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Deposition of arc TiAlN coatings with pulsed bias
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/[£H#OiOM ELSEVIER
Surface and Coatings Technology76-77 (1995) 700-705
Deposition of arc TiA1N coatings with pulsed bias E. Lugscheider, O. Knotek, F. L6ffter, C. Barimani, S. Guerreiro, H. Zimmermann Aachen University of Technology, Materials Science Institute, 52056 Aachen, Germany
Abstract
It is a well-known fact that the aluminium content of TiA1N coatings deposited with the arc physical vapour deposition (PVD) process depends mainly on substrate potential and source-to-substrate distance. To achieve good results in cutting operations with TiA1N-coated tools with a low aluminium content in the film, it is necessary to deposit TiA1N with a relatively high bias voltage which raises the substrate temperature to a level which can cause some damage to the structure of even high speed steel substrates. For high performance cutting operations with TiA1N thin films a high and homogeneous aluminium content in the films, especially on cutting edges, is necessary. Higher aluminium content in arc PVD thin films is achieved with lower bias voltage during deposition which in turn lowers the deposition temperature and consequently enables heat-sensitive substrates to be coated; however, for good adhesion of the deposited films a high bias voltage is required. The application of a pulse bias generator instead of a d.c. bias offers the possibility to decrease the deposition temperature and to obtain more aluminium in the coating. In this paper we compare coating properties of TiAIN deposited with d.c. and pulsed bias source. With pulsed bias it is possible to achieve higher aluminium content in the coating, especially on cutting edges. Analysis of increase in aluminium on cutting edges was carried out by energy-dispersive X-ray analysis.
Keywords: Deposition; Arc; PVD; TiAIN coatings; Pulsed bias voltage
1. Introduction
2. Experimental details
Physical vapour deposition (PVD) processes such as magnetron sputter ion plating or cathodic arc evaporation are increasingly used for the deposition of wearand corrosion-resistant coatings on tools and various components [ 1-3]. The cathodic arc process is especially suitable for the deposition of nitrides and carbides in industry (e.g. TiN, CrN, TiA1N, TiCN) of a wide variety of metals and alloys. Because of the high degree of ionization achieved in the arc evaporated material, the plasma can be easily controlled using a negative substrate bias potential. The benefits of the arc ion plating technique include good coating adhesion and an improved uniformity of deposition on complicated substrate shapes [4]. Thus far not all potential advantages of the use of a bias in a cathodic arc coating process have been fully realized. The application of the cathodic arc deposition of TiN on high speed steel (HSS) plates and drilling tools with a pulsed bias voltage has been reported [5]. In this paper it will be shown that the pulse bias technique enables control of the metal composition in the coating and at the same time good film properties to be achieved. With a pulsed bias voltage it is possible to deposit adherent coatings on heat-sensitive substrates [6].
The depositions were carried out in a Multi Arc PVD 20 in system equipped with a random arc source. The experimental set-up is shown in Fig. 1. TiA1 targets with 50 : 50 (atomic per cent) composition were vaporized. As a modification to the original system, a bipolarly pulsed bias power supply (Eifeler Bipulsar 1000/20), the unipolar mode was used which provides pulses of maximum voltages in the range from 0 to - 1000 V and frequencies from 0 up to 33 kHz. Fig. 2 shows a typical voltage-time diagram of a bipolarly pulsed bias. The Bipulsar 1000/20 can also be used in unipolar or d.c. mode. The pulse bias power supply was only used in unipolar mode. The etching process was carried out also in the unipolar mode. The duty cycle r is the ratio between the time the bias is on to the total time of a pulse cycle. Charged particles are accelerated to the substrates only during the time the bias is on. The arc process is better suited for the application of a pulsed bias source because of its high ionization capability. In the presented investigations only positive ions were accelerated to the substrates owing to the negative voltage applied to them. The duty cycles used
Elsevier ScienceS.A.
E. Lugscheider et al./Surface and Coatings Technology 76-77 (1995) 700-705
701
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N:, CH,, C:H: etc. UB= 0to-1000 v
Uv=-30 V, Iv= 0 to 100A
Fig. 1. Experimental set-up.
