Expandable tool with enhanced expansion capability

An expandable downhole tool features a mandrel that is expanded from within by a swage or other technique. The expansion of the mandrel advances an exterior ring or comparable slidably mounted object against an exterior sleeve that can be impervious or porous. The ring engages a ratchet mechanism to hold its position with respect to the mandrel as the swage advances through the mandrel and expands the ring and the exterior sleeve. The locking of the ring to the mandrel keeps the exterior sleeve from springing back longitudinally as the expansion is concluded.