- Email: [email protected]
TiAlN coatings using unbalanced magnetron sputtering
Surface and Coatings Technology 108–109 (1998) 132–137
Deposition and characterization of TiAlN and multi-layered TiN / TiAlN coatings using unbalanced magnetron sputtering a, a b b J.H. Hsieh *, C. Liang , C.H. Yu , W. Wu a
Gintic Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b I-Shou University, Kaohsiung County, Taiwan 84008, China
Abstract TiAlN coatings have been known to be superior to other coatings such as TiN and TiCN in protecting tools which may be damaged by high thermal load (high cutting speed). Unfortunately, these coatings normally suffer greater damage than TiN and TiCN in more mechanically influenced processes such as interrupted cutting or slow speed cutting. The present study aims at developing multi-layered TiN / TiAlN coatings which may offer a good compromise between the properties of TiN and TiAlN. Three approaches including shutter control, power supply control, and rotational stage control were used to deposit multi-layered TiN / TiAlN coatings using unbalanced magnetrons. These coatings were then characterized using SEM, GDOS, nano-indention system, and tribometer. It was found that, in general, these multi-layered TiN / TiAlN coatings had lower wear rate than single-layered TiAlN within the tested sliding speeds. At certain sliding speeds, these coatings also had lower wear rate than TiN. In some tests, a thin layer (0.1 mm) of TiCN was coated on multi-layered coatings in an attempt to reduce frictional damage particularly during run-in stage. The result shows that the wear resistance of the TiCN–(TiN / TiAlN) coating was significantly improved especially at low sliding speed. 1998 Elsevier Science S.A. All rights reserved. Keywords: TiAlN coating; Multi-layered TiN / TiAlN coatings; Unbalanced magnetron sputtering
1. Introduction Over the past years, hard wear resistant TiN coatings deposited by magnetron sputtering have gained increasing importance in the field of decorative and cutting tool coatings. In general, TiN coating seems to be a good coating to reduce adhesive wear except for some applications. It has been known that TiN coatings tend to oxidize at temperatures beyond 5008C. As a result, poor adherent and brittle rutile phase TiO 2 oxide layer grows on top of the TiN coating upon oxidation, which quickly damage the protectability of the coating [1,2]. The growth rate of these oxide scales increases rapidly with increasing reaction temperature. At temperature as high as 7008C several mm thick TiN coatings oxidize within some minutes, therefore losing their wear protective function completely [3]. The fundamental advantage of TiAlN coating is that it forms a dense, highly adhesive, protective Al 2 O 3 film at its surface when heated, preventing diffusion of oxygen to the coating material. Another advantage for machining applications is its low thermal conductivity. Considerably more
*Corresponding author. Tel.: 165 7938528; fax: 165 7922779; e-mail: [email protected]
heat is dissipated via chip removal. This enables correspondingly higher cutting speeds to be selected, since thermal loading of the substrate is lower [2,3]. However, TiAlN coating, in general, shows poorer performance than TiN in the case of low sliding speed or interrupted cutting process due to its brittleness and high friction coefficient [4]. Recently, several multi-layered coatings have been reported to be a promising approach to optimized and / or enhanced coating’s properties and performance [5,6]. In detail, there are some reasons which show why it may be advantageous to use multi-layered coatings. Firstly, interface layers can be used to improve the adhesion of a coating to the substrate and to ensure a smooth transition from coating properties to substrate properties at the coating-substrate boundary. Secondly, by depositing several thin layers with various mechanical properties on each other the stress concentration in the surface region and the conditions for crack propagation can be changed. Thirdly, the properties of diverse property layer can be improved by depositing layers of coatings that separately have different kinds of effects on the surface, such as corrosion protection, wear protection, thermal isolation, electrical conductivity, diffusion barrier and adhesion to the substrate. Recently, Jensen et al., have shown an example of
0257-8972 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0257-8972( 98 )00684-7
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
depositing of TiN /AlN multi-layered coatings which indeed improved the properties of single-layered TiAlN [7]. The objective of this study is to investigate some possible approaches which can be used to produce multilayered TiN / TiAlN coatings. These approaches include shutter-controlled, stage-controlled, and power-supply controlled processes. Al the above-mentioned coatings were characterized using SEM, GDOS, nano-indention system, and tribometer.
