Browse to the location of the library file in the directory structure of your computer. All material library file names have the extension .adsklib.
Tip: The path to a library shows in a tooltip when you move the cursor over the library name on the Materials tab.
Right-click the library file, click Properties, select the Read-Only check box, and click OK.
If the library you locked is displayed in the library list, the lock icon may not appear beside the library name until you remove that library from the list and then open it again.