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Sol–gel derived, nanostructured oxide lubricant coatings
Surface and Coatings Technology 120–121 (1999) 465–469 www.elsevier.nl/locate/surfcoat
Sol–gel derived, nanostructured oxide lubricant coatings Douglas J. Taylor a, *, Patrick F. Fleig a, Stuart T. Schwab a, Richard A. Page b a Specialty Materials Department, TPL, Incorporated, 3921 Academy Parkway North, Albuquerque, NM 87109, USA b Mechanical and Materials Engineering Division, Bldg. 128, Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228-0510, USA
Abstract A number of studies have shown that the introduction of lubricious solid films, especially metal oxides, can improve elevated temperature tribological performance. Two such studies have identified a class of oxides that offer exceptional tribological performance at elevated temperatures. Certain combinations of ion-implanted titanium and nickel yielded coefficients of friction (COF ) of 0.06–0.09 at 800°C. Examination of the surface layers by Auger spectroscopy indicated that the low friction coefficients were obtained on surfaces with a titanium oxide film that had a substantial nickel content. The second study demonstrated a dramatic reduction in the wear of silicon nitride by adding 15 vol.% titanium carbide. Reductions in the COF by up to 50% were also obtained. Auger spectroscopy of the wear tracks identified a mixed titanium and silicon oxide as the lubricious species in the titanium carbide-containing silicon nitride. We have developed wet chemical methods of depositing the titanium oxide materials believed to be responsible for the improved tribological performance observed in previous studies. Deposition of the oxides as nanostructured materials should promote lubricious behavior at sub-ambient temperatures. The microstructure was examined by X-ray diffraction ( XRD) of powders, energy dispersive spectroscopy (EDS ) in a scanning electron microscope (SEM ), and thermal analysis ( TGA/DSC ). COF was measured by a reciprocating ball-on-plate at 25 and 500°C. Wear tracks were observed with surface profilometry, optical microscopy and SEM. Titania/nickel oxide coatings reduced the COF by at least 50% at both temperatures and demonstrated the best wear resistance. This paper will discuss these results with respect to microstructure and processing parameters. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Lubricious behavior; Nanostructured oxide lubricant coatings; Sol–gel
1. Introduction The reduction of friction and wear is essential to the proper functioning of modern machines. The more complex the machine, the more stringent are the lubrication requirements. Gas turbine engines are among the most complex of machines and, because of their high operating speed and temperature, present one of the most challenging lubrication environments. Hydrocarbon and fluorocarbon materials have been able to satisfy the needs of previous generations of gas turbine engines; however, the operating environment of the next generation of turbine engines will exceed the performance envelope of conventional lubricants. New materials that can provide lubrication and limit wear from sub-ambient to elevated temperatures are essential to * Corresponding author. Tel.: +1-505-342-4428; fax: +1-505-343-1797. E-mail address: [email protected] (D. Taylor)
the successful exploitation of the performance gains obtained through new engine designs. A number of studies have shown that the normally observed wear mechanisms of adhesion and abrasion can be mitigated by environmental factors [1,2]. For example, adsorbed water has been observed to promote near-surface plasticity in alumina, leading to a soft surface layer that appears to prevent adhesion and provide lubrication [3]. The formation of similar, soft lubricious films has been observed in friction and wear tests of a number of ceramic materials, including tungsten carbide ( WC ), titanium carbide ( TiC ), alumina (Al O ), and their cermets [4], as well as silicon nitride 2 3 (Si N ) and silicon carbide (SiC ) [5–7]. These studies 3 4 have shown that the intentional introduction of lubricious solid films, especially metal oxides, can provide an improved elevated temperature tribological performance. In previous studies, it was found that various oxides of titanium and nickel produced low coefficients of
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friction (COFs) [8,9]. Titanium and nickel were ionimplanted into TiC-reinforced Si N monoliths and cre3 4 ated a mixed oxide phase during wear experiments. The current work attempts to deposit coatings of TiO –NiO from wet-chemical synthesis. The metal pre2 cursors were mixed on the molecular level, which resulted in stoichiometric, homogeneous materials. Coatings were deposited onto various substrate materials and geometries. These materials also have the potential to be deposited by spraying the same precursor solutions for application to large areas. Sol–gel techniques were employed to synthesize precursor solutions, which were used to deposit coatings by spinning and dipping [10]. A reciprocating friction/wear tester was used to classify the various compositions and characterize the COF. COFs were measured at room temperature and at elevated temperatures in air. The microstructures of the different materials were analyzed with X-ray diffraction ( XRD) and energy-dispersive spectroscopy ( EDS). Wear tracks were examined with optical microscopy, EDS and optical profilometry. The results of this on-going work are presented and discussed.
