Flexural alumina

Alumina, or aluminum oxide [1344-28-17, has a thermal conductivity 20 times higher than that of most oxides (5). The flexural strength of commercial high alumina ceramics is two to four times greater than those of most oxide ceramics. The drawbacks of alumina ceramics are their relatively high thermal expansion compared to the chip material (siUcon) and their moderately high dielectric constant. 

The use of reinforcing fillers was examined by Seed Wilson (1980). An alumina-fibre cement had a flexural strength of 44 MPa, while one reinforced by carbon fibre had a flexural strength of 53 MPa. Metal reinforcement has also been examined. Seed Wilson (1980) found that a cement reinforced with silver-tin alloy had a flexural strength of 40 MPa. 

Recently, Oldfield Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7-5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus and flexural strength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by voliune of fibre into MP4 glass (Bailey et al, 1991). Results using alumina fibre were less promising as there was no fibre pull-out because of the brittle nature of alumina fibres which fractured under load. 

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. 

Fig. 2.2 Flexural strength of alumina/SiC whisker composite at elevated temperatures and different whisker volume contents.

Thermal shock testing of an alumina/20 vol.% SiC whisker composite showed no decrease in flexural strength with temperature transients up to 900°C.33 Monolithic alumina, on the other hand, shows significant decreases in flexural strength with temperature changes of >400°C. The improvement is a result of interaction between the SiC whiskers and thermal-shock induced cracks in the matrix, which prevents coalescence of the cracks into critical flaws. 

Reinforced fillers are added to improve the tensile and flexural strength of epoxies and fillers used with success are silica, asbestos, and alumina. In recent years, encapsulated fillers containing salts of Ni, Cu, Co and Fe have been found to give excellent reinforcing properties. Anti-corrosion fillers such as AI2O3 are also added to some epoxy formulations. 

Table 4.1 further shows essential mechanical properties of several products of CeramTec s BIOLOX family of alumina-based materials for femoral heads of hip endoprostheses as well as those of BIONIT manufactured by Mathys Orthopadie GmbH (Bettlach, Switzerland). It is evident that decreasing the grain size of the ceramic precursor powders increases both the flexural strength and the fracture toughness of the material dramatically. 

Figure 4.1 shows a cumulative Weibull plot as being representative of the failure probability of typical femoral heads made from high-purity alumina. The very low data scatter and the high average flexural strength around 600 MPa (d50) attest to both the strength and the extraordinary reliability. 

It has been known for some time that considerable improvement of the mechanical properties of alumina in terms of flexural strength, fracture toughness, yield strength and elastic modulus can be achieved by adding particles of stabilised zir-conia as a structural reinforcement (see Chapter 4.1.2). Such ceramics are known as ZTA (zirconia-toughened alumina), duplex ceramics or dispersion ceramics. 

Recently, it has been found that the use of alumina cement and autoclave cure after moist and heat cure are most effective for improving the water resistance of MDF cements.P Figure 8.6 shows the flexural strength of heat plus autoclave cured MDF cements using the alumina cement before and after 48-hour water immersion. 

Figure 8.6 Moist curing period vs. flexural strength of heat plus autoclave cured MDF cements using alumina cement before and after 48-hour water immersion.

FIGURE 39.16 Flexural strength of dense alumina rods after aging under stress in Ringer s solution. Lot 1 and 2 are from different batches of production. (From Krainess F.E. and Knapp W.J. 1978.. Biomed. Mater. Res. 12 245. With... 

Figure 9.9 The strength of many structural ceramics is insensitive to temperature below 1000 °C but is degraded at the higher temperatures, usually by creep-related mechanisms. The trend shown here is typical for the flexural strengths of a dense high-purity alumina.

Figure 9.16 Flexural strength degradation with increasing quench temperature differential for a porous alumina foam. The shaded region is the as-received strengths. (From Orenstein and Green,...

Different cement and polymer types [2, 7-9] can be used for the production of MDF cement. However, highest flexural strengths are always obtained when calcium alumina cements (CAC) as a... 

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