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Effects of temperature and ion-to-atom ratio on the orientation of IBAD MoS2 coatings
Thin Solid Films 260 (1995)
ELSEVIER
Effects of temperature
143-147
and ion-to-atom ratio on the orientation IBAD MoS, coatings
of
L.E. Seitzman, R.N. Bolster, I.L. Singer Nuval Research Laboratory, Received
Code 6170, Washington,
31 March
1994; accepted
DC 20375-5342,
28 October
USA
1994
Abstract MoS, coatings, 55-800 nm thick, were grown by ion-beam-assisted deposition (IBAD) using different ion-to-atom ratios and deposition temperatures. Crystallinity and orientation of the IBAD MoS, coatings were determined by X-ray diffraction (XRD). Only XRD peaks corresponding to (001), (hkO), and amorphous MoS,, and a previously unreported low-20 peak (20 w 10.7”) were observed. The basal (002) peak intensities varied primarily with ion-to-atom ratio; the greatest basal intensity occurred when the ion-to-atom ratio produced about 1 displacement per atom. Although a secondary factor in basal intensity, deposition temperature was the primary factor in edge (100) intensity. Edge intensity increased with increasing temperature; it appears that
the increases are due to annealing of randomly-oriented MoS,, which converts to edge orientation. is unknown, but appears to be associated with the basal planes of MoS,. Keywords: Deposition;
Molybdenum
sulphide; Oxidation
1. Introduction Orientation is an important factor in the performance of MoS, coatings for solid lubrication [ 11, catalysis [2], and photoelectrochemistry [ 31. Usually the preferred orientation is basal, where the (001) planes are parallel to the substrate. However, most deposition processes result in mainly edge orientation, where the (hk0) planes are parallel to the substrate. The ion-beam-assisted deposition (IBAD) process is capable of producing basal-oriented MoS, coatings [4]. In previous studies on IBAD MO&, we have shown that endurance depends on the orientation of the crystalline material [5], and that orientation, in turn, depends on the ion-toatom ratio [4] (a measure of the amount of concurrent ion bombardment). In those studies, however, the deposition temperature was kept to ambient levels. Certain IBAD Mo!$ coatings annealed at 573 K showed improved friction and crystallinity [6] without a reduction in endurance [7]. Perhaps similar improvements can be achieved by tailoring MoS, orientation through optimizing deposition temperature. Deposition temperature is known to influence the microstructure of Elsevier Science S.A. SSDI 0040-6090( 94) 06419-9
The origin of the low-20 peak
metallic and ceramic coatings, including those grown with the IBAD process [S]. This investigation will focus on the effect of both ion-to-atom ratio and deposition temperature on IBAD MoS, orientation. Coatings will be deposited over a range of ion-to-atom ratios at several temperatures up to 573 K. The orientation of the coatings will be investigated by X-ray diffraction (XRD). The relationship between orientation and deposition parameters will be discussed in terms of both ion beam and thermal effects.
2. Experimental
details
MoS, coatings were deposited by the IBAD process, as described elsewhere [4,6]. Two Kaufman ion guns were used to independently sputter MO and S targets with Ar ions. A third Ar-ion gun (the assist gun), aligned near-normal to the sample stage, was used for both sputter cleaning before and ion bombardment during deposition. The base pressure in the system was around lo-’ Pa and the operating pressure around 0.05
L. E. Seitzman
144
et al. I Thin Solid Films 260 (1995) 143-147
a_
.3WK373K
30; a
Ati 10
20
30
40
50
60
70
28 Fig. 1. X-ray diffraction spectra of three IBAD MoS, coatings of similar thickness (320-360 nm) exhibiting (a) low and (b) high intensity (002) peaks and (c) an additional peak at low-20 (10.7”). Peaks marked by 1 are from the coating and unlabeled peaks are from the steel substrate. Solid heavy lines indicate the peak positions of the XRD powder standard.
