IR Transmission, Hardness, and Fracture Toughness of MgAl2O4

Spinel after doping with MgO

George J. Ferko V*

Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA, 18015, United StatesAugust 13, 2009

Abstract

Magnesium aluminate spinel has shown promise in the applications of transparent armor, window, and dome design. Recent work has focused on additives used to alter the sintering kinetics of spinel to make the production of spinel more economically viable and control its final properties. To test the effect of additives on important spinel properties high purity magnesium aluminate spinel powder has been doped with 100 ppm of magnesia and tested to determine its hardness, fracture toughness, transmission in the IR spectrum and microstructure. Evaluation in the SEM has shown that doping with MgO results in increased grain growth and abnormal large grains. Analysis of mechanical properties by Vickers microindentation has shown that the MgO doped spinel has a decreased hardness and slightly increased fracture toughness compared to that of undoped spinel. Transmission testing by FT-IR spectroscopy has determined that doping with 100 ppm of MgO does not significantly reduce the transmission of spinel in the IR spectrum.

1 - Introduction

Polycrystalline magnesium aluminate (MgAl2O4) spinel has been found to have attractive properties for both transparent armor and radar window and dome enclosures. In 1960 Hughes Research Laboratories demonstrated that spinel was capable of transmission further into the infrared (IR) spectrum than sapphire and in 1969 General Electric Co. produced the first visibly transparent polycrystalline spinel sample. The 30 years of intense research that followed has yielded many innovations in spinel processing often with the unfortunate quality of being irreproducible due to impurities in the available powders [6]. Recent availability of high purity spinel powders has brought spinel to the forefront of transparent armor, window, and dome design [11,15]. High purity powders have allowed for spinel windows and domes to be manufactured with high transparency in the midwave infrared (MWIR) range making spinel very attractive for military radome applications. Spinels transmission in the ultraviolet and visible wave (UV-VIS) spectrums make spinel attractive for transparent armor as well [1,2,4,5,6,9]. The relatively low density of spinel compared to other popular transparent armor materials such as AlON or sapphire makes the study of spinel an even more attractive venture [14]. The research done by Technology Assessment & Transfer Incorporated (TA &T) and Surmet is largely responsible for the recent success of spinel. Their production processes have lead to

consistently transparent large scale windows and domes. The recent efforts of TA & T to improve the economics of their manufacturing process by removing the hot isostatic pressing (HIP) step have not been realized [3]. Windows and domes produced by Surmet face the issue of inhomogeneity in select properties due to a bimodal grain size distribution. It is the goal of this research endeavor as well as future research to solve issues with grain size and economics by introducing additives to spinel so that they can be produced satisfactorily by hot pressing, annealing, and surface finishing steps alone. The effects of the additives on optical properties, mechanical properties, and thermal properties must also be studied to ensure that an additive does not negative effect spinels properties and to find any potential improvements in spinels properties that can be induced by doping.

2 - Experimental Methods

2.1 - Sample Preparation

Spinel powder of high purity was obtained from the Baikalox family of products (Baikowki, France). The starting powder for the MgO doped samples consisted of 100 ppm MgO excess powder introduced by mixing the high purity MgO powder with the spinel powder in ethanol and heating in a vacuum until all of the ethanol had evaporated. 0.3-0.5 g of

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powder was poured into a ultra high purity graphite die obtained from POCO Graphite, Inc. (Decatur, TX). A diagram of the die assembly used is shown in figure 1.

Figure 1: Schematic of the die assembly used for all of the spinel samples produced by hot-pressing.

The powder was hot-pressed in a Thermal Technologies LLC high temperature graphite hot zone furnace Model: 1000-2560-FP20 (Santa Rosa, CA) under vacuum. The furnace ramp rate was set to 7.5oC per minute where the powders were first held at 700oC for 3 to 4 hours for degassing. This degassing temperature was used to allow volatiles to escape while preventing the bonding of spinel particles to a degree that would trap gasses in the sample or result in cracking during the pressing operation. Whether or not gas was still evolving from the sample was checked by turning off the vacuum pump and waiting 30 seconds to see if the pressure in the hot-zone would increase. Once the evolution of gas had ceased the ram force in the hot-press was slowly increased to the desired stress. Samples that were hot-pressed at a maximum temperature of 1250oC were pressed using a stress of ~70MPa. The temperature was than increased by 7.5oC per minute to the desired hot-pressing temperature and held for 2 hours. Upon removal from the hot-press each sample had ~0.8mm cut off of each of its 4 sides and ~200µm ground off of each of its 2 surfaces. The cutting and grinding of the samples is necessary to remove the

