添加La_(2)O_(3)快速无压烧结制备高红外透过率AlON陶瓷

Highly Infrared Transparent AlON Ceramics Doped with La_(2)O_(3) via Fast Pressureless Sintering

利用La_(2)O_(3)作为烧结助剂,分别采用D50为1.1μm和2.0μm的AlON粉体,在1880℃保温2.5 h快速无压烧结成功制备了红外透过率达80%~84%(3850 nm处)的AlON陶瓷。基于AlON粉体粒度研究了La_(2)O_(3)掺量对陶瓷透过率的影响规律,并通过研究高透光性AlON陶瓷烧结过程升温阶段的物相组成、微观结构及致密化进程,揭示了其致密化机制。结果表明,La_(2)O_(3)对AlON分解具有一定的抑制作用,使样品中的α-Al_(2)O_(3)含量较低,从而避免了颗粒的早期团聚、粗化等,为后期致密化烧结奠定了良好的基础,同时,高致密度高透光性AlON陶瓷的快速无压烧结制备表明La_(2)O_(3)能为后期烧结提供充足的动力。

Introduction Transparent aluminum oxynitride(AlON)ceramics have superior optical transparency properties,high strength and hardness,having great potential applications as window,dome,and other optical components requiring high strength combined with optical transparency.Recent efforts are made to fabricate transparent AlON ceramics,i.e.,synthesis of high purity AlON powder,optimization of the particle size distribution of AlON powder,and doping with various additives in different amounts.In these works,sintering additives,such as Y_(2)O_(3),Y_(2)O_(3)/La_(2)O_(3),Y_(2)O_(3)/La_(2)O_(3)/MgO,etc.,are commonly used to accelerate densification process of Al ON.However,a long dwelling duration of≥6 h is generally required to eliminate pores to obtain a high transmittance in pressureless sintering.Although the dwelling duration of pressureless sintering AlON ceramics can be shortened to 2.5 h,it is crucial to carefully match the doping amount of Y_(2)O_(3) with the particle size of AlON powder.The excessive Y_(2)O_(3) doping can result in a significant decomposition of AlON intoα-Al_(2)O_(3) and Al N,and the massive formation ofα-Al_(2)O_(3) can lead to particle aggregation/coarsening,and even separation between components during the early stage of sintering,which in turn retards the subsequent densification process.Studies on using CaCO_(3) as an additive to fabricate AlON ceramics indicate that it should be a promising way to fabricate highly transparent AlON ceramics via suppressing AlON decomposition during heating with sintering additive.In this paper,La_(2)O_(3) was respectively doped to AlON powders with median particle size(D_(50))of 1.1μm and 2.0μm to prepared transparent AlON ceramics via pressureless sintering(PS)at 1880℃for 2.5 h.The effect of La_(2)O_(3) doping amount on the transmittance of AlON ceramics was investigated,and the phase composition,microstructure and densification process of samples during heating were monitored to investigate the fast densification mechanism of AlON.Methods Pure AlON powder and La_(2)O_(3) powder were used as starting materials.The AlON powders doped with 0–0.50%La_(2)O_(3)were ground in absolute ethyl alcohol in a grinding mill with Si_(3)N_(4) balls,and AlON powder with D_(50)of 1.1μm and 2.0μm were obtained,referred to as P1.1 and P2.0,respectively.The ground powders were loaded into a pellet of 13 mm in diameter at 50 MPa and then cold isostatically pressed at 120 MPa to prepare green bodies.The green bodies were pressureless sintered in N_(2) in a graphite furnace at a heating rate of 20℃/min.One group of samples were sintered at 1880℃for 2.5 h to prepare transparent AlON ceramics,and another group samples were respectively heated to 1400–1900℃(no dwelling)to investigate the phase transformation,microstructure evolution and densification process during heating.The transparent AlON ceramics were ground and mirror polished on the both sides to a thickness of 2 mm for the optical transmittance measurement.The polished samples were hot etched at 1640℃for 40 min to determine their microstructure.Results and discussion After a 2.5 h dwell at 1880℃,all the La_(2)O_(3) doped AlON ceramics prepared(i.e.,P1.1 and P2.0)show higher transmittance than their undoped counterparts,and the transmittances at 3850 nm enhance from 17%and 0%to 80%and 84%,respectively.Moreover,the doping of La_(2)O_(3) broadens the transmission wavelength range of the fabricated samples at 2500-6000 nm for P1.1 and the optical range at 220–6000 nm for P2.0.Doping amount of La_(2)O_(3) has a similar effect on the transmittance of AlONceramics prepared by P1.1 and P2.0.When the doping amount of La_(2)O_(3)≤0.15%in P1.1 and≤0.40%in P2.0,increasing the doping amount of La_(2)O_(3) contributes to the enhanced transmittance of the AlON ceramics.Doping 0.15%–0.25% and 0.40%La_(2)O_(3) into P1.1and P2.0,respectively,results in the transmittances of AlON ceramics up to 80%–84%at 3850 nm.The high transmittance of fabricated AlON ceramics is mainly due to thehigh relative density.For the two highly transparent AlON ceramics prepared by P1.1doped with 0.20%La_(2)O_(3) and P2.0 doped with 0.40%La_(2)O_(3),their average grain sizes are 97.3μm and 60.2μm,and their Vickers hardness values are(16.00±0.31)GPa and(16.36±0.31)GPa.For the samples prepared by P1.1 doped with 0.20%La_(2)O_(3) and by P2.0 doped with 0.40%La_(2)O_(3),although AlON undergoes decomposition and reformation during heating.α-Al_(2)O_(3) content in the samples does not exceed 25.1%.Moreover,a lower amount ofα-Al_(2)O_(3) appears in the sample P2.0,compared to the sample P1.1 at 1400–1700℃.Furthermore,compared to the samples without additive doping,the two samples above exhibit lowerα-Al_(2)O_(3) contents at≤1600℃.It is indicated that the decomposition of AlONduring heating is slightly suppressed by doping La_(2)O_(3) as an additive.No obvious particle aggregation/coarsening or no composition decomposition occur on the fracture surface of the samples due to the reducedα-Al_(2)O_(3)content in the samples.It is beneficial for the subsequently densification process.Conclusions AlON ceramics with a high infrared transmittance of 80%–84%at 3850 nm were fabricated via pressureless sintering at 1880℃for 2.5 h after doping 0.15%–0.25%and 0.40%La_(2)O_(3) to AlON powders with D_(50)of 1.1μm and 2.0μm,respectively.During heating,La_(2)O_(3) could suppress the decomposition of AlON and lead to a lower amount ofα-Al_(2)O_(3) being decomposed from AlON.This could reduce the particle aggregation/coarsening and even the separation of composition at the early stage of sintering.The obtained exceptional microstructure was beneficial for the subsequent fast densification.Moreover,La_(2)O_(3) could provide sufficient sintering kinetics to effectively eliminate pores at the later stage of sintering,thus promoting the densification of AlON.