Junguo Xu

Formation of tribochemical layer of ceramics sliding in water and its role for low friction

Abstract The formation of a tribochemical layer on silicon nitride sliding in water is studied. The wear mode changes from mechanically dominated wear to tribochemically dominated wear as sliding distance increases. The silica tribochemical layer is formed on the friction surface and reduces the friction. Introducing an additive that accelerates the formation of colloidal silica makes the running-in distance shorter. The activation energy for tribochemical reaction is estimated to be as small as 1/6–1/8 of that for static reaction.

Flying-height reduction of magnetic head slider due to thermal protrusion

Both the heat generated by the current in the write coil and the rise in the surrounding temperature cause local thermal protrusion (TPR) on magnetic-head elements. Such protrusion reduces the flying height of the head slider below the design value, thus reducing the safety margin for head-disk interference. To analyze this problem, we numerically simulated the heat transfer in the head slider, the thermal deformation of the head, and the flying-height change of the slider resulting from the deformation. The parameter study shows that decreasing the thickness of the alumina base coat or increasing the size of the pole and shields can reduce the magnitude of write-current-induced thermal protrusion (W-TPR). On the other hand, a longer pole and shields increase ambient-temperature-induced protrusion (T-TPR). For W-TPR, the reduced flying height is partly compensated by increased air pressure on the air-bearing surface (ABS). However, almost the entire magnitude of T-PTR translates into flying-height reduction.

Spatial and temporal profiling of protrusion in magnetic recording heads

An increase in external temperature or internal heating by writing can induce protrusion or strain of thin film layers in a magnetic recording head. To profile the protrusion, the surface topography and the temperature distribution are imaged in fixed heads by scanning probe and optical microscopy. Near the poles and the shields, the protrusion is at a peak, producing a risk of head-disk interaction. The dynamics are elucidated in flying heads over a disk by measuring the amplitude of a reference pattern with the read head. The temporal profiling technique can be applied both in component testing on spinstands and reliability testing in fully integrated disk drives. For either heating or cooling of the heads, the characteristic time constant for protrusion is 100 /spl mu/s and reaches 95% of the plateau after 1 ms. The protrusion is linear with temperature and is reversible, implying that the strains are within the elastic limit. Furthermore, variation of the write pulse shape suggests that Joule heating is more significant than eddy current or hysteresis losses in inducing protrusion.

Flying-height adjustment of a magnetic head slider with a piezoelectric micro-actuator

In this paper, flying-height adjustment of a magnetic head slider with a piezoelectric micro-actuator is developed.

Scratch–wear resistance of nanoscale super thin carbon nitride overcoat evaluated by AFM with a diamond tip

Abstract Atomic force microscopy (AFM) was used to investigate the scratch–wear resistance of ultrathin superhard carbon nitride overcoats of thickness 1, 2, 4, 6, 8 and 10 nm. When sliding against a diamond tip of radius less than 100 nm in the mode of line scratch, the thin overcoats of thickness 1–4 nm exhibited poor wear resistance, especially at contact pressures larger than 25 GPa, with a wear depth of 4 nm or larger and a specific wear rate up to 0.8×10 −4 mm 3 /nm. Non-contact mode imaging of a scratched surface has shown that a large amount of nanoscale wear debris was formed along the two sides of the scratched grooves, which indicated that the material removal mechanism of such thin overcoats was due to brittle fracture and abrasive wear, both in the nanoscale. In comparison, the overcoats of thickness 6–10 nm exhibited wear resistance with a specific wear rate less than 0.2×10 −4 mm 3 /nm. Instead, the least debris was observed on the scratched surfaces and only shallow grooves were left after scratching. It means that the grooves were formed by both plough and plastic deformation. The micro/nanowear mechanism and thickness effect of coating on scratch resistance were discussed.

Nano-Scale Defect Mapping on a Magnetic Disk Surface Using a Contact Sensor

Targeting both higher touchdown sensitivity and highly accurate nanometer-scale defect detection on a disk surface, a thermal-contact sensor, integrated into a magnetic-head slider, was developed. It was experimentally shown that the contact sensor has sensitivity for detecting head-disk contact at each radial position on a disk surface equivalent to that of a conventional acoustic-emission (AE) sensor. It was also shown that a defect-detection method using the thermal-contact sensor is feasible. Defect mapping, in which a slider inspecting the disk at a certain clearance detects small defects on the disk surface, done with this method was better sensitive with measurements with an optical surface analyzer (OSA). Defect mapping on a proto-type glide tester, based on a conventional glide tester, using the thermal-contact sensor was also demonstrated.

Performance of Sliders Flying Over Discrete-Track Media

A simulation method in which grooves are virtually distributed on the slider air bearing instead of on the grooved medium surface has been developed and used to investigate the performance of sliders flying over the surface of a discrete-track medium. The simulated flying height loss due to a discrete-track medium coincides well with the measured data, whereas the average-estimation method overestimates flying height loss. Among the characteristics of a slider flying over the surface of a discrete-track medium that were studied are the flying attitude, the effect of groove parameters on flying profile, and the flying height losses due to manufacturing variation and altitude. The results indicate that when a slider is flying over the surface of a discrete-track medium, it will have a higher 3σ of flying height, be more sensitive to altitude, and will have a greater flying height loss.

Ultra-low-flying-height design from the viewpoint of contact vibration

Abstract The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.

An active-head slider with a piezoelectric actuator for controlling flying height

The current design of magnetic-disk head sliders needs an extra margin of flying height for manufacturing tolerance or environmental variation. To reduce this margin, we have developed active-head sliders. These sliders carry an unimorph piezoelectric microactuator, and their flying height can be controlled. After simulating the piezoelectric deflection of the actuator and flying characteristics of air-bearing surfaces, we designed two types of active-head sliders (five-pad type and four-pad type) and fabricated them by silicon microelectromechanical system (MEMS) processing. In tests, the sliders flew over a glass disk, and the observed stroke of the actuator (30 nm/5 V) was considered large enough to control the flying height. The large area at the rear end of the four-pad-type slider structure is sufficient to carry conventional read/write heads. Moreover, the slider fabrication process will not destroy the heads. We, thus, conclude that the active-head sliders can be applied for practical use easily, and their flying height will be significantly lower than that of conventional disk head sliders.

Contact/Clearance Sensor for HDI Subnanometer Regime

The fundamental performance of contact/clearance sensor, namely embedded contact sensor (ECS), is addressed in this paper. Both simulation and experiment results revealed that ECS is a promising sensor for low clearance and high reliability at subnanometer regime. The ECS dc signal intrinsically comes from multiple sources including TFC heater, air-bearing surface cooling, and friction heating at head/disk contact. Both ECS dc and ac signals detect head/disk contact. The dc signal comes from the sensor resistance change due to friction heating at contact, but the ac signal is dominated by spacing modulation caused by air-bearing vibration, and partially from the pulse-like friction heating. ECS ac signal responds significantly to disk microwaviness at narrow clearance region. Furthermore, ECS could detect asperities, pit, and lube mogul. The mechanism for asperity detection is friction heating. The mechanism for pit detection is worse cooling when sensor flying over the pit. That for mogul detection is better cooling at narrower spacing when the sensor is flying over the mogul.