Optimisation of the enamelling of an apatite–mullite glass–ceramic coating on Ti<Subscript>6</Subscript>Al<Subscript>4</Subscript>V

Apatite–mullite glass–ceramics (AMGCs) are under investigation as a potential alternative to hydroxyapatite (HA) as a coating for cementless fixation of orthopaedic implants. These materials have tailorable mechanical and chemical properties that make them attractive for use as bioactive coatings. Here, AMGC coatings on Ti₆Al₄V were investigated to determine an improved heat treatment regime using a systematic examination of the different inputs: composition of glass, nucleation hold and crystallisation hold. An upper limit to the heat treatment temperature was determined by the \(\alpha\,+\,\beta\,\longrightarrow\,\beta\) of Ti₆Al₄V at 970°C. The glass composition was modified to produce different crystallisation temperatures and sintering characteristics. A glass was found that is fully crystalline below 970°C and has good sinterability. The effects of different heat treatment time and temperature combinations on the coating and substrate morphologies were examined and the most suitable combination determined. This sample was further investigated and was found to have qualitatively good adhesion and evidence of an interfacial reaction region between the coating and substrate indicating that a chemical reaction had occurred. Oxygen infiltration into the substrate was quantified and the new route was shown to result in a 63% reduction in penetration depth.     

Tribological behavior of alumina-added apatite–wollastonite glass–ceramics in simulated body fluid

Tribological properties of an alumina-added apatite–wollastonite glass–ceramic produced by controlled heat treatment of a glass in the system MgO–CaO–SiO₂–P₂O₅–Al₂O₃ have been evaluated and compared with those of selected commercial dental ceramics, Duceragold and IPS Empress. Tribological tests were performed in dry condition and in simulated body fluid (SBF) using a pin-on-disk apparatus. The friction coefficient and specific wear rate of the tested materials were measured in dry and in artificial saliva (simulated body fluid: SBF) in order to elucidate the appropriateness of the alumina-added apatite–wollastonite (A–W) glass–ceramic for dental applications. Wear rate of the materials investigated varied from 0.96 × 10⁻⁴ mm³ N⁻¹ m to 41.37 × 10⁻⁴ mm³ N⁻¹ m depending on the bioenvironmental test conditions. The results of this study revealed that the alumina-added A–W glass–ceramic becomes more wear resistant as sintering temperature is increased and exhibits tribological properties similar to those of the commercial dental materials investigated.     

Microstructure and stresses in a keatite solid–solution glass–ceramic

The microstructure of a translucent keatite solid–solution glass–ceramic (keatite s.s.) of the LAS-system (Li₂O–Al₂O₃–SiO₂) has been analyzed with SEM, AFM, XRF, XRD, and TEM. The glass–ceramic consists mainly of keatite s.s. with minor secondary phases such as zirconium titanate, gahnite and probably rutile. Furthermore the resistance to temperature differences (RTD) of this glass–ceramic was investigated. It is shown that, in spite of the relatively high coefficient of thermal expansion (CTE) of about 1 × 10⁻⁶ K⁻¹, an improved RTD can be achieved by special ceramization treatment. With this, compressive stresses in the first 100 μm to 150 μm are induced. These stresses can presumably be contributed to a difference in CTE between the surface-near zone and the bulk. Said CTE difference is caused by chemical gradients of CTE-relevant elements, such as Zn, K, and supposedly additional alkali elements such as Li. These stresses are useful to increase the strength and application range of glass–ceramics based on keatite s.s.

