Improvement Of Thermal Shock Resistance Research Articles

Effect of Ge on oxidation behavior of 60% Cu – 40% Zn Muntz metal

Copper–base alloys are widely used in Marine industries and heat exchangers. Muntz Metal 60% Cu – 40% Zn is one of the critical Cu alloys used in decoration, tubing for heat exchangers, and Marine environments, but it is subject to dezincification and stress–correction cracking. This study employed alloying elements (Al, Sn, Ge) to achieve their properties (oxidation behavior, thermal shock resistance, mechanical and corrosion resistance). All results confirmed that an enhancement in properties, Ge, plays an essential role in this enhancement due to the formation of protective non-pours oxide layers from GeO2 and AL2O3 layers. Also, adding (Sn) made the alloy's microstructure very fine. These elements improve hardness and reduce corrosion rate by 83%, while in the oxidation test, the reduction in specific weight change was equal to 99%. Also, a significant improvement in thermal shock resistance concerning the reference sample was achieved.

Improvement of thermal shock resistance and hot mechanical properties by FCC/BCC/B2 multiphase strengthened microstructure in laser cladded high-entropy alloy coatings

NiCoFeCrSiAlxCu0.5TiMoB0.4 high-entropy alloy (HEA) coatings were prepared by laser cladding, and the microstructure, phase structure and thermal properties were studied. The results showed that the solid solution phase of the coating matrix changed from FCC + BCC to BCC as the main phases with increasing Al content. Moreover, the enrichment of Ti resulted in the significant formation of a stacking fault structure in the interdendritic region of the dual-phase coatings. The ordered B2 phase precipitated from the BCC structure exhibited high hardness and crack damage resistance. The wear rates of the HEA-Al0.5 and HEA-Al1.5 coatings at 600 °C were 3.78 × 10−5 and 4.35 × 10−5 mm3/(N·m)−1, respectively. The wear mechanisms of HEA coatings were mainly abrasive wear and oxidation wear. Because of the high microhardness of the BCC phase, cracks initiated and propagated on the worn surface, which aggravated the oxidation wear deterioration of the HEA coating. Overall, this study may provide a theoretical basis for the microstructure-property relationships of HEA coatings and contribute to the development of high-performance coatings.

Effect of C/MgO nanocomposite powders on the properties of low-carbon MgO–C refractories

To produce high performance low carbon MgO–C refractories, the combustion synthesized MgO nanofibers containing C/MgO composite powders (SCM) were introduced. The effect of SCM on the mechanical properties, thermal shock resistance, oxidation resistance and slag resistance of MgO–C refractories has been studied, along with exploring the toughening mechanism on thermal shock resistance. The results showed that the refractories containing 6 wt% SCM (M6) exhibited the most favorable comprehensive properties. Compared to reference sample M0, M6 showed significant enhancements in thermal shock resistance, oxidation resistance, and slag resistance by 38.6%, 29.8%, and 23.9%, respectively. Notably, the unique microstructure of MgO nanofibers and carbon encapsulated MgO played a crucial role in facilitating crack deflection and branching, which promote the generation of microcracks and the consumption of strain energy. These mechanisms contributed to the improvement in thermal shock resistance of refractories.

Cyclic thermal shock resistance for MgAlON–MgO composites obtained with additions of spent MgO–C brick: Microstructure characteristics, thermal shock parameter and thermal shock mechanism

Thermal shock parameters (R, R''', R'''' and Rst) of MgAlON–MgO composites obtained with additions of spent MgO–C brick were calculated using measured mechanical properties and thermal expansion coefficient, determining their resistance to fracture initiation and crack propagation. The cyclic thermal shock experiments of MgAlON–MgO composites performed from 1398 K to ambient temperature indicate that as number of thermal shock cycle increases, retained strength ratio of MgAlON and MgAlON–4.2 wt%MgO sharply decrease and then keep constant, while that of MgAlON–10.5 wt%MgO and MgAlON–15.7 wt%MgO slowly decrease. The reason for the difference is that MgAlON and MgAlON–4.2 wt%MgO show low value of R''' and R'''', and high value of R and Rst. Moreover, precipitation of impurity containing Fe may play a positive role in improvement of thermal shock resistance of MgAlON–MgO composites. MgAlON‒4.2 wt%MgO has the maximum retained strength (55 MPa) even after 5 thermal shock cycles, which is expected to be used in the metallurgical industry.

