Publication information Journal of Materials Processing Technology, 211 (7): 1210-1216Publisher Elsevier Item record/more information http://hdl.handle.net/10197/5260 Publisher's statementThis is the author's version of a work that was accepted for publication in Journal of Materials Processing Technology. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Materials Processing Technology, 211 (7) AbstractThere is considerable interest in processing technologies which can lead to more energy efficient sintering of abrasive metal matrix composites. In this study the use of a novel microwave plasma processing technique called Rapid Discharge Sintering (RDS) for sintering nickel-diamond metal matrix composites (MMCs) is evaluated. Nickel-diamond powder composites (80 -20 % by weight respectively) were uniaxially pressed into 20 mm discs at compaction pressures of 100, 200 and 300 MPa. The discs were sintered using a microwave plasma formed with hydrogen and hydrogen/nitrogen as the discharge gases. For comparison, discs were also sintered using a tube furnace in a gas flow of hydrogen and nitrogen (3:1). Discs pressed to 300 MPa were treated at both 850 and 1000 o C. The properties of the sintered nickel-diamond composites were characterised using density, flexural stress, hardness, wear resistance, SEM and XRD. The RDS samples sintered at 1000 o C achieved the maximum disc strength of approximately 470 MPa within a 20 minute chamber processing time, compared with 6 hours for furnace sintered samples. RDS samples exhibited increased hardness values and a finer nickel matrix over furnace sintered samples. RDS has shown the ability to process nickel-diamond MMCs without oxidation or graphitisation at higher temperatures. As a result, minimal diamond destruction was observed during abrasive wear testing for RDS samples.

Bioactive glass-ceramics have been developed as successful bone graft materials. Although conventional sintering in an electrically-heated furnace is most commonly used, an alternative microwave plasma batch processing technique, known as rapid discharge sintering (RDS), is examined to crystallise the metastable base glass to form one or more ceramic phases. Apatite-mullite glass-ceramics (AMGC) were examined to elucidate the effects of RDS on the crystallization of a bioactive glass-ceramic. By increasing the fluorine content of the glass, the fluorapatite (FAp) and mullite crystallization onset temperatures can be reduced. Samples were sintered in a hydrogen and hydrogen/nitrogen discharge at temperatures of ≈800 and 1000 °C respectively with the higher sintering temperature required to form mullite. Results show that the material can be densified and crystallised using RDS in a considerably shorter time than conventional sintering due to heating and cooling rates of ≈400 °C/min.

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There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. Microwave sintering has recently been shown to reduce energy usage as the volumetric heating process is considerably more efficient than resistance heating. RF plasma sintering meanwhile has been shown to deliver heat via uniform excitation of the processing gas resulting in ion bombardment of the workpiece. In this study the use of a rapid, novel microwave-assisted plasma sintering (MaPS) technology for processing of nickel-diamond metal matrix composites is evaluated. Nickel powder and polycrystalline diamond were mixed to prepare 20 mm discs under uniaxial compaction pressures of 100, 200 and 300 MPa. The discs were fired in a low pressure microwave plasma under a hydrogen atmosphere. For comparison, discs were also sintered using conventional tube furnace firing. The MaPS sintering is very rapid with full disc strength of >1000N, based on 3-point bend tests, being achieved within 10 minutes compared with 8 hours for furnace treatment. This study demonstrates that the microwave-assisted plasma sintered discs produced similar or superior performance to discs fired using furnace firing conditions but with sintering cycle time reduced by up to 95%.

There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. The use of microwave sintering in particular leads to reduced energy usage during sintering as the volumetric heating process is considerably more efficient compared with resistance heating. In this study the use of a novel plasma microwave processing technology for the sintering of nickel powder discs is evaluated. The sintering study was carried out on 20 mm diameter by 2 mm thick pressed discs of nickel powder, with mean particle size of 1 µm. The discs were fired in a 5 cm diameter microwave (2.45 GHz) plasma ball under a hydrogen atmosphere at a pressure of 2 kPa. The same discs were also sintered using both non plasma microwave and tube furnace firing. The microwave plasma sintering is very rapid with full disc strength of approx. 1000 N based on 3--point bend tests being achieved within 10 minutes. In contrast the sintering time in the tube furnace involved treatments of up to 6 hours. The non plasma microwave system involved intermediate treatment periods of 1 to 2 hours.Another advantage of the microwave plasma treatment is that the degree of sintering between the individual nickel powder particles can be precisely controlled by the duration of the treatment time in the plasma. There was a broadly linear increase in fired pellet breaking strength with plasma treatment duration. In addition to breaking load, the mechanical properties of the sintered nickel discs were compared based on Rockwell hardness tests and density measurements. The morphology of the sintered discs was compared using microscopy and SEM.This study demonstrated that the plasma microwave sintered discs produced similar or superior performance (depending on processing conditions) to discs fired using the non-plasma microwave and furnace firing conditions. Accurate control of the sample conditions and structure can easily be controlled with the plasma system compared with the conventional systems. The apparent volumetric heating in the microwave systems give a more uniform heating at lower temperatures and allows for greater control and homogeneity.