Thermal stability and microstructure characterization of CrN/WN multilayer coatings fabricated by ion-beam assisted deposition

Transcription

1 Surface & Coating Technology 200 (2005) Thermal tability and microtructure characterization of CrN/WN multilayer coating fabricated by ion-beam aited depoition Yan-Zuo Tai, Jenq-Gong Duh Department of Material Science and Engineering, National Ting Hua Univerity, Hinchu, Taiwan Available online 13 September 2005 Abtract CrN/WN multilayer coating were depoited on both ilicon (100) and tainle teel ubtrate by the ion-beam aited depoition. The bilayer period were deigned and controlled in the range from 3 nm to 30 nm. The CrN/WN film were annealed from 750 -C to 850 -C for 1 and 4 h in vacuum to evaluate the thermal tability of thee coating. The difference of microtructural characterization between the CrN/ WN multilayer coating and the CrN ingle layer coating wa invetigated by canning electron microcopy. The phae tranformation after annealing wa probed by X-ray diffraction. The hardne of a-depoited CrN/WN coating with 8 nm bilayer period wa much higher than that of CrN and WN ingle layer. The thermal tability of CrN/WN multilayer coating with different bilayer period wa alo dicued in correlation to the aociated microtructural evolution. D 2005 Elevier B.V. All right reerved. Keyword: CrN; CrN/WN; IBAD; Superlattice; Nanolayer; Hardne 1. Introduction Due to the excellent performance in hardne, wear and corroion reitance, the tranition metal nitride have been taken for hard protective coating [1,2]. In the nitride ytem, the chromium nitride (CrN) film have been invetigated for year, and proved to exhibit good mechanical performance, thermal propertie and anti-oxidation behaviour [3,4]. Recent tudie alo how that multilayer coating compoed of two kind of tranition nitride film exhibit uperior mechanical trength, uch a hardne, adheion, and wear reitance, a compared to ingle layer nitride coating due to their pecific interface [5 7]. Several new material ytem, including TiN/AlN, TiN/NbN, CrN/ TiN and CrN/AlN, exhibit evident enhancement of microhardne [7 10]. Theoretically, the enhanced hardne could be explained by dilocation blocking between interface, due to the hear moduli difference, and by coherency train from lattice mimatch of the two different Correponding author. Fax: addre: jgd@mx.nthu.edu.tw (J.-G. Duh). material ytem [11]. The multilayer coating utain high hardne at the room temperature, however, after heat treatment at elevated temperature in vacuum, the trength of multilayer coating would be degraded rapidly, which wa caued by the vanihing of nanolayered tructure due to the inter-diffuion of atom. The thermal tability i thu a critical iue and play an important role on performance of tool operated at the elevated temperature in vacuum. In thi tudy, the combination of chromium nitride and tungten nitride coating wa propoed to form a nanotructure coating ytem. Effort were concentrated on the tructural characterization and thermal tability of both CrN and CrN/ WN coating with bilayer period of 3 nm to 30 nm in vacuum environment. The microtructure of CrN and CrN/ WN multilayer coating were analyzed by X-ray diffraction and SEM. In addition, the trength of the depoit under variou condition wa invetigated by nanoindentation. 2. Experimental detail The CrN ingle layer coating and CrN/WN multilayer coating were fabricated on the ilicon (100) and AISI /$ - ee front matter D 2005 Elevier B.V. All right reerved. doi: /j.urfcoat

