Research ArticleMATERIALS SCIENCE

Boosting contact sliding and wear protection via atomic intermixing and tailoring of nanoscale interfaces

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Science Advances  18 Jan 2019:
Vol. 5, no. 1, eaau7886
DOI: 10.1126/sciadv.aau7886
  • Fig. 1 Schematics of test samples and HRTEM images.

    (A) Schematics of the overcoat designs: Monolithic overcoats 7CP (sample B), 7CF (sample C), and 7SiNx (sample D); carbon superlattice overcoat 2(T1/2CF) (sample E); C/SiNx multilayer overcoats 2(1.5SiNx/2CP) (sample F), 2(1.5SiNx/2CF) (sample G), 2(1SiNx/T2/2CF) (sample H), 0.75SiNx/2.75CF/1.5SiNx/2CF (sample I), 1.5SiNx/2CF/0.75SiNx/2.75CF (sample J), 2(0.75SiNx/2.75CP) (sample K), 2(0.75SiNx/2.75CF) (sample L), and 2(0.7SiNx/T4/2.75CF) (sample M). Sample A is plasma-cleaned bare AlTiC substrate. The prefixed numbers represent the thickness of the respective layer in nanometers, while the letters F and P represent carbon deposition by FCVA (F) and pulsed dc sputtering (P), respectively. High-energy carbon treatment of 350-eV C+ ions (labeled as Tn, where n = 1, 2, 3, and 4) is performed either to enhance the atomic mixing in the C/SiNx multilayer overcoats (T2, T3, and T4) or to construct a carbon superlattice structure (T1, where T1 adds 1.5 nm carbon thickness). The deposition times for T2, T3, and T4 are adjusted in such a way that they contribute to <1-, <0.3-, and <0.5-nm thicknesses, respectively, to the total overcoat thickness. (B) Cross-sectional HRTEM images with measurements of the overcoat thickness in samples C, E, K, and M. In all images, capping layers of tantalum (Ta) are deposited by magnetron sputtering before and after overcoat deposition on Si wafer substrates, as labeled, to provide image contrast for accurate overcoat thickness determination.

  • Fig. 2 TRIM simulations.

    TRIM simulations for selected samples showing the extent of atomic mixing and recoils in the depth profile plots, illustrated using color-coded schematics below each plot (panels 1 to 4). Stoichiometric Si3N4 is available in the TRIM directory and was used to mimic the experimentally grown SiNx. The thickness of SiNx was varied from 1.5 to 0.7 nm, and the energy of incoming carbon atoms was changed from 4 to 350 eV during the TRIM calculations to simulate the experimental structures.

  • Fig. 3 Interfacial and structural properties.

    Deconvolution of various XPS core level spectra recorded without performing a depth profile (0 s) and after an etching time of 100 s for sample G, 125 s for sample L, and 150 s for sample M. (A) Deconvolution of C 1s spectra for sample G (0 and 100 s), sample L (0 and 125 s), and sample M (0 and 150 s). (B) Deconvolution of Si 2p spectra for sample G (0 and 100 s), sample L (0 and 125 s), and sample M (0 and 150 s). (C) Deconvolution of N 1s spectra for sample G (0 and 100 s), sample L (0 and 125 s), and sample M (0 and 150 s). (D) Deconvolution of Ti 2p spectra for sample G (100 s), sample L (125 s), and sample M (150 s). (E) Deconvolution of Al 2p spectra for sample G (100 s), sample L (125 s), and sample M (150 s). (F) Different peaks corresponding to the different chemical bonding species. (G) Variation of sp3C bonding based on Raman and XPS analyses. The three-stage model, proposed by Ferrari and Robertson (25), was used to estimate sp3 bonding by Raman spectroscopy. (H) Ball-and-stick chemical models for samples B, C, and E. Lime, gray, and red color balls correspond to sp2-carbon, sp3-carbon, and oxygen atoms, respectively.

  • Fig. 4 Surface, mechanical, and tribological properties.

