Quantitative evaluation of the adhesion of bioactive glasses onto Ti6Al4V substrates

© 2016 Elsevier Ltd. This paper presents a new method for applying a uniform thickness bioactive glass coating to a Ti6Al4V substrate and postulates a fracture mechanics testing methodology to quantify the glass/metal adhesion in terms of a measured critical strain energy release rate (GIC). Bi-layer double cantilever beam (DCB) test specimens were developed for this purpose and were found to generate repeatable and consistent measures of GIC for the tested system. The measured GIC of the coating decreased significantly from 6.2 to 2.5 J/m2 with an increase in coating thickness from 90 to 390 μm. Since high temperature enamelling processes can potentially introduce significant residual stresses in the glass/metal system, the residual stresses were measured and their impact on the adhesion was assessed. Increases in the coating thickness were found to increase the residual stresses from 6.8 to 17.9 MPa, thus decreasing the adhesion between the glass and the Ti alloy. Finally, the directional stability of the crack within the coating was quantified by calculating the T-stress for coatings with different thicknesses and it was found that as the coating thickness increased, the crack destabilized and tended to kink rather than travel in a straight line

Evaluating the Critical Strain Energy Release Rate of Bioactive Glass Coatings on Ti6Al4V Substrates after Degradation

It has been reported that the adhesion of bioactive glass coatings to Ti6Al4V reduces after degradation, however, this effect has not been quantified. This paper uses bilayer double cantilever (DCB) specimens to determine G IC and G IIC , the critical mode I and mode II strain energy release rates, respectively, of bioactive coating/Ti6Al4V substrate systems degraded to different extents. Three borate-based bioactive glass coatings with increasing amounts of incorporated SrO (0, 15 and 25 mol%) were enamelled onto Ti6Al4V substrates and then immersed in de-ionized water for 2, 6 and 24 h. The weight loss of each glass composition was measured and it was found that the dissolution rate significantly decreased with increasing SrO content. The extent of dissolution was consistent with the hypothesis that the compressive residual stress tends to reduce the dissolution rate of bioactive glasses. After drying, the bilayer DCB specimens were created and subjected to nearly mode I and mode II fracture tests. The toughest coating/substrate system (one composed of the glass containing 25 mol% SrO) lost 80% and 85% of its G IC and G IIC , respectively, in less than 24 h of degradation. The drop in G IC and G IIC occurred even more rapidly for other coating/substrate systems. Therefore, degradation of borate bioactive glass coatings is inversely related to their fracture toughness when coated onto Ti6A4V substrates. Finally, roughening the substrate was found to be inconsequential in increasing the toughness of the system as the fracture toughness was limited by the cohesive toughness of the glass itself

Quantifying the Mode II Critical Strain Energy Release Rate of Borate Bioactive Glass Coatings on Ti6Al4V Substrates

Bioactive glasses have been used as coatings for biomedical implants because they can be formulated to promote osseointegration, antibacterial behavior, bone formation, and tissue healing through the incorporation and subsequent release of certain ions. However, shear loading on coated implants has been reported to cause the delamination and loosening of such coatings. This work uses a recently developed fracture mechanics testing methodology to quantify the critical strain energy release rate under nearly pure mode II conditions, GIIC, of a series of borate-based glass coating/Ti6Al4V alloy substrate systems. Incorporating increasing amounts of SrCO3 in the glass composition was found to increase the GIIC almost twofold, from 25.3 to 46.9 J/m2. The magnitude and distribution of residual stresses in the coating were quantified, and it was found that the residual stresses in all cases distributed uniformly over the cross section of the coating. The crack was driven towards, but not into, the glass/Ti6Al4V substrate interface due to the shear loading. This implied that the interface had a higher fracture toughness than the coating itself

Quantitative Evaluation of the Adhesion of Bioactive Glasses Onto Ti6Al4V Substrates

This paper presents a new method for applying a uniform thickness bioactive glass coating to a Ti6Al4V substrate and postulates a fracture mechanics testing methodology to quantify the glass/metal adhesion in terms of a measured critical strain energy release rate (GIC). Bi-layer double cantilever beam (DCB) test specimens were developed for this purpose and were found to generate repeatable and consistent measures of GIC for the tested system. The measured GIC of the coating decreased significantly from 6.2 to 2.5 J/m2 with an increase in coating thickness from 90 to 390 μm. Since high temperature enamelling processes can potentially introduce significant residual stresses in the glass/metal system, the residual stresses were measured and their impact on the adhesion was assessed. Increases in the coating thickness were found to increase the residual stresses from 6.8 to 17.9 MPa, thus decreasing the adhesion between the glass and the Ti alloy. Finally, the directional stability of the crack within the coating was quantified by calculating the T-stress for coatings with different thicknesses and it was found that as the coating thickness increased, the crack destabilized and tended to kink rather than travel in a straight line

Evaluating the critical strain energy release rate of bioactive glass coatings on Ti6Al4V substrates after degradation

