THE reaction-bonded aluminum oxide (RBAO) process is a

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1 Journal J. Am. Ceram. Soc., 88 [8] (25) DOI: /j x Processing and Properties of ZrO 2 -Containing Reaction-Bonded Aluminum Oxide with High Initial Aluminum Contents Paul M. Sheedy,* Hugo S. Caram, Helen M. Chan,* and Martin P. Harmer*,w Center for Advanced Materials and Nanotechnology, Lehigh University, Bethlehem, Pennsylvania 1815 The reaction-bonded aluminum oxide (RBAO) process relies upon the oxidation of Al/Al 2 O 3 powder compacts, and many of its associated advantages stem from the presence of the aluminum in the green powder. Higher aluminum contents in the starting powders allow for higher green strengths, densities, and lower overall shrinkage, all while producing a fine-grained, high-strength sintered material. However, it is evident that the reaction and sintering of ZrO 2 -containing RBAO with higher aluminum contents are more challenging. Therefore, in this study, the effects of aluminum content on the processing, structure, and properties of RBAO ceramics were comprehensively characterized. It was found that RBAO samples with high aluminum contents were more prone to cracking during reaction and even when successfully fired were not able to be sintered to full density. Despite these characteristics, RBAO samples with increasing aluminum contents showed no significant degradation in mechanical properties. I. Introduction THE reaction-bonded aluminum oxide (RBAO) process is a versatile process for the production of Al 2 O 3 -based ceramics from aluminum-containing precursors. 1,2 There are several advantages to the RBAO process including: lower cost raw materials, high green strengths and densities, low sintering shrinkage, and high strength final ceramics with glass-free grain boundaries. 3 5 The high green strengths and densities result from the deformation of the ductile aluminum during compaction. The aluminum particles bridge together, providing strong bonds throughout the compact. In addition, the B28 volume expansion associated with the oxidation of aluminum reduces the overall shrinkage incurred during sintering. The improved properties of the green compacts enable easy green machining without the use of binders as is common for the conventional processing of Al 2 O 3 ceramics. 6 Combined with the low shrinkage of RBAO samples, this can mean near net-shape-forming capabilities and improved dimensional tolerances. 7 These most attractive advantages of the RBAO process arise from the presence of aluminum in the green compact and are enhanced with increasing aluminum content. However, in most studies, RBAO compositions have typically contained less than 45 vol% aluminum that would have been further reduced as a result of oxidation during the milling process. The milling of RBAO powders is perhaps the most important step in the process, as the powder properties determine the properties of the final ceramic. 8 1 At Lehigh, attrition milling is conducted in mineral spirits for safety reasons, but due to its lower solubility for oxygen and water (as compared with other M. Cinibulk contributing editor Manuscript No Received July 29, 23; approved March 11, 25. Supported by the U.S. Office of Naval Research, under Grant Nos. N and N *Member, American Ceramic Society. w Author to whom correspondence should be addressed. dlh3@lehigh.edu 24 solvents such as acetone and ethanol), less aluminum is oxidized during the milling process. It follows that for identical initial powder compositions and milling conditions, an RBAO powder milled in mineral spirits retains a greater amount of aluminum after the milling process, which will affect the subsequent reaction, sintering, and properties of RBAO ceramics. 8 Therefore, in this study, the effects of high aluminum contents (both initial and retained after milling) were characterized using particle size analysis, thermal analysis techniques, microstructural evaluation, and mechanical property testing. II. Experimental Procedure (1) Powder Processing Three RBAO powders with initial aluminum contents of 45, 55, and 65 vol%, designated,, and, were prepared from starting powders of Al (Powder, spherical, 99.1%, B4 mm, Aldrich Chemical Co. Inc., Milwaukee, WI), a-al 2 O 3 (AKP- 3, %, B.4 mm, Sumitomo Chemical America, Inc., New York, NY) and t-zro 2 (TZ-3Y, B.1 mm, Tosoh USA Inc., Grove City, OH). All three powders also contained 2 vol% t-zro 2 (for improved mechanical properties), with the balance being a-al 2 O 3. The initial compositions for each powder are shown in Table I. All RBAO powders were prepared under similar conditions in 2 g batches. This consisted of attrition milling (Laboratory model 1HD Szegvari Attritor System, Union Process, Akron, OH) at a speed of 58 rpm for 1 h, using 2.5 kg of a mixture of 2 3 mm spherical tetragonal zirconia polycrystal grinding media and 41 ml of mineral spirits as the milling medium. The mill was checked every hour to ensure that the level of the mineral spirits was maintained, since the milling system used was open to the atmosphere. After attrition milling, the grinding media were separated from the powder slurry using a strainer and rinsed with acetone to remove any powder stuck to the media. The powder was then extracted from the slurry using a vacuum filtration technique and rinsed with acetone to ensure that no mineral spirits remained, enabling the powder to dry more easily. The filtered powder was then placed in a glass tray and dried in a fume hood overnight. The resulting powder