Fracture resistance behavior of alumina–zirconia composites

Abstract

R-curve behavior has been examined in alumina–zirconia composites Al₂O₃–10 wt.% ZrO₂ containing two types of ZrO₂ particles: 3 mol% yttria doped ZrO₂ and pure ZrO₂. For comparison alumina ceramics was tested too. The R-curve behavior has been evaluated using two techniques for the different crack-length regimes: a small-crack R-curve by means of indentation-induced controlled Vickers crack growth in bending (ICVC) and pronounced long-crack using load-relaxation technique. The latter technique bases on the three-point bending (3PB) of the single edge notched beam (SENB). A combination of in situ microscopic crack growth observation and mechanical testing enabled measurement of crack growth resistance curves. These observations reveal the existence of increasing R-curve at the long-through thickness cracks method for tested composites. In view of contributions of residual indentation stress intensity and applied bending stress intensity, the residual stress factor χ and the fracture toughness behavior K_R have been determined. SEM micrographs of typical crack profiles in tested ceramic composites were observed. Indentation strength in bending method (ISB) was used for determination fracture toughness as well.

Introduction

Zirconia-toughened alumina (ZTA) cutting tools have firmly established themselves as a good alternative to more traditional hardmetals cutting tools, especially because of their wear rate. One of the primary disadvantages of ceramic materials is their brittle nature, characterized by low fracture toughness. Significant improvements have been made to increase the fracture toughness of ceramics but brittleness continues to keep ceramics from more widespread use. One of the promising means used for solving of this problem is application of ceramic composites. Alumina–zirconia composites, developed by introduction of fine ZrO₂ particles in the alumina matrix, has extensive structural application due to their excellent mechanical properties. Through a suitable adjustment of the concentration of its components, this ceramic composite can offer high level of strength properties, toughness and hardness [1]. Pure alumina ceramics exhibit high hardness and good strength properties but poor toughness, while tetragonal zirconia has high strength properties, high toughness and lower hardness. Then, Al₂O₃–ZrO₂ composites obtained from these ceramic materials give a possibility of a compromise among these mechanical properties. Phase transformation of ZrO₂ from tetragonal (t) to monoclinic (m) has been widely used to improve the toughness of brittle ceramic materials. The improvement is understood as a result of volume expansion during the t → m transformation of ZrO₂ dispersed in the matrix. In an Al₂O₃ matrix, t-ZrO₂ grains undergo the t → m transformation (stress-induced phase transformation) and microcracks form around pretransformed m-ZrO₂ grains (microcrack formation). The stress-induced phase transformation toughening and microcrack toughening are the major toughening mechanism in Al₂O₃–ZrO₂ composite [2]. This mechanism in ceramic-matrix composites often involve change of crack paths by means of stress field from mismatches in thermal and elastic constants of the different phases, such that the net crack driving force is reduced. During cooling of alumina–zirconia composite from the sintering temperature to room temperature the local intrinsic stress fields are generated due to difference in thermal expansion coefficient (CTE). These local intrinsic stress fields in result of affecting with the stress field of propagating crack can alternate it path and leads to the arise R-curve. Increases in the internal stress levels usually result in R-curve enhancement. R-curve behavior is characterized by an increase change of material resistance to crack propagation with crack extension. Alumina–zirconia composites are known to exhibit rising crack growth resistance behavior. This is a strong incentive, to develop and characterize composites with pronounced R-curve behavior. Participation of zirconia grains phase with heigher than alumina coefficient of thermal expansion CTE (${\alpha }_{\text{ZrO2}}=12\times {10}^{-6}{\text{K}}^{-1}$) induced to the matrix additional compressive fields. In compliance with bridging mechanism of fracture toughness such grains subjected to compressive stresses fulfil the part of bridges which resist the crack growth. Crack bridging due to interlocking grains provides the major contribution to increasing of toughness. Various methods to measure fracture toughness using conventional procedures and indentation precracks were tested. Conventional procedures for fracture toughness testing of ceramic composites are based upon the “production of a precrack” in a test specimen (Fig. 1).

The most problematic is creating such a crack that should reflect a natural crack existing in ceramic composites. In conventional method notch with finite width introduced by a saw cut replaces a crack. For this test the single edge notched beam (SENB) specimens are pretty often used. The principal procedure for fracture toughness measurements consists in the three main steps: generation of crack in a test specimen, measurement of load at failure and calculation of the K_IC from failure load or the failure stress, respectively, and crack depth using the relation (1,2) [3]:

$$K_{\text{IC}}=\sigma \sqrt{a}Y$$

$$\text{or}{K}_{\text{IC}}=\frac{P}{B\sqrt{W}}Y$$

where σ=(3PS)/(2BW²), bending stress, P is the failure load, S the support span, B the specimen thickness, W the specimen width, Y the geometric function (a/W), a the crack deep.

The precrack geometry must be well-defined so that stress intensity factor can be accurately evaluated. Straight, through-section cracks in specimen are preferred for most test configurations. Consequently, the standard long-crack fracture toughness test procedure is time-consuming and expensive so the greater attention has been paid on more simple indentation crack methods. Relatively easy way to produce a sharp crack gives the hardness Vickers indentation test with diamond pyramid. In this method the fracture toughness is calculated from the length of cracks which develop during the Vickers indentation test and can be measured optically at the specimen surface. During loading and unloading two perpendicular cracks are initiated starting at the deepest location of the deformation zone and propagated to the specimen surface. A residual driving force results from the elastic–plastic mismatch in the indentation field generates the median/radial and Palmqvist cracks which shape depends on kind of materials and applied loading (Fig. 2).

During unloading the latteral cracks are observed which penetrate up to surface caused chipping of the ceramic composite. Indentation method falls into two main categories: K_IC is evaluated from direct measurements of crack size as a function of indentation load and the indentation crack serves as a controlled flaw in a bending test so that K_IC is determined by a strength measurement. A critical crack intensity factor in any indentation toughness determination is a proper accounting of the residual contact stress in the fracture mechanics formulas. In this paper, the results of the fracture toughness determined for alumina–10 wt.% zirconia composites with pure zirconia and 3 mol% yttria doped zirconia are presented. The fracture toughness was estimated by means of various methods basing on conventional and indentation methods. In the later technique the fracture toughness can be determined by indentation-induced controlled Vickers crack growth in bending (ICVC) or in indentation strength in bending method (ISB). The fracture toughness were interpreted and analyzed in view of the R-curve results. These results indicate that care should be taken for testing the fracture toughness of the composites with rising R-curve because the measured fracture resistance of such materials is a function of the crack size and configuration used.