Heat treatment-improved bond strength of resin cement to lithium disilicate dental glass-ceramic

This study investigated the influence of different hydrofluoric acid (HF) concentrations and heat treatments applied to a lithium disilicate dental glass-ceramic (EMX) on surface morphology and micro-shear bond strength (μSBS) to resin cement. Five HF concentrations (1%, 2.5%, 5%, 7.5% and 10%) and four different heat treatments applied before etching were assessed: 1. etching at room temperature with no previous heat treatment (control group); 2. HF stored at 70 °C for 1 min applied to the ceramic surface at room temperature; 3. HF at room temperature applied after a hot air stream is applied perpendicularly to the ceramic surface for 1 min; 4. the combination of previously heated HF and heated EMX surface. The etching time was fixed for 20 s for all groups. Etched EMX specimens were analyzed on field-emission scanning electron microscope (FE-SEM) and the μSBS was carried out on a universal testing machine at a crosshead speed of 1 mm/min until fracture. For the control groups, FE-SEM images showed greater glassy matrix dissolution and higher μSBS for 7.5% and 10% HF concentrations. The previous heat treatments enhanced the glassy matrix dissolution more evidently for 1%, 2.5% and 5% and yielded increased μSBS values, which were not statistically different for 7.5% and 10% HF concentrations (control group). HF concentrations and previous heat treatments did show to have an influence on the etching/bonding characteristics to lithium disilicate dental glass-ceramic.     

Investigation of mechanical and thermo-mechanical strength of ceramic foam filters (CFFs)

During aluminium production, the molten metal will always contain varying amounts of impurities, e.g., non-metallic inclusions, and for high-quality products removing such inclusions is essential. This can be achieved by filtration using ceramic foam filters (CFFs). However, these filters are highly brittle materials subjected to strong mechanical and thermo-mechanical stresses during transport and operation, which occasionally leads to failure of the filter material. In the present study, the compression strength of five different Al₂O₃-based CFFs was measured at room temperature and elevated temperature (compressed at 730 °C), as well as while submerged in molten aluminium with varying melt compositions (pure aluminium and an aluminium-magnesium alloy). The compression strengths at room temperature were established to be in the range of 1.19–2.09 MPa depending on the filter type tested. In the case of the CFFs compressed at elevated temperature, a reduction in compression strength in the range of 9.2–58.6% was established to exist depending on filter type and heating duration, except in three of the filter/duration-combinations tested. Compression of CFF samples submerged in molten aluminium led to an even further reduction in compression strength in the range of 42.6–69.4% depending on filter type and duration of exposure. With an exposure time of only 5 min, no difference in compression strength was observed between the two aluminium melts.     

Acid etching and silica coating effects on Y-TZP topography and ceramic/resin cement bond strength

Zirconia-based (Y-TZP) dental prostheses' retention loss remains one of the most frequently reported difficulties in dental practice. Selective Infiltration Etching (SIE) treatment has been proposed to alter the final Y-TZP topography improving its bond strength to resin cement. SIE consists of glass film deposition on the Y-TZP surface before glass removal with 10% hydrofluoric acid etching. This study's objective was to investigate the effects of SIE protocol steps on Y-TZP topography and its consequence on its bond strength to resin cement. The SIE protocol was divided into two main steps: silica coating (first step) and glass dissolution by hydrofluoric acid (second step), and the null hypothesis was that the SIE treatment has the same bond quality independently of the HF exposition time. Atomic Force Microscopy (AFM), Scanning Electron and Transmission Electron microscopy were used to characterize the zirconia surfaces. The bond strength was evaluated by the interfacial fracture toughness test, and data were analyzed by ANOVA and Tukey's Test (5%). As the main result, it was shown that, when exposed during sufficient time, the 10% HF can etch zirconia at room temperature, which is responsible for topographic changes, significantly improving the bond quality. Additionally, no glass components or dihedral angles modifications were observed at Y-TZP grain boundaries after silica coating. With the study limitations, it was possible to infer that the surface-modifying agent in SIE protocol is the HF etching, and the null hypothesis was rejected.     

