Static mechanical properties of cast and sinter-annealed cobalt-chromium surgical implants

Porous-surface-layered surgical implants may be produced by sintering at elevated temperatures. An investigation was undertaken to determine the effect of these sintering heat treatments on the tensile properties of the cobalt-chromium casting alloy specified by ASTM F75-76. Specimens which were given a sintering treatment and then rapidly cooled from elevated temperature were found to lack ductility. This was due to the incipient melting of an interdendritic material which was subsequently retained in the grain boundaries as a brittle solid after quenching. Two methods were found which would reduce the amount of this brittle solid: (i) modify the heat treatment to include a slow cooling step to temperatures below that at which incipient melting first occurred; and (ii) reduce the carbon content of the alloy. Reduced-carbon alloys gave the greatest post-sintering ductility, but showed a lower 0.2% yield stress. The techniques of thermal activation analysis were used to investigate the effect of second phases upon the initial low-strain work-hardening rates and the 0.2% yield stress. It was found that the work-hardening rate from the elastic limit to a total strain of about 0.01 to 0.02 depends, in part, directly on the volume fraction of second phase.     

Degradation behaviour and mechanical properties of magnesium implants in rabbit tibiae

To investigate the initial mechanical strength and the degradation behaviour with the associated changes in mechanical properties of magnesium-based osteosynthesis implants, 30 rabbits were implanted with cylindrical pins of the alloys MgCa0.8 (magnesium with 0.8 wt% calcium), LAE442 (magnesium with 4 wt% lithium, 4 wt% aluminium and 2 wt% rare earths) and WE43 (magnesium with 4 wt% yttrium and 3 wt% rare earths). The implants were inserted into the medullary cavity of both tibiae. After 3 and 6 months, each half of the animals was euthanized, respectively, and the implants were taken out. A determination of volume, three-point bending tests, scanning electron microscopy (SEM) and energy dispersive X-ray analyses as well as metallographic and μ-computed tomography examinations were accomplished. All implants were clinically well tolerated. MgCa-implants showed the least initial strength and the highest loss in volume after 6 months. SEM- and μ-computed tomography examinations revealed a pronounced pitting corrosion. Therefore, their use as degradable implant material seems to be limited. LAE442 has the best initial strength which seems to be sufficient for an application in weight-bearing bones. The degradation behaviour is very constant. However, possible unknown side effects of the rare earths have to be excluded in further investigations on biocompatibility. Considering all results of WE43, its application as osteosynthesis material for fracture repair is ineligible due to its heterogeneous and unpredictable degradation behaviour.     

Anisotropy of radiation-induced degradation in mechanical properties of fabric-reinforced polymer-matrix composites

Four kinds of fabric-reinforced polymer-matrix composites (filler: E-glass or carbon fabric; matrix: epoxy or polyimide resin) were irradiated with⁶⁰Co-rays or 2 MeV electrons at room temperature. Three-point bend tests were then carried out at 77 K and at room temperature in a 45° direction from warp to fill. Comparison of the degradation behaviour among these composites reveals that the glass-epoxy and glass-polyimide composites are quite similar to each other in the dose dependence of the ultimate interlaminar shear strength at each test temperature. This result suggests that the radiation damage at the fibre-matrix interface decreases the contribution of the chemical bond mode to the total bond strength at the interface, thus decreasing the composite shear strength with increasing dose. For the carbon-epoxy and carbon-polyimide composites, on the other hand, the shear strength at room temperature changes little even after irradiation up to 140 MGy, while the shear strength at 77 K decreases monotonically with increasing dose. These findings suggest that the fibre-matrix bond strength due to the friction force mode is quite insensitive to radiation, thus resulting in the dose-independent shear strength at room temperature. At 77 K, however, the friction force mode fails to function properly because of the brittleness of the matrix resin, and consequently the composite shear strength decreases with increasing dose owing to a resulting increase in the matrix brittleness.     