Pa of Ar. All three ion guns were operated at 1 kV. The substrates were mounted on a rotatable sample stage and could be heated to 573 + 10 K. In most cases, a thin diffusion barrier [7] of TiN, typically 30 nm thick, was deposited between the substrate and the MoS,. Substrates used in this investigation were 52100, 44OC, M50, and AMS 5749 steels, Ti and Ti alloys. The deposition parameters varied in this study were the ion-to-atom ratio, R, and the deposition temperature. R is a measure of how much concurrent bombardment the growing coating received and is determined using the ratio of the assist-gun current to the deposition rate (coating thickness t deposition time) [4]; as the degree of concurrent bombardment increases, R increases. R values ranged from 0 to 0.12 based on an assist beam current range of 0- 16 mA and a deposition rate range of 0.28-0.45 nm s-‘. Coatings were deposited to thicknesses of 55-800 nm at temperatures between 300 K and 573 K. Some coatings deposited at ambient temperature, 300 < T < 350 K, were later annealed at 573 K for 2 h in the deposition chamber (2 x 10m4 Pa). Also, two depositions, one at 323 K and one at 473 K, were performed on substrates mounted at a 30” angle to the sample stage. The structure and orientation of the coatings were determined by XRD. A Rigaku D/max-B series diffrac-
Fig. 2. Normalized ings as a function ture.
basal (002) XRD intensity of IBAD MoS, coatof ion-to-atom ratio, R, and deposition tempera-
tometer, operated in standard O-20 geometry, was used to identify crystallites oriented parallel to the substrate. A high intensity X-ray source (50 kV and 200 mA), desirable for thin coating measurement, was obtained using a rotating Cu anode. Diffraction slits of l/6” were used to limit the incident beam size at low incidence angles and a monochromator was used to allow only Cu KE X-rays into the detector. Comparison between coatings was made by dividing the MoS, basal (002) and edge ( 100) peak intensities by the steel (200) peak intensity; this normalization procedure accounts for any differences in sampling area.
3. Results 3.1. General observations
Representative spectra from similar thickness IBAD MoS, coatings on steel are shown in Fig. 1. Peaks labelled with arrows correspond to the IBAD coating; unlabelled peaks correspond to the steel substrate. Three types of MoS, peaks were observed: (1) broad basal (001) and edge (hk0) peaks characteristic of small crystallites [ 91; (2) a very broad, weak peak from 30” to 45”, attributed to amorphous MoS, [4,6]; and (3) a previously unreported low-20 peak, seen in Fig. l(c), around 20 = 10.7” (the low-20 peak at 10.7” does not correspond to any major peak of any MO sulfides or oxides in Ref. [7] (JCPDS Powder Diffraction File)). No IBAD MoS, coating exhibited a mixed (hkl) peak. When a TIN intermediate layer was present, the TiN (200) peak at 42” was observed.
145
L. E. Seitzman et al. 1 Thin Solid Films 260 (199.5)143-147 1.5
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”
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ION-TO-ATOM RATIO, R Fig. 4. Normalized basal (002) IBAD MoS, coatings deposited annealed at 573 K (0). Fig. 3. Normalized ings as a function ture.
and edge (100) XRD intensity of at ambient temperature (0) and
edge (100) XRD intensity of IBAD MoS, coatof ion-to-atom ratio, R, and deposition tempera-
The most prominent MoS, peaks, the (002) at 13” and the (100) at 33”, were shifted from the values of a reference standard [lo]. The d-spacing standards are, doo2= 0.6155 nm and d,,, = 0.2738 nm, whereas the TBAD MO& d-spacings ranged from 0.655 < doo2 -C0.707 nm and 0.266 < d,,, < 0.271 nm. These d-spacing shifts are similar to those reported for sputter-deposited MoS, [ 1l- 141. The d-spacing of the low-20 peak ranged from 0.769 to 0.850 nm. Basal intensity varied between about 1 and 38, while the edge intensity ranged from 0 to 1.3. Examples of a low basal and a high basal intensity coating are shown in Figs. l(a) and l(b), respectively. Basal and edge intensity exhibited no thickness dependence. The range of coating thickness where only (001) peaks were found was 55-650 nm, and the range where both (001) and (hk0) peaks occurred was 180-800 nm. Also, the basal and edge intensities exhibited no dependence on coating substrate. Similar MoS, peak intensities were observed when the coating was simultaneously deposited at room temperature on steel with and without a TiN layer, on different steels, or on steel and Ti substrates (again with or without a TiN interlayer). The low-20 peak was observed primarily in coatings deposited at elevated temperatures. The low-20 peak was found in coatings with both low and high basal intensity, and the intensity ratio of low-20/(002) ranged from about 0.3 to 1. One coating with a low20/(002) ratio of 0.5 was sequentially ion thinned and analyzed by XRD; the ratio remained 0.5 even when 240 nm was removed (original coating thickness was 330 nm).