Enhanced mechanical strength of SiC reticulated porous ceramics via addition of in-situ chopped carbon fibers

Due to the poor strength of silicon carbide reticulated porous ceramics prepared by polymer sponge replica method, vacuum infiltration using a low viscosity infiltration slurry is performed to fill the flaws created during pyrolysis of the polymer sponge to improve the compressive strength. However, the flexural strength and thermal shock resistance of the porous silicon carbide ceramic are not significantly improved through this method. This paper investigates the improvement in thermal shock resistance and flexural strength of porous silicon carbide ceramics through the addition of different lengths and amounts of carbon fibers to the infiltration slurry. With the addition of 3 mm long carbon fibers at a concentration of 0.5 wt% to the infiltration slurry, the flexural strength of the porous silicon carbide ceramic was improved by 142%. Moreover, the thermal shock resistance of the SiC specimens was effectively enhanced through the addition of carbon fibers, as evidenced by a 74.4% increase in the residual strength ratio of the specimens. Due to the low thermal expansion coefficient of the carbon fibers, compressive residual stresses were generated in coating layers of SiC RPCs after sintering, which was inhibited the propagation of cracks in the coating layers. Thus, adding carbon fibers to the infiltration slurry can open more possibilities for application of SiC reticulated porous ceramics.

Enhanced thermal shock resistance of ZrO2-reinforced Al2O3–CaAl12O19 composites prepared from ferrotitanium slag: Crack propagation resistance mechanisms

Al2O3–CaAl12O19 composites are ideal high-temperature structural ceramics, however, their poor thermal shock resistance limits their application under thermal shock conditions. In this work, ferrotitanium slag was used as the main raw material for composites preparation. Enhanced thermal shock resistance were obtained by adding 12.5 wt.% unstabilized monoclinic zirconia. A systematic analysis of the phase compositions and microstructural evolution with particular attention to crack propagation was performed. The improvement in thermal shock resistance is attributed to the enhanced crack propagation resistance. Furthermore, the enhanced crack propagation resistance is caused by multiple mechanisms, including the t-ZrO2-related stress-induced t → m phase transformation mechanism and the m-ZrO2-related microcracking mechanism. Coarsened ZrO2 particles doped with TiO2 from the ferrotitanium slag are beneficial to the occurrences of stress-induced transformation and microcracking.

Influence of graphite content on fracture behavior of MgO–C refractories based on wedge splitting test with digital image correlation method and acoustic emission

Mechanical and thermal stresses are one of the main reasons causing the damage of MgO–C refractories in service. An in-depth understanding of the fracture behavior of MgO–C refractories is essential for the further development of high-performance MgO–C refractories. In this paper, effect of graphite content on fracture behavior of MgO–C refractories under wedge splitting test was investigated with the Digital Image Correlation (DIC) and Acoustic Emission (AE) technology. The results showed that the increase of graphite content in MgO–C refractories led to a reduction in brittleness and an improvement of thermal shock resistance. MgO–C refractories with higher graphite content were likely to generate crack at earlier loading stage and to have larger ultimate crack mouth opening displacement. It indicated a lower crack initiation resistance of MgO–C refractories and a better further damage propagation resistance. In addition, the proportion of crack in the MgO grains decreased and the tortuousness of crack path increased with the increasement of graphite content. It promoted the transformation of AE activity and energy dissipation from “burst type” to “slow release type” and the nonlinearity of MgO–C refractories mechanics behavior during failure process.

Preparation and characterization of alumina/calcium-hexaluminate ceramic composites from ferrotitanium slag

Low-cost alumina/calcium-hexaluminate (Al2O3-CaAl12O19) ceramic composites were prepared using ferrotitanium slag in this paper. By making use of the TiO2 and MgO originally existing in ferrotitanium slag, the sintering densification of Al2O3-CaAl12O19 composites was promoted. The results show that the optimum sintering temperature of the composites is 1500 ℃. The dominant sintering mechanism is the solid solution mechanism, i.e., Ti4+ and Mg2+ are dissolved in the CA6 and Al2O3 lattices and generate numerous defects, which ultimately enhance the lattice diffusion coefficient and matter transport. Sintering densification improves the specific heat capacity, thermal conductivity, and thermal shock resistance of Al2O3-CaAl12O19 composites. Thermophysical properties analysis indicates that the composites can be potentially used for thermal storage and the slag-utilized ratio is about 60 wt.%. However, the layered cleavage of CA6 limits further improvement of thermal shock resistance.