Mechanism of radiation-induced degradation in mechanical properties of polymer matrix composites

Four kinds of polymer matrix composites (filler, E-glass or carbon fibre cloth; matrix, epoxy or polyimide resin) and pure epoxy and polyimide resins were irradiated with ⁶⁰Co -rays or 2 MeV electrons at room temperature. Mechanical tests were then carried out at 77 K and at room temperature. Following irradiation, the Young's (tensile) modulus of these composites and pure resins remains practically unchanged even at 170 MGy for both test temperatures. The ultimate strength, however, decreases appreciably with increasing dose. The dose dependence of the composite strength depends not only on the combination of fibre and matrix in the composite but also on the test temperature. A relationship is found between the composite ultimate strain and the matrix ultimate strain, thus indicating that the dose dependence of the composite strength is virtually determined by a change in the matrix ultimate strain due to irradiation. Based on this finding, we propose a mechanism of radiation-induced degradation of a polymer matrix composite in order to explain the dose dependence of the composite strength measured at 77 K and at room temperature.     

Evaluating the microstructure and mechanical properties of friction stir processed Al–Si alloy

In this study, mechanical behaviour and microstructural evolution in friction stir processing (FSP) of casting hypereutectic A390 aluminium alloy have been investigated. The mechanical behaviour of FSP samples was investigated by measuring the strain rate sensitivity using shear punch testing. The room-temperature shear punch tests were conducted at shear strain rates in the range of 10^?4–10^?1?s^?1. The results indicate that the strain rate sensitivity index increases from about 0.015 to 0.120 for as-cast A390 after third FSP pass and then experiences a further growth in FSP passes. The increase in the grain size and CuAl₂ intermetallic particle size result in a reduction in strain sensitivity index as well as shear strength after third FSP pass.     

Aging effect of pH on the mechanical and tribological properties of dental composite resins

This study evaluated the microhardness and wear behavior alterations of dental resins exposed to strong acid and alkaline degradation regimens. Fifty specimens of AP-X, Z350, P60, VITA ZETA and VITA LC resins were distributed into 10 groups. The control group was not subjected to aging treatment, while the other nine groups were assigned to the following pH solutions: 1, 7 or 13 for 1, 13 or 20 d, respectively. Vickers microhardness and wear behavior of materials without aging served as the reference. Repeated measurements were conducted for other specimens. Scanning electron microscopy was used to observe the morphology. Data were analyzed by repeated-measures one-way ANOVA, followed by the least significant difference (LSD) t-test (P?≤?0.05) for multiple comparisons. The aging process was aggravated over time. Alkaline medium significantly decreased the microhardness and wear resistance of resins, and led to serious surface damage. AP-X, P60 and Z350 immersed in different media showed a lower degradation than the nanoresin. The mechanical degradation of resin composites was aggravated with increased aging time. Strong alkaline media induced the largest reduction in mechanical and tribological properties. Hybrid resin composites with high filler loading exhibited better mechanical performance than nanoresins with low filler loading under different pH conditions.     

Influence of boron addition on microstructure and mechanical properties of dental cast titanium alloys

As-cast titanium alloys prepared using a dental cast machine with a series of boron additions have been studied using an optical microscope, XRD, SEM, and hardness and tensile testing. It has been shown that a small amount of boron addition induces a significant refinement of as-cast structure and improvement of mechanical properties. Titanium borides, TiB particles, are observed in Ti matrix. Tensile ductility of cast Ti–B and Ti–0.5Si–B alloys is improved obviously when boron content is about 0.086–0.14 mass%. This is primarily due to the role of borides precipitated at the prior β boundary and refinement of the prior β grains. Cast Ti–B alloys with a good combination of greater tensile ductility and strength can be obtained with very low boron addition.     

Laboratory investigations on the mechanical properties degradation of granite under freeze-thaw cycles

To understand the deteriorate characteristics of granite in cold regions, the influences of freeze-thaw cycles on the mechanical properties need to be investigated. The rock specimens (biotite granite from Tibet in China) were frozen and thawed in a temperature and humidity controlled container where the temperature varied from + 40 to − 40 °C and the humidity was kept at 100% continuously. The freeze-thaw tests were conducted for one cycle including 4 h of freezing and 4 h of thawing, and the number of cycles was from 0 to 150. The deterioration of the specimens was examined by the changes of strength, deformation characteristics, elastic modulus, cohesive strength and internal frictional angle in a series of uniaxial and triaxial compression tests. The experimental results show that (1) the axial strain corresponding to the peak stress increases with an increasing of confining pressure and freeze-thaw cycles; (2) The compressive strength decayed exponentially with the number of freeze-thaw cycles, as well as elastic modulus and cohesive strength; (3) the internal frictional angle remained constant in the process of freeze-thaw cycles; (4) The interrelation among compressive strength, confining pressure and number of cycles can be described by Mohr-Coulomb strength criterion.     

