Graded grain size diamond layer

The field of the present invention relates generally to diamond coatings for cutting tools and wear parts, and more particularly to a polycrystalline diamond coating including a graded diamond layer having a progressively finer grain size in the direction of the outer surface for providing enhanced wear resistance and smoother finishing characteristics. The invention relates to diamond coatings and the growth of diamond coatings suitable for tools, wear parts, and the like. The invention produces polycrystalline coatings having progressively finer grain size in the direction of the outer surface, which enhance the wear resistance and finish characteristics of the parts and tools. In an embodiment, chemical vapor deposition grows a first region over a substrate with a plurality of nucleation sites and the first region transitions into polycrystalline diamond grains growing progressively smaller to an average grain size of less than three microns.