Abstract
BACKGROUND:Machinable bioglass-ceramics became a new inorganic biomaterial; moreover, strength, toughness and machinability are significantly studied.

OBJECTIVE: To observe the effect of ZnO, Fe2O3 and ZrO2 additives on the microstructure and properties of machinable bioglass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system.

DESIGN: Observational contrast study.

SETTING: Department of Orthopaedics, Tongji Hospital Affiliated to Tongji Medical College, Huazhong University of Science and Technology.

MATERIALS: The experiment was carried out in the Laboratory of Materials, Wuhan University of Technology from October 2003 to April 2004. K2O-MgO-CaO-SiO2-P2O5-F system, ZnO, Fe2O3 and ZrO2 additives, D/Max-ⅢA X-ray diffractometer (Japan), JSM-5610LV scanning electron microscope (SEM), HVS-1000 microhardnessmeter and 3257-35 magnetic testing device (Japan) were used in this study.

METHODS: In the glass-ceramic of K2O-MgO-CaO-SiO2-P2O5-F system, three kinds of additives ZnO, ZnO-Fe2O3 and ZnO-Fe2O3-ZrO2 were added respectively. These three kinds of ceramics were prepared by being melted at 1 400 ℃ for 1 hour. After fire, crystal temperature was set based on differential thermal analysis curve. ① Physical properties: Microhardness was determined with microhardnessmeter (HVS-1000). The flexural strength and fracture toughness were established measured with ceramic mechanical test system (MTS) method and single edge notched beam (SENB) method, respectively. Saturation magnetic moment and Curie temperature were determined with 3257-35 magnetism test machine made in Japan. Holing method was used to measure machinability. ② Test of crystalline phase and microstructure: Crystalline phase analysis was carried out with X-ray diffraction (XRD: D/Max-ⅢA) and microstructure analysis were conducted on an etched fracture surface using SEM (JSM-5610LV, Japan).

MAIN OUTCOME MEASURES: Effects of ZnO, Fe2O3 and ZrO2 additives on physical properties, mineral composition and crystalline phase.

RESULTS: ① The addition of ZnO to the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system was beneficial to crystal growth, increasing aspect ratio of crystal, decreasing microhardness and improving toughness and machinability. ② The addition of ZnO-Fe2O3 to the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system can form minority Mg-Zn ferrite with magnetism. The main crystalline phase present in the glass- ceramic was diopside with small size and hardness was improved, which led to worse machinability. ③ Because of the addition of ZnO-Fe2O3-ZrO2, magnesia fluormica and fluorapatite became the main crystalline phases, and the minor phases including Mg-Zn ferrite, t-ZrO2 and m-ZrO2, etc. also presented in the glass-ceramic. This material possessed high strength and toughness, good machinability and magnetism, which could stimulate formation of new bone, and was good substitute of bone restorations. Bioassay and in vitro test indicated that the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system was bioactive and biocompatible.

CONCLUSION: ZnO, Fe2O3 and ZrO2 additives play a significant role in changing crystalline structure, enhancing strength, toughness and machinability of machinable bioglass-ceramic and generating magnetism.

INTRODUCTION

Bioglass-ceramics are among materials that have attracted considerable attention in recent years because of its high mechanical strength, toughness, biocompatibility and chemical stability. These materials were often used as medical implants. But its fragility and difficulty in processing limited its clinic use. Machinable bioglass-ceramics became a new inorganic biomaterial[1].
In the present work, a glass-ceramics composition belonging to the K2O-MgO-CaO-SiO2-P2O5-F system[2], with ZnO, Fe2O3 and ZrO2 as additives, has been considered to enhance strength, toughness, machinability and even magnetism. Three kinds of materials were prepared: samples a-1, b-2 and c-3, respectively. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the crystalline phase and microstructure. At the same time, the physical properties were measured. Physical properties, XRD patterns and SEM microphotograph showed that the machinable bioglass-ceramics in this study is ideal materials being used as bone-repair.

MATERIALS AND METHODS

Materials
The experiment was carried out in the Laboratory of Materials, Wuhan University of Technology from October 2003 to April 2004.
K2O-MgO-CaO-SiO2-P2O5-F system, ZnO, Fe2O3 and ZrO2 additives, D/Max-ⅢA X-ray diffractometer (Japan), JSM-5610LV scanning electron microscope (SEM) and 3257-35 magnetic testing device (Japan) were used in this study.

