Alumina reinforcement mechanism

Very recently, Xia et al.43 reported microstructural investigations on MWCNTs which had been formed within the regular and well-aligned pores of an alumina membrane. The material was too thin (20-90 pm) to permit mechanical measurements, but different possible reinforcement mechanisms induced by the CNTs were evidenced on stressed and damaged materials,... 

Figure 4.2 Reinforcement mechanism of a ZTA (zirconia-toughened alumina) duplex ceramics with agglomerated Y-PSZ particles.

Recently, researches have been made with materials composed of alumina reinforced with zirconia (until 15 % in volume of zirconia) with the purpose of improving the reliability of the single-phase alumina and zirconia ceramic implants. De Aza et. at [38] has put in evidence that, in general, these new composite materials can display not oidy a greater toughness (Kic) that the monolithic materials previously mentioned, otherwise, and what it is more important, a greater tension threshold for the stress intensity factor (Kio), below which some of crack does not take place propagation (Table III). Therefore, in the case of the ceramics prostheses, this tension threshold provides a rank of intensity of tensions of total security for the use of the composite material under mechanical efforts. 

Note The principal reinforcement, with respect to quantity, is glass fibers, but many other types are used (cotton, rayon, polyester/TP, nylon, aluminum, etc.). Of very limited use because of their cost and processing difficulty are whishers (single crystals of alumina, silicon carbide, copper, or others), which have superior mechanical properties. 

Attempts have been made to improve the mechanical properties of these cements by adding reinforcing fillers (Lawrence Smith, 1973 Brown Combe, 1973 Barton et al, 1975). Lawrence Smith (1973) examined alumina, stainless steel fibre, zinc silicate and zinc phosphate. The most effective filler was found to be alumina powder. When added to zinc oxide powder in a 3 2 ratio, compressive strength was increased by 80 % and tensile strength by 100 % (cements were mixed at a powder/liquid ratio of 2 1). Because of the dilution of the zinc oxide, setting time (at 37 °C) was increased by about 100%. As far as is known, this invention has not been exploited commercially. 

Some elucidation of the mechanism of elastomer reinforcement is being obtained by precipitating chemically-generated fillers into network structures rather than blending badly agglomerated filler particles into elastomers prior to their cross-linking. This has been done for a variety of fillers, for example, silica by hydrolysis of organosilicates, titania from titanates, alumina from aluminates, etc. [85-87], A typical, and important, reaction is the acid- or base-catalyzed hydrolysis of tetraethylorthosilicate ... 

In this chapter, we describe the synthesis and characterisation of the microstructure and properties of layered-graded alumina-matrix composites through liquid infiltration. This approach is relatively simple and offers excellent control over the depth of the graded layer. The presence of a graded dispersion of reinforced particles in the alumina matrix has a profound influence on the physical and mechanical properties of the composites. An overview of the infiltration kinetics and the use of the infiltration process as a new philosophy for tailoring novel graded ceramic systems are also presented. 

The initial interest in ceramic matrix nanocomposites arose from reports by Niihara and co-workers indicating enhanced mechanical properties due to the presence of ceramic (SiC) particles.53 With the development of various processing routes to introduce nanometer-sized metal particles in a ceramic matrix, variations in functional (i.e. magnetic) properties are possible. In the following we briefly review the microstructurally dependent properties, with emphasis on the possible mechanisms leading to improved properties and using SiC-reinforced alumina as a point of comparison. 

In addition to the initial work in the alumina and mullite matrix systems previously mentioned, SiC whiskers have also been used to reinforce other ceramic matrices such as silicon nitride,9-13 glass,14 15 magnesia-alumina spinel,16 cordierite,17 zirconia,18 alumina/zirconia,18 19 mullite/zirconia,18-21 and boron carbide.22 A summary of the effect of SiC whisker additions on the mechanical properties of various ceramics is given in Table 2.1. As shown, the addition of whiskers increases the fracture toughness of the ceramics in all cases as compared to the same monolithic materials. In many instances, improvements in the flexural strengths were also observed. Also important is the fact that these improvements over the monolithic materials are retained at elevated temperatures in many cases. 

K. Xia and T. G. Langdon, The Mechanical Properties at High Temperatures of SiC Whisker-Reinforced Alumina, in High Temperature/High Performance Composites, Materials Research Society Symposium Proceedings, Vol. 120, eds. F. D. Lemkey, S. G. Fishman, A. G. Evans, and J. R. Strife, Materials Research Society, Pittsburgh, PA, 1988, pp. 265-271. 

In this chapter, we have sought to provide a state-of-the-art review of the mechanics and micromechanisms of high temperature crack growth in ceramics and discontinuously reinforced ceramic composites. Because of the limited amount of experimental data available in the literature which pertains primarily to oxide cermics and SiC reinforcements, the discussions of crack growth rates and fracture mechanisms have centered around alumina ceramics, with and without SiC reinforcements. However, the generality of the mechan-... 

It is well established that improvements in the room-temperature fracture toughness of ceramics can be achieved by whisker reinforcement, and various mechanisms have been proposed to explain the phenomenon. Recently, two studies have examined the fracture behavior of SiC whisker-reinforced alumina composites at high temperatures to determine the dependence of fracture toughness on temperature, the effect of test atmosphere on fracture toughness, and mechanisms of fracture.33,34 In both studies, single-edged-notched bars... 

A. R. De Arellano-Lopez, A. Dominguez-Rodriguez, K. C. Goretta, and J. Routbort, Plastic Deformation Mechanisms in SiC-Whisker-Reinforced Alumina, J. Am. Ceram. Soc., 76[6], 1425-1432 (1993). 

R. Chaim, L. Baum, and D. G. Brandon, Mechanical Properties and Microstructure of Whisker-Reinforced Alumina-30 vol% Glass Matrix Composite, J. Am. Ceram. Soc., 72[9], 1636-1642 (1989). 

E. R. Fuller, Jr., R. F. Krause, Jr., J. Kelly, R. N. Kacker, E. S. Lagergren, P. S. Wang, J. Barta, P. F. Jahn, T. Y. Tien, and L. Wang, Microstructure, Mechanical Properties, and Machining Performance of Silicon Carbide Whisker-Reinforced Alumina, J. Research NIST, in press. 

Janes, Neumann and Sethna ° reviewed the general subject of solid lubricant composites in polymers and metals. They pointed out that the reduction in mechanical properties with higher concentrations of solid lubricant can be offset by the use of fibre reinforcement. Glass fibre is probably the most commonly used reinforcing fibre, with carbon fibre as a second choice. Metal and ceramic fibres have been used experimentally to reinforce polymers, but have not apparently been used commercially. To some extent powders such as bronze, lead, silica, alumina, titanium oxide or calcium carbonate can be used to improve compressive modulus, hardness and wear rate. 

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