BIPOLARILY PULSED BIAS VOLTAGE
uI t o m i ton~
u ] unipolarily I I I
U U
I DC
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U = 0 - 1000 V f-- 0,33 - 33 kHz or DC t= 5 bts- 3 ms 25 kHz ~ 40 bts 2,5 kHz ~ 400 p.s
Fig. 2. VoItage-time diagram of a bipolarly pulsed bias.
in the coating processes were varied in a range from 15% to 100%. A duty cycle of 100% corresponds to the d.c. mode. Hard metals (HMs, application group K 10) and HSS (HSS M2) substrates with similar shape and size with surface roughness Rz=0.3 lzm were used to be coated with TiA1N. Prior to deposition, the substrates were ultrasonically cleaned in a series of aqueous detergent solutions. The substrates were further cleaned in situ by argon ion etching (pAr=2 Pa). In a second etching step the substrates were bombarded with arc-evaporated titanium and aluminium ions. The bias voltage during both etching phases was -1000 V. The ion bombardment was not continuous, depending on the pulse rate. The substrate temperature was recorded with an IR pyrometer. The deposition time for the TiA1N coatings was chosen to achieve similar coating thicknesses
(around 4 gm) on all substrates. The distance between source and substrates was 190 ram.
3. Results and discussion The effect of a d.c. and a pulse bias source on the films properties was investigated with different bias voltages for the coatings produced with d.c. bias source and, in the case when a pulse bias source was used, with different duty cycles. An evaporation current of 70 A was used for all TiA1N deposited coatings during the heating ion bombardment and deposition phases. The applied bias voltage during the heating phase was - 1000 V for all deposited coatings.
702
E Lugscheider et aI /Swface and Coatings Technology 76-77 (1995) 700-705
3.1. Coating adhesion." critical load L c
The adhesion of the TiA1N films was evaluated by scratch testing with an LSRH Revetest instrument. The critical load Lo is the maximum load with which the film can be scratched without adhesive or cohesive failure. The influence of the applied bias voltage on the films deposited with a d.c. bias source is shown in Fig. 3 (a). A maximum Lo of 70 N was achieved for the coating deposited on HM and 40 N for the that deposited on HSS, both deposited with a d.c. bias voltage of - 2 0 0 V. Higher and lower d.c. bias voltage resulted in lower values for Lo. When using a pulse bias source it is possible to deposit with higher bias voltage and still to achieve good results in terms of adhesion as can be seen in Fig. 3(b). The 80 duty cycle I00 % . . ! . . Ps:~--2"pa J Iv= 70 A ~
~" 60
I ~+__~Si
,..a
4O 2o
100 (a)
200 300 Bias voltage UB [VI
400
80 lb = -300 V pN2=RPa Iv=70A
~" 60
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~-] I ..-'~- I I ttss I
40 20 0
0
25
(b)
50 Duty cycle r [%1
100
75
80 UB= -200 V //! 60 Duty cycle 1 0 0 % / Iv=70A ,d 0
40 20
~s l (c)
2 3 Partial pressure p~2 [Pal
Fig. 3. Critical load as a function of (a) bias voltage with a duty cycle of 100%, (b) duty cycle and (c) nitrogen partial pressure with a duty cycle of 100%.
