Creep sintered ceramic

For second-phase sintered ceramics, these phases control the plasticity and they are responsible for the asymmetric behaviour when deformed in tension or compression, because there is a crucial difference in the microstructure evolution associated with tension and compression creep. There are few explanations for this asymmetry. 

In Sec. 7.4, situations where the fluxes of the different species are coupled are considered. This coupled or ambipolar diffusion is of paramount importance since it is responsible for such diverse phenomena as creep, sintering of ceramics, and high-temperature oxidation of metals. 

This approach that links atom movement to pressure differences has the benefit of directly describing the diffusive deformation of dense polycrystalline materials caused by an applied stress (creep). Sintering rate equations for ceramics have been developed according to the assumption that behaviour caused by pressure differences during sintering is similar to diffusion creep behaviour. The rate of deformation caused by an applied stress is given by the general equation ... 

Fig. 30. Creep behaviour of Si3N4 ceramics with different sintering additives under 150 MPa static load [413]...

The values of the creep parameters (p, n and Q) identifying the superplastic behaviour of ceramic-related materials are not unique to such materials, nor to the same type of materials. As shown in the review papers, these parameters are very similar in tension as in compression in zirconia-based materials (probably the most widely studied ceramics in the widest experimental conditions), although that depends strongly on the purity of the ceramics 5,7 however, their behaviour seems to be very different in compression than in tension when an aid-sintering phase is necessary during the processing, as in silicon carbide and silicon nitride ceramics.8... 

Fig. 19. Creep behavior of silicon nitride ceramics. The composition of this ceramic contains 7 vol % Y3A15Oi2 as a sintering aid. The specimens were hot-pressed at 1750°C under a pressure of 20 Mpa. The holding times are marked on the curve. This curve indicates that crystallization of the grain-boundary phase improves the creep resistance. Garnet crystals were observed at the grain boundaries after annealing [3/]. , Sample A60 , Sample B60 A, Sample B150, x, failure.

Suwanprateeb J, Tanner KE, Ttrmer S, Bortfield W (1995) Creep in polyethylene and hydroxylapatite reinforced polyethylene composites. J Mater Sci Mater in Med 6 804-807 Tagai H, Aoki H (1980) Preparation of synthetic hydroxylapatite and sintering of apatite ceramics. In Mechanical Properties of Biomaterials. GW Hastings, DF Wilhams (eds) John Wiley and Sorts, New York, p 477-488... 

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