Fracture toughness of bone china and hard porcelain

© 2001 IoM Communications L td.
INTRODUCTION
Two high quality tablewares currently in production are bone china and hard porcelain. Bone china was first produced by Josiah Spode II around 1800.1 It is a very white, translucent ware made traditionally from 50 wt-% bone, 25 wt-% Cornish stone, and 25 wt-% china clay. It is normally biscuit fired at about 1230°C for 3 h and glost fired at about 1100°C. Bone china is a highly crystalline whiteware (around 70% crystalline and 30% glass)2 that exhibits good resistance to chipping (strength around 100 MPa) and is composed mainly of anorthite (CaO.Al O .2SiO ), b-tricalcium phosphate (Ca (PO ) ), 2 3 2 3 42 and calcium aluminosilicate glass.3 Dinsdale calculated the proportions of these phases to be 28% anorthite, 43% b-tricalcium phosphate ( b-TCP), and 29% glass.2 More recently Iqbal et al.4 have shown that a bone china body consists of agglomerates of nodular b-TCP (1–3 mm individual crystallites) and tabular anorthite grains (0·4–10 mm), along with 3 vol.-% of unreacted a-quartz grains (∏30 mm), in a calcium aluminosilicate glass. Hard porcelain is the most important type of pottery manufactured on the continent of Europe. Bo ¨ ttger, a Dresden alchemist in the service of August the Strong, Elector of Saxony, was the originator of hard porcelain in Europe.5 A classical composition of hard porcelain is 50% clay, 25% feldspar, and 25% quartz. The composition can vary widely, for example bodies for cookware have a higher percentage of clay and lower percentage of quartz.5 Hard porcelain is first subjected to a low temperature firing (900–1000°C), with the intention of hardening the body to 256
ISSN 0967–9782
T he fracture toughness of unglazed, commercial samples of bone china and hard porcelain produced by powder pressing spray dried granulated bodies has been determined using the V ickers indentation method. Young’s modulus, hardness, and brittleness have also been measured. Although the bone china, with 10% porosity, was considerably more porous than the hard porcelain, with about 5%, its fracture toughness was substantially higher. In addition the brittleness of hard porcelain was higher than that of bone china. It is proposed that these results, which are reflected in the chipping resistance of these whitewares, arise from diVerences in crystallinity, since there is approximately twice the fraction of crystalline material in bone china as in hard porcelain. T he higher porosity of bone china compared to hard porcelain is reflected in the lower Young’s modulus of the former. BCT /486 T he authors are in the Department of Engineering Materials, University of SheYeld, Sir Robert Hadfield Building, Mappin Street, SheYeld S1 3JD, UK (r.hand@). Manuscript received 26 March 2001.
Fracture toughness of bone china Leabharlann nd hard porcelain
S. A. F. Batista, P. F. Messer, and R. J. Hand
make it sufficiently strong to withstand being dipped in glaze slip. The glazed body is then fired at temperatures up to 1400°C. Bloating can occur as a consequence of a release of oxygen owing to the high temperature reduction of the iron impurity from Fe3+ to Fe2+ occurring after open pore closure. To avoid this, hard porcelain is heated in a reducing atmosphere for part of the firing cycle before closure of the open pores. With iron in the ferrous state, the porcelain has a greenish white appearance. Fired porcelain consists of needle shaped mullite crystals and possibly residual quartz grains in a glassy matrix.6 Mullite forming in the clay relict in the course of the kaolinite lattice breakup is initially submicrometre in size and is known as ‘primary’ or ‘scaly’ mullite. The other type of mullite, referred to as ‘secondary’ mullite, consists of long needle shaped crystals that crystallise from the feldspar melt. Schu ¨ ller7 reported that the primary mullite crystals were as small as 10 nm (Ludin8 says <0·5 mm), whilst secondary mullite crystals were large enough to be observed in an optical microscope (>1 mm). The growth of the secondary mullite crystals causes the melt to become less viscous and this more reactive glassy phase progressively dissolves the free silica. At the firing temperatures used for hard porcelain, quartz and secondary mullite are believed to partly dissolve and the primary mullite to coarsen,9 allowing the material to become translucent. Hard porcelain contains approximately 30% crystalline material and 70% glass, the reverse of bone china. In general the crystal/glass ratio should be as high as possible to maximise strength. The fact that normal hard porcelain is much more fragile than bone china was first highlighted in the 1930s by Odelberg10 and Singer.11 Studies using a hammer impact test have shown that the chipping resistance of bone china is over four times greater than that of hard porcelain,10 with bone china having a chipping resistance of 0·543–0·773 N m and hard porcelain 0·109–0·190 N m. It should be noted that there is some confusion in the literature about these chipping resistance values. The original data given by Odelberg10 are in ft lb and are correctly cited by Dinsdale12 (the values being 0·40–0·57 ft lb for bone china and 0·08–0·14 ft lb for hard porcelain). The data have subsequently been given in N m by Rado,5 but unfortunately the conversion factor used was in error and thus the values cited are a factor of 102 too large. It has in addition been reported that the modulus of rupture5,13 and impact strength12 of bone china are about twice those of hard porcelain. It is therefore often stated that hard porcelain is a very brittle whiteware. However, no data on the fracture toughness of these ceramics have been published and so the present study was undertaken using the Vickers indentation toughness test to measure fracture toughness. The Vickers test offers simplicity and economy as a test procedure, at little cost in reliability in comparison with more conventional testing techniques such as single edge notch beam (SENB)14 and double cantilever beam (DCB).15 It also allows the index of brittleness proposed by Lawn and Marshall to be measured (see below).16 A problem with the analysis for this technique is however that there are