Improving Fracture Toughness of Alumina Ceramic with 3D Reinforcement

Alumina ceramic has been widely used in engineering applications owing to its relatively high hardness, wear resistance, low density, chemical inertness, and low cost, among others. However, the major problem in many... [ view full abstract ]

Alumina ceramic has been widely used in engineering applications owing to its relatively high hardness, wear resistance, low density, chemical inertness, and low cost, among others. However, the major problem in many applications is its relatively low fracture toughness, leading to many attempts to create ceramic matrix composites reinforced with ceramic fibers or, more recently, with carbon nanotubes and boron nitride nanotubes. The nanotube approach appears to be quite prospective due to high intrinsic mechanical properties of these materials. However, in numerous publications it has been shown that so-called nano-composite ceramics offered only marginal improvements in fracture toughness and hardness. In this work, a different approach is successfully demonstrated by first depositing carbon nanotubes onto alumina fiber mats and then using these mats to reinforce an alumina matrix in a typical hot press process. As a result, two types of reinforced alumina composites were produced and compared to pure alumina ceramic: a 2D composite consisting of an alumina matrix reinforced with 18 vol% of alumina fiber mats, and a 3D composite having the same amount of fiber mats, but modified with multi-walled carbon nanotubes (MWCNT) vertically grown in a chemical vapour deposition process. The addition of alumina fiber mats in an alumina matrix resulted in 98.6% density of the composite compared to the 100% dense pure alumina due to incorporation of voids between the fibers. However, in the 3D composite this effect was reduced due to presence of MWCNT, producing a composite density of 99.6%. Consequently, the hardness values (HV 1kgf) were slightly higher for the 3D composite (17.97 GPa) than for the 2D composite (17.00 GPa); however, in both cases hardness values were lower compared to the pure alumina (18.88 GPa). Conversely, the improvement in fracture toughness by the 3D reinforcement is almost 74%, from 3.8 MPa.m1/2 for pure alumina to 6.6 MPa.m1/2for the 3D composite, while the 2D composite recorded 4.4 MPa.m1/2 in chevron-notched beam test. In addition, a detailed examination on the fracture surfaces of both 2D and 3D reinforced composites was conducted with scanning electron microscopy to explain the contribution of multi-walled carbon nanotubes and alumina fibers in enhancing the fracture toughness of the alumina matrix.