coatings deposited with pulse bias voltage were heated with the same duty cycle of the deposition phase. The decrease in Lo for the coating deposited with 75% duty cycle on the HM substrate was accompanied by a total loss of adhesion of the coating deposited on the HSS substrate, with the coating peeling off, exhibiting characteristics of high internal stresses [7]. Therefore, the Lo and Vickers hardness values for the HSS substrate could not be presented for the 75% duty cycle. Using a bias voltage of -300 V, the maximum Lo achieved was 50 N for the coating deposited on HM substrates which is higher than that for deposition with a - 3 0 0 V d.c. bias source. As can be seen in Fig. 3(c), for deposition with a d.c. bias source, there was no increase in Lo for deposition with a higher N2 partial pressure than 2 Pa while for deposition with a pressure lower than 2 Pa the critical load is drastically influenced. 3.2. Microhardness: Vickers hardness
Microhardness measurements were carried out with Buehler equipment which has a screen in which the indentation can be easily measured. The N2 partial pressure was again investigated for the coatings deposited with d.c. bias voltage and the results are presented in Fig. 4(a). A similar tendency to that of the adhesion measurements is found for microhardness for pressures higher than 2 Pa. The relative increase in microhardness for deposition with 0.5 Pa of N2 is accompanied by a loss of adhesion (Fig. 3(c)). The microhardness is not strongly influenced by the bias voltage for deposition with the d.c. bias source, as can be seen in Fig. 4(b). It is not strongly influenced by the duty cycle used for deposition with a pulse bias voltage of -300 V and 2 Pa (Fig. 4(c)). The microhardness of the coatings deposited with 15% and 25% duty cycles could n o t be measured owing to the surface quality of the coatings. Comparing the Vickers hardness value of the coating deposited with a - 300 V d.c. bias with that of the coating deposited with the same voltage but with a duty cycle of 50%, one can realize that there is only a slight tendency of decreasing microhardness for deposition with a pulse bias source. 3.3. Deposition temperature
The initial deposition temperature was the same for all deposited coatings. However, the time to achieve it is strongly dependent on the duty cycle used during the heating phase, as a consequence of the reduced time the bias by which the ions are accelerated to the substrate is on. Fig. 5(a) shows the ion bombardment time needed to heat the substrates up to 480 °C in relation to the duty cycle used to ion bombard the substrates during the heating phase. As expected, it takes longer to heat
E. Lugscheider et al,/Surface and Coatings Technology 76-77 (1995) 700-705 3.000 t~
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thus a poor surface quality as shown on the scanning electron microscopy (SEM) photographs in Fig. 6.
3.4. Energy-dispersive X-ray analyses 50
75 Duty cycle r [%1
100
Fig. 4. Microhardness as a function of (a) nitrogen partial pressure with a duty cycle of 100%, (b) bias voltage with a duty cycle of 100% and (c) duty cycle.
the substrates when bombarding them with a lower duty cycle. The rough surfaces, shown in Fig. 6, may be caused by the long ion bombardment time [8]. The much longer heating time achieved with a duty cycle of 15% has consequently promoted a poor surface quality in terms of homogeneity of the coating. It is well known that droplet emissions are stronger when no reactive gas is present [9]. Comparing Figs. 5(a) and 5(b), it is possible to realize the effect of a very low duty cycle during deposition. Fig. 5(b) shows the final deposition temperature for the coatings produced with different duty cycles and for that deposited with a 300 V d.c. bias source. The low deposition temperature found for the duty cycles of 15% and 25% can reduce the surface diffusion of the incoming ions, contributing
The samples coated with a -100 V d.c. bias voltage exhibited an A1 content close to those of samples coated with -300 V pulsed with a duty cycle of 25%, but the coating deposited with a -100 V d.c. bias source displayed poor adhesion as a result of high internal stresses related to the high A1 content achieved I7]. The~results of At content measured by energy-dispersive X-ray (EDX) analysis are shown in Fig. 7. The maximum A1 content was found for the coating produced with a 25% duty cycle but, on looking at the SEM picture shown in Fig. 6, it becomes clear that this coating is not acceptable owing to its high roughness. Through optical analysis it is also possible to compare the homogeneity of the A1 content throughout the whole coating including the edges. Fig. 8 shows the coated HM samples; there is a colour difference between the d.c. coated samples. This can be seen by comparing the coating deposited with - 100 V and that deposited with -300 V. Some colour difference is also found between the edges and the centre of the coating produced with a 300 V d.c. bias source as consequence of the sputtering effect of deposited A1 ions promoted by the incoming
704
E. Lugscheideret al./Smfaee and Coatings Technology 76-77 (1995) 700-705
50 _
4O
"~ •
3O
+ .< 20 ~0!
