Drawn pearlitic wires have remarkable mechanical properties. They combine a very high strength level while retaining ductility and toughness. We propose to fabricate them in thin film form. Pearlitic wires are made from Fe/Fe₃C. We propose to make them using more complex systems: iron/iron carbide and iron/metal carbide. Pearlitic wires are lamellar on the 10 nm or so scale. Multilayers can duplicate this microstructure in a much more controlled manner, and can be artificially varied and made increasingly complex. Such thin films should likewise have remarkable properties.

The thin films will be produced by sputter deposition from multiple targets. Their structure will be varied both in terms of layer spacing, carbide proportion, and carbide composition. Typical carbide forming elements are titanium, chromium, and molybdenum. The structures will be characterized fully by transmission electron microscopy and the mechanical properties assessed by nanoindentation and bulge testing, i.e. standard thin film procedures. The objective is to understand the relationship between structure, constitution, and mechanical properties in order to optimize the latter for thin films and coatings.

Wire drawing of pearlitic steel results in materials with dramatically increased tensile strength while retaining ductility. Such heavily drawn wires are among the strongest metallic materials available at present, and their high level of strength is known to result from the microstructural refinement occurring during drawing. This strength, as well as other mechanical properties, is strongly dependent on the state of the carbide phase. Transmission electron microscopy (TEM) of the highly drawn wires reveals that the microstructure is lamellar with alternating layers of ferrite and carbide. As the degree of drawing strain increases, the layers become correspondingly finer in scale.

We propose to duplicate these structures in thin film form. It is anticipated that their properties, for instance as coatings, will also be extreme. By using sputter deposition, the microstructure and chemistry can be varied at will. We will investigate the structure-property-processing relationships by transmission electron microscopy and nanoindentation techniques.

The natures of the phases present and their corresponding microstructures will have a direct impact on the resulting mechanical properties. Recently, more studies have been made regarding the mechanisms of deformation of pearlite and the evolution of the microstructure during plastic deformation [1—11]. Generally, the dependence of yield and flow stress on the pearlite interlamellar spacing S_p is postulated to obey a Hall-Petch type equation s_f = s ₀ + k*S_p^-m where s₀ and k are constants [12]. There is evidence, however, of dissolution of the carbide phase under severe plastic deformation. It has been proposed that this partial dissolution may result from the interactions between dislocations and carbon atoms [1,2], or from the thermodynamic destabilization of the cementite phase during cold drawing due to the increase of its interfacial free energy [3,4]. In those studies, the only phases observed in the strained state were those of ferrite and cementite, with the ferrite regions displaying a supersaturation of carbon from the dissolved cementite. However, recent studies by K. Makii in Japan have shown evidence of amorphization of the cementite layer [33].

One of the difficulties in isolating the causes and mechanisms in these systems is the presence of impurities in the steel itself. Many of the alloy elements will either promote or impede carbon diffusion or dislocation motion within one or another of the layers, and will therefore introduce further uncertainty in the determination of the basic structure-property relationship. It would then be beneficial to develop a model system which can then be perturbed in a controlled manner to tailor investigations into the questions that still surround the iron-iron carbide and iron-metal carbide systems. As the pearlite system is a lamellar one, this suggests an investigation by means of current thin film synthesis techniques.

Direct construction by sputtering of the lamellar layers would allow for the controlled formation of iron-iron carbide multilayers as the base structure. Substitution of suitable carbide forming elements (e.g. titanium, chromium, molybdenum) would allow building of iron-metal carbide multilayers. This would allow the construction of new thin film materials and the analysis of the effects of varying the periodicity, the concentration, and the composition of the layers. Cross-section and through-foil TEM is ideal for characterizing the structure [21]. Nanoindentation is appropriate for establishing the associated mechanical properties such as hardness and elastic properties [22]. These analyses would engender a greater understanding of the fundamental structure-property relationships in this class of complex materials.

It is proposed to compare the structures and mechanical properties of iron carbide (lamellar) thin films with those produced by wire drawing of steels. In this work, we will prepare artificial iron carbide multilayers with periodicities in the 2-50 nm scale range, varying the composition of both the ferrite (iron) phase and that of the carbide. Sputtering using argon gas is the preferred technique. Sputter deposition of cementite [16,17] and iron [18—20] has been performed by sputtering, but these films were mainly investigated for their magnetic properties. Deposition of metal-carbon multilayers [21] has been quite extensive, but, again, there has been no systematic study of the mechanical properties.

