Hydrogen effects on mechanical responses of small volumes

Available hardness and thin film fracture data as affected by hydrogen at the film substrate interface are reviewed. At the nanometer scale, it appears that hydrogen can increase the yield point near the surface of stainless steels by nearly a factor of three. Additionally, it appears to reduce the resistance to interfacial delamination of Cu films from silicon substrates by about a factor of two. Based upon additional observations of small volumes without hydrogen, interfacial toughness appears to vary with a length scale to the 1/2 power while hardness at nanoscale depths may vary with a length scale to the 2/ 3 power. In both, there is reason to believe that the volume to surface ratio is an appropriate length scale in the nanometer range. Regarding film fracture, this strongly suggests that even in small volumes the volume/surface ratio is truncated by the hydrogen mechanism. Note that this ratio, as a length scale, could be diminished by hydrogen either causing brittle fracture, as in a cleavage or intergranular process, or by localizing fracture, as in a highly localized plastic rupture. The picture is complicated by the hardness in small volumes increasing. Both continuum and discretized dislocation models with a single length scale, a yield strength and a fracture criterion are capable of predicting hydrogen-reduced threshold stress intensities. The connectivity from the mesoscale to the continuum can be shown through the volume to surface ratio concept.