A Thesis Presented To - AURA

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A Thesis Presented To The Faculty of Alfred University

DEVELOPMENT OF IN SITU PLATELET-REINFORCED ALUMINA

by

David Jensen

in partial fulfillment of the requirements for

the Alfred University Honors Program

Alfred, New York

May, 2022 Under the supervision of:

Chair: Dr. William Carty ______________________________________

Committee Members:

Dr. Hyojin Lee ______________________________________

Dr. Garret McGowan ______________________________________

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Acknowledgements

I would like to thank Rachael Moravansky, Dr. Carty, Daniel Delia and Hyojin Lee for their contributions and input, which greatly contributed to the work before you.

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List of Tables

Table I. Fracture toughness of various ceramic materials.¹³ ... 2

Table II. Supply of raw materials, and categorization of particles. ... 3

Table III. Composition ranges investigated (mass %). ... 5

Table IIV. Average platelet grain lengths and widths from SEM images (microns). ... 12

Table V. Calculated density, using ROM and the calculated theoretical maximum percent of CA6 ... 14

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List of Figures

Figure 1. Particle size distributions for as-received alumina on a probability axis after milling for various times, from Whipkey.^1,14 ... 4 Figure 2. SEM image of poorly dispersed sample. ... 5 Figure 3. CaO-Al2O3-SiO2 phase diagram of showing the glass formation boundary,

the SiO2:CaO ratio, and the Hibonite (CaO·6Al2O3) phase field.^1,14 ... 6 Figure 4. SEM images of fractured surface, showing both plate edges (left) and plate

faces (right). ... 9 Figure 5. Microstructures of the three alumina concentrations heat treated at 1500°C

for 1, 3, and 10 hours. (Needs size bar) ... 10 Figure 6. Hillard (circle) and Heyn (random straight lines) methods for grain size

measurement.¹⁷... 11 Figure 7. Ratio of platelets to non-platelets as a function of dwell time in hours. ... 11 Figure 8. Average aspect ratios for all nine samples ... 13 Figure 9. Relative density as a function of dwell time for the three alumina levels

investigated. ... Error! Bookmark not defined.

Figure 10. Energy dispersive spectroscopy map of calcium and backscatter image, 1 hour dwell 96% Al2O3 sample bright purple indicates high calcium levels. ... 15 Figure 11. XRD spectra of 1 hour dwell 88% Al2O3 sample with corundum (C) and

hibonite (H) peaks labeled ... 16 Figure 12. XRD spectra of 10 hour dwell samples, with a change in intensity relating to

the (110) peak at a 2Θ of 31.8° ... 16

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Simplified Introduction

This introduction is written for the benefit of those without any prior knowledge of ceramics or chemistry and can be ignored by those with a more complete

understanding of either subject.

Ceramics are an inherently dense type of material and are well known for their fracture toughness, but there can be times when it is desired to make them more tough and less likely to fracture.

If a material falls from a great height, it can react in a few ways. It could hit the ground and absorb the blow in a plastic manner, absorbing most of the energy within and bouncing, or perhaps by deforming in a way to absorb the energy. Ceramics are widely known for their remarkable ability to do neither of these things, and to instead shatter upon collision.

Sometimes ceramics don’t shatter, and this is due to something called fracture toughness. In a technical sense, this is defined as the critical stress intensity factor where a crack’s propagation becomes rapid and unlimited, and a crack expands through a sample until it separates it into multiple pieces. As you might guess, this can be affected by things such as the thickness of the piece, the temperature the piece is at, the material the piece is made out of, the texture of the piece, and more. Because of this, materials being tested for this are usually held at a constant temperature and polished to set specifications.

In industry and other applications, people often look for ceramics that are resistant to fracture. They can have many good and helpful uses in projects, so it can be ideal. For

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example, suppose you worked for a company that made pizza stones, and you had a new material for your pizza stones, but it was likely to break any time someone set it down.

An employee for the company would be likely to look for a ceramic that can withstand something like this, and so they would be likely to look for a piece with a higher fracture toughness.

