Nickel and zirconia toughened alumina prepared by hydrothermal processing

Abstract

Ceramics are attractive candidates for structural systems due to their strength at high temperatures. However, the major limitation to their application is their low fracture toughness. A major research objective for ceramic communities has, therefore, been to improve the fracture toughness of ceramics. The past two decades have seen the emergence of a number of studies concerned with toughening [1–3]. There are two types of mechanisms to improve the resistance to crack propagation, that is, by increasing the inherent toughness (energy dissipation) of the material and reducing the local crack-tip driving force [4], such as residual stress effects, phase transition toughening, crack deflection, nano-composites toughing, and bridging by ductile particles, fibers, and whiskers [5]. The addition of a dispersed second-phase inclusion, which limits the propagation of cracks is one of the most commonly used approaches. With the addition of either zirconia particles [6] or silicon carbide whiskers [7], the fracture toughness and strength were increased [8, 9]. Alternatively, continuous metal and ceramic phases can improve fracture toughness and creep resistance, respectively. A tri-phase structure is of benefit for the suppression of grain growth [10]. In this paper, two toughening agents, nickel and zirconia particles, were added to an alumina matrix to enhance its toughness. Although the simultaneous use of nickel and zirconia particles is not new, few studies have been reported on triphase composite powders obtained by combining hydrothermal processing with co-precipitation. In the present communication, two-phase and tri-phase composites were investigated. Aluminum hydroxide was prepared by precipitation by slowly adding ammonia liquor into a 1 M Al(NO3)3 9H2O solution while vigorously stirring to attain the desired pH value of the solution in contact with the precipitated solid. In general, the pH value was controlled to be in the range of 10–10.5 [11]. The as-obtained aluminum hydroxide was washed by centrifugal filtration with deionized water and methanol for several times and then washed in methanol accompanied by ultrasonic mixing. The addition of Y2O3 to ZrO2 is known to increase the stability of the t-ZrO2 phase in composite powders [3]. Zirconium oxychloride octahydrate was dissolved in deionized water and yttria was dissolved in a hydrochloric acid solution. The yttrium ionic solution was added into the zirconium oxychloride solution. ZrO2 was doped with yttrium to stabilize its final structure. Colloidal zirconium hydroxide was precipitated by drop-wise addition of 2 M ammonia solution into the mixture under vigorous stirring until precipitation reaction was completed. Colloid was washed by centrifugal filtration with deionized water and methanol for several times and then washed in methanol accompanied by ultrasonic mixing. Nickel chloride hexahydrate was dissolved in absolute ethyl alcohol, as also described by Gibson and Kathy [12]. The nickel was reduced by the drop-wise addition of 2 M NaOH containing hydrazine hydrate. The activation of Ni ions by coordinating and surface absorption may overcome H. Jia X. Liu T. Li H. Yan X. Liu B. Xu (&) Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China e-mail: xubs@public.ty.sx.cn