Fracture behavior and thermal shock resistance of alumina-spinel castables-Effect of added fused zirconia-alumina

This study aims to clarify the influence of fused zirconia-alumina (FZA) on the fracture behavior and thermal shock resistance (TSR) of alumina-spinel castables, and discover the relationship between toughness parameters and TSR. Alumina-spinel castables with different FZA contents (0, 2, 4, 6, 8, and 10 wt.%) were prepared using tabular alumina, fused spinel, calcium aluminate cement, ultrafine alumina powder, and FZA as raw materials. The fracture behavior and TSR of alumina-spinel castables were determined by a wedge splitting test (WST) and a water quenching test, respectively. The results show that the flexibility of alumina-spinel castables can be significantly enhanced by introducing FZA. With increasing FZA content, the load-displacement curves change from typical brittle fracture behavior to non-linear fracture behavior. The flexibility parameter (ratio of GF/σNT) of the castables increases from 8.3 μm to 13.6 μm when FZA content increases from 0 wt.% to 10 wt.%. The flexibility improvement of castables containing FZA can be attributed to the formation of microcracks caused by the transformation of ZrO2. The TSR of alumina-spinel castables can also be significantly improved by adding FZA. The ratio of residual cold modulus of rupture (CMOR) of the castables increases from 12.3% to 34.2% when FZA content increases from 0 wt.% to 10 wt.%. The improvement in TSR can be attributed to a decrease in the stored elastic energy and a subsequent increase in the specific fracture energy of the castables. Generally, alumina-spinel castables with non-linear fracture behavior have good thermal shock resistance. There is a significant correlation between the flexibility parameter (GF/σNT ratio) and the ratio of residual CMOR; the correlation coefficient is 0.96.

Influence of in-situ formed Ba-β-Al2O3 on mechanical properties and thermal shock resistance of ZTA/Ba-β-Al2O3 composites

To improve the mechanical properties and thermal shock resistance of zirconia toughened alumina (ZTA) composites, barium carbonate (BaCO3) was added to yttria-stabilized zirconia (YSZ) and Al2O3 powders to form ZTA/Ba-β-Al2O3 composites, which were prepared by a solid-state reactive sintering method. As BaCO3 content increased, more Ba-β-Al2O3 was formed, resulting in the decreases in relative density and Vickers hardness. The fracture toughness was enhanced with increasing BaCO3 content and reached a peak at 4 wt% BaCO3. The improved fracture toughness is the result of synergistic toughening effects of martensitic transformation of ZrO2 and crack deflection/bridging. After thermal shock tests, the residual strength of ZTA/Ba-β-Al2O3 composites was higher than that of ZTA. The improvement in thermal shock resistance is mainly ascribed to the formation of elongated Ba-β-Al2O3 with a hexagonal structure, which can dissipate the energy associated with crack propagation during thermal shock.

Trace nanoscale Al2O3 in Al2O3-MgAl2O4 castable for improved thermal shock performance

Al2O3–MgAl2O4 castables are prepared using tabular alumina, fused spinel, α-Al2O3 micro-powder, calcium aluminate cement and nano-Al2O3 particles as starting materials. The effects of trace nano-Al2O3 particles (0.2–0.8 wt%) on the thermal shock resistance (TSR) and microstructure of Al2O3–MgAl2O4 castable are investigated. The results show that trace addition of nano-Al2O3 particles contributes to CA6 crystal development and microstructure optimisation. The platelet CA6 crystals form an interlocking network structure creating a toughening effect, which leads to a significant improvement in TSR. In addition, CA6 formation accompanied by volumetric expansion generates many microcracks, which are beneficial for relieving thermal stress during the thermal shock process and enhancing the TSR of the Al2O3–MgAl2O4 castable.

Preparation and thermal shock behavior of nanoscale MgAl2O4 spinel-toughened MgO-based refractory aggregates

Abstract Nanoscale MgAl2O4 (MA) spinel-toughened MgO-based refractory aggregates were prepared by firing lightly calcined magnesia powder and some nanoscale Al2O3 (≤50 nm) particles at high temperature. The effects of various amounts of nanoscale Al2O3 powder on the phase composition, microstructure, physical properties and thermal shock resistance (TSR) of the prepared MgO-based refractory aggregates were investigated. The results showed that the optimum nanoscale Al2O3 powder addition was 10 wt%, and the nanoscale Al2O3 particles reacted with MgO during firing and formed nanoscale MA creating reactive sintering, which contributed to significant increasing in density and cold strength of the prepared aggregates. The thermal mismatch between nano-MA and MgO could produce microcracks created toughening effect, leading to improvement in TSR of the prepared aggregates. And the formed nano MA grains pinned in MgO grain boundaries and on MgO grain surfaces would lead to microcracks deflection and absorbing much more fracture energy, further improving the TSR. The obtained aggregates were trial used in MgO-C slide plate materials, and thermal shock resistance of the materials was improved.

The effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings

The aim of the present research was to investigate the effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings. In this regard, specimens of Inconel 718 were sprayed with Ni–22Cr–10Al–Y bond coat and Al2O3/YSZ top coat using an atmospheric plasma spraying approach. Then, top surfaces of specimens were under exposed to Nd-YAG laser radiation and thereby were glazed. Thermal stabilities of both coatings were evaluated by rising the environment temperature up to 1000 °C, keeping at this temperature for 15 min, and also rapid cooling of specimens in cold water. Using FESEM with EDS and XRD, the microstructure and morphology observations, elemental investigations and also phase identification were carried out. Findings displayed that both sprayed and glazed coatings were damaged as a spallation of top coats. Furthermore, by checking the life-time of coatings, the number of thermal cycles led to 20% destruction of top surfaces of coatings, increased from 232 cycles in sprayed coating to 325 cycles in laser glazed Al2O3/YSZ multilayer coating which displayed 40% improvement of thermal shock resistance after laser glazing process. This enhancement in resistance to thermal shock was due to strain adaptability improvement via a network of segmented cracks on the top surface produced by laser glazing process.

Thermal shock behavior of mixed ytterbium disilicates and ytterbium monosilicates composite environmental barrier coatings

The thermal shock properties of mixed Yb2Si2O7 and Yb2SiO5 composite environmental barrier coating (EBC) topcoat, with various Yb2Si2O7-to-Yb2SiO5 ratios, are investigated between room temperature and 1500 °C. The results show that, with more Yb2Si2O7 introduced, the mixed Yb2Si2O7 and Yb2SiO5 composite topcoats exhibit an increasing thermal shock property. This may be attributed to that, while Yb2Si2O7 mole fraction increases, the thermal expansion coefficient of the mixed Yb2Si2O7 and Yb2SiO5 composite ceramics becomes smaller, more matched to substrates and the fracture toughness of the mixed Yb2Si2O7 and Yb2SiO5 composite ceramics becomes larger, both of which are favoring the improvement of thermal shock resistance.

Optimal design on the high-temperature mechanical properties of porous alumina ceramics based on fractal dimension analysis

Fractal theory and regression analysis were employed for the first time to investigate the effect of pore size and pore distribution on high-temperature mechanical properties of porous alumina ceramics (PAC). In the present work, PAC with the comparable porosity, different pore sizes and pore distributions were prepared using carbon black as the pore-forming agent. Particular emphasis in this study was placed on the establishment of correlation between the thermal shock resistance and pore properties. The relationship between fractal dimension (Df) andthermal shock resistance parameter (Rst) in specimens presented the negative power function, indicating that low Df could benefit the improvement of thermal shock resistance in specimens. The results showed that the increase of pore size and pore sphericity leads to a reduced Df, the enhanced hot modulus of rupture (HMOR) and. The decrease of proportion of micro-pores below 2 μm, the increase of mean pore size and pore sphericity could result in the decrease of Df, and then improve Rst and HMOR of specimens. Based on the correlation between Rst and pore characteristics, PAC with improved thermal shock resistance could be achieved when their pore structure meets the above features.

Tailoring the thermal shock resistance of titanium carbide by reinforcement with tungsten carbide and carbon nanotubes

Titanium carbide (TiC) is reinforced with 3.5wt% of tungsten carbide (WC) and 2wt% of carbon nanotubes (CNTs) in order to improve its thermal shock resistance and mechanical integrity of oxide layer. Three pellets namely, TiC (T-SD), TiC-3.5wt% WC (TW-SD) and TiC-3.5wt% WC-2wt% CNT (TWC-SD) were thermally shocked at 1700°C in an open atmosphere for 10 numbers of thermal cycles. Raman spectra confirmed the retention of CNTs after thermal cycling, which could be attributed to the thermal shielding offered by the molten TiC to CNTs. HR-TEM image infers the collapse of some of the CNTs into carbon structure after the thermal shocks. The mass loss % of T-SD, TW-SD and TWC-SD pellets were found to be 41.5%, 33.3% and 15.13%, respectively after the 10 numbers of thermal cycles. The rate of mass change in T-SD sintered pellet was found to be ~1.4 × 10−3g/min., which got reduced to ~0.4 × 10−3g/min. on addition of 3.5wt% of WC and 2wt% of CNT. Furthermore, reinforcement of 3.5wt% WC and 2wt% CNTs in TiC matrix showed the dramatic increase of 89% in thermal shock resistance parameter (Rst) compared to T-SD pellet. This significant improvement in thermal shock resistance of TWC-SD pellet is mainly attributed to the following three factors viz. (i) higher fracture toughness (ii) higher density and (iii) homogeneous dispersion of CNTs, which hinders the movement of oxygen atoms along the grain boundaries. Hardness of the thermally shocked TWC-SD pellet was found to be 5.1GPa, which was 1.3GPa i.e. ~74% higher compared to thermally shocked T-SD pellet. This increased hardness of thermally shocked TiC matrix has been shown to suppress the crack formation and restrict oxidation.

