Polycrystalline Magnesium Aluminate Spinel Research Articles

Force rheological polishing of polycrystalline magnesium aluminate spinel using agglomerated diamond abrasive

The high hardness and polycrystalline structure of polycrystalline magnesium aluminate spinel (PMAS) present significant challenges in achieving high-precision processing, particularly during polishing. Issues such as low processing efficiency and grain effects can compromise processing accuracy, surface quality, and optical performance. To address these challenges and enhance both polishing efficiency and surface quality, this study employed agglomerated diamond (AD) abrasives and single crystal diamond (SCD) abrasives in force rheological polishing (FRP). The experimental results revealed that the lowest surface roughness (Sa) and optimal selective material removal were achieved using 25 μm AD abrasives, particularly those agglomerated with 0.2 μm SCD abrasives. This approach reduced Sa from 146.1 nm to 4.6 nm, peak-to-valley height difference (Sz) from 1329 nm to 69.3 nm and achieved a material removal rate (MRR) of 228.8 nm/min. Conversely, polishing with 1 μm SCD abrasives resulted in poorer selective material removal, with Sz decreasing from 1265.1 nm to 280.9 nm and Sa decreasing from 138.3 nm to 27.6 nm, although it exhibited the highest MRR of 323.64 nm/min. The superior polishing results obtained with AD abrasives, compared to SCD abrasives of the same size, can be attributed to the polycrystalline-like structure of AD abrasives, which exhibit more micro cutting edges. Additionally, the surface and subsurface damage after polishing with AD abrasives was analyzed. The results indicated that material removal of PMAS using AD abrasives was effective in FRP, resulting in minimal lattice distortion. This study provides both theoretical and experimental foundations for polishing PMAS using larger-sized AD abrasives in FRP.

Two-Photon Polymerization of Nanocomposites for Additive Manufacturing of Transparent Magnesium Aluminate Spinel Ceramics.

Transparent polycrystalline magnesium aluminate (MAS) spinel ceramics are of great interest for industry and academia due to their excellent optical and mechanical properties. However, shaping of MAS is notoriously challenging especially on the microscale requiring hazardous etching methods. Therefore, a photochemically curable nanocomposite is demonstrated that can be structured using high-resolution two-photon lithography. The printed nanocomposites are converted intro transparent MAS by subsequent debinding, sintering, and hot isostatic pressing. The resulting transparent spinel ceramics exhibit a surface roughness Sq of only 10nm and can be shaped with minimum feature sizes of down to 13µm. This technology will be important for the production of microstructured ceramics used for optics, photonics, or photocatalysis.

Transparent LiOH-doped magnesium aluminate spinel produced by spark plasma sintering: Effects of heating rate and dopant concentration

The effects of LiOH doping of magnesium aluminate spinel powders and various Spark Plasma Sintering (SPS) schedules on densification behavior and final transparency of polycrystalline magnesium aluminate spinel were studied. Two commercial magnesium aluminate spinel powders, with different specific surface areas, were doped with up to 0.6 wt% of LiOH and consolidated using SPS with slow (2.75 °C/min) and fast (100 °C/min) heating rates. The slow heating rate was optimal for undoped magnesium aluminate spinel (LiOH-free) with the best real in-line transmittance (RIT) of 84.8% (measured at 633 nm on a disc 0.8 mm thick). For the magnesium aluminate spinel doped with 0.3 wt% of LiOH, the fast heating rate was beneficial, and an RIT of 76.5% was achieved. μ-Raman analysis confirmed that the addition of LiOH suppressed carbon contamination.

Sulfuric acid modified magnetorheological finishing of polycrystalline magnesium aluminate spinel

Polycrystalline magnesium aluminate spinel has excellent optical, mechanical, and chemical properties; making it an ideal material for critical optical parts under extreme conditions, and it has broad application prospects in aerospace. The high hardness and polycrystalline nature of the material bring significant challenges to high-precision spinel processing, especially in magnetorheological finishing (MRF), where low processing efficiency, polishing ripples and grain effect appear during polishing, leading to degradation of processing accuracy, surface quality and optical performance. To achieve high-efficiency and high-quality MRF of spinel, a new method of surface modification of spinel with sulfuric acid followed by MRF is proposed. The effect of sulfuric acid modification on the removal efficiency and surface roughness of spinel MRF was studied. Furthermore, the material properties of the spinel surface before and after modification were analyzed by nanoindentation, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Additionally, the mechanism of sulfuric acid etching assisted MRF was investigated. The sulfuric acid modification softens the spinel surface and removes the surface damage layer, and the modified layer can be used as a sacrificial layer to suppress the grain effect. The results showed that the sulfuric acid modification improved the MRF efficiency by 2.46 times, and decreased the peak-valley values of the grain effect and the average residual stress by 69.6% and 54.5%, respectively.

