Project Objectives

1. Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard CoatingsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Project Objectives A three-year research project on nanotribology of ultra-thin wear and oxidation resistant hard coatings was been initiated. The primary objective of this study is to improve fundamental understanding of the mechanisms underlying the effects of N incorporation on the growth, nanostructure, nanotribological properties, and oxidation resistance of nitrided hard coatings. A secondary objective is to determine to what extent the behavior of thicker films can be extrapolated to the ultra-thin layer regime through an investigation of the thickness dependence of the protective effectiveness of optimized coatings.

2. Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard CoatingsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Significant Accomplishments • Detailed assessment of ambient temperature oxidation resistance of the TiB2 compound. • Nanotribology Studies of CrNx, CrOx, and CN Thin Films using Constant and Ramped Load Nanoscratch Techniques. • Influence of Nitrogen on the Microstructures and Stress Development in Magnetron Sputtered TiCrAl Films.

3. Ambient Temperature Oxidation Resistance of Ultrathin TiB2 FilmsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Motivation • Advanced magnetic storage systems require thermally and dimensionally stable protective overcoats that are <5 nm in thickness. TiB2-based films are viewed as promising candidates. Quantitative investigations of the oxidation of TiB2-based films are lacking. • Nominally 5-50 nm thick TiB2 films were exposed to ambient air for ~2h. The oxidation characteristics are studied by x-ray reflectivity (XRR), x-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). Experimental XRR patterns illustrating the applicability of a two-layer model (i.e., a 2 layer film) Conclusions • Results from our current studies offer a new perspective on the arguments concerning the potential of TiB2 films as protective overcoats for magnetic recording. • Ultrathin TiB2 films oxidize at room temperature as a result of exposure to air. This results in a 9 Å thick erosion of the overcoat and a 6 Å thick surface “pop-out” (swelling). • These observations are independent of the pre-oxidation film thickness. • Since the thickness budget for protective overcoat in head/disk interface has been decreased to 5 nm or even less, the room temperature oxidation of ultrathin TiB2 films revealed in our current studies really negates such a potential. High resolution O 1s XPS spectra for depth profiling illustrating the presence of oxides near the film surface

4. Influence of Nitrogen on the Microstructures and Stress Development in Magnetron Sputtered TiCrAl FilmsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Motivation • Multiphase coatings based on the Ti-Al-Cr alloy system exhibit high resistance to oxidation. Nitrogen additions to these films have been reported to improve the hardness of these films while maintaining the favorable oxidation resistance. The microstructures, stresses, and thermal stability of these coatings have been evaluated for the first time as a function of nitrogen content. • Mechanical properties have been measured using nanoindentation and wafer curvature techniques. Film structure has been investigated using X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Representative stress versus temperature curves for polycrystalline TiAlCr(N) coatings Conclusions • As-deposited nitrogen-free coatings were amorphous and retained their structures after annealing up to 500°C. • After annealing the coatings exhibited higher hardness, larger reduced modulus, and smaller CTE values, which likely result from densification of coatings induced by thermal cycling and surface oxidation. • Nitrogen incorporation results in crystalline films containing a mixture of aluminum and chromium nitrides, which is consistent with prior investigations on TiN-based coatings. • Nitrogen additions increased film hardness and resistance to yielding without degrading oxidation resistance. XRD spectra for TiAlCr(N) films deposited at various nitrogen gas flow ratios, RN.

5. Nanotribological Studies of Thin CrNx, CrOx and CNx Overcoats for Magnetic Recording ApplicationsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Motivation • Chromium-nitride (CrNx) and chromium-oxide (CrOx) based coatings are receiving interest for tribological applications due to their favorable wear behavior and good resistance to oxidation and corrosion. • CrOx and CrNx have been proposed as potential replacements for nitrogenated carbon (CNx) as protective layers on magnetic recording disks. • In this study, the nanotribological properties and thermal stability of CrOx and CrNx films have been investigated in in relation to their potential to replace conventional CNx coatings. • Mechanical properties have been measured using nanoindentation and nanoscratch techniques. Film structure has been investigated using X-ray diffraction (XRD), and x-ray photoelectron spectroscopy (XPS). CNx displays the smallest in situ displacement and residual depth, and highest elastic recovery. The critical load before film fracture for CrOx is the lowest, while CrNx (with 2 nm Cr underlayer) and CNx have similar critical loads. Analysis of wear tracks following nanoscratch experiments show that Conclusions • The hardness and adhesion of CrNx is comparable to CNx. • With a very thin Cr underlayer, CrNx appears to have the potential for magnetic recording applications Hardness and reduced modulus for CrNx and CrOx films in comparison with CNx. The hardness of CrNx is greater than CrOx and comparable to CNx

6. Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard CoatingsM.L. Weaver & J.A. Barnard, The University of Alabama TuscaloosaAward #DAAD 19-99-1-0152 Year Three Publications G. Wei, A. Rar, and J.A. Barnard, “Composition, structure, and nanomechanical properties of DC-sputtered CrNx (0  x  1) thin films,” Thin Solid Films, v 398-399, pages 460-464 (2001) F. Huang, J.A. Barnard, and M.L. Weaver, “Ultrathin TiB2 Protective Films,” Journal of Materials Research v. 16, n. 4, pages 945-954 (2001). F. Huang, G. Wei, J.A. Barnard, and M.L. Weaver, “Microstructure and Stress Development in Magnetron Sputtered TiAlCr(N) Films,” Surface and Coatings Technology, v. 146-147, pages 391-397 (2001). G. Wei, T.W. Scharf, J.N. Zhou, F. Huang, M.L. Weaver, and J.A. Barnard, “Nanotribology Studies of Cr, Cr2N and CrN Thin Films Using Constant and Ramped Load Nanoscratch Techniques,” Surface and Coatings Technology, v. 146-147, pages 357-362 (2001). F. Huang, W. J. Liu, J.F. Sullivan, J.A. Barnard, and M.L. Weaver, “Room Temperature Oxidation Resistance of Ultrathin TiB2 Films: A New Perspective on its Potential as Protective Overcoats in Magnetic Recording,” Journal of Materials Research, v. 17, n. 4, pages 805-813 (2002). F. Huang, J.A. Barnard, and M.L. Weaver, “Mechanical Characterization of DC Magnetron Sputtered Amorphous Ti-Al-Cr Coatings,” Surface and Coatings Technology, v. 155, pages 146-151 (2002). G. Wei, M.L. Weaver, and J.A. Barnard, “Nanotribology Studies of Chromium Thin Films,” Tribology Letters, accepted for publication (2002).