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mental chamber operates in inert or Products Inc., Madison Validating the system reducing atmospheres, or under mild Heights, Mich. vacuum (to about 35 kilopascals). The induction coil The system was validated for flexure tests of zirconium diboride-based A proportional integral derivative presented the most eutectic that occurs around 2390°C limited the upper test temperature–C2ceramics up to 2,300°C using a graphite test fixture. The ZrB controller regulates the temperature. significant design chal- to 2,300°C. The figure shows examples of load–deflection curves for A type-B thermocouple measures lenges. The original two specimen types. The black curve is for a ZrB2 specimen tested at temperatures below 1,600°C, and a induction coil did not 2,200°C in argon, and the red curve is for a ZrB2–30SiC particulate two-color pyrometer measures above have enough electrical composite tested at 1,800°C, also in argon. The average strength of 1,500°C. To date, four-point bend tests insulation, and ambient the ZrB2 was about 300 MPa at 2,200°C. The average strength of the according to ASTM C1211 (“Flexural graphite dust caused ZrB2–SiC was about 220 megapascals at 1,800°C.2 These were the Strength for Advanced Ceramics at it to electrically short first mechanical property measurements for ZrB2 ceramics at tem- peratures above 1,600°C reported since work performed by Rhodes, Elevated Temperature,” ASTM Book to the insulation pack et al., at Manlabs Inc. in 1970.3 of Standards, ASTM International, surrounding the suscep- West Conshohocken, Pa.) have been tor, melting a portion performed, but tensile and compres- of the coil and burn- sion tests are possible with proper test ing a hole through the fixtures. insulation. Now, the coil is wrapped with Design challenges mica tape, then fiber- The system was designed to test at glass tape, and covered temperatures up to 2,500°C. Choosing with Nextel sleeving. induction heating overcame some of A sheet of alumina the limitations of a commercial graph- paper further isolates ite- or refractory-element vacuum the graphite insulation furnace, such as chemical compatibility from the induction coil. high temperatures, heating rate limits, successfully up to (Credit: Missouri S&T.)This design performed with the fixturing and specimens at and limited atmospheres. Induction 2,600°C. heating also allows for changing the We have since made Examples of load–displacement curves for ZrB2 tested at hot-zone material and test fixtures several modifications. 2,200°C and ZrB2–SiC tested at 1,800°C. depending on the sample material and The original design test atmosphere. Finally, because induc- required the ability temperatures to several minutes. tion furnaces have higher heating and to operate under vacuum, which con- The original design did not incorpo- cooling rates than graphite-element strained the size of the load frame, while rate direct strain measurement capabil- resistance furnaces, the system accom- giving maximum space for the furnace, ity. Hence, estimating strain requires modates multiple test runs per day. insulation, and fixturing. However, the compliance-corrected axial displace- The drawback to this approach was original design did not include feed- ment measurements. This restraint that no commercially available systems throughs for water, gas, or instrumenta- limits the precision of elastic moduli appeared to meet our design require- tion. Holes were added for additional calculated from load–displacement ments. access points, but this created concerns curves. Finally, the first test fixture was Students and faculty constructed the regarding the mechanical stability of constructed from graphite, which limits system in several stages over a period the chamber. A graduate student in the the maximum test temperature for ZrB2 of about six years. Several components Mechanical Engineering Department at to 2,300°C. We are fabricating a ZrC were purchased: the environmental Missouri S&T performed finite-element test fixture to address this limitation. chamber, induction power supply, analysis of the chamber under vacuum This fixture should increase the upper pyrometer, temperature controller, stresses to determine whether the holes test temperature for ZrB2 specimens to and load cell. The load frame (Model would compromise the structural integ- about 2,600°C. 33R4204, Instron, Norwood, Mass.) rity of the chamber. Based on the analy- came from another department on sis, operation has been limited to mild Working with the system campus. Graduate students at Missouri vacuum levels (about 35 kilopascals). To perform a test, a specimen is S&T designed and fabricated most of Moreover, the chamber body is not secured to the fixture using a high- the load train assembly, induction coil, actively cooled. Although the induction strength adhesive, such as Super Glue, hot zone, and gas-handling system. coil is capable of sustained operation at and placed in the hot zone. The envi- Graduate students also designed the test temperatures up to 2,600°C, the lack of ronmental chamber is closed, evacu- fixtures and rigid graphite components, cooling loops on the chamber limits the ated, and backfilled with argon several which were fabricated by Graphite time that specimens can be held at test times to remove as much air as possible. American Ceramic Society Bulletin, Vol. 92, No. 1 | www.ceramics.org 37


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