Plasticity in ZrB₂ micropillars induced by anomalous slip activation

Abstract

Room temperature deformation behaviors of ZrB₂ micropillars, fabricated by focused ion beam technique from differently oriented grains of polycrystalline ZrB₂, were investigated under micro compression. Considerable anisotropy was found showing ∼80% higher yield $({\sigma }_Y)$ and rupture $({\sigma }_r)$ stress values for the basal oriented pillars $({\sigma }_{Y,\text{basal}}\text{=13.4}\text{GPa},{\sigma }_{r,\text{basal}}=13.4\text{GPa})$ compared to the prismatic pillars ${(\sigma }_{Y,\text{prism}}=6.4\text{GPa},{\sigma }_{r,\text{prism}}=7.6\text{GPa})$. Micro-scale plasticity was detected in prismatic oriented pillars, revealing the activation of the $\left\{10{\displaystyle \overline{1}}0\right\}〈11{\displaystyle \overline{2}}3〉$ type slip system both in the form of single- and multiple-slip which is anomalous for ZrB₂ and has not been reported so far in the relevant literature.

Introduction

Considerable interest has been recently devoted to ultra-high temperature ceramics (UHTCs) because of their extremely high melting points (>3000 °C) combined with excellent oxidation and ablation resistant properties [1], [2]. They are usually based on the refractory borides, carbides, nitrides and oxides of early transition metals. Zirconium diboride (ZrB₂), owing to its high hardness, electrical and thermal conductivity, is an ideal candidate for UHTCs to withstand extreme chemical and thermal environments. All of these properties make them suitable for a wide range of potential application in the field of cutting tools [3], plasma-arc electrodes [4] and thermal protection system for hypersonic vehicles [5], [6]. Additionally, ZrB₂ has also been investigated as a substrate for heteroepitaxial growth of GaN, because of their small mismatch in lattice constants and coefficients of thermal expansions, to prepare light-emitting diode devices [7].

Single crystals of ZrB₂ are built up of close packed alternating layers of Zr and B atoms giving rise to a hexagonal structure, (space group P6/mmm, No. 191) with lattice parameters of a = 3.17 Å and c = 3.53 Å, respectively [8]. The individual grains within polycrystalline ZrB₂ are essentially single crystals with orientation-dependent mechanical properties. It is necessary to understand these anisotropic deformation behaviors of ZrB₂ grains in order to model and design optimized microstructures with an enhanced combination of strength, fracture toughness and wear resistance according to the requirements of the engineering applications [9].

In the last 40 years, several investigations focused on understanding the hardness anisotropy and possible deformation mechanisms at room and elevated temperatures in ZrB₂ single crystals [10], [11], [12] and polycrystalline composites [13], [14], [15], [16]. One of the first studies on hardness anisotropy was performed by Nakano et al. [10] using Knoop microhardness testing on $\left(0001\right)$ basal, $\left(10{\displaystyle \overline{1}}0\right)$ and $\left(11{\displaystyle \overline{2}}0\right)$ type prismatic oriented planes in ZrB₂ single crystals at room temperature. According to their results, the hardness was very similar on both types of prismatic planes $(H{K}_{\left(10{\displaystyle \overline{1}}0\right)}=20.6-23.5\text{GPa},H{K}_{\left(11{\displaystyle \overline{2}}0\right)}=19.6-22.5\mathrm{G}\mathrm{P}\mathrm{a})$ and lower than that characterizing the basal plane $(H{K}_{\left(10{\displaystyle \overline{1}}0\right)}=21.6-25.5\text{GPa})$ depending on the direction of the long axis of the Knoop indenter. In their work [10], using an analytical method, based on the comparison of two potential slip systems which could contribute to the hardness anisotropy, the activation of the $\left\{10{\displaystyle \overline{1}}0\right\}〈11{\displaystyle \overline{2}}0〉$ slip system was suggested contrary to the $\left(0001\right)〈11{\displaystyle \overline{2}}0〉$ type. At that time, transmission electron microscopy (TEM) investigations confirmed the $\left\{10{\displaystyle \overline{1}}0\right\}〈11{\displaystyle \overline{2}}0〉$ type slip system, which is typical for hexagonal crystals, as the only responsible mechanism for room-temperature plastic deformation in single crystal ZrB₂ [11]. Later, high temperature Vickers microindentation measurements on single crystal ZrB₂, from 25 °C up to 1000 °C, performed by Xuan et al. [12] revealed that the hardness was temperature dependent and the hardness anisotropy was similar in the whole temperature region to that previously reported by Nakano et al. [10] at room temperature.

Recent scratch [13], [14] and indentation [16], [17], [18] investigations on polycrystalline ZrB₂ and ZrB₂–SiC composites have also revealed readily detectable plastic deformation features in the form of slip-lines. Ghosh et al. inferred the $\left\{10{\displaystyle \overline{1}}0\right\}〈11{\displaystyle \overline{2}}0〉$ type slip activation based on TEM observations on scratch-induced plastically deformed grooves and reported for the first time a [0001] type Burgers vector in ZrB₂ grains, which possibly corresponds to the $\left\{10{\displaystyle \overline{1}}0\right\}\left[0001\right]$ system [14]. First-principles stress-strain calculations also revealed that the theoretical weakest shear strength corresponds to the $\left\{10{\displaystyle \overline{1}}0\right\}〈11{\displaystyle \overline{2}}0〉$ type prismatic slip system compared to the $\left(0001\right)〈11{\displaystyle \overline{2}}0〉$ and $\left(0001\right)〈10{\displaystyle \overline{1}}0〉$ basal plane slips [15]. Similar plastic features were revealed in the form of pop-in phenomena during low-load indentation by Guicciardi et al. [16] and post-yield hardening on plastically deformed isolated ZrB₂ grains by Blaber et al. [17].

The plasticity of brittle materials has been studied extensively in the last ten years using micropillar compression. At first, the technique was applied to metals [19], [20], [21], and later it was successfully adopted for ceramics [22], [23], [24], [25], [26] to investigate microscopic volumes under relatively homogenous stress condition and in a much wider plastic region than that could be achieved by indentation and scratch tests. Micropillar compression is especially important in this regard, since the stress–strain relationships of the compressed grains (small volumes, ∼10⁻¹⁵ m³) are significantly different from the deformation characteristics of macroscopic single crystals. Additionally, the properties of differently oriented grains are essential to be able to model the mechanical response of polycrystalline ceramics under macroscopic loading. The work presented here is the first reported investigation of the deformation behavior of ZrB₂ micropillars.

The aim of the present work is to study the room temperature deformation anisotropy of ZrB₂ grains in polycrystalline ZrB₂ under micropillar compression.