Direct 3D Printing of Silica Doped Transparent Magnesium Aluminate Spinel Ceramics.

John M Pappas,Xiangyang Dong

doi:10.3390/ma13214810

Abstract

Transparent magnesium aluminate spinel ceramics were additively manufactured via a laser direct deposition method in this study. With a minimum porosity of 0.3% achieved, highly transparent spinel samples with the highest total optical transmittance of 82% at a wavelength of 632.8 nm, were obtained by a 3D printing approach. However, cracking was found to be a major issue affecting printed spinel samples. To control prevalent cracking, the effect of silica dopants was investigated. Increased silica dopants reduced average total crack length by up to 79% and average crack density by up to 71%. However, a high dopant level limited optical transmission, attributed to increased porosity and formation of secondary phase. Further investigation found that with decreased average fracture toughness, from 2.4 to 1.9 , the obvious reduction in crack formation after doping was related to decreased grain size and introduction of softer secondary phase during deposition. The study demonstrated the feasibility of the proposed laser direct deposition method in directly fabricating transparent spinel ceramics while dopants showed potentials in addressing cracking issues.

Highlights

Transparent ceramics are of great interest for a variety of applications including transparent armor [1,2] as well as windows and domes for aircraft and defense [3,4,5]
As observed in both pure spinel and defect of of printed cylindrical samples presence a non-transparent region on top, the top, i.e., thethe printed cylindrical samples waswas thethe presence of aofnon-transparent region on the i.e., “hat formation of this region wasattributed attributedtotoshrinkage shrinkage cavities cavities formed formed during region”
Shows crack density measured for spinel samples printed at a laser power length, and (B) shows crack density measured for spinel samples printed at a laser power of of 580

Summary

Introduction

Transparent ceramics are of great interest for a variety of applications including transparent armor [1,2] as well as windows and domes for aircraft and defense [3,4,5]. The most prominent candidates include magnesium aluminate spinel (MgAl2 O4 , referred to as “spinel”), aluminum oxynitride (ALON), and sapphire. Cubic-polycrystalline transparent ceramics like spinel and ALON have several distinct advantages over non-cubic ceramics like sapphire (alumina). Non-cubic transparent ceramics are birefringent in nature [6]. Fabrication of polycrystalline alumina presents its own challenges, requiring grains smaller than one-tenth the optical wavelength of interest to achieve transparency [6,10,11]. Cubic-polycrystalline transparent ceramics including spinel and ALON do not exhibit birefringence. Solid solution spinel is typically represented as MgO·nAl2 O3 , and the useful range for transparent ceramics is typically 0.98 < n

Methods

Results

Conclusion