The Effect of Secondary Phases and Birefringence on Visible Light Transmission in Translucent α’-sialon Ceramics

Open Access
Author:
Morandi, Carl Sebastian
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 16, 2013
Committee Members:
  • Gary Lynn Messing, Thesis Advisor
Keywords:
  • α’-SiAlON
  • Ceramics
  • Light Transmission
  • Transparent α’-SiAlON
  • Translucent α’-SiAlON
  • AlN-Polytypoid
  • Microstructure Control
Abstract:
α’-SiAlON ceramics have the potential to replace modern transparent ceramic ballistic windows such as AlON, spinel, and sapphire due to their superior hardness and lower density. However, very limited research has been done to understand the processing-microscructure-property relationships that govern the optical properties of α’-SiAlON ceramics. In order to understand these relationships, this thesis explores the factors that control the formation of AlN-polytypoids and the dominant sources of visible light transmission loss in α’-SiAlON ceramics. A preliminary study was performed to produce transparent α’-SiAlON with equivalent or superior forward light transmission of α’-SiAlONs reported in the published literature. The resulting samples were analyzed using backscatter electron microscopy and energy dispersive spectroscopy. Residual AlN-polytypoid and amorphous glass were observed as secondary phases. Due to the lack of knowledge about the refractive indices for AlN-polytypoids, the effects of the AlN-polytypoids on visible light transmission in α’-SiAlON were explored by attempting to remove the phase from the microstructure. Based on a review of the literature, it is proposed that AlN-polytypoid formation occurs via heterogeneous nucleation on large AlN-particles. Various compositions were made to reduce the AlN-polytypoid concentration based on the α’-SiAlON formation reaction sequence and the heterogeneous nucleation hypothesis. Utilizing backscatter electron microscopy, the amount of AlN-polytypoids was determined for each composition. The results show that large AlN particles nucleate AlN-polytypoid phases. When utilizing excess liquid phase, the AlN-polytypoid concentration is dependent on the relative composition of the liquid phase. When the samples are AlN deficient, there is a significant decrease in the AlN-polytypoid concentration due to the increased driving force for AlN particle dissolution and the presence of fewer AlN particles. The optical properties of the compositionally shifted α’-SiAlON ceramics were compared utilizing a backlit background and viewing the textual contrast with the sample 10 cm above the background. UV-visbile spectroscopy was utilized to quantify the optical properties on the ceramics. A correlation was not found between the visible light transmission of the various compositions and the AlN-polytypoid concentration. Stronger absorptions and other potential scattering sources were observed in samples with excessive residual amorphous glass phase. A trend was found between the residual amorphous glass phase concentration and the light transmission, but only at high (>2-3 vol%) concentrations. It was concluded that neither the amorphous glassy grain boundary phase nor AlN-polytypoids were acting as the dominant light scattering source in this study’s α’-SiAlON ceramics. Absorption by free Si is believed to account for some of the transmission losses based on the absorbance peaks around 300 nm to 320 nm in all samples. The effects of birefringent scattering on the light transmission were explored by analyzing the grain size of the various α’-SiAlON ceramics. A general trend among all samples could not be found due to either the small differences in grain size or the presence of other scattering/absorption sites. The aluminum nitride deficient samples showed the largest changes in grain size and improvement in the transmission. Based on these samples, an attempt was made to reduce the grain size further by seeding the microstructure of α’-SiAlON with excess α-Si3N4 -from the commercial powder. The minimum α-Si3N4 nuclei size in the samples was determined to be ≈400 nm. The addition of 17.13 wt% excess α-Si3N4 powder with respect to the sample weight was predicted to reduce the grain size to 0.7 μm. The resulting seeded sample had a grain size of 0.77 μm. It was concluded that the larger particles in the initial α-Si3N4 powder limited the reduction of the average grain size. Additionally, x-ray diffraction found that J’ phase formed as an intermediate phase during sintering which may have further reduced the potential number of excess α-Si3N4 nuclei. Qualitatively, the excess Si3N4 sample shows better textual contrast than the AlN deficient samples when looking at backlit text, but the UV-visible spectroscopy analysis shows less total transmission in the seeded sample with respect to the AlN deficient samples, which was attributed to greater absorption processes from free Si. The improvement in the total transmission in the aluminum nitride deficient and seeded samples with respect to the other samples was attributed to a reduction in the grain size. It was concluded that birefringence and absoprtion processes by free Si were the dominant sources of visible light transmission loss in the samples.