JOURNAL OF MATERIALS SCIENCE 21 (1986) 4019-4023 Vickers microhardness indentation and fracture mechanics of chalcogenide arsenic-selenium glasses M. HAMMAM,* J. J. SANTIAGO Center of Chemical Electronics, Electrical Engineering Department, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA Measurements of Vickers microhardness have been carried out on AsxSel-x glass (0.28 < x < 0.60). The diamond pyramid hardness number as a function of composition revealed a maximum at 40% As, the stoichiometric composition, indicating that this com- position is the most ordered and strongest of the alloys. Deviation from stoichiometry was found to increase the disorder and introduce weaker bonds. An attempt was made to use the indentation approach to determine the fracture toughness of the investigated glasses. Therefore, the extent of surface traces of well-developed penny-like (conchoidal) cracks extending from the corners of Vickers indents were measured and found to obey Lawn's relation P/C 3/2 = constant (where P is the indenter load and C is the characteristic crack dimension). An approximate value of the fracture toughness was inferred from these measure- ments. 1. Introduction The Vickers hardness test method is one of the most common and reliable methods for hardness measure- ments. It provides useful information concerning the mechanical behaviour of brittle solids [1]. Moreover, the indentation microhardness is important for under- standing the mechanisms of deformation and fracture of materials [2]. Studies on the indentation fracture in brittle materials have also shown that indentation testing is a simple technique for characterizing the fracture behaviour of glass and ceramics [3]. The indentation is generally carried out using sharp indenters such as a cone or pyramid, because of the geometrical similarity of the residual impressions. The contact pressure with such a geometry is independent of indent size and thus affords a convenient measure of hardness [1]. In particular, the hardness of glass is of direct prac- tical importance since it is apparently related to bond- ing in these materials. It has often been used as an approximate measure of strength [4, 5]. Although hardness is extensively measured and many techniques are available for its measurement, there is still no satisfactory definition as far as glass is concerned [6]. A fracture mechanics analysis of the indentation fracture problem has been developed, and the micro- fracture patterns in brittle solids have been related to the contact load, in terms of standard material par-' ameters. This has enabled fracture toughness data to be obtained by individual techniques [3]. Strength- related properties of ceramics and glasses were recently reviewed by Lawn [7]. He considered that indentation with a sharp, fixed-profile diamond pyramid (Vickers or Knoop) is the most practical way of introducing controlled flows for strength testing, requiring only access to a routine hardness-testing facility. Where the indenter in a contact system is considered to be sharp, indentation fracture mechanics has been shown to be characterized by lateral, median and radial com- ponents. During the course of propagation both the median and the radial components attain an equilib- rium state. This has recently been analysed as growing primarily as a result of relief to residual stress sur- rounding the constrained plastic zones [8, 9], then tending to the well-developed configuration of stably propagating half-pennies with linear radial traces extending from the impression corners of the specimen surface. It is this stage of crack propagation which can be readily amenable to fracture mechanics analysis [3]. Based upon ideal "sharp indented" geometry, Lawn and Fuller [10] have provided a simple formulation for the well-developed stage of indentation fracture. The extent of surface traces of well-developed median cracks can be related to the contact load in terms of fracture toughness as follows: P / C 3/2 = constant = k K ~ 3/2 tan ~k (1) Here P is the indenter load, C is the characteristic crack dimension defined in Fig. 1, K is the fracture toughness, ~ is the half-angle of the indenter between opposing pyramid edges and k is a small correction factor. A literature survey has revealed that there has been little study of micromechanical behaviour, hardness variation and crack patterns for chalcogenide glasses. *Permanent address: Department of Physics, Faculty of Science, Universityof Mansoura, Mansoura, Egypt. 0022-2461/86 $03.00 + .12 9 1986 Chapman and Hall Ltd. 4019