© 2017 Elsevier Ltd It has been reported that the adhesion of bioactive glass coatings to Ti6Al4V reduces after degradation, however, this effect has not been quantified. This paper uses bilayer double cantilever (DCB) specimens to determine G IC and G IIC , the critical mode I and mode II strain energy release rates, respectively, of bioactive coating/Ti6Al4V substrate systems degraded to different extents. Three borate-based bioactive glass coatings with increasing amounts of incorporated SrO (0, 15 and 25 mol%) were enamelled onto Ti6Al4V substrates and then immersed in de-ionized water for 2, 6 and 24 h. The weight loss of each glass composition was measured and it was found that the dissolution rate significantly decreased with increasing SrO content. The extent of dissolution was consistent with the hypothesis that the compressive residual stress tends to reduce the dissolution rate of bioactive glasses. After drying, the bilayer DCB specimens were created and subjected to nearly mode I and mode II fracture tests. The toughest coating/substrate system (one composed of the glass containing 25 mol% SrO) lost 80% and 85% of its G IC and G IIC , respectively, in less than 24 h of degradation. The drop in G IC and G IIC occurred even more rapidly for other coating/substrate systems. Therefore, degradation of borate bioactive glass coatings is inversely related to their fracture toughness when coated onto Ti6A4V substrates. Finally, roughening the substrate was found to be inconsequential in increasing the toughness of the system as the fracture toughness was limited by the cohesive toughness of the glass itself

Quantifying the mode II critical strain energy release rate of borate bioactive glass coatings on Ti6Al4V substrates

© 2017 Elsevier Ltd Bioactive glasses have been used as coatings for biomedical implants because they can be formulated to promote osseointegration, antibacterial behavior, bone formation, and tissue healing through the incorporation and subsequent release of certain ions. However, shear loading on coated implants has been reported to cause the delamination and loosening of such coatings. This work uses a recently developed fracture mechanics testing methodology to quantify the critical strain energy release rate under nearly pure mode II conditions, GIIC, of a series of borate-based glass coating/Ti6Al4V alloy substrate systems. Incorporating increasing amounts of SrCO3 in the glass composition was found to increase the GIIC almost twofold, from 25.3 to 46.9 J/m2. The magnitude and distribution of residual stresses in the coating were quantified, and it was found that the residual stresses in all cases distributed uniformly over the cross section of the coating. The crack was driven towards, but not into, the glass/Ti6Al4V substrate interface due to the shear loading. This implied that the interface had a higher fracture toughness than the coating itself

Characterization and fracture property of different strontium-containing borate-based glass coatings for Ti6Al4V substrates

© 2016 Elsevier B.V. This work considered the effect of increasing Strontium ion (Sr2+) content on the structure of a series of glasses based on the B2O3-P2O5-CaCO3-Na2CO3-TiO2-SrCO3 series and their resultant fracture toughness when coated onto a surgical metal substrate. Six glasses with increasing Sr2+ content (0 to 25 mol%) were synthesized and subsequently characterized by X-ray Diffraction (XRD), Differential Thermal Analysis (DTA) and both Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Raman Spectroscopy. These techniques confirmed that increased Sr2+ content induced more non-bridging oxygens (NBOs) into the glass network. This would be expected to lead to de-polymerization of the glass structure, as would be evinced by lower glass transition temperatures (Tgs) as Sr2+ increased within the glass series. However, Tg was found to increase with Sr2+ addition, inferring that the strength of the ionic bond between strontium and oxygen (Sr[sbnd]O) enhanced network rigidity. The glasses were coated onto Ti6Al4V substrates using an enamelling technique, and the critical strain energy release rates (GIC) of the resultant coating/substrate constructs were measured. The incorporation of 15–25 mol% Sr2+ into the glass network was found to significantly increased the toughness of the glass/Ti6Al4V constructs

Silica-Based and Borate-Based, Titania-Containing Bioactive Coatings Characterization: Critical Strain Energy Release Rate, Residual Stresses, Hardness, and Thermal Expansion

Silica-based and borate-based glass series, with increasing amounts of TiO2 incorporated, are characterized in terms of their mechanical properties relevant to their use as metallic coating materials. It is observed that borate-based glasses exhibit CTE (Coefficient of Thermal Expansion) closer to the substrate’s (Ti6Al4V) CTE, translating into higher mode I critical strain energy release rates of glasses and compressive residual stresses and strains at the coating/substrate interface, outperforming the silica-based glasses counterparts. An increase in the content of TiO2 in the glasses results in an increase in the mode I critical strain energy release rate for both the bulk glass and for the coating/substrate system, proving that the addition of TiO2 to the glass structure enhances its toughness, while decreasing its bulk hardness. Borate-based glass BRT3, with 15 mol % TiO2 incorporated, exhibits superior properties overall compared to the other proposed glasses in this work, as well as 45S5 Bioglass® and Pyrex