Development and mechanical testing of a novel model system for investigating Orowan strengthening
The enhancement of the durability and strength of thin films and coatings is possible with several methods. The theoretical models describing these methods for thin films, however, lack refinement, as model systems to study the strengthening mechanisms are missing. In most applications the enhancement of coating strength and durability require several consecutive procedures including complex heating and/or mechanical processes. Furthermore, in most cases, the variables influencing the final strengthening effects cannot be independently manipulated. Our com-pact high vacuum system, containing both a gas aggregation nanoparticle source and a physical vapor deposition magnetron, enables us to create nanoparticle reinforced thin films in an additive combinatory manner, with great flexibility in choosing the constituents, without significant thermal or mechanical loads due to manufacturing. We have shown that the hardness and structure of nanograined copper can be retained after heating by incorporating about 1 vol% of tungsten nanoparticles. In situ high temperature indentation measurements were performed up to 400°C. By comparing the hardness evolution versus the temperature of the Cu film containing a high concentration of W nanoparticles with an area of low W concentration, and a pure Cu film with nano-sized twins, differences in the deformation mechanism become apparent. In the as-deposited state, both samples containing nano-particles exhibited an identical hardness of 3.2 GPa, while the pure Cu film showed a hardness of around 2.7 GPa. When the temperature increased, the hardness decreased by equal amounts in both nanoparticle containing films. From 200°C onwards, however, the drop in hardness was notably sharper in the film with low W nanoparticle concentration and the pure Cu film. After cool down, the hardness was found to be reduced by 50% in the pure Cu and low nanoparticle concentration films, whereas the sample with high nanoparticle concentration had retained more than 90% of its initial hardness value. TEM imaging showed that the microstructure consisting of columnar grains with a high density of nanotwins was not changed after the thermal exposure. The pure copper films showed larger grains and exhibited a completely changed texture. We proved that by incorporating as little as 1 vol% of second phase particles in nanocrystalline copper, the microstructure can be stabilized even at high temperatures.