Stress-corrosion crack growth of Si-Na-K-Mg-Ca-P-O bioactive glasses in simulated human physiological environment

Abstract

This paper describes research on the stress-corrosion crack growth (SCCG) behavior of a new series of bioactive glasses designed to fabricate coatings on Ti and Co-Cr-based implant alloys. These glasses should provide improved implant fixation between implant and exhibit good mechanical stability in vivo. It is then important to develop an understanding of the mechanisms that control environmentally assisted crack growth in this new family of glasses and its effect on their reliability. Several compositions have been tested in both static and cyclic loading in simulated body fluid. These show only small dependences of SCCG behavior on the composition. Traditional SCCG mechanisms for silicate glasses appear to be operative for the new bioactive glasses studied here. At higher velocities, hydrodynamic effects reduce growth rates under conditions that would rarely pertain for small natural flaws in devices.

Figure 1

The DCB geometry used for crack-growth study. Dimensions were thickness, B, of 5 ± 0.5 mm, height, 2h, of 8 ± 0.04 mm, and length, L, of 50 ± 5 mm.

Figure 2

The average SCCG data for glasses in Table 1 (n designates the number of samples in each population if greater than one). The gray band describes data for Bioglass^® 45S5 taken from ref. [31], and solid line describes data for soda-lime glass taken from ref. [32]. All data except for soda-lime glass were taken in SBF at 37°C.

Figure 3

Crack-growth rates measured under cyclic loading for 6P61 glass are compared with predictions of growth rates under fatigue loading based on data from static crack-growth rates computed for two cases: i) no hydrodynamic forces acting at crack flanks, ii) the crack tip at the mean load throughout the applied loading cycle. These results show that for no shielding, the predictions overestimate cyclic crack-growth rates while with high shielding the cyclic growth rates are underpredicted.

Figure 4

The mechanisms of reaction of the Si-Na-K-Ca-Mg-P-O glasses with SBF are consistent with those described by Hench for Bioglass® [20,37,38]. The steps involved are: the exchange of Na⁺ and K⁺ from the glass with H⁺ or H₃O⁺ from solution, accompanied by the loss of soluble silica into the solution and the formation of silanols on the glass surface; condensation and repolymerization of a SiO₂-rich layer on the surface; migration of Ca²⁺ and PO^3-₄ through the silica-rich layer forming a CaO-P₂O₅-rich film that incorporates calcium and phosphates from solution; finally, the crystallization of the amorphous calcium phosphate film to form an apatite layer. De Aza et al. [39] have pointed out that the increase in pH on the glass surface due to the ionic exchange between the labile cations Na⁺, K⁺, Ca⁺ etc., is necessary for the partial dissolution of the silica-rich layer and the subsequent apatite precipitation.

Figure 5

Range of strain energy release rates observed at a crack-growth velocity of 10^-5 m/s plotted against thermal expansion coefficient (CTE) of the glass. The glass thermal expansion range covers the entire range between Ti-based to Co-Cr-based alloys.

Figure 6

(a) Plot showing a decline of activation area for crack growth versus CTE of the glass, indicating indirectly that the parameter A* decreases with increasing reactivity of the glass caused by higher alkali and alkaline earth oxide contents. (b) Dependence of the activation area with $G$, according to the data shown in Fig. 2. The lines are theoretical boundaries ( $A\ast =\text{mkT}/2G$) taking m from the fittings summarized in Table 2.

Figure 7

Logarithmic $v-K$ curve showing deviation from the straight line at the slower speeds for highly reactive glasses.

Figure 8

Sample predictions of $v-G$ curves for the stress-independent regime of crack growth using Eqs. 7 and 8.

Figure 9

Computed values of normalized crack-tip stress-intensity range vs. frequency of applied loading and vs. crack length for an applied ΔK = 0.5 MPa√m.