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The effect of ZrO2 and TiO2 on solubility and strength of apatite–mullite glass–ceramics for dental applications

The effect of ZrO₂ and TiO₂ on the chemical and mechanical properties of apatite–mullite glass–ceramics was investigated after sample preparation according to the ISO (2768:2008) recommendations for dental ceramics. All materials were characterized using differential thermal analysis, X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. X-ray fluorescence spectroscopy was used to determine the concentrations of elements present in all materials produced. The chemical solubility test and the biaxial flexural strength (BFS) test were then carried out on all the samples. The best solubility value of 242 ± 61 μg/cm² was obtained when HG1T was heat-treated for 1 h at the glass transition temperature plus 20 °C (T_g + 20 °C) followed by 5 h at 1200 °C. The highest BFS value of 174 ± 38 MPa was achieved when HG1Z and HG1Z+T were heat-treated for 1 h at the T_g + 20 °C followed by 7 h at 1200 °C. The present study has demonstrated that the addition of TiO₂ to the reference composition showed promise in both the glass and heat-treated samples. However, ZrO₂ is an effective agent for developing the solubility or the mechanical properties of an apatite–mullite glass–ceramic separately but does not improve the solubility and the BFS simultaneously.     

Effect of glass–ceramic microstructure on its in vitro bioactivity

Two routes were used to obtain a glass–ceramic composed of 43.5 wt % SiO₂ – 43.5 wt % CaO – 13 wt % ZrO₂. Heat treatment of a glass monolith produced a glass–ceramic (WZ1) containing wollastonite-2M and tetragonal zirconia as crystalline phases. The WZ1 did not display bioactivity in vitro. Ceramizing the glass via powder technology routes formed a bioactive glass–ceramic (WZ2). The two glass–ceramics, WZ1 and WZ2, were composed of the same crystalline phases, but differed in microstructure. The in vitro studies carried out on WZ2 showed the formation of an apatite-like layer on its surface during exposure to a simulated body fluid. This paper examined the influence of both chemical and morphological factors on the in vitro bioactivitity. The interfacial reaction product was examined by scanning and transmission electron microscopy. Both instruments were fitted with energy-dispersive X-ray analyzers. Measurements of the pH made directly at the interface of the two glass–ceramics were important in understanding their different behavior during exposure to the same physiological environment.     

Gold nanoparticles developed in sol–gel derived apatite—bioactive glass composites

The study is focussed on synthesis and characterisation of a new sol–gel derived composite system consisting of nanocrystalline apatite, bioactive glass and gold nanoparticles, which are of interest both for regenerative medicine and for specific medical applications of the releasable gold nanoparticles. Samples dried at 110°C and then heat treated for 30 min at 300 and 500°C were investigated by thermal analysis (DTA/TG), X-ray diffraction (XRD), UV–VIS–NIR, Fourier Transform Infrared (FTIR) spectroscopy, X-ray Photoelectron(XPS) spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Gold nanoparticles and nanocrystalline apatite are developed already after heat treatment at 300°C. XPS analysis clearly revealed the presence of both metallic and ionic gold species. The development of gold nanoparticles was evidenced by UV–VIS–NIR and TEM analysis, and their size increased from few nanometers to 25 nm by increasing the treatment temperature from 300 to 500°C. The bioactivity of the samples immersed in simulated body fluid was demonstrated by XRD and SEM results.     

Nanostructured glass–ceramic coatings for orthopaedic applications

Glass–ceramics have attracted much attention in the biomedical field, as they provide great possibilities to manipulate their properties by post-treatments, including strength, degradation rate and coefficient of thermal expansion. In this work, hardystonite (HT; Ca₂ZnSi₂O₇) and sphene (SP; CaTiSiO₅) glass–ceramic coatings with nanostructures were prepared by a plasma spray technique using conventional powders. The bonding strength and Vickers hardness for HT and SP coatings are higher than the reported values for plasma-sprayed hydroxyapatite coatings. Both types of coatings release bioactive calcium (Ca) and silicon (Si) ions into the surrounding environment. Mineralization test in cell-free culture medium showed that many mushroom-like Ca and phosphorus compounds formed on the HT coatings after 5 h, suggesting its high acellular mineralization ability. Primary human osteoblasts attach, spread and proliferate well on both types of coatings. Higher proliferation rate was observed on the HT coatings compared with the SP coatings and uncoated Ti-6Al-4V alloy, probably due to the zinc ions released from the HT coatings. Higher expression levels of Runx2, osteopontin and type I collagen were observed on both types of coatings compared with Ti-6Al-4V alloy, possibly due to the Ca and Si released from the coatings. Results of this study point to the potential use of HT and SP coatings for orthopaedic applications.     

Biocompatible glass–ceramic materials for bone substitution

A new bioactive glass composition (CEL2) in the SiO₂–P₂O₅–CaO–MgO–K₂O–Na₂O system was tailored to control pH variations due to ion leaching phenomena when the glass is in contact with physiological fluids. CEL2 was prepared by a traditional melting-quenching process obtaining slices that were heat-treated to obtain a glass-ceramic material (CEL2GC) that was characterized thorough SEM analysis. Pre-treatment of CEL2GC with SBF was found to enhance its biocompatibility, as assessed by in vitro tests. CEL2 powder was then used to synthesize macroporous glass–ceramic scaffolds. To this end, CEL2 powders were mixed with polyethylene particles within the 300–600 μm size-range and then pressed to obtain crack-free compacted powders (green). This was heat-treated to remove the organic phase and to sinter the inorganic phase, leaving a porous structure. The biomaterial thus obtained was characterized by X-ray diffraction, SEM equipped with EDS, density measurement, image analysis, mechanical testing and in vitro evaluation, and found to be a glass–ceramic macroporous scaffold with uniformly distributed and highly interconnected porosity. The extent and size-range of the porosity can be tailored by varying the amount and size of the polyethylene particles.     

Study of glass-nanocomposite and glass–ceramic containing ferroelectric phase

Transparent glass nanocomposite in the pseudo binary system (100 − x) Li₂B₄O₇–xBaTiO₃ with x = 0 and 60 (in mol%) were prepared. Amorphous and glassy characteristics of the as-prepared samples were established via X-ray powder diffraction (XRD) and differential scanning calorimetry (DSC) respectively. The precipitated BaTiO₃ nanocrystal phase embedded in the glass sample at x = 60 mol% was identified by transmission electron microscopic (TEM). The optical transmission bands at 598 and 660 nm were assigned to Ti³⁺ ions in tetragonal distorted octahedral sites. The precipitated Li₂B₄O₇, BaTi(BO₃)₂ and BaTiO₃ nanocrystallites phases with heat-treatment at 923 K for 6 h (HT923) in glass–ceramic were identified by XRD, TEM and infrared absorption spectroscopy. The as-prepared at x = 60 mol% and the HT923 samples exhibit broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature. The results demonstrate that the method presented may be an effective way to fabricate ferroelectric host and development of multifunctional ferroelectrics.     

Crystallization of a Bi–Pb–Fe–Cd–O glass under magnetic annealing

Magnetic annealing with a tunable solenoid magnetic field from 0–240 G, was conducted on a Bi–Pb–Fe–Cd–O glass containing 20% Fe2O3, which was prepared by the melt-quenching process. The crystalline phases of the annealed samples were identified as -Bi2O3 and BiFeO3. Evidence of the formation of the crystalline BiFeO3 which was strongly magnetically enhanced at the surface of the samples, was obtained from X-ray diffraction patterns and EPR spectra. Based on the structure transition of Fe3+ ions, a crystallization mechanism for the BiFeO3 crystals under magnetic annealing has been proposed.     