Characterisation and properties of geopolymer composites. Part 2: Role of cordierite-mullite reinforcement

Our previous research paper on geopolymer-mullite composites showed promising results on compressive strength and fire resistance. However, no improvement in thermal shock resistance was observed in the afore mentioned study. In this study, further attempts to improve thermal shock resistance of the geopolymer were explored. The research was performed by compositing a fly ash-based geopolymer with cordierite-mullite at 20, 40 and 60wt% replacement. X-ray diffraction (XRD) of the cured geopolymer composite specimens showed the existence of cordierite, mullite, quartz, cancrinite and lazurite. It was found that compressive strength and strength retention after thermal exposure at 400°C were improved in the geopolymer composite specimens, especially those with 20–40wt% replacement. Upon further heating to 600°C, all geopolymer specimens showed insignificant differences in compressive strength. Fire resistance was found to improve with increasing proportion of replacement contents.

Thermal Shock Resistance of Si3N4/h-BN Composites Prepared via Catalytic Reaction-Bonding Route

Si3N4/h-BN ceramic matrix composites were prepared via a catalytic reaction-bonding route by using ZrO2 as nitridation catalyst, and the water quenching (fast cooling) and molten aluminum quenching tests (fast heating) were carried out to evaluate the thermal shock resistance of the composites. The results showed that the thermal shock resistance was improved obviously with the increase in h-BN content, and the critical thermal shock temperature difference (ΔT c) reaches as high as 780 °C when the h-BN content was 30 wt.%. The improvement of thermal shock resistance of the composites was mainly due to the crack tending to quasi static propagating at weak bonding interface between Si3N4 and h-BN with the increase in h-BN content. For the molten aluminum quenching test, the residual strength showed no obvious decrease compared with water quenching test, which could be caused by the mild stress condition on the surface. In addition, a calculated parameter, volumetric crack density (N f), was presented to quantitative evaluating the thermal shock resistance of the composites in contrast to the conventional R parameter.

Synergic effects of nano carbon sources on thermal shock resistance of Al2O3-C refractories

Abstract The fracture behavior and thermal shock resistance of low-carbon containing Al 2 O 3 -C refractories with various nano carbon sources were comparatively investigated by means of three-point bending test and wedge splitting test. The results show that synergic enhancing mechanisms, including “pull-out”, “crack bridging”, “crack deflection” and “crack branching”, can be developed with the application of multiple nano carbon sources. Therefore, the carbon black, expanded graphite, in-situ multi-walled carbon nanotubes and SiC whiskers containing Al 2 O 3 -C refractories have the highest residual strengths of 11.16 MPa. Furthermore, the wedge splitting test results suggest that the improvement of thermal shock resistance is mainly associated with an increase of crack propagation resistance, which can be fulfilled through decreasing the brittleness and inducing tortuous crack propagation along grain/matrix. Additionally, it is demonstrated that the wedge splitting test after thermal shock treatment should be a feasible way to evaluate the thermal shock cycling resistance of Al 2 O 3 -C refractories.

Thermal shock behavior of rare earth modified alumina ceramic composites

AbstractAlumina matrix ceramic composites toughened by AlTiC master alloys, diopside and rare earths were fabricated by hot-pressing and their thermal shock behavior was investigated and compared with that of monolithic alumina. Results showed that the critical thermal shock temperature (ΔT) of monolithic alumina was 400 °C. However, it decreased to 300 °C for alumina incorporating only AlTiC master alloys, and increased with further addition of diopside and rare earths. Improvement of thermal shock resistance was obtained for alumina ceramic composites containing 9.5 wt.% AlTiC master alloys and 0.5 wt.% rare earth additions, which was mainly attributed to the formation of elongated grains in the composites.