Waste recycling of coal fly ash: A novel approach to prepare hierarchically porous coal fly ash/Al2O3 ceramic composite with high porosity and high strength templated by emulsion-assisted self-assembly

The coal fly ash (CFA) produced from coal-fired power generation is classified as a common solid waste; thus, improving the recovery and utilization rate of CFA is highly desirable. In this study, a novel strategy using CFA and Al₂O₃ as raw materials, to prepare hierarchically porous ceramic composites that serve as potential candidates for future building materials is developed. In this process, the well-developed self-assembly method in which an anionic modifier is used to prepare hydrophobic powders that form an attractive oil/water network via electrostatic interactions, thereby yielding honeycomb-like structures. In order to explore the mechanism of preparation, five samples with different mixture ratios of alumina and CFA were prepared according to 1: 0, 2: 1, 1: 1, 1: 2, and 0: 1 (Alumina: CFA). Compared with the sample prepared with pure CFA, the as-prepared CFA/Al₂O₃ composite exhibited both superior porosity and high mechanical property. When the porosity is as high as 73 ± 0.17%, the compressive strength is as high as 80.9 ± 3.4mpa (alumina: CFA = 1:1). As the porosity decreases to 49.3 ± 0.7%, the compressive strength reaches 159.33 ± 36.89mpa (alumina: CFA = 1:2). Moreover, this work obtains the highest compressive strength-porosity related B-value in comparison to previously reported CFA-based composites and provides a new insight into the effective recycling of CFA and offers a novel approach to prepare CFA/Al₂O₃ composite with excellent overall mechanical properties.     

Thermal shock effects on the impact resistance,tolerance and flexural strength of alumina based oxide/oxide ceramic matrix composites

In this study the effects of thermal shock on the impact damage resistance, damage tolerance and flexural strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates with balanced and symmetric layup were gradually heated to 1200°C in an air-based furnace and held for at least 30 min before being removed and immersed in water at room temperature. The laminates were then subjected to low velocity impacts via a hemispherical steel impactor. The resultant damage was characterized non-destructively, following which the laminates were subjected to compression tests. Three-point bend tests were also performed to evaluate the effect of thermal shock on the flexural strength and related failure modes of the laminates. Thermally shocked laminates showed smaller internal damage and larger external damage areas in comparison to their pristine counterparts. For the impact energy and resultant damage size considered, the residual compressive strengths for the thermally shocked and pristine laminates were similar.     

Effect of ceramic surface treatments on the bond strength of different composite resins

This study was aimed to observe the relationship between the different surface treatments and the bond strength of both composite based adhesive cement and zirconia ceramic. Thirty-two zirconia ceramic discs were fabricated by following the instructions of manufacturer (5 × 5 × 1.5 mm). Four subgroups were obtained from the specimens according to the specified surface treatments respectively: (a) C: control groups: no treatment; (b) SB: sandblasting with 125 μm aluminum oxide particles for 10 s; (c) SC: silica coating for 10 s; (d) Nd :YAG laser . The composite resin specimens Panavia F and Clearfil SA were introduced and polymerized to the treated bonding areas. Afterwards the specimens were stored in distilled water at 37 °C during 24 h, and the shear test was applied. The data were statistically analyzed by ANOVA and Duncan tests. The bond strength was stated significantly higher in silica coating/Panavia F group (23.35 MPa). The lowest bond strength was stated in control groups cemented with Clearfil SA (12.25 MPa). As a result it was determined that the bond strength has affected the both surface treatments and cement types (p < 0.001). The silica coating –treated zirconia ceramic recorded a significant increase in mean bond strength values.     

Influence of sample size on strength distribution of advanced ceramics

Strength distribution of advanced ceramics is mostly characterized by Weibull distribution function. The question whether the Weibull distribution always gives the best fit to strength data has been being considered in the last years. The sample size affects the reliable decision of discrimination of different distribution functions (e.g. normal, log-normal, gamma or Weibull). In this paper, 5100 experimental alumina strength data and virtual strength data generated by Monte Carlo simulations are used in order to investigate the effect of sample size on strength distribution of advanced ceramics. It is suggested that, at least 150–200 samples should be used for determination of best fitting distribution function with a statistical fallibility of 10%. Extreme Value Analysis performed with the experimental strength data showed that the Weibull distribution fits the data best and difference between the Weibull and Gumbel distributions appear at the tails.     

