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The effect of transport ratio and ion energy on the mechanical properties of IBAD niobium nitride coatings
Surface and Coatings Technology 146 – 147 (2001) 243–249
The effect of transport ratio and ion energy on the mechanical properties of IBAD niobium nitride coatings M.L. Klingenberga,*, J.D. Demareeb a
b
Concurrent Technologies Corporation, 100 CTC Drive, Johnstown, PA 15904, USA Army Research Laboratory, AMSRL-WM-MC, Aberdeen Proving Ground, Aberdeen, MD 21005, USA
Abstract Niobium nitride films were produced by ion beam assisted deposition (IBAD) using low (90 eV) and moderate (750–800 eV) energy nitrogen ion beams and electron-beam evaporated niobium. The transport ratios (ion to atom arrival ratios) used were approximately 0.062, 0.125, 0.25, 0.33, 0.50, 0.75 and 1.0. The resulting microstructure, crystal phase, and composition of the films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry (RBS). The hardness, coefficient of friction, and wear resistance were assessed. The research detailed herein describes relationships between IBAD process parameters, NbN film structure and phase, and the mechanical properties of the films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Niobium nitride films; Ion beam assisted deposition (IBAD); Nitrogen ion beams
1. Introduction The Group V transition metal nitrides are known for their high hardness, toughness, and wear resistance, but these properties can vary significantly with composition w1–5x. Ion beam assisted deposition (IBAD) is known to provide increased control of composition, internal stress, morphology, and texture, leading to improved adhesion, coating density, and mechanical properties by varying the deposition species, ion mass, ion energy, and substrate temperature w6,7x. Amorphous, nanocrystalline, textured crystalline, epitaxial, or metastable crystalline coatings can all be made by altering these variables w8,9x. Therefore, it is important to understand the effect of varying processing parameters on the resulting coating composition and mechanical properties. Niobium nitride is an excellent candidate for such studies since it can assume seven crystalline phases w10x. Niobium nitride has long been valued for its superconducting properties, but more recently Wong et al. * Corresponding author. Tel.: q1-814-269-6415; fax: q1-814-2692798. E-mail address: [email protected] (M.L. Klingenberg).
recognized the applicability of this refractory material’s high strength, high melting point, and high transition temperature to a variety of wear applications w4,11,12x. Previous studies with sputtered NbN have obtained a variety of cubic, hexagonal close packed (hcp), and amorphous structures with varying hardness values w4,13–15x. Baba et al. used 2–20-keV IBAD to deposit NbN films and characterized the corrosion resistance of the hcp d9-NbN, hcp b-Nb2N, and bcc solid solutions of Nb–N films w1x. The focus of the present study was to characterize the tribological properties of NbN films produced using lower-energy IBAD processing. 2. Experimental Niobium nitride coatings were deposited on a variety of substrates including AISI 4340 steel, single crystal sodium chloride, and glass using an e-beam ion beam processing system. Cryopumps were used to achieve a base pressure of 3.6–7.3=10y5 Pa. A residual gas analyzer (RGA) indicated that water and oxygen levels did not exceed 4.8=10y6 and 2.7=10y6 Pa, respectively, and the nitrogen partial pressure ranged from 4– 13=10y6 Pa. Upon achieving base vacuum, the low
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M.L. Klingenberg, J.D. Demaree / Surface and Coatings Technology 146 – 147 (2001) 243–249
energy (90 eV), end-hall ion source was operated at current densities of 0.34–0.37 mAycm2, and the moderate energy (750–800 eV), RF ion source was operated at current densities of 0.09–0.18 mAycm2 during sputter cleaning. Current density calculations are based on the ion current, as measured by the source, and the area that the beam intersected at the platen. The IBAD total operating pressure ranged from 1.6–14=10y3 Pa with partial pressures of nitrogen, oxygen, and water ranging from 9.7–71=10y4, 1.3–72=10y7, and 7.2–10=10y6 Pa, respectively. Niobium deposition rates were varied to obtain total ion-to-atom transport ratios of 0.062, 0.125, 0.25, 0.33, 0.50, 0.75 and 1.0 and ranged from 0.09 to 0.45 nmys. Although previous studies have shown that plasma sources yield approximately 1.89 nitrogen atoms per unit charge, only the total beam current was used to calculate the nitrogen arrival ratios w16x. The transport ratios cited in this paper, however, do account for the 308 angle of incidence (off of the substrate normal) of the ion beam. Sodium chloride crystals received a targeted thickness of 75 nm, and metal and glass substrates were coated to targeted thicknesses of 1000 and 2000 nm. The final film thickness varied for the coatings due to nitrogen and oxygen uptake, film morphology, and ion beam sputtering. Compositional and thickness analyses of the 75-nm films were performed using Rutherford backscattering spectrometry (RBS), using a 2-MeV