3.2. Intensity
vs. R and temperature
No change in peak width with R or deposition temperature was observed. However, the intensities of three MoS, peaks changed as follows: the broad peak (observed from 30” to 45”) decreased with increasing temperature, and the basal and edge peaks varied with R and/or deposition temperature as shown in Figs. 2 and 3, respectively. In Fig. 2, basal intensity increased then decreased with increasing R. At each deposition temperature, the basal intensity reached a maximum around R = 0.04. The greatest basal intensity occurred in coatings grown at elevated temperature ( 2 423 K). Fig. 3 shows that edge intensity increased with increasing temperature, but exhibited no apparent dependence on R at any temperature. 3.3. Annealing
efSects
The normalized XRD intensities of coatings deposited at ambient temperature and subsequently annealed are plotted in Fig. 4 as closed and open circles, respectively. Annealing typically produced only small ( < 20%) increases or decreases in basal intensity but large (up to 4000%) increases in edge intensity. The increased edge intensity was greatest for coatings exhibiting low basal intensity in the as-deposited coating. While peak intensities changed, peak widths remained constant.
4. Discussion
This XRD investigation of temperature effects has added three new observations to those of previous
146
L. E. Seitzman et al. I Thin Solid Films 260 (1995) 143-147
IBAD MoS, studies [4,6]: (1) the presence of a low20 peak; (2) basal (002) intensity increases when deposition temperature is raised; and (3) edge intensity increases with increasing deposition and annealing temperature. Although the results clearly indicate that the low-20 peak is associated with the basal (002) peak, we do not know the origin of the low-20 peak at this time. Possible explanations include a second MoS, basal peak representing a discrete expansion of the c axis or diffraction from trapped Ar atoms. Further work is required in order to determine the cause of the low-20 peak. The remainder of the discussion will focus on the observed ion bombardment and temperature effects on basal and edge intensities. When the deposition temperature was raised to 423 K or greater the basal intensity increased, especially in the range 0.03 < R < 0.05. This is the same range of R where basal intensity was greatest at all temperatures. Both facts indicate that although temperature is a factor, bombardment effects are primarily responsible for basal orientation. Theoretical and experimental studies have shown that concurrent ion bombardment can affect the degree of preferred orientation in coatings [8]. Preferred orientation was often greatest when R corresponded to a critical value of one displacement per coating atom ( 1 dpa). The R value corresponding to 1 dpa can be determined by calculating the average number of displaced atoms, v, due to collision with an Ar ion of energy E,: R( 1 dpa) = l/v(E,,) The average number of displaced atoms can be estimated by using a simple Kinchin-Pease model [ 151 and ignoring the differences in atomic mass between Ar, MO, and S. Then v(E,) = E,/2E,, where Ed is the energy needed to displace a lattice atom. Assuming Ed = 25 eV, R( 1 dpa) = 0.05. A more detailed calculation using the TRIM code [ 161 yields a similar result. The secondary factor in determining basal intensity is temperature. Temperature can increase both bulk atom and adatom mobility. If bulk atom mobility had increased significantly, the crystallite size would have increased; however, the basal (002) peak widths did not narrow with temperature indicating constant crystallite size. We speculate that the increased basal intensity at higher temperatures was due to increased adatom mobility. Temperature was the primary factor influencing edge intensity of IBAD MO&. The increasing edge intensity with temperature can be due to increased edge crystallite growth or nucleation. Growth, as explained above, is unlikely because the (100) peak widths remained broad at all temperatures. Edge nucleation can occur either in or below the ion-bombarded layer. Based on