cake was subsequently pulverized with a mortar and pestle and the powder was passed through a #8 sieve (18 mm). The particle sizes of the powders were determined using a laser scattering particle size analyzer (LA-91, Horiba Instruments, Inc., Irvine, CA). A few milligrams of the sieved powders were dispersed in 5 ml of ethanol and placed in an ultrasonic bath for 3 min prior to measurement. (2) Thermal Analysis Thermogravimetric (TG) (STA 49, Netzsch Instruments, Burlington, MA) measurements were conducted on B8 mg of loose RBAO powder and pressed disc samples. The disc samples (B22. mm diameter 2. mm thickness) were fabricated by uniaxially pressing B2. g of powder in a 25.4 mm cylindrical die at 5 MPa followed by cold isostatic pressing at 29 MPa. For these samples, a standard TG-only plate sample stand was modified with three 3 mm alumina milling balls to provide a base for

2 August 25 Processing and Properties of RBAO with High Initial Aluminum Contents 241 the sample to rest (similar to a ball-on-ring for a biaxial flexure test), while allowing sufficient air flow to the entire sample. For all thermogravimetric measurements, the samples were heated at a rate of 11C/min in air. Various isothermal holds were also incorporated into the heating cycles as indicated throughout the text. Dilatometry measurements were made using a push-rod dilatometer (DIL 42C, Netzsch Instruments, Burlington, MA) on rectangular samples cut from the pressed discs. The samples were sanded to ensure that the two sides perpendicular to the pushrod were straight and parallel. Based on the results for TG, the following heating cycle was used for dilatometer measurements: 11C/min to 111C with a 1 h isotherm at 451C, 51C/ min to 1551C, 2 h isotherm, and 51C/min cooling to room temperature. Samples for in situ temperature measurements (B24 mm diameter 2.5 mm thickness) were fabricated by placing a type-b thermocouple inside a rubber mold filled with B2.5 g ofpowder that was cold isostatically pressed at 29 MPa, thereby embedding the thermocouple in the sample. The temperature measurements were made on the samples containing the thermocouple by recording the thermocouple output during reaction in a tube furnace (Model 59256, Lindberg Co., Riverside, MI). The readout from the thermocouple was recorded by a strip chart recorder. The temperature of the furnace was recorded by a second thermocouple situated next to the sample in the hot zone, but far enough to avoid thermal effects from the reacting samples. (3) Microstructure and Properties Disc samples for microstructure and property measurements were fired in a box furnace (Rapid Temp Model 17, C. M. Furnaces Inc., Bloomfield, NJ) using the same heating cycle as the dilatometry measurements. The samples were prepared for microscopy by grinding and polishing to a 1 mm finish, thermal etching at 141C for 3 min, and coating with carbon. Microstructural analysis was conducted using a field emission scanning electron microscope (63f, JEOL USA, Inc., Peabody, MA) operated at 3.1 kv. Grain size measurements were made on 1 random fields encompassing at least 15 grains. Tracings of the microstructure were made using a transparency. An image analysis program (NIH Image) was used to measure the grain area distributions, which were converted into grain size by calculating the equivalent spherical diameter for the respective areas. Various properties of both the green and final fired RBAO samples were determined. The green densities were calculated using the measured mass and dimensions of the green samples. Final densities were calculated using the Archimedes technique, with water 1.1% wetting agent used as the immersion medium. The green and final strengths of the samples were measured as-pressed and as-fired, respectively, using a biaxial flexure technique. The testing rig consisted of three 3.18 mm stainless-steel balls placed in a mm diameter support radius and a 9.53 mm stainless-steel ball with a loading flat of 3.13 mm for the punch. The failure load was applied by a servohydraulic mechanical test frame (Model 135, Instron Corp., Canton, MA) and the output was recorded by an oscilloscope. The fracture strength was calculated in accordance with ASTM standard F Samples for hardness and fracture toughness were taken from those prepared for microstructural analysis and indented with a 1 N load using a Vickers diamond indenter (V-1A, Leco Corporation, St. Joseph, MI) on the polished surface. Both the lengths of the indentation diagonals and associated radial cracks were recorded. The hardness values were calculated as H 5 P/2a 2 and the toughness values were calculated using the relationship K IC 5 wp/c 3/2, with dimensionless constants w 5 x(e/h) 1/2 and x The elastic moduli of the samples were measured using an impulse excitation technique (Grindosonic MK-5 Industrial, J.W. Lemmens, Inc., St. Louis, MO) according to ASTM Standard C , 12 on rectangularshaped samples similar to those prepared for dilatometry, which were ground and polished to a 1 mm finish. The critical flaw size of the samples was determined from the strength, elastic moduli, and hardness values according to the relationship K IC /p 1/2 s f a 1/2, assuming that a half-penny surface flaw was responsible for failure. III. Results and Discussion (1) Effects of Aluminum Content on the Powder Processing of RBAO The green compositions and properties of the,, and powders and pressed discs are summarized in Table I. Note that during the milling process, B2 25% of the aluminum was oxidized (as a percentage of