Effects of different surface treatments on shear bond strength between ceramic systems and metal brackets

The aim of this study was to evaluate the shear bond strength of orthodontic brackets bonded to different kinds of ceramic surfaces after different surface conditioning methods. A total of 120 ceramic disks were divided into two main groups in terms of feldspathic or lithium disilicate. Each ceramic group was further subdivided into six subgroups depending on surface treatment (n = 10). The ceramic surfaces were conditioned by one of the following methods: Group C: control group; Group P: %37.5 orthophosphoric acid; Group HF: %9.6 hydrofluoric acid; Group L: Nd-YAG laser irradiation; Group SB: sandblasting with 50 µm Al₂O₃ particles; and Group DB: grinding with a diamond bur. Surface roughness value was evaluated with a digital profilometer. Surface topographies of one specimen from each group were observed by atomic force microscopy (AFM) after surface treatments. All samples were primed with silane before the bracket bonding, including the control group. Metal brackets were bonded to the specimens with a light curing composite resin. The samples were stored in distilled water for 24?h and thermocycled 2500× at 5 and 55 ºC for 30?s. Shear bond strengths between the ceramic surface and the bracket were measured with a universal testing machine at a crosshead speed of 0.5 mm/min. Failure modes were classified as adhesive, cohesive, or mixed. Data were analyzed using ANOVA and Tukey's tests (α = .05). Group SB had significantly rougher surface compared with the other groups in each ceramic system (p < .05), and Group SB demonstrated significantly higher shear bond strengths than other groups as well. Within the limitations of this study, surface conditioning methods, except for sandblasting and grinding, were associated with lower shear bond strengths; however, thermocycling may have had negative effects on bond strengths of specimens. Furthermore, in each ceramic system, there was a significant difference between surface-conditioning methods and surface roughness with regard to shear bond strength.     

Comparison of the physicochemical properties and mineralogy of Chinese (Beihai) and Brazilian kaolin

Beihai area (China) has about 500 million tons kaolin reserves with potential applications in ceramics, paint, plastic and rubber-filler industries, but unlike the Brazilian kaolin, it is not good in paper-coating industry. In order to understand the differences between Chinese (Beihai) and Brazilian kaolin, their individual physicochemical properties, morphologies, crystal structures, and surface characteristics were systematically investigated and compared. The results showed that the viscosity concentration, specific surface area and zeta potential of Brazilian kaolin were higher than those of Beihai kaolin. SEM and TEM images revealed that Brazilian kaolin had more regular particle shape, smoother surface and larger diameter–thickness ratio than those of Beihai kaolin. Moreover, XRD and TG/DSC analyses exhibited that Beihai kaolin was low ordered kaolinite with lower value of Hinckley index, average flake thickness along c-axis and endothermic peak temperature as compared to Brazilian kaolin. Furthermore, the crystal structures refined by the Rietveld method of Beihai kaolin showed a greater distortion of [SiO₄] tetrahedron than that of Brazilian kaolin. It can be concluded that low viscosity concentration, low degree of order, and irregular particle shape of Chinese (Beihai) kaolin are the reasons why it is not suitable for use in the paper-coating industry.     

Material removal and surface damage in EDM of Ti3SiC2 ceramic

Material removal and surface damage of Ti₃SiC₂ ceramic during electrical discharge machining (EDM) were investigated. Melting and decomposition were found to be the main material removal mechanisms during the machining process. Material removal rate was enhanced acceleratively with increasing discharge current, i_e, working voltage, u_i, but increased deceleratively with pulse duration, t_e. Microcracks in the surface and loose grains in the subsurface resulted from thermal shock were confirmed, and the surface damage in Ti₃SiC₂ ceramic led to a degradation of both strength and reliability.     

Corrosion behavior in simulated environments of ITZO films prepared by two targets co-sputtering

Indium tin zirconium oxide (ITZO) films were deposited by a co-sputtering technique with ITO and zirconium targets. The stability and corrosion behavior of films in simulated environments were studied on account of microstructure and optical–electrical properties. The results show that ITZO films possess a better crystalline structure and optical–electrical properties. Zirconium-doping changes the preferred orientation of ITO films, and ITZO films under the optimum parameters have sheet resistance of 10 Ω/sq and transmittance of above 85%. According to the polarization measurements and the relative resistance change of the films in simulated environments such as acidic climate, oceanic climate and industrial climate, the doping films show better chemical and thermal stabilities than ITO films. Besides the influence of crystal structure, the better stability of zirconium oxide can improve the chemical and thermal stabilities. ITZO films have better electrical stability and chemical antcaustic properties, and the films could find more extensive applications.     

