Water-quenching Method Research Articles

Microstructure and high-temperature sintering properties of APS coatings prepared from spherical thin-walled hollow-shell powders

High-performance thermal barrier coatings (TBCs) made of 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder with a spherical thin-walled hollow-shell (STHS) structure exhibited a special microstructure different from the conventional lamellar structure of air plasma-sprayed (APS) coatings. The as-sprayed STHS APS coatings had a completely tetragonal prime (t′) structure and non-lamellated closed-cell structure with high porosity, which resulted in relatively low thermal conductivity (∼1.0 W m−1 K−1) and high Vicker's hardness (∼6 GPa). The influences of high-temperature aging on the microstructure stability, phase stability, and sintering capability were investigated after long-time heat treatment at different temperatures. The characterization results indicated that the pore content was basically constant, and it was less than 0.5% for sintered linear shrinkage of the STHS coatings after heat treatment at 1500 °C for 100 h. Furthermore, no spalling appeared in the STHS APS coating with the t′ phase structure after 101 thermal cycles of the water-quenching method at 1050 °C, and no monoclinic ZrO2 (m-ZrO2) phase was present in all of the STHS coatings after aging at 1200 °C for 1–1100 h. The excellent anti-sintering properties and phase stability of the STHS coatings are attributed to the closed-pore microstructure and the highly pure t′ phase composition with uniform distribution of ions, respectively. The results suggested that the non-lamellated closed-cell microstructure is beneficial for improving the coating properties, and the results also provide guidelines for microstructure design of TBCs using a feedstock powder.

Investigating the effect of residual stress on hydrogen cracking in multi-pass robotic welding through process compatible non-destructive testing

Abstract In this paper, the effect of Welding Residual Stress (WRS) on the size and morphology of hydrogen-induced cracks (HIC) is studied. Four samples were manufactured using a 6-axis welding robot and in two separate batches. The difference between the two batches was the clamping system used, which resulted in different amounts of welding deformation and WRS. The hydrogen cracks were intentionally manufactured in the samples using a localised water-quenching method, where water was sprayed over a specific weld pass in a predetermined position. The Phased-Array Ultrasonic Testing (PAUT) system was implemented during the welding process (high-temperature in-process method), to detect the HIC in real-time. The WRS in both batches was measured using the hole-drilling method, where a difference in transversal residual stress of 78 MPa was found between the two samples. Based upon both the PAUT results and microscopic investigations, the batch with higher WRS resulted in larger size and number of HIC. For the first time, the negative effect of WRS on HIC has been monitored in real-time using high-temperature in-process inspection. This was achieved using an innovative approach, introduced in this paper, to repeatably manufacture high and low WRS samples in order to control the size and location of subsequent HIC.

Thermal stability and thermal shock resistance of Fe2AlB2

The nanolaminated ternary boride Fe2AlB2 exhibits a combination of damage tolerance, electrical conductivity, and machinability at room temperature. This work reports for the first time on the thermal stability and thermal shock resistance of Fe2AlB2 at high temperatures. The thermal stability of Fe2AlB2 at temperatures up to 1300 °C in Ar atmosphere was studied. Fe2AlB2 intensively decomposed at 1236 °C in Ar atmosphere. The thermal shock resistance was investigated using the water-quenching method in the temperature range 600–1200 °C. The residual strength of the quenched Fe2AlB2 decreased continuously with increasing temperatures. The phase composition, phase transformation and microstructure were characterized with X-ray diffractometry, differential scanning calorimetry and scanning electron microscopy.

Thermal shock resistance of bimodal structured thermal barrier coatings by atmospheric plasma spraying using nanostructured partially stabilized zirconia

In this article, an agglomerated and sintered Y2O3-stabilized ZrO2 (YSZ) powder was introduced as a feedstock for atmospheric plasma spraying, and bimodal structured thermal barrier coatings with different contents of nanostructure were deposited. Water-quenching method was carried out to study the effect of nanostructure content on thermal shock resistance of the coating. Results showed that the thermal cycle life of the coating is significantly influenced by the microstructure of the coating. When the nanostructure content was relatively low (about 39%), the coating showed high microhardness and poor thermal cycle life (failure at 11cycles); however, when the nanostructure content was relatively high (about 60%), the coating showed lower microhardness and improved thermal cycle life (failure at 42cycles); the coating with medium nanostructure (about 52%) showed the best thermal shock resistance (no failure at 50cycles). The thermal stability of nanostructured coating was also investigated. After heat treatment at 1300°C for 10h, the nanoparticle size increased and the hardness of the coating increased noticeably, indicating the sintering effect of the nanostructured coating.

