What causes single handle mixer faucet's cartridge to become stiff during the years?
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There are "o" rings and rubber bushing inside the cartridge that will deteriorate over time with age and water treatment chemicals. They will expand and become brittle and can tighten the operation of the cartridge. You can usually get a repair kit for a cartridge that includes the rubber parts and some plumber's grease and fix it yourself. You can also just take the cartridge to a plumbing / home store and get a new one.
Ceramic discs are a different animal. They don't have replaceable seals but they do have seals inside. there is a pivot point in the handle that can get wore and might stiffen the movement up. The faucets usually have a lifetime warranty so get with them and they might send you a new cartridge