    (A) (i) Rq and nanoscale friction, (ii) hardness and elastic modulus, and (iii) CNP and CNF for various samples. (B) (i) Schematic of the experimental setup for ball-on-disk tribological tests. (ii to ix) Ball-on-disk microscale tribology showing the frictional curves and COF values for different samples. One cycle corresponds to a distance of 1.257 cm. All the experimental conditions were kept the same to limit any external influence on microscale tribology (see Materials and Methods and Supplementary Discussion 2). The nanoscale COF was found to be similar in these samples, although closer inspection revealed that it was marginally higher in the carbon superlattice overcoat (sample E) and C/SiNx multilayer overcoats than in the monolithic carbon overcoats (samples B and C). Furthermore, in a microscale friction test environment, sample M exhibited the lowest and most stable friction.

  • Fig. 5 Wear analysis after microscale ball-on-disk tribological tests.

    (A) Optical images of the balls (rows 1 and 3) and sample surfaces (rows 2 and 4) after the tribological measurements. The images reveal the amount of debris transferred to the balls and the severity of the wear tracks formed on the sample surfaces. Scale bars, 100 μm. Bare AlTiC showed significant wear, whereas sample M yielded the least wear. (B) Raman spectra of selected samples recorded from outside the wear track and at various locations (Loc) on the wear track to examine the wear in relation to the overcoat design. The sputter-processed C/SiNx multilayer in sample F experienced significant wear, indicating a considerable amount of overcoat removal and damage to the carbon layers, as visualized by the noisy Raman spectra on the wear tracks when compared to outside of the wear track. However, the wear was minimized in sample K because of the tailoring of the carbon and SiNx thicknesses, resulting in a small amount of carbon removal and minor structural damage to the carbon layers, although a D peak emerged when Raman spectra were recorded from the wear tracks, suggesting a higher amount of disordered sp2 carbon. A similar behavior to the sputter-processed overcoats was observed for the FCVA-processed C/SiNx multilayers, where sample G showed higher wear, but the wear was minimized by tuning the carbon and SiNx thicknesses in sample J (see Supplementary Discussion 11 for a detailed wear track analysis by Raman spectroscopy).

  • Fig. 6 Long-term THW test and wear analysis.

    (A) Schematic of the THW test setup used in the present work comprising tape supply/take-up reel components, tape guides, and the tape head to be tested, which was kept in contact with the sliding tape media at a constant tension. This simulates the operation of actual TDs. Since most of the previous thicker (~20 to 100 nm) overcoats could survive up to a maximum of 1.7 to 5 Mm of tape sliding in the THW test, we performed THW tests up to a tape sliding distance of 5 Mm to analyze the wear of our ~7- to 8-nm overcoats. (B) Optical microscope images of selected tape heads after THW tests. The brighter and darker portions indicate the presence and removal of overcoats, respectively. Although sample G demonstrated good wear resistance, it yielded very severe wear near the AlTiC substrate/closure edges adjacent to the Al2O3 gap region and inside the gap. In contrast, the wear was found to be minimal in samples E and M, suggesting higher wear resistance. (C) AES images to assess wear based on elemental mapping of C, O, Si, and Al. Before AES imaging, SEM images were recorded. The wear was evaluated on the basis of the intensities of C (from the overcoat) and Al/O (from the substrate), where a warmer/brighter color represents higher intensities and cooler/darker color represents lower intensities, according to the color scale bar. The C/SiNx overcoat in sample M demonstrated the best wear resistance.

  • Fig. 7 AES depth profiling to examine overcoat wear after long-term THW tests.

    After 5 Mm of THW tests, AES depth profiles were performed at selected locations on the head as highlighted by the red dot in each SEM image of Fig. 6C. The corresponding AES depth profile from an unworn region outside of the area of magnetic tape/head interaction for each sample was taken as a reference for comparison purposes. The depth profiles of samples C, E, G, and M are shown here, while those of samples F, H, K, and L are presented in fig. S10D. AES depth profiles for C/SiNx multilayer structures reveal the fluctuation of C, Si, and N intensities when moving away from the surface, approaching/crossing the C/SiNx interfaces, and reaching the film/substrate boundary, corroborating well with the actual designs of these multilayer structures. The immediate drop in C intensity and the immediate increase in O intensity in the first 50 s of most of the depth profiles after 5-Mm THW tests indicate that most of the head overcoats had been completely worn off, except in samples C, E, and M. Sample M with just a ~7- to 8-nm-thick overcoat displayed the best wear resistance (about 62% overcoat is still present after 5 Mm of THW) out of all the overcoats used in the present work and even surpasses the wear resistance of previously reported thicker and sputter-deposited ~20- to 100-nm coatings of CrNx, CrOx, CNx, c-BN, yttrium-stabilized ZrO2, W-C:H, a-C:H, Al2O3, and TiN (1316).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/1/eaau7886/DC1

    Supplementary Discussion 1. List of samples and their nomenclature.