2 1684 Y.-Z. Tai, J.-G. Duh / Surface & Coating Technology 200 (2005) Table 1 Layer configuration and microhardne of the CrN/WN multilayer coating Sample name Bilayer period (nominal) (nm) Bilayer period (calculated) (nm) D2h (deg) Microhardne (GPa) 420 tool teel ubtrate by ion-beam aited depoition (IBAD). Metallurgical finihing and polihing with SiC andpaper of #120, #240, #400, #600, #800, and #1200 were ued to remove the contamination on the urface of teel ubtrate. To obtain an adequate urface condition for the following tet and property evaluation, the ground ubtrate were then polihed with a 1.0 Am diamond powder. After grinding and polihing treatment, the ubtrate were cleaned by ultraonic vibration cleaning in acetone to remove all contaminate. Both chromium and tungten target of 3 in. in diameter were wt.% in purity. After loading of the ubtrate and target, the vacuum chamber wa degaed down to Pa, followed by the inlet of argon and nitrogen gae a plama ource and reactive ga, repectively, to a working preure of Pa. The target-to-ubtrate ditance wa fixed at 100 mm from puttering target ource and ion gun. Before depoition, both chromium and tungten target were pre-puttered for 2 min to clean the target urface, and then Cr interlayer wa depoited with a power of 300 W for 2 min. An aited ion ource (Mark II Gridle ion ource, Veeco) wa adopted during puttering. The current of aited ion beam and electron beam were 4.0 and 2.7 A, repectively. Sputtering of Cr and W wa proceeded alternately to form the equential CrN/WN multilayer coating. Both the input power on Cr and W target were fixed at 300 W. The depoition time of individual nitride layer of the multilayer CrN/WN coating during equential puttering wa modified from 8 to 93. The total thickne of every multilayer coating with different bilayer period wa controlled around 1.0 Am. The CrN/WN multilayer coating were then annealed at 750, 800 and 850 -C in a vacuum chamber with a preure of Pa for 1 h and 4 h. The multilayer tructure and crytallographic phae of the thin film were identified by low-angle and high angle diffraction, repectively, with an X-ray diffractometer (Shimadzu, XRD6000) under h/2h mode. The coating thickne and cro-ection image were oberved with a field emiion canning electron microcope (FESEM, JSM- 6500, JEOL, Japan). The microhardne of the coating wa analyzed with a nanoindentation apparatu (TriboScope, Hyitron, Minneapoli, MN) equipped with a Berkovich indenter. The maximum load adopted for all the coating wa fixed at 3000 AN. 3. Reult and dicuion 3.1. Low-angle X-ray diffraction The CrN/WN multilayer coating were fabricated with total thickne about 1.0 Am by IBAD proce. By controlling the depoition time of individual nitride layer, a) diffraction peak 3nm Relative intenity c) d) 5nm 8nm 10nm e) 30nm Fig. 1. Low-angle X-ray diffraction pectra of CrN/WN multilayer coating with adjuted bilayer period (a) K =3, ( K =5, (c) K =8, (d) K =10 and (e) K =30 nm.

3 Y.-Z. Tai, J.-G. Duh / Surface & Coating Technology 200 (2005) the bilayer period wa adjuted. The nominal bilayer period are lited in Table 1. The low-angle X-ray diffraction pectra of CrN/WN multilayer coating with adjuted bilayer period K = 3, 5, 8, 10 and 30 nm i preented in Fig. 1. It wa demontrated by Li et al. [12] that modulation interface of multilayer coating with compoitionally modulated tructure can produce diffraction of X-ray. Becaue the interplanar ditance of crytal are uually maller than bilayer period of multilayer coating, only at low angle can the diffraction peak reulting from modulation interface be oberved. There are two evident diffraction peak found in the low-angle XRD pattern for K =3 nm, and four peak for K =8 nm, and ix peak for K =30 nm. With increaing bilayer period, the amount of diffraction peak increaed, while the pace between two peak decreaed in every lowangle XRD pattern. According to the low-angle XRD peak, it i apparent that CrN/WN multilayer coating prepared by IBAD poe a dene and well-packed layered coating configuration. Thu, mooth and clear interface between CrN and WN layer are evident. Fig. 2a and b how the low-angle X-ray diffraction pectra of CrN/WN multilayer coating with bilayer period a) 850 o C 1h 800 o C 1h Relative Intenity 750 o C 1h a-depoited annealing 4h Intenity annealing 1h a-depoited Theta Fig. 2. Low-angle X-ray diffraction pectra of CrN/WN multilayer coating (a) with bilayer period of 3 nm after annealing at variou temperature and ( with bilayer period of 30 nm after annealing for different time.

