In this paper, we present a simple annealing procedure (which we refer to as ''flash-annealing'' because of short duration) that results in the formation of an ultralow friction surface film on vanadium diboride (VB{sub 2}) surfaces. This annealing is done in a box furnace at 800 C for a period of 5 min. During annealing, the exposed surface of the VB{sub 2} undergoes oxidation and forms a layer of boron oxide (B{sub 2}O{sub 3}). In open air, the B{sub 2}O{sub 3} layer reacts spontaneously with moisture and forms a boric acid (H{sub 3}BO{sub 3}) film. The friction coefficient of a 440C steel pin against this H{sub 3}BO{sub 3} film is {approx}0.05, compared to 0.8 against the as-received VB{sub 2}. Based on Raman spectroscopy and electron microscopy studies, we elucidate the ultralow friction mechanism of the flash-annealed VB{sub 2} surfaces

Erdemir, A.

Fenske, G. R.

Halter, M.

UNT Digital Library

PREPARATION OF ULTRALOW FRICTION SURFACE FILMS ONVANADIUM DIBORIDE*A. Erdemir~ M. Halter,+ and G. R. Fenske. .. .....August 1996The submittedmanuscripthas been authored by acontractor of the U. S. Government under contract No.W-31 -109-ENG-38. Accordingly, the U. S. Governmentretainsa nonexclusive, royalty-freelicense to publish orreproduce“thepublishedform of this ecmtribution,orallow others to do so, for U. S. Government purposes.For presentation at 1lth International Conference on Wear of Materials, San Diego,CA, April 21-24, 1997.*Work supported by the U.S. Department of Energy under Contract W-31-1 09-Eng-38.#To whom all correspondence should be sent.+ Permanent Address: Purdue University, Department of Materials En@eering,West Lafayette, IN 60515.#w-@q’~f’ ‘“’uAud*, .bIDISCLAIMERThis report was prepared as an account of work sponsoredby an agency of the United States Government. Neither theUnited States Government nor any agency thereof, nor anyof their employees, make any warranty, express or implied,or assumes any legal liability or responsibility for theaccuracy, completeness, or usefulness of any information,apparatus, product, or process disclosed, or represents thatits use would not infringe privately owned rights. Referenceherein to any specific commercial product, process, orservice by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply itsendorsement, recommendation, or favoring. by the UnitedStates Government or any agency thereof. The views andopinions of authors expressed herein do not necessarilystate or ref Iect those of the United States Government orany agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic imageproduced from thedocument.products. Images arebest available original*I*.-1“PREPARATION OF ULTRALOW FRICTION SURFACE FILhIS ONVANADIUM DIBORIDEA. Erdemir, M. Halter, and G. R. FenskeArgonne National LaboratoryEnergy Technology DivisionArgonne, IL 60439. .. .ABStiCTIn this paper, we present a simple annealing procedure (which we refer to as “flash-amealing”because of short duration) that results in the formation of an ultraiow fiction surface film onvanadium diboride (VB2) surfaces. This annealing is done in a box fimace at 800°C for a periodof 5 min. During annealing, the exposed surface of the VB2 undergoes oxidation and forms a layerof boron oxide (B203). In open air, the B203 layer reacts spontaneously with moisture and forms aboric acid (H3B03) film. The fi-iction coefficient of a 440C steel pin against this H3B03 film is=0.05, compared to 0.8 against the as-received V& Based on Raman spectroscopy and electronmicroscopy studies, we elucidate the uhralow friction mechanism of the flash-annealed VB2surfaces...~ ‘“INTRODUCTIONMany transition metal diborides, i.e., TiB2, ZrB2, VBz, etc. possess exceptional hardness, highelastic modulus, and excellent thermal and electrical conductivity [1,2]. Chemically, most of themare ve~ inert and provide excellent protection against oxidation at elevated temperatures. Someborides (e.g., TiB2 and ZrBJ are metallic conductors. [2]. Owing to a high elastic modulus and.