Effect of hot pressing and hot isostatic pressing on the microstructure, hardness, and wear behavior of nickel
Introduction
Nickel is widely used in powder metallurgy as an alloying element for sintered steels [1], [2], in the form of self-lubricating nickel composites [3] and nickel-based super alloys [4], amongst other applications [5], [6]. Nanocrystalline nickel and Ni-based coatings have been proven to have an excellent tribological behavior in pin-on-disk and scratch tests [7], [8]. It has been found that the coefficient of friction in these coatings decrease with crystallite size due to the short range deformation processes undergone by Ninc during tribological testing when compared to the energy consuming large scale plastic deformation process present in conventional microcrystalline Nickel [9].
However, up to this point the feasibility of using Ni(nc) is limited to coatings, as bulk-processing techniques does not usually allow the obtaining of nanocrystalline Ni with complex geometries due to grain growth during processing [10]. Powder metallurgy (PM) allows obtaining bulk materials with complex geometries while using low temperatures. Hot pressing (HP) is a PM effective technique which allows obtaining fine grained materials through rapid heating of powders under moderate loads [11]. Another interesting technique for densification of PM materials, is Hot Isostatic Pressing (HIP) which consist in placing a preform into a high pressure chamber, where high pressure and temperature is applied through gas, allowing to obtain almost completely dense parts [12].
The aim of this study is to obtain nanocrystalline Ni trough high-energy milling and then sintering it using the HP and HIP techniques in order to characterize their microstructure, mechanical properties, and tribological behavior.
Section snippets
Experimental
Ni powders (99% at., <10 µm, Merck) were mechanically milled under argon atmosphere using a SPEX 8000D high-energy mill. The milling was conducted using ball/powder ratio was 10:1, Stearic acid being added as a controlling agent, and the milling time of five hours. The resulting powders were sintered using Uni-axial Hot Pressing (HP) at a pressure of 900 MPa at 300 °C and 2 min of holding time [13], and also using Hot Isostatic Pressing (HIP) at a pressure of 150 MPa and 900 °C for 1 h.
The
Results and discussion
Fig. 1a displays the X-ray diffraction patterns of nickel before and after the different consolidation methods. The milling process produced a decrease in the crystallite size, as well as an increased microstrain, which is reflected in the broadening of the peaks than the as-received sample. After sintering, the crystallite size increases and the number of crystalline defects is reduced, this produces narrow and slightly more intense peaks. Furthermore, Ni_HP presents small amounts of
Conclusions
Ninc samples were prepared by high-energy milling, hot pressing, and hot isostatic pressing. The HIP samples were more densified than the HP samples; however, the hardness and elastic modulus were higher for the HP samples due to smaller crystallite sizes and higher microstrain. The higher hardness of the HP specimens translated into lower COF and wear after the scratch tests. The results of this study show that high-energy milling, combined with hot pressing, is a powerful method for producing
CRediT authorship contribution statement
Carola Martínez: Conceptualization, Methodology, Validation, Investigation, Writing - review & editing, Visualization. Francisco Briones: Methodology, Validation, Writing - original draft. Claudio Aguilar: . Nicolás Araya: Validation, Writing - original draft. Iñigo Iturriza: Provision of resources for fabrication of nickel samples. Izabel Machado:Auxiliary edition of the manuscript. Paula Rojas:Resources, Auxiliary edition of the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financial support by FONDECYT (Grant 3140207; 1130475) and PhD student, CNPq and BNDES.
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