John Robert Brandon

Properties of freestanding CVD diamond optical components

This paper describes polished CVD polycrystalline diamond components, many in excess of 1 mm in thickness, being produced in the sizes and geometries required for many infrared window and dome applications. For instance, 100 mm diameter flats can now be fabricated routinely and 70 mm diameter hemispherical shells are being developed. It is essential that the properties of the material are suitable for such applications and this paper describes the characterization of these properties. Data on the materials optical transmission properties, absorption coefficients (at a wavelength of 10.6 micrometers) and imaging properties is presented. The dielectric properties, such as loss tangent and permittivity, have been assessed at microwave and millimeter wave frequencies and the suitability of CVD diamond for multispectral and multimode window applications is discussed. The strength of the material is also of considerable importance and this parameter has been evaluated as a function of sample thickness and grain size and the specific problem of assessing the strength of curved samples is also considered. Numerous curved and several hundred planar samples have been strength tested using a three point bend geometry and a statistical analysis (Weibull) has been applied to the results. The average strength of the CVD diamond growth surface, for samples of thickness 0.4 to 1.4 mm, has been determined to be 470 to 390 MPa respectively, with a Weibull modulus of 23 (when systematic variations in sample strength with thickness are taken into account), while the nucleation surface was found to have a higher average strength of 1000 to 770 MPa (at 0.4 and 1.4 mm thickness respectively) but a lower modulus of 11. The cause of this difference in strength is discussed in detail.