2. Experimental 2.1. Sample preparation Silica, titania and nickel oxide were made in binary and tertiary compositions. Coating precursors for the various metal oxides were prepared using wet chemical processing. All of the chemicals were used as received, except ethanol, which was distilled. Nickel (II ) acetate tetrahydrate was purchased from Alfa Aesar; tetraethoxysilane ( TEOS), titanium (IV ) isopropoxide, and nitric acid were purchased from Aldrich Chemical Company; and McCormick brand ethanol (200 proof, anhydrous) was obtained from a local source. Solutions were mixed in Pyrex round bottom flasks sealed with rubber septa. The final compositions are given in weight per cent (wt%) of the oxides. Silica–titania compositions were made by first mixing TEOS and ethanol. Acidified water was added to the mixture in a 4:1 molar ratio, water:TEOS. This intermediate solution was sealed and allowed to stir for several hours. Titanium (IV ) isopropoxide was added so that the final oxide concentration would result. The solution was allowed to stir for an hour before coating. For nickel oxide containing compositions, nickel (II ) acetate tetrahydrate was mixed with anhydrous (200 proof ) ethanol, followed by a sufficient amount of concentrated nitric acid to completely dissolve the nickel precursor. The quantity of ethanol varied, depending on the mole percentage of solids in the sol. Complete dissolution of the nickel (II ) acetate typically required
a minimum of 12 h. A sufficient amount of TEOS and/or titanium (IV ) isopropoxide was added to achieve the desired final oxide composition. For tertiary compositions, TEOS was added to the nickel oxide mixture first and allowed to stir before the titanium ( IV ) isopropoxide was added. The solution was allowed to stir for 1 h before coating. Coupons were coated by dipping or spinning from their respective precursor solutions. Coatings for friction measurements were deposited using a computer-controlled dip coater in a HEPA-filtered chamber. Samples were withdrawn from the coating solutions at a rate of 5 mm/s. To increase the overall thickness, multiple layers were applied. Between layers, the coatings were heated to 400°C for 20 min. Final firing was done in air at 800°C for 30 min. Witness samples were made on silicon wafers to facilitate film measurement by ellipsometry and Fourier transform infra-red ( FTIR) spectroscopy. 2.2. Characterization Coupons for the pin on flat wear tests were polished Hastalloy plates, cut into rectangular sections (5.08 cm×0.635 cm). Coated specimens were measured on a reciprocating friction and wear apparatus, the details and operation of which are described elsewhere [11]. The pin was a TiC hemisphere with a tip radius of 0.63 cm. The pin was loaded with a 1 N normal force and traveled over a distance of 1 cm at a frequency of 1 Hz. The coefficient of friction was continuously monitored throughout the test and calculated by dividing the measured tangential force by the applied normal force. Specimens were tested at room temperature and at 500°C in air for 60 min each. The microstructures of the various compositions were probed with XRD and FTIR spectroscopy. Powder samples for XRD were made by drying the precursor solutions at room temperature and firing them at 800°C, in a manner similar to that used for processing the coated coupons. FTIR spectroscopy was performed in transmission mode on the witness samples. Witness samples were also measured by multi-angle ellipsometry to determine the layer thickness and index of refraction. Psi and Delta values were obtained at 60, 65 and 70° from the normal with 632.8 nm radiation. Together with these analytical techniques, differential scanning calorimetry (DSC ) was helpful in determining crystallization and phase-change information. DSC measurements were performed using dried, but unfired, powders that were heated to 800°C at 10°C/min. Wear tracks were examined with optical microscopy, optical surface profilometry, and EDS in a scanning electron microscope (SEM ). The dimensions of the wear tracks were obtained using a Wyko RST Plus optical profiler system. Wear rates were determined from the depths of the wear track, unless the depth of the wear
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track exceeded the coating thickness. Elemental analysis using EDS indicated the presence of the coating material inside the wear track.