large amount of carbon contamination that exists at the surfaces where the graphite die touches the spinel sample. Following cutting and grinding each sample was cleaned ultrasonically in the VWR B3500A-DTH (Batavia, IL) using deionized water, acetone, and ethanol to ensure that no contaminates remained on the samples. Great care was exerted to keep the samples clean at all stages of their processing and analysis because of the importance of maintaining very high purity samples. Following the cleaning step some of the samples were annealed in a CM Inc Rapid Temp Box Furnace (Model: 950065, Bloomfield, NJ) under an air atmosphere. The annealing step was left out for some samples so that they could be characterized without the added grain growth that occurs during the annealing process or the potential contamination that was thought to occur from being in the box furnace. The box furnace was set to a ramp rate of 15oC per minute and a dwell temperature of 1000oC. After this step the samples were polished to a surface finish of 50nm and thermally etched. The thermal etching of each sample was performed from temperatures of 1000oC to 1100oC for no longer than 60 minutes.

2.2 - Scanning Electron Microscopy

The Following thermal etching the samples were coated with Ir to prepare for analysis in the SEM using the Hitachi S-4300 CFE (Japan). The high resolution images obtained were analyzed for differences in grain size and grain size distribution using the line intercept method and percent porosity using the 100 point count method.

2.3 - Transmission

Following SEM analysis the samples were re-polished to remove the Ir conductive coating and cleaned using the same cleaning process previously mentioned. The samples were then mounted on a sample holder over a 4mm diameter pinhole using two-sided tape. A Varian 7000e FT-IR spectrometer (Palo Alto, CA) was used to measure the percent transmission of each sample in both the infrared (IR) spectrum and the ultra-violet/visible (UV-VIS) spectrum. Samples containing absorption peaks were run multiple times to ensure that the peaks were not the result of surface contamination or surface roughness. The transmission spectra of each sample were plotted on the same chart to show differences in transmission and the relevant absorption peaks were labeled.

2.4 - Mechanical Properties

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Following transmission testing the samples were cleaned using the same method that has been previously mentioned. Each sample was then mounted on an aluminum slug using an amorphous adhesive to prevent any sample movement during micro-indentation. Micro-indentation was performed using a LECO M-400 microhardness tester (St. Joseph, MI). The indenter was equipped with a diamond Vickers indenter tip. Before the indenter could be used a stereo-micrograph of the indenter tip was taken to ensure the tip was of the proper geometry, shown below in figure 2. Due to spinels hygroscopic nature samples were indented while in immersion oil to prevent humidity in the air from causing crack growth [20].

Figure 2: Stereo-Micrograph of the Vickers indenter tip showing that the geometry has not been affected by regular use. The samples were indented at loads of 300, 500, and 1000gf. It was reported by Parimal J. Patel, et al. that below a load of 388gf the indentation size effect (ISE) would artificially inflate the hardness values for the samples [33]. For this reason indentations made at 300gf were used only to verify that the ISE was inflating the hardness and not for actual hardness calculations. Indentations were used to calculate both the hardness and the fracture toughness of the samples using as described in the literature [20,31,22]. In order for the indentations to be used in calculations of hardness and fracture toughness the indentation had to meet the following ideal conditions. All cracks had to propagate straight out of the indentation corners. All cracks had to be of the same length. No secondary cracks could be observed on or below the sample surface. Each indentation had to be perfectly square [22]. An

ideal crack and indentation is pictured in figure 3.

Figure 3: Light optical micrograph of an ideal indentation and crack propagation.

In some samples up to 80 indentations were required to obtain indentations and cracks of an ideal nature. For each sample three separate ideal indentations were measured five times each and their values were averaged. The area of the indentation was used to calculate hardness through equation 1 where P is the force incident on the sample and A is the area of the indentation.

