Processing by Cold Sintering Reduction of Surface Oxides

Consolidation to full density requires densification with the formation of strong adhesive bonds between the particles. When oxide- and contamination-free surfaces are brought into intimate contact, strong chemical bonds arise resulting in good integrity among the powder particles. Plastic deformation, through the breaking of oxide layers, can result in an intimate contact between the freshly formed surfaces. At higher temperatures, diffusional flow can result in dissolution or spheroidization of oxide layers on metal particle surfaces. Processing routes involving a pronounced shear deformation provide better conditions for breaking of surface oxide.

For a number of pure metals and alloys, oxide layers can be removed by heat treatment in hydrogen or in vacuum at relatively low temperatures, which guarantees the retention of the fine microstructure. Adhesion is considerably improved after such treatment. This has been demonstrated by the high bending strength (irTRs > 600 MPa) and high ductility (>20%) of cold sintered iron, nickel, and copper powders treated in hydrogen prior to consolidation (Ref 13). The formation of bonded chains of very fine oxide-free particles produced by vapor condensation was observed for Fe, Co, Ni, Ag, Cu, Mg, Zn, and Al by Tholen (Ref 27). For a number of alloys (e.g. aluminum and magnesium alloys), oxide layers cannot be removed by heat treatment. For stainless steels, chromium-containing high-strength or high-speed steels, superalloys and titanium alloys, reduction of surface oxides can require unduly high processing temperatures (Ref 28, 29, 30).

Good adhesion is also important to prevent partial fracture of adhesive bonds as a result of the elastic springback of consolidated powder particles. Elastic springback can be responsible for the lower effective density of consolidated powders after unloading, as compared to the density measured under load. A higher density was obtained in copper and nickel powders high pressure consolidated at 2 GPa, when the powders were treated in hydrogen at T = 200 °C and kept in vacuum prior to consolidation, as compared to the powders left in open air for h after the H2 treatment (—99.5 versus ~-98.5%). The effect of springback is even more pronounced when consolidation of nanoscale metal powders is considered.

Reduction diagrams can be used to establish temperature limits within which surface oxides on metal, carbide, or nitride powders are reducible (Ref 30, 31). An apparatus used for experimental determination of such diagrams is shown schematically in Fig. 7. A sample (a loose powder or a compact with an interconnected system of pores) is placed into a quartz reactor in a stream of hydrogen. The temperature of the furnace can be increased at a constant rate, so that the reduction behavior can be studied with a steady increase of temperature. In the hydrogen atmosphere, reduction of oxides, sulfides, carbides, etc. can take place, as well as reaction of hydrogen with dissolved carbon, nitrogen, or other elements with the formation of water vapor, hydrogen sulfide, methane, and other gases. The amount of the gaseous reaction product is proportional to the amount of the compound reacted with hydrogen. For example, the amount of water vapor is proportional to the amount of oxides reduced. Reaction kinetics can be studied by measuring the amount of gas released as function of time at a constant temperature. This is done by connecting the reactor to a thermal conductivity (TC) detector; the TC signal being proportional to the amount of evolved gases. The starting temperature for a reaction depends on thermodynamic parameters of the reagents and products. Thus, for each reaction, a peak of the TC signal versus temperature can be found within a specific range. Peaks corresponding to different reactions can be separated by using a number of cold traps at different temperatures. For example, if a dry ice cold trap is placed between the reactor and the TC detector, water vapor is removed and the peak corresponding to oxide reduction does not appear in the diagram. At the same time, peaks corresponding to reactions of hydrogen with carbon (CH4), nitrogen (NH3), etc. will remain. Examples of reduction diagrams for high-speed T15 steel and 410L stainless steel (12 wt% Cr) are shown in Fig. 8. In both cases, the first peak corresponds to the removal of iron oxide and the second to the reduction of mixed oxides (e.g. iron-chromium spinel, FeCr204). Figure 9 shows the room temperature transverse rupture strength, (Tms, of cold sintered T15 and 410L steel samples as a function of the preconsolidation reduction treatment temperature (80% dense compacts in hydrogen flow, 1 h). In both cases, an increase of £T"trs was observed in agreement with the reduction peaks in Fig. 8. T15 samples cold sintered after reduction treatment at T> 1000 °C measured irTRS > 600 MPa, and a very high value of £7xrs > 1500 MPa was obtained for 410 steel cold sintered after reduction treatment at T > 1100 °C. The high strength of these cold sintered steels is the result of good adhesion between the surface of oxide-free alloy particles.

Fig. 7 Schematic of the apparatus used for investigating the reactions of hydrogen with powders and powder compacts and for construction of reduction diagrams
Fig. 8 Reduction diagrams for 80% dense compacts with an interconnected system of pores (water-atomized powders) (a) high speed steel T15, and (b) 410L stainless steel
Fig. 9 Room temperature transverse rupture strength, (Ttrs, as a function of the reduction heat treatment temperature (1 h anneal in an H2-flow)

The apparatus for reduction studies shown in Fig. 7 was also used for the investigation of decomposition of hydrates in aluminum alloys (Ref 4). Removal of hydrates formed on the surface of aluminum alloy powders in the process of atomization is important for improving the bonding integrity of consolidated materials (Ref 32, 33). Degassing leading to dehydration of atomized aluminum alloys is usually done at temperatures >400 °C, which is higher than the planned service temperature. At such temperatures the coarsening of fine metastable microstructures can take place (Ref 4). Decomposition diagrams of Al(OH)3 and of hydrates obtained employing the apparatus with the TC detector for a number of aluminum alloy powders are shown in Fig. 10. It can be seen that, for a number of the aluminum alloys, degassing can be performed at <300 °C.

Fig. 10 Decomposition diagram for aluminum hydrates obtained by employing the apparatus shown in Fig. 7

Determination of a temperature range necessary for the reduction of surface oxides is important for retention of fine microstructures of rapidly solidified/mechanically alloyed or of blends of very fine powders.

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