Heq beam at a backscattering angle of 1708, the results were analyzed using the simulation program RUMP w17x. Film morphology was analyzed using 20-keV electron beam SEM. TEM and X-ray diffraction (XRD) were used to identify the crystalline phase of the films. A 120-keV electron beam was used to obtain the diffraction patterns using SAD. For XRD analysis, Cu Ka radiation was used over a scan range of 2us20–808, and the X-ray tube was set at a 58 take-off angle. Hardness was measured on AISI 4340 steel substrates using the continuous stiffness to constant depth (200 nm) mode of a NanoIndenter. Stress was analyzed by measuring the radius of curvature of the 1000-nm-thick films on glass cover slips. The residual film stress was calculated by using the Stoney equation and knowing the film thickness, substrate thickness, modulus of the substrate, and Poisson’s ratio of the substrate w18x. Dry sliding wear tests were performed to assess the wear resistance and the frictional coefficients of the coatings. The tests were performed using a ball-on-disk tribometer, in which 440C stainless balls were worn, under 50–150-g loads with no lubrication, against the coatings. The sliding speed ranged from 4.61 to 5.10 cmys, and the test radius was set at 3 mm. All tests were run for 1 h. The wear scars were analyzed using profilometry, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Profilometry
techniques were used to measure the depth of the scar while SEM enabled analysis of the wear mode and estimation of the depths of very shallow wear scars. SEM images were taken in the backscattering electron imaging (BEI) mode to ascertain the topography of the scar and in secondary electron imaging (SEI) mode to better define surface effects. EDS was performed using a 35-keV electron beam and was operated in spot mode to determine the source of any adherent debris. 3. Results and discussion 3.1. Composition and microstructure RBS showed that the films contained 30–40 at.% nitrogen even at the lowest ion to atom ratios (e.g. Rs 0.062 and 0.125) using the 90-eV beam, indicating significant reactivity with the nitrogen gas background coming from ion source operation. Upon increasing the transport ratio to 0.25, nitrogen content increased to 48 at.%, which remained unchanged with increasing transport ratio. The 750–800-eV coatings also displayed increased incorporation of nitrogen with increased transport ratio, from 34 at.% nitrogen at Rs0.062 to 60 at.% nitrogen at Rs1.0. Most coatings contained approximately 9–17 at.% oxygen, with the lower transport ratio coatings containing the most oxygen. However, the 750– 800-eV coating of Rs0.75 contained anomalously large amounts of oxygen (14 at.%), but still a substantial (51 at.%) amount of nitrogen, indicating some unexplained inconsistencies in the deposition. TEM and XRD revealed that all the films were microcrystalline, which is consistent with Baba’s observation of NbN films produced with higher energy ions (2–20 keV) w1x. XRD (Fig. 1) of most films (excepting the 90-eV films of Rs0.062 and 0.125), produced patterns that were most consistent with the fcc structure of d-NbN. Grain orientation was either random or displayed texturing in the N111M or N200M directions normal to the substrate. The pattern displayed by the low energy films of Rs0.062 and 0.125 was a result of its tetragonal g-Nb4N3 structure with preferred orientation in the N111M direction and considerable peak broadening. SEM examination of fractured coatings revealed that all of the 90-eV coatings and the 750–800-eV coatings with Rs0.062, 0.125 and 0.50 displayed a densely packed columnar microstructure (Fig. 2) w19x, similar to Zone 2 metallic films. These structures are expected for the 90-eV films and the 750–800-eV films with low transport ratio due to high operating pressures and low energy bombardment, and low ion fluxes, respectively. The unexpected columnar structure found in the 750– 800-eV, Rs0.50 film may be result of the high operating pressure experienced during this particular deposition
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245
Fig. 1. XRD patterns for the 90 and 750–800-eV coatings.
trial, which might have led to charge exchange neutralization w20x. The 750–800-eV coatings with Rs0.25, 0.33 and 0.75 displayed a non-columnar growth morphology, as is shown in Fig. 3 w19x. The increased energy, combined with increased ion flux, was evidently sufficient to enable diffusional processes, leading to very dense films. The 750–800-eV coatings with Rs0.75 and 1.0 were too thin for SEM analysis due to sputtering loss.
Fig. 2. SEM cross-section of 90-eV coating with Rs0.25.
3.2. Residual stress and hardness Stress analysis showed that the 90-eV coatings Rs 0.062–0.25 exhibited extremely high tensile stress with the films cracking and peeling on the substrates. With increased R, e.g. 0.33 and 0.50, the films continued to relieve stress through cracking, but did not peel. Films of Rs0.75 and 1.0 did not crack and yielded compressive stress values 1.8 and 0.49 GPa, respectively. This suggests that increasing ion bombardment initially
Fig. 3. SEM cross-section of 750–800-eV coating with Rs0.25.
