Diamond windows and domes: flexural strength and thermal shock

Ring-on-ring flexural strength measurements performed on free-standing specimens of chemically vapor-deposited (CVD) diamond indicate that if fracture originates at the nucleation surface, the strength is thickness independent and best described as follows: characteristic strength of approximately 300 MPa: and a Weibull modulus of approximately three. In a test configuration that puts the growth surface in tension, the strength decreases with increasing thickness and drops to less than 100 MPa for specimens thicker than 0.5 mm. These strengths are much lower than anticipated and therefore, invalidate earlier speculations on the thermal shock resistance capability of CVD-diamond windows and domes. The purpose of this contribution is to evaluate the results of strength measurements and to provide an updated assessment of the thermal shock performance. In this context, it is essential to keep in mind that the transient stress intensity critically depends on the nature of the heat flow as characterized by the Biot number. Considering that any viable sensor window must also be able to withstand the mechanical Stresses induced by aerodynamic pressure. it is demonstrated that in a thermally thin regime. CVD-diamond plates or shells have unmatched figures of merit, which is of significance because presently available infrared-transmitting window/dome materials for the long-wavelength IR may not be able to handle the thermo-structural loads encountered on projected flight trajectories. (C) 2002 Elsevier Science B.V. All rights reserved.