[Fracture toughness of porcelain using indentation method]

Material resistance to brittle fracture was quantitatively evaluated in the commercial porcelains, CERA 8 (CE 8), VITA DUR (VITD), VITA VMK 68 (VIT), CERAMCO II (CE II), UNIBOND (UNB), NORITAKE SUPER PORCELAIN AAA (AAA), PENCRAFT (FEN), COSMOTECH (COM) and OPTEC HSP OPT) from values of fracture toughness (KIC) obtained in the crack on a mode I, determined upon insertion of Vickers indenter. The results are summarized as follows: 1) Mean maximum and minimum values of Vickers hardness degree of 9 kinds of commercial porcelains at 5kgf of load for 15s were 1348 (SD 98.1) for OPT abd 666 (SD 74.6) for CE 8, respectively. 2) The value of half of the diagonal of indentation (a) ranged from 42 (SD 1.5) to 58 (SD 3.3) microns, and that of half of the crack length (c) ranged from 101 (SD 4.0) to 175 (SD 17.2) microns. 3) The ratio of (c) to (a) (c/a ratio) was within the range of 2.3 to 3.3, and median cracks were present. 4) KIC in the commercial porcelains determined by the indentation method was within the range of 2.04 to 4.69MNm-3/2, showing a maximum for OPT and minimum for VITD. 5) KIC of OPT was significantly greater than that of any other material. 6) The porcelains were divided by fractography of the direction of crack course into 2 groups: a group of intra-granular fracture showing linear cracks (AAA, COM and OPT) and a group of Inter-granular fracture showing a range of non-linear cracks (CE 8, VITD, VIT, CE II, UNB ND PEN).