Determination of relative in vivo osteoconductivity of modified potassium fluorrichterite glass–ceramics compared with 45S5 bioglass

Potassium fluorrichterite (KNaCaMg₅Si₈O₂₂F₂) glass–ceramics were modified by either increasing the concentration of calcium (GC5) or by the addition of P₂O₅ (GP2). Rods (2?×?4?mm) of stoichiometric fluorrichterite (GST), modified compositions (GC5 and GP2) and 45S5 bioglass, which was used as the reference material, were prepared using a conventional lost-wax technique. Osteoconductivity was investigated by implantation into healing defects in the midshaft of rabbit femora. Specimens were harvested at 4 and 12?weeks following implantation and tissue response was investigated using computed microtomography (μCT) and histological analyses. The results showed greatest bone to implant contact in the 45S5 bioglass reference material at 4 and 12?weeks following implantation, however, GST, GC5 and GP2 all showed direct bone tissue contact with evidence of new bone formation and cell proliferation along the implant surface into the medullary space. There was no evidence of bone necrosis or fibrous tissue encapsulation around the test specimens. Of the modified potassium fluorrichterite compositions, GP2 showed the greatest promise as a bone substitute material due to its osteoconductive potential and superior mechanical properties.     

Long-term conversion of 45S5 bioactive glass–ceramic microspheres in aqueous phosphate solution

The conversion of 45S5 glass and glass–ceramics to a hydroxyapatite (HA)-like material in vitro has been studied extensively, but only for short reaction times (typically <3 months). In this paper, we report for the first time on the long-term conversion of 45S5 glass–ceramic microspheres (designated 45S5c) in an aqueous phosphate solution. Microspheres of 45S5c (75–150 μm) were immersed for 10 years at room temperature (~25 °C) in K₂HPO₄ solution with a concentration of 0.01 M or 1.0 M, and with a starting pH of 7.0 or 9.5. The reacted 45S5c microspheres and solutions were analyzed using structural and analytical techniques. Only 25–45 vol% of the 45S5c microspheres were converted to an HA-like material after the 10 year reaction. In solutions with a starting pH of 9.5, an increase in the K₂HPO₄ concentration from 0.01 to 1.0 M resulted in a doubling of the volume of the microspheres converted to an HA-like material but had little effect on the composition of the HA-like product. In comparison, reaction of the 45S5c microspheres in the solution with a starting pH of 7.0 resulted in an HA-like product in the 0.01 M K₂HPO₄ solution but a calcium pyrophosphate product, Ca₁₀K₄(P₂O₇)₆.9H₂O, in the 1.0 M solution. The consequences of these results for the long-term use of 45S5 glass–ceramics in biomedical applications are discussed.     

Bioactive glass induced in vitro apatite formation on composite GBR membranes

The aim of this study was to investigate in vitro bioactivity of different thermoplastic biodegradable barrier membranes. Three experimental GBR membranes were fabricated using Poly(epsilon-caprolactone-co-D: ,L-lactide) P(CL/DL-LA) and particulate bioactive glass S53P4 (BAG; granule size 90-315 microm): (A) composite membrane with 60-wt.% of BAG, (B) membrane coated with BAG; and (C) copolymer membrane without BAG. Membranes were immersed in simulated body fluid (SBF), and their surfaces were characterized with SEM, XRD and EDS after 6 and 12 h and after 1, 3, 5, 7, and 14 days. Calcium phosphate (Ca-P) surface formation was observed on both composite membranes (A and B) but not on the copolymer membrane without bioactive glass (C). The Ca-P precipitation appeared to be initiated on the bioactive glass followed by growth of the layer along the polymer surface. In 6-12 h ion dissolution of the bioactive glass led to formation of the silica rich layer on the surface of the exposed glass granules on composite membrane B whereas only small amounts of silica was observed on the polymer surface of the composite membrane A. At 24 h nucleation of Ca-P precipitation was observed, and by 3-5 days membrane surface was covered with a uniform Ca-P layer transforming from amorphous to low crystalline structure. At 7 days composition and structure of the apatite surface resembled the apatite in bone. Once nucleated, the surface topography seemed to have significant effect on the growth of the apatite layer.     