Influence of addition curing silicone formulation and surface aging of aluminum adherends on bond strength

In spite of many studies, knowledge about the fundamental factors influencing adhesion between addition curing silicones and aluminum substrates is very limited. The aim of this publication is to evaluate the influence of the formulation and the surface state of the adherend on bond strength. For this purpose, the composition of an addition curing silicone was systematically varied and the effects on both material and bond properties were examined. Additionally, the influence of surface aging at different humidities (0% r. h., 34% r. h., 82% r. h.) of acid etch pretreated aluminum substrates was considered. It is shown that the mechanical properties of the silicone material can be easily adjusted over a wide range by changing the formulation. Although high tensile strengths up to 9.2 MPa for the silicone material can be achieved, lap-shear strengths remain moderate at approximately 3.5 MPa. Predominant adhesive failures show the limited adhesive strength of the basic formulation without additives. Basic ingredients of addition curing silicones without additives are able to reach a certain adhesive strength. However, this strength was quite limited and adhesion promoters are required to further improve adhesion. The humidity at which the pretreated substrates are stored has an overall minor influence on bond strength. Surprisingly, bond strength tends to increase with the storage time of aluminum substrates despite lower surface energies in comparison to freshly pretreated substrates. All in all, the storage conditions of aluminum had a rather small influence on adhesion, whereas the composition of the silicone adhesive strongly influences bond strength.     

Hydroxyapatite coatings by pulsed ultrasonic spray pyrolysis

Hydroxyapatite coatings on 316L stainless steel substrates were prepared using ultrasonic spray deposition with substrate temperature kept at 300 °C. Two spray patterns, continuous and pulsed (intermittent), were used coupled with precisely control of substrate movement. It was determined that the ability to form continuous and relatively dense, well bonded coatings, was related primarily to the control of the flowrate of precursor solution and temperature and movement of the substrate. The microstructure and bonding strength of the coatings were evaluated in relationship with continuous and pulsed (intermittent) spray routes. Coatings prepared by continuous spray contained relatively large cracks while coatings by pulse spray exhibited finer cracks and higher bonding strength.     

Influence of surface defects on the biaxial strength of a silicon nitride ceramic - Increase of strength by crack healing

Failure of brittle materials starts in general from defects which exist in the volume or on the surface of the specimens. Surface flaws, which are more dangerous than volume flaws, can be introduced by machining. They decrease the strength of specimens and components.For this investigation silicon nitride specimens were produced using different machining conditions. About half of them were strength tested by use of the biaxial ball-on-three balls (B3B) test. It has been shown that better (more gentle) machining increases the strength but may also cause an increased scatter of strength data.The remaining specimens were heat treated (annealed) at 1000 °C in air and afterwards also strength tested using the B3B test. Compared to the non heat treated specimens a significant increase in strength could be proven, which was - depending on the machining conditions - between almost 300 MPa and more than 500 MPa. The scatter of strength data was largely decreased.The improvement was caused by the formation of a thin (0.5-2 μm) glassy layer which filled surface cracks and surface related pores during annealing.     

High temperature flexural strength,microstructure, phase evolution and anti-oxidation mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C

Carbon fiber/phenolic resin composites (CF/Ph) have attracted great interests in the field of thermal protection materials for their characteristics of high specific strength and easy manufacturing. However, CF/Ph are inherently susceptible to oxidation failure at elevated temperatures. In this study, a novel Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB₂ and B₄C was fabricated by an impregnating and compression molding route. Thermal stability, flexural strength, microstructure and phase evolution of the resulting ceramizable composite were studied. The residue yield at 1400 °C and flexural strength after treated at 1400 °C for 15min was 90.4% and 53.1 MPa, respectively, which was increased by 15.9% and 532.1% than that without ceramizable fillers. Surface defects generated by matrix pyrolysis were well healed, and PyC and carbon fibers were covered with dense ceramic layers while the fracture surface was covered with relatively continuous ceramic layers without visible pores. Multiphase ceramics composed of TiB₂, TiO₂, TiC and PyC were identified. Furthermore, oxidation failure and anti-oxidation mechanism was revealed based on the aforementioned characterizations and thermodynamic calculation results. Oxidation resistance got enhanced markedly for synergistic effects of oxygen consuming, carbon fixation, oxygen barrier and endothermic effect, which were derived from ceramization reactions between TiB₂, B₄C, O₂, Al and PyC.     

