Corroboration and extension of analysis of c-axis sapphire filament fractures from pores

Abstract

Newcomb’s thesis [1] and subsequent publications with Tressler [2, 3] provide very useful data on the tensile fracture of c-axis sapphire filaments (Saphikon, Inc., Milford, NH) between 22 and (mostly 800) 1500 8C at strain rates of 4.8 3 10y1 to 9.6 3 10y5. While this data (and most other referenced data) is generally higher in strength than earlier data of Shahinian [4], it is consistent with the general trend of his data, i.e. a rapid tensile strength drop from 22 8C to a minimum at ,400 8C, then a maximum at 800–1000 8C before again decreasing. Major observations and conclusions of their work [1–3] were as follows. Internal pores, singly or as pairs closely spaced or partially joined, were about half of the sources of failure identified fractographically [2] (feasible primarily from ,800 8C and above). Fracture features over the mirror region on both fracture halves, rather than being mirror images of each other, matched each other, as previously suggested by Rice and Becher [5], indicating propagation of a sharp crack at all temperatures, and did not reflect changing to a finger crack growth mechanism as proposed by Heuer and Firestone [6, 7]. Until ,1400 8C (or lower temperatures at higher strain rates) pores at origins were the only discernible feature there with dimensions consistent with the flaw size, indicating that the pores acted as sharp flaws. Above this strain-rate-dependent transition temperature range, peripheral cracks around the pores were found, whose size was consistent with the flaw size for brittle fracture. Fracture toughness was approximately constant at 1.4 MPa m1=2 for fracture of c-axis filaments (mostly 1200–1500 8C), reasonably consistent with the data of Iwasa and Bradt [8]. The substantial fractographic data [1] allows additional evaluation corroborating and extending the above results by themselves and when compared to other data. Such evaluation is important not only for its relevance to the mechanical behaviour of sapphire, but also to two other areas of long interest: (i) failure from pores [9] and (ii) fractographic behaviour and features of single crystals [10, 11]. This letter summarizes such additional evaluation, drawing particularly on the substantial fractographic data [1], other literature data and data published here. Newcomb’s substantial strength and fracture mirror radius data at (primarily) 1000–1500 8C and several strain rates (Fig. 1) corroborates that the fracture is brittle, and that the KIC is approximately constant at ,1.4 Mpa m1=2 since it fits the typical relationship: