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, 2004, 46, . 4 Nanodiamonds in magnetic recording system technologies V.I. Kurmachev, Y.V. Timoshkov, T.I. Orehovskaja, V.Y. Timoshkov PLASMOTEG Engineering Center, National Academy of Science Belarus, Minsk, Belarus Minsk Institute of Management, Minsk, Belarus Belarus State University of Informatics and Radioelectronics, Minsk, Belarus E-mail: pec@mserv.bas-net.by The features of codeposition process of Ni, NiFe, CoP, CoFeP, NiP, Ni, and Cu matrixes with ultra dispersed diamond particles from sulphate, chloride, acetate, glycine, citrate, sulphamate bath were investigated in view of magnetic recording systems application. The cation and anion surfactants were used to study the liophobic colloid systems and the behavior of ultra fine particles, to prevent agglomeration and sedimentation and to control the particle incorporation into metal matrix. Mechanical properties of composite films were described from the point of view of applications in high-tech devices. It was determined that wear resistance increased 22.5 times, microhardness increased 2 times, coefficient of friction and corrosion current were reduced by factor 1.5 and 1.6 respectively.

Application of nanodiamond particles in technology of composite functional layers of hard disks, magnetic heads, micromotors, and micromechanical components allows increasing reliability of storage information systems considerably.

1. Introduction and hard magnetic films, as well as composite conducting films with inclusion of nanodiamond particles.

The problems of static and dynamic friction, as well as wear of functional layers are fundamental for high-tech devices and magnetic recording systems, in particular for 2. Experimental details magnetic hard disks, heads, micromotors, and micromechanical systems of positioning. It is connected with the The electrodeposition of composite coatings containing rising requirements of the record density (longitudinal and ultra-fine diamond particles was performed from sulfate, track), the decrease of the bit dimensions and minimization glycine, acetic, and Watts bathes. Soft magnetic (NiFe, of displacements and errors [1]. CoFeP, CoP) and hard magnetic (CoNiP, CoW, CoP) alloys as well as conductive matrix of Cu and Ni were One of the approaches to solve the problem of friction investigated. The influence of the ampholyte (pyridine carand wear of mechanically moving elements of micron dimensions for high-tech devices is the use of the com- boxylic acid, aminoacetic acid), cation-active (monoethanol posite materials, in particular, codeposited metal and alloy amide), anion-active (CTAC, SLS) was estimated. Concentration of ultra-fine diamond particles was varied from with inert hard particles by electroless or electrochemical to 20 g dm-3 (dry substance). Nanodiamond particles processes [25]. The methods of electrochemical synthesis of magnetic films for magnetic recording systems are well were obtaines by a detonation process [8].

known since sixties [6]. From the period, this method was The amount of codeposited ultrafine diamond particles considered to be a competitive with vacuum methods of film was determined both by integral Couloumbmetric analysis deposition. The same situation took place in the mentioned with the express analyser AH-7529 (USSR) and by local period related to hybrid and silicon ICs. However finally, it Auger spectroscopy (PHI-660 Perkin Elmer Corp., USA).

became obvious for both cases that the every method should The Vickers microhardness of coating was measured at be considered in viewpoint of concrete application field. At a load of 0.5N with MICROMET-II (Buehler-Met, CH). The present, the electrochemical methods due to their obvious structure of the deposits was examined by TEM (EM-125, advantages allow for fabrication of magnetic films not only USSR). The coefficient of friction and the wear were in the traditional fields of application (magnetic tapes, disks, evaluated by a FRETTING II test machine (KU Leuven, heads). One of the most exciting applications of the BE) [9]. A ball-on-flat geometry was used for the low electrodeposition methods is the MicroElectroMechanical amplitude oscillating sliding wear tests. Corundum balls system that has a tremendous potential in future. Now, of 10 nm diameter (Ceratec, N1) were loaded on top of the micromotors are already created and used in storage the vibrating surface of samples with constant normal force devices for precise positioning magnetic heads. Indeed, the of 2N. Displacement strokes of 100 and 500 m were used peculiarities of the electrodeposition process make possible at frequencies of 8 and 2 Hz respectively. The tests were fabrication of high-quality 3D structures (LIGA Process) [7]. performed in ambient air at 20C and a relative humidity The aim of the present work is to carry out integrated of 50%. Wear volumes were estimated by the RM600 laser researches on technology and application of composite soft profilometry (Rodenstok, D) after 100 000 fretting cycles.

