Understanding the Failure Mechanism of Thermal Barrier Coatings Considering the Local Bulge at the Interface between YSZ Ceramic and Bond Layer

The TC/BC interface morphology in APS TBC is one of the important factors leading to crack propagation and coating failure. Long cracks are found near the bulge on the TC/BC interface. In this study, the TBC model with the bulge on the interface is developed to explore the influence of the bulge on the coating failure. Dynamic TGO growth and crack propagation are considered in the model. The effects of the bulge on the stress state and crack propagation in the ceramic layer are examined. Moreover, the effects of the distribution and number of bulges are also investigated. The results show that the bulge on the interface results in the redistribution of local stress. The early cracking of the ceramic layer occurs near the top of the bulge. One bulge near the peak or valley of the interface leads to a coating life reduction of about 75% compared with that without a bulge. The increase in the number of bulges further decreases the coating life, which is independent of the bulge location. The results in this work indicate that a smooth TC/BC interface obtained by some possible surface treatments may be an optional scenario for improving coating life.

New refined analytical models for various bonding conditions of an adhesively bonded smart PZT transducer using the EMI technique

The electro-mechanical impedance (EMI) technique has emerged as a cost-effective and non-destructive technique to detect the possible damages in the structure using a piezoelectric transducer, especially, lead zirconate titanate (PZT). The adhesive bond layer plays an important role in the PZT patch-host structure interaction for monitoring structural damage. Two bonding conditions are investigated in this research paper. Primarily, the debonding phenomenon of the adhesive bond layer may misinterpret the EMI response on the damage caused in structure. Subsequently, the investigation included the protective layer at the top of the PZT transducer to avoid sensor degradation. However, the analytical models developed so far have not considered a protective layer at the top of the PZT transducer. This paper presents the novel two-dimensional (2D) analytical model for incorporating debonding concepts and the new refined 2D analytical model to include a protective layer in the study of surface-bonded PZT transducers. The proposed analytical models are verified with the experimental studies. The experimental and analytical results show good agreement, which confirms the effectiveness of the new models. This paper also incorporated the effect of each bonding condition for monitoring structural damage by implementing the EMI technique. For the simulation, the numerical investigations on the PZT transducer bonded on the metallic (aluminum and steel) and concrete blocks are performed using coupled field analysis through finite element (FE) modeling. It is found that each bonding condition has influenced the resulting signatures. The signatures obtained from developed theoretical models and numerical simulations using three-dimensional FE models for each bonding condition are compared to highlight the influence on structural damage detection. The trend of signatures is found to be matching satisfactory. Several parametric studies have been conducted to show the efficacy of the new refined model with a protective layer. It considers the different input properties of an adhesive layer, host structure, and temperature conditions. The influence of debonding of the protective layer is also studied, and the obtained results support the need for a protective layer in the models.

Analysis and Research on Fracture Cause of Fixed Shaft of Torsion Arm of Wind Turbine Gearbox

In a wind power plant of a wind power plant limited liability company, the fixed shaft of the torsion arm of the gear box broke during the operation of a wind power generator set. In order to find out the cause of fracture, the fracture fixed shaft of torsion arm was comprehensively detected and analyzed by means of appearance morphology analysis, chemical composition analysis, mechanical properties testing, microstructure testing and fracture micro-area analysis. The results show that the main reasons for the fracture of the fixed shaft of the torsion arm of the fan gear box are as follows: improper heat treatment process of the fixed shaft of the torsion arm of the gear box causes a large amount of massive ferrite in the material structure, resulting in insufficient strength of the material; the inclusion in the material is serious, resulting in unqualified impact toughness; Shaft surface surfacing Cr - Mn stainless steel material causes the fusion zone C migration form brittle layer, at the same time the vast difference between the state of welding layer and substrate organization fusion zone caused by larger remnants stress, the embrittlement in bond layer to form the intergranular crack crack source, and in the process of the equipment operation under the action of cyclic torsional and impact load, Cracks propagate in a fatiguing manner and lead to eventual fracture.

Analytical and experimental investigation of cracking in two-way reinforced concrete panels

In this study, the cracking behaviour of RC panels under a two-way loading condition is investigated analytically and experimentally. A modified FE bond layer model is introduced and shown to be much simpler to define, and yet, more accurate than perfect bond or link element bond models. FE analysis suggests a bi-linear curve for approximating the distribution of bond stresses at stabilized cracking. Several FE parametric studies are performed on a one-way tension member to obtain peak shear bond stresses, concrete tensile stresses due to internal restrained shrinkage, splitting tensile stresses caused by radial bond stresses, and the reduction in bond strength caused by longitudinal splitting cracks. The FE analysis of flexural beams suggests a new factor for estimation of the depth of effective tension area, which is defined through FE parametric study. In the experimental phase, three medium-scale RC panels are subjected to direct tension in one direction and bending in the perpendicular direction. The collected data consist of applied loads, steel and concrete stresses, crack propagation sequence, crack pattern, crack width, total elongation, and leakage observations. The crack pattern is mainly influenced by the shape of reinforcement mesh due to the presence of splitting tensile stresses. The formation of diagonal cracks depends on principal stresses that can be influenced by reinforcement ratio, ratio of loads in two directions, and clear concrete cover to bar diameter ratio. The bond strength is significantly weakened by the formation of orthogonal cracks along reinforcing bars. The crack width is increased under repeated loading especially at the first cycle. Residual crack widths remain in place even after complete unloading. Observations indicate that the water leakage is influenced by the crack width gradient. Finally, it is shown that one-way crack prediction models underestimate the crack width of two-way panels. A new set of analytical equations is developed for the prediction of cracking load, minimum and maximum spacing of cracks, and maximum crack width in a two-way panel. These equations are shown to predict the cracking behaviour of the tested panels more accurately than any other previously proposed model.