HV =

P (sin( π360

68o)) (1000 )

A ( kgfmm2 ) (equ. 1)

Once the hardness was calculated equation 2 was used to calculate the fracture toughness of the sample where Kc is the fracture toughness, α is a constant equal to 0.016, E is the Young’s modulus, H is the Vickers hardness, P is the load applied by the indenter, and c is the length of the crack measured from the center of the indentation to the tip of the crack.

K c=α( EH )

12 ( P

c3/2 ) (kgf √mm ) (equ. 2)

Unfortunately a suitable method of non-destructively determining the Young’s moduli of the samples was not able to be performed and the modulus was held constant at 275GPa for all samples.

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3 - Results

3.1 - Microscopy

A micrograph from sample SsqA7 is shown in figure 1. The sample SsqA7 has not been doped and has not been densified to the extent of transparency. This sample was hot-pressed at 1050oC with 100MPa of pressure.

Figure 1: Micrograph of sample SsqA7 showing sub-micron size grains and a pore size on the order of 10 nm.

The roughness seen on the grains is a result of over etching a problem which is difficult to solve because the low hot-pressing temperature limits the thermal etching temperature that can be used without causing additional grain growth. The porosity is about 0.3% and the grain size is ***. The small grain size and angularity of the grains is a desirable feature in spinel for increased hardness, according to the Hall-Petch relationship, and increased fracture toughness when the mode of fracture in spinel is intergranular. The grain size and morphology in this sample would appear to be desirable; however, a significantly smaller amount of porosity is needed to achieve transparency.

Shown in figure 2 is a micrograph of sample SsqA3. This sample has been hot-pressed at 1250oC with 70MPa of pressure. The result is a sample that has densified to a percent porosity that is immeasurable by graphical analysis. This sample was translucent upon removal from the hot-press and after polishing appeared transparent to the naked eye.

Figure 2: Micrograph of sample SsqA3 showing no noticeable porosity and a grain size of **.

A slight gray color was observed in the sample suggesting that some carbon contamination occurred during hot-pressing. For the purpose of this study and future studies on doping spinel it is very important to have as little contamination as possible. To remove the carbon contamination from the hot-pressed samples it was decided that the carbon could be annealed out of the sample.

Shown in figure 3 is sample SsqA4. This sample has undergone hot-pressing at 1250oC with a pressure of 70MPa, just as sample SsqA3 has. This sample has also been annealed at 1000oC for 18 hours in a box furnace with an air atmosphere.

Figure 3: Micrograph of sample SsqA4 after undergoing carbon decontamination by annealing showing no noticeable porosity and a grain size of **.

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The unfortunate effect of annealing is that a significant amount of grain growth has occurred. The largest grains in sample SsqA4 are more than three times smaller than the reported grain size for commercially available spinel, such as those from TA & T. Fortunately annealing produced a sample lacking in gray coloring, suggesting that the carbon contamination was successfully removed. The tradeoff between grain size and contamination was deemed necessary so that a sample of lower contamination could be used for this study.

Shown in figure 4 is sample SsqA6. This sample has the same thermal history as sample SsqA4. It has been hot-pressed at 1250oC with 70MPa of pressure and annealed at 1000oC for 18 hours.

Figure 4: Micrograph of sample SsqA6 showing some visible porosity as well as enhanced grain growth above that which was expected from the thermal operations performed.

In figure 4 it can be seen that the same hot-pressing and annealing operations that yielded the sample in figure 3 have produced a different grain morphology. The micrograph of sample SsqA4 in figure 3 shows a normal grain size distribution with an acceptable average and maximum value, however; figure 4 shows sample SsqA6 which has a bimodal grain size distribution and much higher average and maximum grain size values. Both samples were also thermally etched using the same parameters for temperature and time yet the sample in figure 4 appears to have undergone very little etching on select grain boundaries. The result is a poor image that is useless for image analysis, but it can still be discerned from the image that the grain growth kinetics have unexpectedly changed. The residual porosity that was observed is also a troubling result.

Shown in figure 5 is sample SM100sqA2. The starting powder for this sample was doped with 100ppm of MgO before hot-pressing. The hot-pressing and annealing operations for this sample are the same as those in samples SsqA4 and SsqA6.

Figure 5: Micrographs of sample SM100sqA2 showing very large grains relative to the other samples with a pronounced bimodal grain size distribution and no noticeable porosity at 5k, 2k, and 800x magnifications.