the initial aluminum content). Therefore, the actual post-milling or green aluminum contents for the,, and powders were B34%, 43%, and 51%, respectively. The data also showed that for the same milling conditions, less aluminum was oxidized (on a percentage basis) as the initial aluminum content increased from to, to. The particle size distributions (Fig. 1) increased slightly with increasing aluminum content, with the samples,, and having mean particle sizes of 1.7, 2.9, and 4. mm, respectively. Therefore, one consequence of higher initial aluminum contents was a lowered milling efficiency. This was likely due to the fact that there were less ceramic fines in the powder mixture to aid in the communition of the more ductile aluminum particles that would have more of a tendency to coldweld together. Thus, the reduction in the amount of aluminum oxidized during milling (with increasing aluminum content) can be explained by the decrease in milling efficiency, since less aluminum surface area was exposed to oxidation. However, as opposed to milling in acetone, where a powder with the same composition as powder would typically contain less than 5% of the initial aluminum concentration, powders milled in mineral spirits ultimately retained more of the initial aluminum content. 8 Powders,, and retained 75%, 78%, and 79% of the initial aluminum concentrations, respectively. The bulk densities of the green bodies formed from each powder were 2.4 g/cm 3, which corresponded to theoretical green densities of 6.7%, 62.6%, and 64.4% for the,, and samples, respectively. The corresponding green strengths were also enhanced with increasing aluminum content and were 12.3, 15.8, and 22.5 MPa for the,, and samples, respectively. (2) Reaction of RBAO with Increasing Al Content (A) Reaction of Powders: Despite the slight differences in green particle size, the three powders show similar reaction Frequency (%) Fig Particle Size (microns) Particle size distributions for the,, and powders.

3 242 Journal of the American Ceramic Society Sheedy et al. Vol. 88, No. 8 behavior. The differential thermal analysis (DTA, mv/mg), thermogravimetric (TG, % weight change), and differential thermogravimetric (DTG, %/min) curves for loose powder samples of,, and (reacted at 11C/min) are shown in Fig. 2. Aside from the slight melting endotherms, the DTA curves were mirrored by the DTG, which indicated that the reaction measured by the DTA corresponded to the oxidation of aluminum. As expected, each powder displayed the traditional two-stage reaction behavior, characterized by separate solid gas and liquid gas oxidation reactions (i.e., above and below the melting point). The fastest reaction rate during the solid gas reaction occurred at slightly higher temperatures with increasing aluminum content (5191C, 5211C, 5261C), and the magnitude of the DTA and DTG peaks increased with increasing aluminum content as well. The percentages of the total aluminum content (i.e., fraction converted) oxidized below the melting point for the,, and were B76%, 72%, and 68%, respectively. The amount that reacted below the melting point typically correlated to the particle size of the RBAO powder, so this result was not unexpected. 4 (B) Reaction of Bulk Samples Content: Another consequence of the higher initial aluminum contents was the difficulty in reacting bulk (disc) samples. Although the DTA measurements showed a substantial exothermic reaction associated with DTG /(%/min) TG, % DTA /(uv/mg) Fig. 2. DTA, TG, and DTG curves (top to bottom) for the,, and powders heated at 11C/min to 121C. Fig. 3. B22 mm diameter B2. mm thick discs of,, and samples reacted in a box furnace heated at 11C/min. the RBAO process, the relatively small sample volumes (powders) used were not able to completely capture the violent reaction that occurred. Figure 3 shows a photo of larger disc samples that were fired in a box furnace at 11C/min for microscopy and property characterization. The observed mud-like cracking behavior always occurred for samples with initial aluminum contents greater than 45 vol% (which had been milled in mineral spirits), and became increasingly more severe with increased aluminum content. This cracking intensity correlated with the in situ temperature measurements made on similarly large samples. Figure 4 shows the actual sample temperatures of the,, and samples as measured by an attached thermocouple plotted against the measured furnace temperature. As mentioned above, the differential thermal analysis showed an exothermic reaction that corresponded to a large weight gain, but had not indicated such dramatic temperature changes as measured by the embedded thermocouple. Similar to the DTA behavior, both the temperatures of the peak reactions and the magnitudes of the peaks increased with increasing aluminum content. Self-heating of the,, and samples produced DT s of B691C, 741C, and nearly 81C, respectively. In a similar fashion, Fig. 5 shows the TG and DTG behavior of larger disc samples fired, which showed results significantly different from the loose powder. Each sample showed a period of rapid oxidation that was quickly extinguished as evidenced by the DTG curves. The reaction was more pronounced with increasing aluminum content; all the samples cracked in the TG in a fashion similar to those shown in Fig. 3. In fact, the sample cracking was so severe that much of it fell off the balance at about 81C and corrupted the measurement. Sample Furnace Temperature, C Fig. 4. In situ temperature measurements for -, -, and - pressed samples.