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M.L. Klingenberg, J.D. Demaree / Surface and Coatings Technology 146 – 147 (2001) 243–249
Fig. 4. Residual stress in films deposited with 750–800-eV IBAD.
reduced the tensile stress of the evaporated coatings, and subsequently led to compressively stressed coatings at high R values. None of the 750–800-eV films cracked or peeled, and all displayed compressive stress (Fig. 4). Stress in the Rs0.062 coating was highly asymmetric, such that two values were obtained when rotating the circular coverslip 908. All of the other 750–800-eV coatings deformed the glass cover slips uniformly. A minimum compressive stress value was obtained at Rs0.25, before and after which the compressive stress values gradually increased. Compressive stress at low R values (e.g. 0.062 and 0.125) could be due to oxygen and nitrogen incorporation into the coating, which is consistent with the observations of Cuomo et al. w21x. With a slight increase of ion flux to Rs0.25, oxygen incorporation decreased, leading to slightly lower levels of compressive stress. Further increases in transport ratio resulted in increased incorporation of nitrogen and densification of the film, producing increased compressive stress. Coatings produced at Rs0.75 and 1.0 were, however, too thin for accurate measurement of the coating stress due to sputtering. Hardness measurements, as depicted in Fig. 5, show increasing hardness with increasing transport ratio for the 90-eV coatings, while the 750–800-eV coatings exhibited a more complex behavior that is thought to be the result of the combined effects of intrinsic film stress, coating thickness, and crystalline phase. The highest moderate energy hardness value was obtained for Rs
0.33, an amorphous coating, as determined by XRD. The measurements obtained for Rs0.33–1.0 are the result of substrate influence. The 200-nm penetration depth was greater than 1y10 the actual coating thickness due to sputtering effects. Therefore, the much softer (5.74 GPa) 4340 steel substrate hardness influenced the measurements. The hardness value for the moderate energy film of Rs1.0 suggests the possibility that the 4340 steel was annealed by the heat induced by the ions at the surface. The hardness of all other films is thought to be due to the competition between increasing nitride character and stress. 3.3. Friction and wear Coatings were wear tested at increasing loads under unlubricated sliding wear conditions to compare and rank them for wear endurance while monitoring their frictional behavior. Two coatings, the 750–800-eV at Rs0.25, and 90-eV at Rs0.062, performed significantly better than all other coatings during ball-on-disc wear testing (indicated by arrows in Fig. 6). The 750–800eV (Rs0.25) coating demonstrated the best overall abrasive wear resistance against 440C stainless steel under heavy loading conditions, even though adhesive wear was evident. The 90-eV (Rs0.062) coating provided better abrasive wear characteristics at loads of 50, 100, and 125 g, but completely delaminated upon testing at a 150-g load. The 750–800-eV (Rs0.25) coating displayed much less compressive stress than the other
M.L. Klingenberg, J.D. Demaree / Surface and Coatings Technology 146 – 147 (2001) 243–249
247
Fig. 5. Coating microhardness as a function of transport ratio and ion energy.
moderate energy coatings and lower overall stress than most of the low energy coatings. The 90-eV (Rs0.062) coating had the highest stress level overall, but still provided a high level of unlubricated sliding wear
resistance, indicating that the coating was more coherentyadherent than most. The complex nature of sliding wear is demonstrated by the fact that the two most wear resistant coatings had different composition,
Fig. 6. Coating wear loss after ball-on-disc testing against 440C at various loads.
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Table 1 Unlubricated frictional coefficients against 440C stainless steel Coating type
m50
m100
ME, Rs1.0 ME, Rs0.75 ME, Rs0.50 ME, Rs0.33 ME, Rs0.25 ME, Rs0.125 ME, Rs0.062 LE, Rs1.0 LE, Rs0.75 LE, Rs0.50 LE, Rs0.33 LE, Rs0.25 LE, Rs0.125 LE, Rs0.062
0.70 0.78 0.90 0.84 0.55 0.21 0.80 0.85 0.23 0.98 0.20 0.22 0.23 0.24
1.35
0.43 1.63
m125
m150
2.20
2.80
1.64 1.10 1.75 0.34 0.37
2.30 2.10 0.75
2.65
ME, moderate energy; LE, low energy.