Joining of Al2O3 ceramic to Cu using refractory metal foil

Joining of Al₂O₃ ceramic to Cu has been conducted with Ag-26.7Cu-4.5Ti braze and refractory metal (W or Ta) foil. The interfacial microstructure in the joint with W foil is similar to that with Ta foil. The joining region in the joint consists of a reaction zone, braze zone I, refractory metal layer and braze zone II. The reaction zone of Cu₃Ti₃O with a thickness of about 5 μm develops close to Al₂O₃ side due to the reactions of Ti and Cu in the braze with Al₂O₃ substrate. The braze zones I and II are mainly composed of Ag- and Cu-based solid solutions. For the joint with W foil, the adsorption of Ti at the braze/W interfaces followed by the Ti diffusion into W foil occurs, whilst slight dissolution and diffusion of Ta into the brazes take place in the joint with Ta foil. The average shear strengths of joints with W and Ta foils are much higher than those without refractory metal foil, indicating the contribution of the refractory metal foil to the improvement of joint mechanical strength. Introduction of refractory metal foil in Al₂O₃/Cu joining is beneficial for the shift of joint residual stress distribution and the decrease of stress concentration in the joint since the coefficient of thermal expansion (CTE) of refractory metal layer approximately approaches that of Al₂O₃. Furthermore, a slight thickness increase of the Cu₃Ti₃O reaction zone in the joint with refractory metal foil may also give rise to the joint strength promotion.     

Mechanical behaviour of a silicon nitride particulate ceramic composite

Mechanical behaviour of a particulate ceramic composite (Si₃N₄ + SiC) was investigated. Its strength and fracture toughness on heating up to 1300 °C were determined as well as stress–strain curves plotted for this temperature range were analyzed. It is emphasized that this material is not only heterogeneous but also inelastic, and its deformation and fracture behaviour differ considerably from those of conventional ceramics. It was established that SENB fracture toughness measurements on notched specimens in flexure were quite reliable. Thus, there is no need in employing sophisticated standard test methods for this purpose. Fracture resistance estimates by the edge fracture (EF) method demonstrated that this material exhibited a lower barrier to the onset of fracture and a nonlinearly rising R-line, i.e., it displayed the ability to resist crack propagation (R-curve effect). The fracture resistance F_R and initial fracture toughness K_Ii were also determined. This information is rather useful for analysis of its actual performance under mechanical loading. The model of a nozzle vane of the gas turbine was employed to illustrate that the EF method was appropriate for evaluating the uniformity of ceramic items by their fracture resistance.     

Mechanical investigation of weak regions in a wound oxide-oxide ceramic matrix composite

The main aim of the investigation was to quantify the influence of production-related cross-lines on static mechanical properties (tensile, flexural and shear) of an oxide-oxide CMC as a comparison between specimens with cross-lines and specimens without cross-lines in tested regions. Investigated material was a weak-matrix oxide-oxide CMC (WHIPOX?) made of Nextel? 610 fibers (3000 denier) and alumina matrix with a special winding pattern. Mechanical tests at room temperature revealed that cross-lines were local weak regions in a wound component. Spatial separation of the cross-line within the composite (2?mm shift from layer to layer) did not improve the negative influence of the cross-lines on mechanical properties. Fractographic investigations revealed that cross-lines acted as a trigger of material failure.     

Fabrication of novel superhydrophilic and underwater superoleophobic hierarchically structured ceramic membrane and its separation performance of oily wastewater

Hierarchically engineered asymmetric ceramic membrane is successfully fabricated, and special wettability is endowed to the membrane by introducing superhydrophilic and underwater superoleophobic silica nanoparticles through dip-coating technology onto the active separation layer. Effect of reaction temperature on the wettability of the silica nanoparticles is discussed. It has been found that the hydrophilicity of the silica increases with increasing the temperature and superhydrophilicity can be obtained when the temperature reaches to 60 °C. Underwater oil contact measurements are performed using a series of oils and it turns out the as-prepared material bear excellent oil repellency towards various oil droplets. Separation performance of the membrane for oily wastewater treatment was investigated. It has been revealed that the as-prepared membrane can successfully separate the oil contaminated wastewater within one step under a small applied pressure (0.1 MPa) with oil rejection coefficient R>99.95%, indicating an extremely high separation efficiency.