4 1686 Y.-Z. Tai, J.-G. Duh / Surface & Coating Technology 200 (2005) of 3 nm after annealing at different temperature and that with bilayer period of 30 nm after annealing for different time, repectively. In Fig. 2a, the low-angle XRD peak do not vanih after heat treatment at 850 -C for 1 h, and nothing more than the econd peak of thi pectra i weakened lightly. Due to the exitence of the low-angle diffraction peak, the individual layer of chromium nitride and tungten nitride hould be unmixed after annealing. No interdiffuion between CrN and WN interface in the coating with bilayer period of 3 nm wa revealed after thermal treatment, o the thermal tability of the film wa evidenced. The low-angle XRD peak of coating with bilayer period of 30 nm after 800 -C annealing for 4 h hown in Fig. 2b are weaker and broader than that of adepoited film. The harpne of interlayer wa decreaed due to the interdiffuion after heat treatment. Neverthele, the uperlattice tructure till exited. The bilayer period of multilayer film can be enumerated from Bragg Law [12]: 2 in 2 h ¼ nk þ 2d ð1þ 2K where K i the bilayer period, k i the wavelength of Cu Ka=0.154 nm, d i the real part of the average refractive index of the coating and h i the interval of two peak of the low-angle XRD pattern. Both the emumerated bilayer period and h are lited in Table 1. The difference between nominal and enumerated bilayer period i no more than 10%. Since the cale of modulation period i mall in the nanocale, it i apparent that the meaured value of bilayer thickne in thi work were very cloe to the expected one. Table 1 alo lit the microhardne of CrN/WN multilayer film with different modulation period. The hardne of multilayer coating with modulation period of 8 nm i about 29 GPa. It i argued that when the multilayer formed a uperlattice tructure, the hardne of thi coating would increae High angle X-ray diffraction The high angle X-ray diffraction pattern of uperlattice film with modulation period of 8 nm at a-depoited tate and 800 -C annealing for 1 h in vacuum are hown in Fig. 3. Four broadened diffraction peak are found approximately at 2h =37-, 43-, 62- and 74- in the a-depoited film, o the face centered cubic phae of CrN/WN multilayer i evidenced. Although it wa difficult to identify chromium and tungten nitride due to overlapped diffraction peak (CrN (111): 37.5-, WN (111): [18]) by XRD pattern, the atellite peak near the peak (111) of the f.c.c. tructure were found, which confirmed the formation of uperlattice tructure [13]. Becaue of the broadening and the weak intenity for the peak, the grain ize of the film wa uppoed to be mall. ubtrate atellite peak 800 o C 1h Relative Intenity a) (111) (200) (220) a-depoited (311) Fig. 3. High angle X-ray diffraction pattern of uperlattice film with modulation period of 8 nm (a) a-depoited tate, and ( 800 -C annealing for 1 h in vacuum.

5 Y.-Z. Tai, J.-G. Duh / Surface & Coating Technology 200 (2005) It wa alo found that the atellite peak till exited and intenity of every Bragg diffraction peak increaed after annealing at 800 -C for 1 h in vacuum. It wa argued that the uperlattice tructure prevailed and the grain growth occurred after heat treatment. Becaue an in-plane compreive tre uually exited in crytalline coating depoited by puttering [14], all the four Bragg diffraction peak were maller than the f.c.c. diffraction peak of CrN (or WN) by 1- or 2-. After heat treatment, the four main peak hifted to higher angle apparently, becaue the compreive tre wa releaed. In fact, the main diffraction peak poition of thi film after thermal treatment hifted toward that of CrN(or WN). Similar obervation could be revealed in the multilayer film with modulation period of 3 nm. Fig. 4 how the high angle X-ray diffraction pattern of multilayer film with bilayer period of 3 nm at a-depoited tate and 800 -C annealing for 1 and 4 h in vacuum. The four Bragg diffraction peak of CrN/WN uperlattice exited and atellite peak could alo be found. By heat treatment, the compreive tre wa releaed o the four diffraction peak hifted to higher angle obviouly. With increaing thermal treatment time, more compreive tre wa releaed and thu the peak wa hifted more toward higher angle. The poition of the four main peak then moved toward that of CrN (or WN). It wa proved that the multilayer film with bilayer period of 3 nm exhibited uperlattice tructure by the appearance of Hardne (GPa) CrN the atellite peak around (111) and (311) peak of f.c.c. tructure. The poition of the atellite peak i given by [15]: inh F ¼ inh B Fmk=2K Bilayer period (nm) 800 C for 1h a-depoited CrN Fig. 5. The hardne v. bilayer period of CrN/WN multilayer coating. ð2þ where h B i the poition of main peak, k i the wave length of Cu Ka=0.154 nm, K i the modulation period and h T i the poition of atellite peak. If the modulation period of multilayer coating decreaed, the pace between the atellite peak and the main Bragg diffraction peak increaed. Neverthele, due to the broadened (311) peak ubtrate atellite peak c) (111) (200) (220) (311) 800 o C 4h Relative Intenity 800 o C 1h a) a-depoited Fig. 4. High angle X-ray diffraction pattern of multilayer coating with bilayer period of 3 nm (a) a-depoited tate, ( 800 -C annealing for 1 h in vacuum, and (c) 800 -C annealing for 4 h in vacuum.