Doping with MgO has resulted in very large grains and a grain size distribution that is bimodal. The smaller grains in the sample appear in clusters and are very similarly sized to those in the undoped sample seen in figure 3. The smaller grains in the sample are unfortunately difficult to resolve at magnifications

where the large grains can be imaged. This meant that accurate grain size measurements were not able to be made by the linear intercept method. The large grain shown on the far right image in figure 5 shows one of the large grains in the sample. This grain has not only undergone a relatively large amount of grain

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growth compared to that in the undoped samples, but it has also lost its angular grain morphology. The shapes of the larger grains in the sample shown in figure 5 appear round.

Sample SX100/111sqA2 is shown in figures 6 and 7. This sample is a bi-crystal single crystal polycrystal hybrid with the polycrystalline regions of the sample sandwiched between two single crystals. The polycrystalline region that was against the <111> direction of the single crystal is shown in figure 6 and the polycrystalline region grown against the single crystal oriented in the <100> direction is shown in figure 7.

Figure 6: Micrograph of sample SX100/111sqA2 showing the polycrystalline region against the single crystal that was oriented in the <111> direction with a bimodal grain size distribution.

Figure 7: Micrograph of sample SX100/111sqA2 showing the polycrystalline region against the single crystal that was oriented in the <100> direction with a bimodal grain size distribution.

The grain size distribution in the two samples is bimodal, not unlike that of sample SM10sqA2 in figure 5. The average grain size of the large grains in figure 6 is ** and the average grain size of the large grains in figure 7 is **. Other images of the polycrystalline regions indicate that the grains closest to the <111> oriented single crystal have experienced more grain growth than those closest to the <100> oriented single crystal. It should be noted that it is probable that the crystal direction does not play a role in the grain growth of the sample and the final grain size. This difference in grain growth effects is likely due to differing contamination levels on the two different single crystal faces.

3.2 - Transmission

Shown in figure 8 are the transmission spectra for several important samples that were tested. The single crystal sample was polished using the same technique as the other samples to ensure that surface finish would not be at fault for any of the results. Others have reported that uncoated single crystal spinel should have a transmission of about 87% [5,10,32]. The single crystal tested reaches a maximum transmission of 85.91% which indicates that there may be some reflection or beam diffraction issues and a higher transmission could be obtained from the same samples with a difference surface finishing technique. There is some slight noise between the wavelengths of 2.57 and 2.80 µm that is most likely ethanol that remained on the sample from cleaning. While measuring the spectra it was observed that the noise decreased with time and eventually disappeared as a result of the ethanol on the sample surface evaporating. The two absorption peaks at 3.50 and 3.42 µm are thought to be from water or carbon dioxide that was still in the testing chamber. These peaks also disappeared with time except in the case of sample SM100sqA2. Sample SM100sqA2 was doped with 100ppm MgO, hot-pressed at 1250oC with 70MPa of pressure, and annealed at 1000oC for 18 hours. In sample SM100sqA2 not only did the absorption peaks at 3.50 and 3.42 µm not change in intensity with time, but the absorption peak at 2.96 µm also remained at the same intensity over time. To insure that the peaks weren’t part of some kind of surface contamination the sample was cleaned with ethanol, placed under a heat lamp for an hour to obtain a more complete evaporation of the ethanol, and then tested again. After cleaning and heating intensity of the absorption peak remained the same.

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Figure 8: Figure showing the transmission spectra for six samples in the IR range.

The samples in figure 8 that did not appear in the section 3.1 on microscopy are the single crystal, SM100sqA3, SsqA8, and ST1. The single crystal was obtained from **. SM100sqA3 is the same as SM100sqA2 only this sample has not been annealed. This sample was created to determine if the absorption peak in SM100sqA2 was due to doping or if it was from unintentional contamination during thermal processing. SsqA8 is a replacement for SsqA3 which was contaminated in a thermal etching operation and could no longer be used for further testing. The sample labeled ST1 was obtained from TA & T. The thermal history of the sample is not known, however the cloudy appearance of the sample is similar to that of other TA & T samples immediately after hot-pressing and before annealing or hot-isostatic-pressing. It should be noted that this sample is not indicative of the performance of TA & T’s commercial grade spinel products and is of a lower quality.

