July_August_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | J U L Y / A U G U S T 2 0 1 9 4 8 iTSSe TSS iTSSe TSS FEATURE 8 new water-cooled specimen holder is being installed, which will enable the use of more relevant materials such as steels in the next research phase. Coatings sprayed using pure W feedstock were relatively dense, exhibiting typical splat structure originating from the impact of molten W droplets (Fig. 4). Coating thickness ranged from 50 µm to 15 mm. Coatings were wire-cut into 3 × 4 × 32 mm 3 single-edge notched specimens, which were tested for cyclic pure bending fatigue and fracture toughness using an in- house computer-controlled setup [7] . Test results suggest good mechanical endurance and an elastic modulus of ∼ 40% of the corresponding bulk; this would enable easier mate- rial conformation on prospective dimensional changes of the underlying PFCs. Importantly, the coatings were not oxi- dized (determined using EDX and XRD) and retained metal- lic coloration. The microstructure, ease of excessive thick- ness build-up, and prevention of oxidation all confirm RF-ICP technology to be a promising method for producing W armor coatings [8] . Fine-tuning the spray parameters of blended, co-milled, or mechanically alloyed (metallurgically bonded) W-Cr feed- stock presented a new challenge. One issue to resolve con- cerned the melting temperature of tungsten, which is ap- proximately 1500 K higher than that of chromium (roughly 750 K higher than the boiling point of chromium, in fact). Thus, initial attempts frequently resulted in significant porosity in the deposits. Despite the uniform coating thickness exceeding 500 µm, the coatings were not suitable for PFC armor due to their extremely high specific surface area. Further tuning of the parameters resulted in a better mutual connection of Wand Cr splats and a reduction of pore content (Fig. 4). As in the case of pure tungsten coatings, the protective atmosphere in the chamber prevented oxidation of both W and Cr. An example of a deliberately oxidized sample of the W coating is shown in Fig. 4. Fig. 4 — RFICP sprayed coatings: (top) pure W coating; (middle) WCr powder blend; and (bottom) illustration of deliberately oxidized coating, which shows excessive growth of columnar structure, analogous to the phenomenon encountered in suspension plasma spraying. Fig. 3 — Different powder morphologies used for RF-ICP deposition. Pure W powder (left) and a cross section of a layered W-12Cr structure after co-milling (right); bright phase is tungsten.

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