One way to increase the fracture toughness of a material is to make it harder for a crack to propagate through the material. This may sound like a strange way to word it, but picture the ceramic as being the room before a ride at Disney World, where the crack is the crowd trying to get on the ride. You’re likely to have a large crowd, so if you leave the room empty they’ll just walk in a straight line to the door to the ride and get there in no time. However, if you make a more complicated queue and put up obstacles in the way of the line, they have to walk around those obstacles and will take a much longer time to reach the ride. Similarly, if you add some sort of obstacle to the crack’s

propagation in the ceramic, it can prevent the crack from making it entirely through the material.

One of the most effective ways to create an obstacle of this type is to insert a dopant into the system, which can create a secondary phase within what’s called the matrix. In a material, the matrix refers to the consistent material which makes up the bulk of the material and which is relatively uniform in composition. Within this material are fibers, which are composed of a different material and structure, and which can have different properties than the matrix, such as increased strength, hardness, or toughness.

More importantly for this study, they can make it harder for a crack or fracture to

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propagate in a straight line, instead forcing it to weave and move at sharp angles that would require more energy than proceeding straight.

Alumina is a very commonly used ceramic. Among ceramics, it is usually well- regarded for how well it resists scratching and wearing, and it has an inherently high strength. It is commonly used for technical applications due to its high electrical resistance, and is very resistant to chemical wear from things like acids and other

degrading chemicals that wear other ceramics and materials down. When it is not used on its own it is often found as a component of things like glasses, glazes, and other more specific ceramics. However, even with its high usability and wide usage, alumina holds room for improvement in toughness.

This study involved adding small amounts of a dopant to alumina to investigate the formation of precipitates formed within the material. In this case, the dopant was chosen based on the work of a recent Alfred University doctoral graduate, Sarah

Whipkey, who observed and noted the formation of precipitates in a specific composition region of her samples. By composition region, I’m referring to a portion of a phase diagram. A phase diagram acts to map out what ceramics form under what combination of compositions and temperatures, and in this case the map plotted a region composed largely of alumina but with a small amount of calcia, or CaO, and silica, or SiO2.

Sarah Whipkey noticed that these precipitates were forming in a specific region of the phase diagram where they had not been extensively studied before, and made a note of it. By analyzing this in further depth, we hoped to determine to a greater degree how the precipitates form and are affected by the change in alumina percent. It took some time

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to determine how to go about this, but in the end the process was to create multiple

samples and vary the composition in a set amount each time to create three compositions.

People often propose that changes in a sample are not due as much to the

composition or minerology of a sample as to the firing conditions. ‘Firing conditions’ is a general term which can describe a wide variety of factors occurring during the firing process. This is the process where a green, or unfired, ceramic body is placed into a furnace (or kiln, as artists prefer to call them) and heated to the point where sintering occurs. In ceramics, sintering is a process occurring on a microscopic level at high temperatures, where separate particles in a dry or wet mix begin to diffuse and compress, forming the body into a more dense and solid material. While it is most commonly associated with ceramics, sintering occurs in many forms, ranging from metals to plastics. Interestingly, even snow can sinter at temperatures around -10 degrees Celsius, where individual snowflakes coalesce into a icy whole. Although it may seem easy to place the blame for certain material results on firing conditions, in general it has been found that most ceramic properties are determined from either the composition or the forming process.

In order to account for the possibility, however remote, that the changes noted in this particular ceramic were due to firing conditions, the samples were fired with three different soak times. Soak times refers to the amount of time a sample is held at the same temperature during firing. For instance, a sample could be fired at 1500 degrees Celsius and held at that temperature for three hours, which would mean it had a soak time of three hours. This can also be called a hold time.

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This project was ultimately a way to determine how accurate our theories about the behavior of this material in these conditions was.

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Abstract

It has been shown that alumina is a strong ceramic material, but it typically exhibits poor fracture toughness. Recently, studies have found that liquid-phase sintered alumina containing an invert glass (i.e., high calcium with low silica) can precipitate what appear to be high-aspect ratio needle-like crystals that have been determined to be

calcium hexaluminate (CaO·6Al2O3, CA6). In this study, the chemistry and processing conditions used to create the first specimens were revisited and it was determined how the sintering conditions contributed to densification of the alumina matrix and the size and population of the CA6 precipitates. While keeping the ratio of CaO:SiO2 constant, it was observed that the number of CA6 precipitates appeared to increase in number and size with increased CaO and SiO2 level (that is, with more liquid phase). In addition, it appears that the precipitates are nont needles, but platelets. The sintering conditions had little contribution to the size and concentration of the precipitates but were found to aid in the densification of the alumina matrix. The ability to control the size and concentration of these grains may have the potential to affect the fracture toughness of alumina, but this is beyond the scope of this work.

