This paper reports the results of drilling tests on various workpiece materials using diamond coated tungsten carbide drills. The performance of the coated drills were compared with uncoated control drills. Hot filament chemical vapor deposition was used for coating the pre-treated drills and film coating morphology and stress characteristics were studied prior to drilling. Forces and torques were measured during drilling and the results indicate catastrophic failure and short tool-life of the coated drills for all workpiece materials. Drill failure reasons are attributed to crystal clustering and point loading of the cutting edges. Further research issues are also identified.