2. Experimental
2.1. Sample preparation TiN, TiAlN and Multi-layered coatings were deposited on mirror-finished sub-micron grade tungsten carbide (CD650) substrates. The coating system was a Teer 550 deposition unit with optical emission spectrometer (OES) controller to monitor flow-rate of reactive gas. Details of the deposition system can be found elsewhere [8]. Prior to sputtering, the chamber was evacuated to less than 8310 26 Torr. Once the desired vacuum was reached, the chamber was then back-filled with argon to 3310 23 torr, and the substrates were sputter cleaned for 30 min using 200 W r.f. bias (equivalent to 240 DC). After a thin layer of titanium was deposited on the substrate, high purity nitrogen gas was injected into the deposition chamber for reactive deposition of TiN, gradient layer and TiN / TiAlN multilayers. Multi-layered coatings were deposited using three approaches, namely: shutter-controlled (SC), power-supplycontrolled (PSC) and rotational-stage-controlled (RSC). This is illustrated in Fig. 1. These approaches were used to start and stop depositing Al during reactive sputtering deposition. For SC process, the shutter was placed in front of Al target, and was periodically open and close to control deposition of Al. Therefore, a multi-layered TiN / TiAlN
Fig. 1. Schematic drawing of the deposition chamber (top view).
133
can be made. For PSC process, the power supply to the Al target was turned up and down periodically to produce similar effect. For RSC process, the rotational stage was simply changed to 1-axis (center) rotation where the substrate always faced the targets. Consequently, a multilayered coating can be made. The total deposition time take 90 min which corresponds to 2.5 to 3.0 mm in coating thickness coatings. For SC and PSC process, a total of 50 layers were deposited following Ti interlayer, TiN, and Al ramp-up layer. For RSC process, an estimate of 200 layers was formed. During deposition, the substrates were biased with 90 W r.f. The thickness and composition of films were then examined using glow discharged spectrometer (GDOS). Scanning electron microscopy (SEM) was used to study microstructure.
2.2. Wear test Tribological behaviors of all the coatings were characterized using a ball-on-disk (ASTM wear testing standard G99) tribological wear tester. The samples were fixed at the center of the rotating disk of the tribological wear tester and rotated horizontally at five different linear speed 30, 20, 15, 10 and 5 cm s 21 . All wear tests were carried out under dry running conditions against Al 2 O 3 ceramic balls at a load of 2.5 N. These balls were well-polished with 10 mm in diameter. During wear tests, the humidity was 40%–50% at room temperature. The amount of wear is determined by measuring appropriate linear dimensions of specimens before and after the test using Talysurf surface profilometer. Linear measurement of wear are then converted to wear volume by using appropriate geometric relations.
3. Results and discussion Fig. 2 shows the cross-sectional SEM photos of TiAlN and multi-layered TiN / TiAlN coatings. The layered structure can be seen clearly. The corresponding GDOS profiles are shown in Fig. 3. The results of wear tests are presented in Fig. 4. Overall, all three multi-layer coatings show lower wear rate than TiAlN coatings under the testing conditions. However, These multi-layered coatings show a tendency of high wear rate comparing with TiN at low sliding speeds, particularly at 5 cm s 21 . In extreme cases, some multilayered coatings broke down during the run-in stage at low sliding speed. This could be related to their high friction force during run-in stage and low cohesive strength as characterized by scratch testing. For scratch testing, Lc was determined as the lowest load at which the first damage on the coatings occurs. This damage can be a spalling off of the coating at the interface (adhesive type of failure), or chipping within the coating itself (cohesive type of failure). The test results of nanoindentation and scratch testing are listed in Table 1. The
134
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
Fig. 2. (a) Cross-sectional SEM micrograph of a TiAlN coating. (b) Cross-sectional SEM micrograph of a multi-layered TiN / TiAlN coating.
