Inherent polycrystalline layers 20 to 40 μm thick were produced on Cu single crystal substrates by a controlled surface abrasion and annealing technique. A surface zone produced in this fashion theoretically minimizes certain parameters associated with film strengthening such as, film-substrate modulus differences, chemical composition gradients, film decohesion, lattice parameter misfit and surface energy variations. Investigations of the mechanical behavior of Cu single crystals surrounded with such polycrystalline layers were compared with uncoated single crystals and the results correlated with etch-pit observations and mechanical relaxation measurements. The results indicated that plastic constraints were the principle mechanism responsible for the excess hardening of crystals with a polycrystalline surface zone. The constraint effect was anisotropic with enhanced higher order slip activity, observed in the substrate as an excess forest dislocation density with depth, being principally associated with primary screw dislocation accumulations at the coating-substrate interface. Correlation of strain recovery studies with observations of slip development in the surface layer showed that initially the polycrystalline zone acted as an impenetrable barrier which eventually degenerated when slip activated in the surface grains at strains ≈ 3 x 10-4.