calculations using the TRIM code [ 161, the bombarded layer is the outermost 6 nm of the growing coating. Edge nucleation in the bombarded layer is probably insignificant, since edge intensity is independent of ionto-atom ratio. Therefore, edge nucleation must be occurring by conversion of material underlying the bombarded layer. This is primarily a thermal effect, as evidenced by the increased edge intensity in the coatings annealed at 573 K. The underlying material which converted to edge orientation must have been initially either amorphous or crystalline with a non-edge orientation, such as basal. Certainly amorphous-to-edge conversion is a possibility; the decline in intensity of the 30” to 45” peak with increasing deposition temperature is consistent with decreasing amorphous content. On the other hand, basal-to-edge conversion cannot be the only mechanism because some annealed coatings showed increased edge intensity without a significant change in basal intensity. Crystallites with random orientation might also convert to edge orientation, although random orientation would appear to be ruled out by the absence of (hkl) peaks in the diffraction spectra. However, there is a structure which can be randomly oriented yet not give mixed (hkf) XRD peaks: turbostratic MoS,. The turbostratic structure is a two-dimensional arrangement, where the basal planes are randomly rotated about the c axis like a poorly stacked deck of cards [ 17,181. Warren [ 191 has shown that turbostratic structure gives only broad (001) and (hk0) diffraction peaks; turbostratic crystallites with the basal planes neither parallel nor perpendicular to the substrate will contribute only to the background of the diffraction spectrum. Therefore, the decrease in the 30” to 45” peak intensity with temperature may also be attributed to random-to-edge conversion of turbostratic material. We note that the turbostratic MoS, may also be present in sputter-deposited MoS,. The turbostratic structure of sputter-deposited MO&, in fact, was reported in one investigation [20], although the diffraction spectrum was not shown. XRD spectra of sputter-deposited MoS, usually contain only (001) and (hk0) peaks [ ll-14,21,22]; the absence of (hkl) peaks has historically been interpreted to mean only basal and edge orientations exist. However, as noted above, these XRD features can also be explained by the presence of turbostratic MO&. In fact, azimuthal disorder of sputter-deposited MoS, has been reported [23,24], and crystallites with mixed orientation have been imaged in the electron microscope, although the electron diffraction yields no (hkl) intensity [21,23]. Further investigation is required to confirm the existence of the turbostratic structure in sputter-deposited and IBAD MO&.
L. E. Seitzman et al. I Thin Solid Films 260 (1995) 143-147
5. Summary and conclusions
Orientation of IBAD MO&, as determined by XRD, is a function of both ion-to-atom ratio R and deposition temperature, but not coating thickness or substrate chemistry. Ion-to-atom ratio is the primary factor in controlling basal intensity, and deposition temperature plays a secondary role. The greatest basal intensity occurs when the ion bombardment produces about 1 displacement per atom (0.03 < R < 0.05)in the coating and the temperature is greater than or equal to 423 K. Temperature is the primary factor in controlling edge intensity. The observed increasing edge intensity with temperature may result from increasing conversion of amorphous or randomly-oriented turbostratic MoS, to edge orientation. A new, unexplained low-20 peak (20 z 10.7”) is observed in IBAD MO&, usually when the deposition temperature exceeds 373 K. This low-20 peak appears to be associated with the basal planes of MO&.
Acknowledgments
The authors would like to thank the Office of Naval Research for funding and L.E.S. would like to thank the National Research Council for support through a post-doctoral fellowship. This work was partly supported by the SD10 tribomaterials group.
References [l] W.O. Winer,
Wear, IO (1965) 422.
141
