Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard Coatings M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #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.

Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard Coatings M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #DAAD 19-99-1-0152

Significant Accomplishments

 Detailed assessment of ambient temperature oxidation resistance of the TiB 2 compound.  Nanotribology Studies of CrN x , CrO x , 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 .

Ambient Temperature Oxidation Resistance of Ultrathin TiB 2 M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #DAAD 19-99-1-0152 Films Motivation

 Advanced magnetic storage systems require thermally and dimensionally stable protective overcoats that are <5 nm in thickness. TiB TiB 2 2 -based films are viewed as promising candidates. Quantitative investigations of the oxidation of -based films are lacking.

 Nominally 5-50 nm thick TiB 2 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)

SCR Fit 5 nm 0.00

0.05

0.10

0.15

0.20

q z

(Å -1 ) 0.25

10 nm 15 nm 20 nm 0.30

25 nm 0.35

35 nm 0.40

B 2 O 3 TiO 2 e (120 sec) d (90 sec) c (60 sec) b (30 sec) a (0 sec) 555 550 545 540 535 530

Binding energy

(eV) 525 520

High resolution O 1s XPS spectra for depth profiling illustrating the presence of oxides near the film surface

Conclusions

 Results from our current studies offer a new perspective on the arguments concerning the potential of TiB 2 films as protective overcoats for magnetic recording.  Ultrathin TiB 2 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 TiB 2 films revealed in our current studies really negates such a potential.

Influence of Nitrogen on the Microstructures and Stress Development in Magnetron Sputtered TiCrAl Films M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #DAAD 19-99-1-0152 Motivation

500  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.

Representative stress versus temperature curves for polycrystalline TiAlCr(N) coatings

0 -500 -1000 -1500 Coolin g R N = 0.12

R N = 0.15

R N = 0.20

R N = 0.25

Heatin g  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).

-2000 0 100

Conclusions

200 300

Temperature

(°C) 400 500 35 40 45 R N = 0.25

R N = 0.20

R N = 0.15

R N = 0.12

R N = 0.08

R N = 0.04

R N = 0.00

50 55 60 65 70 75

2

q (degrees) 80 85 TiN(cubic) CrN(cubic) AlN(hex.) 90 95 100 105

XRD spectra for TiAlCr(N) films deposited at various nitrogen gas flow ratios, R N .

 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.

Nanotribological Studies of Thin CrN x , CrO x and CN x Overcoats for Magnetic Recording Applications M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #DAAD 19-99-1-0152 Motivation

 Chromium-nitride (CrN x ) and chromium-oxide (CrO x ) based coatings are receiving interest for tribological applications due to their favorable wear behavior and good resistance to oxidation and corrosion.

 CrO x and CrN x have been proposed as potential replacements for nitrogenated carbon (CN x ) as protective layers on magnetic recording disks.

 In this study, the nanotribological properties and thermal stability of CrO x and CrN x films have been investigated in in relation to their potential to replace conventional CN x coatings.

TABLE I. Tribological Properties of CrN x , CrO x and CN x .

In situ displacement ( nm ) (  1 nm ) Residual depth ( nm ) (  1 nm ) Elastic recovery rate ( % ) (  0.2% ) Critical load ( mN ) (  0.1 mN ) Friction coefficient (  0.02 ) CrN x -96 -50 47.9 1.9 0.21 CrO x -107 -58 45.8 1.4 0.26 CN x -88 -38 56.8 1.8 0.15  CN x displays the smallest in situ displacement and residual depth, and highest elastic recovery.

 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).

 The critical load before film fracture for CrO while CrN x (with 2 nm Cr underlayer) and CN x critical loads.

x is the lowest, have similar  Analysis of wear tracks following nanoscratch experiments show that

Hardness and reduced modulus for CrN CN x . The hardness of CrN x x and CrO x is greater than CrO x films in comparison with and comparable to CN x

Conclusions

 The hardness and adhesion of CrN x is comparable to CN x .  With a very thin Cr underlayer, CrN x appears to have the potential for magnetic recording applications

Nanotribology of Ultra-thin Wear and Oxidation Resistant Hard Coatings M.L. Weaver & J.A. Barnard, The University of Alabama Tuscaloosa Award #DAAD 19-99-1-0152

1.

2.

3.

4.

5.

6.

7.

Year Three Publications

G. Wei, A. Rar, and J.A. Barnard, “Composition, structure, and nanomechanical properties of DC sputtered CrN x (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 TiB

Research

v. 16, n. 4, pages 945-954 (2001). 2 Protective Films,”

Journal of Materials

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, Cr 2 N 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 TiB 2 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).