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Introduction

Alumina is one of the most widely used ceramics in industry, however there are still many aspects of its microstructure evolution behavior that are not understood. Recent work investigated the role of amorphous grain boundaries on the sintering of alumina, that observed the precipitation of a second phase at high calcium dopant levels (and low silica).¹ This simulates an alumina body that would be received in industrial practices, where receiving a high-purity alumina (>99.999%)¹⁹, would not be feasible.² With the addition of CaO and SiO2, the samples were observed to show smaller average grain size and decreased densification compared to as-received Al2O3.^1,2 These effects took place during liquid-phase sintering of the ceramics. CaO acts as a flux for the silica within the system, which allows it to melt at a much lower temperature.^3,4 This presence of a liquid phase can allow for the formation of a second phase: CaO·6Al2O3 (CA6).^5-11

With few exceptions, such as Si3N4 and YSZ (Yttria-stabilized ZrO2), ceramic materials exhibit low fracture toughness as presented in Table I. This toughness can be altered, either positively or negatively, by the addition of dopants to either chemically toughen the material, or by adding microstructural inclusions. Microstructural inclusions, such as platelets, have the potential to act as obstacles to crack propagation, slowing down fracture propagation and absorbing energy.^11,12

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Table I. Fracture toughness of various ceramic materials.¹³ Material Fracture Toughness

(MPa·m^1/2)

Amorphous Silica 1.15

Al2O3 5.50

ZrO2 (TZP) 23.0

SiC 32.4

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Experimental Procedure

Raw materials

The raw materials used, and selected powder properties, are listed in Table II.

Table II. Supply of raw materials, and categorization of particles.

Material As received Post-milling Manufacturer

Al2O3 0.425μm 0.326 μm A-16 S.G., Almatis Inc., Leetsdale, PA CaCO3 52 μm N/A Castle Carb 18, Castle Minerals Limited,

West Perth, WA

SiO2 63 μm N/A SI-CO-SIL, US Silica, Lamesa, TX

Powder preparation

As-received Al2O3 powder was vibratory milled to reduce agglomeration. After the material had been milled for 100 hours, a small portion was removed from the bulk sample and dried to be used as a powder. A milling study was conducted, as shown in Figure 1, to eliminate agglomerates.

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A 20-volume percent suspension was prepared and the pH was lowered to 6.0 through the addition of HNO3. CaO and SiO2 were added with a fixed ratio of SiO2:CaO

= 1:3 to create samples composed 92% Al2O3 by mass. Following the addition of silica and calcium carbonate, NH4-PMAA polyelectrolyte (Darvan C-N, Vanderbilt Minerals, LLC, Norwalk, CT) was added to disperse the alumina. The first samples mixed using ultrasonication were not fully dispersed and analysis demonstrated uneven densification as shown in Figure 2. Large pores, and agglomerates of CA6 grains surrounded by smaller alumina particles were observed within these samples. These were then divided into three groups, which were then fired with three different soak times of 1 hour, 3 hours, and 10 hours.

Figure 1. Particle size distributions for as-received alumina on a probability axis after milling for various times, from

Whipkey.^1,14

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Figure 2. SEM image of poorly dispersed sample.

A second set of samples were produced using the same method with the exception that, rather than ultrasonicating to combine, the 20-volume percent alumina suspension containing PMAA was milled for an hour before adding a 20v/o suspension of CaO and SiO2. The mixed suspension was then milled for an additional 10 hours. The batch compositions are listed in Table III, and maximum mass fraction of CA6 possible, assuming all of the CaO was used to form CA6 is listed in Error! Reference source not found.. At high percentages of CaO, >8.3%, alumina becomes the limiting reactant, causing the maximum amount of CA6 to be decreased with increasing CaO. The sample chemistries all reside within the hibonite phase field, as shown in Figure 3.

