**3. Drawbacks in conventional sinterization of HA/ZrO2 composites**

The inherent low strength and fracture toughness of synthetic HA have restricted its use to high load resistant implants (bones) due to its inferior mechanical properties (especially fracture toughness) compared to cortical bone (LeGeros et al., 1993). For example, its fracture toughness (*KIc*) is 1 MPa.m1/2, below the minimum of the range of values for human bones: 2–12 MPa.m1/2. Moreover, the Weibull modulus (*n*) is low in wet environments (*n* = 5–12) (DeWith et al., 1981) which indicates a weak structure of the HA implants. On the other hand, synthetic HA is more isotropic with a larger grain size than the biological HA (Cao & Hench, 1996). Further, bone is a complex composite of an organic (collagen) and an inorganic (biological apatite) component. Therefore, the synthesis of composites made of HA have been put forward, in which the addition of second phase into HA for improving its fracture toughness was widely used and investigated (Kong et al., 1999; H.W. Kim et al., 2003; Shen et al., 2001; Adolfsson et al., 2005; Kumar et al., 2005; Rao et al., 2002; Wu et al., 1998). For example, both the fracture toughness and flexural strength of HA can be improved substantially by the addition of ZrO2 (Yu et al., 2003). It was widely reported that that HA/ZrO2 composites show significantly higher mechanical properties, in particular bending strength, micro-hardness and Young's modules have proved to be bigger than those of pure HA (Miao et al., 2004). The incorporation of ZrO2 into HA enhances its mechanical properties and will not affect its biocompatibility. On the other hand, implants made of the HA/ZrO2 composites are highly biocompatible with no adverse reactions when used in rabbit mandibles (Vaidhyanathan et al., 1997). Further, the Y2O3 addition into ZrO2 can stabilize the tetragonal phase at room temperature (YSZ), and the tetragonal phase plays a major role in the fracture toughness increase. The toughening mechanisms include transformation and crack deflection toughening. Transformation toughening mechanism is an important mechanism in ceramic composites (Garvie et al., 1975). A great deal of energy is absorbed due to the volume expansion during the transformation of t-ZrO2 to m-ZrO2 so that the fracture toughness is enhanced. On the other hand, the ZrO2 particles dispersed in matrix would disturb the crack tip stress field and make the crack deflect and/or curve, then the driving force of crack propagation decreased and newborn surface area of crack increased, therefore the toughness was enhanced (Faber & Evans, 1983). The change of crack propagation mode, the increase of crack length and the formation of newborn surface area of crack at crack tip will consume energy. The main factor causing crack deflection is the

Description of sintering of particle-reinforced composites, for a large extent of densification and if no interfacial reaction occurs between the matrix and reinforcement, can be made in terms of a simple rule of mixtures approach (Edrees et al., 1998), as shown by equation (1):

> *<sup>Δ</sup><sup>V</sup> <sup>Δ</sup><sup>V</sup> ( p) V V c m*

where the subscripts *c* and *m* refer to composite and matrix, respectively, and where *p* is the volume fraction of the filler; therefore, the shrinkage of the composite after sintering is equal to the shrinkage that would be found in the isolated matrix times the matrix volume fraction

The inherent low strength and fracture toughness of synthetic HA have restricted its use to high load resistant implants (bones) due to its inferior mechanical properties (especially fracture toughness) compared to cortical bone (LeGeros et al., 1993). For example, its fracture toughness (*KIc*) is 1 MPa.m1/2, below the minimum of the range of values for human bones: 2–12 MPa.m1/2. Moreover, the Weibull modulus (*n*) is low in wet environments (*n* = 5–12) (DeWith et al., 1981) which indicates a weak structure of the HA implants. On the other hand, synthetic HA is more isotropic with a larger grain size than the biological HA (Cao & Hench, 1996). Further, bone is a complex composite of an organic (collagen) and an inorganic (biological apatite) component. Therefore, the synthesis of composites made of HA have been put forward, in which the addition of second phase into HA for improving its fracture toughness was widely used and investigated (Kong et al., 1999; H.W. Kim et al., 2003; Shen et al., 2001; Adolfsson et al., 2005; Kumar et al., 2005; Rao et al., 2002; Wu et al., 1998). For example, both the fracture toughness and flexural strength of HA can be improved substantially by the addition of ZrO2 (Yu et al., 2003). It was widely reported that that HA/ZrO2 composites show significantly higher mechanical properties, in particular bending strength, micro-hardness and Young's modules have proved to be bigger than those of pure HA (Miao et al., 2004). The incorporation of ZrO2 into HA enhances its mechanical properties and will not affect its biocompatibility. On the other hand, implants made of the HA/ZrO2 composites are highly biocompatible with no adverse reactions when used in rabbit mandibles (Vaidhyanathan et al., 1997). Further, the Y2O3 addition into ZrO2 can stabilize the tetragonal phase at room temperature (YSZ), and the tetragonal phase plays a major role in the fracture toughness increase. The toughening mechanisms include transformation and crack deflection toughening. Transformation toughening mechanism is an important mechanism in ceramic composites (Garvie et al., 1975). A great deal of energy is absorbed due to the volume expansion during the transformation of t-ZrO2 to m-ZrO2 so that the fracture toughness is enhanced. On the other hand, the ZrO2 particles dispersed in matrix would disturb the crack tip stress field and make the crack deflect and/or curve, then the driving force of crack propagation decreased and newborn surface area of crack increased, therefore the toughness was enhanced (Faber & Evans, 1983). The change of crack propagation mode, the increase of crack length and the formation of newborn surface area of crack at crack tip will consume energy. The main factor causing crack deflection is the

0 0

**3. Drawbacks in conventional sinterization of HA/ZrO2 composites** 

of the composite.

