Atomic layer deposited (ALD) films are an ideal choice for enhancing the performance and reliability of microsystem devices. The self-limiting chemistry results in conformal coatings of high aspect ratio structures with monolayer precision. ALD tungsten films are of particular interest for MEMS and LIGA applications due to their good wear resistance. However, property data is extremely limited as most conventional test methods are difficult to apply to these very thin films. We therefore began a study of ALD tungsten films on silicon substrate properties as a function of film thickness using nanoindentation and nanoscratch techniques. Nanoindentation showed that elastic modulus and hardness increased with film thickness. For the thickest films, these values were substantially higher than those of the substrate and hardness equaled values for bulk and sputter deposited films. Of particular concern, nanoscratch tests triggered channel cracking and delamination in the thickest film. These cracks formed in succession during the scratch tests closely following the path established by the previous cracks. Mechanics-based cracking and delamination models showed that the films fractured under steady state conditions at an energy of 0.4 J/m2. The corresponding interfacial fracture energy for film delamination between the channel cracks was substantially less at 50 mJ/m2 with a mode I component equal to 30 mJ/m2. This value is less than the work of adhesion for sputter deposited tungsten films on silicon substrates but may accurately reflect the influence of lower film density along the substrate interface.