3. Results The coefficient of friction values ranged from 0.08 to roughly 0.9, which appeared to be the value of the TiC pin on bare Hastalloy. Figs. 1 and 2 show COF as a function of time in the friction tester at room temperature and at 500°C, respectively. From the graphs, the material with the best performance at both temperatures was the titania-rich TiO –NiO composition. The 67 and 2 75 wt% TiO (not shown) compositions (balance nickel ) 2 had similar performance, and both had lower COFs
Fig. 1. Sliding coefficient of friction between wet-chemically derived oxide coatings on Hastalloy and a TiC pin as a function of time. Test conditions: room temperature, 0 in air, 1 N load, 2 cm/s, and 1 Hz oscillation frequency. The binary compositions are listed by weight per cent, where TN is TiO /NiO and ST is SiO /TiO , and the ternary is 2 2 2 33 wt% each of TiO , SiO , and NiO. 2 2
Fig. 2. Sliding coefficient of friction between wet-chemically derived oxide coatings on Hastalloy and a TiC pin as a function of time. Test conditions: 500°C, in air, 1 N load, 2 cm/s, and 1 Hz oscillation frequency. The binary compositions are listed by weight per cent, where TN is TiO /NiO and ST is SiO /TiO , and the ternary is 33 wt% each 2 2 2 of TiO , SiO , and NiO. 2 2
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than the 50 wt% composition. Nickel-rich compositions (not shown) had COFs between the titania-rich compositions and the 50 wt% composition. All of the TiO –NiO compositions and the 33 wt% ternary com2 position had excellent wear resistance; specifically, no wear tracks were evident after 1 h, or 3600 cycles, of testing. However, the SiO –TiO compositions and the 2 2 SiO –NiO composition had high wear rates. 2 Optical profilometry was employed to evaluate wear tracks that resulted after 3600 cycles on the friction tester. Wear scars varied from tracks with no discernible depth to tracks that were deeper than the thickness of the coating, which indicated complete coating removal during wear/friction testing. To confirm the presence or absence of a coating, the surface composition was measured by EDS in the SEM. For TiO –NiO coatings, 2 the spectra were identical inside and outside the wear track. Nickel, titanium, oxygen, molybdenum and chromium were found inside and outside the wear tracks. The Hastalloy X substrate contains 49% Ni, 22% Cr, 20% Fe and 9% Mo. These elements were detected with EDS because the depth of the sampling volume (~1 mm) was greater than the thickness of the coating. Since nickel was common to both coating and substrate, it was the strong presence of titanium and oxygen that indicated the presence of the coating material in the wear track. Concerning the counterfaces, optical microscopy showed little wear on the TiC pins. X-ray diffraction spectra showed the crystalline phases present in each composition. The crystallite size was calculated from the diffraction peaks using the Sherrer formula [12]. These results covering the initial range of compositions are listed in Table 1 and show that the oxide compositions produced in this work are nano-structured. Crystallite sizes ranged from roughly 6 to 75 nm with this heat treatment. Since the TiO –NiO materials possessed the best friction/wear 2
Fig. 3. X-ray diffraction spectrum of 50 wt% TiO –NiO made by sol– 2 gel methods. Labels identify the material responsible for each peak as follows: NT=nickel titanate (NiTiO ), R=rutile ( TiO ), and B=bun3 2 senite (NiO).
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Table 1 Crystallinity of wet-chemically derived oxides Composition (wt%)
Phase (S )
Crystallite size (nm)
50% TiO –50% SiO 2 2 50% SiO –50% NiO 2 50% TiO –50% NiO 2
Amorphous NiO NiTiO 3 TiO (rutile) 2 NiO TiO (anatase) 2 TiO (rutile) 2 NiO Ni TiO 3 5 TiO (rutile) 2 NiTiO 3 TiO (anatase) 2 TiO (rutile) 2 NiO
n/a 17 47 47 51 6.3 Trace 33 Trace 7.6 18 8.2 11 28
67% TiO –33% SiO 2 2 33% TiO –33% SiO –33% NiO 2 2 50% TiO –25% SiO –25% NiO 2 2
behavior, other compositions of this binary system were investigated. Crystallization data for the TiO –NiO 2 system are listed in Table 2. Only titania and nickel oxide combined to form a mixed metal oxide phase, nickel titanate in this case (NiTiO ). Fig. 3 displays an 3 XRD spectrum for 75 wt% TiO –25 wt% NiO showing 2 the formation of nickel titanate. Crystallite sizes for TiO , NiO and NiTiO were consistently between 40 2 3 and 50 nm. Infra-red spectroscopy confirmed intimate mixing of the metal ions by the presence of TiMOMNi bonds. However, SiMOMNi bonds, for example, were not evident in the silica/nickel oxide composition, nor did mixed oxide phases develop; only NiO crystalline peaks were detected. Crystallization of NiO into bunsenite occurred at roughly 540°C, according to DSC. Thermal analysis alone was unable to determine the crystallization temperatures for all of the other phases without more rigorous XRD experiments. Ellipsometry was employed to determine the thickness and index of refraction of the coatings. The individual layer thickness depended upon each precursor solution and the deposition conditions. When the thickness was Table 2 Crystallinity of wet-chemically derived nickel titantes Composition (wt%)
Phase (S)
Crystallite size (nm)
25% TiO –75% NiO 2
NiTiO 3 NiO NiTiO 3 NiO NiTiO 3 TiO (rutile) 2 NiO NiTiO 3 TiO (rutile) 2 NiO NiTiO 3 TiO (rutile) 2
44 44 49 45 47 47 51 42 40 48 48 44
33% TiO –67% NiO 2 50% TiO –50% NiO 2 67% TiO –33% NiO 2 75% TiO –25% NiO 2
below 0.5 mm, multiple layers were deposited to produce coatings that were at least this thick. The refractive index (RI ) varied with composition. For example, the RI of the 50 wt% SiO –TiO composition was measured 2 2 to be 1.75, which indicated 6% porosity, based on the theoretical RI for this composition. The porosity was calculated using literature values and the Maxwell Garnett effective medium approximation model. This model assumes spherical inclusions (pores in this case) and dipole interactions, which are standard first approximations that give reasonably accurate results [13].