The maximum transmission of the TA & T spinel sample was found to be 68.86% at a wavelength of 4.14 µm. The spinel samples produced for this study all have a maximum transmission that is superior to the TA & T sample. Sample SM100sqA2 has a maximum transmission of 75.37% at a wavelength of 4.11 µm. Sample SsqA8 has a maximum

transmission of 74.75% at a wavelength of 4.16 µm. Sample SM100sqA3 has a maximum transmission of 76.76% at a wavelength of 4.03 µm. Sample SsqA4 has a maximum transmission of 79.05% at a wavelength of 4.23 µm.

The UV-Vis range was also tested, however; the results were not reportable due to the incident beam being diffracted. The diffraction of the beam caused the detector to produce values for transmission that were either extremely low or extremely high. Although these values could not be used to make any conclusions the results are still available in appendix *

3.3 - Mechanical Testing

The results for the samples that underwent mechanical testing are displayed in table 1.

Table 1: Hardness by Vickers indentation and fracture toughness by the Vickers Indentation Method for four samples.

Sample Hardness (kgf/mm2)Fracture Toughness

(MPa√m)

SsqA8 1625.3 ± 27.8 1.15

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SsqA4 1493.7 ± 28.8 1.15

SM100sqA3 1544.3 ± 20.5 1.16

ST1 1423.7 ± 44.9 1.05

The data shows that the undoped and unannealed sample, SsqA8, has the highest hardness. The MgO doped and unannealed sample, SM100sqA3, has a lower hardness than SsqA8 indicating that doping spinel with MgO may be lowering the hardness. The sample obtained from TA & T, ST1, was found to have the lowest hardness, however, ST1s hardness is not significantly lower than the hardness of sample SsqA4. Aside from the nominal empirical data it was also observed that as the average grain size of the sample became larger and the grains became abnormal the ideal crack geometry needed to make the measurements in table 1 became more difficult to obtain. Larger and more abnormal grained samples would exhibit crack geometries similar to the geometry shown in figure 9 obtained from a study done by G.R. Anstis, et al. [20].

Figure 9: Micrograph obtained from a study on grain size dependence on the affectivity of the Vickers indentation method on measuring fracture toughness authored by G.R. Antis, et al., width of field 200 μm [20].

4 - Discussion

4.1 - Grain Growth

The increased hot-pressing temperature has resulted in a larger grain size and greater densification in undoped spinel due to an increase in grain boundary mobility. The difference in grain size between sample SsqA7, shown in figure 1, and sample SsqA3, shown in figure 2, appears consistent with the theoretical temperature dependence of grain growth rate and densification rate, shown below in equations 3 and 4.

Densification Rate=(constant )× DL × γ × Ω

L3 ×k ×T(equ. 3)

dGdt

=(constant )× Db

⊥× γk× T ×G

(equ. 4)

In both equations 3 and 4 the effect that temperature has on the defect concentration, diffusivity, and interfacial energy is great and so the grain growth and densification increase with temperature, both across the grain boundary and through the lattice. The difference between sample SsqA3 in figure 2 and SsqA4 in figure 3 shows a continued grain growth during the annealing process similar to that which is described in studies done by Chiang and Kingery [33,34]. Chaing and Kingery observed further grain growth during annealing with an increase in grain size distribution, but did not report any abnormally large grains. In this study the same effect is observed.

Sample SsqA6, shown in figure 4, exhibited grain growth behavior inconsistent with previous studies []. Spinel has been found to be an impurity tolerant system, however; this undoped sample has undergone abnormal grain growth. Throughout the sample there are abnormally large grains. There are also grains of the expected grain size in clusters. The abnormal grain growth must be contributed to contamination during hot-pressing or annealing. This sample has shown that greater care in preventing contamination must be observed when working with the spinel system.

Doping with MgO has yielded even larger abnormal grains than in the contaminated sample. The same bimodal distribution of grain size is observed in sample SM100sqA2 as in the contaminated sample with even larger grains. The larger grains indicate a greater amount of grain boundary mobility in the MgO doped sample than in the undoped samples. This suggests that MgO doping is increasing the defect concentration in the spinel sample and thus altering the diffusivity. Doping with MgO cannot be held responsible for the abnormal grain growth with certainty. The contaminated undoped sample has proven that abnormal grain growth can be observed without any intentional

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doping. Despite this it is suspected that the MgO is, in part, responsible for the abnormal grains.

