Curious stress reduction with W incorporation of WC-C nanocomposite films by hybrid ion beam deposition A. Y. Wang a), H. S. Ahn a), K. R. Lee a), J. P. Ahn b) a) Future Technology Research Division, b) Nano-materials Research Center, Korea Institute of Science and Technology. KVS

Heart valve Watch bracelet Hard disk MEMS package Spur gear Properties of DLC hardness up to 4000 HV deposition temperature ca. 100 °C temperature stability < 350 °C tribology low friction and wear stable against acids and alkalines biocompatible, haemocompatible Hip joints Introduction

Key obstacle to applications  Urgent dillemma: how to reduce stress without degrading mechanical properties ?  High residual stress, poor thermal stability and adhesion to the steel substrate. M.W.Moon, Acta Mater., 50 (2002) 1219.

Concept of Me-DLC nanocomposite Refractory phases: nc-TiN, TiC, WC, … Primary phases: a-C, C:H, Si 3 N 4 … 2~10 nm   ~2nm  Hardness: no grain boundary sliding, no dislocations activity, strong interface, strong segregation (immiscibility).  Profound effects on the stress, thermal stability, friction coefficient et al !!

Previous works C. S. Lee et al., Diamond Rel. Mater. 11 (2002) 198. B. Shi et al, Appl. Phy. Lett. 81(2002)352. B. Feng, Thin Solid Films, (2001)210 Si atom substitute the carbon atom of sp 3 site. Percolation structure transition from a-C:H like to TiC like occurs.

Previous works P. Zhang et al, J. Vac. Sci. Technol. A 20(2002)1390.  Increase in sp 2 site and fraction release stress.  Due to the absence of hard carbides, soft Al incorporation has a remarkable stress reduction while Ti and Si are inferior.  Decrease of mechanical properties in cases of Ti and Si.

Motivations  Synthesis of WC-C nanocomposite films by hybrid ion beam deposition.  Characterization of structure and properties of WC-C films.  To examine the role of W on stress reduction.  To note the stress reduction origin.  To bring forward the generic origin of stress reduction with W incorporation regardless of deposition process.

Synthesis process  Working gas: Ar, C 6 H 6 (total: 12sccm)  Base pressure : 2.0  Torr  Deposition Pressure : 0.6 ~ 1  Torr  Power density of target: 4.2~7.3 W/cm 2  RF Bias : V  Thickness: 350±50nm  Sputter gun : Third elements addition to DLC (W, Ti, Si …);  Ion gun : Easy controlling the ion bombardment energy with high flux. W n+, Ar H +, C m+

W composition C Si in substrate Ar W

Mechanical properties ±3 GPa 170±15 GPa

Atomic bond structure ?? ** ** The undegrading mechanical properties is related with the unvaired 3-D interlink of atomic bond structure.

Role of W on bond structure ??  Region 1 : W essentially act as the relaxation site of nearby carbon network, which is now proposed a generic origin in other W-DLC films. S. J. Park. Diamond Relat. Mater. 11(2002) 1747

(c) (d)(e)  -W 2 C (101) 2 nm2 nm 2 nm2 nm 2 nm2 nm 4 nm4 nm 4 nm4 nm 4 nm4 nm 4 nm4 nm 4 nm4 nm (e) 4 nm  -W 2 C (102)  -W 2 C (101) 4 nm (c)  -W 2 C (101) 4 nm (b) (a) 4 nm TEM-microstructure changes ?? 4 nm (d)  -W 2 C (101)  -W 2 C (102)  Region 1 : Amorphous carbides formed  Region 2 : crystalline carbides begun emerging.  Region 3 : crystalline carbides phase presented. (a) 3.0, (b) 4.2, (c) 5.1, (d) 8.7, (d) 12.5 at.% 12 3

GIXRD-phase identification ?? amorphous structure weak crystallinity high crysallinity

 Region 2: the emergence of crystalline carbides enhanced the distortion of local carbon bond network, which caused the temporary increase in stress.  Region 3: the increase in fraction, grain size of the crystalline carbides relief the stress again. 2 3 Stress reduction origins in region 2, 3 The random amorphous network are generally softer than the equivalent crystal one in spite of containing the same atomic bonds. J. Robertson, Mater. Sci. Eng. R 37 (2002) 129.

Conclusions  WC-C nanocomposite films prepared by hybrid ion beam deposition.  The significant stress reduction by W incorporation.  The stress is reduced by 50% at 4.2 at.% W, while hardness is only reduced by 20%.  W essentially acts as the relaxation site of surrounding carbon network via forming more flexible C-W bond, proposed by a generic origin of stress reduction with W incorporation.  Evolution of crystalline carbides plays a subsidiary role on the stress in case of the larger W concentration.  W concentration is varied by the Ar fraction in gas mixture.

Thanks for attention !!! Questions ??