Number Of Thermal Cycles Research Articles

Study on properties of pre-alloy powder of TiNiHf high temperature shape memory alloy for additive manufacturing by preheating treatment

In this work, pre-alloyed powder high-temperature shape memory alloy (Ti30Ni50Hf20) for additive manufacturing was preheated at 300–800°C and thermoscycled at 650°C to examine the influence of preheating temperature and thermal cycle number on the phase composition and phase transformation behavior. The results showed that the H phase precipitation occurred in the matrix with the rising preheating temperature within 300–600°C, and the phase transformation temperature of the pre-alloyed powders also increased. When the preheating temperature exceeded 600°C, the H phase dissolved back into the matrix, and the phase transformation temperature decreased. A total of 50 thermal cycles were conducted on the pre-alloyed powders at 650°C, and the phase transformation temperature and phase composition remained stable after cycling. It can be concluded that the preheating temperature greatly affected the phase composition and phase transformation behavior of Ti30Ni50Hf20 high-temperature shape memory alloy pre-alloyed powders, while the number of thermal cycles delivered a minor effect.

Can Chitosan Be Depolymerized by Thermal Shock?

Chitosan is a biopolymer with a wide range of applications. It typically requires depolymerization to achieve a desired molecular weight for specific uses. This study investigated the potential for depolymerizing chitosan by thermal shock and grinding to produce nanochitosan. A series of thermal shock cycles combined with grinding were performed to assess the influence of drying temperature, residence time, and number of thermal cycles on the molecular weight, particle size, and crystallinity of chitosan. The thermal shock reduced the molecular weight and particle size of chitosan within the first hour of treatment, with optimal conditions achieved at a drying temperature of 90 °C and residence time inside the oven of 5 min. These conditions resulted in a molecular weight of 15.0 kDa with an average diameter of 136 nm. Thermal shock can be considered an effective method for chitosan depolymerization with grinding serving to standardize the particle size. This optimized process offers promising applications where low-molecular-weight chitosan is required, including biomedical, agricultural, and food industries, as well as the potential for reducing time and energy consumption.

Effect of thermal shocks on the interfacial bond strength of sandwich composites built with rigid and soft materials produced through extrusion process

The mechanical behavior and adhesion strength of the sandwich composite, which consists of a flexible and a rigid material, were examined using extruded polylactic acid (PLA) and thermoplastic polyurethane (TPU). Three types of binders were combined with the appropriate hardeners to bond the extruded materials. The fabricated specimens are subjected to a different number of thermal shock cycles, with a maximum and minimum temperature of 80 °C and −18 °C, respectively. The sandwich composite's mechanical properties, including tensile, flexural, and impact strength, were assessed after 20, 40, and 80 cycles of thermal shock. The results indicated that the ductility of PLA material was significantly impacted by the thermal shock cycles. After completing the maximal thermal shock cycles, the composites joined with polyester resin exhibited a lower strength reduction in the impact specimens. Maximum tensile strength of 22.3 MPa, flexural strength of 31.6 MPa, and impact strength of 8.3 J were recorded prior to the application of thermal shocks to the specimens. The specimens that were bonded with epoxy resin recorded the maximal tensile, flexural, and impact strength. Following the thermal shock cycles, the composites bonded with epoxy resin exhibited a lesser reduction in tensile and flexural strength, while the composites bonded with polyester resin showed a lower strength reduction for impact specimens. The tensile and flexural specimens encountered a 7.54 % and 20.24 % reduction in strength, respectively, after undergoing 80 cycles of thermal shocks. The impact specimens experienced an 8.62 % reduction in strength.

THz-TDS characterization of stress evolution of EB-PVD TBCs under thermal cycling

The failure of thermal barrier coatings (TBCs) during service is a critical issue affecting aeroengine efficiency. In this study, electron beam physical vapor deposition (EB-PVD) TBCs were subjected to thermal cycling at 1100 °C. Terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the relationship between the evolution of characteristic frequency and the number of thermal cycles, along with the development of stress in the ceramic layer. In situ tensile test revealed a linear correlation between the characteristic frequency and micro strain derived from THz-TDS data, with a proportional coefficient of −9.94 between the characteristic frequency and the slope of the refractive index curve. The results show that the stress evolution can be effectively monitored by recording the trend of characteristic frequency. During the non-failure stage, the characteristic frequency followed a parabolic trend as thermal cycles increased, influenced by the growth of thermally grown oxide (TGO), ceramic layer sintering, and thermal mismatch. Approaching the failure stage, stress evolution was dominated by the expansion of vertical cracks in the ceramic layer and the widening of transverse cracks at the interface, which corresponded to a sudden change in the characteristic frequency.