—-good chemical stability at elevated temperatures, TiB2 fibers and pardculates are used extensivelyin the production of reitiorced ceramic composites [3], In short, because of their unique thermal,mechanical, electrical, and chemical properties, borides are expected to find increased usage in avariety of industrial applications.In past years, coatings of transition metal dlborides (e.g., TiBz, ZrBz etc.) have been produced byreactive sputtering and chemical vapor deposition on a variety of metalforming dies and machineparts to combat wear [4,5]. Despite their excellent chemical stability and high wear resistanceover a wide range of temperatures, most borides cannot afford low friction to dry sliding surfaces.In this paper, we introduce a very simple annealing procedure that results in the formation of alow filction film on the surilace of vanadium debarred (VBz). In a series of studies, Bindal andErdernir [6], and Erdemir et al. [7] used a similar annealing procedure to achieve ultralow ftictionsurface films on borided steels and boron carbide. The annealing procedure of this study results inthe formation of a lubricious surface film providhg friction coefficients as low as 0.05. The goalof this paper is to elucidate the formation and ultralow friction mechanisms of thk surface film onVBz surfaces..-EXPERIM.ENTAL DETAILSThe VBz material used in our experiments was in the form of a 200 pm thick coating attached to acopper plate by means of explosion bonding. In order to achieve strong adhesion between VB2and copper plate, a layer (50 pm thick) of pure vanadium was first applied onto copper plateagain by explosion bonding. The test pieces for tribotesting were in the shape of squares with-nominal dimensions of 40 X 40 mm and 6 mm. The surface finish of the test pieces was in therange of 0.2 to 0.3 pm center-line-average (CLA). The annealing of the test pieces was performedin a box firnace at 800°C for 5 min. Specifically, the VBz test pieces were placed on a ceramicplate and inserted into the box fbmace at 800”C. After 5 min exposure time, they were taken outof the fimace and cooled to room temperature in open air.Friction and wear tests were pefiormed in a triborneter with ball-on-disk type contact geomet~under a load of 5 N and at room temperature (about 23”C). The relative humidi~ of the test .chamber varied between 38 and 84°/0. Rotational velocity was 6 to 8 rev. rein-l which translatedinto a sliding velocity of 5.3 rnm.s-l. The counterface materials were made of 440C steel pins (9.5“mm in diameter). One end of each pin was rounded to a radius of curvature of 127 mm and usedas the contact surface. It was polished to a surface finish of 0.01 pm CLA roughness. The slidingdistance was 275 m (or 5000 cycles). Before each sliding test and annealing heat - treatment, theceramic pins and flats were cleaned ultrasonically in acetone and methanol for 300 s each and thenoven-dried at 11O“C for 10 min. Laser-Raman spectroscopy was used to characterize the structureand chemical nature of the sliding surfaces. The Raman spectroscope used an HeNe laser at 632.8~.nm with an output power of 25 mW focused to a spot size of 2 to 3 ~m. Duplicate tests were runwith flash-amealed VI+ surfaces to check the reproducibility of test results.RESULTS AND DISCUSSIONFigure 1 shows the variation of friction coefficients of 440C steel pins during sliding against an as--received and flash-annealed VB2 sample. The fiction coefficient of steel pin is initially 0.15against V132,but increases rapidly as sliding continues and reaches a value of 0.8 toward the endof the test. The initially low friction coefficient may have been due to the presence of somesurface adsorbates and/or a thin chemical film. As the adsorbed