6 1688 Y.-Z. Tai, J.-G. Duh / Surface & Coating Technology 200 (2005) of the coating with bilayer period of 8 nm, the atellite peak were overlapped and could not be oberved Microhardne and SEM image The hardne of CrN/WN multilayer coating a a function of modulation period a well a the microhardne of CrN i hown in Fig. 5. The hardne of CrN i about 18 GPa and thoe of all the multilayer coating are over 24 GPa. The nanolayered tructure of the multilayer film indeed promoted the hardne of the coating. The microhardne of coating with modulation period of 8 nm i 29.1 GPa. The abruptly raied hardne have been reported [5,8,15,16] and the uperlattice enhancement of mechanical propertie wa propoed. In theory, the enhanced hardne could be explained by dilocation blocking between interface due to the different hear moduli, and by coherency train from lattice mimatch of the two different material ytem [11]. It i intereting to point out that in thi tudy the hardne eemed to increae after 800 -C annealing for 1 h, intead of falling-off. The uperlattice tructure wa maintained after annealing, indicating that the thermal tability of the multilayer film i quite uperior. It i noted that the peak intenity increaed after heat treatment, a hown in Fig. 4. Thi would be attributed to the increae of crytallinity, leading to the enhancement of microhardne, a indicated in Fig. 5. The cro-ection SEM image of CrN and CrN/WN multilayer coating with bilayer period of 8 nm are hown in Fig. 6a and b, repectively. Smooth and dene coating configuration were developed by IBAD proce. CrN depoit revealed trong columnar tructure in Fig. 6a, and a tight and dene tructure of CrN/WN multilayer coating i exhibited in Fig. 6b. The difference of hardne between CrN and CrN/WN multilayer coating may be acribed to the cro-ectional tructural variation. The uperior microhardne of multilayer coating with modulation period of 8 nm wa due to the dene tructure. However, owing to the reolution limit, the nanolayered tructure of multilayer film could not be oberved by SEM only. More attempt in the TEM analyi for the microtructure detail of the multilayer film hould be cheduled in the future work. 4. Concluion The CrN/WN multilayer coating fabricated by ion-beam aited depoition (IBAD) proce exhibited a dene and well-packed layered tructure, and the interface between CrN and WN layer were mooth and evident. Thi confirmation of the layered tructure of CrN/WN coating could be achieved by low-angle X-ray diffraction technique. It wa believed that the CrN/WN multilayer film formed a uperlattice tructure by the appearance of atellite peak from high angle X-ray diffraction pattern. By comparing the cro-ection image of CrN coating and CrN/WN multilayer film, it wa apparent that the uperior microhardne howed up with the nanolayered tructure. The microhardne wa enhanced due to the exitence of uperlattice tructure. The evidence of uperlattice tructure wa alo revealed by the hardne enhancement in the multilayer coating with modulation period of 8 nm. After heat treatment at variou temperature, all the pecific characteritic peak of lowangle and high angle X-ray diffraction pattern till prevailed without appreciable diminihing. The hardne enhancement of multilayer coating till exited and eemed to be even higher after 800 -C annealing for 1 h. A a reult, the thermal tability of the CrN/WN multilayer coating wa excellent a compared with other nitride multilayer film [17], and thi material ytem could be employed for high temperature application. Acknowledgment Fig. 6. Cro-ectional SEM image of (a) CrN coating and ( CrN/WN multilayer coating with modulation period of 8 nm. Thi upport of work from the National Science Council, Taiwan, under Contract No. NSC E i appreciated.

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