H. Hadraba et al /Ceramics International 30(2004)853-863 3Y ZrO2-T --- HP AL.O-T LC HP/Y-T OTHOzu-> 王-12+4P4Q,T=10% TE(HP ALO3T)=9010°K 16 C HP/3Y-T)=-1478% TE(LC HP/3Y-T)=9.710*KT Yzo2T)=2559% 02004006008001000120014001600 TEMPERATURE [C] ce of relative length change of alumina/zirconia layered composite LC HP/3Y and single-component alumina and zirconia deposits on sintering temperature. The relative density of layered composite LC HP/3Y was cause of residual thermal stresses(or) in the layers. The 7%TD Single-component deposits had a relative density magnitude of such stress in ZrO2 can be calculated from the of 99.2%(type HP AlO3) or 99.9%TD(type 3Y ZrO2) relation [91 11. The higher porosity of the laminated composite was probably due to the inhomogeneities at the interface of in- orZo,&(CTEzrO2-CTEAlO3)ATEZrO2 dividual phases. However, these inhomogeneities were re- 1一vrO2 sponsible for about 2% of the porosity, the bond of layers in tzrO EzrO,/(I-vZ the composite was thus of good quality, as shown in Fig. 2 Fig. 3 gives for the layered alumina/zirconia compos- ite the dependence of shrinkage in parallel direction to the where tZrO2 and tAl203 are the average layer thickness val- layers(transversal direction-T)on the temperature in the ues, VZrO, and VAl2O, are the Poisson ratios and Ezron and course of sintering and cooling. In this work the abbrevia- EAl, are the elasticity moduli of ZrO and Al,O3 of the tions t(transversal direction) and L (longitudinal direction) composite(the stress in the Al2O3 phase, orAb, is obtained will have the same meaning as in the previous paper [11]. by interchanging the subscripts of the quantities) The shrinkage of the alumina/zirconia layered composite in Employing the values given in Table 1, we obtain for the this direction was given by the shrinkage of Al2O3, which Al2 O3 layer ssion stress orAl203 =-362 MPa and shrinks less than Zro(for comparison, the sintering curves for ZrO tensile stress orZrO2 for one-component deposits in transversal direction are also are parallel to the interface of Al2 O3 and ZrOz layers,the given in Fig. 3). This result lends support to the consider- tensile stresses(which originated in ZrO2) are more dan ations in the introduction to this chapter, namely that the gerous. By analysing Eq (I)it can be shown that in zro2 appearance of cracks in ZrO2 layers more than 50 Hm thick residual stress increases with decreasing thickness of the was due to different green densities of individual layers. A ZrO2 layer. By contrast, in the case of stresses appearing an be seen from Fig 3, in the course of sintering the ZrO2 in the course of sintering it was shown that with increasing layer should shrink more than the surrounding Al2O3 lay thickness of the ZrO2 layer the danger of cracks appearing ers in the composite permitted, which led to tensile stress in this layer increased. From the viewpoint of defect-free in the layer In the case of thicker layers this tension led in layer a layered composite must thus have a certain optimum turn to the appearance of cracks. Whether in thinner layers his stress remained or relaxed at the sintering temperature Properties of the ceramic materials being deposited (for example, via diffusion processes on grain boundaries) remains unanswered in this paper Al O3 e The slope or the cooling curve was used to calculate Elasticity module, E(GPa)[8] 380.0 210.0 CTE. As is obvious from Fig. 3, the CTE of the LC Poisson ratio,v[8 0.31 HP/3Y composite was roughly an average of CTEAL,O, and CTE(x10-K)[11] CTEzrO, which differed by about 13%. This fact was the Average layer thickness, I(um)856 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 3. Dependence of relative length change of alumina/zirconia layered composite LC HP/3Y and single-component alumina and zirconia deposits on sintering temperature. The relative density of layered composite LC HP/3Y was 97%TD. Single-component deposits had a relative density of 99.2% (type HP Al2O3) or 99.9%TD (type 3Y ZrO2) [11]. The higher porosity of the laminated composite was probably due to the inhomogeneities at the interface of in￾dividual phases. However, these inhomogeneities were re￾sponsible for about 2% of the porosity, the bond of layers in the composite was thus of good quality, as shown in Fig. 2. Fig. 3 gives for the layered alumina/zirconia compos￾ite the dependence of shrinkage in parallel direction to the layers (transversal direction—T) on the temperature in the course of sintering and cooling. In this work the abbrevia￾tions T (transversal direction) and L (longitudinal direction) will have the same meaning as in the previous paper [11]. The shrinkage of the alumina/zirconia layered composite in this direction was given by the shrinkage of Al2O3, which shrinks less than ZrO2 (for comparison, the sintering curves for one-component deposits in transversal direction are also given in Fig. 3). This result lends support to the consider￾ations in the introduction to this chapter, namely that the appearance of cracks in ZrO2 layers more than 50
m thick was due to different green densities of individual layers. As can be seen from Fig. 3, in the course of sintering the ZrO2 layer should shrink more than the surrounding Al2O3 lay￾ers in the composite permitted, which led to tensile stress in the layer. In the case of thicker layers this tension led in turn to the appearance of cracks. Whether in thinner layers this stress remained or relaxed at the sintering temperature (for example, via diffusion processes on grain boundaries) remains unanswered in this paper. The slope of the cooling curve was used to calculate the CTE. As is obvious from Fig. 3, the CTE of the LC HP/3Y composite was roughly an average of CTEAl2O3 and CTEZrO2 , which differed by about 13%. This fact was the cause of residual thermal stresses (σr) in the layers. The magnitude of such stress in ZrO2 can be calculated from the relation [9]: σrZrO2 = (CTEZrO2 − CTEAl2O3 ) TEZrO2 1 − νZrO2 ×
1 + tZrO2 tAl2O3 EZrO2 /(1 − νZrO2 ) EAl2O3 /(1 − νAl2O3 ) −1 (1) where tZrO2 and tAl2O3 are the average layer thickness val￾ues, νZrO2 and νAl2O3 are the Poisson ratios and EZrO2 and EAl2O3 are the elasticity moduli of ZrO2 and Al2O3 of the composite (the stress in the Al2O3 phase, σrAl2O3 is obtained by interchanging the subscripts of the quantities). Employing the values given in Table 1, we obtain for the Al2O3 layer compression stress σrAl2O3 = −362 MPa and for ZrO2 tensile stress σrZrO2 = +373 MPa. These stresses are parallel to the interface of Al2O3 and ZrO2 layers; the tensile stresses (which originated in ZrO2) are more dan￾gerous. By analysing Eq. (1) it can be shown that in ZrO2 residual stress increases with decreasing thickness of the ZrO2 layer. By contrast, in the case of stresses appearing in the course of sintering it was shown that with increasing thickness of the ZrO2 layer the danger of cracks appearing in this layer increased. From the viewpoint of defect-free layer a layered composite must thus have a certain optimum Table 1 Properties of the ceramic materials being deposited Al2O3 ZrO2 Elasticity module, E (GPa) [8] 380.0 210.0 Poisson ratio, ν [8] 0.26 0.31 CTE (×10−6 K−1) [11] 9.0 10.3 Average layer thickness, t (
m) 41.5 42.8