[2] 2. Shuxian, W.K. Hall, G. Ertl and H. Kniizinger, J. Catal., 100 (1986) 167. [3] G. Djemal, N. Miiller, U. Lachish and D. Cahen, Solar Energy. Mater., 5 (1981) 403. [4] L.E. Seitzman, R.N. Bolster and IL. Singer, Surf. Coat. Technol., 52 (1992) 93. [S] L.E. Seitzman, R.N. Bolster, I.L. Singer and J.C. Wegand, Tribol. Trans., in press. [6] R.N. Bolster, IL Singer, J.C. Wegand, S. Fayeulle and CR. Gossett, Surf. Coat. Technol., 46 (1991) 207. [7] L.E. Seitzman, I.L. Singer, R.N. Bolster and C.R. Gossett, Surf: Coat. Technol., 51 (1991) 232. [8] F.A. Smidt, In?. Mater. Rev., 35(2) (1990) 61. [9] B.D. Cullity, Elements of X-ray Dzfiacrion, 2nd edn., Addison Wesley, Reading, MA, 1978, p. 102. [lo] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, International Center for Diffraction Data, Swarthmore, PA, 1987, Card 37-1492. [ 111 H. Dimigen, H. Hiibsch, P. Willich and K. Reichelt, Thin Solid Films, 129 (1985) 79. [ 121 J.R. Lince and P.D. Fleischauer, J. Mater. Res., 2(6) (1987) 827. [ 131 P.A. Bertrand, J. Mater. Res., 4(l) (1989) 180. [ 141 V. Buck, Surf. Coal. Technol., 57 (1993) 163. [ 151 M.W. Thompson, Defects and Radiation Damage in Metals, Cambridge University Press, Cambridge, UK, 1969. [ 161 J.P. Biersack and L.C. Haggmark, Mucl. Instr. Meth., 174 (1980) 257. [ 171 G.A. Tsigdinos, Aspects of Molybdenum and Related Chemistry, Springer-Verlag, New York, 1978 p. 65. [ 181 J.C. Wildervanck and F. Jellinek, Zeit. Anorg. Allgem. Chemie, 328 (1964) 309. [19] B.E. Warren, Phys. Rev., 59(9) (1941) 693. [20] E. Bergmann, G. Melet, C. Mtiller and A. Simon-Vermot, Tribol. Int., 14(6) (1981) 329. [21] M.R. Hilton and P.D. Fleischauer, J. Mafer. Res., Z(2) (1990) 406. [22] Q. Cong, D. Yu, J. Wang and Y.J. Ou, Thin Solid Films, 209 (1992) I. [23] C. Donnet, Th. LeMogne and J.M. Martin, Surf: Coat. Technol., 62 (1993) 406. [24] J. Moser and F. Levy, J. Mater. Res, 7(3) (1992) 734.
ELSEVIER
Effects of temperature
143-147
and ion-to-atom ratio on the orientation IBAD MoS, coatings
of
L.E. Seitzman, R.N. Bolster, I.L. Singer Nuval Research Laboratory, Received
Code 6170, Washington,
31 March
1994; accepted
DC 20375-5342,
28 October
USA
1994
Abstract MoS, coatings, 55-800 nm thick, were grown by ion-beam-assisted deposition (IBAD) using different ion-to-atom ratios and deposition temperatures. Crystallinity and orientation of the IBAD MoS, coatings were determined by X-ray diffraction (XRD). Only XRD peaks corresponding to (001), (hkO), and amorphous MoS,, and a previously unreported low-20 peak (20 w 10.7”) were observed. The basal (002) peak intensities varied primarily with ion-to-atom ratio; the greatest basal intensity occurred when the ion-to-atom ratio produced about 1 displacement per atom. Although a secondary factor in basal intensity, deposition temperature was the primary factor in edge (100) intensity. Edge intensity increased with increasing temperature; it appears that
the increases are due to annealing of randomly-oriented MoS,, which converts to edge orientation. is unknown, but appears to be associated with the basal planes of MoS,. Keywords: Deposition;
Molybdenum
sulphide; Oxidation
1. Introduction Orientation is an important factor in the performance of MoS, coatings for solid lubrication [ 11, catalysis [2], and photoelectrochemistry [ 31. Usually the preferred orientation is basal, where the (001) planes are parallel to the substrate. However, most deposition processes result in mainly edge orientation, where the (hk0) planes are parallel to the substrate. The ion-beam-assisted deposition (IBAD) process is capable of producing basal-oriented MoS, coatings [4]. In previous studies on IBAD MO&, we have shown that endurance depends on the orientation of the crystalline material [5], and that orientation, in turn, depends on the ion-toatom ratio [4] (a measure of the amount of concurrent ion bombardment). In those studies, however, the deposition temperature was kept to ambient levels. Certain IBAD Mo!$ coatings annealed at 573 K showed improved friction and crystallinity [6] without a reduction in endurance [7]. Perhaps similar improvements can be achieved by tailoring MoS, orientation through optimizing deposition temperature. Deposition temperature is known to influence the microstructure of Elsevier Science S.A. SSDI 0040-6090( 94) 06419-9
The origin of the low-20 peak
metallic and ceramic coatings, including those grown with the IBAD process [S]. This investigation will focus on the effect of both ion-to-atom ratio and deposition temperature on IBAD MoS, orientation. Coatings will be deposited over a range of ion-to-atom ratios at several temperatures up to 573 K. The orientation of the coatings will be investigated by X-ray diffraction (XRD). The relationship between orientation and deposition parameters will be discussed in terms of both ion beam and thermal effects.
