High Temperature Mechanical Loss of Nanostructured Yttria Stabilized Zirconia (3Y‐TZP) Reinforced with Carbon Nanotubes

Abstract

High temperature mechanical spectroscopy measurements were conducted on carbon nanotubereinforced fine-grained 3Y-TZP ceramics processed by conventional (CS, grain size ~ 350 nm) and spark plasma (SPS, grain size ~ 100 nm) sintering. The mechanical loss spectra are composed of a peak and an exponential background appearing at low frequencies or high temperatures. The SPS samples showed a much higher level of internal friction and lower creep resistance, which can be attributed to the easier grain boundary sliding in nanosize-grained specimens. The addition of carbon nanotubes resulted in a decrease in damping with respect to the high purity of zirconia powder and possibly in a reduction of creep. INTRODUCTION Toughness improvement of ceramics, even the more classical ones, is still nowadays a subject of interest . Large increase in strength and fracture-toughness is frequently documented for ultra-fine ceramic/metallic structures; a phenomenon that is explained in part by small grain sizes using the wellknown Hall–Petch relationship. Grain refining is a promising route for simultaneous increase of mechanical strength and fracture toughness. Such improvement can be attributed to the fact that grain boundaries act as obstacles against deformation. For instance, application of two-step sintering method resulted in processing nanostructured cubic stabilized zirconia, which exhibits up to ~ 96% increase in the fracture toughness (i.e. from 1.61 to 3.16 MPa m), when the grain size is reduced from ~ 2.15 μm to ~ 295 nm. In addition to mechanical advantages, using nanopowder for fabrication of nanocrystalline parts, considerably, enhances sinter ability at lower temperatures rather than those of micrometric grains. Spark Plasma Sintering (SPS) is a promising technique for production of nanostructured ceramics. SPS is a newly developed sintering process that combines the use of mechanical pressure and microscopic electric discharge between the particles. The enhanced densification in this process has been attributed to localized self-heat generation by the discharge, activation of the particle surfaces, and the high speed of mass and heat transfer during the sintering process. As a result, samples can rapidly reach full density (in a few minutes) at relatively low temperatures. This process has been used to prepare a large variety of nanograined ceramics, including 3Y-TZP, Al2O3 and BaTiO3. According to the above introduction, processing of nanostructured ceramics (instead of micrometric grains), can increase fracture toughness at room temperature. On the other hand, high temperature mechanical properties of ceramics and especially fine-grained 3 mol% yttria stabilized tetragonal zirconia polycrystals (3Y-TZP) depend highly on grain boundary (GB) properties. GB sliding is an important mechanism of plastic deformation for fine-grained ceramics. It is mostly attributed to the fact that, when nanostructure is concerned, GB sliding can be activated at much lower temperatures than in the case of coarser grain size . Hence, it seems that application of nano-structured ceramics at room temperature is reasonable. But, the domination of GB sliding at high temperature will deteriorate