Stiction

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Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. [1] The term is a portmanteau of the words static and friction, [2] perhaps also influenced by the verb stick .

Contents

Any solid objects pressing against each other (but not sliding) will require some threshold of force parallel to the surface of contact in order to overcome static cohesion. Stiction is a threshold, not a continuous force.

In situations where two surfaces with areas below the micrometer scale come into close proximity (as in an accelerometer), they may adhere together. At this scale, electrostatic and/or Van der Waals and hydrogen bonding forces become significant. The phenomenon of two such surfaces being adhered together in this manner is also called stiction. Stiction may be related to hydrogen bonding or residual contamination.

Automobiles

Stiction is also the same threshold at which a rolling object would begin to slide over a surface rather than rolling at the expected rate (and in the case of a wheel, in the expected direction). In this case, it's called "rolling friction" or μr.

This is why driver training courses teach that if a car begins to slide sideways, the driver should try to steer in the same direction as the slide with no brakes. It gives the wheels a chance to regain static contact by rolling, which gives the driver some control again. An overenthusiastic driver may "squeal" the driving wheels trying to get a rapid start but this impressive display of noise and smoke is less effective than maintaining static contact with the road. Many stunt-driving techniques are also done by deliberately breaking and/or regaining this rolling friction.

A car on a slippery surface can slide a long way if the driver "locks" the wheels in stationary positions by pressing hard on the brakes. Anti-lock braking systems use wheel speed sensors and vehicle speed sensors to determine if any of the wheels have stopped turning. The ABS Module then briefly releases pressure to that wheel to allow the wheel to begin turning again. Anti-lock brakes can be much more effective than cadence braking which is essentially a non-automatic technique for doing the same thing.

Examples

Engineering

Stiction refers to the characteristic of stop and start type motion as a force overcomes static friction and causes a part to accelerate under dynamic friction, but the force cannot keep up with the speed of the moving part so the part tends to stop again until the force catches up, and it happens repeatedly. Stiction is a problem for the design and materials science of many moving linkages. This is particularly the case for linear sliding joints, rather than rotating pivots. Owing to simple geometry, the moving distance of a sliding joint in two comparable linkages is longer than the circumferential travel of a pivoting bearing, thus the forces involved (for equivalent work) are lower and stiction forces become proportionally more significant. This issue has often led to linkages being redesigned from sliding to purely pivoted structures, just to avoid problems with stiction. An example is the Chapman strut, a suspension linkage developed by Colin Chapman of Lotus cars. [3]

Surface micromachining

Stiction or adhesion between the substrate (usually silicon based) and the microstructure occurs during the isotropic wet etching of the sacrificial layer. The capillary forces due to the surface tension of the liquid between the microstructure and substrate during drying of the wet etchant cause the two surfaces to adhere together. Separating the two surfaces is often complicated due to the fragile nature of the microstructure. Stiction is often circumvented by the use of a sublimating fluid (often supercritical CO2, which has extremely low surface tension) drying process where the liquid phase is bypassed. CO2 displaces the rinsing fluid and is heated past the supercritical point. As the chamber pressure is slowly released the CO2 sublimates thereby preventing stiction.

Precision boring

Many components will lock together with stiction even though they have sufficient theoretical clearance.

Polished glass

Polished glass is especially prone to stiction.

Hard disk drives

In the context of hard disk drives, stiction refers to the tendency of read/write heads to stick to the platters. Stiction is a result of smoothness and is exacerbated by humidity and other liquids condensing at the head-disk interface. Once the heads have stuck to the platters, the disk can be prevented from spinning up and can cause physical damage to the media and the slider. Other forces considered as responsible for stiction include electrostatic forces.[ citation needed ]

In the early models of hard disk drives, stiction was known to cause read/write heads to stick to the platters of the hard drive due to the breakdown of lubricants used to coat the platters. During the late 1980s and early 1990s, as the size of hard drive platters decreased from the older 8" and 5.25" sizes to 3.5" and smaller, manufacturers continued to use the same calendering processes and lubricants used on the older, larger drives. The much tighter space caused much higher internal operating temperatures in these newer smaller drives, often leading to an accelerated breakdown of the surface lubricants into their much stickier components. When the drive was powered off and would cool down (for example at the end of the day when a user went home and shut off their PC), these now-broken-down lubricants would become quite viscous and sticky, sometimes causing the read/write heads to literally stick to the platter. One response to this problem was to remove the affected drive and strike it gently but firmly on the side, then try to start it while connected to but not necessarily fitted inside the machine. This might break the heads free for long enough to spin up the drive and recover the data from it without powering it down. Once started, it would continue to run indefinitely, but might not start again if powered down. Instead of tapping the drive, rotating it sharply by hand could start it. In most Maxtor hard drives, if the heads are stuck to the platters, the drive might make "music" from either the heads trying to move or from the platters trying to spin up.