crystalline phases, and morphologies, as described below. During the initial wear test at 50 g load, six coatings displayed relatively high frictional coefficients (Table 1), e.g. greater than 0.70, and severe abrasive or adhesive wear (see Fig. 6 — first row). The six coatings included two 90-eV coatings, Rs0.50 and 1.0, and four 750–800-eV coatings, Rs0.062, 0.33, 0.50 and 0.75. The 90-eV coatings, Rs0.50 and 1.0, both displayed abrasive wear and some delamination. Plowing was evident in Rs0.50, and some adhesive wear was noted in Rs1.0. The 750–800-eV coatings Rs0.062 and 0.33 displayed abrasive wear, and significant loss of cohesion was noted in Rs0.33. At Rs0.50, the wear mode appeared to change from abrasive to adhesive wear, with 440C stainless steel deposits welding to the coating. EDS confirmed the composition of the debris. Because no scoring was evident, no wear bar is shown in Fig. 6. The hard, brittle, Rs0.75 coating suffered severe abrasive and adhesive wear with plowing and delamination. Although no wear was visually observed for the moderate energy coating of Rs1.0, the EDS method of detecting subtle differences in coating thickness in the wear tracks could not be used due to the very thin layer of coating present. As a result, there is no wear bar shown for the moderate energy coating of Rs1.0 in Fig. 6. Upon testing using a 100-g load, four additional coatings exhibited coefficients of friction greater than 1 and displayed measurable adhesive and abrasive wear (Table 1 and Fig. 6 — second row). The 750–800-eV coating, Rs0.125, experienced decohesion in selected areas, although most of the film had been uniformly worn. The extremely thin and brittle, moderate energy coating, Rs1.0, was completely worn away with deep gouges in the base material and welding of the stainless steel ball. Again, no wear bar is shown for this coating because the coating was too thin to use the EDS method
of removal and the complete removal of the coating made measurement using EDS impossible. Areas of the low energy coating, Rs0.25, were removed and redeposited elsewhere, and the stainless steel ball had welded to the scar area. The 90-eV coating, Rs0.75, experienced mild to severe abrasive wear, as well as adhesive welding of the stainless steel ball. Although the 90-eV coating of Rs0.33 displayed a high frictional coefficient, no scar could be measured using profilometry or SEM, substantiating that friction and wear are not necessarily related. Three of the four remaining coatings exhibited frictional coefficients greater than 2 when tested using a 125-g load. SEMyEDS analysis confirmed that adhesive welding occurred for these three coatings, but profilometry indicated that only the low energy coatings had measurable wear. Because the 750–800-eV coating, Rs 0.25, and the 90-eV coating, Rs0.062 displayed minimal abrasive wear in comparison to the other coatings, each was tested further. Both remaining coatings displayed coefficients of friction greater than 2 upon testing at 150 g load. SEM yEDS confirmed that much 440C stainless steel had welded to both coatings. The 90-eV coating experienced complete delamination in areas; however, only a minor amount of coating material was removed from the 750– 800-eV coating. SEM imaging indicated some loss of cohesion for Rs0.25, but the appearance was very similar to that displayed for the 125-g load. 4. Conclusions These studies have shown that hard, wear resistant niobium nitride films can be deposited onto 4340 steel using both low energy and moderate energy ion sources. Appropriate deposition parameters for synthesizing these films have been identified, correlating composition and structure to the beam energy and transport ratio. The wear characteristics of the niobium nitride films could not be correlated with hardness, stress, morphology, or composition. The 750–800-eV (Rs0.25) coating demonstrated the best abrasive wear resistance against 440C stainless steel under heavy loading conditions, but adhesive wear was evident. The 90-eV (Rs0.062) coating provided better abrasive wear characteristics at lighter loads, but ultimately failed catastrophically at the highest load. The coatings were dramatically different in terms of composition, morphology, crystalline phase, hardness, and stress. The 90-eV coating exhibited less nitride character, was more ductile, and was highly stressed, as indicated by cracking. These characteristics enabled the film to deform plastically, until a critical load was reached where the shear stresses were greater than the bond strength of the film, leading to delamination. In the 750–800-eV coating, the relatively low stress, near-stoichiometric NbN composition, intermedi-
M.L. Klingenberg, J.D. Demaree / Surface and Coatings Technology 146 – 147 (2001) 243–249
ate modulus, and moderate hardness, led to the improved wear properties. Acknowledgements The authors would like to acknowledge the work of Mr Jerry Stem who assisted with the deposition of the niobium nitride films, Dr James K. Hirvonen who provided guidance during testing. Support also was provided Dr Joseph Argento of Army Industrial Ecology Center at Picatinny Arsenal, NJ who supported the activities reported on herein through the Sustainable Green Manufacturing project. Gratitude also is expressed to Dr Jogender Singh who provided academic support. References w1x K. Baba et al., Nucl. Instrum. Meth., B 127y128 (1997) 841. w2x M. Gupta, V.A. Gubanov, D.E. Ellis, J. Phys. Chem. Solids 38 (5) (1977) 499. w3x L.E. Toth, Transition Metal Carbides and Nitrides: A Series of Monographs, 7, Academic Press, NY, NY, 1971, pp. 1–9. w4x K.S. Havey, J.S. Zabinski, S.D. Walck, Thin Solid Films 303 (1-2) (1997) 238.
