Weibull modulus and fracture strength of highly porous hydroxyapatite.

Porous hydroxyapatite (HA) is used in a variety of applications including biomedical materials such as engineered bone materials and microbe filters. Despite the utility of the Weibull modulus, m, as a gauge of the mechanical reliability of brittle solids, there have been very few studies of m for porous HA. A recent study of porous HA that included the current authors (Fan, X., Case, E.D., Ren, F., Shu, Y., Baumann, M.J., 2012a. Journal of the Mechanical Behavior of Biomedical Materials. 8, 21-36) showed increases in m for porosity, P, approaching PG, the porosity of the green (unfired) specimen. In this paper, 18 groups of highly porous HA specimens (12 groups fabricated in this study and 6 groups from Fan et al., 2012a) were analyzed with P values from 0.59 to 0.62, where PG=0.62. The partially sintered HA specimens were fractured in biaxial flexure using a ring-on-ring test fixture. The fracture strength decreased monotonically with decreasing sintering temperature, Tsinter, from 4.8MPa for specimens sintered at 1025°C-0.66MPa for specimens sintered at 350°C. However, the Weibull modulus remained surprisingly high, ranging from 6.6 to 15.5. In comparison, for HA specimens with intermediate values of P, from about 0.1-0.55, the Weibull modulus tended to be lower (ranging from about 4 to 11) than the highly porous specimens included in this study.

Part I: porosity dependence of the Weibull modulus for hydroxyapatite and other brittle materials.

Porous brittle materials are used as filters, catalyst supports, solid oxide fuel cells and biomedical materials. However the literature on the Weibull modulus, m, versus volume fraction porosity, P, is extremely limited despite the importance of m as a gauge of mechanical reliability. In Part I of this study, m is determined for 441 sintered hydroxyapatite (HA) specimens fractured in biaxial flexure for 0.08 ≤ P ≤ 0.62. In this study, we analyze a combined data set collected from the literature that represents work from a total of 17 different research groups (including the present authors), eight different materials and more than 1560 oxide and non-oxide specimens, the m versus P plot is "U-shaped" with a wide band of m values for P<0.1 (Region I) and P>0.55 (Region III), and a narrower band of m values in the intermediate porosity region of 0.1, and the Young's modulus E for the HA specimens tested in Part I along with literature data for other brittle materials. Both <σ(f)> and E are power law functions of the degree of densification, ϕ, where ϕ=1-P/P(G) and P(G) is the green (unfired) porosity.

Part II: fracture strength and elastic modulus as a function of porosity for hydroxyapatite and other brittle materials.

Part I of this paper discussed the Weibull modulus m, versus porosity P behavior of brittle materials, including HA. While the Weibull modulus m deals with the scatter in fracture strength data, this paper (Part II) focuses on two additional key mechanical properties of porous materials, namely the average fracture strength <σ(f)>, and Young's modulus E, for P in the interval from P≈ zero to P≈P(G) (the porosity of the unfired compacts). The <σ(f)> versus P data for HA from this study and the literature data for alumina, yttria stabilized zirconia (YSZ) and silicon nitride are described well by functions of ϕ, where ϕ=1-P/P(G)= the degree of densification. A similar function of ϕ applies to the versus P behavior of HA from this study and data from the literature for alumina, titanium and YSZ. All of the data analyzed in this study (Part II) are based on partially and fully sintered powder compacts (excluding green powder compacts), thus the <σ(f)>/σ(0) versus ϕ and /E(0) versus ϕ relationships may apply only to such specimens.

Self-joining of zirconia/hydroxyapatite composites using plastic deformation process.

A plastic deformation process was demonstrated to self-join 3 mol.% yttria partially stabilized zirconia (3Y-TZP)/hydroxyapatite (HA) composites. The 3Y-TZP/40 vol.% HA composites were fabricated by conventional ceramic processing by cold pressing premixed 3Y-TZP and HA powders into pellets. Densification ( approximately 90%) of composites was achieved by sintering composite powder compacts at 1450 degrees C for 5h. Optimum self-joining of 3Y-TZP/40 vol.% HA composites was obtained at 1300 degrees C for a strain rate of 5 x 10(-5)/s. The flow stress during joining was 40 MPa. Microstructural and mechanical characterizations of the joint interface demonstrated that there were no discernible differences between the joint and the composite material away from the interface.

Confocal laser scanning microscopy as a tool for imaging cancellous bone.

Understanding the bimodal structure of cancellous bone is important for tissue engineering in order to more accurately fabricate scaffolds to promote bone ingrowth and vascularization in newly forming bone. In this study, confocal laser scanning microscopy (CLSM) was used to create detailed images of the bimodally porous intertrabecular space of defatted and deproteinized cancellous canine bone taken from the epiphysis of the humerus. The bimodal pore structure was imaged using both reflective and fluorescent modes in CLSM, resulting in four different, but complementary image types: (1) a Z-stack overlay, (2) a phi-Z scan, (3) a topographical map, and (4) a contour map. Submerging the bone in rhodamine B dye prior to fluorescent imaging enhanced the pore surface details, giving a more accurate pore size measurement. The average macropore diameter was found to be 260 +/- 97 microm while the average micropore diameter was 13 +/- 10 microm. When compared with common techniques, including microcomputed tomography, magnetic resonance imaging, scanning electron microscopy, and environmental scanning electron microscopy, for imaging cancellous bone, CLSM was found to be an effective tool, given its ability to nondestructively image the surface and near-surface pore structure.