    Supplementary Discussion 2. Sample preparation and characterization.

    Supplementary Discussion 3. HRTEM results and analysis.

    Supplementary Discussion 4. TRIM calculations results and analysis.

    Supplementary Discussion 5. XPS spectra at various depths and consideration of various effects during analysis.

    Supplementary Discussion 6. XPS results and analysis for chemical and interfacial bonding.

    Supplementary Discussion 7. Structural properties by XPS and Raman spectroscopy.

    Supplementary Discussion 8. Surface roughness of the auxiliary samples.

    Supplementary Discussion 9. Nanomechanical and nanotribological properties results and analysis.

    Supplementary Discussion 10. Ball-on-disk microscale tribological results and analysis.

    Supplementary Discussion 11. Wear track analysis by Raman spectroscopy.

    Supplementary Discussion 12. Fundamentals of TD technology and tape heads and evaluation of THW by optical microscopy, AES imaging, and AES depth profiles.

    Table S1. Description and nomenclature of all samples.

    Fig. S1. HRTEM imaging.

    Fig. S2A. TRIM calculations.

    Fig. S2B. TRIM calculations.

    Fig. S2C. TRIM calculations.

    Fig. S2D. TRIM calculations.

    Fig. S2E. TRIM calculations.

    Fig. S2F. TRIM calculations.

    Fig. S2G. TRIM calculations.

    Fig. S2H. TRIM calculations.

    Fig. S3. XPS analysis.

    Fig. S4. Interface analysis by XPS.

    Fig. S5. Structural properties.

    Fig. S6. Surface roughness measurements of the auxiliary samples.

    Fig. S7. AFM imaging before and after nanoindentation and nanoscratch tests.

    Fig. S8. Comprehensive friction and wear results for all samples.

    Fig. S9. Raman spectroscopy for wear analysis of various samples.

    Fig. S10. Fundamentals of magnetic tape heads and analyses of THW by optical microscopy, AES imaging, and AES depth profiles.

    References (4152)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Discussion 1. List of samples and their nomenclature.
    • Supplementary Discussion 2. Sample preparation and characterization.
    • Supplementary Discussion 3. HRTEM results and analysis.
    • Supplementary Discussion 4. TRIM calculations results and analysis.
    • Supplementary Discussion 5. XPS spectra at various depths and consideration of various effects during analysis.
    • Supplementary Discussion 6. XPS results and analysis for chemical and interfacial bonding.
    • Supplementary Discussion 7. Structural properties by XPS and Raman spectroscopy.
    • Supplementary Discussion 8. Surface roughness of the auxiliary samples.
    • Supplementary Discussion 9. Nanomechanical and nanotribological properties results and analysis.
    • Supplementary Discussion 10. Ball-on-disk microscale tribological results and analysis.
    • Supplementary Discussion 11. Wear track analysis by Raman spectroscopy.
    • Supplementary Discussion 12. Fundamentals of TD technology and tape heads and evaluation of THW by optical microscopy, AES imaging, and AES depth profiles.
    • Table S1. Description and nomenclature of all samples.
    • Fig. S1. HRTEM imaging.
    • Fig. S2A. TRIM calculations.
    • Fig. S2B. TRIM calculations.
    • Fig. S2C. TRIM calculations.
    • Fig. S2D. TRIM calculations.
    • Fig. S2E. TRIM calculations.
    • Fig. S2F. TRIM calculations.
    • Fig. S2G. TRIM calculations.
    • Fig. S2H. TRIM calculations.
    • Fig. S3. XPS analysis.
    • Fig. S4. Interface analysis by XPS.
    • Fig. S5. Structural properties.
    • Fig. S6. Surface roughness measurements of the auxiliary samples.
    • Fig. S7. AFM imaging before and after nanoindentation and nanoscratch tests.
    • Fig. S8. Comprehensive friction and wear results for all samples.
    • Fig. S9. Raman spectroscopy for wear analysis of various samples.
    • Fig. S10. Fundamentals of magnetic tape heads and analyses of THW by optical microscopy, AES imaging, and AES depth profiles.
    • References (4152)

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