Alumina disks with different surface finish: thermal shock behavior

Disks of commercial alumina powder were fabricated by slip casting and sintering (1600°C, 2 h). Two different surface treatments were performed: a coarse wear, using a 70 grit diamond wheel (CAW) and a fine one with SiC paper of 120 and 320 grit (FAW). The thermal shock resistance of the worn specimens was evaluated testing the disks by sudden cooling with a high-velocity air jet. The critical temperature differential (ΔT_C) was determined increasing the initial temperature of the sample in 10°C until the crack propagation was detected. The values of the mean ΔT_C were 940°C for FAW specimens and 765°C for CAW ones. The statistical distributions of the results were analyzed in function of the fracture strength of the specimens measured in biaxial flexion and the characteristics of the different surface finish determined by scanning electron microscopy, profilometry, and residual stresses measured by X-ray diffraction.     

A study of the effect of surface pre-treatment on the adhesion of coatings

This paper deals with the study of effects of mechanical surface preparation on the adhesion of coating with high content of zinc dust. Five kinds of mechanically blasted surfaces were studied. The following were used as abrasives: steel shot, steel grit, brown corundum oxide and zirblast. The last surface type was modified by MBX Blaster technology (mechanical bristle blasting). The surfaces topography was quantified by a roughness profilometer. The shape and size of the incurred inequalities on the modified surfaces were studied using 3D microscope images of the surface. The purity of the surfaces after pre-treatment was evaluated by impurity glued on the tape and measuring the reflection of light from the surface. Fractal analysis was used to evaluate the diversity of inequalities on the prepared surfaces. Cross-sections were also taken of the prepared surfaces. The prepared surfaces were coated with zinc-filled coating. The adhesion of the coating to the substrate was evaluated by a pull-off test after curing the coating (as sprayed), as well as after exposure to severe corrosive environments. The best adhesion of the coating was found for the coating applied to the substrate which had been pre-treated with brown corundum and steel grit.     

Effect of Joint Thickness and Residual Stresses on the Properties of Ceramic Adhesive Joints: II, Experimental Results

The residual stresses in joints were varied by varying the thickness and thermal expansion mismatch of joints prepared using alumina adherends and silicate glass adhesives. An apparent fracture toughness was measured by the single-edge notchedbeam method; strengths were measured in flexure, and the fracture surfaces were studied. Stress distributions, determined using the finite element method in Part I, together with the results of literature analyses for stresses in joints subjected to externally applied loads, were used to aid in interpreting the experimental observations. The measured fracture toughness and strength of ceramic adhesive joints increase with decreasing adhesive thickness and decrease with increasing thermal expansion mismatch (residual stress), both positive and negative.     

Fracture of Soda-Lime Glass Tubes by Field-Assisted Ion Exchange

The field-assisted ion exchange of K⁺ and Na⁺ in soda-lime glass tubes fdled with and immersed in molten KNO₃ was Investigated. Stress distribution deduced from photoelastic measurements agrees well with that calculated from analogy to thermal stresses. Fracture was observed when a critical exchange depth was reached. The effects of tube dimension, surface condition, temperature, electric current density, current interruption, and current reversal on the critical exchange depth were studied. Calculated stresses for fracture originating on the tensile side of tubes exchanged on one side only agree with the independently measured fracture stresses of similar tubes. Fracture originating on the compressive side is believed to be related to "spalling" under biaxial compressive stresses which were frequently observed on the compressive surface.     

Assessment of Surface Flaws in Glass via Thermal Downshock Test

The severity of surface flaws in a sodium aluminosilicate glass was assessed by subjecting flat glass plates to successively higher thermal downshock and determining the temperature range that causes failure. The temperature data are then translated to short-lived tensile stresses, which, in turn, provide an estimate of flaw depth using Griffith's fracture criterion. The data show that the failure occurs from the edge zone indicating a more severe flaw population associated with edge finishing than that with manufactured flat surfaces. Etching of the edge region reduces flaw severity and increases the failure temperature nearly twofold. The observed fragmentation density appears to vary with stored elastic energy. The estimated thermal stresses, flaw depth, and elastic energy during thermal downshock are discussed.     