4. Discussion Work by Page et al. [8,9] found that ion-implanted Ni and Ti significantly lowered the COF of a TiC pin on Si N . Auger spectroscopy indicated that it was the 3 4 oxides of Ni and Ti that were responsible for the improved tribological properties; however, relative amounts of each atom and their microstructure were not determined. In this work, Ni and Ti oxides were synthesized by wet-chemical techniques to imitate those oxides formed by ion implantation in the cited study. The results showed that Ni/Ti oxide was produced in a number of microstructures. In the same study, it was found that TiC-reinforced Si N reduced the COF from 0.72 to 0.43 and reduced 3 4 the wear to practically zero. Auger spectroscopy found Si and Ti oxides on the wear surface. Again, the composition (quantitative) and microstructure were not investigated. Therefore, silica/titania was synthesized to determine whether similar properties could be repeated in the wet-chemically derived coatings. Several binary and ternary combinations of SiO , 2 TiO and NiO were synthesized and examined. Any 2 coating that contained both silica and nickel oxide was worn away during the friction tests. Although hardness testing was not performed, this combination of materials appeared to produce soft coatings. Eliminating compositions that contained both SiO and NiO left two combi2 nations: SiO /TiO and TiO /NiO. The former produced 2 2 2 a COF between 0.2 and 0.3 at room temperature for a few minutes before wearing away. At 500°C, it maintained a moderate COF of 0.58 (average) throughout the entire 60 min test, in agreement with the previous findings. However, due to its poor wear properties at room temperature, SiO /TiO was not selected for fur2 2 ther study. Our results confirm the earlier findings that combinations of titania and nickel oxide provide low COF surfaces. Coupons coated with TiO –NiO had the lowest 2 COFs and the best wear behavior of all the compositions tested. This was the only composition in which the pin did not wear through to the substrate during the 3600 cycle test. The improved friction/wear properties pro-
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vided by the TiO /NiO coatings may be due to the 2 formation of a mixed metal oxide phase, NiTiO . This 3 phase, while it does not have a mineral name, has an ilmenite structure where nickel atoms sit on the iron sites [14]. Several of the compositions contained NiTiO with other phases, which may have enhanced 3 the friction and wear behavior by preventing growth of the nano-sized crystallites. At this time, the tribological performance of pure NiTiO coatings is unknown. 3 5. Summary Nano-structured oxide coatings were made by wet chemical techniques to imitate compositions previously noted to improve wear properties. Coatings that contained both silica and nickel oxide had poor wear characteristics and high COFs. Silica-titania had a COF <0.20 at room temperature, but wore off after only 200 test cycles. Compositions that contained titanium and nickel produced the only mixed oxide crystalline phase, NiTiO . Nickel titanate coatings had good friction/wear 3 properties at room temperature and at 500°C.
Acknowledgements This material is based upon work supported by the Air Force Office of Scientific Research under Contract
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No. F49620-98-C-0065. The authors thank Major H. DeLong for his contributions to this project and M.N. Gardos for many helpful discussions.
