Investigation of interfacial microstructure of CrN coatings on HSS substrates pretreated by HIPIMS for adhesion enhancement

In this study, six dc Magnetron Sputtered (dcMS) CrN hard coatings were deposited on pretreated High Speed Steel (HSS) to achieve different interface architectures. The aim was to correlate the interfacial microstructure to the adhesion of the coatings. The substrates were pretreatment using the Ionized Physical Vapor Deposition (IPVD) method High Power Impulse Magnetron Sputtering (HIPIMS) using a Cr target in an inert atmosphere varying the substrate bias ($U_b$) between 0 V and 1100 V at ambient temperature as well as at a substrate temperature of 400$^\circ$C. The deposition parameters were chosen to show how kinetically induced diffusion, etching and implantation changes the interface chemistry and structure and to investigate their effect on the adhesion on the film. At elevated temperatures, the diffusion will be thermally driven. Annealing of the deposited samples were, therefore, performed at 900 K in an Ar atmosphere. The films were characterized employing XRD, HR-TEM, A-STEM and by scratch test measurements to see how the the interface microstructure can be correlated to the adhesion of the coating. The study shows that a sputter cleaned substrate surface with well preserved crystal structure of the substrate enhances the adhesion of the coating by promotion of local epitaxial growth. However, annealing was also shown to have a large effect on the adhesion enhancement by allowing for interdiffusion in the interface region and due to promotion of interface strain relaxation. Implantation of target material on the other hand had limited influence on the adhesion compared to the clean oxide free surfaces. The low adhesion improvement when gradually changing the chemical composition at the interface is assumed to stem from that the radiation induced defects and strain diminished the positive effect of this gradient

Helmersson Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering Plasma Sources Science and Technology

Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering, 2009, PLASMA SOURCES SCIENCE and TECHNOLOGY, (18) Abstract Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 µs. Two different current regimes could clearly be distinguished during the pulses: (1) A high-current transient followed by (2) a plateau at lower current. These results provide a link between the HiPIMS and the direct current magnetron sputtering (DCMS) discharge regimes. At high applied negative voltages the high-current transient had the characteristics of HiPIMS pulses, while at lower voltages the plateau values agreed with currents in DCMS using the same applied voltage. The current behavior was found to be strongly correlated with the chamber gas pressure, where increasing gas pressure resulted in increasing peak current and plateau current. Based on these experiments it is here suggested that the high-current transients cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high voltage, lower current regime. Confidential: not for distribution

Transition between the discharge regimes of high power impulse magnetron sputtering and conventional direct current magnetron sputtering

Current and voltage have been measured in a pulsed high power impulse magnetron sputtering (HiPIMS) system for discharge pulses longer than 100 mu s. Two different current regimes could clearly be distinguished during the pulses: (1) a high-current transient followed by (2) a plateau at lower currents. These results provide a link between the HiPIMS and the direct current magnetron sputtering (DCMS) discharge regimes. At high applied negative voltages the high-current transient had the characteristics of HiPIMS pulses, while at lower voltages the plateau values agreed with currents in DCMS using the same applied voltage. The current behavior was found to be strongly correlated with the chamber gas pressure, where increasing gas pressure resulted in increasing peak current and plateau current. Based on these experiments it is suggested here that the high-current transients cause a depletion of the working gas in the area in front of the target, and thereby a transition to a DCMS-like high-voltage, lower current regime

Feasibility study of gamma-ray micro-densitometry for the examination of nuclear fuel swelling

   Nuclear fuel undergoes several thermo-mechanical changes during irradiation in a nuclear reactor, such as change of density, caused by solid and gaseous swelling. This affects the heat transport within the pellet and, when leading to the pellet-cladding gap closure, it also affects the gap conductance, causing stress in the cladding.    The density of irradiated fuel pellets can be measured in post-irradiation examination using several methods. In this work, a feasibility study was made using the gamma-ray transmission micro-densitometry technique. This is based on the comparison of two intensity measurements, with and without a sample with well-characterized thickness. Using a collimated source, a local examination of the density can be performed, scanning a pellet slice radially. The proposed technique aims to obtain a spatial resolution of cca. 100 microns.    In this work, the parameters of the setup, such as the source activity, detector counting time, slit dimensions, collimator length, and sample thickness, are used to predict detector efficiency and expected count rates. The obtainable precision of the density is assessed by first-order uncertainty propagation of counting errors in the gamma-ray detection to the density estimate.    A collimator design was presented that achieves a reasonable compromise between time requirements, precision, and spatial resolution. The sensitivity of the performance to set-up parameters was investigated. In addition, a realistic setup was modeled in MCNP6 for validation of the peak count-rate, and to ensure that the total spectrum count-rate is within typical throughput capabilities of HPGe detectors. The MCNP model was also used to confirm that the assumed attenuation law is valid in a relevant geometry, and to assess the spatial resolution, using the 10-90% edge spread of an Edge Spread Function.    It is concluded that fuel density can be determined with <1 % precision, using a 100-micron wide slit, and 1 hour of measurement

