What wears out in an HDD?

Giganews Newsgroups
Subject: What wears out in an HDD?
Posted by:  micky (NONONOmisc…@bigfoot.com)
Date: Thu, 14 Jan 2016

What wears out in an HDD?  Is it only the tone arm that breaks?  or
can the bearings the platter rides on break???    Good watches use
jewels, rubies, as bearings; and cheap watches use metal.  What do
hard drives use?

I googled but couldn't find much about this.  Does the spindle really
ride on an air cushion?    Even when the drive is positioned sideways?