Silicon nitride nanoceramics densified by dynamic grain sliding

Abstract

The densification behaviors of two silicon nitride nanopowder mixtures based respectively on a-Si3N4 and ß-Si3N4 as the major phase constituent were studied by spark plasma sintering. Sintering conditions were established where a low viscous liquid not in equilibrium with the main crystalline constituent(s) stimulated the grain sliding yet did not activate the reprecipitation mechanism that unavoidably yields grain growth. By this way of dynamic grain sliding full densification of silicon nitride nanoceramics was achieved with no noticeable involvement of a- to ß-Si3N4 phase transformation and grain growth. This processing principle opens the way toward flexible and precise tailoring of the microstructures and properties of Si3N4 ceramics. The obtained silicon nitride nanoceramics showed improved wear resistance, particularly under higher Hertzian stresses

Field‐Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

“…As was shown for Si 3 N 4 -materials nearly complete densification can be achieved by rearrangement accelerated by viscous flow of the liquid with less than 10 vol% of the grains originating from solutionprecipitation. [37,38] A similar behavior was found for liquid phase sintered SiC. In liquid-phase-assisted FAST/SPS grain growth can be minimized due to the applied pressure and fast heating, but the liquid phase can be partially squeezed out if its amount is too high or the pressure-time cycles are not selected carefully.…”

“…The early stages of densification of nanopowder compacts proceed either by the plastic deformation of particles, grain rotation, and sliding, aided by softening of the particle surfaces. [37,130] These processes allow fast densification with limited grain growth. In some cases, grain coarsening may be further reduced by optimizing the temperature schedule.…”

“…Large differences are commonly found between thermocouple and pyrometer measurements or between the temperature measured inside the tool and on the outer surface of the pressing die. By using the phase transformation from duplex a þ g-to lamellar a-structure in Ti 48 Al 48 Cr 2 Nb 2 to track the true temperature in the sample, Voisin et al [249] showed that the radial pyrometer underestimated the temperature by [250] This is due to the face that temperature was measured at different positions. Furthermore, Munir et al [251] found temperature differences of over 150°C when heating up an alumina sample at 200°C min -1 up to 1350°C and reading the temperature with a thermocouple inside the material and an optical pyrometer on the outer diameter of the die.…”

Tribological behaviour of Si3N4-based nanocomposites

“…In general, the nanocrystalline ceramics win in hardness and bend strength, when the so-called "size effect" appears, but lose in fracture toughness compared to coarse grained ceramics. Nanocomposites made with non-oxide ceramic compounds exhibit extremely high hardness and wear resistance, low specific gravity, and maintain mechanical properties at high temperatures without oxidation [1][2][3][4][5][6][7][8][9][10]. Unique properties of Si 3 N 4 -based nanocomposites found an application in novel wear-resistive components, capable to withstand severe operating conditions such as cryogenic and high temperatures, boiling sea water, bases, and acids [1][2][3][4][10][11][12][13].…”

“…Due to such selflubricating phenomenon, cylindrical rollers made with Si 3 N 4 -TiN composites exhibited enhanced tribological behaviour and 1.5-2 times higher lifespan compared with conventional Si 3 N 4 -based materials [3,6]. Traditionally, Si 3 N 4 -based nanocomposites for wearproof applications are manufactured using high pressure (HP) or spark plasma sintering (SPS) technologies [1][2][3][4]6,13]. These technologies, however, are limited due to strict requirements for the shape of consolidated parts and the necessity of special equipment.…”