U~ = -300 V pN: = 2 Pa Iv = 70 A HM substrate
0
25
50 Duty cycle r [%]
75
100
Fig. 7. Aluminium content as a function of duty cycle.
yellowish colour than the centre. The relatively low A1 content on the cutting edges is related to the bias source applied to the substrate [7]. A similar colour difference is found when comparing the coatings produced with duty cycles of 50% and 75%. The coating produced with a 75% duty cycle is darker than that produced with the same bias voltage in a d.c. mode, which exhibits a slight difference between edges and the centre; the coating produced with 50% duty cycle is darker than that produced with 75% and the edges and the centre have the same colour. This was confirmed by the EDX analysis carried out, where no difference between the A1 contents on the edges and on the centre for the sample coated with 50% duty cycle was found.
3.5. Scanning microscopy analysis SEM analysis of the coating surfaces revealed the influence of a longer heating phase and the deposition temperature on the surface quality. The poorest surface was found for the coating deposited with the lowest duty cycle used, 15%. This was produced with the longest heating time and the lowest deposition temperature, as a consequence of the low energy incoming ions. The surface morphology for this coating showed that there were not enough energy for surface diffusion for the depositing ions. The condensation conditions for the incoming ions seem to be better when using higher duty cycles; this can be seen comparing the SEM photographs shown in Fig. 6. The surface is smoother for higher duty cycles.
Fig. 6. SEM photographs of the TiA1N films deposited with different duty cycles.
heavier Ti ions. Some colour difference is also found between the edges and the centre of the coating deposited with a - 100 V d.c. bias source; the edges have a more
4. C o n c l u s i o n s
The possibility of achieving higher AI contents in TiAIN coatings deposited with a pulse bias source with arc evaporation was confirmed even on the edges of the coated samples. A TiA1N coating deposited with a duty cycle of 75% on an HSS substrate is highly stressed and
E. Lugscheider et al./Smfaee and Coatings Technology 76-77 (1995) 700-705
705
r = 2 5 % [1 r=50% -100 V dc!J
-400 V dc I
r=75%
Fig. 8. Colour differenceof the deposited filmsfor various duty cycles. has very poor adhesion, while when deposited on an H M substrate the film adheres but still exhibits low Lo. The duty cycle of 50% produced a higher Lo for deposition under - 300 V bias, 70 A evaporation current and 2 Pa N2 partial pressure. The microhardness of the coatings appears to be affected by variations in pulse bias voltage. The deposition temperature decreases, as expected, with decreasing duty cycle, with longer ion bombardment heating times for lower duty cycles. EDX analysis carried out on the edges and on the centre of the coatings produced with a 50% duty cycle showed that there was little difference in A1 content between them. With SEM analysis a roughening of the films deposited with 15% and 25% duty cycles was found, which can be explained by the too low energy of incoming ions during deposition. With the 50% duty cycle the surface of the film becomes acceptable and comparable with that obtained with a d.c. bias voltage. It appears to be possible that better properties and
surface quality can be achieved when depositing with a low duty cycle if the heating phase can be carried out by other means by which the substrates do not have to be ion bombarded for too long.
References [1] J. Vetter and A.J. Perry, Swf. Coat. Technol., 61 (1993) 305. [2] O. Knotek, W.D. Manz and T. Leyendecker,J. Vac. Sci. Technol. A, 5 (4)(1987) 2173. [3] O. Knotek, F. L~Sfl~erand G. Kr~imer,Surf. Coat. TechnoI., 49 (1991) 325. [4] P.A. Lindfors, W.M. Mutarie and G.K. Wehner, Swf. Coat. TeehnoI., 29 (1986) 275. ['5] J. Fessmann, W. Olbrich, G. Kampschulte and J. Ebberink, Mater. Sci. Eng., A104 (1990) 830. [6] R. Gr~in,German Patent DE 37 O0 633 C1, January 12, 1987. [7] H.G. Prengel, VDI-Fortschrittberichte, Reihe 5, No. 205, VDI, Dasseldorf, 1990. [8] O.A. Johansen, LH. Dondje and L.D. Zenner, Thin Solid Films, I53 (1987) 75. [9] J. Vyskociland J. Musil, Surf. Coat. Technol., 43-44 (1990) 299.