4 August 25 Processing and Properties of RBAO with High Initial Aluminum Contents 243 TG, % DTG, %/min Fig. 5. TG/DTG curves for B22 mm diameter 2. mm thick pressed discs of,, and samples heated at 11C/min. DTG, %/min TG /% No Hold 45 C, 1 hrs 4 C, 1 hrs Time/min Fig. 6. TG/DTG data for B22 mm diameter 2. mm thick discs fired at a heating rate of 11C/min with no hold, and 1 h isotherms at 451 and 41C incorporated into the heating cycle. This type of reaction behavior has been linked to a self-propagating high-temperature synthesis (SHS) reaction occurring between aluminum and ZrO In such cases, it was shown that the initial oxidation of aluminum on the surface of the samples (with O 2(g) ) exceeded the rate of oxygen diffusion into the sample. At this point, the oxidation of aluminum continued, but now by an oxidation/reduction reaction with ZrO 2.Other investigators 14,15 have also shown the propensity for such a reaction to occur between Al and ZrO 2 in inert atmospheres and under vacuum. The overall Al/ZrO 2 ratio to form Al 2 O 3 and Al 3 Zr from such a reaction between Al and ZrO 2 is The Al/ ZrO 2 ratios for the,, and were B3.5, 4.5, and 5.5, respectively, and therefore contained excess aluminum that reacted with the zirconium to form Al x Zr y intermetallics, mostly Al 3 Zr (elemental zirconium had never been observed experimentally). From the ratios, it was expected that Al 2 Zr could have been found in the sample, but perhaps as seen in other work, the formation of Al 3 Zr was the preferred phase to form. 16,17 (C) Controlled Firing: The occurrence of the reduction reaction with ZrO 2 has been linked to the cracking shown in Fig. 3 and must be avoided to obtain dense, crack-free samples. To assess the validity of the low-temperature isotherms commonly used to fire RBAO, 18 disc samples of were subjected to 1 h isothermal treatments at 41 and 451C, as shown in Fig. 6. The results showed that during the isotherm at 451C, substantial aluminum was oxidized and runaway oxidation prevented further heatup, which allowed for the production of crack-free samples. Conversely, the pressed disc fired with a 41C isothermal treatment displayed reaction behavior similar to firing without an isotherm present at all. Since only a minute amount of aluminum was oxidized during the isotherm at 41C, a rapid reaction ensued on heat up and the sample cracked. It is interesting to note that, as shown in Fig. 7, the larger sample behaved identically to the powder sample (i.e., DTGs overlapped) when the appropriate temperature controls (as little as a 3 h hold at 451C) were used. For all subsequent samples, a 1 h isothermal hold at 451Cwasaddedtothe normal heating cycle. Figure 8 shows the TG/DTG results for the controlled firing of the,, and samples. (D) Microstructure & Properties: Once controlled firing of crack-free samples was achieved, the appropriate heating schedule was used for dilatometry measurements as shown in Fig. 9, a plot of the dimensional change vs. time for the,, and samples. This heating cycle was also used to fire crack-free samples for microstructural and property analysis. The dilatometry results (for temperatures o111c) in Fig. 9 were expected to parallel the thermogravimetric behavior of Fig. 8, since the oxidation of aluminum was accompanied by a 28% volume expansion. However, the expansion reaction did not exactly mirror the weight gain. This was clearly seen during the isothermal hold where significant weight gain did not translate into much macroscopic dimensional change. This signified that concurrent densification of the aluminum, or filling of the pores with oxidation product, had occurred. 4,19 At temperatures above 111C, the completely reacted samples began to densify. The phenomenon that occurred at the beginning of densification, which was slightly more pronounced with increasing aluminum content, was found by XRD to be due to the g to a-al 2 O 3 transformation. 4 After this transformation, there was a clear difference in the rate of shrinkage among the samples, with the samples with higher initial aluminum contents shrinking less and at a slower rate. While lower overall shrinkage was to be expected due to an offset from the expansion of the oxidation reaction, the apparent rate of densification observed in the dilatometry curves of Fig. 9 indicated a difference in the subsequent densification rates. In a companion paper considering the densification behavior of RBAO materials, these phenomena are considered further. 2