From nanoporous to transparent MgAl2O4 spinel—Nanostructural flexibility by reaction densification of metastable powders

Polycrystalline magnesium aluminate spinel (MgAl2O4) is one of the most important structural ceramics with proven applications as a hard, temperature resistant material and as a high strength optical window. Here, we present a study of the reaction densification of nano-sized MgO (nMgO) and metastable γ-Al2O3 to form MgAl2O4 instead of the typical stable α-Al2O3 and micrometer-sized MgO (μMgO). We show that the reaction kinetics of γ-Al2O3 – nMgO are substantially faster and occur at significantly lower temperatures compared to previous kinetic studies of α-Al2O3 – μMgO, revealing that they are not controlled by the same mechanism. We propose that the enhanced reaction rates are caused by short diffusion distances, faster diffusion through defected γ-Al2O3 as well as similarities in crystal structure between products and reactants. Lastly, we show that understanding the reaction and densification kinetics of this reactant combination can be leveraged to produce a wide range of microstructures leading to nanoporous high temperature energy absorbing materials and highly dense optically transparent spinel.

Porosity characterization of additively manufactured transparent MgAl2O4 spinel by laser direct deposition

Polycrystalline magnesium aluminate (MgAl2O4) spinel has established itself as one of the leading candidates for use as transparent, high strength ceramics. In this study, additive manufacturing of spinel ceramics by laser direct deposition was investigated with a particular focus on porosity characterization. Residual porosity was the most significant factor hindering transparency of spinel ceramics. Hence, this paper systematically studied how processing conditions and initial powder particle size affected porosity and densification of laser direct deposited spinel samples. Scan speed, laser power, and powder flow rate, along with powder size, were all shown to have substantial influences on density and porosity of produced parts. High density bulk spinel ceramics, with average densities of nearly 98%, were produced at a low powder flow rate of 0.58 g/min. In addition, the pore size distribution was studied under various processing conditions as it was closely related to transmission properties of transparent spinel at different wavelengths. The reduction in average porosity was mainly attributed to a significant reduction in large pores (>30 μm) whereas less obvious effects were found on smaller pores. An increase of scan speed resulted in an initial increase in density, followed by a subsequent decrease at scan speeds higher than 2000 mm/min. No obvious difference in porosity was observed between pulsed mode and continuous mode during laser deposition. The use of nanoparticles in initial MgO–Al2O3 mixtures for deposition also promoted densification of the deposited samples. A systematic understanding of residual porosity in this study can help process optimization to minimize porosity in additively manufactured transparent spinel ceramics, potentially eliminating the needs of time-consuming and expensive post-processing procedures.

Bimodal grain structure effect on the static and dynamic mechanical properties of transparent polycrystalline magnesium aluminate (spinel)

Abstract Transparent polycrystalline magnesium aluminate (spinel) with bimodal and unimodal grain structures were prepared. The influence of grain size distribution on static and dynamic mechanical properties were systematically investigated. The results showed that bimodal grain structure spinel has larger flexural strength (236.31 MPa) compared to unimodal grain structure spinel (221.38 MPa). Whereas, their values of hardness are very similar (15.1 vs 14.7 GPa) and fracture toughness remains unchanged (1.1 MPa∙m1/2 for both spinel). Although static compression strength of bimodal grain structure spinel (1236 MPa) is higher than that of unimodal one (1078 MPa) due to a smaller average grain size in the former, the negative effect of bimodal grain structure reduced the spinel strength compared to theoretically predicted value. Bimodal grain structure spinel shows slightly lower increment (49%) in compression strength from static to dynamic loading compared to that of unimodal one (57%) due to a decreased strain-rate sensitivity ascribed to bimodal grain structure. A brittle mode in inelastic deformation at Hugoniot elastic limit was demonstrated in both bimodal and unimodal grain structures. Bimodal grain structure has an influence on the Hall-Petch-like relation of yield strength under planar impact loading.

Terahertz Dielectric Properties of Polycrystalline MgAl2O4 Spinel Obtained by Microwave Sintering and Hot Pressing

This paper describes the fabrication of polycrystalline magnesium aluminate spinel (MgAl2O4) by microwave sintering and hot pressing and the investigation of its dielectric properties in the millimeter-wave and terahertz frequency range. The dielectric properties were studied in the frequency range 50–300 GHz using a spectrometer based on an open Fabry–Perot resonator with a high quality factor and in the range 0.6–3.3 THz using the time-domain spectroscopy method. The terahertz radiation was generated as a result of air breakdown using two-color laser pulses with carrier wavelengths of 800 and 400 nm. The dielectric characteristics of MgAl2O4 ceramics obtained from high-purity nanosized powders by microwave sintering and by hot pressing are compared. The refractive index of the materials varies from 2.85 to 2.95, and the dielectric loss tangent increases from 1.5 × 10−4 to 1.5 × 10−2 within the frequency range 0.05–3.3 THz. The possible use of magnesium aluminate spinel for millimeter-wave and terahertz applications is discussed.