2. Experimental
details
MoS, coatings were deposited by the IBAD process, as described elsewhere [4,6]. Two Kaufman ion guns were used to independently sputter MO and S targets with Ar ions. A third Ar-ion gun (the assist gun), aligned near-normal to the sample stage, was used for both sputter cleaning before and ion bombardment during deposition. The base pressure in the system was around lo-’ Pa and the operating pressure around 0.05
L. E. Seitzman
144
et al. I Thin Solid Films 260 (1995) 143-147
a_
.3WK373K
30; a
Ati 10
20
30
40
50
60
70
28 Fig. 1. X-ray diffraction spectra of three IBAD MoS, coatings of similar thickness (320-360 nm) exhibiting (a) low and (b) high intensity (002) peaks and (c) an additional peak at low-20 (10.7”). Peaks marked by 1 are from the coating and unlabeled peaks are from the steel substrate. Solid heavy lines indicate the peak positions of the XRD powder standard.
Pa of Ar. All three ion guns were operated at 1 kV. The substrates were mounted on a rotatable sample stage and could be heated to 573 + 10 K. In most cases, a thin diffusion barrier [7] of TiN, typically 30 nm thick, was deposited between the substrate and the MoS,. Substrates used in this investigation were 52100, 44OC, M50, and AMS 5749 steels, Ti and Ti alloys. The deposition parameters varied in this study were the ion-to-atom ratio, R, and the deposition temperature. R is a measure of how much concurrent bombardment the growing coating received and is determined using the ratio of the assist-gun current to the deposition rate (coating thickness t deposition time) [4]; as the degree of concurrent bombardment increases, R increases. R values ranged from 0 to 0.12 based on an assist beam current range of 0- 16 mA and a deposition rate range of 0.28-0.45 nm s-‘. Coatings were deposited to thicknesses of 55-800 nm at temperatures between 300 K and 573 K. Some coatings deposited at ambient temperature, 300 < T < 350 K, were later annealed at 573 K for 2 h in the deposition chamber (2 x 10m4 Pa). Also, two depositions, one at 323 K and one at 473 K, were performed on substrates mounted at a 30” angle to the sample stage. The structure and orientation of the coatings were determined by XRD. A Rigaku D/max-B series diffrac-
Fig. 2. Normalized ings as a function ture.
basal (002) XRD intensity of IBAD MoS, coatof ion-to-atom ratio, R, and deposition tempera-
tometer, operated in standard O-20 geometry, was used to identify crystallites oriented parallel to the substrate. A high intensity X-ray source (50 kV and 200 mA), desirable for thin coating measurement, was obtained using a rotating Cu anode. Diffraction slits of l/6” were used to limit the incident beam size at low incidence angles and a monochromator was used to allow only Cu KE X-rays into the detector. Comparison between coatings was made by dividing the MoS, basal (002) and edge ( 100) peak intensities by the steel (200) peak intensity; this normalization procedure accounts for any differences in sampling area.
3. Results 3.1. General observations
Representative spectra from similar thickness IBAD MoS, coatings on steel are shown in Fig. 1. Peaks labelled with arrows correspond to the IBAD coating; unlabelled peaks correspond to the steel substrate. Three types of MoS, peaks were observed: (1) broad basal (001) and edge (hk0) peaks characteristic of small crystallites [ 91; (2) a very broad, weak peak from 30” to 45”, attributed to amorphous MoS, [4,6]; and (3) a previously unreported low-20 peak, seen in Fig. l(c), around 20 = 10.7” (the low-20 peak at 10.7” does not correspond to any major peak of any MO sulfides or oxides in Ref. [7] (JCPDS Powder Diffraction File)). No IBAD MoS, coating exhibited a mixed (hkl) peak. When a TIN intermediate layer was present, the TiN (200) peak at 42” was observed.
145
L. E. Seitzman et al. 1 Thin Solid Films 260 (199.5)143-147 1.5
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”
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02.