Small-scale physical modelling of vertically loaded, cyclically thermally-activated helical piles

To investigate the use of thermally-activated helical piles in shallow geothermal energy systems, a 1-g modelling study was conducted. Helical piles with either a single- or double- helix were installed in a medium dense, dry sand, and subjected to mechanical, thermal only and thermo-mechanical loading. The results indicate that during the thermal tests (1 – 3 cycles), a small upwards residual displacement was observed and pile head movements ranged between about 90% and 100% of the free expansion of the pile shaft above the shallowest helix, suggesting that the helices fixed the shaft and little restraint was offered by the surrounding soil. In the thermo-mechanical tests (30 thermal cycles), the pile head developed irrecoverable settlement as a function of the number of helices (more helices, less settlement) and initial load (higher load, greater settlement). No significant alteration in pile axial stiffness or resistance was found for piles with zero mechanical load that underwent only a few thermal cycles; however, an increase in stiffness and resistance, beyond that due to inherent variability in the test setup, was observed for piles with an initial load and following a large number of thermal cycles. The testing of thermally-activated helical piles in sand has confirmed that the response is similar to conventional piles and that thermal ratcheting effects can be managed by the application of suitable margins of safety in design and/or the use of multi-helix piles.

Synthesis and thermophysical properties of rare-earth tantalate ZrO2-Dy3TaO7 ceramics

Abstract Thermal Barrier Coatings (TBCs) are of major concern to researchers because of their outstanding properties, such as high-temperature resistance, compressive resistance, hardness, and toughness. Its applications are wide-ranging and critical. Examples include hypersonic vehicles, aero-engines, and service in high-temperature and high-impact extreme environments. This work focuses on the preparation, thermal properties (coefficient of thermal expansion, thermal conductivity), and performance evaluation (thermal shock resistance, thermal shock resistance, and bonding) of ZrO2-Dy3TaO7 rare earth tantalate ceramic coating materials. The results show that ZrO2-Dy3TaO7 possesses a low thermal conductivity (1.25-1.50 W K−1 m−1, 900°C), a high thermal expansion coefficient (10.80-11.14×10-6 K−1, 1200°C), which is about half of the value of the thermal conductivity of 7YSZ (2.8-2.2 W K−1m−1). It is better than that of the 7YSZ (10.04×10−6 K−1, 1200°C), which meets the performance requirements of the new thermal barrier coating. In addition, ZrO2-Dy3TaO7 can achieve more than 6, 000 thermal fatigue cycles at 1200°C, and the ideal number of thermal shock cycles is 4, 040 cycles at the same temperature. These outstanding test results demonstrate that the ceramic coating can effectively protect the substrate material and provide long-term service in high-temperature copper liquid. This research lays the theoretical foundation for the development of next-generation coating materials, particularly for thermal barrier/environmental barrier integrated coating materials.

Thermophysical-mechanical behaviors of hot dry granite subjected to thermal shock cycles and dynamic loadings

Exploring dynamic mechanical responses and failure behaviors of hot dry rock (HDR) is significant for geothermal exploitation and stability assessment. In this study, via the split Hopkinson pressure bar (SHPB) system, a series of dynamic compression tests were conducted on granite treated by cyclic thermal shocks at different temperatures. We analyzed the effects of cyclic thermal shock on the thermal-related physical and dynamic mechanical behaviors of granite. Specifically, the P-wave velocity, dynamic strength, and elastic modulus of the tested granite decrease with increasing temperature and cycle number, while porosity and peak strain increase. The degradation law of dynamic mechanical properties could be described by a cubic polynomial. Cyclic thermal shock promotes shear cracks propagation, causing dynamic failure mode of granite to transition from splitting to tensile-shear composite failure, accompanied by surface spalling and debris splashing. Moreover, the thermal shock damage evolution and coupled failure mechanism of tested granite are discussed. The evolution of thermal shock damage with thermal shock cycle numbers shows an obvious S-shaped surface, featured by an exponential correlation with dynamic mechanical parameters. In addition, with increasing thermal shock temperature and cycles, granite mineral species barely change, but the length and width of thermal cracks increase significantly. The non-uniform expansion of minerals, thermal shock-induced cracking, and water-rock interaction are primary factors for deteriorating dynamic mechanical properties of granite under cyclic thermal shock.