Transparent Polycrystalline Magnesium Aluminate Spinel Fabricated by Spark Plasma Sintering.

Polycrystalline magnesium aluminate (MgAl2 O4 ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for lithography, electroinsulation, and structural windows for both VIS and IR region radiation and armor applications. Conventional two-stage processing of PMAS involves prolonged pressureless sintering followed by hot isostatic pressing. The costly processing of high-quality transparent PMAS ceramic is the main reason for the limited usage of this material in industry. Spark plasma sintering (SPS) is a relatively novel one-stage, rapid, and cost-effective sintering technique, which holds great potential for producing high-quality optical materials. Here, recent advances in the fabrication of transparent PMAS by the SPS approach, the influence of sintering parameters on microstructure evolution during densification, and their effects on the optical and mechanical properties of the material are reviewed.

Low energy cathodoluminescence analysis of damage build-up in ion irradiated spinel mono- and polycrystals

Abstract In contrast to single crystalline materials a quantitative determination of defect formation in polycrystals is challenging. Here we use low-energy (3 keV) cathodoluminescence (CL) for structural characterization of single and polycrystalline magnesium aluminate spinel (MgAl2O4) exposed to irradiation with 300 keV Ar+ ions at room temperature in a wide fluence range of 1 × 1012–2 × 1016 cm−2. The results for single crystalline materials are also compared with conventional Rutherford Backscattering and Channeling spectrometry (RBS/C). It is demonstrated that damage formation in both single and polycrystalline spinels exhibits a multi-step character where polycrystalline material is more susceptible to ion irradiation. The results also imply that the low-energy CL is more sensitive to low damage levels as compared to RBS/C and it can be efficiently used as a complementary tool showing new perspectives in the damage accumulation studies in single- and polycrystalline materials.

Transparent ceramic and glass-ceramic materials for armor applications

Abstract Transparent ceramics and glass-ceramics intended for armor applications still present limitations such as high production cost, manufacture of larger parts, among other challenges discussed in this review. The information concerning this topics is highly scattered, and to some extent scarce in specific points, scenario that make difficult to understand and cope the abovementioned challenges. Herein we intend to simplify the landscape of transparent armor materials and manufacture. In the first section, guidelines for materials development are discussed in the form of a filter-like methodology related with the material properties assessment, which is organized from the straightforward, economic evaluation to the most complex, expensive analysis. In the second section, the materials outlook is reviewed with special attention to Polycrystalline Magnesium Aluminate Spinel (PMAS) and Magnesium-Aluminum-Silicate (MAS). For either PMAS or MAS, the manufacture state of the technique is also reviewed to establish their scale-up potential. In summary, here we give some tutorial information about materials structure, fabrication and properties, which may be useful for those interested in entering the field. The state of the technique, including current challenges, technological gaps, limitations and the most recent findings, show the further field development towards simplified fabrication and low-cost production.

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400MPa pressure at 1200°C was about 170nm, while for samples fabricated under 1000MPa at 1000°C the average grain size was remarkably smaller (about 50nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.

Creep of Polycrystalline Magnesium Aluminate Spinel Studied by an SPS Apparatus.

A spark plasma sintering (SPS) apparatus was used for the first time as an analytical testing tool for studying creep in ceramics at elevated temperatures. Compression creep experiments on a fine-grained (250 nm) polycrystalline magnesium aluminate spinel were successfully performed in the 1100–1200 °C temperature range, under an applied stress of 120–200 MPa. It was found that the stress exponent and activation energy depended on temperature and applied stress, respectively. The deformed samples were characterized by high resolution scanning electron microscope (HRSEM) and high resolution transmission electron microscope (HRTEM). The results indicate that the creep mechanism was related to grain boundary sliding, accommodated by dislocation slip and climb. The experimental results, extrapolated to higher temperatures and lower stresses, were in good agreement with data reported in the literature.

Electrochemical Impedance Spectroscopy of Transparent Polycrystalline Magnesium Aluminate (MgAl2O4) Spinel

Transparent polycrystalline magnesium aluminate spinel is one of few materials capable of fulfilling certain demanding optical applications. However, LiF additive typically required to enhance sintering and impart transparency also degrades mechanical properties and causes scatter, precluding wider application. To shed light on the cause of altered properties, electrochemical impedance spectroscopy was performed between 500°C and 900°C, in oxidizing and reducing atmosphere, on fully dense, polycrystalline spinel compacts hot‐pressed with and without LiF. In combination with electron microscopy, chemical analysis, and secondary‐ion mass spectroscopy, bulk and grain‐boundary dielectric behavior were related to microstructure and chemistry. Decades higher conductivity than for comparable single crystals correlated with Al2O3‐rich stoichiometry, impurities, and small grain size, and suggested increased conduction and current‐line detouring along Mg‐depleted, impurity‐rich, field‐parallel grain boundaries. LiF addition reduced conductivity by one decade and increased the activation energy for conductivity, attributed to impurity removal, larger grain size, and point defects caused by lithium incorporation.