Surface Energy and Adhesion in Composite–Composite Adhesive Bonds

In the absence of weak boundary layers, surface energy can be an excellent indicator of the suitability of a fiber-reinforced composite surface for adhesive bonding. Mechanical surface treatments such as grit blasting are effective and commonly used to prepare composite surfaces, but the roughness introduced by these treatments makes quantification of the surface energy by contact angle methods difficult. This paper shows that the diameter of a small drop of a low-viscosity fluid chosen to have surface tension characteristics very similar to the adhesive can be used as an effective predictor of adhesive bond fracture energy. This technique could form the basis of a sensitive quality assurance tool for manufacturing.     

Shear decohesion of clamped abraded steel interfaces reinforced with epoxy adhesive

This paper presents an experimental assessment of quasi-static shear strength of the combined mechanically clamped and epoxy adhesive reinforced steel interfaces. The effect of the surface roughness and clamping load on the combined interfacial decohesion and slipping is investigated. The maximum shear strength of the adhesive reinforced specimens with fine ground, coarse ground and grit blasted contact surface finishes is reported with comparison to the results of the identical non-reinforced specimens. Results have been assessed both in terms of calculated fracture energy and interface decohesion. The bonded interfaces with grit blasted finish showed considerably higher maximum shear stresses as compared to the identical ground cases. The shear strength contributions of strong clamping and reinforcing conformed well to the principle of superposition for all experimented interface types.     

Study on the role of laser surface irradiation on damage and decohesion of Al/epoxy joints

In this work we investigate the effect of laser irradiation on the bond toughness of aluminum/epoxy bonded joints. The evolution of substrate surface morphology and wettability, for various sets of laser process parameters (i.e. laser power, line spacing, scan speed), was investigated by means of Scanning Electron Microscopy (SEM) and contact angle measurements. A proper combination of power, line spacing and scan speed was then selected and adhesive bonded Al/epoxy T-peel joints were prepared and tested. For comparison, similar samples were produced using substrates with classical grit blasting surface treatment. Finally, post-failure SEM analyses of fracture surfaces were performed, and in order to typify the increase in bond toughness of the joints, finite element simulations were carried out using a potential based cohesive zone model of fracture.     

梯度功能陶瓷圆球的抗热震性

推导和提出了第三类边界条件下梯度功能材料(functionally gradient materials,FGM)圆球的瞬态温度场及瞬态热应力场表达式.基于陶瓷材料的临界应力断裂判据,通过求解梯度功能陶瓷圆球表面达到其局部断裂强度的时间,建立了引起其表面临界热应力的临界温差△Tc的表达式,并以此作为梯度功能陶瓷圆球的抗热震参数.通过计算实例并与均质陶瓷圆球对比,分析了材料的热-物理性能分布规律对其抗热震性的影响,并提出了高抗热震性FGM陶瓷圆球的设计原则:线膨胀系数和热扩散率应由表及里增大,而弹性模量应由表及里减小.     

Effect of Grain-Boundary Oxidation on Fracture Toughness of SiC

Hot-pressed SiC was oxidized at temperatures from 900° to 1300°C, and the fracture toughness was determined by the indentation method. The apparent fracture toughness of the surface layers increased with oxidation temperature and then decreased. The observed variations in apparent fracture toughness were consistent with earlier observations of strength variations in oxidized SiC. Some specimens were fractured, and the fracture surfaces were characterized. Based on these observations, the variations in fracture toughness were attributed to variations in residual compressive stresses induced by oxidation.     

Elastic Energy at Fracture and Surface Energy as Design Criteria for Thermal Shock

The physical properties which affect the propagation of cracks in specimens fractured by thermal shock are discussed. The driving force for crack propagation is provided by the elastic energy stored at fracture. The mechanism of energy dissipation which will tend to arrest the propagating cracks is provided by the "effective surface energy" required to produce the newly formed crack surfaces. An expression is derived applicable to a body of spherical shape for the mean area traversed by cracks nucleated by thermal shock. Three numerical examples are given for materials with widely different physical properties, and their fracture behavior is predicted. Good agreement with experiment was obtained. Thermal shock damage resistance parameters suitable for the relative comparison of the "degree of damage" to be expected in materials fractured by thermal shock are proposed. The criteria for a low degree of damage are high values of Young's modulus of elasticity, Poisson's ratio, and effective surface energy and low values of strength. Recommendations are made for the selection of materials for severe thermal shock, where the best materials available are known to fail.     