Pcfw+L
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0.06
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ION-TO-ATOM RATIO, R Fig. 4. Normalized basal (002) IBAD MoS, coatings deposited annealed at 573 K (0). Fig. 3. Normalized ings as a function ture.
and edge (100) XRD intensity of at ambient temperature (0) and
edge (100) XRD intensity of IBAD MoS, coatof ion-to-atom ratio, R, and deposition tempera-
The most prominent MoS, peaks, the (002) at 13” and the (100) at 33”, were shifted from the values of a reference standard [lo]. The d-spacing standards are, doo2= 0.6155 nm and d,,, = 0.2738 nm, whereas the TBAD MO& d-spacings ranged from 0.655 < doo2 -C0.707 nm and 0.266 < d,,, < 0.271 nm. These d-spacing shifts are similar to those reported for sputter-deposited MoS, [ 1l- 141. The d-spacing of the low-20 peak ranged from 0.769 to 0.850 nm. Basal intensity varied between about 1 and 38, while the edge intensity ranged from 0 to 1.3. Examples of a low basal and a high basal intensity coating are shown in Figs. l(a) and l(b), respectively. Basal and edge intensity exhibited no thickness dependence. The range of coating thickness where only (001) peaks were found was 55-650 nm, and the range where both (001) and (hk0) peaks occurred was 180-800 nm. Also, the basal and edge intensities exhibited no dependence on coating substrate. Similar MoS, peak intensities were observed when the coating was simultaneously deposited at room temperature on steel with and without a TiN layer, on different steels, or on steel and Ti substrates (again with or without a TiN interlayer). The low-20 peak was observed primarily in coatings deposited at elevated temperatures. The low-20 peak was found in coatings with both low and high basal intensity, and the intensity ratio of low-20/(002) ranged from about 0.3 to 1. One coating with a low20/(002) ratio of 0.5 was sequentially ion thinned and analyzed by XRD; the ratio remained 0.5 even when 240 nm was removed (original coating thickness was 330 nm).
3.2. Intensity
vs. R and temperature
No change in peak width with R or deposition temperature was observed. However, the intensities of three MoS, peaks changed as follows: the broad peak (observed from 30” to 45”) decreased with increasing temperature, and the basal and edge peaks varied with R and/or deposition temperature as shown in Figs. 2 and 3, respectively. In Fig. 2, basal intensity increased then decreased with increasing R. At each deposition temperature, the basal intensity reached a maximum around R = 0.04. The greatest basal intensity occurred in coatings grown at elevated temperature ( 2 423 K). Fig. 3 shows that edge intensity increased with increasing temperature, but exhibited no apparent dependence on R at any temperature. 3.3. Annealing
efSects
The normalized XRD intensities of coatings deposited at ambient temperature and subsequently annealed are plotted in Fig. 4 as closed and open circles, respectively. Annealing typically produced only small ( < 20%) increases or decreases in basal intensity but large (up to 4000%) increases in edge intensity. The increased edge intensity was greatest for coatings exhibiting low basal intensity in the as-deposited coating. While peak intensities changed, peak widths remained constant.
4. Discussion
This XRD investigation of temperature effects has added three new observations to those of previous
146
L. E. Seitzman et al. I Thin Solid Films 260 (1995) 143-147
IBAD MoS, studies [4,6]: (1) the presence of a low20 peak; (2) basal (002) intensity increases when deposition temperature is raised; and (3) edge intensity increases with increasing deposition and annealing temperature. Although the results clearly indicate that the low-20 peak is associated with the basal (002) peak, we do not know the origin of the low-20 peak at this time. Possible explanations include a second MoS, basal peak representing a discrete expansion of the c axis or diffraction from trapped Ar atoms. Further work is required in order to determine the cause of the low-20 peak. The remainder of the discussion will focus on the observed ion bombardment and temperature effects on basal and edge intensities. When the deposition temperature was raised to 423 K or greater the basal intensity increased, especially in the range 0.03 < R < 0.05. This is the same range of R where basal intensity was greatest at all temperatures. Both facts indicate that although temperature is a factor, bombardment effects are primarily responsible for basal orientation. Theoretical and experimental studies have shown that concurrent ion bombardment can affect the degree of preferred orientation in coatings [8]. Preferred orientation was often greatest when R corresponded to a critical value of one displacement per coating atom ( 1 dpa). The R value corresponding to 1 dpa can be determined by calculating the average number of displaced atoms, v, due to collision with an Ar ion of energy E,: R( 1 dpa) = l/v(E,,) The average number of displaced atoms can be estimated by using a simple Kinchin-Pease model [ 151 and ignoring the differences in atomic