Etienne Bousser

Hybrid Co-Cr/W-WC and Ni-W-Cr-B/W-WC Coating Systems

The aim of this study was to investigate the effects of subsurface materials on the performance of a wear-resistant thin film. The mechanical, tribological, and corrosion properties were assessed for two hybrid coating systems: (1) W-WC thin film and a laser cladded Co-Cr interlayer applied to a 316 stainless steel substrate and (2) the same W-WC thin film and a spray-and-fused Ni-W-Cr-B interlayer applied to an Inconel® 718 substrate. They were then compared to the same systems without an interlayer. The microstructures were analyzed by XRD, EDS, and SEM. The hardness and surface load-carrying capacity of the coating systems were determined by micro- and macrohardness testing. Rockwell indentation was used to assess coating adhesion (CEN/TS 1071-8). Tribological properties were assessed with a reciprocating tribometer, and corrosion resistance was determined by potentiodynamic polarization and electrochemical impedance spectroscopy. The originality of this work lies in the finding that, except for the Inconel 718/Ni-W-Cr-B/W-WC system, the wear rate decreases with decreasing hardness of the subsurface materials carrying the thin film due to the decreasing contact pressure for a given load. Another novel finding is the stress-induced phase transformation of the Co-Cr interlayer, which occurs beneath the thin film under high load.

Predicting the Load-Carrying Capacity and Wear Resistance of Duplex-Coated Low-Strength Alloys for Severe Service Ball Valves

The load-carrying capacity and wear resistance of a duplex-coated 316 stainless steel were determined, and a finite element numerical approach was developed to predict and corroborate experimental observations. Low-strength alloys are generally used for highly demanding valve applications due to their superior chemical stability, galvanic corrosion resistance, and lower susceptibility to stress corrosion cracking failure. Hardfacing (using thermal spraying, laser cladding, or plasma transferred arc welding) is currently the most common solution to protect valve components. Hardfacing provides a thick, hardened case that significantly improves tribological performance. However, hardfaced layers provide lower wear resistance compared to vacuum-deposited hard coatings. One solution to further improve hardfacing performance is a duplex approach, which combines the two processes. This study investigates the following materials: a 316 stainless steel base hardfaced with laser-cladded Co-Cr superalloy and topped with a CVD nanostructured W-WC coating. Tribological properties of three configurations were assessed for their ability to delay initiation of plastic deformation and surface cracking under quasistatic loading and for their resistance to dry reciprocal sliding wear. The results demonstrate that finite element modeling allows numerical prediction and comparison of the load-carrying capacity and wear resistance of duplex-coated AISI 316 stainless steel.

Tribo Mechanical Properties of CoCr and NiWCrB Hardfacing Superalloy Coating Systems

Wear of materials is a serious problem facing industry especially in mechanical applications where moving parts are continuously subjected to friction. Hard coatings prepared by a variety of processes are nowadays considered as effective solutions to protect components against wear. Examples of such processes are: thermal spray coating, vacuum-based coating and hardfacing. In this paper, we study the mechanical, tribological and corrosion properties of two hard coating systems: CoCr Stellite 6 (ST6) hardfacing on 316 stainless steel and NiWCrB Colmonoy 88 (C88) thermal spray coating on Inconel 718. The effect of gas nitriding on the microstructure and wear performance of these coating systems is investigated. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were used for microstructural analysis. Micro-indentation technique was utilized to measure the surface and cross-sectional hardness of the coatings. Rockwell indentation technique was used to evaluate coating adhesion in accordance with CEN/TS 1071-8. Pin-on-disk tests were conducted to assess the tribological performance of the coatings. Microstructural analysis showed that ST6 has a cobalt matrix in the form of dendrites reinforced with metal carbide particles whereas C88 has a Nickel matrix reinforced mainly with metal boride particles. ST6 and C88 improved significantly the wear resistance of their corresponding substrates. This is mainly due to good adhesion and high hardness of the coatings; HR15N values of ST6 and C88 were almost 85 as compared to 61 and 80 for 316 and INC substrates, respectively. ST6 was found to improve significantly the corrosion resistance of 316 whereas C88 decreased the corrosion performance of INC. Moreover, nitriding treatment was found to improve significantly the wear resistance of 316 and INC, however, in the case of ST6, nitriding was beneficial in terms of wear resistance only at relatively low load.Copyright

Characterization of Calcified Plaques Retrieved From Occluded Arteries and Comparison With Potential Artificial Analogues

Cardiovascular disease is the leading cause of death worldwide. This disease includes chronic total occlusion (CTO), which is a complete blockage of an artery. Unlike partial occlusions, CTOs are difficult to cross percutaneously using conventional guidewires (thin and flexible wires) because of the fibrotic and calcified nature of the blockage. The lack of data regarding the mechanical properties of CTO limits the development of new technologies in the field of percutaneous coronary intervention (PCI) and percutaneous peripheral intervention (PPI). In this study, calcified plaques retrieved from occluded arteries are analyzed in order to better understand their mechanical properties and to help propose an artificial analogue. Calcified plaques samples were collected from the superficial femoral artery wall within one hour following a lower limb amputation surgery. These samples were studied to determine their composition and mechanical properties. The same characterization procedures were performed on various potential artificial analogues. These analogues include three plaster materials and dense hydroxyapatite blocks. The results were then compared with those of the calcified plaques in order to determine the more favorable analogue. This mechanical analysis and the proposal of a potential analogue for the calcified plaques found in occluded arteries could benefit the development of new technologies and devices in the field PCI and PPI.Copyright