Durability assessment of laser treated aluminium bonded joints

Structural adhesives are widely used in industrial applications dealing with the problem of assembly and bonding. Despite the several advantages brought by this technique, one of the main issue is represented by the need of the surface to be mechanically and/or chemically treated with the aim to make it suitable for the adhesive deposition. Previous works demonstrate how the surface treatment by laser ablation seems to enhance the joint strength with respect to the untreated material. In particular, the effect of pulsed Yb-laser ablated aluminium surfaces over the mode I energy release rate of Double Cantilever Beam (DCB) specimens was investigated and a significant growth of the fracture toughness compared with the untreated and the grit blasted joints was observed. In this work, an investigation concerning the durability of the mechanical properties of aluminium joints treated with several representative parameters configurations was conduced. These setting configurations were used in an experimental campaign with the aim of verifying their suitability varying the type of the test (fatigue tests) and the environmental conditioning of the specimens (quasi-static tests after ageing cycles). Concerning the fatigue behavior, the ranking of the laser parameters configuration according to the increase of toughness with respect the degreased and the grit blasted samples seemed quite consistent with the results obtained in the quasi-static test campaign. When an accelerated ageing cycle in control of temperature and relative humidity was applied, a general lowering of toughness affected every tested specimen. This effect was however more marked in the grit blasted sample than in the laser treated ones. Therefore, a relative improvement of the mechanical performance when using some laser ablation configurations instead of the grit blasting, under the same conditions, when the adhesively bonded joints were aimed to undergo some critical environmental exposure, was recorded.     

In Memoriam

The surface modification and adhesive bonding of a carbon fiber reinforced plastic (CFRP) composite has been investigated. Wettability studies showed that plasma-treated specimens provide a significant increment in the surface energy, relative to untreated material. The surface modification resulted in significantly improved adhesion between the composite and an applied toughened acrylic adhesive; a considerable increase in fracture energy was observed following grit blasting and grit blasting plus silane treatments. Specimens treated with atmospheric plasma showed a slight increment in fracture energy, usually failing adhesively. The durability was tested using a wedge test. Specimens degreased and treated with atmospheric plasma showed the greatest crack growth and failed in an adhesive mode.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

Crack-Tip Structure in Soda–Lime–Silicate Glass

The topology of crack tips in soda–lime–silicate glass was investigated using atomic force microscopy (AFM). Studies were conducted on cracks that were first propagated in water and then subjected to stress intensity factors either at or below the crack growth threshold. Exposure to loads at the crack growth threshold resulted in long delays to restart crack growth after increasing the stress intensity factor to higher values. After breaking the fracture specimen in two, the "upper" and "lower" fracture surfaces were mapped and compared using AFM. Fracture surfaces matched to an accuracy of better than 0.5 nm normal to the fracture plane and 5 nm within the fracture plane. Displacements between the upper and lower fracture surfaces that developed after a critical holding time were independent of distance from the crack tip, and increased with holding time. Despite the surface displacement, crack tips appeared to be sharp. Results are discussed in terms of a hydronium ion–alkali ion exchange along the crack surfaces and corrosion of the glass surface near the crack tip by hydroxyl ions.     

Thermal Shock Resistance of Sintered Alumina/Silicon Carbide Nanocomposites Evaluated by Indentation Techniques

The thermal shock resistance of sintered Al₂O₃/1, 2.5, and 5 vol% SiC nanocomposites was studied using two indentation techniques. In the first technique, "indentation thermal shock" measurements were made of the extension of median/radial cracks around Vickers indentations after quenching from various temperatures (up to 480°C) into a bath of boiling water. This technique allowed a critical thermal shock temperature, Δ T _C^Ind, to be quantitatively evaluated. In the second technique, "indentation fatigue" tests were conducted on the thermally shocked specimens; repeated indentations were made at the same site, and the number of load cycles needed to initiate lateral fracture was measured. The results showed that nanocomposites with an addition of SiC nanophase as low as 1 vol% had a thermal shock resistance superior to that of pure Al₂O₃.     

