Hard drive stiction

[ShaneWard]

How does the heads get stuck on a spinning disk.

From what I have read, the heads get stuck when the drive is dropped or shaken, but how does the heads get stuck if the disk is spinning.

Does it happen when the Heads heat up, then when the drive is turned off and the heads park, is this when the heads meld with the disk? and what sticks to the disk on the heads?

Would be interesting to know how this happens in detail.

Many thanks

[ShaneWard]

Found an article on Wikipedia: -

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.

In the early models of hard disk drives stiction was known to cause read/write heads to stick 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(say 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 is 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.