Wear corrosion properties of DLC films with different C 2 H 2 gas flows and CN layer of magnetic recording disksby C. P. Chen, A. H. Tan

Surface Engineering

Text

Wear corrosion properties of DLC films with different C2H2 gas flows and CN layer of magnetic recording disks

C. P. Chen and A. H. Tan*

The wear corrosion properties of magnetic recording disks which correlate to hardness, lubricant bonded ratio and friction coefficient of ultra thin diamond like carbon films deposited with ion beam hydrogenated carbon and sputtering nitrogenated carbon respectively when, deposited on

NiP/Al substrates were investigated. Results show that the ion beam deposition hydrogenated carbon (IBD–CH) films prepared at lower C2H2 gas flowrates or 1 nm sputter nitrogenated carbon (CN) deposited on 2 nm IBD–CH layers, exhibit a higher Raman intensity ratio Id/Ig and G peak frequency. The hardness of diamond like carbon films increases with decreasing G peak frequency and intensity ratio Id/Ig. The lubricant bonded ratio is dramatically increased from 12 to 38% when a 1?0 nm CN is deposited on a 2?0 nm IBD–CH layer. IBD–CH with 35 sccm C2H2 gas flowrate which shows the lowest weight loss, the lowest friction coefficient and the best wear corrosion resistance because of the optimised lubricant bonded ratio and the highest hardness.

Keywords: Wear corrosion, Diamond like carbon, C2H2 gas flow, CN, Hard disk

Introduction

Wear resistant and corrosion resistant diamond like carbon (DLC) coatings are frequently used in magnetic recording disk technology to improve disk quality and life.1,2 They allow disk products to survive in adverse environments where they are subjected to high wear or corrosion. The magnetic recording disk properties, such as disk read/write, durability and reliability performance, are strongly related to the disk surface protective overcoat. Diamond like carbon overcoats of a sputtered amorphous hydrogenated carbon (a-C:H) overcoat3 has long served the disk industry as the protective overcoat for the underlying magnetic layer from the mid-1990s but this has recently been replaced by ion beam deposition hydrogenated carbon (IBD–CH).4 Sputtering nitrogenated carbon thin films (CNx) have been extensively studied in recent years.5,6 Adding an appropriate amount of nitrogen during the DLC deposition process increased the lubrication effect and the wear resistance of the thin film.7 The incorporated N2 could lead to formation of

C5N and C;N bonds, which at the same time creates unsaturated spins and mid-gap states resulting in a substantial decrease of electrical resistance of the film.8

Compared with carbon overcoats, carbon nitride has a different surface polarity, which results in a dramatically different lubricant overcoat interface and this in turn influences the amount of bonded lubricant.9 Ion beam deposition can produce very hard, smooth and highly dense amorphous carbon films compared with usual deposition methods, i.e. sputtering.10,11 Further increases in the magnetic storage density of future head disk drives will necessitate a reduction in the thickness of protective overcoat. Corrosion resistance and durability become very challenged when carbon overcoat is lowered to an ultra thin thickness (,5 nm).12 Developing a thinner overcoat with better corrosion and wear resistance is therefore a necessity to meet the challenges of current and future technology.

Most contributors when focusing on overcoat relate to the corrosion or wear performance13,14 separately.

Dorner-Reisela et al.15 studied the characterisation of nitrogen modified DLC films by changing the C2H2 flow ratio in radio frequency plasma enhanced chemical vapour deposition. Liao et al.16 investigated the effect of

H2/C2H2 ratio on the structure and tribological properties of carbon thin films prepared by plasma based ion implantation. It has not yet been explored whether enhanced wear corrosion resistance can be achieved through overcoat properties in the IBD process. The

DLC film has a widespread potential use in industry, and by adjusting the C2H2 gas flow in the deposition process one can affect the microstructure and various characteristics of the thin film, such as mechanical and wear characteristics, but knowledge of this effect on the wear corrosion characteristics of media remains lacking.

Therefore, this study is to investigate the wear corrosion which is then correlated to the mechanical properties and lubricant bonded ratio of single layer IBD–CH films and dual layers of sputtered nitrogenated carbon (CN) deposited on IBD–CH film surfaces. The effect of C2H2 gas flow on mechanical properties and wear corrosion

Ching Yun University, 229 Chien Hsin Road, Jungli 320, Taiwan *Corresponding author, email ahtan@cyu.edu.tw  2009 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 3 September 2008; accepted 19 September 2008 496 Surface Engineering 2009 VOL 25 NO 7 DOI 10.1179/026708408X370212 properties of IBD–CH film were further investigated.

This can be expected to be utilised as the next generation deposition technique for an ultra thin disk overcoat as this technology evolves.

Experimental

A NiP layer was deposited electrolessly on a commercial grade 5088 Al alloy substrate. Standard polish processes on a NiP layer with a thickness of 10 mm on aluminium substrates are used in this paper. Atomic force microscopy roughness Ra of the polished NiP/Al substrates surface was measure to be 0?3 nm, as shown in Fig. 1a.

All substrates were sequentially deposited with various carbon overcoat types and thicknesses using an Intevac 250B disk dc sputtering system. Diamond like carbon overcoats of CH and CN films were deposited by IBD and sputter respectively. In IBD–CH deposition, the process settings are acetylene (C2H2) gas flowrate of 25 and 35 sccm, 0?5 A emission current, 2150 V (bias voltage), which obtained 2?0 to 3?0 nm IBD–CH thicknesses using different deposition times. The correlation between IBD–CH film thickness and deposition during various times at 25 and 35 sccm gas flowrate are shown in Fig. 2. In the sputtered CN layer, A 30 wt%N2 content in the CN film was controlled by adjusting the ratio of Ar and N2 mixture gases. The DLC structures were monitored using a Raman spectrum, taken at 514 nm wavelength. The sp3 content of the three samples was determined by electron energy loss spectroscopy (EELS). EELS measurements were carried out on a scanning transmission electron microscope with a dedicated parallel EELS spectrometer of the