Fracture behaviour of alumina and zirconia thin layered laminate

Laminates with strong bonds between thin layers were examined in this work to explore the influence of developed internal stresses on the fracture behaviour. A set of laminates having different level of internal stresses were prepared. Alumina and zirconia were the model materials for evenly alternating layers. The electrophoretic deposition technique was used for manufacturing of the laminates. The basic mechanical properties as elastic modulus and flexural strength were determined for all prepared materials. The crack propagation changes due to effect of internal stresses, elastic mismatch and surface effects were investigated using modified single edge notched beam technique. An extensive fractographical analysis of fracture surfaces was undertaken using laser confocal microscopy. The changes of the crack direction when crack propagates through alternating layers under different angels were described. Further, the effect of the internal stresses level within individual layers was reported.     

Behaviour of cordierite materials under mechanical and thermal biaxial stress

Abstract

A commercial cordierite powder (< 0.17 wt-% impurities) was selected for a study of material behaviour under mechanical and thermal stresses. Disks were slip cast, sintered for 2 h at 1450°C, and indented (Vickers, 44.1 N) at the centre of the surface to be subjected to mechanical and thermal shock tests. The sintered bodies (84 wt-% cordierite, 10 wt-% mullite, 6 wt-% glass) reached 95% of theoretical density. The microstructure consisted of homogeneous, mainly equiaxed grains (mean size ≈0.5 μm) and a few elongated grains (aspect ratio ≈1.9). A glass phase was identified at triple points, and intergranular pores (< 10 μm) and a few isolated larger pores (up to 40 μm) were observed. The fracture strength σ_F was measured by biaxial flexure, employing a ball on discontinuous ring configuration with displacement con1 trol (0.05 mm min ^-1). In each thermal shock test, the indented specimen was heated to a selected temperature and the disk centre was then suddenly cooled using a high velocity air jet at room temperature. The initial temperature was increased by increments of 10°C until crack propagation was detected and the value of the thermal shock resistance Δ T_C was evaluated. The values obtained were compared with cordierite disks without indents and with alumina materials. The fracture features of the specimens broken in both mechanical and thermal shock tests (crack patterns and fracture surfaces) were characterised, taking into account the developed microstructures (grains, phases, pores) and the fracture origin at the controlled size defect introduced by indentation.     

Wettability and bonding quality of exterior coatings on jabon and sengon wood surfaces

Wettability and bonding quality of exterior coatings on fast-growing wood surfaces were studied. Samples of air-dried flat-grained (tangential surface) and edge-grained (radial surface) pattern of jabon (Anthocephalus cadamba) and sengon (Paraserianthes falcataria) woods were used. Before application of exterior coatings, the surfaces of the lumber samples were sanded. To provide wood surfaces with various degrees of roughness, abrasive papers of 120, 240, and 360 grits were used for the surface preparation. The wettability of two exterior coatings (water-based acrylic and oil-based alkyd varnishes) on the sanded wood surfaces was measured using a sessile drop contact angle method. The Shi and Gardner (S/G) model was used to evaluate and compare the wettability of the surface coatings on the wood. The sanded wood samples were coated with the two coatings (two layers each). Bonding quality of the coating layers was measured using a crosscut tape test method. Experimental results show that constant contact angle change rate (K value) of the S/G model decreased as the grit number of abrasive paper increased. This indicates that the wettability decreased as the roughness of the surface decreased (surface becomes smoother). There was no evidence of differences in wettability between tangential and radial wood surfaces. The oil-based alkyd coating generated better wettability compared to the water-based acrylic. The crosscut tests showed that the bonding quality of the coating films on both jabon and sengon wood decreased as the surface became smoother. The sengon wood compared to jabon wood provided better coating wettability and bonding quality. Wettability in terms of the K values was a good indication for determining the bonding quality of the two varnish layers.     

Thermal shock of ground and polished alumina and Al2O3/SiC nanocomposites

The thermal shock behaviour of sintered alumina and alumina/SiC nanocomposites with 1, 2.5 and 5 vol.% SiC was studied. The thermal shock testing was carried out by means of quenching into water from high temperatures (ΔT in the range 0–750 °C). Both single shocks and repeated shocks were used. The damage introduced by thermal shock was characterised by degradation of strength in four-point bending and by changes in Young's modulus. The effects of the surface finish of the test specimens (either ground or highly polished surfaces) on the thermal shock resistance were also studied. In both alumina and nanocomposite materials, specimens with ground surfaces showed a better resistance to thermal shocks than specimens with polished surfaces. However, the resistance of the nanocomposite material to single and repeated thermal shocks was no better than that of the pure alumina.