ICMCTF2017 Session G2: Components Coatings

The industrial demand to manufacture complex, heterogeneous devices has grown exponentially. Such devices typically comprise various materials with different heat sensitivities and thermal expansion coefficients. Thus, novel approaches towards joining of complex multi-materials at ever-reduced temperatures are emerging. Among the others, a promising strategy involves the use of nanostructured brazing fillers in the form of coatings consisting of Nano-MultiLayers (NMLs) of metallic brazing filler and a chemically inert barrier. The interplay between spatial confinement, internal stress gradients and the processing environment can stimulate phase-transitions and/or enhanced kinetics associated with a significant outflow of the confined metallic brazing filler to the surface at reduced temperatures. This phenomenon could be exploited for joining materials well below the melting point of the bulk constituents.

Here, we present a comprehensive investigation of the microstructural evolution of (Ag/AlN)5nm/10nm NML coatings upon heating in air. SEM/TEM results evidence the strong migration of Ag from the inner part of the NML to the surface. Silver particles as large as 1 µm are found after a heat treatment in air up to 420 °C. XRD characterization and pole figures confirm that Ag and AlN are initially strongly textured. The in-plane texture is partially lost upon heating in air, as a consequence of the Ag migration and the partial oxidation of AlN. The microstructural evolution of the Ag/AlN NML during annealing was monitored by real-time XRD collected at the synchrotron. Beyond this temperature, a strong increase in the Ag coherency domain is registered. Such increase correlates with the Ag particle appearing on the surface and subsequently coarsening. The average stress state in the Ag layers has been qualitatively evaluated using the real-time XRD data. The results indicate an accumulation of (thermal) stress between 200-280 °C which is then released at higher temperatures, triggering the massive Ag migration. Identical experiments carried out in vacuum or in absence of multilayered structure indicate that no Ag migration takes place. To elucidate the crucial role of oxygen on the Ag mobility at low temperatures, an extensive XPS analysis was carried out on samples heated at different temperatures (from 200 °C to 420°C). The results indicate that oxygen is penetrating through the NML structure and (partially) reacting with both AlN (forming AlOx) and Ag (being adsorbed and/or incorporated at Ag layers surface). The adsorption and dilution of oxygen in Ag can strongly enhance its atomic mobility, thus further easing its relocation on the NML surface.

Natural gas (NG) consumption has been grown rapidly during the last decade, and NG production is projected to further increase by 44% through 2040. Consequently, the potential for methane emission (which can deter GHG benefits of using NG) is also expected to increase throughout the supply chain. Accordingly, the NG industry is facing tough challenges toward mitigating methane emissions in the form of not only adopting new low-emission gas compression technologies but also upgrading or enhancing the currently installed compressors in the field. NG industry uses two types of compressors in the production, delivery and storage of NG. Most common one is the reciprocating NG compressors which account for the largest amount of leakage of methane. Primary leak source of those compressors are wear and scratches on rod and seal material surfaces in piston rod packing systems. One approach is to apply hard coatings onto rod surfaces to prevent wear on the rod side, however it could be detrimental on the delicate Teflon based counter face seal side.

New coatings and surface modification techniques are in need to prevent wear on both surfaces. One approach would be to develop a hard coating that generates tribo-films that beneficial to Teflon based seal side as well, this would minimize wear on both sliding surfaces, or another approach would be to use already known solid lubricant coating such as diamond like carbon. Accordingly, in this paper, we concentrate on the friction and wear performance of DLC, and catalytically active nano-composite nitride based coatings against PTFE type seal materials filled with different fillers (Carbon, MoS2, glass, etc.). A series of vanadium nitride – copper nano-composite, and hydrogenated DLC films were prepared using high power impulse magnetron sputtering (HPIMS). The films were grown on 52100 steel substrates for tribological tests. X-Ray Diffraction (XRD), raman Spectroscopy, nano-indentation techniques were used to characterize the structural, mechanical and chemical nature of the resultant coatings. Bench-top tribological tests were conducted by using oil lubricated reciprocating test rig. Overall, the effect of sliding speed, contact pressure, type of PTFE fillers on the tribolocigal behavior of coatings were investigated. Both nitride and carbon based coatings improved the wear performance of the system depending on the type PTFE composite counter-face used. At the end of each test, confocal raman was used to evaluate sliding interfaces and any structural changes resulting from the tribological tests to shed more insight into the possible mechanisms responsible.