Disc springs that are resistant to premature failure caused by friction wear, disc spring assemblies and disc spring stacks including the disc springs, and methods of making and using said assemblies and stacks are disclosed. Slideably mounting a lock washer over the drain pipe and threadably tightening the lock washer against a lower surface of the bottom wall of a bathtub. The photochromic materials each exhibit a first \u201cinvisible\u201d photochromic state at a first absorption spectrum and at least a second \u201cvisible\u201d photochromic state at a second absorption spectrum. 45, and M1 and M2 represent the content of iron and manganese, respectively, in a carrier particle. The present invention also provides a process for the purification of pazopanib hydrochloride.