film is removed or worn through,the ftiction coefficient increases and the fictional trace becomes very unsteady, possibly becauseof increased metal-to-VB2 contact and greater stick-slip. In short, the test result confirms thatVB2 is not a low friction material and cannot provide lubrication under dry sliding conditions. Thewear rate of steel pin used against the as-received VB2 was 1.7 X 104 mm3.N-1.m-1.The initial friction coefficient of a 440C steel pin sliding against the flash-annealed VBz surface is0.1, but decreases to 0.05 after about 8000 s and remains constant for the rest of the test. Thisresult confirms that the simple annealing procedure used in our study produces a slippery film onthe sliding contact surface of VBz. Also, the fictional behavior of this test pair is very steady anddoes not show much fluctuation. The wear rate of440C pin used against the flash-annealed VB2is 3.5 X 10< mm3.N1. m-l. Thk is nearly 50 times lower than that of the pin slid against the as-received V&..5. .In an attempt to elucidate the structural chemistry as well as the fimdamental mechanisms of thelow friction films on annealed VBz surfaces, we used a Raman spectroscope equipped with anHeNe laser operating at 632.8 nm with an output power of 25 mW focused to a spot size of 2 to3 pm. The Raman spectrum of the flash-annealed VB2 sutiace revealed two strong Raman bandscentered at approximately 498 and 880 cm-l (see Fig. 2a). We found that these values were veryclose to those (i.e., 500 and 881 cm-l) of the bulk boric acid (H~BOJ reported in Refs. 8 and 9.—-Furthermore, in previous studies, we have verified that films forming on BZ03 has a similar H3B03structure [5,6].In order to better make sure and fiu-ther verifi that the surilace reaction fiim was indeed HJB03,we obtained Raman spectrum of a piece of standard boric acid and have included its spectrum in.Fig. 2b for comparison. As is clear, the principal Raman bands of this spectrum match ped?ectlywith those from on the surface of the annealed VBZ. Note that the Raman spectrum of as-receivedVB2 is very different from those of the boric acid standard and annealed VBz sample. In short, thelow ftiction behavior of annealed VB2 surface (see Fig. 1) must have been closely related to theformation of a thin boric acid film on the exposed surface.Based on some thermodynamics and kinetics considerations, we can provide some explanation forthe formation of Lubricious HJBOJ film on VB2 surface. We believe that during exposure to 800”C,the boron atoms within VB2 structure gain high mobility and activation energies for diflbsion.Those boron atoms reaching the sutiace can readily react with oxygen in open air to form B203(e.g., layer 1 in Fig. 3). The standard heat of reaction for B,O, formation at 800°C is -290.6.-.~..kcal/mol [10], hence the formation of BZOJ is thermodynamically favorable at 800°C. As indicatedin Ref. 10, V can also readily oxidizes at 800”C, and the range of vanad~um oxides includes V2Qand V203. The standard heats of reaction for V20~ formation is -291.5 kcal/mol and for V203formation is -362.3 kcal/mol [10]. During our annealing experiments, we noticed that if we heldthe VBz sample in the fimace for more than 5 rein, we would see the formation of some Iiquidlikeislands on the surface. We believe that these islands were primarily composed of vanadium oxides.