ICMCTF2003 Session E2: Tribological Coatings in Severe Environments

Tuesday, April 29, 2003 10:30 AM in Room Royal Palm 1-3

Tuesday Morning

Time Period TuM Sessions | Abstract Timeline | Topic E Sessions | Time Periods | Topics | ICMCTF2003 Schedule

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10:30 AM E2-7 Tribology of Head/Disk Interface
F.E. Talke (Universityof California, San Diego)

During the last decade, the recording density in magnetic disk drives has continued to increase at an astonishing pace. At present, recording densities on the order of 50 Gbits/inch2 are shipped in state-of-the-art disk drives, and densities on the order of 100 Gbits/inch2 are anticipated to appear in commercially available products within one to two years. Recording densities of 100 Gbit/inch2 correspond to track densities of 150k to 200k tracks per inch (6k to 8k tracks/mm) and linear densities of 500k to 600k bits per inch (20k to 30k bits/mm). Thus, in terms of the size of a single bit, areal densities of 100 Gbit/inch2 correspond to a bit dimension on the order of 125 nm in width and 50 nm in length.

One of the key factors in achieving recording densities on the order of 100 Gbits/inch2 is related to the fact that magnetic recording heads can be designed to "fly" over the disk surface without touching the disk. In order to obtain maximum signal resolution, it is necessary that magnetic recording heads are designed to fly over the disk surface at the smallest possible spacing without causing wear and signal degradation. In fact, from a magnetics point of view, the best head/disk interface is achieved if sliding contact between the slider and the disk were to exist at all times. However, continuous contact between slider and disk causes wear, resulting in signal degradation and failure of the disk drive. Thus, a compromise between signal resolution and flying height must be made, i. e., the head/disk interface must be designed so that the slider flies over the disk at a spacing that is large enough to prevent detrimental contacts between slider and disk, but at the same time is small enough to allow reading and writing at high resolution. Presently, flying heights in commercially available hard drives are on the order of 12 nm and flying heights on the order of 3 nm must be realized in order to achieve areal densities of 500 Gbit/inch2.

In this presentation, tribology problems of the head/disk interface are discussed. The degradation of the head/disk interface is examined, and the dependence of friction and wear on slider and disk parameters is investigated. The effect of lubricant in reducing friction and wear of the head/disk interface is analyzed, and the dependence of lubricant loss on disk and slider parameters is investigated.

A reduction in head/disk spacing requires also that the carbon layer on the slider and disk surface be decreased to the order of 2 to 3 nm. Since films of such small thickness can not be easily measured and characterized, it is apparent that the tribological challenges encountered in the magnetic recording industry will continue to increase. New and improved instrumentation for measuring flying heights and lubricant thickness will be required together with an improved understanding of lubricant/materials interactions and lubricant chemistry. Furthermore, with film thickness on the order of several atomic layers, corrosion will become an even more important area.