Modern hard drives have mostly solved the stiction problem by using ramps to "unload" the heads from the disk surface on power-down. These ramps ensure the heads are not touching the platters, which not only prevents stiction but also keeps abrasion from kicking up microscopic particulates that can later contaminate the drive mechanism. Parking the heads in this manner also allows the voice coil actuator to be shut down to save power, so the heads are also frequently unloaded when the drive is idle. A competing solution is based on laser textured landing zones near the ID of the platter where no data are stored. The heads are parked in that zone, and the actuator is latched until the next start-up. The landing zone consists of a controlled array of nanometer-level 'bumps' on the disk surface produced during manufacturing of the disk using a local substrate melting process employing suitable laser-based equipment. The method was pioneered by IBM around 1995 and is still widely in use in most desktop and server class HDDs. [4]

Digital storage tapes

Stiction may also manifest itself on computer tapes (9 track tape etc.). The magnetic surface of the tape would be heated against the read head in the tape deck, and when the tape stopped moving would cool slightly and "glue" onto the read head. This could be avoided by configuring the software so that the tape could be read continuously. [5]

Amateur astronomy

The term "stiction" has come into use in amateur astronomy circles to describe a characteristic of Dobsonian style altazimuth telescope mounts. These mounts can resist initial movement by the user, making it difficult to track an object in the sky. There is backlash; breaking this resistance requires enough force to cause the observer to overshoot the object.

Stereolithography

Typically the phenomenon occurs when “green” epoxy photopolymer components are left in direct contact with each other. If left long enough, it appears that “cross-linking” of the polymer takes place in the region of contact. This effectively “welds” or more appropriately “glues” the parts together. This issue can have a significant impact on models where testing of kinematics are required. To avoid stiction in stereolithography, clean, and more importantly, fully cure all parts prior to assembly.

Biology

Stiction happens with the human body in situations where two surfaces press together to the point any lubrication is excluded, such as in ball joints for hip replacements or in post-operative patients using smooth plastic dilators.

See also

Related Research Articles

Disk storage storage mechanisms that record data on the surface of rotating disks

Disk storage is a general category of storage mechanisms where data is recorded by various electronic, magnetic, optical, or mechanical changes to a surface layer of one or more rotating disks. A disk drive is a device implementing such a storage mechanism. Notable types are the hard disk drive (HDD) containing a non-removable disk, the floppy disk drive (FDD) and its removable floppy disk, and various optical disc drives (ODD) and associated optical disc media.

Friction Force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

Hard disk drive platter circular disk on which magnetic data is stored in a hard disk drive

A hard disk drive platter is the circular disk on which magnetic data is stored in a hard disk drive. The rigid nature of the platters in a hard drive is what gives them their name. Hard drives typically have several platters which are mounted on the same spindle. A platter can store information on both sides, requiring two heads per platter.

Disc brake

A disc brake is a type of brake that uses the calipers to squeeze pairs of pads against a disc or "rotor" to create friction. This action slows the rotation of a shaft, such as a vehicle axle, either to reduce its rotational speed or to hold it stationary. The energy of motion is converted into waste heat which must be dispersed.

Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings.

Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication, and wear. Tribology is highly interdisciplinary. It draws on many academic fields, including physics, chemistry, materials science, mathematics, biology, and engineering. People who work in the field of tribology are referred to as tribologists.

Head crash

A head crash is a hard-disk failure that occurs when a read–write head of a hard disk drive comes in contact with its rotating platter, resulting in permanent and usually irreparable damage to the magnetic media on the platter surface. It is most commonly caused by a sudden severe motion of the disk, for example the jolt caused by dropping a laptop to the ground while it is operating or physically shocking a computer.

Magnetic storage storage of data on a magnetized medium

Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetisable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads.

Rolling type of motion that combines rotation and translation of an object with respect to a surface with which it is in contact

Rolling is a type of motion that combines rotation and translation of that object with respect to a surface, such that, if ideal conditions exist, the two are in contact with each other without sliding.