/[£H#OiOM ELSEVIER
Surface and Coatings Technology76-77 (1995) 700-705
Deposition of arc TiA1N coatings with pulsed bias E. Lugscheider, O. Knotek, F. L6ffter, C. Barimani, S. Guerreiro, H. Zimmermann Aachen University of Technology, Materials Science Institute, 52056 Aachen, Germany
Abstract
It is a well-known fact that the aluminium content of TiA1N coatings deposited with the arc physical vapour deposition (PVD) process depends mainly on substrate potential and source-to-substrate distance. To achieve good results in cutting operations with TiA1N-coated tools with a low aluminium content in the film, it is necessary to deposit TiA1N with a relatively high bias voltage which raises the substrate temperature to a level which can cause some damage to the structure of even high speed steel substrates. For high performance cutting operations with TiA1N thin films a high and homogeneous aluminium content in the films, especially on cutting edges, is necessary. Higher aluminium content in arc PVD thin films is achieved with lower bias voltage during deposition which in turn lowers the deposition temperature and consequently enables heat-sensitive substrates to be coated; however, for good adhesion of the deposited films a high bias voltage is required. The application of a pulse bias generator instead of a d.c. bias offers the possibility to decrease the deposition temperature and to obtain more aluminium in the coating. In this paper we compare coating properties of TiAIN deposited with d.c. and pulsed bias source. With pulsed bias it is possible to achieve higher aluminium content in the coating, especially on cutting edges. Analysis of increase in aluminium on cutting edges was carried out by energy-dispersive X-ray analysis.
Keywords: Deposition; Arc; PVD; TiAIN coatings; Pulsed bias voltage
1. Introduction
2. Experimental details
Physical vapour deposition (PVD) processes such as magnetron sputter ion plating or cathodic arc evaporation are increasingly used for the deposition of wearand corrosion-resistant coatings on tools and various components [ 1-3]. The cathodic arc process is especially suitable for the deposition of nitrides and carbides in industry (e.g. TiN, CrN, TiA1N, TiCN) of a wide variety of metals and alloys. Because of the high degree of ionization achieved in the arc evaporated material, the plasma can be easily controlled using a negative substrate bias potential. The benefits of the arc ion plating technique include good coating adhesion and an improved uniformity of deposition on complicated substrate shapes [4]. Thus far not all potential advantages of the use of a bias in a cathodic arc coating process have been fully realized. The application of the cathodic arc deposition of TiN on high speed steel (HSS) plates and drilling tools with a pulsed bias voltage has been reported [5]. In this paper it will be shown that the pulse bias technique enables control of the metal composition in the coating and at the same time good film properties to be achieved. With a pulsed bias voltage it is possible to deposit adherent coatings on heat-sensitive substrates [6].
The depositions were carried out in a Multi Arc PVD 20 in system equipped with a random arc source. The experimental set-up is shown in Fig. 1. TiA1 targets with 50 : 50 (atomic per cent) composition were vaporized. As a modification to the original system, a bipolarly pulsed bias power supply (Eifeler Bipulsar 1000/20), the unipolar mode was used which provides pulses of maximum voltages in the range from 0 to - 1000 V and frequencies from 0 up to 33 kHz. Fig. 2 shows a typical voltage-time diagram of a bipolarly pulsed bias. The Bipulsar 1000/20 can also be used in unipolar or d.c. mode. The pulse bias power supply was only used in unipolar mode. The etching process was carried out also in the unipolar mode. The duty cycle r is the ratio between the time the bias is on to the total time of a pulse cycle. Charged particles are accelerated to the substrates only during the time the bias is on. The arc process is better suited for the application of a pulsed bias source because of its high ionization capability. In the presented investigations only positive ions were accelerated to the substrates owing to the negative voltage applied to them. The duty cycles used
Elsevier ScienceS.A.
E. Lugscheider et al./Surface and Coatings Technology 76-77 (1995) 700-705
701
THE CATHODIC ARC PVD PROCESS /5
4
/6
Jn
1
"
7
8
/
10
t
/
1 2 3 4 5 6 7 8 9 10
•
cathodeholder permanent magnet target insulator chamberwall cathodeshield arc ionized metal vapour substrate substrate holder
N:, CH,, C:H: etc. UB= 0to-1000 v
Uv=-30 V, Iv= 0 to 100A
Fig. 1. Experimental set-up.
