The influence of oxygen and nitrogen contamination on the densification behavior of cryomilled copper powders during spark plasma sintering

Abstract

It has been found difficult to fully densify some mechanically milled pure metal powders by spark plasma sintering (SPS). In this study, the densification behavior of cryomilled, nanostructured (NS) Cu powders during SPS was related to changes to the chemistry of the powders. The results showed that the presence of very small amounts of O and N in the powders, which were introduced during cryomilling and handling, significantly influenced the densification response. Moreover, reduction/removal of O/N via thermal annealing of the powders before SPS led to complete densification of the powders during subsequent SPS. The mechanisms responsible for this behavior were ascertained: O and N existed in the cryomilled powders in the form of thermally unstable compounds, and the subsequent thermal decomposition of these compounds during SPS generated the gaseous species, leading to porosity formation and incomplete densification; annealing of the powders before SPS removed the gases which resulted from thermal decomposition, thereby facilitating complete consolidation during subsequent SPS.

Influence of Ti addition and sintering method on microstructure and mechanical behavior of a medium-entropy Al 0.6 CoNiFe alloy

“…(2), the relative densities of HP'ed and SPS'ed Al 0.6 CoNiFe mediumentropy alloy and HP'ed Ti 0.4 Al 0.6 CoNiFe HEA are 97.8%, 98.3%, and 98.3%, respectively. It is evident that SPS'ed Al 0.6 CoNiFe shows higher density than the HP'ed counterpart, which may be attributed to the fact that mass transport and therefore densification is enhanced during SPS [36,37]. Fig.…”

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].…”

“…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].…”

“…Many researchers have focused their attention on the spark plasma sintering behaviour of ultra-fine and nanosized copper [10][11][12][13][14][15][16], paying attention mainly to the evolution of the grain size and the microstructural characteristics during sintering. Less attention has been dedicated to the study of the influence of the powder oxygen content on the milling and sintering processes.…”

Spark plasma sintering of cryomilled copper powder

“…20,21 The cryomilled powder is contaminated by nitrogen and oxygen. The influence of these elements on sintering and densification by SPS was investigated by Wen et al: 22 the adsorption of oxygen and nitrogen at powder surfaces leads to the formation of thermally unstable compounds; their decomposition during SPS produces gaseous species that cause the formation of pores and reduce the final density attainable. Figure 5 shows the results of DSC and TGA measurements, along with the records of the displacement of the lower punch and of the chamber pressure during SPS.…”