table shows that most of the hardness are in the range of 20 to 25 GPa. The coatings also show low cohesive critical load which is directly related to fracture toughness and cohesive strength between layers. This could be one of the reasons contributing to high wear rates of multi-layer coatings tested under low sliding speeds. At low sliding speeds, it is speculated that these multilayer coatings suffer some degree of cohesive failure or fracture, resulting in high wear rate. The situation is worsened during run-in stage when the sliding condition is not stable and the coatings are subjected to severe impact. It is hence believed that a smoother run-in stage can help improving wear resistance. To achieve this goal, a thin TiCN layer (0.1 mm) was deposited on top of a multi-layer coating to reduce friction coefficient, especially during run-in stage. TiCN is distinguished by low surface roughness and exceptional frictional behavior in low temperature machining processes [4]. The wear test result is presented
in Fig. 5. In most of the cases, the friction coefficient during run-in stage decrease from 1.2 to 0.5 or lower. The wear resistance for this TiCN-coated multilayer coating is hence greatly improved.
4. Conclusions Three approaches including shutter-controlled, powersupply controlled, and rotational-stage controlled processes were used to deposit multi-layered TiN / TiAlN coatings. These coatings were then characterized using SEM, GDOS, nano-indention system, and tribometer. In general, these multi-layered TiN / TiAlN coatings had lower wear rate than single-layered TiAlN with various sliding speeds from 5 cm s 21 to 30 cm s 21 . At a sliding speed greater than 20 cm s 21 , these coatings also had lower wear rate
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
135
Fig. 3. (a) GDOS depth-profile of a single layer TiAlN coating (in atomic % / 100). (b) GDOS depth-profile of a multi-layered TiAlN coating (in atomic % / 100).
than TiN. However, at a sliding speed less than 10 cm s 21 , the coatings showed sudden increase in wear rate, comparing with TiN coating. To overcome this problem, a thin layer (0.1 mm) of TiCN was coated on a multi-layered
coating in an attempt to reduce friction particularly during run-in stage. The result shows that the wear resistance of the TiCN–(TiN / TiAlN) coating was significantly improved especially at low sliding speed.
136
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
Fig. 4. Wear rate vs. sliding speed for various coatings.
Fig. 5. Wear rate vs. sliding speed for TiN, SC multi-layered TiN / TiAlN, and TiCN-coated multi-layered coating, sliding against alumina ball.
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
137
Table 1 Nano-hardness and critical loads for coatings Coatings
Nano-hardness (GPa)
Cohesive critical load (N)
Adhesive critical load (N)
TiN TiAlN Shutter-controlled PS-controlled Stage-controlled
20.5 25 23.5 22 21
50 19 35 18 61
53 .100 .100 65 70
Acknowledgements The authors are grateful to Ms. Y. C. Liu and Mr. Anthony Yeo at the Gintic Institute of Manufacturing Technology for their assistance. A review by Dr. Chang Soo Kong is also acknowledged.
References [1] U. Wahlstrom, L. Hultman, J.E. Sundgren, F. Adibi, I. Petrov, J.E. Green, Thin Solid Films 235 (1993) 62.
[2] R. Wuhrer, W.Y. Yeung, M.R. Philips, G. McCredie, Thin Solid Films 290 (1996) 339. [3] W.D. Munz, Werkstoffe und Korrosion 41 (1990) 753. [4] M.V. Stappen, L.M. Stals, M. Kerkhofs, C. Quaeyhaegens, Surf. Coat. Technol. 74–75 (1995) 629. [5] O. Knotek, F. Loffler, G. Kramer, Surf. Coat. Technol. 59 (1993) 14. [6] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996) 173. [7] H. Jensen, J. Sobota, G. Sorensen, J. Vac. Sci. Tech. 15A (1997) 941. [8] D.P. Monaghan, et al., Surf. Coat. Technol. 60 (1993) 525.
