Keywords

Introduction

Due to its versatility, polycrystalline alumina (Al2O3) ceramics are used in many commercial applications that require materials with good wear resistance, dielectric strength, and thermal conductivity. In addition, its strength and hardness properties have made it a desirable material for structural and armor applications. At the Army as well as at other institutions, the microstructure [1], mechanical properties [2,3,4], shock wave loading on alumina tiles [5, 6] and projectile penetration behavior [7,8,9,10] into alumina targets have been studied for many years. The grade of alumina often studied in these projects [3,4,5,6,7,8, 10] was the Coorstek AD995 grade material, which has a nominal composition of 99.5% alumina . Microstructural effects were broadly mentioned but were not specifically studied. Hence the idea to study two compositions of alumina in an attempt to discern the effects of microstructure on the properties and of the quasi-static and dynamic compressive strength was pursued. The CoorsTek AD995 was chosen as one of the composition, and the other was the AD85, the nominally 85% alumina composition.

Experimental Methods

To prepare the AD85 and AD995 samples for characterization, samples were mechanically polished using the Allied HighTech MetPrep 4™ PH-6 polisher, starting with the 600 grit silicon carbide and progressing down to the 0.25 micron diamond surface. Microstructural characterization of both compositions, AD995 and AD85, were conducted using the FEI Nova NanoSEM 600 scanning electron microscope (SEM). Secondary electron micrographs were used to determine the grain size using the linear intercept method [11]. Energy dispersive spectroscopy (EDS) elemental analyses and texture analysis using the electron backscattered diffraction (EBSD) were conducted with the Pegasus XM4 analysis system from EDAX Inc. attached to the SEM. X-ray diffraction (XRD) for phase analysis was conducted with the PANalytical X’Pert Pro x-ray, from a 2θ of 20°–120°. For 3-dimentional microstructural and defect analysis, micro computed tomography (micro-CT) with the Zeiss Xradia 520 Versa x-ray microscope was utilized. Knoop hardness testing was performed on a Wilson Tukon testing machine with a load of 2 kg according to the ASTM standard C1326 [12]. Knoop indents falling on surface flaws, such as a pore, were rejected. Low strain rate compression testing was done in a quasi-static MTS 810 testing machine at 10−3 s−1, while higher rate testing, at strain rates between 500 and 900 s−1, was done in a Split-Hopkinson pressure bar system.

Results

A representative micrograph of the AD85 alumina sample is shown in Fig. 1a and of AD995 in Fig. 1b. The brighter colored areas around the grains in AD85 is the liquid phase, indicating that this alumina was liquid phase sintered. The dark areas were determined to be porosity for both compositions. AD85 has significant porosity and a more elongated grain morphology compared to AD995. From 4 SEM micrographs of each composition, the average grain size was measured to be 2.95 μm for the AD85 and 7.75 μm for the AD995. These grain sizes differ from Coorstek’s reported grain size of 6 μm for both compositions [13]. In addition, as can be deduced from Fig. 1b, the AD995 appears to have a bimodal grain size distribution, or population of abnormal grain size in the microstructure . Hence the average grain size is not an adequate indicator of the grain size distribution for AD995.

Fig. 1
figure 1

Representative SEM micrographs of a AD85 and b AD995

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To identify the secondary phase present in the AD85 material, EDS analysis was conducted. The AD85 EDS results indicated the presence of Mg, and Si, in addition to Al and O. The EDS results of AD995 did not show the presence of Mg, but did show Si peaks. The XRD results shown in Fig. 2a confirmed the presence of Mg and Si in AD85, identifying the secondary phase as Mg2SiO4. It is likely that MgO and SiO2 reacted during the process to form the Mg2SiO4 phase. However the XRD results of AD995 shown in Fig. 2b only identified Al2O3 , and did not identify any silicate phases.