5 244 Journal of the American Ceramic Society Sheedy et al. Vol. 88, No. 8 DTG, %/min Small Large dl /Lo, % Time, min Fig. 9. Dilatometry data for,, and samples using the standard heating cycle for controlled firing (i.e., 1 h isotherm at 451C) Time, minutes Fig. 7. DTG data for B22 mm diameter B2. mm thick disc and a powder sample of showing similar behavior when appropriate temperature controls (3 h isotherm at 451C) are used. DTG, %/min TG, % Time, min Fig. 8. TG/DTG data for controlled firing of,, and samples using a 1 h isotherm at 451C. Fig. 1. Microstructure of (a), (b), and (c) samples sintered at 1551C for 2 h (thermally etched at 141C for 3 min).

6 August 25 Processing and Properties of RBAO with High Initial Aluminum Contents 245 Figure 1 shows typical final microstructures of samples,, and sintered for 2 h at 1551C. The final properties associated with these samples are listed in Table II. Clearly, it was evident that the density of the samples decreased with increasing aluminum content from 95.1% to 91.3% to 88.1% of theoretical density. At first glance, there also appeared to be a correlation between the aluminum content and grain size, but despite the variations in density, a stereological study revealed no significant differences among the three samples, with mean grain sizes of.64,.68, and.67 mm for the,, and, respectively. As expected, given the density differences among the samples, the values for elastic modulus decreased with increasing aluminum content (36 to 284 to 267 GPa) as did the hardness values (16. to 13.1 to 11.4 GPa). Surprisingly, the as-fired biaxial flexure strengths of the samples increased slightly from 52 to 58 to 599 MPa for the,, and samples, respectively (despite the increased size and frequency of porosity), and although the samples had similar grain size distributions. It is surmised that the slight variation in the ZrO 2 content among the samples vol% may have contributed to the differences. 21,22 In addition, the lower elastic modulus and increase in porosity may have allowed for the generation of higher stresses in the matrix, which could have induced a marginal contribution from transformation toughening. 23 In fact, there was a corresponding slight increase in fracture toughness with increasing aluminum content (3.1 to 3.5 to 3.6 MPa m 1/2 ), which correlated to the differences in the biaxial fracture strengths. This change in toughness certainly supports this hypothesis; however, no systematic study was conducted to determine how much, if any, t-zro 2 transformed during mechanical testing. However, another calculation that supported these trends was the determination of the average critical flaw size from the strength, toughness, hardness, and elastic moduli measurements. Assuming that a half-penny geometry surface flaw was the cause of failure, the critical flaw size for all samples was found to be B22 mm. In each case, the critical flaw size is substantially larger than any observed microstructural features. This may indicate that a processing flaw (on the sample surface) was the cause of failure, which, given the identical processing conditions, could be expected. IV. Summary The reaction-bonded aluminum oxide process is a robust process for producing Al 2 O 3 -based ceramics. By increasing the initial aluminum content, an attempt was made to further exploit the advantages of the process. However, many new difficulties arose during the processing of RBAO ceramics with higher aluminum content, which were not previously observed. These included the occurrence of a self-propagating high-temperature synthesis reaction during oxidation in air, which lead to cracking of pressed samples, as well as the inhibition of densification. The mechanical properties of the RBAO samples with increased aluminum content were exceptional for Al 2 O 3 ceramics with the levels of porosity that were contained in the samples. In fact, the strength and toughness of the samples seemed to show slight increases with increasing aluminum content and decreasing relative density. Other property data were consistent with this phenomenon, and possible explanations for these trends were given. References 1 N. Claussen, T. Le, and S. Wu, Low-Shrinkage Reaction-Bonded Alumina, J. Eur. Ceram. Soc., 5, (1989). 2 N. Claussen, R. Janssen, and D. 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