Deposition and characterization of TiAlN and multi-layered TiN / TiAlN coatings using unbalanced magnetron sputtering a, a b b J.H. Hsieh *, C. Liang , C.H. Yu , W. Wu a
Gintic Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore b I-Shou University, Kaohsiung County, Taiwan 84008, China
Abstract TiAlN coatings have been known to be superior to other coatings such as TiN and TiCN in protecting tools which may be damaged by high thermal load (high cutting speed). Unfortunately, these coatings normally suffer greater damage than TiN and TiCN in more mechanically influenced processes such as interrupted cutting or slow speed cutting. The present study aims at developing multi-layered TiN / TiAlN coatings which may offer a good compromise between the properties of TiN and TiAlN. Three approaches including shutter control, power supply control, and rotational stage control were used to deposit multi-layered TiN / TiAlN coatings using unbalanced magnetrons. These coatings were then characterized using SEM, GDOS, nano-indention system, and tribometer. It was found that, in general, these multi-layered TiN / TiAlN coatings had lower wear rate than single-layered TiAlN within the tested sliding speeds. At certain sliding speeds, these coatings also had lower wear rate than TiN. In some tests, a thin layer (0.1 mm) of TiCN was coated on multi-layered coatings in an attempt to reduce frictional damage particularly during run-in stage. The result shows that the wear resistance of the TiCN–(TiN / TiAlN) coating was significantly improved especially at low sliding speed. 1998 Elsevier Science S.A. All rights reserved. Keywords: TiAlN coating; Multi-layered TiN / TiAlN coatings; Unbalanced magnetron sputtering
1. Introduction Over the past years, hard wear resistant TiN coatings deposited by magnetron sputtering have gained increasing importance in the field of decorative and cutting tool coatings. In general, TiN coating seems to be a good coating to reduce adhesive wear except for some applications. It has been known that TiN coatings tend to oxidize at temperatures beyond 5008C. As a result, poor adherent and brittle rutile phase TiO 2 oxide layer grows on top of the TiN coating upon oxidation, which quickly damage the protectability of the coating [1,2]. The growth rate of these oxide scales increases rapidly with increasing reaction temperature. At temperature as high as 7008C several mm thick TiN coatings oxidize within some minutes, therefore losing their wear protective function completely [3]. The fundamental advantage of TiAlN coating is that it forms a dense, highly adhesive, protective Al 2 O 3 film at its surface when heated, preventing diffusion of oxygen to the coating material. Another advantage for machining applications is its low thermal conductivity. Considerably more
*Corresponding author. Tel.: 165 7938528; fax: 165 7922779; e-mail: [email protected]
heat is dissipated via chip removal. This enables correspondingly higher cutting speeds to be selected, since thermal loading of the substrate is lower [2,3]. However, TiAlN coating, in general, shows poorer performance than TiN in the case of low sliding speed or interrupted cutting process due to its brittleness and high friction coefficient [4]. Recently, several multi-layered coatings have been reported to be a promising approach to optimized and / or enhanced coating’s properties and performance [5,6]. In detail, there are some reasons which show why it may be advantageous to use multi-layered coatings. Firstly, interface layers can be used to improve the adhesion of a coating to the substrate and to ensure a smooth transition from coating properties to substrate properties at the coating-substrate boundary. Secondly, by depositing several thin layers with various mechanical properties on each other the stress concentration in the surface region and the conditions for crack propagation can be changed. Thirdly, the properties of diverse property layer can be improved by depositing layers of coatings that separately have different kinds of effects on the surface, such as corrosion protection, wear protection, thermal isolation, electrical conductivity, diffusion barrier and adhesion to the substrate. Recently, Jensen et al., have shown an example of
0257-8972 / 98 / $ – see front matter 1998 Elsevier Science S.A. All rights reserved. PII: S0257-8972( 98 )00684-7
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
depositing of TiN /AlN multi-layered coatings which indeed improved the properties of single-layered TiAlN [7]. The objective of this study is to investigate some possible approaches which can be used to produce multilayered TiN / TiAlN coatings. These approaches include shutter-controlled, stage-controlled, and power-supply controlled processes. Al the above-mentioned coatings were characterized using SEM, GDOS, nano-indention system, and tribometer.
