Microstructural and Mechanical Properties of Al2O3 and Al2O3/TiB2 Ceramics Consolidated by Plasma Pressure Compaction

Abstract

Alumina oxide ceramics were produced by plasma pressure compaction (P2C) sintering process. Two types of pure α-alumina (Al2O3) and a mixture of alumina and titanium diboride (TiB2) powders were used as starting materials. Microstructure and mechanical properties, namely hardness, elastic modulus, and fracture toughness, were analyzed and correlated to the type of the sintered powders and the adopted manufacturing route. The microstructural development and the chemical composition variation induced by the sintering process were assessed by using scanning electron microscopy and x-ray diffraction. Nano-indentation and Chevron notch beam techniques were adopted to estimate the mechanical properties of the sintered ceramics. The conducted analyses show the capability of P2C technique to produce sound alumina ceramics. Pure alumina bulks exhibit a good level of compaction and mechanical properties close to those achievable with conventional sintering processes, such as hot isostatic pressing or spark plasma sintering. No significant alterations in the chemical composition of the ceramics were observed. The addition of the titanium diboride in the alumina powders caused a moderate increase in the grain size lowering the hardness and Young’s modulus of the sintered alumina and, at the same time, increased its fracture toughness to the occurrence of toughening mechanisms, like crack bridging and crack deflection.

Preparation and toughening mechanism of Al 2O 3 composite ceramics toughened by B 4C@TiB 2 core–shell units

“…First, the B 4 C@TiB 2 core-shell unit is regarded as a unified entity. When cracks traverse these units, they generate crack bridges and forks around them, which is similar to the crack propagation pattern observed in Al 2 O 3 ceramics toughened by independently distributed B 4 C or TiB 2 phases[4,11]. However, further analysis of crack propagation within the interior of these core-shell units is warranted upon considering the B 4 C@TiB 2 core-shell units as composite structures.…”

“…The relative density, Vickers hardness, and fracture toughness of the Al 2 O 3 -B 4 C@TiB 2 composite ceramics, along with other referenced Al 2 O 3 composite ceramics [10,11,16,17,21], are listed in Table 1. The relative density of the Al 2 O 3 -B 4 C@TiB 2 composite ceramics is 99.7%±0.2%, indicating approximately complete density.…”

“…However, its inherent low fracture toughness hampers its overall mechanical performance and restricts its broader utility. Numerous endeavors have aimed to enhance the fracture toughness of Al 2 O 3 ceramics, including techniques, such as refining grain size through specialized powder preparation methods and advanced sintering technologies [7][8][9], and incorporating diverse secondary phases, such as TiB 2 , B 4 C, SiC, ZrO 2 , BN, and C with the forms of particles, whiskers, fibers, tubes, or plate-like materials [10][11][12][13][14][15][16]. Despite varying degrees of success attained by these toughening methods, the improvements are often marginal, accompanied by elevated production costs or challenges in large-scale manufacturing.…”

“…The optimal fracture toughness, 5.2 MPa•m 1/2 , is realized when the TiB 2 content is 30 vol%. Currently, despite the preparation of Al 2 O 3 composite ceramics toughened by various ceramic particles, the toughening effects of these ceramic particles have yet to yield substantial breakthroughs; the achieved fracture toughness for Al 2 O 3 composite ceramics range from 3.8 to 6.2 MPa•m 1/2 over years [10][11][12][13][14][15][16][17][18][19]. The primary reason for this limitation lies in the prevailing toughening structure shared by various Al 2 O 3 composite ceramics toughened, wherein the secondary phases are independently dispersed within the Al 2 O 3 matrix, as shown in Fig.…”