Centrifugal casting of ceramics and the properties of the castings

Abstract

In a recent study [I], we established the main regularities of the centrifugal casting process and some of the technological parameters required for casting the highly concentrated binder suspensions (HCBS) of fused quartz.This paper deals with a specific study of the effect of the technological and the theological properties of HCBS of fused quartz on the porosity Pcsg and the strength of the castings based on them. Some aspects of the strengthening process of-the castings used for obtaining unfired ceramics [2] are also examined.During centrifugal casting, besides the concentration of HCBS and the coefficient of excess solid phase Kex s [i, 2], the circumferential velocity ~ has a significant effect on the parameter Pcsg (Fig. I, region i). In this region, the range of variation of Pcsg at different values of ~ is determined by various technological factors (for example, Kexs). However, a regular increase of the porosity of the castings is observed with increasing of the casting process. On increasing ~ from 3 up to 20 m/sec, Pcsg increases from 11-14 up to 19-22%. The corresponding values of the apparent density of the castings are found to decrease from 1.86-1.89 up to 1.71-1.78 g/cm s. The aforementioned regularity owes to the rheologicai properties of the HGBS (in particular, their dilatancy) and to the excessive layering of the castings (that, in turn, hinders dense packing of particles) at high values of ~ of the process.It is known [3,4] that the increased porosity of the slip-cast quartz ceramics in the limits indicated in Fig. 1 (see region i) is accompanied by a significant loss of their strength.During centrifugal casting, abn d of the castings in the as-dried condition (region 2) as well as in the strengthened condition (region 3) remain unaltered. The aforementioned anomaly possibly owes to the structure (in particular, the specific features of the grain size distribution in the castings) and requires a special investigation.Just as in the case of slip casting, the pH value of the original HCBS (that, in turn, determines the rheolQgical properties [5,6]) is another significant factor. Figure 2 shows that thixotropic properties (in the acidic region) and distinct dilatant characteristics (in the alkaline region) become apparent depending on the pH value of the suspension. Figure 3 shows the results of a comparative study of the porosity of the castings obtained by slip casting and centrifugal casting from a given HCBS over a wide range of pH values. In contrast to the slip casting process during which a significant increase of Pcsg is observed in the region of low pH values (during thixotropic flow of the original HCBS), even a decrease of Pcsg can occur during the centrifugal casting process. On the other hand, in the pH range corresponding to dilatant flow, an increase of Pcsg is observed during centrifugal casting whereas in the case of slip casting, minimum values of Pcsg are observed in the same range.The aforementioned difference can be attributed to the 'deformation' characteristics of the pr...