References [1] [2] [3] [4]
A.F. McLean, Am. Ceram. Soc. Bull. 61 (1982) 861. D. Tabor, J. Lub. Technol. 103 (1981) 169. R.P. Steijn, Wear 7 (1964) 48. H. Shimura, T. Tsuya, in: Proc. Int. Conf. Wear of Materials, ASME, New York, 1977, p. 452. [5] T.E. Fischer, H. Tomizawa, Wear 105 (1985) 29. [6 ] P. Sutor, Proc. Ceram. Eng. Sci. 5 (1984) 460. [7] D.C. Cranmer, J. Mater. Sci. 20 (1985) 2029. [8] R.A. Page, J. Lankford, C.R. Blanchard, Adv. Eng. Tribol. SP-31 (1990) 145. [9] C.R. Blanchard-Ardid, J. Lankford, R.A. Page, L. Fehrenbacher, W.O. Winer ( Eds.), New Materials Approaches to Tribology: Theory and Applications Vol. 140, Materials Research Society, Pittsburgh, PA, 1989, p. 345. [10] C.J. Brinker, G.W. Scherer, Sol–gel Science, Academic Press, New York, 1989. [11] R.A. Page, C.R. Blanchard-Ardid, W. Wei, J. Mater. Sci. 23 (1988) 946. [12] B.D. Cullity, in: Elements of X-ray Diffraction, 2nd edition, Addison-Wesley, Reading, MA, 1978, p. 102. [13] H.G. Tompkins, A User’s Guide to Ellipsometry, Academic Press, San Diego, CA, 1993. [14] M. Lerch, H. Boysen, R. Neder, F. Frey, W. Laqua, J. Phys. Chem. Solids 53 (1992) 1153.
Sol–gel derived, nanostructured oxide lubricant coatings Douglas J. Taylor a, *, Patrick F. Fleig a, Stuart T. Schwab a, Richard A. Page b a Specialty Materials Department, TPL, Incorporated, 3921 Academy Parkway North, Albuquerque, NM 87109, USA b Mechanical and Materials Engineering Division, Bldg. 128, Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228-0510, USA
Abstract A number of studies have shown that the introduction of lubricious solid films, especially metal oxides, can improve elevated temperature tribological performance. Two such studies have identified a class of oxides that offer exceptional tribological performance at elevated temperatures. Certain combinations of ion-implanted titanium and nickel yielded coefficients of friction (COF ) of 0.06–0.09 at 800°C. Examination of the surface layers by Auger spectroscopy indicated that the low friction coefficients were obtained on surfaces with a titanium oxide film that had a substantial nickel content. The second study demonstrated a dramatic reduction in the wear of silicon nitride by adding 15 vol.% titanium carbide. Reductions in the COF by up to 50% were also obtained. Auger spectroscopy of the wear tracks identified a mixed titanium and silicon oxide as the lubricious species in the titanium carbide-containing silicon nitride. We have developed wet chemical methods of depositing the titanium oxide materials believed to be responsible for the improved tribological performance observed in previous studies. Deposition of the oxides as nanostructured materials should promote lubricious behavior at sub-ambient temperatures. The microstructure was examined by X-ray diffraction ( XRD) of powders, energy dispersive spectroscopy (EDS ) in a scanning electron microscope (SEM ), and thermal analysis ( TGA/DSC ). COF was measured by a reciprocating ball-on-plate at 25 and 500°C. Wear tracks were observed with surface profilometry, optical microscopy and SEM. Titania/nickel oxide coatings reduced the COF by at least 50% at both temperatures and demonstrated the best wear resistance. This paper will discuss these results with respect to microstructure and processing parameters. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Lubricious behavior; Nanostructured oxide lubricant coatings; Sol–gel
1. Introduction The reduction of friction and wear is essential to the proper functioning of modern machines. The more complex the machine, the more stringent are the lubrication requirements. Gas turbine engines are among the most complex of machines and, because of their high operating speed and temperature, present one of the most challenging lubrication environments. Hydrocarbon and fluorocarbon materials have been able to satisfy the needs of previous generations of gas turbine engines; however, the operating environment of the next generation of turbine engines will exceed the performance envelope of conventional lubricants. New materials that can provide lubrication and limit wear from sub-ambient to elevated temperatures are essential to * Corresponding author. Tel.: +1-505-342-4428; fax: +1-505-343-1797. E-mail address: [email protected] (D. Taylor)
the successful exploitation of the performance gains obtained through new engine designs. A number of studies have shown that the normally observed wear mechanisms of adhesion and abrasion can be mitigated by environmental factors [1,2]. For example, adsorbed water has been observed to promote near-surface plasticity in alumina, leading to a soft surface layer that appears to prevent adhesion and provide lubrication [3]. The formation of similar, soft lubricious films has been observed in friction and wear tests of a number of ceramic materials, including tungsten carbide ( WC ), titanium carbide ( TiC ), alumina (Al O ), and their cermets [4], as well as silicon nitride 2 3 (Si N ) and silicon carbide (SiC ) [5–7]. These studies 3 4 have shown that the intentional introduction of lubricious solid films, especially metal oxides, can provide an improved elevated temperature tribological performance. In previous studies, it was found that various oxides of titanium and nickel produced low coefficients of
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friction (COFs) [8,9]. Titanium and nickel were ionimplanted into TiC-reinforced Si N monoliths and cre3 4 ated a mixed oxide phase during wear experiments. The current work attempts to deposit coatings of TiO –NiO from wet-chemical synthesis. The metal pre2 cursors were mixed on the molecular level, which resulted in stoichiometric, homogeneous materials. Coatings were deposited onto various substrate materials and geometries. These materials also have the potential to be deposited by spraying the same precursor solutions for application to large areas. Sol–gel techniques were employed to synthesize precursor solutions, which were used to deposit coatings by spinning and dipping [10]. A reciprocating friction/wear tester was used to classify the various compositions and characterize the COF. COFs were measured at room temperature and at elevated temperatures in air. The microstructures of the different materials were analyzed with X-ray diffraction ( XRD) and energy-dispersive spectroscopy ( EDS). Wear tracks were examined with optical microscopy, EDS and optical profilometry. The results of this on-going work are presented and discussed.
