ALON™ Optical Ceramic is a durable window material for UV, Visible and Mid IR window and dome applications. The mechanical, thermal, and optical properties of ALON products produced commercially by Surmet Corporation have been measured and this new data will be presented. Comparisons to previously measured data will be made.
Optical quality, low scatter ALON having high strength that is nearly double previously reported has been made. Average strength values of 700 MPa at 21°C and 631 MPa at 500°C have been measured for ALON specimens prepared by precision surface finishing techniques. Polished optical domes tested have survived severe thermal shock tests. These strength levels are comparable to those for single crystal sapphire. Strength, thermal conductivity, thermal expansion, refractive index, emissivity and absorption coefficient will be presented. The possible mechanisms for the increased strength will be discussed.
Aluminum oxynitride, ALON is a transparent polycrystalline ceramic which has high strength and hardness. It is transparent from ultraviolet (UV) wavelengths to mid infrared wavelengths (MWIR). High speed IR missile dome applications require a durable transparent material that can withstand severe thermal shock due to aerodynamic heating, remain transparent throughout the entire flight trajectory and be highly resistant to rain and sand damage. Light weight transparent armor applications require high strength and hardness in the outer layers of a ballistic laminate to defeat high velocity projectiles. Lighter weight ballistic laminate designs can be achieved by using thinner ALON panels having greater fracture strength.
The technology for the making ALON transparent ceramics was transferred from Raytheon Company to Surmet Corporation in 2002. The manufacture of dozens of large ALON panels for armor performance evaluations and the production of hundreds of domes, lenses and point of sale (POS) scanner windows have been achieved at Surmet over the past two years. In light of this recent manufacturing scale-up of the production of ALON it is important to reevaluate the critical material properties such as strength, thermal conductivity, thermal expansion, and transparency and demonstrate that the material is as good as or better than that which was produced in a laboratory/pilot scale process.
The data and results presented in this paper are part of Air Force sponsored Phase II SBIR program structured to addresses the need for an up to date and accurate material properties database for currently produced transparent ALON. Results of these measurements will be used in the future to model and predict the performance of ALON in missile dome and transparent armor applications. This materials properties study also successfully demonstrated that the same deterministic grinding and finishing operations could be used to produce high strength test samples as well as low-cost high strength thermal shock resistant missile domes.
2.0 MECHANICAL PROPERTIES
Mechanical Properties Overview:
Mechanical properties testing of ALON was performed at the University of Dayton Research Institute (UDRI) in Dayton, Ohio. Biaxial Flexural Strength (at 21°C and 500°C), Young’s Modulus, shear Modulus and Poisson’s Ratio were measured using standard ASTM C1499-03(1) and ASTM C1259-01(2) test methods. Weibull modulus and characteristic strength were determined by fitting the measured data to a standard two parameter Weibull distribution given by Equation 1, ASTM C 1239-94a(3). Figure 1 shows the test fixture used for the biaxial flexure strength testing at room temperature and 500°C. Equation 2 is used to calculate the strengths from the sample and fixture dimensions (see Table 1) and the breaking load. Grafoil ™ discs having a thickness of 0.005” thick were placed between the sample disc and the load and support rings to reduce the contact stresses that are thought to cause some samples to fail under the loading ring or support ring.
Elastic Properties Measurements
The elastic properties of ALON were measured according to ASTM C1259-01 using the same disc specimens (1.25” diameter x 0.055” thick) used for biaxial flexure testing. The values measured at UDRI for the room temperature Young’s Modulus, shear modulus and Poisson’s ratio are given in Table 2 with comparisons to previously measured values. These current measured values are only slightly different from historical values (4, 5, 6, and 7). Test specimens in all three set of measurements were 100% dense transparent ALON. The difference may be in the due to the greater accuracy of the flexural resonance method used in the current measurements.
Biaxial Flexure Strength Measurements
The testing program initially set out to compare three different grades of ALON and two test temperatures (RT and 500°C). The results of these measurements (May 2004) showed average strengths comparable to historical values (Historical Raytheon 2002) (8), however, the Weibull modulus was very low and the there were some very low and some very high strengths measured. Since the material is transparent it is relatively easy to inspect for the presence of strength limiting flaws. Having not found anything of significance the next step was to examine other possible reasons for the wide variation in strengths. There were some concerns that there was damage introduced into the samples by the fixed abrasive grinding process. Typically free abrasive grinding is used to lap the material prior to final finish. There were several other concerns which included the one-inch sample diameter, and the possibility of damage introduced during the edge beveling step.
It was decided that the next set of tests compare both fixed and free abrasive grinding as well as the benefits or detriments of chemical etching the ALON prior to mechanical testing. Chemical etching or a combination of chemical etching followed by a light polish has been demonstrated to reduce the average flaw size and increase the fracture strength of garnet (YAG, GSGG, and GGG) slab lasers (9). Table 3 and Table 4 describe the complete set of samples and test conditions evaluated in this study.