-V20~ melts at 670”C [10].It is our opinion that B20~ layer forming at 800°C during short duration results from the oxidationof interstitially accommodated boron atoms Withh the VBz structure. The oxidation of VBz is alsopossible at high temperatures (especially when exposure time and/or temperature is higher than 5min or 800”C) but as a very stable compound, it will be rather difficult to extract free boron atomsfrom the highly stable V&. Obviously, the thermodynamics and kinetics of the physical/chemicalevents taking place at 800°C are rather complex and require more in-depth studies, but in generalthe short-duration annealing procedure adopted in our study seems to result in B203 formation.As illustrated in Fig. 3 and described in detail in Refs. 9 and 11, boron oxide undergoes aseconda~ reaction with moisture in surrounding air (because of a negative standard heat ofreaction). The end product is a thin boric acid film on the upper surface as verified by Ramanspectroscopy in Fig. 2.The uhralow fiction mechanism of boric acid film forming on the surface can be explained as,:.follows. Boric acid has a layered triclinic crystal structure as described in Ref. 12. The atomiclayers in its crystal structure are parallei to the basal plane and they are made up of boron, oxygen,and hydrogen atoms. These atoms are closely packed and strongly bonded to each other bycovalent, ionic, and hydrogen bonds, whereas the atomic layers are widely spaced (i.e., 0.318 nm)and held together by weak forces, e.g., van der Waals (Fig. 4). In a sense, the layered-crystalstructure of boric acid resembles those of MoS2, graphite, and hexagonal boron nitride which are-well established as lamellar solid lubricants. Therefore, we believe that the ultralow fiction..behavior of the flash-annealed VB2 surfaces is associated with the formation of a thin boric acidfilm that has a layered crystal structure. Mechanistically, we propose that under shear forces,atomic layers of boric acid crystal align themselves parallel to the direction of relative motion;once so aligned, they can slide over one another with relative ease to provide the low ftiction.coefficient shown in Fig. 1.SUMMARYIn this study, we demonstrated that ultralow friction coefficients can be achieved on VB2 surfacesafter short-duration annealing. We believe that the sequential formation of first a boric oxidelayer during amealing at 800”C and then a boric acid film during cooling on VBZsurface are themain reason for low friction properties. The boric acid film formed on the surface has a layeredcrystal structure. Boron, oxygen, and hydrogen atoms making up the layers are closely packedand strongly bonded to each other, while the layers themselves are relatively apart and bondedtogether with weak van der Waals forces. When present at a sliding surface, the layers can shear.8“easily, thus providing low ftiction. The findings of this study suggest that other transition metaldiborides, such as TiBz and ZrBz can also be made slippery by performing a similar annealingprocedure at elevated temperatures.ACKNOWLEDGMENTS.-- This work was supported by the U.S. Department of Energy under Contract W-3 l-109-Eng-38.REFERENCES1. Boron, Metallo-boron Compounds and Boranes, R. M. Adams, Interscience Publ., New York1964, pp. 335-364.2. R. A. Cutler, Engineering Properties of Borides, Engineered Materials Handbook Vol. 4,Ceramics and Glasses, S. J. Schneider, Jr., (Ed, chairman), ASM International, Metals Park, OH.,1991, pp, 787-8033. C. H. McMurtry, W. D. G. Boecker, S. G. Seshadry, J. S. Zanghi, and J. E. Gamier,“Microstructure and Material Properties of SiC-TiBz Particulate Composites,” Ceram. Bull.,66(1987) pp. 325-3294. W. Herr, B. Matthes, E. Broszeit, and K. H. K1OOS,“Fundamental properties of wear resistanceof r.f. sputtered TiBz and Ti(B,N) coatings,” Mat. Sci. Eng., A140( 1991) pp. 616-624.5. H. Deng, J. Chen, R. B. Inturi, J. A. Barnard, “Structure, mechanical and Tribologicalproperties of d.c. magnetron sputtered TiBz and TiB2 (N) thin films, Surf Coat. Technol., 76-77(1995) pp. 609-614.6. C. Bindal and A. Erdemir, “Ultralow friction behavior of borided steel surfaces after flashannealing,” Appl. Phys. Lett., 68 (1996), pp. 923-925.7. A. Erdemir, C. Bindal, and G. R. Fenske, “Formation of ultralow friction surface films on-.. . -boron carbide,” Appl. Phys. Lett., 68 (1996), pp. 1637-1639.8. R. Janda