The bi-crystal in figures 6 and 7 indicates that spinel shows differing grain growth rates dependant on crystallographic orientation. The grains that nucleated near the <111> oriented single crystal and those near the <100> oriented single crystal are of differing average grain size. This part of the experiment has been repeated in unpublished work by A. Kundu yielding similar results. These results suggest that spinel may exhibit some anisotropic grain growth that has been previously unobserved. However, it is far more likely that the difference in grain growth between the <111> and <100> directions is due to differing levels of contamination between the two single crystal surfaces.

4.2 - Transmission

The IR transmission spectra for samples SsqA8 and SsqA4, shown in figure 8, indicate that annealing spinel increases the percent transmission. This increase in transmission occurs for two reasons. First, impurities that act as scattering sites have been removed during the annealing process. Second, the grain boundary surface area has been reduced by grain growth resulting in less scattering on the grain boundaries.

This effect has not been observed among the MgO doped samples. SM100sqA2 has been annealed and SM100sqA3 has not been annealed yet SM100sqA3 has a higher transmission. This discrepancy is believed to be due to contamination in the annealed sample. The annealed sample also exhibits a large absorption peak at a wavelength of 2.96 μm. This absorption peak must be due to contamination of the sample and cannot be a result of doping with MgO because the unannealed sample has no suck absorption peak. The peak must therefore be caused by contamination from annealing in the box furnace, surface finishing, or cleaning. The location in the spectra of the absorption peak is characteristic of O-H bonds commonly found in alcohols, however the depth and width of the peak are uncharacteristically large [**]. This sample must be re-polished and re-cleaned to isolate the cause of the peak. It is possible that future work may find a relationship between different dopants and the absorption peaks they cause. This would be valuable to the application of spinel as a dome cover for multi-mode sensors. The transmission spectra for the unannealed MgO doped sample lies in close proximity to that of the undoped unannealed sample. This suggests that doping with 100ppm MgO does not cause any significant additional scattering in the IR spectrum. This result is promising as it indicates that the spinel structure has not been significantly altered enough to reduce its transmission. All of the samples tested for

transmission in this study have been found to have greater transmission then the sample obtained from TA & T, label ST1 in figure 8. This is largely due to the fact that the TA & T sample was much thicker than the other samples tested. It was also observed in a light optical microscope that the TA & T sample had undergone some differential polishing. The differential polishing allowed for the grains to be resolved and it was noted that they were extremely large compared to those produced for this study. The differential polishing that occurred on the TA & T sample indicates that a greater surface roughness was present in ST1 than in the other samples, which may be contributing to the low percent transmission in sample ST1. Sample ST1 also contained some visible opaque scattering sites the composition of which is unknown. These sites are also a likely candidate for the reduced transmission in the TA & T sample.

4.3 - Mechanical Properties

One of the observations made during indentation was that as the grain size of the spinel sample got larger the cracks propagating away from the indentation got more irregular. This is believed to occur because the indentation is only hitting a small number or only one grain at a time. Hitting a small number of grains with the indentation is resulting in the cracks only propagating along the preferred cleavage planes of one grain rather than many grains. The result is cracks that don’t necessarily propagate straight out of the indentation corners. A schematic of irregular crack propagation is shown in figure 10. The irregular nature of the cracks does not mean that samples with large grains cannot be tested. If the test is repeated many times than the indentation corners eventually will roughly line up with the preferred cleavage planes in the sample and by luck the cracks will propagate straight out from the indentation corners. It is assumed that the energy dissipated by a crack moving through a spinel grain is roughly the same as the energy dissipated by cracking along the grain boundaries at room temperature because of the mixed mode cracking ( inter- and trans-granular fracture) that has been observed at room temperature. This mixed mode fracture might show that the fracture toughness is not biased dependant on how many grains the cracks are propagating through, of course this cannot be known for certain.

The decrease in hardness observed during mechanical testing in some samples can be explained by the relationship between hardness and grain size. Both doping with MgO and annealing have caused further grain growth in the spinel samples and thus decreased hardness. This data shows that spinel hardness exhibits a normal inverse relationship with grain size.

