Low Temperature Preparation Of Yttrium Aluminum Garnet Transparent Ceramics And Investigation On The Densification Mechanisms

Yttrium aluminum garnet(Y₃Al₅O₁₂,YAG)ceramics with excellent optical and mechanical properties have been considered as a promising class of transparent materials.High-performance transparent ceramics with good optical transmittance show much better mechanical properties?e.g.hardness and thermal shock resistance?over the conventional single crystal or glass.It is noted that the mechanical properties are closely related to the grain size.And reducing grain size without deterioration of the densification contributes to the increase of mechanical properties.However,in the conventional route,highly transparent YAG ceramics were prepared by the vacuum sintering or hot isostatic pressing?HIP?in the presence of additives such as SiO₂ and MgO.These sintering techniques usually involve prolonged dwell time at relatively high temperatures?>1700°C?,which may result in coarse-grained ceramics.It is suggested that phase transformation-assisted spark plasma sintering has been available to fabricate fully dense ceramics with fine microstructure at relatively low temperatures.During phase transformation accompanied by the bond breakage and rearrangement,the atoms are very mobile,and an enhanced densification rate is expected at lower temperatures.The metastable YAlO₃?YAH?powders were synthesized by a novel method combining sol-gel combustion and field-assisted rapid synthesis technique.And transparent YAG ceramics were prepared by phase transformation-assisted spark plasma sintering using the synthesized YAH powders as raw materials.The sintering process shows two-step shrinkage.It is suggested that the densification curve of the first shrinkage corresponds to the transformation from the metastable hexagonal phase to the garnet phase accompanied by particle rearrangement.The second shrinkage is characterized by a rapid densification of the YAG phase.During sintering,the YAH phase transforms to the YAG phase,which leads to the enhancement of densification rate.And the sintering temperature can be decreased by approximately 300°C compared with the sintering of YAG powders.The main results are described as follows:?1?The phase evolution by field-assisted rapid synthesis technique was investigated using the gel-combusted powders as raw materials.For comparison,phase transformation by conventionally slow heating was studied.The effect of synthetic process on crystallization route was also discussed.The results indicate that the crystallization pathways are closely related to the atmosphere and heating rate.In an oxygen-free atmosphere,phase evolution via the intermediate YAH phase is observed,which completely transforms to the YAG phase at higher temperatures.It is noted that phase formation follows an amorphous?YAH?YAG route in the case of field-assisted rapid heating under vacuum.However,there are two crystallization routes by conventionally slow heating.YAG is directly crystallized from amorphous precursors without any intermediate phases in air.While the amorphous?YAH+YAG?YAG route is followed in Ar,the YAH phase co-exists with YAG phase.Moreover,hexagonal YAH is a metastable phase and only exists with rapid heating for a short span of time.?2?The effects of ball-milling process on the calcined YAH powders as well as the subsequent sintering process were systematically studied.The key to the ball-milling is that the YAH phase should transform in a controlled way during sintering to the YAG phase.Based on the results of thermal analysis,the influence rule of ball-milling on the phase transformation of YAH?YAG was established.It is noted that the activation energy of phase transformation decreases as the increase of ball-milling speed.As a result,the two-step densification of YAH powders changes suddenly to one-step of rapid densification.The extremely rapid shrinkage during sintering suggests that the exterior of the sample densifies much faster than the interior.Consequently,the fully dense outside will constrain the inside to further densification.Hence large pores trapped in the interior lead the sample to be opaque.Only proper ball-milling parameters would enhance phase transformation,hence promote densification.The homogeneous microstructure could be obtained by optimizing the ball-milling parameters.?3?Transparent YAG ceramics were prepared by spark plasma sintering assisted by phase transformation from YAH to YAG.A systematic study of temperature,dwell time,pressure and heating rate was conducted to their effect on microstructure and transmittance.The samples sintered at 1050°C with a heating rate of 10°C/min for 20 min under 80 MPa have the homogeneous microstructures with the average grain size of 375 nm.The Vickers hardness reaches 16.67±0.7 GPa.And the in-line transmittances are 55%and 71%at 680 nm and 1000 nm,respectively.?4?Based on the Coble creep model and S.L.Kang pore-dragged model,the densification mechanism and grain growth kinetics of phase transformation-assisted spark plasma sintered YAG ceramics were studied.As a result,densification mechanism for YAG ceramics is controlled by grain-boundary diffusion.As the sintering proceeds,the grain growth is inevitable during the final stage.The grain growth kinetics is dominated by grain-boundary migration.