BIPOLARILY PULSED BIAS VOLTAGE
uI t o m i ton~
u ] unipolarily I I I
U U
I DC
t
U = 0 - 1000 V f-- 0,33 - 33 kHz or DC t= 5 bts- 3 ms 25 kHz ~ 40 bts 2,5 kHz ~ 400 p.s
Fig. 2. VoItage-time diagram of a bipolarly pulsed bias.
in the coating processes were varied in a range from 15% to 100%. A duty cycle of 100% corresponds to the d.c. mode. Hard metals (HMs, application group K 10) and HSS (HSS M2) substrates with similar shape and size with surface roughness Rz=0.3 lzm were used to be coated with TiA1N. Prior to deposition, the substrates were ultrasonically cleaned in a series of aqueous detergent solutions. The substrates were further cleaned in situ by argon ion etching (pAr=2 Pa). In a second etching step the substrates were bombarded with arc-evaporated titanium and aluminium ions. The bias voltage during both etching phases was -1000 V. The ion bombardment was not continuous, depending on the pulse rate. The substrate temperature was recorded with an IR pyrometer. The deposition time for the TiA1N coatings was chosen to achieve similar coating thicknesses
(around 4 gm) on all substrates. The distance between source and substrates was 190 ram.
3. Results and discussion The effect of a d.c. and a pulse bias source on the films properties was investigated with different bias voltages for the coatings produced with d.c. bias source and, in the case when a pulse bias source was used, with different duty cycles. An evaporation current of 70 A was used for all TiA1N deposited coatings during the heating ion bombardment and deposition phases. The applied bias voltage during the heating phase was - 1000 V for all deposited coatings.
702
E Lugscheider et aI /Swface and Coatings Technology 76-77 (1995) 700-705
3.1. Coating adhesion." critical load L c
The adhesion of the TiA1N films was evaluated by scratch testing with an LSRH Revetest instrument. The critical load Lo is the maximum load with which the film can be scratched without adhesive or cohesive failure. The influence of the applied bias voltage on the films deposited with a d.c. bias source is shown in Fig. 3 (a). A maximum Lo of 70 N was achieved for the coating deposited on HM and 40 N for the that deposited on HSS, both deposited with a d.c. bias voltage of - 2 0 0 V. Higher and lower d.c. bias voltage resulted in lower values for Lo. When using a pulse bias source it is possible to deposit with higher bias voltage and still to achieve good results in terms of adhesion as can be seen in Fig. 3(b). The 80 duty cycle I00 % . . ! . . Ps:~--2"pa J Iv= 70 A ~
~" 60
I ~+__~Si
,..a
4O 2o
100 (a)
200 300 Bias voltage UB [VI
400
80 lb = -300 V pN2=RPa Iv=70A
~" 60
e ..
~-] I ..-'~- I I ttss I
40 20 0
0
25
(b)
50 Duty cycle r [%1
100
75
80 UB= -200 V //! 60 Duty cycle 1 0 0 % / Iv=70A ,d 0
40 20
~s l (c)
2 3 Partial pressure p~2 [Pal
Fig. 3. Critical load as a function of (a) bias voltage with a duty cycle of 100%, (b) duty cycle and (c) nitrogen partial pressure with a duty cycle of 100%.
