bromark, P. Hedenqvist, S. Hogmark and G. Hakansson, Surf Coat. Technol., 76/77 (1995) 174-180. [15] A. Ehrlich, M. Kuhn, F. Ricter and W. Hoyer, Surf Coat. Technol., 76/77 (1995) 280-286. [16] J.A. Thornton, Annual Review of Materials Science, Annual Reviews Inc., Palo Alto, CA, 1977. [17] B. Navinsek, P. Panjan and A. Cvelbar, Surf Coat. Teehnol., 74/75 (1995) 155 161.