Methods
In the glass-ceramic of K2O-MgO-CaO-SiO2-P2O5-F system, three kinds of materials were prepared by adding ZnO, ZnO-Fe2O3 and ZnO-Fe2O3-ZrO2 respectively. The compositions were melted at 1 400 ℃ for 1 hour. Differential thermal analysis was performed on the glass. Samples were treated at various temperatures. The chemical composition is shown in Table 1.

Physical properties
Microhardness was determined with microhardnessmeter (HVS-1000) (test force: 9.8 N; test time: 40 s). The flexural strength and fracture toughness were measured with MTS method and SENB (single edge notched beam) method (with a span of 20 mm, rate of 0.5 mm/minute), respectively. Saturation magnetic moment and Curie temperature were determined with 3257-35 magnetism test machine made in Japan (according to HG/T2347.2-92). Holing method was used to measure machinability. The physical properties were shown in Table 2.

Test of crystalline phase and microstructure
The properties of the machinable glass-ceramic were closely related to the crystalline phase and microstructure. Crystalline phase analysis was carried out with X-ray diffraction (XRD: D/Max-ⅢA) and microstructure analysis were conducted on an etched fracture surface using SEM (JSM-5610LV, Japan).

Biology test
To evaluate the biological compatibility of magnetic machinable bioactive glass-ceramic, a series of experiments was made (Table 3).

RESULTS AND DISCUSSION

Effect of ZnO on the crystallization and microstructure of the glass-ceramic
Basic glass and glass-ceramic of a-1 appeared transparent and cream white respectively. According to XRD of Figure 1, the main crystalline phases were composed of fluorapatite [Ca5(PO4)3F], magnesium fluormica [K2Mg5(Si8O20)F4] and β-TCP. The shape of fluorapatite [Ca5(PO4)3F] crystal was hexagonal column, magnesia fluormica crystal was column or sheet structure and β-TCP crystal was spherulite. All the crystals formed framework structure. Theoretically, framework structure was beneficial to improving flexural strength, fracture toughness and machinability. Figure 2 showed that there were transcrystalline cracks. The reason was that the thermal expansion coefficient of crystal is lower than that of glass. All the radial stress between two crystal phases is compressive stress, while peripheral stress is tensile stress in glass and compressive stress in crystal. Consequently, the crack parallel to crystal would be expected to turn to run along the crystal/crystal interface. While stress perpendicular to glass/crystal interface is compressive stress, which may lead to transcrystalline fracture[3].

The crystal size has great effect on the strength of material. The toughness and strength of the flurophore glass-ceramic which crystal size was 4－5 μm, in comparison with that of other glass-ceramics which size was 1－2 μm, was higher[4]. Crossman has studied tetrasilica fluormica glass-ceramics, and found out that if d < 4.5 μm the strength of material was in direct proportion to d0.2, and if d > 4.5 μm, the strength did in direct proportion to d－1[5]. For the glass-ceramics, in which one of crystalline phases was mica, if crystal size was bigger, the comparative content of crystalline phase was also higher, the strength of material would be improved[6]. Figure 2 shows that magnesia fluormica was long column shape; the ratio of crystal was large, which was efficient in forming framework structure with interlocking: Because β-TCP and hydroxyapatite were filled in framework, the strength and toughness will be improved. According to Crossman[5.6], if there was framework structure with enough aspect ratio crystal, the glass-ceramics in which the content of mica is only 1/3 had good machinability. On the other hand, crystal with large aspect ratio was beneficial to crack deflection and played a bridge role in deflection process, which would improve toughness of materials.