High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)

Abstract A high pressure SPS (spark plasma sintering) process was applied for consolidation of un-doped polycrystalline magnesium aluminate spinel. This approach allows fabricating a fully dense transparent ceramic with submicron grain size and high hardness values at a relatively low temperature (1200 °C). The light transmittance of the specimens increases with increasing applied pressure, while the hardness gradually decreases. The optimal combination of properties was achieved after sintering at 1200 °C at a heating rate of 5°/min, a holding time of 15 min and an applied pressure of 350–400 MPa. The specimens display the level of transmittance in the visible wavelengths and hardness values comparable with the best results reported in the literature for the two-stage fabrication process (pressureless sintering and hot isostatic pressing).

Cover Image, Volume 97, Issue 2

False‐color scanning electron microscopy image of a fracture surface in a transparent polycrystalline magnesium aluminate spinel. Areas of sub‐micrometer grains (~500 nm) were observed within a matrix of larger grains (~25 μm); suggesting the presence of an impurity phase. Impurities were detected at grain boundaries and triple junctions of the smaller grains with transmission electron microscopy and these can lead to opacity in the spinel. Micrograph courtesy of Marc Rubat du Merac. image

Optical and mechanical properties of compaction and slip cast processed transparent polycrystalline spinel ceramics

Transparent polycrystalline Magnesium Aluminate Spinel (MgAl2O4) specimens were formed by colloidal slip casting (SC) and uniaxial powder compaction (UDP) routes using powder with identical properties. SC and UDP formed specimens which were pressureless sintered and further subjected to hot isostatic pressing to eliminate the residual porosities and to achieve the transparency. SC samples exhibited better transmission compared to UDP samples. Mechanical properties were found to be lower by 10–15% for UDP samples and are complemented by the total fracture energy release rate (Jc) and JR–δ curves. Finally, the optical and mechanical properties were correlated with the processing routes.

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near‐theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis–processing–property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green‐body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an exposé of the state of the art in transparent polycrystalline spinel fabrication.

The Effect of Grain Size on the Mechanical and Optical Properties of Spark Plasma Sintering‐Processed Magnesium Aluminate Spinel MgAl2O4

The study deals with the effect of the SPS parameters and LiF doping on the mechanical and optical properties polycrystalline magnesium aluminate spinel (PMAS) with emphasis on the grain size of the final product. Sintering at 1300°C of undoped powder yielded fully dense submicrometer (0.4–0.6 μm) samples with elevated mechanical properties (1600HV and 300MPa bending strength). Doped samples had a larger, 40 μm grain size, lower, 1450HV, hardness and 150MPa bending strength. The transmittance of the doped samples (80% at 500 nm wavelength) was higher than that of the undoped ones. Thus, the required functionality of the ceramic dictates the choice of parameters for the fabrication of dense transparent PMAS.

Rate Sensitive Indentation Response of a Coarse‐Grained Magnesium Aluminate Spinel

The effect of loading rate on indentation hardness, brittleness, and fracture modes during static and dynamic Vickers indentation of a transparent coarse grain (250 μm average grain size) polycrystalline magnesium aluminate spinel was investigated. Static hardness was measured using a conventional Vickers tester while the dynamic hardness was measured using a custom test fixture based on split Hopkinson pressure bar technique with the capability to produce single indentations at strain rates of roughly 103 s−1. Static and dynamic hardness values as well as characteristic fracture response are compared at a range of indentation loads. It was found that Vickers micro hardness increased by an average of 5% over the entire load range, while the calculated brittleness factor increased when subjected to dynamic indentation loads. Static and dynamic indentations were found to produce radial and lateral cracks with evidence of spall appearing only during dynamic loading. Dynamic indentations also revealed an increased level of transgranular cracking, whereas intergranular cracking was more dominant during static indentations. Indentation cracks generated due to static indentations placed in the vicinity of a grain boundary deflected along a grain boundary, whereas these cracks cut across the grain boundary into the neighboring grain during dynamic indentations. Static and dynamic indentations that were placed directly on grain boundaries generated intergranular (grain boundary) cracks that were on average 18% longer than accompanying intragranular cracks. The implications of these fracture modes on dynamic impact response of polycrystalline spinel are discussed.