Metric Serrated flange lock nuts. This lock nut has an integral flange built into it and the backside is serrated. They are typically used in applications where you have some type of metal sheeting or something that has metal. After you tighten it down, the serrations will prevent the nut from backing off. 

They also make these without the serrations but those are not technically locking and.used when the object is not a locking feature but a larger bearing surface.

Talk to Hardware Everywhere’s knowledgeable customer service team for help choosing the fasteners that match your application. Call us at 1-800-432-8979 or contact us online.