An anisotropic texture is formed on a principal surface in the circumferential direction of the principal surface. The circumferential roughness of the principal surface increases from an outer circumferential section toward an inner circumferential section of the principal surface. The ratio [Ra-c/Ra-r] of the circumferential roughness (Ra-c) to the radial roughness (Ra-r) of the principal surface increases from the outer circumferential section toward the inner circumferential section.