 = − 

1

(1)

internal stress caused by the interface effect, especially the interaction between internal stress/interface and cracks. The reason for the improvement of fracture toughness by this mechanism is the growth of surface area of crack and the change of the stress field distribution due to the crack propagating by deflection.

In general, the mechanical properties decrease significantly with increasing content of porosity and grain size, while a high crystallinity, a low porosity and small grain size tend to give a higher stiffness, a higher compressive and tensile strength and a greater fracture toughness. Recently it were obtained HA/YSZ composites with improved mechanical properties, flexural strength of ~155 MPa and fracture toughness of ∼2*.*1 MP m1*/*2, due to the contribution of 25 wt% YSZ component (Sung et al., 2007). The increased fracture toughness would result from the stress-induced tetragonal to monoclinic phase transformation in the YSZ component of HA/YSZ composites. In spite of a better mechanical behaviour, HA/YSZ composites must display uniform microstructures with a high degree of dispersion and without decomposition of the HA, during the sintering process. There are, however, some problems concerning the ceramic processing of HA/ZrO2 composite that should be solved. These problems are related to the influence synthesis and sintering conditions on phase stability of HA and *t*-ZrO2. The conventional mechanical mixing of HA and YSZ powders has been reported for the preparation of HA/YSZ composites through sintering (H.W. Kim et al., 2005; Kumar et al., 2005; Rapacz-Kmita et al., 2006). However, this mechanical mixing would cause low sintering density and/or non-uniform YSZ phase distribution in the sintered HA matrix due to the large particle size and/or segregation of YSZ particles, as shown in both micrographs of Fig.3.

Non-uniform distribution of the YSZ phase in the HA matrix, as shown in Fig.3, would seriously deteriorate the mechanical properties of HA/YSZ composites. Rather, the addition of ZrO2 particles reduces the size of HA grains, ZrO2 can thus act as an effective grain growth inhibitor to the HA grains. However, such composites cannot be sintered at temperatures same as the sintering of pure HA, while the sintering temperature for pure HA does not exceed the 1000 ºC, the specimens made of HA–20 vol.% YSZ must be necessarily heated up to 1100 ºC to obtain near full dense compacts (Khalil et al., 2007). Furthermore, the higher sintering temperature of HA/ZrO2 composite, frequently, causes an extreme grain coarsening, also illustrated in Fig.2, and HA phase decomposition, leading to a considerable deterioration of mechanical properties. Also, at high temperature the calcium, which is the main component of the HA, diffuses into YSZ and changes its tetragonal structure into the stable cubic phase, hindering the transformation toughening mechanism (Wu & Yeh, 1988). Furthermore, calcium diffusion also favours the decomposition of HA, leading to the formation of β-tricalcium phosphate (β-TCP, Ca3(PO4)2) and calcium zirconate (CZ, CaZrO3). The presence of such phases in the composite drastically reduces it mechanical behaviour (LeGeros, 1993). The formation of other phases among HA also interferes with the sinterability of the composites due to differential shrinkage, leading to overall reduced density as compared to that of pure HA. For instance, the sinterability of a HA–20wt.%ZrO2 (PSZ; 3 mol% Y2O3-doped) composite was affected by the differential shrinkage due to HA thermal decomposition (Wu & Yeh, 1988). Therefore, if there is no reaction between HA and ZrO2, the strength and toughness of HA are indeed improved significantly. HA and ZrO2 readily react with each other above 1000 ºC (Shen et al., 2001). However, reducing the temperature, the densification is

New Challenges in the Sintering of HA/ZrO2 Composites 187

To overcome the problem of grain growth, unconventional sintering and densification techniques have been proposed. The literature shows that dense HA bodies can be produced by a range of thermal treatments in addition to pressureless air sintering such as conventional HIP, microwave and SPS. However, the application of high temperatures

To minimize these reactions, some efforts have been made toward reducing the sintering temperature and holding time. Simple economical routes to microstructural improvement are therefore worth investigating. One alternative is the use the sol-gel process to synthesize a large variety of compositions carefully doped with additional phases, allowing obtain HA nanosized particles and a greater control of the morphology and microstructure of the

Dense ceramic-ceramic composites are usually obtained by pressing and conventional sintering of powders using pressure assisted methods, such as hot pressing, hot isostatic

The high sintering temperatures and long sintering times required for the consolidation of HAP/ZrO2 powders often result in excessive grain coarsening and decomposition of the HAP into second phases including phase transformation of *t*-ZrO2 to cubic phase during sintering in air, which are characteristic for conventional sintering methods and results in