Biological behavior of sol-gel coated dental implants

The biocompatibility of dental implants coated with titania/hydroxyapatite (HA) and titania/bioactive glass (BG) composites obtained via sol-gel process was investigated using an in vitro and in vivo model. A device for the in vitro testing of screw-shaped dental implants was developed, in order to well compare the two experimental models studying the behavior of human MG63 osteoblast-like cells seeded onto a particular geometry. The expression of some biochemical parameters of osteoblastic phenotype (alkaline phosphatase specific activity, collagen and osteocalcin production) and some indications on cells morphology obtained by scanning electron microscopy were evaluated. The in vitro and in vivo models were compared after implants insertion in rabbit tibia and femur. The removal torque and histomorphometric parameters (percentage of bone in contact with implant surface and the amount of bone inside the threaded area) were examined. A good agreement was found between the in vitro and in vivo models. These experiments showed better performances of HA and BG sol-gel coated dental implants with respect to uncoated titanium; in particular, it was found that in vitro the HA coating stimulates osteoblastic cells in producing higher level of ALP and collagen, whereas in vivo this surface modification resulted in a higher removal torque and a larger bone-implant contact area. This behavior could be ascribed to the morphology and the chemical composition of the implants with rough and bioactive surfaces.     

Changes in mechanical properties of dental alloys induced by saliva and oral probiotic supplements

The effects of the saliva and oral probiotic supplements on roughness, friction and microhardness of the stainless steel (SS) and nickel-titanium (NiTi) alloys used in dentistry was studied. The specimens of stainless steel, uncoated, rhodium-coated and nitrided NiTi were exposed to artificial saliva with pH 4.8 and artificial saliva with addition of probiotic supplements containing bacteria Lactobacillus reuteri Prodentis through 28 days. First 5 days specimens were subjected to thermocycling to simulate intraoral conditions, 2500 cycles from 5 °C to 50 °C and the following days to the temperature of the 37±2 °C. Analyses demonstrated that oral probiotic supplements do not influence microhardness, roughness or friction of stainless steel above the influence of saliva. Probiotics increase roughness in NiTi, but without significant influence on friction, while microhardness in NiTi is not influenced. Surface nitriding reduces the influence of probiotics on roughness while rhodium coating increases it.     

玻璃纤维增强光固化树脂基齿科生物复合材料的半互穿网络结构界面的形成及力学性能

采用不同浓度的聚甲基丙烯酸甲酯(PMMA)丙酮溶液对玻璃纤维进行表面处理,并对不同处理条件下的玻璃纤维表面化学组成、PMMA的吸附量及齿科树脂基复合材料的力学和界面性能进行了分析和测试。结果表明,经过表面处理,玻璃纤维表面吸附上PMMA,且吸附量随PMMA溶液浓度的增大而增大。控制玻纤表面吸附的PMMA质量分数在1%左右,可以设计其与齿科树脂形成半互穿网络结构的良好界面。与未处理的玻纤复合材料相比,用质量分数为5%的PMMA溶液处理的玻纤/光固化树脂基复合材料的弯曲强度提高29.6%,弯曲模量提高30%,可以作为一种齿科修复用的新型生物复合材料应用。     

Novel dental nanocomposites: fabrication and investigation of their physicochemical,mechanical and biological properties

The aim of this study was to investigate biological, physicochemical and mechanical characteristics of a series of novel dental restorative nanocomposites that comprise dendritic methacrylate end-caped monomers, triethylene glycol dimethacrylate (TEGDMA; as diluting monomer) and modified silica nanoparticles (\(\hbox {M-SiO}_{2}\); as inorganic filler). The cytotoxicity effects of the monomers and fabricated nanocomposites were examined against NIH3T3 cells (the standard fibroblast cell line) through MTT and trypan blue cell viability tests, respectively. The antibacterial activities of the monomers were evaluated against Lactobacillus plantarum by standard agar disk diffusion approach. The mechanical properties (flexural strength (FS) and compressive strength (CS)) as well as some physicochemical characteristics such as water sorption (WS), sol fraction (SF) and double bond conversion (DC) were also investigated, and compared with corresponding characteristics of 3M Filtek Z250 as a reference. Thus, the fabricated nanocomposites have potential as dental restorative materials mainly due to their suitable biological, physicochemical and mechanical properties.     

