This will depend on what kind of scales the dragon has.
An easy way to make it specials is to consider if they have steel that can cut dragon scales why are they using dragon scale instead of that steel. youSo make the dragon scales harder/tougher than steel. You may say only adamantine can cut dragon scales but dragonhide is easier to come by than enough adamantine to make a full set of armor. Your players may have to have a quest just to find adamantine tools or an armor make who has such tools. Alternatively you could require that they use diamond tipped tools, magic, or the dragons own teeth to drill the scales.
Consider also how you dragon scales work, if they work like most reptile scales they are just thickened skin, that is the scales and skin are one contiguous material. Crocodilians have bony core to their scutes although the covering is the same. Fish scales are often made of enamel like teeth and they grow out of the skin and can be individually pulled out and overlap naturally. emulating theseEmulating fish scales may be your best bet, since they are made of the hardest material available in the body.