The advantages of the hot pressing technique are the enhancement of the densification kinetics and the limiting of grain growth, while the disadvantages are the limited geometry of the end product and the expensive equipment required. Ahn et al., 2005, studied the effect of nano ZrO2 reinforcement on the strength of hot pressed HA/ZrO2 composites containing 1.5 to 8 wt% ZrO2. They observed the highest strength and hardness at 3wt% ZrO2 which subsequently decreased on higher ZrO2 loading. They also reported that lower volume fraction of ZrO2 addition will help to retain both HA and *t*- ZrO2 phase, which may be due to the combined effect of matrix constraint and uniform dispersion of fine ZrO2 particles. However, there are still controversial results in the literature about the phase stability of these composites prepared by pressure sintering methods (e.g HIP, hot pressing). Indeed, some reaction has been reported between HA and ZrO2 by some authors (Veljovic et al., 2007) but not by others (Li et al., 1996). However, there is a general conformity that HA/ZrO2 composites were considerably stable especially after HIP (Wu & Yen, 1988), even though traces of reaction may be expected due to the increased contact area between HA matrix and dispersed ZrO2 particles (Adolfsson et al., 2000). On the other hand, a partial reaction between HA and ZrO2 was observed in the composites prepared by hot pressing but still much less than observed in sintering in air (Evis et al., 2005, 2007). Therefore, the sintering environment and statistical effects on the ZrO2 distribution within matrix are important parameters that affect the thermal stability of HA and ZrO2 in the composites. Hot pressing of HA was found to allow the occurrence of densification at temperatures much lower than during conventional sintering (Halouani et al., 1994; Veljovic et al., 2009)

**4. Enhancements in the sinterability of HA/ZrO2 ceramic composites** 

during processing may cause decomposition of the principal components.

the deterioration of the mechanical properties of such ceramics.

derived HA/ZrO2 compacts.

pressing, etc.

**4.1 Hot Isostatic Pressing (HIP)** 

usually difficult unless very special processes, such as HIP, SPS, or other approaches are employed. Hence, another disadvantage of such materials is related to its processing. It is difficult to densify the HA/ZrO2 by pressureless sintering. However, despite the use of a cold isostatic pressing, a poor densification (apparent porosity ~30%) have been obtained in HA–20-vol%-ZrO2 composites, after sintering in air at 1350°C for 1 h, (H. W. Kim et al., 2002a) (Fig. 4).

Fig. 3. SEM microstructures of HA–15 vol.% YSZ, sintered at 1300ºC, 1h.

Fig. 4. SEM images showing the microstructure of HA–20- vol%-ZrO2 composite after sintering at 1350°C for 1 h (source: H. W. Kim et al., 2002a).

A number of approaches have been tried to prepare HA/ZrO2 composites containing higher fraction of ZrO2 (20-40 vol%) (Rao & Kannan, 2002; J. Li et al., 1996). However, in these composites, the major problem encountered were the densification of the composites and the decomposition of HA to β-TCP. Actually, the decomposition of HA to β-TCP is accelerated at the higher sintering temperatures (>1300ºC). Much effort has been exerted to improve the densification an overcome the reaction between HA and ZrO2. However, in most cases, hot-pressing (Kong et al., 1999) or HIP (Takaki et al., 1992) processes are still necessary to obtain HA–ZrO2 composites with high densities. Nevertheless, there is still a desire for a novel technique to produce dense bodies that use lower temperatures in order to facilitate consolidation but that removes any phase decomposition.

usually difficult unless very special processes, such as HIP, SPS, or other approaches are employed. Hence, another disadvantage of such materials is related to its processing. It is difficult to densify the HA/ZrO2 by pressureless sintering. However, despite the use of a cold isostatic pressing, a poor densification (apparent porosity ~30%) have been obtained in HA–20-vol%-ZrO2 composites, after sintering in air at 1350°C for 1 h, (H. W. Kim et al.,

Fig. 3. SEM microstructures of HA–15 vol.% YSZ, sintered at 1300ºC, 1h.

Fig. 4. SEM images showing the microstructure of HA–20- vol%-ZrO2 composite after

A number of approaches have been tried to prepare HA/ZrO2 composites containing higher fraction of ZrO2 (20-40 vol%) (Rao & Kannan, 2002; J. Li et al., 1996). However, in these composites, the major problem encountered were the densification of the composites and the decomposition of HA to β-TCP. Actually, the decomposition of HA to β-TCP is accelerated at the higher sintering temperatures (>1300ºC). Much effort has been exerted to improve the densification an overcome the reaction between HA and ZrO2. However, in most cases, hot-pressing (Kong et al., 1999) or HIP (Takaki et al., 1992) processes are still necessary to obtain HA–ZrO2 composites with high densities. Nevertheless, there is still a desire for a novel technique to produce dense bodies that use lower temperatures in order to facilitate consolidation but that removes any phase

sintering at 1350°C for 1 h (source: H. W. Kim et al., 2002a).

2002a) (Fig. 4).

decomposition.
