Study Of Stability Of Lubricant Layer And Gas Bearing In Large Capacity Hard Disk Drives Based On MD And DSMC Method

With the increasing requirement for digital media contents,high-density magnetic recording technology is in demand.To make the data storage density up to 2 Tbits/in2,the head of hard disk has to fly at a nano-hight which is below 3 nm.Because of such a low height,the stability of flight height(FH)of head is more and more important to ensure the performance of hard disk drive(HDD).The stability of FH is determined by the gas bearing in the head-disk interface(HDI).On the other hand,such a low FH can make the wear between head and disk more notable.Thus the study of stability of lubricant layer on the disk that prevents the disk from wear is more and more important.While the lubricant molecule can desorb from the lubricant layer due to the decomposition an the evaporation which can deteriorate the stability of gas bearing in the HDI and then impact the reading and writing performance of HDD.And in the heat assistant magnetic recording(HAMR)HDD,the laser heating will increase the amount of desorbed lubricant molecules.Due to the low FH,the desorbed lubricant molecules may adhered to the bottom surface of head and form the lubricant bridge connecting the head to the disk which can still further deteriorate the stability of gas bearing.To improve the digital density of HDD,it is important to study the stability of gas bearing in HDI and lubricant layer on the disk.Up to date many studies have been done on the field of lubricant layer on the disk and the gas bearing in the HDI.But studies on stability of lubricant layer heated by HAMR laser have not been found.Formation and development of lubricant bridge and the influence of lubricant bridge on the gas bearing is also not found in previous studies.Furthermore,fewer researchers studied the stability of gas bearing pressure in helium filling HDDs and bit-pattern media(BPM).Aiming at the above problems,this paper respectively studies the stability of the lubricant layer on disk and the gas bearing in HDI of large capacity HDDs.A full-atom model for the three-dimensional slider-lubricants-disk substrate system was built for studying the consumption and recovery of the lubricant perfluoropolyethers(PFPE)layer under rapid heating and cooling through HAMR.It is found that in the HAMR laser heating process,if the lubricant temperature rises above 700 K,the amount of escaped PFPE molecules of lubricant will significantly increase.The loss rate of remain lubricant decreases gradually in the heating after the lubricant temperature rises to target temperature.At last the lubricant layer reaches stable state.And the lubricant's weight percentage on the disk substrate rises up again when the cooling occurs.On the other hand,the bearing pressure reduces the degree of consumption of the PFPE lubricant by laser heating and makes the lubricant layer reaches stable state faster.The thickness and the flying height of HDD can also affect the stability of lubricant layer.This paper also builds a enlarged full-atom model and a coarse-grained bead-spring model of disk-lubricant-slider system.And the molecular dynamic method is used to study the formation mechanism of the lubricant bridge under HAME laser heating and room temperature.Furthermore,the influence of lubricant bridge on the pressure acting on the bottom surface of head is also studied.It can be found in the results that in the HAMR laser heating case,the increase of heating time,the increase of bearing pressure and the increase of disk velocity can prevent the formation of lubricant bridge.In the room temperature condition,the slider FH and disk rotation speed can affect the formation of the lubricant bridge.However,the effect of disk velocity is less obvious when the FH is higher.Furthermore,the lubricant bridge significantly affects the pressure distribution of slider bottom surface.The average pressure of slider bottom surface significantly decreases after the lubricant bridge being formed.A HDI model based on DSMC method is also built to investigate the bearing force over bit-patterned media(BPM)in the filling gases of air and helium under different conditions,including groove depth,ambient temperature,slider surface temperature,rotation speed of the disk,and the working altitude of HDD.The results show that the bearing force reduces with the increase of the BPM groove depth.The change rate of bearing force decreases with the increase of the BPM groove depth.The gas bearing in helium filling HDD is more stable than that in air filling HDD.The resistant force and the tangential component of bearing force in helium filling HDD are 40%and 50%of that in air filling HDD respectively.The influences caused by ambient temperature,head temperature,disk rotation speed and working altitude are significant.