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w5x J.E. Sundgren, L Hultman, in: Y. Pauleau (Ed.), Materials and Processes for Surface and Interface Engineering, Kluwer Academic Publishers, The Netherlands, 1995, p. 453. w6x G.K. Wolf, J. Vac. Sci. Technol. A10 (4) (1992) 1757. w7x G. Fenske, R.A. Erck, Adv. Ceramics Rep. March (1991) 25. w8x G.K. Hubler, J.K. Hirvonen, in: F. Reidenbach (Ed.), ASM Handbook, 5, ASM International, Materials Park, OH, 1994, p. 593. w9x G.K. Wolf, Mater. Sci. Eng. A115 (1989) 118. w10x H.O. Pierson, Handbook of Refractory Carbides and Nitrides, Noyes Publications, Westwood, NJ, 1996, p. 170. w11x Z. Wang et al., J. Appl. Phys. 79 (10) (1996) 7837. w12x C.H. Winter, K.C. Jayaratne, J.W. Proscia, Mater. Res. Soc. Symp. Proc. (1994) 103. w13x M.S. Wong, W.D. Sproul, J. Vac. Sci. Technol. 11 (4) (1993) 1528. w14x S.A. Barnett, W.D. Sproul, M.S. Wong, Deposition and Properties of Novel Nitride Superlattice Coatings, Department of Energy Report No. DOEyERy45434-5, 1996, pp. 5–6. w15x I. Hotovy et al., Phys. Status Solidi A161 (1) (1997) 97. w16x D. VanVechten et al., J. Vac. Sci. Technol. A8 (2) (1990) 821. w17x L.R. Doolittle, Nucl. Instrum. Meth. B15 (1986) 227. w18x H. Ljungcrantz et al., J. Vac. Sci. Technol. A16 (5) (1998) 3106. w19x B.A. Movchan, A.V. Demchisin, Phys. Met. Metallogr. 28 (1969) 83. w20x G.K. Hubler et al., J. Vac. Sci. Technol. A8 (2) (1990) 831. w21x F.A. Smidt, Int. Mater. Rev. 35 (2) (1990) 108.
The effect of transport ratio and ion energy on the mechanical properties of IBAD niobium nitride coatings M.L. Klingenberga,*, J.D. Demareeb a
b
Concurrent Technologies Corporation, 100 CTC Drive, Johnstown, PA 15904, USA Army Research Laboratory, AMSRL-WM-MC, Aberdeen Proving Ground, Aberdeen, MD 21005, USA
Abstract Niobium nitride films were produced by ion beam assisted deposition (IBAD) using low (90 eV) and moderate (750–800 eV) energy nitrogen ion beams and electron-beam evaporated niobium. The transport ratios (ion to atom arrival ratios) used were approximately 0.062, 0.125, 0.25, 0.33, 0.50, 0.75 and 1.0. The resulting microstructure, crystal phase, and composition of the films were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry (RBS). The hardness, coefficient of friction, and wear resistance were assessed. The research detailed herein describes relationships between IBAD process parameters, NbN film structure and phase, and the mechanical properties of the films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Niobium nitride films; Ion beam assisted deposition (IBAD); Nitrogen ion beams
1. Introduction The Group V transition metal nitrides are known for their high hardness, toughness, and wear resistance, but these properties can vary significantly with composition w1–5x. Ion beam assisted deposition (IBAD) is known to provide increased control of composition, internal stress, morphology, and texture, leading to improved adhesion, coating density, and mechanical properties by varying the deposition species, ion mass, ion energy, and substrate temperature w6,7x. Amorphous, nanocrystalline, textured crystalline, epitaxial, or metastable crystalline coatings can all be made by altering these variables w8,9x. Therefore, it is important to understand the effect of varying processing parameters on the resulting coating composition and mechanical properties. Niobium nitride is an excellent candidate for such studies since it can assume seven crystalline phases w10x. Niobium nitride has long been valued for its superconducting properties, but more recently Wong et al. * Corresponding author. Tel.: q1-814-269-6415; fax: q1-814-2692798. E-mail address: [email protected] (M.L. Klingenberg).