Effect of Additives and Carbon Resources on Thermal Shock Resistance of Spinel-C Material

Spinel-C material is important for monolithic stopper and submerged nozzle, which usually determine the application effects. Spinel-C specimens with 8% flake graphite is characterized by good corrosion resistance but poor thermal shock resistance. The effects of additives (Al-Si alloy, silicon powder, B4C and SiC), flake graphite (+199/-199), ultrafine flake graphite, expanded graphite and nickel nitrate + pitch powder on thermal shock resistance of spinel-C material were investigated by using water-quenching method. The results show that: (1) The addition of Al-Si alloy and B4C can significantly decrease the residual strength ratios (RSR) of the specimens. The addition of SiC and silicon powder can improve the RSR obviously. (2) The flake graphite of different particle sizes has little effect on the RSR of the specimens. With the increasing of the expanded graphite content, the RSR of the specimens increases. (3) The addition of mesophase pitch and Carboresâ P can receive the RSR rise of 74% and 59% respectively.

Thermal Cycling Behavior of Quasi-Columnar YSZ Coatings Deposited by PS-PVD

Columnar-structured thermal barrier coatings, owing to their high strain tolerance, are expected for their potential possibilities to substantially extend turbine lives and improve engine efficiencies. In this paper, plasma spray-physical vapor deposition (PS-PVD) process was used to deposit yttria partially stabilized zirconia (YSZ) coatings with quasi-columnar structures. Thermal cyclic tests on burner rigs and thermal shock tests by heating and water-quenching method were involved to evaluate the thermal cycling and thermal shock behaviors of such kind of structured thermal barrier coatings (TBCs). Evolution of the microstructures, phase composition, residual stresses and failure behaviors of quasi-columnar YSZ coatings before and after the thermal tests was investigated. The quasi-columnar coating obtained had an average life of around 623 cycles when the spallation area reached about 10% of the total coating surface during burner rig tests with the coating surface temperature of ~1250 °C. Failure of the coating is mainly due to the break and pull-out of center columnar segments.

The thermal shock behaviors of yb:YAG transparent ceramics

The thermal shock resistance is a major concern of the Yb:Y3Al5O12 (Yb:YAG) transparent ceramic considered as a potential solid-state laser material. Its thermal shock behavior was investigated by the standard water-quenching and indentation-quenching thermal shock measuring methods. The residual flexural strength and Vickers radial cracks were measured as a function of thermal shock temperature difference (ΔT). It showed that after the thermal shocking of ΔT=130°C a reduction in residual flexural strength was observed, and there was a ΔT of 120°C atwhich the longitudinal cracks (aligned parallel to the sample's length) grew radically. A good correspondence was found between the results of the thermal shock critical temperature difference (ΔTc) obtained by the standard water-quenching method and estimated by the indentation-quenching test. The analysis of the indentation fracture mechanics was used to account for the cracking responses. The analysis confirmed that the thermal stress induced by the thermal shock is much greater in the transverse direction than in the longitudinal direction, resulting in the unstable longitudinal cracks propagated at first.

Low-field magnetization process and complex permeability of FeCoBSiTa wires coated with hard magnetic CoNi layer

Biphase wires consisting of a soft magnetic amorphous nucleus surrounded by a hard magnetic CoNi layer of variable thickness were obtained by means of rotating water-quenching method and subsequent electroplating technique. Magnetization processes for all the biphase wires were resolved in terms of reversible bulging of magnetic domains and spin rotation by means of complex permeability measurements within the frequency range of 10 Hz–13 MHz. Results are interpreted in terms of CoNi layer effect on the magnetic anisotropy of the soft core.

Thermal shock resistance of ZrC matrix ceramics

A ZrC-20 vol% SiC based ceramic matrix composite containing 10 vol% graphite flake was fabricated by hot press sintering. The thermal shock resistance of the materials was investigated through the water-quench method and subsequent three-point bend testing of flexural strength diminution. The ZrC-20 vol% SiC composite containing 10 vol% graphite flake exhibited higher critical temperature difference and higher residual ratio of strength compared with that of ZrC-20 vol% SiC composite. It is the main reason that addition of the graphite flake provides a weak interface into the material which acts to deflect propagating cracks, and the bridging of the cracks occurred. Meanwhile, a laminar structure of the graphite flake would relax the interface stress by sliding of interlayers and provide favorable sites for the dissipation of energy associated with crack growth during fracture.