coatings deposited with pulse bias voltage were heated with the same duty cycle of the deposition phase. The decrease in Lo for the coating deposited with 75% duty cycle on the HM substrate was accompanied by a total loss of adhesion of the coating deposited on the HSS substrate, with the coating peeling off, exhibiting characteristics of high internal stresses [7]. Therefore, the Lo and Vickers hardness values for the HSS substrate could not be presented for the 75% duty cycle. Using a bias voltage of -300 V, the maximum Lo achieved was 50 N for the coating deposited on HM substrates which is higher than that for deposition with a - 3 0 0 V d.c. bias source. As can be seen in Fig. 3(c), for deposition with a d.c. bias source, there was no increase in Lo for deposition with a higher N2 partial pressure than 2 Pa while for deposition with a pressure lower than 2 Pa the critical load is drastically influenced. 3.2. Microhardness: Vickers hardness
Microhardness measurements were carried out with Buehler equipment which has a screen in which the indentation can be easily measured. The N2 partial pressure was again investigated for the coatings deposited with d.c. bias voltage and the results are presented in Fig. 4(a). A similar tendency to that of the adhesion measurements is found for microhardness for pressures higher than 2 Pa. The relative increase in microhardness for deposition with 0.5 Pa of N2 is accompanied by a loss of adhesion (Fig. 3(c)). The microhardness is not strongly influenced by the bias voltage for deposition with the d.c. bias source, as can be seen in Fig. 4(b). It is not strongly influenced by the duty cycle used for deposition with a pulse bias voltage of -300 V and 2 Pa (Fig. 4(c)). The microhardness of the coatings deposited with 15% and 25% duty cycles could n o t be measured owing to the surface quality of the coatings. Comparing the Vickers hardness value of the coating deposited with a - 300 V d.c. bias with that of the coating deposited with the same voltage but with a duty cycle of 50%, one can realize that there is only a slight tendency of decreasing microhardness for deposition with a pulse bias source. 3.3. Deposition temperature
The initial deposition temperature was the same for all deposited coatings. However, the time to achieve it is strongly dependent on the duty cycle used during the heating phase, as a consequence of the reduced time the bias by which the ions are accelerated to the substrate is on. Fig. 5(a) shows the ion bombardment time needed to heat the substrates up to 480 °C in relation to the duty cycle used to ion bombard the substrates during the heating phase. As expected, it takes longer to heat
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.e
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25
50 Duty cycle r [%1
75
100
Fig. 5. (a) Heating time and (b) final deposition temperature as functions of duty cycle.
2,000
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thus a poor surface quality as shown on the scanning electron microscopy (SEM) photographs in Fig. 6.
3.4. Energy-dispersive X-ray analyses 50
75 Duty cycle r [%1
100
Fig. 4. Microhardness as a function of (a) nitrogen partial pressure with a duty cycle of 100%, (b) bias voltage with a duty cycle of 100% and (c) duty cycle.
the substrates when bombarding them with a lower duty cycle. The rough surfaces, shown in Fig. 6, may be caused by the long ion bombardment time [8]. The much longer heating time achieved with a duty cycle of 15% has consequently promoted a poor surface quality in terms of homogeneity of the coating. It is well known that droplet emissions are stronger when no reactive gas is present [9]. Comparing Figs. 5(a) and 5(b), it is possible to realize the effect of a very low duty cycle during deposition. Fig. 5(b) shows the final deposition temperature for the coatings produced with different duty cycles and for that deposited with a 300 V d.c. bias source. The low deposition temperature found for the duty cycles of 15% and 25% can reduce the surface diffusion of the incoming ions, contributing
The samples coated with a -100 V d.c. bias voltage exhibited an A1 content close to those of samples coated with -300 V pulsed with a duty cycle of 25%, but the coating deposited with a -100 V d.c. bias source displayed poor adhesion as a result of high internal stresses related to the high A1 content achieved I7]. The~results of At content measured by energy-dispersive X-ray (EDX) analysis are shown in Fig. 7. The maximum A1 content was found for the coating produced with a 25% duty cycle but, on looking at the SEM picture shown in Fig. 6, it becomes clear that this coating is not acceptable owing to its high roughness. Through optical analysis it is also possible to compare the homogeneity of the A1 content throughout the whole coating including the edges. Fig. 8 shows the coated HM samples; there is a colour difference between the d.c. coated samples. This can be seen by comparing the coating deposited with - 100 V and that deposited with -300 V. Some colour difference is also found between the edges and the centre of the coating produced with a 300 V d.c. bias source as consequence of the sputtering effect of deposited A1 ions promoted by the incoming
704
E. Lugscheideret al./Smfaee and Coatings Technology 76-77 (1995) 700-705
50 _
4O
"~ •
3O
+ .< 20 ~0!