Effect of ZnO-Fe2O3 on microstructure and properties of glass-ceramic
The basic glass of sample b-2 was black, and became black and opaque glass-ceramics after crystallization. According to (b) in Figure 1, because of addition of ZnO-Fe2O3 additives, the main crystalline phases consisted of diopside [Ca(Mg,Fe)Si2O6], fluorapatite [Ca5(PO4)3F], magnesium fluormica and β-TCP, and the minor was comprised of γ- Fe2O3 and Mg-Zn ferrite. The addition of Fe2O3 and more MgO resulted in crystallization of diospide. Fluorapatite and β-TCP of bioglass-ceramic in used bioactivity, magnesium fluormica did machinability, γ- Fe2O3 and Mg-Zn ferrite did magnetism stimulating formation of new bone. The structure of γ- Fe2O3 was one of those structures of Fe2O3 crystals and cubic symmetry. The structure and magnetism of γ- Fe2O3 was similar to those of Fe3O4. Mg-Zn ferrite [MxZn1-xFe2O4] was a solid solution in which Mg was bivalent ion. The saturation magnetic moment of Mg-Zn ferrite changed with the increase of the content of ZnO. With the increase of the content of ZnO, magnetization intensity increased when it less than 40% and decreased when it more than 40%. Magnetization intensity decreased to zero when the content of ZnO was 100%.
The additive of ZnO-Fe2O3, which was compound, could improve nucleation. Compound effect led to greater density of compound nucleus than that of singular nucleus with same total concentration.
In Figure 3, there were minor microcrystal with sheet structure which indorsed machinability to the certain extent, and more diopside with fine crystals, structure of diopside were short column, bulb and redial which was beneficial to improving tensile strength, with the exception of impact strength. Hardness of diopside was bigger than that of magnesium fluormica. Therefore, the machniability and toughness of this material were worse.

Effect of ZnO-Fe2O3-ZrO2 on crystallization and microstructure of glass-ceramic
Because of the addition of ZrO2, F- together with ZrO2 could promoted the formation of mica with claviform and great aspect ratio with interlocking. This structure was efficient in improving machinability, strength and toughness[5,7]. According to XRD of Figure 1, crystalline of sample c-3 were composed of magnesium fluormica, fluorapatite, Mg-Zn ferrite, t-ZrO2 and m-ZrO2. The former two were main crystalline phase. In Figure 4, the structure of magnesium fluormica was claviform and sheet structure which afforded machinability; that of fluorapatite was hexagon column which did bioactivity; Mg-Zn ferrite did magnetism which could excite formation of new bone, t-ZrO2 and m-ZrO2 could promoted strength and toughness of glass-ceramic. The main toughening mechanisms were dispersion and transformation toughening. Some Si4+ might be substituted for Zr4+, which was beneficial to improving strength and tough too.

Machinability glass-ceramic was investigated with holing method. It could be found with microscope that wall hole of sample a-1 and b-2 had microcrack, but that of sample c-3 nothing. Consequently, it could be concluded that ZrO2 of sample c-3 played a role in toughening and improving machinability.

CONCLUSION

Firstly, the addition of ZnO to the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system was beneficial to crystal growth, increasing aspect ratio of crystal, decreasing microhardness and improving toughness and machinability.
Secondly, the addition of ZnO-Fe2O3 to the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system can form minority Mg-Zn ferrite with magnetism. The main crystalline phase present in the glass- ceramic was diopside with small size and large hardness, which led to worse machinability.
Thirdly, because of the addition of ZnO-Fe2O3-ZrO2, magnesia fluormica and fluorapatite became the main crystalline phases, and the minor phases including Mg-Zn ferrite, t-ZrO2 and m-ZrO2, etc. also presented in the glass-ceramic. The material of this system possessed of large strength and toughness, good machinability and magnetism, which could stimulate formation of new bone, and was good substitute of bone restorations. Bioassay and in vitro test indicated that the glass-ceramic in K2O-MgO-CaO-SiO2-P2O5-F system was bioactive and biocompatible.