Analytical and experimental investigation of cracking in two-way reinforced concrete panels

In this study, the cracking behaviour of RC panels under a two-way loading condition is investigated analytically and experimentally. A modified FE bond layer model is introduced and shown to be much simpler to define, and yet, more accurate than perfect bond or link element bond models. FE analysis suggests a bi-linear curve for approximating the distribution of bond stresses at stabilized cracking. Several FE parametric studies are performed on a one-way tension member to obtain peak shear bond stresses, concrete tensile stresses due to internal restrained shrinkage, splitting tensile stresses caused by radial bond stresses, and the reduction in bond strength caused by longitudinal splitting cracks. The FE analysis of flexural beams suggests a new factor for estimation of the depth of effective tension area, which is defined through FE parametric study. In the experimental phase, three medium-scale RC panels are subjected to direct tension in one direction and bending in the perpendicular direction. The collected data consist of applied loads, steel and concrete stresses, crack propagation sequence, crack pattern, crack width, total elongation, and leakage observations. The crack pattern is mainly influenced by the shape of reinforcement mesh due to the presence of splitting tensile stresses. The formation of diagonal cracks depends on principal stresses that can be influenced by reinforcement ratio, ratio of loads in two directions, and clear concrete cover to bar diameter ratio. The bond strength is significantly weakened by the formation of orthogonal cracks along reinforcing bars. The crack width is increased under repeated loading especially at the first cycle. Residual crack widths remain in place even after complete unloading. Observations indicate that the water leakage is influenced by the crack width gradient. Finally, it is shown that one-way crack prediction models underestimate the crack width of two-way panels. A new set of analytical equations is developed for the prediction of cracking load, minimum and maximum spacing of cracks, and maximum crack width in a two-way panel. These equations are shown to predict the cracking behaviour of the tested panels more accurately than any other previously proposed model.

Galvanic Corrosion Performance of an Al–BN Abradable Seal Coating System in Chloride Solution

In this study, we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. The results indicated a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage II under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.

Improving wear resistance of plasma-sprayed calcia and magnesia-stabilized zirconia mixed coating: roles of phase stability and microstructure

The phase stability and microstructure of ZrO2–5CaO and ZrO2–24MgO mixed coating (wt%) by air plasma spraying on 304 stainless steel substrates were investigated. A Ni–5Al (wt%) metallic bond coating was firstly sprayed between the substrate and the ceramic top layer. The results were compared with the individual coatings of ZrO2–5CaO and ZrO2–24MgO for a better understanding of the correlation between their microstructures and mechanical properties. Mixed zirconia coating was found to have a mixture of cubic and tetragonal phases that stabilized under different plasma spray conditions. Microscopic observations and elemental composition analysis of as-sprayed mixed coating showed that modified ceramic-matrix grains had been formed. Microsized ZrO2–5CaO particles were embedded in the matrix grain creating an intragranular microstructure. Results indicated that ceramic-matrix grains provided a diffusion barrier for the growth of oxides induced stress near and onto the bond layer that reduced cracks, thereby overcoming the top delamination of the ceramic coating. Moreover, disparity in wear resistance and microhardness behavior of the coatings was influenced by initial feedstock powder and matrix microstructures. Improvement in the wear resistance of the mixed zirconia coating was attributed to a decrease in oxide content, which resulted in an increase in intersplat cohesive strength.

Characterization and Prevention of Metal Overflow in Ultra-Thin Au-Sn Eutectic Chip Bonding for Packaging and Integration of Extreme Heat Flux Micro-Coolers

In this study, a detailed characterization of Au-Sn eutectic ultra-thin metal stack (∼ 1 μm) bonding has been performed between Pyrex and silicon substrates using a commercial flip-chip bonder. A thorough recipe characterization and development was performed on three different bond sizes of 9, 49 and 100 mm2 by varying bonding temperature between 320 and 380°C with pressure ranging between 2 to 10 MPa. Results indicate that better bond quality was observed at higher temperatures but was relatively unaffected by the bond pressure magnitude. It was also found that flatness of contact is one of the most important parameters that determine the bond uniformity and thus the quality, which is especially important for ultra-thin metal bonding. In addition, this study puts special emphasis on observing the bond uniformity and metal overflow through the transparent Pyrex top substrate. The mean overflow width increased with increasing temperature, reaching as high as 300 μm at 380°C, but was not significantly affected by the bond pressure applied. Simultaneously, the ultra-thin bond layer made it possible for us to observe several different types of microstructures forming within the bond zone, which provided crucial information about sample cool down rate, grain size and intermetallic composition in the eutectic alloy. For a specific case, Kirkendall voids were observed under the optical microscope at the interface between Pyrex and bonded metal because of dissimilar rates of migration of Au and Sn during the eutectic reaction. We believe that this is the first successful observation of voids in bond alloy using non-destructive optical imaging techniques. Following successful characterization of metal reflow from the bond site, a simple method to control this overflow has been demonstrated by precisely controlled misalignment of the two complementary chips. This fundamental study on eutectic bonding aims to further the understanding of eutectic bonding process as well as facilitate development of effective ultra-thin layer, high strength bonding recipes between chips for versatile applications in the electronic packaging industry.