Bioactive glass–ceramic coated titanium implants prepared by electrophoretic deposition

Surface modification of Ti alloys towards an improved osteoinductive behaviour is one of the major challenges in orthopaedic implant technology nowadays. One way to achieve this is by applying a bioactive coating which can increase the rate of osseointegration and chemical bonding of surrounding bone to the implant. In the present work, the production of a bioactive glass–ceramic coating on flat Ti alloys by electrophoretic deposition is demonstrated. The coatings are applied by cathodic deposition from non-aqueous suspensions followed by sintering in vacuum, avoiding uncontrolled oxidation of the Ti substrates. The use of non-aqueous suspensions both allowed to reduce the deposition time and yielded homogeneous coatings with a uniform thickness of 8 μm. Evaluation of the coating adhesion confirmed the good mechanical performance of the coatings with a tensile bond strength of 41.0 ± 11.1 MPa. Additionally, a feasibility study demonstrated the potential of electrophoretic deposition as a coating technique for commercial complex implants.     

Laboratory scale erosion testing of a wear resistant glass–ceramic

Abstract

The testing of erosion resistant materials is usually performed on laboratory scale test rigs under accelerated wear conditions. The validity of this is questionable because of the dependence of wear mechanisms on a variety of impact parameters. Data are presented that have been obtained from such tests on a glass–ceramic (Silceram) which has been developed as a low-cost erosion resistant lining material. Various impact conditions have been investigated, including impact angle, particle velocity, and impact frequency. The data concerning the effects of particle velocity show very good agreement with one particular erosion model, although there is an apparent dependence on other test variables.

MST/611     

Low temperature sintering and properties of Ca–Al–B–Si–O glass/ceramic composites with various ceramic fillers

Low-temperature sintering and properties of low temperature co-fired ceramics materials based on a typical Ca–Al–B–Si–O glass and various ceramic fillers such as (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ were investigated. Densification, crystallization and dielectric properties are found to strongly depend on the type of filler. The densification process of glass/ceramic composites with different ceramic fillers is mainly from 600 to 925 °C, and the initial compacting temperature of samples is 600 °C. The initial rapid densification of samples starts after glass softening temperature of samples. The XRD patterns of (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ samples demonstrate crystalline phases, CaTiO(SiO₄) and CaTiSiO₅, respectively, as a result of firing at 875 °C for 15 min. The high dielectric constant fillers produce high ε_r values of the dielectric samples. The maximum dielectric constant of samples for (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ filler is 14.02, 16.21, 18.64 and 23.78, respectively. Comparing with other samples, the specimens for (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ ceramic filler have lower dielectric loss. Especially, the sample for (Ca_0.5Mg_0.5)TiO₃ filler exhibits the lowest dielectric loss of 0.00011.     

Effect of cyclic loading on interfacial shear stress in SiC–CAS glass ceramic matrix composite

Abstract

Mechanical fatigue has been observed to occur in the Nicalon–CAS continuous fibre reinforced glass ceramic matrix composite under cyclic loading at room temperature, and both microcrack proliferation and propagation are induced. In situ fibre push down tests within a scanning electron microscope have then been used to assess changes in interfacial properties as a result of this mechanical cyclic loading. Both the interfacial shear stress and the interfacial fracture energy decrease when specimens are subjected to mechanical cyclic loading. It is deduced that a decrease in interfacial shear stress is the most likely mechanism driving stable and progressive microcrack propagation.     

Micromechanical testing of glass–ceramic sealants for solid oxide fuel cells

The understanding of the mechanical behavior of sealants is a prerequisite for the improvement of the integrity and reliability of solid oxide fuel cell stacks. The glass–ceramic sealant material in a SOFC stack is usually a thin layer; hence, micromechanical testing methods need to be applied for characterization. Indentation testing is used in the current study to determine elastic modulus and fracture toughness. The properties of sealant materials in stack typical thin layer geometry are compared with sintered bars. In addition to tests of as-joined material, the effect of stack operation on the properties is assessed.