mass between Ar, MO, and S. Then v(E,) = E,/2E,, where Ed is the energy needed to displace a lattice atom. Assuming Ed = 25 eV, R( 1 dpa) = 0.05. A more detailed calculation using the TRIM code [ 161 yields a similar result. The secondary factor in determining basal intensity is temperature. Temperature can increase both bulk atom and adatom mobility. If bulk atom mobility had increased significantly, the crystallite size would have increased; however, the basal (002) peak widths did not narrow with temperature indicating constant crystallite size. We speculate that the increased basal intensity at higher temperatures was due to increased adatom mobility. Temperature was the primary factor influencing edge intensity of IBAD MO&. The increasing edge intensity with temperature can be due to increased edge crystallite growth or nucleation. Growth, as explained above, is unlikely because the (100) peak widths remained broad at all temperatures. Edge nucleation can occur either in or below the ion-bombarded layer. Based on
calculations using the TRIM code [ 161, the bombarded layer is the outermost 6 nm of the growing coating. Edge nucleation in the bombarded layer is probably insignificant, since edge intensity is independent of ionto-atom ratio. Therefore, edge nucleation must be occurring by conversion of material underlying the bombarded layer. This is primarily a thermal effect, as evidenced by the increased edge intensity in the coatings annealed at 573 K. The underlying material which converted to edge orientation must have been initially either amorphous or crystalline with a non-edge orientation, such as basal. Certainly amorphous-to-edge conversion is a possibility; the decline in intensity of the 30” to 45” peak with increasing deposition temperature is consistent with decreasing amorphous content. On the other hand, basal-to-edge conversion cannot be the only mechanism because some annealed coatings showed increased edge intensity without a significant change in basal intensity. Crystallites with random orientation might also convert to edge orientation, although random orientation would appear to be ruled out by the absence of (hkl) peaks in the diffraction spectra. However, there is a structure which can be randomly oriented yet not give mixed (hkf) XRD peaks: turbostratic MoS,. The turbostratic structure is a two-dimensional arrangement, where the basal planes are randomly rotated about the c axis like a poorly stacked deck of cards [ 17,181. Warren [ 191 has shown that turbostratic structure gives only broad (001) and (hk0) diffraction peaks; turbostratic crystallites with the basal planes neither parallel nor perpendicular to the substrate will contribute only to the background of the diffraction spectrum. Therefore, the decrease in the 30” to 45” peak intensity with temperature may also be attributed to random-to-edge conversion of turbostratic material. We note that the turbostratic MoS, may also be present in sputter-deposited MoS,. The turbostratic structure of sputter-deposited MO&, in fact, was reported in one investigation [20], although the diffraction spectrum was not shown. XRD spectra of sputter-deposited MoS, usually contain only (001) and (hk0) peaks [ ll-14,21,22]; the absence of (hkl) peaks has historically been interpreted to mean only basal and edge orientations exist. However, as noted above, these XRD features can also be explained by the presence of turbostratic MO&. In fact, azimuthal disorder of sputter-deposited MoS, has been reported [23,24], and crystallites with mixed orientation have been imaged in the electron microscope, although the electron diffraction yields no (hkl) intensity [21,23]. Further investigation is required to confirm the existence of the turbostratic structure in sputter-deposited and IBAD MO&.
L. E. Seitzman et al. I Thin Solid Films 260 (1995) 143-147
5. Summary and conclusions
Orientation of IBAD MO&, as determined by XRD, is a function of both ion-to-atom ratio R and deposition temperature, but not coating thickness or substrate chemistry. Ion-to-atom ratio is the primary factor in controlling basal intensity, and deposition temperature plays a secondary role. The greatest basal intensity occurs when the ion bombardment produces about 1 displacement per atom (0.03 < R < 0.05)in the coating and the temperature is greater than or equal to 423 K. Temperature is the primary factor in controlling edge intensity. The observed increasing edge intensity with temperature may result from increasing conversion of amorphous or randomly-oriented turbostratic MoS, to edge orientation. A new, unexplained low-20 peak (20 z 10.7”) is observed in IBAD MO&, usually when the deposition temperature exceeds 373 K. This low-20 peak appears to be associated with the basal planes of MO&.
Acknowledgments
The authors would like to thank the Office of Naval Research for funding and L.E.S. would like to thank the National Research Council for support through a post-doctoral fellowship. This work was partly supported by the SD10 tribomaterials group.
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