2. Experimental 2.1. Sample preparation Silica, titania and nickel oxide were made in binary and tertiary compositions. Coating precursors for the various metal oxides were prepared using wet chemical processing. All of the chemicals were used as received, except ethanol, which was distilled. Nickel (II ) acetate tetrahydrate was purchased from Alfa Aesar; tetraethoxysilane ( TEOS), titanium (IV ) isopropoxide, and nitric acid were purchased from Aldrich Chemical Company; and McCormick brand ethanol (200 proof, anhydrous) was obtained from a local source. Solutions were mixed in Pyrex round bottom flasks sealed with rubber septa. The final compositions are given in weight per cent (wt%) of the oxides. Silica–titania compositions were made by first mixing TEOS and ethanol. Acidified water was added to the mixture in a 4:1 molar ratio, water:TEOS. This intermediate solution was sealed and allowed to stir for several hours. Titanium (IV ) isopropoxide was added so that the final oxide concentration would result. The solution was allowed to stir for an hour before coating. For nickel oxide containing compositions, nickel (II ) acetate tetrahydrate was mixed with anhydrous (200 proof ) ethanol, followed by a sufficient amount of concentrated nitric acid to completely dissolve the nickel precursor. The quantity of ethanol varied, depending on the mole percentage of solids in the sol. Complete dissolution of the nickel (II ) acetate typically required
a minimum of 12 h. A sufficient amount of TEOS and/or titanium (IV ) isopropoxide was added to achieve the desired final oxide composition. For tertiary compositions, TEOS was added to the nickel oxide mixture first and allowed to stir before the titanium ( IV ) isopropoxide was added. The solution was allowed to stir for 1 h before coating. Coupons were coated by dipping or spinning from their respective precursor solutions. Coatings for friction measurements were deposited using a computer-controlled dip coater in a HEPA-filtered chamber. Samples were withdrawn from the coating solutions at a rate of 5 mm/s. To increase the overall thickness, multiple layers were applied. Between layers, the coatings were heated to 400°C for 20 min. Final firing was done in air at 800°C for 30 min. Witness samples were made on silicon wafers to facilitate film measurement by ellipsometry and Fourier transform infra-red ( FTIR) spectroscopy. 2.2. Characterization Coupons for the pin on flat wear tests were polished Hastalloy plates, cut into rectangular sections (5.08 cm×0.635 cm). Coated specimens were measured on a reciprocating friction and wear apparatus, the details and operation of which are described elsewhere [11]. The pin was a TiC hemisphere with a tip radius of 0.63 cm. The pin was loaded with a 1 N normal force and traveled over a distance of 1 cm at a frequency of 1 Hz. The coefficient of friction was continuously monitored throughout the test and calculated by dividing the measured tangential force by the applied normal force. Specimens were tested at room temperature and at 500°C in air for 60 min each. The microstructures of the various compositions were probed with XRD and FTIR spectroscopy. Powder samples for XRD were made by drying the precursor solutions at room temperature and firing them at 800°C, in a manner similar to that used for processing the coated coupons. FTIR spectroscopy was performed in transmission mode on the witness samples. Witness samples were also measured by multi-angle ellipsometry to determine the layer thickness and index of refraction. Psi and Delta values were obtained at 60, 65 and 70° from the normal with 632.8 nm radiation. Together with these analytical techniques, differential scanning calorimetry (DSC ) was helpful in determining crystallization and phase-change information. DSC measurements were performed using dried, but unfired, powders that were heated to 800°C at 10°C/min. Wear tracks were examined with optical microscopy, optical surface profilometry, and EDS in a scanning electron microscope (SEM ). The dimensions of the wear tracks were obtained using a Wyko RST Plus optical profiler system. Wear rates were determined from the depths of the wear track, unless the depth of the wear
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track exceeded the coating thickness. Elemental analysis using EDS indicated the presence of the coating material inside the wear track.