Table III. Composition ranges investigated (mass %).

Al2O3 CaO SiO2 Maximum

CA6

96% 3% 1% 35.7%

92% 6% 2% 71.5%

88% 9% 3% 96.1%

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Figure 3: CaO-Al2O3-SiO2 phase diagram of showing the glass formation boundary, the SiO2:CaO ratio, and the Hibonite (CaO·6Al2O3) phase field.^1,14

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7 Specimen fabrication

The samples were then slip cast and fired in an electric furnace. The firing cycles were previously determined by Whipkey, and the three cycles chosen had needle-like structures appear in microstructure analysis.¹ Three sets of samples were sintered at 1500°C with hold times of 1 hour, 3 and 10 hours.

Characterization of Sintered Samples

After bulk density measurements (using an immersion method modified for small specimen size¹⁵) specimens were selected at random and polished. Polished specimens were chemically etched for 10 seconds with 10% hydrofluoric acid (HF) to remove the amorphous grain boundary phase, then thermally etched for 30 minutes at 1450°C.¹⁶ A Scanning Electron Microscope (SEM) (JEOL 6010LA, JEOL, Tokyo, Japan) was used to image the samples. Sample images were then analyzed through image analysis software (ImageJ, V.1.8.0 NIH Image, Bethesda, MD) to determine precipitate size.

Skeletal density measurements were obtained using a pycnometer method (He- Pycnometer, AccuPyc II 1340, Micromeritics Instrument Corp., Norcross, GA). Due to limited quantity of samples they were not ground into a powder, but remained in a pellet form.

Mineralogy was confirmed using X-Ray Diffraction (XRD) (D2 PHASER, Bruker Corporation, Billerica, MA), with phase ID conducted using software (Diffrac.EVA, Bruker Corporation, Billerica, MA) and the PDF 4+ 2021 database (ICDD, Newton Square, PA). Bulk samples were used instead of powders, in part due to

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scarcity of materials and in part to account for any specific orientation that may exist within the samples.

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Results and Discussion

Microstructure

The initial samples to be analyzed using the SEM were the poorly dispersed samples, exhibiting porosity visible to the naked eye. Under SEM analysis, it was clear that the needle-like grains observed by Whipkey were actually platelets, as shown by the images in Figure 2.¹

Analysis of a fractured surface, as shown in Figure 4, which revealed that the samples had a very large quantity of these platelets, and they appeared to display random orientation and could likely have been mistaken for normal grains in previous samples when viewed from certain angles. Microstructures of all nine samples were then compared, as presented in Figure 5.

Figure 4. SEM images of fractured surface, showing both plate edges (left) and plate faces (right).

Plate edge

Plate face

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88% Al2O3 92% Al2O3 96% Al2O3

1 hour3 hours 10 hours

Figure 5. Microstructures of the three alumina concentrations heat treated at 1500°C for 1, 3, and 10 hours.

The grains were counted using the Hillard and the Heyn line methods, as illustrated in Error! Reference source not found., which depicts both methods.¹⁷ Platelets are defined as any grain having an aspect ratio greater than 5. It could not be determined whether non-platelet grains (grains defined as having an aspect ratio less than

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2) were also plate faces, and so they were counted as non-platelets. The intersections were then counted and plotted against firing time in Figure 7. There was no consistent change in number of grains with dwell time, however, increasing the dopant level increased the percentage of grains that were seen as platelets (otherwise labeled plate edges).

Figure 6. Hillard (circle) and Heyn (random straight lines) methods for grain size measurement.¹⁷

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Figure 7. Ratio of platelets to non-platelets as a function of dwell time in hours.

The aspect ratio of the grains is presented in Figure 8. The dimensions of at least 35 grains were compiled and are presented as an overall average and standard deviation of length and width in Table IV. Platelets were not seen to grow in length or coarsen with respect to dwell time.

Table IIV. Average platelet grain lengths and widths from SEM images (microns).