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Figure 10: Schematic of the cracks propagating irregularly away from the indentation in a large grained spinel sample.

The last result worth noting from the indentation testing is the increase in fracture toughness found to occur with MgO doping. The theoretical effect of such small amounts of dopant on fracture toughness is not well explained in the literature. It cannot be determined from this data the exact cause of the increase in fracture toughness. One theory is that the MgO dopant has not completely segregated to the grain boundaries and is in the lattice effecting the bond energy between ions. This change in bond energy or bond type may be causing more energy to be dissipated during fracture. Ting and Lu have provided the possible defect reactions that may be occurring for MgO in spinel, they are shown below in equations 5, 6, and 7.

4 MgO→ 2 MgAl' +MgMg

x +4OOx +Mgi

… (equ. 5)

4 MgO+Al Alx →3 MgAl

' +MgMgx +4OO

x + Ali…

(equ. 6)

3 MgO →2 MgAl' +MgMg

x +3 OOx +V O

..

(equ. 7)

These equations have been used to describe the rate controlling mechanism during sintering in previous papers. The change in energy caused by defects that results in a change in sintering kinetics may also be effecting the kinetics of fracture.

5 - Conclusions

The objective of this study was to analyze the affect of doping magnesium aluminate spinel with MgO on mechanical properties and transmission. This study has led to the following conclusions:1. The doping of spinel with 100ppm MgO does not cause a

significant reduction in transmission throughout the IR range.

2. The sensitivity of the spinel system to contamination is greater than was previously thought.

3. Annealing results in higher transmission in the IR range due to grain growth and decreased concentration of impurities.

4. Doping with 100 ppm MgO produces a sample with lower hardness, but higher fracture toughness.

5. Doping with 100 ppm MgO causes abnormal large grains as well as a larger grain size throughout all grains.

6 - Future Work

7 - Acknowledgements

It is my pleasure to acknowledge Animesh Kundu PhD for the abundance of technical advice throughout this research endeavor. His contribution to what I’ve learned about the spinel system and research in general has been almost entirely responsible for my growth as a student of ceramics and has led me to be capable enough to perform research in the ceramics field.

I would also like to thank Eva Campo PhD for bringing me this opportunity to work on spinel and for her aid in keeping my work focused. Her ability to inspire my persistence with this project was vital to its progress. It is because of her direction that the breadth of my knowledge has increased so greatly in only a few short months.

I would like to thank Professor Martin Harmer for his class room teachings and the example that he sets for me and his other students. His success has enabled my research and the research of many others in the ceramics field.

I am grateful to ShuaiLei Ma for her instruction in previous work, her appreciation of my work, and her advice throughout this study.

8 - References

1. Mark C.L.Patterson, et al., An Investigation of the Transmission Properties and Ballistic Performance of Hot Pressed Spinel, Proceedings of the Ceramic Armor Materials

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Cleavage Plane

Irregular Crack

by Design Symposium, Vol. 134, American Ceramic Society, pp. 595-608, 2002.

2. Parimal J. Patel, et al., Transparent Armor Materials: Needs and Requirements, Proceedings of the Ceramic Armor Materials by Design Symposium, Vol. 134, American Ceramic Society, pp. 573-586, 2002.

3. A. LaRoche, et al., An Economic Comparison of Hot Pressing vs. Pressureless Sintering for Transparent Spinel Armor, Advances in Ceramic Armor IV: Ceramic Engineering and Science Proceedings, Vol 29, Iss. 6, American Ceramic Society, pp. 55-62, 2008.

4. Juan L. Sepulveda, et al., Defect Free Spinel Ceramics of High Strength and High Transparency, Advances in Ceramic Armor IV: Ceramic Engineering and Science Proceedings, Vol 29, Iss. 6, American Ceramic Society, pp. 75-85, 2008.

5. Guillermo Villalobos, et al., Analysis of Scattering Sites in Transparent Magnesium Aluminate Spinel, Advances in Ceramic Armor: Ceramic Engineering and Science Proceedings, Vol 26, No. 7, American Ceramic Society, pp. 293-298, 2005.

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Appendix

* Undergraduate Student – Department of Materials Science and Engineering – Lehigh UniversityTel.: 610-597-8007E-mail:gjf210@lehigh.edu