recognized the applicability of this refractory material’s high strength, high melting point, and high transition temperature to a variety of wear applications w4,11,12x. Previous studies with sputtered NbN have obtained a variety of cubic, hexagonal close packed (hcp), and amorphous structures with varying hardness values w4,13–15x. Baba et al. used 2–20-keV IBAD to deposit NbN films and characterized the corrosion resistance of the hcp d9-NbN, hcp b-Nb2N, and bcc solid solutions of Nb–N films w1x. The focus of the present study was to characterize the tribological properties of NbN films produced using lower-energy IBAD processing. 2. Experimental Niobium nitride coatings were deposited on a variety of substrates including AISI 4340 steel, single crystal sodium chloride, and glass using an e-beam ion beam processing system. Cryopumps were used to achieve a base pressure of 3.6–7.3=10y5 Pa. A residual gas analyzer (RGA) indicated that water and oxygen levels did not exceed 4.8=10y6 and 2.7=10y6 Pa, respectively, and the nitrogen partial pressure ranged from 4– 13=10y6 Pa. Upon achieving base vacuum, the low
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energy (90 eV), end-hall ion source was operated at current densities of 0.34–0.37 mAycm2, and the moderate energy (750–800 eV), RF ion source was operated at current densities of 0.09–0.18 mAycm2 during sputter cleaning. Current density calculations are based on the ion current, as measured by the source, and the area that the beam intersected at the platen. The IBAD total operating pressure ranged from 1.6–14=10y3 Pa with partial pressures of nitrogen, oxygen, and water ranging from 9.7–71=10y4, 1.3–72=10y7, and 7.2–10=10y6 Pa, respectively. Niobium deposition rates were varied to obtain total ion-to-atom transport ratios of 0.062, 0.125, 0.25, 0.33, 0.50, 0.75 and 1.0 and ranged from 0.09 to 0.45 nmys. Although previous studies have shown that plasma sources yield approximately 1.89 nitrogen atoms per unit charge, only the total beam current was used to calculate the nitrogen arrival ratios w16x. The transport ratios cited in this paper, however, do account for the 308 angle of incidence (off of the substrate normal) of the ion beam. Sodium chloride crystals received a targeted thickness of 75 nm, and metal and glass substrates were coated to targeted thicknesses of 1000 and 2000 nm. The final film thickness varied for the coatings due to nitrogen and oxygen uptake, film morphology, and ion beam sputtering. Compositional and thickness analyses of the 75-nm films were performed using Rutherford backscattering spectrometry (RBS), using a 2-MeV Heq beam at a backscattering angle of 1708, the results were analyzed using the simulation program RUMP w17x. Film morphology was analyzed using 20-keV electron beam SEM. TEM and X-ray diffraction (XRD) were used to identify the crystalline phase of the films. A 120-keV electron beam was used to obtain the diffraction patterns using SAD. For XRD analysis, Cu Ka radiation was used over a scan range of 2us20–808, and the X-ray tube was set at a 58 take-off angle. Hardness was measured on AISI 4340 steel substrates using the continuous stiffness to constant depth (200 nm) mode of a NanoIndenter. Stress was analyzed by measuring the radius of curvature of the 1000-nm-thick films on glass cover slips. The residual film stress was calculated by using the Stoney equation and knowing the film thickness, substrate thickness, modulus of the substrate, and Poisson’s ratio of the substrate w18x. Dry sliding wear tests were performed to assess the wear resistance and the frictional coefficients of the coatings. The tests were performed using a ball-on-disk tribometer, in which 440C stainless balls were worn, under 50–150-g loads with no lubrication, against the coatings. The sliding speed ranged from 4.61 to 5.10 cmys, and the test radius was set at 3 mm. All tests were run for 1 h. The wear scars were analyzed using profilometry, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Profilometry
techniques were used to measure the depth of the scar while SEM enabled analysis of the wear mode and estimation of the depths of very shallow wear scars. SEM images were taken in the backscattering electron imaging (BEI) mode to ascertain the topography of the scar and in secondary electron imaging (SEI) mode to better define surface effects. EDS was performed using a 35-keV electron beam and was operated in spot mode to determine the source of any adherent debris. 3. Results and discussion 3.1. Composition and microstructure RBS showed that the films contained 30–40 at.% nitrogen even at the lowest ion to atom ratios (e.g. Rs 0.062 and 0.125) using the 90-eV beam, indicating significant reactivity with the nitrogen gas background coming from ion source operation. Upon increasing the transport ratio to 0.25, nitrogen content increased to 48 at.%, which remained unchanged with increasing transport ratio. The 750–800-eV coatings also displayed increased incorporation of nitrogen with increased transport ratio, from 34 at.% nitrogen at Rs0.062 to 60 at.% nitrogen at Rs1.0. Most coatings contained approximately 9–17 at.% oxygen, with the lower transport ratio coatings containing the most oxygen. However, the 750– 800-eV coating of Rs0.75 contained anomalously large amounts of oxygen (14 at.%), but still a substantial (51 at.%) amount of nitrogen, indicating some unexplained inconsistencies in the deposition. TEM and XRD revealed that all the films were microcrystalline, which is consistent with Baba’s observation of NbN films produced with higher energy ions (2–20 keV) w1x. XRD (Fig. 1) of most films (excepting the 90-eV films of Rs0.062 and 0.125), produced patterns that were most consistent with the fcc structure of d-NbN. Grain orientation was either random or displayed texturing in the N111M or N200M directions normal to the substrate. The pattern displayed by the low energy films of Rs0.062 and 0.125 was a result of its tetragonal g-Nb4N3 structure with preferred orientation in the N111M direction and considerable peak broadening. SEM examination of fractured coatings revealed that all of the 90-eV coatings and the 750–800-eV coatings with Rs0.062, 0.125 and 0.50 displayed a densely packed columnar microstructure (Fig. 2) w19x, similar to Zone 2 metallic films. These structures are expected for the 90-eV films and the 750–800-eV films with low transport ratio due to high operating pressures and low energy bombardment, and low ion fluxes, respectively. The unexpected columnar structure found in the 750– 800-eV, Rs0.50 film may be result of the high operating pressure experienced during this particular deposition
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245
Fig. 1. XRD patterns for the 90 and 750–800-eV coatings.
