Surface terraces in pure tungsten formed by high temperature oxidation

Abstract

We observe large-scale surface terraces in tungsten oxidised at high temperature and in high vacuum. Their formation is highly dependent on crystal orientation, with only {111} grains showing prominent terraces. Terrace facets are aligned with {100} crystallographic planes, leading to an increase in total surface energy, making a diffusion-driven formation mechanism unlikely. Instead we hypothesize that preferential oxidation of {100} crystal planes controls terrace formation. Grain height profiles after oxidation and the morphology of samples heat treated with limited oxygen supply are consistent with this hypothesis. Our observations have important implications for the use of tungsten in extreme environments. Main text:Tungsten (W) is scientifically interesting as it has the highest melting and boiling temperatures and the highest yield strength among the un-alloyed elements. Additional beneficial properties of high thermal and electric conductivity [1] make it the front-runner material for many high-temperature applications.Some examples include light bulbs, rocket engine nozzles, accelerator targets, and electron microscopy gun filaments and tips [2,3]. Recently, tungsten has also captured the interest of the nuclear fusion industry, where it is being considered as candidate for plasma-facing armour components [4][5][6][7].

Contact Guidance Effect and Prevention of Microfouling on a Beta Titanium Alloy Surface Structured by Electron-Beam Technology

“…A similar effect was observed on Ti6Al4V, as illustrated in Figure 2 and described in previous work [33]. This surface morphology has been barely studied in metals; some recent works on oxide formation in tungsten [44,45] and thin films [46] aim to manipulate the properties of the material by a similar terrace/stair formation.…”

An oxidation mechanism map for tungsten

“…E.g., the sample size [20], [21], [25], [34] and surface roughness [20], [31], [34] can affect reported rates of oxidation and sublimation. They also vary depending on the crystallographic orientation [20], [37], [38] and grain size [21], [31] of the material; and on the flow rate [39], the partial pressure [40], [41] and the moisture content [29], [42], [43] of the purge gas.…”

Surface engineering of metals via metallographic etching for microstructural characterization

“…The oxide film on (100) surface is stable and prevents the aluminium surface from further dissolving, resulting in etch pits being bounded by ( 100) facets [133]. Sample etched with KR for 3 minutes shows that the etch pits started to appear, but the geometries for differently oriented grains are not distinctive (Figure 4.11).…”