and G. Heller, “Ill- und Ramanspectren isotop markerter Tetra- und Pentaborate,”Spectrochim. Acts, A., 36,997 (1981).9. A. Erdemir, G. R. Fenske, and R. A- Erck, “ A Study of the Formation and Self-lubricatingMechanisms of Boric Acid Films on Boric Oxide Coatings,”, Surf Coat. Techno., Volume 43/44(1990) pp. 588-596.10. C. E. Wicks and F. E. Block “Thermodynamic Properties of 65 Elements-Their Oxides,Halides, Carbides, and Nitrides,” U.S. Bureau of Mkes, Bulletin # 605, pages 22 and 131 (1963)11.A. Erdemir, G. R. Fenske, R. A. Erck, F. A. Nichols, and D. E. Busch, “TribologicalProperties of Boric Acid and Boric-Acid-Forming Surfaces. Part 11. Mechanisms of Formationand Self-Lubrication of Boric Acid Films on Boron- and Boric Oxide-Containing Sufiaces,” LubrEng., 47 (1991) 179-183.12. A. Erdemir, “Tribological Properties of Boric Acid and Boric-Acid-Forming Surfaces. Pafi I:Crystal Chemistry and Mechanism of Self-1ubrication of Boric Acid,” Lubr. Eng., 47 (1991), 168-178.FIGURE CAPTIONS:. .Fig. 1. Variation of fiction coefilcients of 440C steel pins during sliding against as-received andflash-annealed V&Fig. 2. Raman spectra of (a) as-received and flash-annealed VB2 and (b) standard H3B03 samples.Fig. 3. Schematic illustration of formation of B20~ and H~BOl films on VB2 surfaces.Fig. 4. Layered triclinic csystal structure of H3B03..‘E(1””.-C)10.80.60.4IL0.200. ............... ..........t. ............... .... ...-.. ...... .. . ... .... .................... .....F.......+..................... ....... .... ..... .............. ................ ....... ............................. ....... ............... ...........50As-rece...........................Annea100 150Distance (m)fed VB2..............................>d VB2P .-.............200 250 300Fig. 1. Variation of friction coefficients of 440C steel pins during sliding against as-receivedflash-annealed VBz,iand700600. .. .““20010001- I [ i I I I i I I878, !/..........8t-t:~498,*s’,,210 ~*S.--------.--.--..---------------.--.p-------------------------------------f, 1!,.#S..*,*.4.,~~.~~.,*#*x ~‘* M*.,,1/*“*.’+,%J‘‘%Ll_.____j..--_.—_.-._.—Ar@ealed VB2.-—_.....-.....—-.-.-&_-_._ _—__——........-..--—-------- ..............8*\*,4’\..F.*- “-%.-,,,.—..-...--—-...—--------I I I I.. . . . . .. . . . .. . .. . .. . .. . . . . . .. . .. . . . . . . . . .. . . . . . . .. . . . . . .. . .. . . . . . . . . . . . . . . . . . . . ...—.. . . . . . . . . .. . . .. . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . .. . . . . .. . . . . .. . . ..—‘.*v*.-—.-----—-.. --.--. --. -.—“+0. ,,* ,“ ‘“-%.,.,**......... ............ ...... .......... .1 I t Io 500 1000 1500 2000RAMAN SHIFT, cm-’(a)#. .u)“ “e- ~...““‘2’cdLga8007006005004003002001000I I I t. . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . .. .—... -. . . . . . .. . . . . . . . . . . . . . .. . .. . . . .—... __ . . .. . . . . ..-. .___. ------. . . . . .. . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . .880.............................. ......... . .... .... . .... ... .498.. ....... ... .. ........ .. . . ...... ............... ....... . .. ........... ... .. ..! i..........................: H3B03 standa~do 500Fig. 2. Raman spectra of(a) as-received1000RAMAN SHIFT, cm-’(b)1500 2000and flash-annealed VBZ and (b) standard H3B03 samples..- - .-:.-.~H$~ B203+- H20 -+ H3B03 :B90q 2B + 3/2 024 B@ ~VB SUBSTRATE2“IFig. 3. Schematic illustration of formation of BzO~ and H3BOJ films on VB2 sutiaces.- .-LcbaI0.318 nmINTERLAYER BONDING : van der WaalsQ = 92.580 a = 0.7039 nm/3”10117°● b = 0.7053 nmy = I 19.830 c = 0.6578 nmFig. 4. Layered triclinic crystal structure of HJBOJ,

Preparation of ultralow-friction surface films on vanadium diboride.

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Preparation of ultralow-friction surface films on vanadium diboride.

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