2. Experimental
2.1. Sample preparation TiN, TiAlN and Multi-layered coatings were deposited on mirror-finished sub-micron grade tungsten carbide (CD650) substrates. The coating system was a Teer 550 deposition unit with optical emission spectrometer (OES) controller to monitor flow-rate of reactive gas. Details of the deposition system can be found elsewhere [8]. Prior to sputtering, the chamber was evacuated to less than 8310 26 Torr. Once the desired vacuum was reached, the chamber was then back-filled with argon to 3310 23 torr, and the substrates were sputter cleaned for 30 min using 200 W r.f. bias (equivalent to 240 DC). After a thin layer of titanium was deposited on the substrate, high purity nitrogen gas was injected into the deposition chamber for reactive deposition of TiN, gradient layer and TiN / TiAlN multilayers. Multi-layered coatings were deposited using three approaches, namely: shutter-controlled (SC), power-supplycontrolled (PSC) and rotational-stage-controlled (RSC). This is illustrated in Fig. 1. These approaches were used to start and stop depositing Al during reactive sputtering deposition. For SC process, the shutter was placed in front of Al target, and was periodically open and close to control deposition of Al. Therefore, a multi-layered TiN / TiAlN
Fig. 1. Schematic drawing of the deposition chamber (top view).
133
can be made. For PSC process, the power supply to the Al target was turned up and down periodically to produce similar effect. For RSC process, the rotational stage was simply changed to 1-axis (center) rotation where the substrate always faced the targets. Consequently, a multilayered coating can be made. The total deposition time take 90 min which corresponds to 2.5 to 3.0 mm in coating thickness coatings. For SC and PSC process, a total of 50 layers were deposited following Ti interlayer, TiN, and Al ramp-up layer. For RSC process, an estimate of 200 layers was formed. During deposition, the substrates were biased with 90 W r.f. The thickness and composition of films were then examined using glow discharged spectrometer (GDOS). Scanning electron microscopy (SEM) was used to study microstructure.
2.2. Wear test Tribological behaviors of all the coatings were characterized using a ball-on-disk (ASTM wear testing standard G99) tribological wear tester. The samples were fixed at the center of the rotating disk of the tribological wear tester and rotated horizontally at five different linear speed 30, 20, 15, 10 and 5 cm s 21 . All wear tests were carried out under dry running conditions against Al 2 O 3 ceramic balls at a load of 2.5 N. These balls were well-polished with 10 mm in diameter. During wear tests, the humidity was 40%–50% at room temperature. The amount of wear is determined by measuring appropriate linear dimensions of specimens before and after the test using Talysurf surface profilometer. Linear measurement of wear are then converted to wear volume by using appropriate geometric relations.
3. Results and discussion Fig. 2 shows the cross-sectional SEM photos of TiAlN and multi-layered TiN / TiAlN coatings. The layered structure can be seen clearly. The corresponding GDOS profiles are shown in Fig. 3. The results of wear tests are presented in Fig. 4. Overall, all three multi-layer coatings show lower wear rate than TiAlN coatings under the testing conditions. However, These multi-layered coatings show a tendency of high wear rate comparing with TiN at low sliding speeds, particularly at 5 cm s 21 . In extreme cases, some multilayered coatings broke down during the run-in stage at low sliding speed. This could be related to their high friction force during run-in stage and low cohesive strength as characterized by scratch testing. For scratch testing, Lc was determined as the lowest load at which the first damage on the coatings occurs. This damage can be a spalling off of the coating at the interface (adhesive type of failure), or chipping within the coating itself (cohesive type of failure). The test results of nanoindentation and scratch testing are listed in Table 1. The
134
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
Fig. 2. (a) Cross-sectional SEM micrograph of a TiAlN coating. (b) Cross-sectional SEM micrograph of a multi-layered TiN / TiAlN coating.
