Elsevier

Applied Surface Science

Volume 477, 31 May 2019, Pages 232-240
Applied Surface Science

Full Length Article
Effect of processing factors on the microstructure and gradual diffusion of tungstenized layers

https://doi.org/10.1016/j.apsusc.2017.10.040Get rights and content

Highlights

  • The gradual Ti-W composition coatings was performed on Ti-Al-Nb alloy.

  • TixW1-x composition coatings with the excellent adhesion were prepared.

  • The optimal process parameters of the tungstenized layer were obtained.

  • The gradual diffusion modeling of W atoms in the Ti-W composition coatings is presented.

Abstract

The refractory tungsten (W) coating due to its low sputtering yield is not easy to be obtained, and the bonds between the W coating and its substrate are weak and easily broken. A novel gradual tungstenized layer as the plasma facing armor material is prepared by using the double glow plasma surface alloying technology (DGPA). The effects of process factors on the microstructure and micro-hardness of the alloyed layers are investigated. It is found that TixW1-x gradual layers are formed in the alloyed layer. Due to the fact that the tungsten content in the gradual tungstenized layer gradually decreases from surface to the substrate, the layer is composed of both pure tungsten coating and transitional coatings (e.g. TixW1-x (x > 0.5) and TixW1-x (x < 0.5) coatings). Hence, the gradual layer can enhance the bond strength between the layer and substrate, and the improved mechanical properties such as high hardness can also be obtained. The fact is that vacancies, dislocation and crystal defects could form on the surface of the substrate due to the effect of the double glow plasma bombardment. Therefore, a particular attention is paid to the diffusion mechanisms of W atoms in the W gradual layers and a diffusion model is proposed to explain the forming process of the gradual layers according to the Fick's first law. The findings reported here demonstrate the potential of an effective surface modification method for improving the diffusivity of W atoms. The present work provides a better understanding of the gradual diffusion process of tungsten element.

Introduction

Owing to its high physical sputtering threshold energy, good thermal properties under high temperature condition, and low erosion rate under plasma loading, tungsten is considered as a prime candidate for plasma facing armor material in nuclear fusion reactors [1], [2]. Nonetheless, the drawbacks of tungsten bulk material as the plasma facing material are the high ductile to brittle transition temperature (DBTT), heavy weight, and difficulties in machining and welding [3]. A promising method for industrial applications of tungsten is to coat the designated part with a thin tungsten layer. Some researchers have reported that the W coatings are commonly obtained with the plasma sprayed, magnetron sputtering and chemical vapor deposition (CVD) and electrodeposition [4], [5], [6], [7]. However, the high elastic modulus and thermal expansion mismatch with other metals make the joining of W to heat sink metals really challenging [8]. A high thermal stress would be built between heterogeneous metals after heating them, which will certainly damage the coating-substrate joint. The tungsten coating delaminating from the substrates frequently appears when using conventional methods mentioned above.

The double glow plasma surface alloying (DGPA) can produce two cross-linked glow discharges, which mainly originate from two negative bias of the substrate and the source electrode. One glow discharge heats the work piece and the other one strikes the source electrode. The potential difference between the substrate and the target leads to an unequal electronic potential hollow cathode effect [9]. The effect provides the capability of enhancing the sputtering yield and the activities of substrate surface, which has become one of the most attractive techniques in surface alloying or composite coatings preparation [10], [11], [12]. Therefore, more W ions or atoms sputtered from the source electrode can be deposited on the substrate and diffuse into its inner structures. In addition, plasma surface alloying can achieve a certain thickness of the alloyed layers varied from several microns to 500 μm with the concentration of the alloying elements being from 5% to more than 90%. It is also obtained for desired alloyed layers on the different substrates. However, the role of the diffusion behavior of the alloying elements in the DGPA has not been understood so far. Especially, the forming processing of W alloying element, as a typical low sputtering yield and the refractory material, is still rarely found in the open literature.

In view of this, an effort was made to produce a tungsten coating on Ti-Al-Nb alloy substrate by the DGPA technology. The effects of the processing factors on the microstructure and micro-hardness properties of the alloyed layers are systematically investigated. Meanwhile, a gradual-diffusion model of the W atoms in the plasma surface alloying is proposed and the diffusion mechanisms of other transition metals can be explained.

Section snippets

Material and the process

The substrate was Ti–Al–Nb alloy, including 47 wt.% for titanium, 10.82 wt.% for aluminum and 42.12 wt.% for niobium. The specimens were machined into 10 mm × 10 mm × 5 mm in dimension. The specimens were mechanically polished via a standard metallographic procedure and then were cleaned with acetone before the plasma surface alloying.

The sputtering target of W plate (Ф100 mm × 5 mm) with the purity of 99.9% prepared by powder metallurgy was used as the source electrode to supply alloying elements. The

Effect of the temperature on the tungstenized layer

According to the Ti-W equilibrium phase diagrams, titanium is important as an alloying agent with W and can be stabilized in the β phase. β0 + O binary phase exhibits excellent mechanical properties that the good balance among tensile, creep and fracture toughness can be obtained at the temperature of 900–1000 °C [13], while the brittleness increases as β transforms to β0 when the temperature is above 1050 °C. To study the effect of the temperature on the microstructure of the alloyed layers, a

Conclusions

In the current study, the gradual tungstenized layers were carried out by the DGPA technology. We compared the effects of temperature, the spacing distance between electrodes and the pressure on the surface microstructure and micro-hardness of the alloyed layers. The diffusion modeling of W atoms in the Ti-based alloys is proposed based on the Fick ‘s first law and the diffusion process of the TixW1-x gradual layers is discussed. Major findings of the study are summarized as follows:

  • (1)

    The optimal

Acknowledgements

This project is supported by National Natural Science Foundation of China (No. 51405242), Natural Science Foundation-Outstanding Youth Foundation of Jiangsu Province of China (BK20160091) and the Six Talent Peaks Project of Jiangsu Province, China (No. GDZB-046).

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