trial, which might have led to charge exchange neutralization w20x. The 750–800-eV coatings with Rs0.25, 0.33 and 0.75 displayed a non-columnar growth morphology, as is shown in Fig. 3 w19x. The increased energy, combined with increased ion flux, was evidently sufficient to enable diffusional processes, leading to very dense films. The 750–800-eV coatings with Rs0.75 and 1.0 were too thin for SEM analysis due to sputtering loss.
Fig. 2. SEM cross-section of 90-eV coating with Rs0.25.
3.2. Residual stress and hardness Stress analysis showed that the 90-eV coatings Rs 0.062–0.25 exhibited extremely high tensile stress with the films cracking and peeling on the substrates. With increased R, e.g. 0.33 and 0.50, the films continued to relieve stress through cracking, but did not peel. Films of Rs0.75 and 1.0 did not crack and yielded compressive stress values 1.8 and 0.49 GPa, respectively. This suggests that increasing ion bombardment initially
Fig. 3. SEM cross-section of 750–800-eV coating with Rs0.25.
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Fig. 4. Residual stress in films deposited with 750–800-eV IBAD.
reduced the tensile stress of the evaporated coatings, and subsequently led to compressively stressed coatings at high R values. None of the 750–800-eV films cracked or peeled, and all displayed compressive stress (Fig. 4). Stress in the Rs0.062 coating was highly asymmetric, such that two values were obtained when rotating the circular coverslip 908. All of the other 750–800-eV coatings deformed the glass cover slips uniformly. A minimum compressive stress value was obtained at Rs0.25, before and after which the compressive stress values gradually increased. Compressive stress at low R values (e.g. 0.062 and 0.125) could be due to oxygen and nitrogen incorporation into the coating, which is consistent with the observations of Cuomo et al. w21x. With a slight increase of ion flux to Rs0.25, oxygen incorporation decreased, leading to slightly lower levels of compressive stress. Further increases in transport ratio resulted in increased incorporation of nitrogen and densification of the film, producing increased compressive stress. Coatings produced at Rs0.75 and 1.0 were, however, too thin for accurate measurement of the coating stress due to sputtering. Hardness measurements, as depicted in Fig. 5, show increasing hardness with increasing transport ratio for the 90-eV coatings, while the 750–800-eV coatings exhibited a more complex behavior that is thought to be the result of the combined effects of intrinsic film stress, coating thickness, and crystalline phase. The highest moderate energy hardness value was obtained for Rs
0.33, an amorphous coating, as determined by XRD. The measurements obtained for Rs0.33–1.0 are the result of substrate influence. The 200-nm penetration depth was greater than 1y10 the actual coating thickness due to sputtering effects. Therefore, the much softer (5.74 GPa) 4340 steel substrate hardness influenced the measurements. The hardness value for the moderate energy film of Rs1.0 suggests the possibility that the 4340 steel was annealed by the heat induced by the ions at the surface. The hardness of all other films is thought to be due to the competition between increasing nitride character and stress. 3.3. Friction and wear Coatings were wear tested at increasing loads under unlubricated sliding wear conditions to compare and rank them for wear endurance while monitoring their frictional behavior. Two coatings, the 750–800-eV at Rs0.25, and 90-eV at Rs0.062, performed significantly better than all other coatings during ball-on-disc wear testing (indicated by arrows in Fig. 6). The 750–800eV (Rs0.25) coating demonstrated the best overall abrasive wear resistance against 440C stainless steel under heavy loading conditions, even though adhesive wear was evident. The 90-eV (Rs0.062) coating provided better abrasive wear characteristics at loads of 50, 100, and 125 g, but completely delaminated upon testing at a 150-g load. The 750–800-eV (Rs0.25) coating displayed much less compressive stress than the other
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247
Fig. 5. Coating microhardness as a function of transport ratio and ion energy.
moderate energy coatings and lower overall stress than most of the low energy coatings. The 90-eV (Rs0.062) coating had the highest stress level overall, but still provided a high level of unlubricated sliding wear
resistance, indicating that the coating was more coherentyadherent than most. The complex nature of sliding wear is demonstrated by the fact that the two most wear resistant coatings had different composition,
Fig. 6. Coating wear loss after ball-on-disc testing against 440C at various loads.
