Rogul T. Structure and micromechanical behavior of deposited chromium

The thesis presents results of complex analysis of structure and mechanical behavior of chromium produced by electric arc and magnetron sputtering, gas-flame spraying. The correlation has been revealed between structural transformations and microhardness.The chromium coatings produced by gas-flame spraying have composite structure which consists of crystallized drops and chromium oxycarbonitrides. The crystallized drops have fine-grained structure (the grain sizes ranged from 200 to 400 nm), and disperse chromium oxides are present inside in grains. The microhardness of gas-flame sprayed chromium coatings reaches the value of 8.7 GPa. High hardness of coatings retains up to 1273 K in contrast to electrolytic chromium coatings for which hardness decreases at temperature 873 К. Nonequilibrium crystallization conditions of electric arc sputtered in vacuum materials is real reason for the specific structure formation, which is characterized by high concentration of vacancies. The vacancies form dislocation prismatic loops at condensation temperature higher than 473 К. The dislocation prismatic loops have Burgers vector b=<100> and lie in {100} plane of chromium lattice. Maximum density of dislocation loops is at condensation temperature 573 К . The microhardness of electric arc sputtered chromium coatings reaches the value of 4.1 GPa. The density and size of dislocation loops are define the type of variation of hardness with condensation or annealing temperature variation. New mode of magnetron sputtering for nanocrystalline chromium layers preparation was proposed. The grain size of these coatings ranged from 40 to 60 nm. In this case, the volume concentration value of chromium oxides phase in the coating has been estimated at less than 3%. The microhardness of the 40-nm thick chromium coatings produced by magnetron cyclic sputtering reaches the value of 18.7 GPa, that is 10 times as high as that of cast chromium. To clarify the nature of the extremely high hardness of fine-grained chromium coatings, thestructure, mechanical behavior and nanohardness of nanocrystalline films of chromium and molybdenum, which are metals-analogues, have been studied. Comparative analysis has shown that chromium films are of lower plasticity and higher hardness than molybdenum films, which have been produced under the same conditions and are characterized by even more fine-grained structure. The hardening of chromium coatings is a result of oxygen atoms embedding in nanocrystalline boundaries, i.e. "healing" of defects at "weak" points on grain boundaries. It is possible due to strong chemical bond in the Cr-O system. The polycrystalline chromium is characterized by a stronger Me-O bond as compared with Me-Me bond, in contrast to polycrystalline molybdenum. The conception of "useful" impurities is suggested on the assumption of this mechanism. It permits to employ grain boundary engineering for hardness increasing in nanocrystalline materials. "Healing" of "weak" places in "bad" material is possible due to enrichment of material by "useful" impurities. The influence of elastic-plastic substrate characteristics on nanocrystal chromium film-substrate system was investigated. Analysis of loading-unloading diagrams of the substrate and chromium film-substrate samples and of ACP versus indenter displacement for chromium film-substrate samples shows that the substrate elastic-plastic properties affect the micromechanical behavior of system even at the early stage of the nanoindentation. In this case, the effect of soft substrates and substrates having a low elastic modulus is observed at lower indenter displacement. The closest to the reference data are the Young modulus values calculated by the Hertz formula for the early stages of the nanoindentation on the assumption that the blunting radius of the Berkovich indenter equals the effective radius of the sphere.