Sputter deposition would be carried out in collaboration with, and in the research laboratories of, Dr. Troy Barbee, Jr. (Lawrence Livermore National Laboratory). The Stanford group has over ten years experience in the deposition of metal-metal, metal-metalloid, and metal-carbon layered systems [23—26]. However, the scope and complexity of the present project rather precludes the use of university-based systems. We would start with the straightforward deposition of metal-carbon and metal-iron carbide (cementite composition) multilayers. These would be chosen to be close to the pearlitic structure at first, but deviated from subsequently (i.e. increasing the carbide proportion). However, the later stages would involve the formation of iron-metal carbide multilayers, to duplicate he pearlitic structure with strong carbides. Classical metallurgical elements will be employed, with varying degrees of carbide-forming tendency. Titanium and molybdenum are strong carbide formers, chromium is intermediate with several metastable phases [21, 27] (as indeed is iron itself), and it may even be interesting to extend to elements with only highly metastable carbides such as cobalt [29] and nickel [28].

The multilayer (i.e. lamellar) periodicity is the primary experimental variable, but other avenues to be pursued include the proportion of the carbide phase and the carbide composition itself. The iron phase (ferrite) is expected to equilibrate its composition (carbon content) rapidly with respect to the carbide. Finally, of course, the effect of the iron phase can be studied by inclusion of austenite stabilizers to assess whether FCC-carbide multilayers are superior to the primary BCC-carbide structures.

Film thicknesses in the range of 0.1—1.0µm will be fabricated on amenable substrates (i.e. oxidized silicon wafers). Ultimately, once optimal structures are identified, we would "scale-up" the thickness, which once again requires the superior facilities at Lawrence Livermore National Laboratory.

Characterization of these thin films, as well as characterization of the fine lamellar structure in highly drawn pearlite, requires techniques that will allow for resolution down to the sub-nanometer scale. TEM has proven itself a critical imaging and analytic technique in this range, with which the Stanford group has extensive experience. Furthermore, with the development of in situ techniques, it allows for concurrent experimentation on the stability of the sample structure while observing the microstructural changes thus created. The structures produced will be characterized by TEM, both in through-foil (for phase identification) and in cross-section (for multilayer structure) orientation. We will employ both high resolution imaging (Philips EM430 TEM) and nanoprobe-analysis techniques (Philips CM20FEG TEM) and complement the data with additional characterization at the National Center for Electron Microscopy in the Lawrence Berkeley National Laboratory (e.g. energy filtered imaging). The results will be compared with those obtained in drawn steel wires, in terms of phase identity, composition, interface structure, etc.

There has been intense study of mechanical properties testing techniques for thin films in recent years. This has been motivated by the use of thin film materials in small-scale devices, especially the continuing goals miniaturization in the semiconductor industry. These techniques include nanoindentation, bulge testing, and optical level techniques. Nanoindentation determines hardness as a function of yield strength, stiffness, and crystallography [30]. Sampling a range of indent sizes and depths allows for the observation and removal of substrate effects. Bulge testing involves applying pressure to one side of a freestanding "window", an area from which the substrate has been etched away, causing it to bulge upward [31]. The center deflection and applied pressure are then directly related to the stress and strain in the window. Wafer curvature techniques involve scanning a laser across a wafer, where the angle of reflection as a function of position gives the curvature, which can then be related to stress by the Stoney relation[32]. These techniques are all routinely available at Stanford, through the group of Professor W. D. Nix which manages the MTS Nanoinstruments Nanoindenter and built the bulge tester and the wafer curvature instruments used at Stanford University.

The first phase of the research is highly dependent on the material preparation. Accordingly, varying the sputtering parameters to duplicate as far as possible the "natural" structures, and indeed to extend them to further regimes, is the major task. In addition, perfecting the TEM specimen preparation of the thin films and of the wires requires careful attention, as does optimization of the TEM experiments themselves.

The second phase is the determination of the mechanical properties and the correlation to their microstructures. We will employ nanoindentation as well as optical level techniques to determine properties such as hardness and tensile stress. We will then be in a position to correlate the mechanical properties of the structures with their material microstructures.

The final phase is increasing the complexity of the system. After thorough study of the iron-iron carbide system, we will have the tools and techniques in place to easily extend the study into further systems.

1. V. N. Gridnev and V. G. Gavrilyuk, "Cementite Decomposition in Steel under Plastic Deformation (a Review)," Phys. Metals, 4:3, 531—551 (1982).

2. H. G. Read, et al. "APFIM and TEM Studies of Drawn Pearlitic Wire," Scripta Mater., 37:8, 1221—1230 (1997).

3. J. Languillaume, G. Kapelski, and B. Baudelet, "Cementite Dissolution in Heavily Cold Drawn Pearlitic Steel Wires," Acta Mater., 45:3, 1201—1212 (1997).

4. J. Languillaume, G. Kapelski, and B. Baudelet, "Evolution of the tensile strength in heavily cold drawn and annealed pearlitic steel wires," Mater. Lett., 33:3—4, 241—245 (1997).

5. A. V. Korznikov, et al. "Influence of Severe Plastic Deformation on Structure and Phase Composition of Carbon Steel," NanoStructured Materials, 4:2, 159—167 (1994).

6. J. Toribio and E. Ovejero, "Microstructure orientation in a pearlitic steel subjected to progressive plastic deformation," J. Mater. Sci. Lett., 17:12, 1037—1040 (1998).