table shows that most of the hardness are in the range of 20 to 25 GPa. The coatings also show low cohesive critical load which is directly related to fracture toughness and cohesive strength between layers. This could be one of the reasons contributing to high wear rates of multi-layer coatings tested under low sliding speeds. At low sliding speeds, it is speculated that these multilayer coatings suffer some degree of cohesive failure or fracture, resulting in high wear rate. The situation is worsened during run-in stage when the sliding condition is not stable and the coatings are subjected to severe impact. It is hence believed that a smoother run-in stage can help improving wear resistance. To achieve this goal, a thin TiCN layer (0.1 mm) was deposited on top of a multi-layer coating to reduce friction coefficient, especially during run-in stage. TiCN is distinguished by low surface roughness and exceptional frictional behavior in low temperature machining processes [4]. The wear test result is presented
in Fig. 5. In most of the cases, the friction coefficient during run-in stage decrease from 1.2 to 0.5 or lower. The wear resistance for this TiCN-coated multilayer coating is hence greatly improved.
4. Conclusions Three approaches including shutter-controlled, powersupply controlled, and rotational-stage controlled processes were used to deposit multi-layered TiN / TiAlN coatings. These coatings were then characterized using SEM, GDOS, nano-indention system, and tribometer. In general, these multi-layered TiN / TiAlN coatings had lower wear rate than single-layered TiAlN with various sliding speeds from 5 cm s 21 to 30 cm s 21 . At a sliding speed greater than 20 cm s 21 , these coatings also had lower wear rate
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
135
Fig. 3. (a) GDOS depth-profile of a single layer TiAlN coating (in atomic % / 100). (b) GDOS depth-profile of a multi-layered TiAlN coating (in atomic % / 100).
than TiN. However, at a sliding speed less than 10 cm s 21 , the coatings showed sudden increase in wear rate, comparing with TiN coating. To overcome this problem, a thin layer (0.1 mm) of TiCN was coated on a multi-layered
coating in an attempt to reduce friction particularly during run-in stage. The result shows that the wear resistance of the TiCN–(TiN / TiAlN) coating was significantly improved especially at low sliding speed.
136
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
Fig. 4. Wear rate vs. sliding speed for various coatings.
Fig. 5. Wear rate vs. sliding speed for TiN, SC multi-layered TiN / TiAlN, and TiCN-coated multi-layered coating, sliding against alumina ball.
J.H. Hsieh et al. / Surface and Coatings Technology 108 – 109 (1998) 132 – 137
137
Table 1 Nano-hardness and critical loads for coatings Coatings
Nano-hardness (GPa)
Cohesive critical load (N)
Adhesive critical load (N)
TiN TiAlN Shutter-controlled PS-controlled Stage-controlled
20.5 25 23.5 22 21
50 19 35 18 61
53 .100 .100 65 70
Acknowledgements The authors are grateful to Ms. Y. C. Liu and Mr. Anthony Yeo at the Gintic Institute of Manufacturing Technology for their assistance. A review by Dr. Chang Soo Kong is also acknowledged.
References [1] U. Wahlstrom, L. Hultman, J.E. Sundgren, F. Adibi, I. Petrov, J.E. Green, Thin Solid Films 235 (1993) 62.
[2] R. Wuhrer, W.Y. Yeung, M.R. Philips, G. McCredie, Thin Solid Films 290 (1996) 339. [3] W.D. Munz, Werkstoffe und Korrosion 41 (1990) 753. [4] M.V. Stappen, L.M. Stals, M. Kerkhofs, C. Quaeyhaegens, Surf. Coat. Technol. 74–75 (1995) 629. [5] O. Knotek, F. Loffler, G. Kramer, Surf. Coat. Technol. 59 (1993) 14. [6] S.J. Bull, A.M. Jones, Surf. Coat. Technol. 78 (1996) 173. [7] H. Jensen, J. Sobota, G. Sorensen, J. Vac. Sci. Tech. 15A (1997) 941. [8] D.P. Monaghan, et al., Surf. Coat. Technol. 60 (1993) 525.