A comparison of mechanical properties of discontinuous Kevlar 29 fibre reinforced bone and dental cements

A comparative study of the fracture behaviour of Kevlar 29 reinforced bone and dental cements is undertaken using both linear elastic and non-linear elastic fracture mechanics approaches. Results from both approaches reflect improved fracture toughness at very low fibre contents. Flexural modulus is not apparently improved in either system, and flexural strength is only improved in the bone cement system probably because of poor interfacial bonding and the presence of voids in the dental cement. In all cases, however, bone cement is seen to be superior to dental cement. This is interpreted in terms of smaller voids and better fibre distribution due to the lower viscosity of the bone cement material. When compared to carbon-polymethyl methacrylate (PMMA) cements, Kevlar 29 reinforced systems appear to be superior. More work is underway to optimize the properties of these systems with regard to structural parameters.     

Thermal degradation of pearlitic steels: influence on mechanical properties including fatigue behaviour

With the aim to predict the durability of railway wheels, thermomechanical damage was studied for two steels with different alloying levels of silicon and manganese in the temperature range of 500–725°C. Softening caused by cementite spheroidisation in pearlite leads to changes in the mechanical behaviour and an accompanying decrease in fatigue lifetimes. It was found that higher contents of Si and Mn lead to better resistance to softening of both virgin and plastically deformed material. Correspondingly, the high Si-Mn alloyed steel loses much less in fatigue lifetime than the lower alloyed steel.     

An application of non-asbestos papers to the dental field: mechanical properties and structural change

New non-asbestos papers composed of glass fibres and ceramic fibres were examined for dental application. The fibre liners showed a larger deformation ability than conventional asbestos paper, and had a smaller water uptake than the conventional liner. In addition, the setting expansion in new non-asbestos papers was increased at 60 min after investing, similarly to the setting expansion behaviour in a conventional liner. After casting a silver alloy using the new fibre liners, a microstructure mainly of primary solid formed, with a smaller amount of complex phases. The microstructures formed were similar to those in the conventional asbestos liner.     

掺杂Fe2O3的牙科用氧化锆陶瓷着色研究

采用粉体掺杂方法在3Y-TZP陶瓷中添加不同质量分数的Fe2O3，研究Fe2O3对材料的着色效果及力学性能的影响。结果表明，掺杂Fe2O3着色后的3Y-TZP陶瓷三点弯曲强度略有下降，但不同掺杂量的测试结果均在1000MPa以上。肉眼观察，掺杂Fe2O3的氧化锆陶瓷呈黄色，颜色均匀，透光性良好，正反面色差小；色度值测试结果显示，随Fe2O3含量的增加陶瓷呈颜色由浅到深、亮度逐渐降低的梯度变化，且L^＊值和b^＊值均在中国人牙相应色度值范围内。Fe2O3着色3Y-TZP陶瓷接近自然牙色，且着色后材料的强度仍能满足牙科修复材料的要求。     

Antibacterial hydroxyapatite coatings on titanium dental implants

Titanium and its alloys are often used as substrates for dental implants due to their excellent mechanical properties and good biocompatibility. However, their ability to bind to neighboring bone is limited due to the lack of biological activity. At the same time, they show poor antibacterial ability which can easily cause bacterial infection and chronic inflammation, eventually resulting in implant failure. The preparation of composite hydroxyapatite coatings with antibacterial ability can effectively figure out these concerns. In this review, the research status and development trends of antibacterial hydroxyapatite coatings constructed on titanium and its alloys are analyzed and reviewed. This review may provide valuable reference for the preparation and application of high-performance and multi-functional dental implant coatings in the future.