680 V.I. Kurmachev, Y.V. Timoshkov, T.I. Orehovskaja, V.Y. Timoshkov Figure 1. AES sputter-etch elemental profiles (a) and local X-ray analysis (b) of pure and composite Ni coatings containing nanodiamond particles.

3. Results and discussion dislocation aggregates inside the grains, and a concentration of solitary dislocations and dislocation walls of 20 nm thick along the grain boundaries. The average grain size is In general, during the electrolytic codeposition, the about 500 nm. As for composite coatings the grain size suspended diamond particles interact with the surface of the growing film due to hydrodynamic, molecular and reduces up to 30-100 nm. An accumulation of ball-type electrostatic forces [9]. This complex process results in the dislocations along the grain boundaries takes place. Thus, formation of composite coatings. Auger profiles and local for the first time it was determined that during codeposition X-ray analysis demonstrate that ultra-fine diamond particles of matrix and nanodiamond particles, nanocrystalline Ni are effectively incorporated into the meal matrix (Fig. 1).

electrodeposites were formed. The electron diffraction patterns of the coating confirm the presence of cubic carbon Visually the composite coatings have grey apppearance.

Pitting was not noticed on the surface. A structural inves- according to ASTM 6-675, indicative of the incorporation of tigation shows (Fig. 2) that pure Ni coatings contain twins, diamond particles into the nickel matrix.

, 2004, 46, . Nanodiamonds in magnetic recording system technologies Figure 2. TEM micrographs of pure and nanocrystalline composite Ni coatings [5].

Based on the experimental data [5], the qualitative vails at intermediate distences. The primary minimum codeposition model of the composite coatings with the 1 corresponds to direct sticking of particle. In the case, ultra-fine particles is proposed. The peculiarities of the the particles are irreversibly stuck (coalescence). The ultra-fine particles behaviour are considered in the model.

secondary minimum 2 corresponds to attraction through The model worked out is based on the assumption the interlayer of invironment. It this case, the aggregates may be codeposition of ultra-fine particles proceeds through the counteracted relatively easily. The maximum corresponding following stages (Fig. 3):

to intermediate distances characterizes the potential barrier, 1) coagulation of ultra-fine particles in plating solution;

which prevents sticking the particles. Forces of interaction 2) formation of quasi-stable aggragates and therefore are extended for hundreds of nanometers.

change of system dispersion constitution;

In the consideration of interaction between the particles, 3) transport of the aggregates to the cathode surface by the following conclusions from the DLVO theory should be convection, migration and diffusion;

mentioned as well. The height of the energy maximum and 4) disintegration of the aggregates in the near-cathode the depth of the primary and secondary maxima depend surface;

on the parameters of the systems, namely the zeta potential, 5) weak adsorption of ultra-fine particles and aggregate fragments onto the cathode surface;

6) strong adsorption of spersion fraction (embedment).

Hydrophobic colloidal systems are thermodynamically unstable due to the surplus of the surface energy. They exist owing to stabilization by protective ionic and molecular layers. In general, in the bulk of suspension the particles encounter one another due to the Brownian movement, gravity and convection. The forces between them determine whether the encounters result in sticking the particles or the particles remain free.

