Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17¡2 nm and dislocation density of 6?26¡0?04610 16 m 22 ) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400uC. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.

Spark plasma sintering behaviour of copper powders having different particle sizes and oxygen contents

“…The nanopores in the sintered specimens may originate from both the internal voids of the powder agglomerates and by the entrapment of gases during sintering [15,16,22]. Intra-agglomerate pores may only be eliminated if the agglomerates are fragmented during sintering; the other pores are mostly formed during sintering if the products of degassing are entrapped by the densifying material when the process is not properly conducted (i.e., the gas emission is prevented by the application of pressure) [16].…”

Effect of sintering temperature on the microstructure and mechanical properties of Fe–30%Ni alloys produced by spark plasma sintering

“…1). These unique features of SPS result in a low ''heat input'', which is a favorable prerequisite to sinter powders with a controlled grain size [18][19][20][21][22] and limited chemical interaction with different constituents [22,23] . In SPS a minimum pressure is required to establish the electrical contact between punches and powders and densification is progressively promoted by rearrangement of the powder particles, localized deformation at the contact points and bulk deformation of the particles [24] .…”

A critical review on spark plasma sintering of copper and its alloys

“…Accordingly, the greater resistance of the contact points enhances the bulk compact electrical resistance when a pulsed direct current is passed through the particles. Hence, such an electrical current can produce more homogenous heat generation and becomes more suitable for sintering [59,[75][76][77][78].…”

“…According to the Hall-Petch relationship, finergrained compacts are more resistant to plastic deformation at low temperatures [73,75]. However, it is demonstrated that the strengthening effect of the grain boundaries turns into softening effect at the evaluated temperatures, where annihilation of dislocations is facilitated by climb at grain boundaries [79].…”

“…Another factor influencing the sintering behavior of Cu powders is the contamination content, which predominantly originates from the raw powder. Contaminants and impurities on the particle surface can adversely affect the sinterability of Cu powders owing to several factors: (i) They can act as a barrier for electrical and thermal flows [82]; (ii) they can effectively decelerate all of the mass transport mechanisms responsible for neck formation between Cu particles [78]; and (iii) they can form pores and increase the sample porosity due to the decomposition of thermally unstable compounds during SPS treatment [75]. One of the most critical contaminating sources is oxygen or the presence of oxides on the superficial layers of Cu particles.…”