3. Results The coefficient of friction values ranged from 0.08 to roughly 0.9, which appeared to be the value of the TiC pin on bare Hastalloy. Figs. 1 and 2 show COF as a function of time in the friction tester at room temperature and at 500°C, respectively. From the graphs, the material with the best performance at both temperatures was the titania-rich TiO –NiO composition. The 67 and 2 75 wt% TiO (not shown) compositions (balance nickel ) 2 had similar performance, and both had lower COFs
Fig. 1. Sliding coefficient of friction between wet-chemically derived oxide coatings on Hastalloy and a TiC pin as a function of time. Test conditions: room temperature, 0 in air, 1 N load, 2 cm/s, and 1 Hz oscillation frequency. The binary compositions are listed by weight per cent, where TN is TiO /NiO and ST is SiO /TiO , and the ternary is 2 2 2 33 wt% each of TiO , SiO , and NiO. 2 2
Fig. 2. Sliding coefficient of friction between wet-chemically derived oxide coatings on Hastalloy and a TiC pin as a function of time. Test conditions: 500°C, in air, 1 N load, 2 cm/s, and 1 Hz oscillation frequency. The binary compositions are listed by weight per cent, where TN is TiO /NiO and ST is SiO /TiO , and the ternary is 33 wt% each 2 2 2 of TiO , SiO , and NiO. 2 2
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than the 50 wt% composition. Nickel-rich compositions (not shown) had COFs between the titania-rich compositions and the 50 wt% composition. All of the TiO –NiO compositions and the 33 wt% ternary com2 position had excellent wear resistance; specifically, no wear tracks were evident after 1 h, or 3600 cycles, of testing. However, the SiO –TiO compositions and the 2 2 SiO –NiO composition had high wear rates. 2 Optical profilometry was employed to evaluate wear tracks that resulted after 3600 cycles on the friction tester. Wear scars varied from tracks with no discernible depth to tracks that were deeper than the thickness of the coating, which indicated complete coating removal during wear/friction testing. To confirm the presence or absence of a coating, the surface composition was measured by EDS in the SEM. For TiO –NiO coatings, 2 the spectra were identical inside and outside the wear track. Nickel, titanium, oxygen, molybdenum and chromium were found inside and outside the wear tracks. The Hastalloy X substrate contains 49% Ni, 22% Cr, 20% Fe and 9% Mo. These elements were detected with EDS because the depth of the sampling volume (~1 mm) was greater than the thickness of the coating. Since nickel was common to both coating and substrate, it was the strong presence of titanium and oxygen that indicated the presence of the coating material in the wear track. Concerning the counterfaces, optical microscopy showed little wear on the TiC pins. X-ray diffraction spectra showed the crystalline phases present in each composition. The crystallite size was calculated from the diffraction peaks using the Sherrer formula [12]. These results covering the initial range of compositions are listed in Table 1 and show that the oxide compositions produced in this work are nano-structured. Crystallite sizes ranged from roughly 6 to 75 nm with this heat treatment. Since the TiO –NiO materials possessed the best friction/wear 2
Fig. 3. X-ray diffraction spectrum of 50 wt% TiO –NiO made by sol– 2 gel methods. Labels identify the material responsible for each peak as follows: NT=nickel titanate (NiTiO ), R=rutile ( TiO ), and B=bun3 2 senite (NiO).