Feasibility study of gamma-ray micro-densitometry for the examination of nuclear fuel swelling

   Nuclear fuel undergoes several thermo-mechanical changes during irradiation in a nuclear reactor, such as change of density, caused by solid and gaseous swelling. This affects the heat transport within the pellet and, when leading to the pellet-cladding gap closure, it also affects the gap conductance, causing stress in the cladding.    The density of irradiated fuel pellets can be measured in post-irradiation examination using several methods. In this work, a feasibility study was made using the gamma-ray transmission micro-densitometry technique. This is based on the comparison of two intensity measurements, with and without a sample with well-characterized thickness. Using a collimated source, a local examination of the density can be performed, scanning a pellet slice radially. The proposed technique aims to obtain a spatial resolution of cca. 100 microns.    In this work, the parameters of the setup, such as the source activity, detector counting time, slit dimensions, collimator length, and sample thickness, are used to predict detector efficiency and expected count rates. The obtainable precision of the density is assessed by first-order uncertainty propagation of counting errors in the gamma-ray detection to the density estimate.    A collimator design was presented that achieves a reasonable compromise between time requirements, precision, and spatial resolution. The sensitivity of the performance to set-up parameters was investigated. In addition, a realistic setup was modeled in MCNP6 for validation of the peak count-rate, and to ensure that the total spectrum count-rate is within typical throughput capabilities of HPGe detectors. The MCNP model was also used to confirm that the assumed attenuation law is valid in a relevant geometry, and to assess the spatial resolution, using the 10-90% edge spread of an Edge Spread Function.    It is concluded that fuel density can be determined with <1 % precision, using a 100-micron wide slit, and 1 hour of measurement

Performance evaluation of a novel gamma transmission micro-densitometer for PIE of nuclear fuel

Collimated Gamma Transmission Micro-Densitometry (GTMD) is a novel technique proposed to investigate local density variations of nuclear fuel in PIE, with a high spatial resolution. In this work, the first experimental tests of a gamma micro-densitometer are presented and the performance is characterized. The experimental procedures are described, including the aligning process and the calibration methodology. The results demonstrated that for the calibration samples with a thickness above 5 mm, a local density was obtained with a maximum discrepancy of about 2% and a spatial resolution of about 280 µm. The setup was used for the first test on an irradiated ADOPTTM fuel pellet slice. From the measurement, an average bulk density of about 9.58 g/cm3 was calculated and local density features were observed, possibly related to rim effects or the presence of local cracks. The information acquired also presented valuable information for possible improvements in the setup’s performance

Crystallographic characterization of U2CrN3: A neutron diffraction and transmission electron microscopy approach

In this study, neutron diffraction and transmission electron microscopy (TEM) have been implemented to study the crystallographic structure of the ternary phase U2CrN3 from pellet to nano scale respectively. Recently microstructural evaluation of this ternary phase has been performed for the first time in pellet condition, overcoming the Cr evaporation issue during the conventional sintering process. In this work for the first time, the crystallographic structure of the ordered ternary U2CrN3 phase, stabilized in pellet condition, has been obtained by implementing neutron diffraction. For this study, pellets of the composite material UN with 20 vol% CrN were fabricated by powder metallurgy by mixing UN and CrN powders followed by Spark Plasma Sintering (SPS). TEM was used to investigate the nanoscale structure with a thin lamella of the order of 100–140 nm produced by focused ion beam (FIB). The neutron data revealed the phase composition of the pellet to be primarily 54(8) wt.% U2CrN3, in good agreement with the stoichiometry of starting reagents (UN and CrN powder) and metallographic analysis. Neutron data analysis confirms that all the crystallographic sites in U2CrN3 phase are fully occupied reinforcing the fully stoichiometric composition of this phase, however, the position of the N at the 4i site was found to be closer to the Cr than previously thought. TEM and selected area electron diffraction rendered nano-level information and revealed the presence of nano domains along grain boundaries of UN and U2CrN3, indicating a formation mechanism of the ternary phase, where the phase likely nucleates as nano domains in UN grains from migration of Cr