7. J. Toribio and E. Ovejero, "Microstructure evolution in a pearlitic steel subjected to progressive plastic deformation," Mater. Sci. and Engin. A, 234—236, 579—582 (1997).

8. B. L. Bramfitt and A. R. Marder, "A Transmission-Electron-Microscopy Study of the Substructure of High-Purity Pearlite," Mater. Charact., 39:2—5, 199—207 (1997).

9. C. M. Bae, W. J. Nam, and C. S. Lee, "Effect of Interlamellar Spacing on the Delamination of Pearlitic Steel Wires," Scripta Mater., 35:5, 641—646 (1996).

10. W. J. Nam and C. M. Bae, "Void initiation and microstructural changes during wire drawing of pearlitic steels," Mater. Sci. and Engin. A, 203:1—2, 278—285 (1995).

11. M. Dollar, I. M. Bernstein, and A. W. Thompson, "Influence of Deformation Substructure on Flow and Fracture of Fully Pearlitic Steel," Acta Metall., 36:2, 311—320 (1988).

12. J. D. Embury and R. M. Fisher, "The Structure and Properties of Drawn Pearlite," Acta Metall., 14, 147—159 (1966).

13. C. Sella, et al., "Microstructural and magnetic studies of the interfaces in sputtered metal/carbon and metallic multilayer films," Surf. Coat. Technol., 60:1—3, 379—384 (1993).

14. K. Van Acker, et al., "Neutron Diffraction Measurement of the Residual Stress in the Cementite and Ferrite Phases of Cold-Drawn Steel Wires," Acta Mater., 44:10, 4039—4049 (1996).

15. B. Goranchev, et al., "Some Properties of R.F. Reactively Sputtered Magnetic Fe-C Films," Thin Solid Films, 100:3, 257—269 (1983).

16. S. Tajima and S. Hirano, "Synthesis and properties of Fe₃C film by r.f. magnetron sputtering," J. Mater. Sci., 28:10, 2715—2720 (1993).

17. C. Lin, et al., "Magnetron facing target sputtering system for fabricating single-crystal films," Thin Solid Films, 279:1—2, 49—52 (1996).

18. X. X. Zhang and E. Y. Jiang, "The nucleation and initial growth of iron films deposited by facing targets sputtering," J. Magn. Magn. Mater., 121:1—3, 46—48 (1993).

19. K. Okamoto, et al., "Crystallographic Investigation of the Iron Films Deposited Obliquely by Sputtering," Thin Solid Films, 176, 255—262 (1989).

20. S. Yaegashi, T. Kurihara, and H. Segawa, "Epitaxial growth and magnetic properties of Fe(211)," J. Appl. Phys., 74:7, 4506—4512 (1993).,

21. T. Itoh, Study of reactions in transition metal/amorphous carbon layered thin films using transmission electron microscopy, Ph.D. Thesis, Stanford University, 1996.

22. J. J. Vlassak, New Experimental Techniques and Analysis Methods for the Study of the Mechanical Properties of Materials in Small Volumes, Ph.D. Thesis, Stanford University, 1994.

23. G. A. Bertero, et al., "(Pt/Co/Pt)/X multilayer films with high Kerr rotations and large perpendicular magnetic anisotropies," Appl. Phys. Lett., 64:24, 3337—3339 (1994).

24. T. J. Konno, et al., "Crystallization of co-sputtered amorphous cobalt-carbon alloys: morphology and kinetics of spherulitic growth," Mater. Sci. and Engin. A, 179—180, 297-302 (1994).

25. K. Holloway, et al., "Interfacial reactions on annealing molybdenum-silicon multilayers," J. Appl. Phys., 65:2, 474—480 (1989).

26. T. Itoh and R. Sinclair, "In Situ TEM Study of Reactions in Iron/Amorphous Carbon Layered Thin Films," Mat. Res. Soc. Symp. Proc., 382, pp. 45—50 (1995).

27. K. Bongartz, "A finite difference model describing carburization in high-temperature alloys," Corrosion, 42:7, 390—397 (1986).

28. F. J. Derbyshire, et al., "The formation of graphite films by precipitation of carbon from nickel foils," Carbon, 10:1, 114—115 (1972).

29. T. J. Konno, et al., "In situ atomic resolution electron microscopy of metal-mediated crystallization of semiconductors," Mater. Sci. Forum, 204—206, 749—754 (1996).

30. M. F. Doerner and W. D. Nix, "A method for interpreting the data from depth-sensing indentation instruments," J. Mater. Res., 1:4, 601—609 (1986

31. O. Tabata, et al., "Mechanical property measurement of thin films using load-deflection of composite rectangular membranes," Sensors and Actuators, 20:1—2, 135—141 (1989).

32. P. A. Flinn, et al., "Measurement and interpretation of stress in aluminum-based metallization as a function of thermal history," IEEE Trans. Electr. Dev., ED-34:3, 689—699 (1987).

33. K. Makii, personal communication.