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Table 1 Crystallinity of wet-chemically derived oxides Composition (wt%)
Phase (S )
Crystallite size (nm)
50% TiO –50% SiO 2 2 50% SiO –50% NiO 2 50% TiO –50% NiO 2
Amorphous NiO NiTiO 3 TiO (rutile) 2 NiO TiO (anatase) 2 TiO (rutile) 2 NiO Ni TiO 3 5 TiO (rutile) 2 NiTiO 3 TiO (anatase) 2 TiO (rutile) 2 NiO
n/a 17 47 47 51 6.3 Trace 33 Trace 7.6 18 8.2 11 28
67% TiO –33% SiO 2 2 33% TiO –33% SiO –33% NiO 2 2 50% TiO –25% SiO –25% NiO 2 2
behavior, other compositions of this binary system were investigated. Crystallization data for the TiO –NiO 2 system are listed in Table 2. Only titania and nickel oxide combined to form a mixed metal oxide phase, nickel titanate in this case (NiTiO ). Fig. 3 displays an 3 XRD spectrum for 75 wt% TiO –25 wt% NiO showing 2 the formation of nickel titanate. Crystallite sizes for TiO , NiO and NiTiO were consistently between 40 2 3 and 50 nm. Infra-red spectroscopy confirmed intimate mixing of the metal ions by the presence of TiMOMNi bonds. However, SiMOMNi bonds, for example, were not evident in the silica/nickel oxide composition, nor did mixed oxide phases develop; only NiO crystalline peaks were detected. Crystallization of NiO into bunsenite occurred at roughly 540°C, according to DSC. Thermal analysis alone was unable to determine the crystallization temperatures for all of the other phases without more rigorous XRD experiments. Ellipsometry was employed to determine the thickness and index of refraction of the coatings. The individual layer thickness depended upon each precursor solution and the deposition conditions. When the thickness was Table 2 Crystallinity of wet-chemically derived nickel titantes Composition (wt%)
Phase (S)
Crystallite size (nm)
25% TiO –75% NiO 2
NiTiO 3 NiO NiTiO 3 NiO NiTiO 3 TiO (rutile) 2 NiO NiTiO 3 TiO (rutile) 2 NiO NiTiO 3 TiO (rutile) 2
44 44 49 45 47 47 51 42 40 48 48 44
33% TiO –67% NiO 2 50% TiO –50% NiO 2 67% TiO –33% NiO 2 75% TiO –25% NiO 2
below 0.5 mm, multiple layers were deposited to produce coatings that were at least this thick. The refractive index (RI ) varied with composition. For example, the RI of the 50 wt% SiO –TiO composition was measured 2 2 to be 1.75, which indicated 6% porosity, based on the theoretical RI for this composition. The porosity was calculated using literature values and the Maxwell Garnett effective medium approximation model. This model assumes spherical inclusions (pores in this case) and dipole interactions, which are standard first approximations that give reasonably accurate results [13].
4. Discussion Work by Page et al. [8,9] found that ion-implanted Ni and Ti significantly lowered the COF of a TiC pin on Si N . Auger spectroscopy indicated that it was the 3 4 oxides of Ni and Ti that were responsible for the improved tribological properties; however, relative amounts of each atom and their microstructure were not determined. In this work, Ni and Ti oxides were synthesized by wet-chemical techniques to imitate those oxides formed by ion implantation in the cited study. The results showed that Ni/Ti oxide was produced in a number of microstructures. In the same study, it was found that TiC-reinforced Si N reduced the COF from 0.72 to 0.43 and reduced 3 4 the wear to practically zero. Auger spectroscopy found Si and Ti oxides on the wear surface. Again, the composition (quantitative) and microstructure were not investigated. Therefore, silica/titania was synthesized to determine whether similar properties could be repeated in the wet-chemically derived coatings. Several binary and ternary combinations of SiO , 2 TiO and NiO were synthesized and examined. Any 2 coating that contained both silica and nickel oxide was worn away during the friction tests. Although hardness testing was not performed, this combination of materials appeared to produce soft coatings. Eliminating compositions that contained both SiO and NiO left two combi2 nations: SiO /TiO and TiO /NiO. The former produced 2 2 2 a COF between 0.2 and 0.3 at room temperature for a few minutes before wearing away. At 500°C, it maintained a moderate COF of 0.58 (average) throughout the entire 60 min test, in agreement with the previous findings. However, due to its poor wear properties at room temperature, SiO /TiO was not selected for fur2 2 ther study. Our results confirm the earlier findings that combinations of titania and nickel oxide provide low COF surfaces. Coupons coated with TiO –NiO had the lowest 2 COFs and the best wear behavior of all the compositions tested. This was the only composition in which the pin did not wear through to the substrate during the 3600 cycle test. The improved friction/wear properties pro-
D.J. Taylor et al. / Surface and Coatings Technology 120–121 (1999) 465–469
vided by the TiO /NiO coatings may be due to the 2 formation of a mixed metal oxide phase, NiTiO . This 3 phase, while it does not have a mineral name, has an ilmenite structure where nickel atoms sit on the iron sites [14]. Several of the compositions contained NiTiO with other phases, which may have enhanced 3 the friction and wear behavior by preventing growth of the nano-sized crystallites. At this time, the tribological performance of pure NiTiO coatings is unknown. 3 5. Summary Nano-structured oxide coatings were made by wet chemical techniques to imitate compositions previously noted to improve wear properties. Coatings that contained both silica and nickel oxide had poor wear characteristics and high COFs. Silica-titania had a COF <0.20 at room temperature, but wore off after only 200 test cycles. Compositions that contained titanium and nickel produced the only mixed oxide crystalline phase, NiTiO . Nickel titanate coatings had good friction/wear 3 properties at room temperature and at 500°C.
Acknowledgements This material is based upon work supported by the Air Force Office of Scientific Research under Contract
469
No. F49620-98-C-0065. The authors thank Major H. DeLong for his contributions to this project and M.N. Gardos for many helpful discussions.
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