Accurate measurement of strength and fracture toughness for miniature-sizethick diamond-film samples by three-point bending at constant loading rate

Fracture strength and fracture toughness (K-CI) of miniature-size thick fre estanding diamond film samples were accurately measured by three-point bend ing at a constant loading rate. A specially designed testing rig, capable o f achieving a constant loading rate as small as < 0.5 N s(-1) was used, whi ch is beneficial for accurate measurement of the fracture strength and toug hness for brittle materials like diamond films. A novel method was employed to nucleate and accurately control the length of the pre-crack. A speciall y designed loading fixture was used to ensure that the diamond film sample was tested in a near-perfect vertical position, as required by the ASTM E39 9 standards. Effects of sample dimensions on the measurement of fracture to ughness and fracture strength are discussed; it was shown that miniature-si ze freestanding diamond film samples can be used for the accurate determina tion of fracture strength and fracture toughness. However, for accurate mea surement of fracture toughness, the use of pre-cracked diamond film samples is absolutely necessary. The effect of loading configuration on the measur ement of fracture strength is discussed in detail. In addition to the frequ ently reported two loading configurations (with the nucleation and growth s urface in tension, respectively), a third loading configuration with the ed ge surface in tension is also discussed. The Weibel modulus was also found to be strongly dependent on the loading configuration. The significance of these findings is discussed. (C) 2001 Elsevier Science B.V. All rights rese rved.