The fracture toughness (KIc) of LiF single crystal was evaluated by the indentation microfracture technique (IMT) and the indentation strength technique (IST) at room temperature. The indentation crack on the (001) of ‹100› specimen resulted in the lowest KIc of about 0.5MNm-1.5 in both IMT and IST because the {110} is the preferred cleavage plane of this crystal. The Vickers hardness (HV) for the (001) was obtained to be about 0.9GPa. A fairly smooth planar was provided by the crack-propagation originated from the initial crack on the (001) of ‹110› specimen. However, cracks in the ‹110› (110) specimen became eventually angled at 45° to the original flaw. As the result, KIc≈0.7MNm-1.5, HV≈0.9GPa for the former, and KIc≈0.8MNm-1.5, HV≈1.2GPa for the latter. The indentation crack on both the (110) and (112) of ‹111› specimen didn't propagate along the extention of Vickers indentation diagonal and produced the rough surface after fracture. The KIc values for ‹111› (110) and ‹111› (112) were determined to be 1.5 and 1.8MNm-1.5, respectively, while HV≈1GPa. These results indicate that the KIc of single crystals is affected by the cleavability of crystals and the geometric configuration between the cleavage plane and the direction of crack propagation.
