Date of Award

Spring 2018

Project Type

Thesis

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

James E Krzanowski

Second Advisor

Olof E Echt

Third Advisor

William F Hersman

Abstract

Since the initial discovery of the S-phase in 1985, understanding the structural nature of this phase and the anomalous shift of the (200) diffraction peaks has been a challenging problem. Austenitic stainless steels, ternary Fe–Cr–Ni alloys, like AISI 304, demonstrate excellent corrosion resistance and relatively good levels of toughness and strength. For this reason, they are widely used engineering materials in areas such as aerospace, construction buildings, piping, telecommunications, chemical and petrochemical applications. However, stainless steels have a relatively low hardness, and this leads to a poor wear resistance, resulting in a short lifetime that limits its use in industrial applications. Therefore, surface treatment methods have been developed to improve its mechanical properties without loss of corrosion resistance. Surface hardening of stainless steels can be accomplished using a combination of nitrogen implantation and diffusion to create a hardened surface layer. The incorporation of nitrogen into stainless steels by these techniques results in expansion of the fcc (austenite) lattice; this phase is referred to as “expanded austenite,’’ or the “S-phase’’. A notable feature of the S-phase is the displacement of the (200) reflection from its expected position. The reactive magnetron sputtering process has been used to deposit thin films of nitrogen-supersaturated stainless steels. In addition, new hybrid coatings were studied by combining stainless steel targets with other transition metals, as well as carbon, in the deposition process. A variety of advanced characterization methods were used to examine the structural, compositional, mechanical and tribological properties of these films. These techniques include x-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy with energy dispersive x-ray spectroscopy (SEM-EDX), transmission electron microscopy (TEM), micro-hardness (Knoop indenter), nano-Indentation, and both pin-on-disk and optical microscopy tests for tribological evaluations. In addition, the structural nature of the films was further examined using area-detector based x-ray diffractometry. Using 304 stainless steel sputtering targets, films were deposited in a mixed Ar/N atmosphere using a variety of Ar/N ratios, as well as parametric variations in substrate bias and temperatures and sputter gun power ratios. XPS analysis showed nitrogen supersaturation levels near 40 at.% in these films. X-ray diffraction analysis showed the structures of the films were strongly temperature dependent: above 450 °C, the films were a mixture of CrN, bcc-Fe, and Ni; below 450 °C, the films were nominally fcc-structured. However, the common anomalous deviation in the position of the (200) reflection was observed, indicating the presence of the S-phase. Area-detector based X-ray diffraction studies, which allowed peak position measurements as a function of the inclination of the diffraction vector (angle ψ), showed a200 declined with increasing ψ, but always remained greater than a111, which was relatively constant with ψ. Hardness was measured and also found to be a strong function of substrate temperature, with the highest hardness of 2100 kg/mm2 obtained for films deposited at room temperature. SEM and TEM cross-section samples showed uncommon morphological features which provided insight into the structural nature of the S-phase. Hybrid stainless steel /titanium nitride (SS-Ti-N) films, as well as a hybrid stainless steel/chromium nitride (SS-Cr-N) coatings were investigated and showed superior mechanical properties that may be promising new coatings. The S-phase was also produced in these hybrids coatings. In the SS-Ti-N, titanium concentrations of up to ~14 at.% were obtained, in which case the nitrogen levels were near stoichiometric (50 at.%N). Hardness levels of 18-24 GPa (~1800-2500 Kg/mm2) were obtained for the films that had titanium concentrations between 10-14 at.%. These S-phase films made by co-sputteirng from both stainless steel and titanium targets could increase the hardness by nearly 100% compared to films made with only stainless steel. A tribological analysis of the films was conducted using a pin-on-disk test with an alumina ball, and the optimal results were obtained on a SS-Ti-N film deposited at 150oC/ -140V, where the average friction coefficient was 0.39. It should be noted that the average of regular stainless steel is 0.6 For the SS-Cr-N films, chromium concentrations of up to 54% were obtained and showed a maximum hardness of ~ 4639.8 Kg/mm2 for a film deposited at 250C and -140V. These films tend to have a nitrogen concentration of ~ 40%. The S-phase was formed in these coatings and the (200) peak also shifted from expected positions. The friction coefficient of the SS-Cr-N coated films was examined and showed an improved friction coefficient (0.41) at film deposited at 150C. Further studies of N-supersaturated films deposited stainless steel and stainless steel co-sputtered with titanium were conducted to better understand the structural nature of the S-phase. In order to quantify the peak shift in these films, a term denoted the “R-value” was used, which for an FCC structure is given by: (1) An R-value of 0.75 is expected for normal fcc structures; a value of R>0.75 indicates the presence of the S-phase. The effect of nitrogen and titanium concentrations, substrate temperature and the morphology on R-value was investigated. R-values were generally > 0.75, indicating a deviation from the common fcc structure. The samples with R closest to 0.75 were films with higher titanium levels (10-14 at.% Ti), and these films had stoichiometric nitrogen concentration levels (~50 at.% N). Also, films that have a nitrogen content of 30-43% do not show a consistent relationship to high or low R-values. SEM cross-section of the S-phase films deposited at lower bias showed a layered or ribbed morphology in the coarse columns. TEM images revealed a central spine and branched structure in films deposited at 150C and 250 oC, with fewer branches at 350C. Additionally, increasing the substrate temperature from 150 to 350 oC led to a decrease in the R-value (from 0.802 to 0.779) made the films denser. The effect on the peak shift (2) calculated and the shift was 0.022o, however, this number was far from the value of 2 measured from our XRD data. It was concluded that the observed layered morphology does not explain the measured R-values. Films of stainless steel/carbon were also deposited by co-sputtering. This was done because carbon offers another way to make an alternative version of S-phase using carbon instead of nitrogen. These films maintained S-phase structure when deposited below 450oC. Carbon concentrations near 50% were obtained in several cases, and the hardness of these films reached a maximum value of 2256 Kgf/mm2 at a deposition temperature of 250oC. In comparison to SSN, SSC has an improved hardness.

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