Fig. 2
figure 2

XRD peaks identified as Al2O3 and Mg2SiO4 for a AD85 and only Al2O3 for b AD995

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Knoop hardness for AD85 and AD995 were reported by CoorsTek but at the 1 kg load. They were 9.4 and 14.1 GPa, respectively [13]. The 2 kg load (19.6 N) was chosen for this study since a ceramic hardness evaluation work by Swab [14] established that at 2 kg, the hardness curves would be in the load-independent plateau region. At the 2 kg load, the hardness of AD85 was 6.4 GPa with a standard deviation of 0.3 GPa. For AD995, the hardness was 9.6 GPa with a standard deviation of 1.8 GPa. The low hardness value of AD85 can be attributed to the numerous pores in the sample. However for AD995, the minimal amount of pores cannot explain the much lower hardness values compared to the manufacturer reported value. Jones et al. [7] determined their AD995 sample to have a Knoop hardness value of 13.2 GPa with a standard deviation of 1.3 GPa, a value much closer to that reported by CoorsTek. Unfortunately Jones et al. did not include a micrograph of their material in their paper for microstructural comparison to the current AD995. It is possible that in the current study, some of the Knoop indents were falling on the abnormal grains, lowering the hardness values.

Micro-CT scans were conducted on both AD85 and AD995. As expected from the SEM micrographs, only small pores were observed throughout the AD995 sample volume. However in the AD85, large pores were found throughout the volume of the sample. The micro-CT scans at a resolution of 2 μm/pixel were analyzed to determine the volume fraction of pores. The pores were segmented from the scans using grayscale intensity based thresholding. The binary images of the segmented inclusions were then combined to form a 3D reconstruction, as shown in Fig. 3a, from which sizes of the pores were characterized. The volume distribution of the pores was produced and is shown in Fig. 3b.

Fig. 3
figure 3

a 3D representation of alumina pores in AD85 reconstructed using micro-CT scans and b Volume histogram of the pores characterized using the 3D reconstruction

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Under quasi-static conditions AD85 demonstrated a compressive strength of 1970 ± 100 MPa and AD995 a strength of 2360 ± 180 MPa, both in line with manufacturer’s reported values. Under dynamic conditions both see a rise in strength, with AD85 having an average strength of 3180 ± 110 MPa and AD995 a strength of 3880 ± 670 MPa. Eliminating the potential outlier in AD995 from consideration gives an average strength of 3430 ± 230 MPa. These results plotted in Fig. 4, show that strength increases with strain rate for these materials, although only 2 strain rates were tested. While AD85 has lower strength under both quasi-static and dynamic conditions, over the range tested it also demonstrates a greater proportional increase in strength in comparison to AD995.

Fig. 4
figure 4

Compressive strength versus strain rate for AD85 (circles) and AD995 (diamonds)

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Discussions

The two alumina microstructures were drastically different. This is attributed to not only the compositional purity of the alumina samples, but the processing and sintering aids added. Goswami et al. [1] have shown the influence of oxide additions and their amounts on the morphology of alumina grains. It is safe to deduce that in the current AD85 material, the elongated grain shapes and the porosity is due to the liquid phase sintering process. In the case of AD995, the microstructure showed the presence of abnormal grains. This can be explained with the Si (in the form of silica) addition in the microstructure found by EDS analysis. Dillon et al. [15,16,17] have investigated the role of additives such as silica on grain boundaries and grain growth of alumina . They determined that additives, such as silica cause complexion transitions to occur, inducing abnormal grain growth for some grains, depending on the grain boundary energy of those grains.

The Knoop hardness and the quasi-static and dynamic compressive strength is correlated to the different microstructures . The porosity played a large role in the lower hardness values measured on the AD85 sample. However, although the compressive strength were lower than those of AD995, the porosity found throughout the sample of AD85 did not appear to significantly influence the strength. The preliminary results show that the compressive strength increased with strain rate for both compositions. More research is necessary to understand the role of the homogeneously distributed pores on the quasi-static and high strain rate compressive strengths , and additional compression testing is required to definitively link the increase in the compressive strengths to the strain rate.

Conclusions

Two compositions, nominally 85 and 99.5%, of alumina were characterized. The AD85 sample showed a liquid phased sintered microstructure , consisting of a Mg2SiO4 secondary phase surrounding elongated alumina grains. The AD995 sample showed evidence of abnormal grain growth, most likely due to the silica sintering additives causing complexion transitions. There were distinct differences in the Knoop hardness values. The lower hardness of the AD85 material was attributed to the porosity. The compressive strengths were measured and determined to increase with increasing strain rate. The AD85 alumina had lower but not significantly lower compressive strengths compared to those of AD995. More experiments are necessary to deduce the influence of the porosity and microstructural differences on the quasi-static and dynamic compressive strengths of alumina .