11:10 AM E2-9 Deposition of Hard Coatings on Inner Surfaces of Tubes using Magnetic Field Enhanced Plasma
R. Wei (Southwest Research Institute)
Deposition of hard coatings onto inner diameter (ID) of tubes with high aspect ratios is always a challenging task. Previously, some techniques have been attempted. One of them is an ion implantation-sputtering process. In this process a high-energy ion beam is collimated into the tube, where a metal rod with a 90° cone at the end is located in the center. When the ion beam strikes the metal surface, which has a 45° angle with respect to the ion beam as well as the ID of the tube, the metal is sputtered away from the cone and deposited on to the ID. Another method is a plasma immersion ion process. In this process, an auxiliary electrode (a wire or a rod) is inserted into the center of the tube. With the proper electrical connections between the electrode and the tube, ions can be generated and directed to the tube, thereby resulting in the formation of a coating on the ID. In this paper, we will present a novel plasma enhanced chemical vapor deposition (PECVD) method in which a magnetic field is employed to enhance the plasma production inside a long tube. The magnetic field also helps sustain glow discharge inside the tube. By selecting proper precursors, hard coatings such as diamondlike carbon and silicon carbide can be deposited on to the ID of tubes. Using this method we have deposited DLC films on the inner walls of tubes with aspect ratio up to 20 (1.26 cm in diameter x 15 cm, and 3 cm in diameter x 61 cm long). In this paper, we will also discuss the design and the construction of the set-up, properties of the plasma and characteristics of the coatings obtained. Finally, we will present some practical applications.
11:30 AM E2-10 Coating Toughness Determined by Vickers Indentation: an Important Parameter in Cavitation Erosion Resistance of WC-Co Thermally Sprayed Coatings
M.M. Lima, C. Godoy, P.J. Modenesi (Universidade Federal de Minas Gerais, Brazil); J.C. Avelar Batista (University of Hull, United Kingdom); A. Matthews (The University of Sheffield, United Kingdom)
The deficient performance of thermally sprayed coatings in cavitation erosion tests is often attributed to the nature of their lamellar microstructure. Poor coating/substrate adhesion, low toughness and tensile residual stresses, which are introduced during the deposition process, can also adversely affect their cavitation resistance. In order to improve the cavitation performance of such coatings, it is also important to control some of the coating properties such as elastic modulus (E) and hardness (H). By reducing E and increasing H (i.e., to ensure a higher H/E ratio), a better tribological performance is usually achieved. In this work, an attempt to improve the cavitation resistance of WC-Co coatings was made by either modifying coating composition (and therefore modifying some coating properties such as hardness, elastic modulus and toughness) or by carrying out a 'melt' post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating. Four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC-12Co; (ii) as-sprayed 50%(WC-12Co) + 50%(NiCr), (iii) post-melted 50%(WC-12Co) + 50%(NiCr) and (iv) a duplex system comprising a WC-12Co top layer and a NiCrAl interlayer. The 'post-melted' coating produced from the pre-alloyed powder 50%(WC-12Co) + 50%(NiCr) displayed a higher elastic modulus (measured by Knoop indentation) and a lower hardness (and thus a lower H/E ratio) than the WC-12Co. Also, the fracture toughness of the latter (measured by Vickers indentation tests) was increased from 1.6 ± 0.9 MPa m1/2 to 32 ±12 MPa m1/2. The worst performance in cavitation erosion tests was achieved by the WC-12Co coating, which showed the highest mass loss throughout the test. Conversely, the 'post-melted' 50%(WC-12Co) + 50%(NiCr) coating exhibited the best cavitation resistance and a correlation between coating toughness and cavitation resistance could be established.
11:50 AM E2-11 Spin Coated Tin Oxide Gas Sensors by Sol-gel Process for the Detection of Alcohol
C. Ghanshyam, S. Mishra, N. Ram, R.P. Bajpai (Microelectronics Group, India); R.K. Bedi (Guru Nank Dev University, India)
In this paper, the surface morphology and sensing characteristics of tin oxide thin films prepared by spin coating using sol-gel technique are reported. The morphology of the films was studied by scanning electron microscopy. The structural analysis of the films is made using X-ray diffractometer. The sensing characteristics were studied and it was found that the sensitivity towards alcohol vapour was maximum at 673K.At this temperature the response time was found to be 30sec and it was able to detect the ethanol vapour at a level of 100 ppm. The results obtained by sol-gel technique are promising for the preparation of sensitive and low cost ethanol sensors.
12:10 PM E2-12 Mechanical Properties of Oxide Ceramic - Diamonds Composite Coatings
M.V. Kireitseu, I.A. Nemerenco, L.V. Yerakhavets (Institute of Machine Reliability, Belarus)
The use of Hertzian indentation with rheological models as a method for determining the fracture of any multi-layered brittle coating is described. The advantage of the method revealed here is that the only quantity to be measured is a deformation or a fracture load. Use of a refined stress intensity factor formulation for surface-breaking cracks in steep stress gradients has enabled accurate estimates to be made of the minimum loads necessary to propagate cracks by Hertzian indentation. At present the analysis can be applied to sphere/substrate systems with any value of Poisson's ratio. By measuring this minimum load (and that is the only quantity that must be measured) an accurate estimate of fracture may easily be made with application of rheological modeling. The only other requirement is that the surface should be coarsely polished and a sphere of relatively small radius (<5mm) should be used. The existence of an absolute minimum fracture load, for a given sphere size, suggests that the Hertzian indentation test could find use as a localized proof-test.


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