REFERENCES

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Yue WH, Chen T.Medical synthetic material. Wuhan: Publishing House of Wuhan University 1988;307-311
Mcmilan PW. Glass Ceramics. Beijing: Publishing House of China Architectural Industry 1998;199-202
Vogel W. Heidenreich, Grellner. Germany patent 113885，1973
Grossman DG. Machinable glass-cermic based on tetrasilicic mica. J Am Ceram Soc 1972;55(9):446-449
Li H, Ran JG, Gou L. Study on crystallization of machinable glass-ceramics in the CaO-MgO-Al2O3-SiO2-F System. Cailiao Kexue yu Gongcheng 2002;20(2):28-30
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添加剂对可切削生物微晶玻璃结构的影响☆

周新华1，褚 颖2，陈安民3，孙淑珍2
1北京积水潭医院矫形骨科，北京市 100035； 2武汉理工大学材料学院，湖北省武汉市 430070； 3华中科技大学同济医学院附属同济医院骨科，湖北省武汉市 430030
周新华☆，男，汉族，1967年生，湖北省黄梅县人，2004年华中科技大学同济医学院毕业，博士，主治医师，主要从事生物材料、人工关节的研究。
摘要
背景：医用可加工微晶玻璃是一种新型的无机生物新材料，可切削生物微晶玻璃的强度、韧性和可切削加工性能值得研究。
目的：分析ZnO、Fe2O3和ZrO2添加剂对 K2O-MgO-CaO-SiO2-P2O5-F系统可切削微晶玻璃的结构和性能的影响。
设计：观察对比实验。
单位：华中科技大学同济医学院附属同济医院骨科。
材料：实验于2003－10/2004－04在在武汉理工大学材料学院实验室完成。K2O-MgO-CaO-SiO2-P2O5-F系统及ZnO、Fe2O3和ZrO2添加剂，日本理学D/Max-ⅢA型X射线衍射仪及JSM-5610LV型扫描电镜，HVS-1000显微硬度计，日产3257-35磁性测试仪。
方法：采用K2O-MgO-CaO-SiO2-P2O5-F系统可切削生物微晶玻璃，分别加入适量的ZnO、ZnO-Fe2O3和ZnO-Fe2O3-ZrO2添加剂，在1 400 ℃下熔制，保温1 h，浇铸成型，经退火后根据差热分析曲线制定晶化温度制度，制备了3组不同组分的微晶玻璃。① 物理性能的测试：采用HVS-1000显微硬度计测定显微硬度、MTS陶瓷试验系统三点弯曲法及单面切口三点弯曲法测定抗弯强度和断裂韧性、日产3257-35磁性测试仪测定饱和磁矩及居里温度、钻孔法测定了可切削性。②矿物组成及晶体形貌的测定：采用日本理学D/Max-ⅢA型X射线衍射仪及JSM-5610LV型扫描电镜分别测定了3种试样的矿物组成及晶体的形貌。
主要观察指标：ZnO、ZnO-Fe2O3和ZnO-Fe2O3-ZrO2添加剂制备不同组分的微晶玻璃物理性能、矿物组成及晶体形貌的测定结果。
结果：① ZnO加入K2O-MgO-CaO-SiO2-P2O5-F微晶玻璃中，有利于晶体成长，增加晶体的长径比，显微硬度低，韧性较好，易于切削。②ZnO-Fe2O3加入K2O-MgO-CaO-SiO2-P2O5-F微晶玻璃中，形成少量镁锌铁尖晶石铁氧体，具有磁性。主晶相为透辉石，晶粒细小，硬度较大，切削性能下降。③ ZnO-Fe2O3-ZrO2加入K2O-MgO-CaO-SiO2-P2O5-F微晶玻璃中，析晶后主晶相为氟镁白云母、氟磷灰石，另有少量的镁锌铁尖晶石铁氧体、t-ZrO2、m-ZrO2等。材料强度高、韧性及可切削性能良好，具有的磁性能激发新骨形成，用其做骨替代材料可加速骨缺损的修复。
结论： ZnO、ZnO-Fe2O3和ZnO-Fe2O3-ZrO2添加剂的加入对改变生物微晶玻璃的晶体结构，提高生物微晶玻璃的强度、韧性和可切削性具有重要意义，并同时赋予了磁性。
关键词：添加剂；可切削生物微晶玻璃；显微结构
中图分类号:R318.08 文献标识码:A 文章编号:1673-8225(2007)22-04443-04
周新华，褚颖，陈安民，孙淑珍.添加剂对可切削生物微晶玻璃结构的影响[J].中国组织工程研究与临床康复，2007，11(22):4443-4446
[www.zglckf.com/zglckf/ejournal/upfiles/07-22/22k-4443(ps).pdf]
(Edited by Lu L/Ji H/Wang L)