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Table 1 Unlubricated frictional coefficients against 440C stainless steel Coating type
m50
m100
ME, Rs1.0 ME, Rs0.75 ME, Rs0.50 ME, Rs0.33 ME, Rs0.25 ME, Rs0.125 ME, Rs0.062 LE, Rs1.0 LE, Rs0.75 LE, Rs0.50 LE, Rs0.33 LE, Rs0.25 LE, Rs0.125 LE, Rs0.062
0.70 0.78 0.90 0.84 0.55 0.21 0.80 0.85 0.23 0.98 0.20 0.22 0.23 0.24
1.35
0.43 1.63
m125
m150
2.20
2.80
1.64 1.10 1.75 0.34 0.37
2.30 2.10 0.75
2.65
ME, moderate energy; LE, low energy.
crystalline phases, and morphologies, as described below. During the initial wear test at 50 g load, six coatings displayed relatively high frictional coefficients (Table 1), e.g. greater than 0.70, and severe abrasive or adhesive wear (see Fig. 6 — first row). The six coatings included two 90-eV coatings, Rs0.50 and 1.0, and four 750–800-eV coatings, Rs0.062, 0.33, 0.50 and 0.75. The 90-eV coatings, Rs0.50 and 1.0, both displayed abrasive wear and some delamination. Plowing was evident in Rs0.50, and some adhesive wear was noted in Rs1.0. The 750–800-eV coatings Rs0.062 and 0.33 displayed abrasive wear, and significant loss of cohesion was noted in Rs0.33. At Rs0.50, the wear mode appeared to change from abrasive to adhesive wear, with 440C stainless steel deposits welding to the coating. EDS confirmed the composition of the debris. Because no scoring was evident, no wear bar is shown in Fig. 6. The hard, brittle, Rs0.75 coating suffered severe abrasive and adhesive wear with plowing and delamination. Although no wear was visually observed for the moderate energy coating of Rs1.0, the EDS method of detecting subtle differences in coating thickness in the wear tracks could not be used due to the very thin layer of coating present. As a result, there is no wear bar shown for the moderate energy coating of Rs1.0 in Fig. 6. Upon testing using a 100-g load, four additional coatings exhibited coefficients of friction greater than 1 and displayed measurable adhesive and abrasive wear (Table 1 and Fig. 6 — second row). The 750–800-eV coating, Rs0.125, experienced decohesion in selected areas, although most of the film had been uniformly worn. The extremely thin and brittle, moderate energy coating, Rs1.0, was completely worn away with deep gouges in the base material and welding of the stainless steel ball. Again, no wear bar is shown for this coating because the coating was too thin to use the EDS method
of removal and the complete removal of the coating made measurement using EDS impossible. Areas of the low energy coating, Rs0.25, were removed and redeposited elsewhere, and the stainless steel ball had welded to the scar area. The 90-eV coating, Rs0.75, experienced mild to severe abrasive wear, as well as adhesive welding of the stainless steel ball. Although the 90-eV coating of Rs0.33 displayed a high frictional coefficient, no scar could be measured using profilometry or SEM, substantiating that friction and wear are not necessarily related. Three of the four remaining coatings exhibited frictional coefficients greater than 2 when tested using a 125-g load. SEMyEDS analysis confirmed that adhesive welding occurred for these three coatings, but profilometry indicated that only the low energy coatings had measurable wear. Because the 750–800-eV coating, Rs 0.25, and the 90-eV coating, Rs0.062 displayed minimal abrasive wear in comparison to the other coatings, each was tested further. Both remaining coatings displayed coefficients of friction greater than 2 upon testing at 150 g load. SEM yEDS confirmed that much 440C stainless steel had welded to both coatings. The 90-eV coating experienced complete delamination in areas; however, only a minor amount of coating material was removed from the 750– 800-eV coating. SEM imaging indicated some loss of cohesion for Rs0.25, but the appearance was very similar to that displayed for the 125-g load. 4. Conclusions These studies have shown that hard, wear resistant niobium nitride films can be deposited onto 4340 steel using both low energy and moderate energy ion sources. Appropriate deposition parameters for synthesizing these films have been identified, correlating composition and structure to the beam energy and transport ratio. The wear characteristics of the niobium nitride films could not be correlated with hardness, stress, morphology, or composition. The 750–800-eV (Rs0.25) coating demonstrated the best abrasive wear resistance against 440C stainless steel under heavy loading conditions, but adhesive wear was evident. The 90-eV (Rs0.062) coating provided better abrasive wear characteristics at lighter loads, but ultimately failed catastrophically at the highest load. The coatings were dramatically different in terms of composition, morphology, crystalline phase, hardness, and stress. The 90-eV coating exhibited less nitride character, was more ductile, and was highly stressed, as indicated by cracking. These characteristics enabled the film to deform plastically, until a critical load was reached where the shear stresses were greater than the bond strength of the film, leading to delamination. In the 750–800-eV coating, the relatively low stress, near-stoichiometric NbN composition, intermedi-
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