Thermal shock and thermal fatigue behavior of Si3N4–TiC nano-composites

Abstract Nano monolithic Si 3 N 4 and Si 3 N 4 –TiC nano-composites were fabricated by hot press sintering. The thermal shock and thermal fatigue behaviors of monolithic Si 3 N 4 nano-ceramic and Si 3 N 4 –TiC nano-composites were investigated through the water-quench method. Si 3 N 4 –TiC nano-composites showed both higher critical temperature difference and higher residual strength compared with that of monolithic silicon nitride nano-ceramic. The highest critical temperature difference is 550 °C, which obtained from Si 3 N 4 –TiC nano-composite with 10 wt% TiC addition.

Microstructure and mechanical properties of an HfB 2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB 2–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB 2 + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min −1 heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO 2. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB 2. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.

Synthesis, sintering and thermal shock resistance estimation of porous cordierite by IR heating technique

A thermal shock fracture test performed by centrally heating disk shaped specimens is found to have advantages over traditional water quench method. Thermal shock property evaluation of porous materials; especially porous cordierite is seldom undergone. In the present work, porous cordierite has been prepared by super critical (ethanol) drying and freeze drying of triphasic sol. Even after high temperature sintering cordierite aerogel could not be well densified because of their inherent textural features. Thermal properties of porous cordierite such as thermal conductivity ( λ), thermal diffusivity ( κ) and specific heat ( C p) were evaluated by laser flash technique. Thermal shock resistance ( R 1c) was estimated by infrared radiation heating (IRH) technique by centrally heating disk shaped specimens and evaluating the temperature profile during cracking with the help of acoustic emission (AE) sensor connected to the specimen. Here, synthesis, sintering and thermal property evaluation has been briefly discussed.

Bulk amorphous Pd–Ni–Fe–P alloys: Preparation and characterization

Bulk amorphous alloys of PdxNiyFe80−x−yP20 (25 ≤ x ≤ 60, 20 ≤ y ≤ 55, x + y ≥ 60) were prepared by a flux-melting and water-quenching method. Seven-mm diameter glassy rods of Pd40Ni40−xFexP20 (0 ≤ x ≤ 20) were studied in greater detail. For these alloys, the difference between the crystallization and glass transition temperatures ranges from 102 K for x = 0 to 53 K for x = 20. In this composition range, the reduced glass transition temperature, Trg, ranges from 0.66 to 0.57. The change in density upon crystallization ranges from 0.24 ± 0.04% for x = 0 to 1.33 ± 0.24% for x = 10. The partial molar volume of Fe in amorphous Pd40Ni40−xFexP20 alloys is significantly larger than the molar volume of (metastable) fcc Fe. This, as well as a comparison with the molar volumes of crystalline compounds, suggests chemically selective Fe–Pd bonding in these glasses.

Evaluation of Thermal Shock Resistance of Ceramics by Up-Shock Method and Water-Quench Method.

An up-shock method was proposed as a new technique for estimating the thermal shock resistance of ceramics. The method uses a disk-shaped specimen which is heated by infrared ray and by a constant heat flux at the central area. This technique enables an evaluation of thermal shock strength, Rc, directly from the power charge, despite the fact that Rc consists of some of the properties of the tested material. In this paper, the thermal shock resistance of Al2O3 and Si3N4 are estimated by both the up-shock method and the improved water-quench method which was presented earlier. The results of the infrared radiation heating method show that the thermal shock strength of Si3N4 was 7 times as high as Al2O3's one. In the water quench method the heat transfer coefficient of water was indispensable for the quantitative estimation. In this study we show that the up-shock method is more suitable for the evaluation of the thermal shock resistance for ceramics.

Cyclic Crack Growth Behavior and Dynamic Thermal Stresses of Silicon Nitride under Repeated Thermal Shock Tests.

Dynamic thermal stresses and cyclic crack growth behavior produced by a single and repeated thermal shocks were studied on silicon nitride. Thermal shock was applied to a square shaped bar specimen using the improved water-quenching method. It was clarified that the cyclic crack growth rates, da/dN, under the repeated thermal shock tests are represented by the following expression, da/dN=9.35×10-6(Kmax-1.36)3.14. In addition, the experimentally observed maximum thermal stresses, σmax, for silicon nitride in the present study as well as those for cemented carbides and cermets in the previous report can be approximated by the expression, σmax=0.704αEΔT-57.74, where α, E, ΔT denote a linear coefficient of expansion, Young's modulus of the materials and applied temperature differences at thermal shocks, respectively. By using this expression we can directly evaluate the maximum thermal stresses induced by the thermal shocks without temperature measurements.

