Toughening of alumina ceramics by yttrium containing zirconium oxynitride

Abstract

Recently it was shown that the fracture toughness of alumina ceramics can be enhanced by oxidation-induced compressive surface stresses [1]. These stresses are induced by oxidation of a zirconium oxynitride dispersed phase which is accompanied by a volume increase. The mechanism is called chemical surface layer toughening and was also described for the system Si3N4-ZrO2 [2]. In this article, it is shown that yttrium containing zirconium oxynitrides also enhance alumina ceramics using the same toughening mechanism. The results are discussed in comparison to the recent report [1]. Yttrium containing zirconium oxynitrides can be synthesized by nitridation of yttria stabilized zirconia at temperatures above 1400◦C in a nitrogen atmosphere using a graphite heated resistance furnace [3–5]. The process results in compounds with the general composition YmZr1−mN2n/3O2−(m/2)−n , showing randomly distributed anion vacancies and a tetragonal fluoritetype structure. Upon heating above 600◦C in air, yttria containing zirconium oxynitride reacts to yttria stabilized zirconia with a volume increase of about 3–4% [5]. These oxynitride phases do not show a martensitic phase transformation. The fracture toughened ceramics were synthesized using two different routes. The first way contains three steps. In the first step, yttria stabilized zirconia was nitrided for 1 h at 1750◦C in nitrogen atmosphere. The obtained phase with the analysed composition Y0.4Zr6.6N1.1O12.1 was mixed with fine-grained alumina in various proportions. The mixtures were homogenized in an attritor, isostatically pressed to rods, and, in a second step, sintered in a graphite heated resistance furnace for 1 h at 1400◦C in nitrogen atmosphere. The heating rate was 20 K min−1. In a third step, the ceramic rods were polished and heat treated after that for different periods of time at 700 or 900◦C in air. The second way of ceramic synthesis contains only two steps. First the green ceramics, which contain various mixtures of alumina and yttria stabilized zirconia, were sintered for 1 h at 1400 or 1600◦C in nitrogen atmosphere. The heating rate was varied between 8 and 40 K min−1. In one step, the nitridation and densification occurred at the same time. As in the first synthesis route, a heat treatment step followed at 700 and 900◦C, respectively. Quantitative phase analysis was carried out by X-ray powder methods (Cu Kα1 radiation). The nitrogen content was measured by hot gas extraction. The critical stress intensity factors were determined from the crack size of vickers indentations (measuring load: 98 N), the fracture strength from four-point-bending-tests of the rods (size: 3× 4× 45 mm). For both routes of synthesis, the uniformly grey samples show a homogeneous microstructure with a very small porosity after sintering of the mixture of zirconia containing phase and alumina in a nitrogen atmosphere. The density varies between 96 and 98% of the theoretical value. With regard to the second way of synthesis needing only two steps, it should be mentioned that the right choice of the heating rate is important in order to achieve the incorporation of a high amount of nitrogen and the densification at the same time. A fast heating process up to 1400◦C results in a white ceramic without nitrogen. The fracture strength of this ceramic with 70 wt% alumina amounts to about 1000 MPa. This high strength is caused by the well known transformation toughening and microcracking mechanisms [6]. Only after slow heating up to 1600◦C, a dark-grey ceramic is produced which contains α-Al2O3 and the compound Y0.4Zr6.6N1.1O12.1. The surface was covered by a thin layer of AlN, which can be easily removed by grinding. The resulting ceramic and pure alumina have similar fracture toughness and strength. After tempering the dark-grey samples at 900◦C in air, the color of the ceramics becomes brighter and the nitrogen content has decreased. The loss of nitrogen can be measured by hot gas extraction and by powder X-ray diffraction. Fig. 1 presents the diffraction patterns of the ceramic