U~ = -300 V pN: = 2 Pa Iv = 70 A HM substrate
0
25
50 Duty cycle r [%]
75
100
Fig. 7. Aluminium content as a function of duty cycle.
yellowish colour than the centre. The relatively low A1 content on the cutting edges is related to the bias source applied to the substrate [7]. A similar colour difference is found when comparing the coatings produced with duty cycles of 50% and 75%. The coating produced with a 75% duty cycle is darker than that produced with the same bias voltage in a d.c. mode, which exhibits a slight difference between edges and the centre; the coating produced with 50% duty cycle is darker than that produced with 75% and the edges and the centre have the same colour. This was confirmed by the EDX analysis carried out, where no difference between the A1 contents on the edges and on the centre for the sample coated with 50% duty cycle was found.
3.5. Scanning microscopy analysis SEM analysis of the coating surfaces revealed the influence of a longer heating phase and the deposition temperature on the surface quality. The poorest surface was found for the coating deposited with the lowest duty cycle used, 15%. This was produced with the longest heating time and the lowest deposition temperature, as a consequence of the low energy incoming ions. The surface morphology for this coating showed that there were not enough energy for surface diffusion for the depositing ions. The condensation conditions for the incoming ions seem to be better when using higher duty cycles; this can be seen comparing the SEM photographs shown in Fig. 6. The surface is smoother for higher duty cycles.
Fig. 6. SEM photographs of the TiA1N films deposited with different duty cycles.
heavier Ti ions. Some colour difference is also found between the edges and the centre of the coating deposited with a - 100 V d.c. bias source; the edges have a more
4. C o n c l u s i o n s
The possibility of achieving higher AI contents in TiAIN coatings deposited with a pulse bias source with arc evaporation was confirmed even on the edges of the coated samples. A TiA1N coating deposited with a duty cycle of 75% on an HSS substrate is highly stressed and
E. Lugscheider et al./Smfaee and Coatings Technology 76-77 (1995) 700-705
705
r = 2 5 % [1 r=50% -100 V dc!J
-400 V dc I
r=75%
Fig. 8. Colour differenceof the deposited filmsfor various duty cycles. has very poor adhesion, while when deposited on an H M substrate the film adheres but still exhibits low Lo. The duty cycle of 50% produced a higher Lo for deposition under - 300 V bias, 70 A evaporation current and 2 Pa N2 partial pressure. The microhardness of the coatings appears to be affected by variations in pulse bias voltage. The deposition temperature decreases, as expected, with decreasing duty cycle, with longer ion bombardment heating times for lower duty cycles. EDX analysis carried out on the edges and on the centre of the coatings produced with a 50% duty cycle showed that there was little difference in A1 content between them. With SEM analysis a roughening of the films deposited with 15% and 25% duty cycles was found, which can be explained by the too low energy of incoming ions during deposition. With the 50% duty cycle the surface of the film becomes acceptable and comparable with that obtained with a d.c. bias voltage. It appears to be possible that better properties and
surface quality can be achieved when depositing with a low duty cycle if the heating phase can be carried out by other means by which the substrates do not have to be ion bombarded for too long.
References [1] J. Vetter and A.J. Perry, Swf. Coat. Technol., 61 (1993) 305. [2] O. Knotek, W.D. Manz and T. Leyendecker,J. Vac. Sci. Technol. A, 5 (4)(1987) 2173. [3] O. Knotek, F. L~Sfl~erand G. Kr~imer,Surf. Coat. TechnoI., 49 (1991) 325. [4] P.A. Lindfors, W.M. Mutarie and G.K. Wehner, Swf. Coat. TeehnoI., 29 (1986) 275. ['5] J. Fessmann, W. Olbrich, G. Kampschulte and J. Ebberink, Mater. Sci. Eng., A104 (1990) 830. [6] R. Gr~in,German Patent DE 37 O0 633 C1, January 12, 1987. [7] H.G. Prengel, VDI-Fortschrittberichte, Reihe 5, No. 205, VDI, Dasseldorf, 1990. [8] O.A. Johansen, LH. Dondje and L.D. Zenner, Thin Solid Films, I53 (1987) 75. [9] J. Vyskociland J. Musil, Surf. Coat. Technol., 43-44 (1990) 299.