Research PaperThe improved mechanical properties of β-CaSiO3 bioceramics with Si3N4 addition
Graphical abstract
Introduction
Over the past two decades, calcium silicate (Ca–Si) based bioceramics have been introduced as potential bioactive materials for bone tissue regeneration due to their superior bone bioactivity compared to hydroxyapatite (HA) (Deaza et al., 1994, Ni et al., 2007, Oonishi et al., 2000). As one of the most important calcium silicate, β-CaSiO3 is widely used for industrial ceramics. In recent years, β-CaSiO3 has been investigated as bioactive biomaterials, and has drawn growing attention for its promising applications in bone tissue regeneration because of its good bioactivity, biocompatibility and biodegradability (De Aza et al., 1999, Ni et al., 2006, Ni et al., 2007). Nevertheless, the extensive use of β-CaSiO3 is limited by their inadequate strength (Mehrali et al., 2014). Difficulties in preparing dense β-CaSiO3 ceramics with improved mechanical properties make them suitable only for low-bearing applications (Endo et al., 1994, Shirazi et al., 2014b)
In order to improve the mechanical properties of β-CaSiO3 ceramics, one of the effective methods is the incorporation of a second phase with good mechanical properties into β-CaSiO3 (Mehrali et al., 2014, Shirazi et al., 2015). Using glasses as sintering additives has been considered as an effective way to promote the sintering properties of ceramics by liquid-phase sintering (Lin et al., 2009), whereas glassy phase in the ceramic matrix may be disadvantageous to the mechanical strength of ceramics. Some researchers investigated the alumina reinforced β-CaSiO3 and demonstrated that alumina particles could improve its hardness and fracture toughness (Shirazi et al., 2014a). Nevertheless, the sintering temperature of β-CaSiO3 ceramics needed to be higher than 1125 °C, at which the flexural strength of the composites was considerably degraded by the transformation of β-phase into α-phase of CaSiO3. Silicon nitride (Si3N4) is a kind of ceramics known for its high performance characterized by fracture toughness, high wear resistance and low coefficient of friction. Several works on biocompatibility and bioactivity of Si3N4 outlined that Si3N4-based ceramics can be used as materials in the field of hard tissues surgery. Si3N4-based ceramics can be used as toxic free materials which has already been testified (Silva et al., 2004). In vivo tests, implanting Si3N4 pieces into the femurs of rabbits had demonstrated good bone/implant attachment and no adverse cell reactions (Guedes e Silva et al., 2008). What is more, at low temperature, the surface of Si3N4 particles can be oxidized to form SiO2 with high reaction activity and the oxidation-derived SiO2 can act as bonding phases to improve the mechanical strength of a material. Numerous works utilizing the oxidation of Si3N4 have already been done to prepare Si3N4 ceramics and have demonstrated improved mechanical properties (Ding et al., 2007, Gao et al., 2002, Zymla et al., 2002).
Taking into consideration the above-mentioned concepts, Si3N4 is an ideal reinforcing phase of β-CaSiO3. The motivation of this work is to study the effects of Si3N4 on the sinterability of β-CaSiO3 ceramics. The microstructures of obtained specimens were analyzed. The mechanical properties were characterized.
Section snippets
Synthesis of β-CaSiO3 powder
β-CaSiO3 powder was synthesized by the chemical precipitation method (Lin et al., 2004). Na2SiO3·9H2O and Ca(NO3)2·4H2O were used as the source of Si and Ca, respectively. The Ca(NO3)2 solution was added dropwise into the Na2SiO3 solution to produce a white suspension under rigorous stirring. The white suspension was further stirred for 24 h followed by washing three times with deionized water and two times with pure ethanol. After washing, the remaining liquid were removed by vacuum filtration,
Characteristics of β-CaSiO3 powders
The XRD patterns of the prepared powders were shown in Fig. 1(a). The result showed that the as-prepared powder was β-CaSiO3 and no other phases were detected. Fig. 1(b) exhibits a typical SEM image of the as-prepared powder. The morphology indicated that the as-prepared powders were composed of irregularly shaped particles, and the granularity of powder was below 5 μm.
Microstructure and phase composition of β-CaSiO3 ceramics
The sintering process of β-CaSiO3 ceramics with 3 wt% Si3N4 addition and the oxidation behavior of pure Si3N4 in air were both
Conclusion
Calcium silicate ceramics with different addition concentration of Si3N4 were prepared by pressureless-sintering. The main crystal phase of the ceramics was β-CaSiO3. The concentration of Si3N4 had great effect on the mechanical properties of the β-CaSiO3 ceramics due the SiO2 originated from Si3N4 oxidization. When a 3 wt% content of Si3N4 and a sintering temperature of 1100 °C were used, a flexural strength, hardness and fracture toughness of 157.2 MPa, 4.4 GPa and 2.3 MPa m1/2 were successfully
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