Rolling resistance rolling friction or rolling drag, is the force resisting the motion when a body (such as a ball, tire, or wheel) rolls on a surface

Rolling resistance, sometimes called rolling friction or rolling drag, is the force resisting the motion when a body rolls on a surface. It is mainly caused by non-plastic effects; that is, not all the energy needed for deformation of the wheel, roadbed, etc. is recovered when the pressure is removed. Two forms of this are hysteresis losses, and permanent (plastic) deformation of the object or the surface. Another cause of rolling resistance lies in the slippage between the wheel and the surface, which dissipates energy. Note that only the last of these effects involves friction, therefore the name "rolling friction" is to an extent a misnomer.

Adhesion railway Railway which relies on adhesion traction to move a train

An adhesion railway relies on adhesion traction to move the train. Adhesion traction is the friction between the drive wheels and the steel rail. The term "adhesion railway" is only used when there is need to distinguish adhesion railways from railways moved by other means, e.g. by a stationary engine pulling on a cable attached to the cars, by railways which are moved by a pinion meshing with a rack, etc.

Cadence braking or stutter braking is a driving technique that involves pumping the brake pedal and is used to allow a car to both steer and brake on a slippery surface. It is used to effect an emergency stop where traction is limited to reduce the effect of skidding from road wheels locking up under braking. This can be a particular problem when different tires have different traction, such as on patchy ice for example. Its use in an emergency requires a presence of mind that the situation itself might preclude. Cadence braking is supposed to maximize the time for the driver to steer around the obstacle ahead, as it allows the driver to steer while slowing. It needs to be learned and practiced. For most drivers of modern cars, it has been entirely superseded by ABS, however it is still a valuable skill for drivers of non-ABS equipped vehicles such as classic cars.

Traction, or tractive force, is the force used to generate motion between a body and a tangential surface, through the use of dry friction, though the use of shear force of the surface is also commonly used.

Threshold braking or limit braking is a driving technique most commonly used in motor racing, but also practiced in road vehicles to slow a vehicle at the maximum rate using the brakes. The technique involves the driver controlling the brake pedal pressure to maximize the braking force developed by the tires. The optimal amount of braking force is developed at the point when the wheel just begins to slip.

Fishtailing is a vehicle handling problem which occurs when the rear wheels lose traction, resulting in oversteer. This can be caused by low friction surfaces. Rear-drive vehicles with sufficient power can induce this loss of traction on any surface, which is called power-oversteer.

Hard disk drive failure failure mode of hard disks

A hard disk drive failure occurs when a hard disk drive malfunctions and the stored information cannot be accessed with a properly configured computer.

The stick-slip phenomenon, also known as the slip-stick phenomenon or simply stick-slip, is the spontaneous jerking motion that can occur while two objects are sliding over each other.

Burnishing (metal)

Burnishing is the plastic deformation of a surface due to sliding contact with another object. It smooths the surface and makes it shinier. Burnishing may occur on any sliding surface if the contact stress locally exceeds the yield strength of the material. The phenomenon can occur both unintentionally as a failure mode, and intentionally as part of a manufacturing process. It is a squeezing operation under cold working.

The flying height or floating height or head gap is the distance between the disk read/write head on a hard disk drive and the platter. The first commercial hard-disk drive, the IBM 305 RAMAC, used forced air to maintain a 0.002 inch (51 μm) spacing between the head and disk. The IBM 1301, introduced in 1961, was the first disk drive in which the head was attached to a "hydrodynamic air bearing slider," which generates its own cushion of pressurized air, allowing the slider and head to fly much closer, 0.00025 inches (6.35 μm) above the disk surface.

Sticky pad is a friction device used to prevent objects from sliding on a surface, by effectively increasing the friction between the object and the surface.

References

  1. "Stiction, n." The Free Dictionary . Retrieved 23 May 2012.
  2. "Stiction". Merriam-Webster . Retrieved 23 May 2012.
  3. Ludvigsen, Karl (2010). Colin Chapman: Inside the Innovator. Haynes Publishing. p. 121. ISBN   978-1-84425-413-2.
  4. A new laser texturing technique for high performance magnetic disk drives, Baumgart, P.; Krajnovich, D.J.; Nguyen, T.A.; Tam, A.G.; IEEE Trans. Magn.
  5. Discussion by data recovery firm