Improvement of physical properties of Cu-infiltrated W compacts via electroless nickel plating of primary tungsten powder

doi:10.1016/j.surfcoat.2009.02.055

Surface and Coatings Technology

Volume 203, Issue 16, 30 May 2009, Pages 2333-2336

https://doi.org/10.1016/j.surfcoat.2009.02.055 Get rights and content

Abstract

In the present research, tungsten particles were coated using nickel/nickel–phosphorus electroless plating technique. The coated tungsten powders were pressed under constant pressure to achieve compact material of cylindrical shape with same porosity. Then, attained compacts were infiltrated/penetrated by liquid copper under the hydrogen atmosphere in order to obtain W–15 wt.% Cu composites. The coated/uncoated powders as well as its infiltrated compacts were characterized by optical microscopy (OM) as well as scanning electron microscopy (SEM), EDS and XRD methods. The microstructure, relative density and specific resistivity of composites were compared. The microstructural observations revealed that the infiltration behavior can be improved in the compacts prepared by both nickel and nickel–phosphorus coated tungsten powders, in comparison with uncoated ones. In addition, it was found that relative density may be raised from < 85% to > 95% by nickel electroless plating, that leads to decrease specific resistivity from 6 to 4 µΩ cm. Enhancement of electrical conductivity of infiltrated W–15 wt.% Cu compacts prepared by electroless nickel coated tungsten powders was related to its higher density.

Introduction

Tungsten–copper composites have been widely used for heavy-duty electrical contacts and arcing electrodes [1], [2]. Tungsten–copper electrodes are used for electro-discharge machining of steel and tungsten carbide dies owing to spark erosion resistance of tungsten as well as high thermal conductivity of copper [3], [4], [5], [6], [7], [8]. Also, superior thermal properties and high microwave absorption capacity of tungsten–copper materials has been documented by authors recently [9]. Tungsten–copper composites are generally fabricated by infiltration/penetration of copper liquid into tungsten compacts [1], [9]. However, the tungsten–copper system exhibits immiscibility and therefore the compacts show very poor sinterability, even after liquid phase sintering [10], [11]. Residual porosity in copper infiltrated tungsten composites may be due to incomplete densification. This leads to accelerated arc erosion during electrical discharge phenomenon. The low wettability of tungsten particles impedes the rearrangement/solution–reprecipitation mechanism during liquid phase sintering [12].

It is well known that sinterability/densification of tungsten–copper compacts can be improved using fine particles, particularly where rearrangement is the dominant mechanism through sintering [13]. The sinterability of tungsten–copper compacts can be also improved by thermo-chemical and activated sintering processes [7], [8]. According to reports, additions of small amount of active elements (i.e. cobalt, nickel, phosphorus, iron etc.) to tungsten–copper powders mixture leads to better sintering behavior. On the other hand, these elements increase wettability of tungsten particles. Consequently, preformed tungsten compacts can be filled completely by infiltrator/penetrator component (i.e. melted copper). So infiltration efficiency is enhanced [14], [15], [16].

Active elements may be added to tungsten powder by the electroless plating technique. It has been documented that electroless nickel plating process can be successfully applied on the tungsten powder [16], [17].

This research is done with the aim of improvement of infiltration behavior of tungsten–copper composites as well as its electrical properties by electroless nickel plating of primary tungsten powder.

Section snippets

Experimental procedures

The characteristics of tungsten powder are presented in Table 1. The nickel and nickel–phosphorus coatings were separately employed on the primary tungsten powder using hydrazine and hypophosphite based electroless plating, respectively. Table 2 illustrates composition/conditions of plating processes. Before electroless plating, the powders were activated/sensitized by hydrofluoric acid [16], [17], [18].

The coated/uncoated tungsten powders were pressed to obtain compact material of cylindrical

Analysis

Scanning electron microscopy images of nickel and nickel–phosphorus coated tungsten powders in comparison with uncoated ones are shown in Fig. 2a–c. As seen, the morphological characteristics of nickel–phosphorus coated particles as well as degree of agglomeration of nickel coated particles are different with uncoated ones. The shape of tungsten particles can be changed from polyhedral to semispheroidal due to formation of continuous/homogenous nickel layer around tungsten particles. The

Conclusions

The effect of electroless nickel/nickel–phosphorus plating on the infiltration behavior and physical properties of infiltrated W–15 wt.% Cu composites was investigated. Following results were found:

1.
The morphology as well as the agglomeration degree of primary tungsten powder can be affected by electroless plating process.
2.
Improvement in infiltration behavior of primary tungsten powder may be attained by the presence of nickel/nickel–phosphorus (0.5–1 wt.%) on the surface of tungsten powder, due

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Improvement of physical properties of Cu-infiltrated W compacts via electroless nickel plating of primary tungsten powder

Abstract

Introduction

Section snippets

Experimental procedures

Analysis

Conclusions

Mater. Lett.

Mater. Lett.

Rare Met.

J. Univ. Sci. Technol. Beijing

I. J. Refract. Met. Hard Mater.

J. Mater. Process. Technol.

Scr. Mater.

Mater. Sci. Eng. A

Scr. Mater.

Int. J. Powder Metall.

J. Am. Ceram. Soc.

Powder Metall.