Behaviour of dispersed systems is described by the DLVO theory. Stability or coagulation rate of suspensions depends on sign and magnitude of the overall potential energy of interaction between the particles. Positive electrostatic repulsion energy Ur (h) decreases by exponential law, whereas negative molecular attraction energy Ua(h) is in inverse proportion to squared distance. As a result, at small distances (h 0, Ur(h) const, Ua(h) -) and large distances (exponent diminishes much rapidly than power function), the attraction energy between the Figure 3. Mechanism of coodeposion of metal matrix and particles prevails. The electrostatic repulsion energy pre- nanodiamond particles.

, 2004, 46, . 682 V.I. Kurmachev, Y.V. Timoshkov, T.I. Orehovskaja, V.Y. Timoshkov particle size, electrolyte concentration (and valence) and the the result of the rotational motion. The transverse particle Hamaker constant. At low electrolyte concentrations, the migration results from pressure drop on the side where energy maximum may reach high values and this prevents the sum of the tangential velocity components of flowing the particle aggregation. At increase of the electrolyte past and rotating the particle reaches the maximum. The concentration, the height of the energy maximum decreases transverse particle migration is directed always toward this and disappeares at a critical concentration (which depends maximum. In the case being considered particle moves on valence of electrolyte). The coagulation becomes more away from the cathode surface.

rapid. Thus, to enhance the suspension stability, one needs When the particle is trapped by cathode, the longitudinal to reduce the electrolyte concentration and increase the zeta force by flow the plating solution affects the particle. If this potential.

force exceeds the friction force keeping the particle onto the The transport of the particles toward the cathode surface cathode surface, the particle is detached from the growing occurs by convection, migration, diffusion, and the Browdeposit.

nian movement.

Besides the forces connected to interaction between the Migration is the movement of cations, anions or charged particle and hydrodynamic flow, the gravity and buoyancy particles through the solution under influence of the applied contribute to the particle motion. The sum of forces by potential between the electrodes in the solution. Diffusion gravity and buoyancy results in the sedimentation force.

is the second process. Electrode reaction deplete the concentration of oxidant or reluctant at en electrode surface and produces a concentration gradient there. This gives the rise to the movement species from the higher to the lower concentration. Unlike migration, which only occurs for charged particles, diffusion occurs for both charged and uncharged particles. Convection includes thermal and stirring effects, which can arise extraneously through vibration, shock and other types of stirring and temperature gradients. At last, the Brownian movement, as known, is affected strongly by the particle size, and may be neglected in the case of the particles size is above 1 m.

The stirring mode is required to be the transition mode between the turbulent flow and the laminar flow in the bulk of plating bath. Such a mode is of most practical significance. It is because the laminar flow does not provide the sufficient stirring of the electrolyte suspension. Alternatively, at the stirring rate corresponding to the turbulent mode, the conditions preventing totally the particles embedment appear.

In the near-cathode region, the aggregate is influenced by the forces of different nature and direction. Motion of the aggregate is determined by resultant force, and integrity of aggregate by sum of forces values as well. For the investigated system, the following forces are considered:

1) mechanical forces, associated with interaction with the fluid flow and other particles, gravity and buoyancy;

2) electrical forces, connected with the electric field that presents in the plating solution during electrodeposition process;

3) molecular forces acting on the particle in the vicinity of the cathode surface.

Mechanical forces. At the laminar liquid flow in the boundary hydrodynamic layer the law of viscous friction is followed in this region, and the boundary conditions are following: V = 0 at y = 0 and V = V0 at y =.

If the particle moves in the flow having the transverse velocity gradient of liquid movement, the rotation motion can be imparted to the particle because the different Figure 4. Physico-mechanical properties of the composite velocities of flow past a particle from the top and from coatings: a coefficient of friction, b wear volume, c SEM the bottom. The transverse particle migration appears as images of wear scars after 100 000 fretting cycles.

, 2004, 46, . Nanodiamonds in magnetic recording system technologies Figure 5. Some applications of nano-diamonds in magnetic recording systems: a thin film magnetic heads; b hard disk;

c, d MEMS elements [11].

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