88% Al2O3 92% Al2O3 96% Al2O3

Dwell Time

(hours) L W L W L W

1.0 7.18 0.55 4.87 0.36 4.13 0.59

3.0 8.92 0.46 6.43 0.58 5.03 0.74

10 7.3 0.54 7.03 0.64 4.54 0.74

0 10 20 30 40 50 60

0.3 3 30

Percent Platelets (#platelets/#non-platelets, %)

Dwell Time (hours)

88% Al2O3 92% Al2O3 96% Al2O3

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Overall average grain length 6.16 ± 1.60 Overall average grain width 0.58 ± 0.12

Figure 8. Average aspect ratios for all nine samples

Densification

The fired samples displayed increased density with increasing alumina content.

Additionally, density increased marginally with increasing soak time, as shown in Error!

Reference source not found.. Relative density was calculated as a function of the measured density relative to the calculated density based on Rule of Mixtures (ROM) (Equation 1):

𝑓_{𝑚𝑎𝑠𝑠,𝐴𝑙} 1

𝜌_𝐴𝑙+ 𝑓𝑚𝑎𝑠𝑠,ℎ𝑖𝑏𝑜𝑛𝑖𝑡𝑒

1

𝜌_{ℎ𝑖𝑏𝑜𝑛𝑖𝑡𝑒}= 1

𝜌_{𝑠𝑎𝑚𝑝𝑙𝑒} (1)

0 5 10 15 20 25 30

82 84 86 88 90 92 94 96

Aspect Ratio (L/W)

Relative Density (Measured/ROM calculated, %) 88% AL2O3 92% Al2O3 96% AL2O3

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Where f mass, Al is the mass fraction of alumina, fmass, hibonite is the mass fraction of hibonite in the sample, ρAl is the density of alumina, ρhibonite is the density of hibonite, and ρsample is the calculated density of the sample.

Table V. Calculated density, using ROM and the calculated theoretical maximum percent of CA6

Theoretical CA6 Max Al2O3 Theoretical Density (g/cm³) (ROM)

35.7% 64.3% 3.88

71.5% 28.5% 3.74

96.1% 3.90% 3.71

80 82 84 86 88 90 92 94 96 98 100

0.3 3 30

Relative Density (Measured/ROM Calculated, %)

Dwell Time (hours)

88% Al2O3 92% Al2O3 96% Al2O3

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Figure 9. Relative density as a function of dwell time for the three alumina levels investigated.

The poor densification observed with increasing hibonite levels suggests that hot pressing may be necessary to reach full densification. This may be a critical step to obtaining high density specimens for mechanical testing.

Composition Analysis

EDS maps were taken of all samples to evaluate the calcium distribution, and along with backscatter imaging (Figure 10), these results correlated with the presence of a second phase and the apparent lack of CaO in the grain boundaries.

Figure 10. Energy dispersive spectroscopy map of calcium and backscatter image, 1 hour dwell 96% Al2O3 sample bright purple indicates high calcium levels.

Semi-quantitative analysis supported a target ratio of SiO2:CaO at 1:3. XRD analysis was conducted to confirm the presence of a second phase, and the labeled

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spectra of 1-hour samples is presented in Figure 11. As dwell time increases, the peaks remain similar in height and width, however, as dopant concentrations increase, the peaks increase in intensity, consistent with expectations, as shown in Figure 12.

Figure 11. XRD spectra of 1 hour dwell 88% Al2O3 sample with corundum (C) and hibonite (H) peaks labeled

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Figure 12. XRD spectra of 10 hour dwell samples, with a change in intensity relating to the (110) peak at a 2Θ of 31.8°

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Conclusions

Hibonite, or CaO·6Al2O3, is found to precipitate in alumina with high CaO:SiO2

dopant ratios, where the compositions reside in the hibonite phase field. These

precipitates are in the shape of platelets. Hibonite concentration increases with increasing dopant concentration but longer dwell times from one to ten hours do not appear to coarsen the platelets, maintaining a constant aspect ratio. These platelets form an interconnected network that hinders densification of the alumina matrix, suggesting that hot pressing may be necessary to achieve full density.

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Suggestions for Future Work

The addition of platelets into the alumina body is likely to significantly improve the fracture toughness. It is recommended that specimens be created for strength and fracture toughness measurements. It is possible, however, that the hindered densification observed in this thesis may require hot pressing to obtain fully dense parts that would be necessary for mechanical property measurement.

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