The effect of grain size, porosity and yttria content on the thermal conductivity of nanocrystalline zirconia

High-performance thermal barrier coatings (TBCs) made of 4 mol.% Y2O3–stabilized ZrO2 (4YSZ) powder with a spherical thin-walled hollow-shell (STHS) structure exhibited a special microstructure different from the conventional lamellar structure of air plasma-sprayed (APS) coatings. The as-sprayed STHS APS coatings had a completely tetragonal prime (t′) structure and non-lamellated closed-cell structure with high porosity, which resulted in relatively low thermal conductivity (∼1.0 W m−1 K−1) and high Vicker's hardness (∼6 GPa). The influences of high-temperature aging on the microstructure stability, phase stability, and sintering capability were investigated after long-time heat treatment at different temperatures. The characterization results indicated that the pore content was basically constant, and it was less than 0.5% for sintered linear shrinkage of the STHS coatings after heat treatment at 1500 °C for 100 h. Furthermore, no spalling appeared in the STHS APS coating with the t′ phase structure after 101 thermal cycles of the water-quenching method at 1050 °C, and no monoclinic ZrO2 (m-ZrO2) phase was present in all of the STHS coatings after aging at 1200 °C for 1–1100 h. The excellent anti-sintering properties and phase stability of the STHS coatings are attributed to the closed-pore microstructure and the highly pure t′ phase composition with uniform distribution of ions, respectively. The results suggested that the non-lamellated closed-cell microstructure is beneficial for improving the coating properties, and the results also provide guidelines for microstructure design of TBCs using a feedstock powder.

Size Effect on Thermal Shock Behavior of an Alumina

This study investigated the degradation in mechanical properties of an alumina subjected to thermal shock loading by systematic experiments on bend-bar specimens of two sizes, 3×4×45mm and 9×12×45mm. Thermal loading was applied by water-quench method with several well defined temperature differences. Critical temperature difference (ΔTc) was defined as the lowest applied temperature difference above which a loss in flexure strength occurred. ΔTc values for the large and small specimens were found to be 175 and 200°C, respectively. The reduction in thermal shock resistance with increasing sample size can be well described by the thermal elastic and fracture energy theories. However, if the size effect on the initial, unshocked fracture strength is considered, the size effect on the transport of heat during quench test might play a less important role in reducing the thermal shock resistance of large specimen.

Thermal shock resistance of [formula omitted] nanocomposites fabricated from amorphous Si-C-N precursor powders

The thermal shock resistance of Si 3N 4 SiC nanocomposites fabricated from the amorphous Si-C-N precursor powder was investigated by a water-quenching method. The SiC dispersion decreased the critical temperature difference ( ΔT c ) of Si 3N 4, although it improved significantly the fracture strength. This decrease in ΔT c was discussed associated with the variations in mechanical and thermal properties by the nano-sized SiC dispersions as well as temperature difference dependence of the surface heat transfer coefficient at the water-ceramic interface.

Scattering of Residual Strengh of Ceramics Subjected to Repeated Thermal Shook.

Residual strengths after repeated thermal shocks were studied using Al2O3 ceramics specimens. Thermal shocks were repeated by the water quench method under two temperature difference conditions less than the critical temperature difference ΔTC at which the residual strength degradation was recognized after a single thermal shock. Residual strengths were significantly reduced by repetition of thermal shock and show wide scattering. After a relatively large thermal shock was repeated, the residual strentgth of the specimen showed a minimum value which was lower than that by single thermal shock. The results were discussed by means of the stable and the unstable crack growth behavior based on the thermal shock fracture theory proposed by Hasselman.

Thermal shock behavior of Y 2O 3-partially stabilized zirconia

The thermal shock behavior of 3mol% Y2O3-partially stabilized zirconia with average grain size of 0.4μm (Z3Y-I) and 1.0μm (Z3Y-II) has been studied by the water quench method. Twenty three specimens of rectangular cross-section bar (3 by 3 by 45mm) were quenched at 300°, 350° and 400deg;C temperature differentials (ΔT) and their retained strength (σf) were plotted using the Weibull distribution function and doubly exponential distribution function. The retained strength distribution at ΔT=350°C are similar to the original strength distribution. On the other hand, the distribution at ΔT=350° and 400°C showed a shoulder at F=0.458 and 0.875, respectively, They are the initiation points of thermal shock damage, Assumming the surface heat transfer coefficient (h) of 0.19 and 0.4cal/cm2·°C·s, calculated stress intensity factors KI's for crack initiation by water quench were 73-75% and 91-92% of the critical stress intensity factor KIC for Z3 Y-I, and 69-70% and 86-88% for Z3 Y-II, respectively. A σf-δT curve for a certain failure probability level F showed instantaneous decline at a critical temperature differential δTc. High failure probability level F resulted in a high critical temperature diffrential δTc. Estimated δTc's are 310°, 360° and 400°C for F=0.2, 0.5 and 0.9, respectively.