Analysis of BGA Lead-Free solder joints failure behavior based on thermal shock testing

The interconnected solder joints within automotive electronic components are the most vulnerable parts of the structure, and their reliability is crucial for the development of new energy vehicles. This study focuses on automotive ADAS (Advanced Driver Assistance System) devices, analyzing the failure behavior of internal BGA (Ball Grid Array) lead-free solder joints under thermal shock conditions.Through thermal shock testing from −40℃ to 95℃, it was observed that microcracks first appeared in the upper intermetallic compound (IMC) layer of corner solder joints after 1600 cycles. As the number of thermal shock cycles increased, the net-like β-Sn phase in the solder joints gradually transformed into block and dot-like structures. Due to the relatively weak strength between the IMC layer and the Ag-Sn compound region, cracks typically propagated along the boundaries of these areas. Recrystallization occurred in the interface between the solder joint and the upper pad, with the proportion of high-angle grain boundaries increasing from 10.74% to 72.43%. The crack propagation mode changed from initial intergranular to transgranular fracture. Throughout the thermal shock test, the upper IMC layer remained thicker than the lower one, with the lower IMC layer thickness showing a minimum point at 750 cycles.The average thrust force between the solder joints and the substrate decreased more rapidly with thermal shock cycles, indicating that non-solder mask defined pads are more effective in improving structural strength.

Evaluation of pore characteristics evolution and damage mechanism of granite under thermal-cooling cycle based on nuclear magnetic resonance technology

During the development process of deep geothermal energy (Hot dry rock, HDR), the high-temperature granite matrix in the wellbore needs to withstand thermal-cooling cycles, easily leading to wellbore instability and collapse. Considering the actual drilling engineering using different types of drilling fluid, thermal-cooling cycle experiments (cycles of 1, 2, 4, 8, 12 times) of granite were conducted using natural, water, and liquid nitrogen (LN2) cooling methods. Based on the low-field nuclear magnetic resonance measurement, the development and evolution of pore size, quantity, and distribution under different cooling ways and thermal cycles were studied, and the changes in rock mechanical parameters under different cooling methods and thermal cycling conditions were quantified. Then, the rock damage coefficients based on the variation patterns of different types of pores were calculated, and a quantitative relationship was established among T2 spectrum, pore, and mechanical damage. Finally, the proton weighting method was used to analyze the changes in the internal bearing area of rocks, revealing the deterioration mechanism of rock mechanical properties under different cooling ways and thermal cycling conditions. The results showed that the porosity increased with thermal cycles during the thermal-cooling cycle treatment, and the growth rate was first fast and then slow. Under the same number of thermal cycles, the groundwater cooling porosity was the highest, followed by LN2 and natural cooling. The relationship between T2 spectral area and rock damage was established on this basis, and the rock damage value increased with thermal cycles in an exponential function, and the proportion of large pores significantly affected the pore damage. A relationship (positively proportional linear) between pore damage and the mechanical degradation coefficient of high-temperature cooling cycle granite and a method for predicting rock degradation through pore damage were established, the deterioration mechanism of rock mechanical properties under thermal cycle treatment was revealed.

Effect of Rotational Shear and Heat Input on the Microstructure and Mechanical Properties of Large-Diameter 6061 Aluminium Alloy Additive Friction Stir Deposition

Additive friction stir deposition (AFSD), in which molten metal materials are formed into free-form stacked structural parts according to the path design, may have a wide range of applications in high-efficiency mass production. In this study, experiments were conducted for the rotational speed in the AFSD parameters of 6061 aluminium alloy bars to investigate the effects of different rotational shear conditions and heat inputs on the properties of the deposited layer for diameter bars based on the analysis of the micro-morphology, micro-tissue composition, and mechanical properties. The width and thickness of each layer were constant, approximately 40 mm wide and 2.5 mm thick. The particle undulations on the surface of the deposited layer were positively correlated with the AFSD rotational speed. Continuous dynamic recrystallisation in the AFSD process can achieve more than 90% grain refinement. When the rotational speed increases, it causes localised significant orientation and secondary deformation within the recrystallised grains. The ultimate tensile strength of the deposited layer was positively correlated with the rotational speed, reaching a maximum of 211 MPa, and the elongation was negatively correlated with the rotational speed, with a maximum material elongation of 37%. The cross-section hardness of the deposited layer was negatively correlated with the number of thermal cycles, with the lowest hardness being about 45% of the base material and the highest hardness being about 80% of the base material.

Thermal shock resistance of carbon-based composites reinforced by HfC nanowires under extreme conditions

Lightweight composites with excellent thermal shock performance exhibit significant potential for thermal structural components. Herein, HfC nanowires modified carbon/carbon (HfCNWs-C/C) composites are fabricated, establishing a three-dimensional network within the matrix. The microstructure of the composites before and after thermal shock cycles were characterized by SEM, XRD and TEM. Additionally, the flexural strength with varied thermal shock were analyzed, which were initially increasing before subsequently decreasing with the number of thermal shock cycles under Ar atmosphere, exhibiting thermal shock strengthening phenomena. While within oxy-acetylene system, the strength of HfCNWs-C/C exhibit an increasement of 75 % compared with C/C after ablation for 6 cycles, which can be attributed to the sintering and the generation of molten film of partial HfO2 phases that can mitigates oxidation of the matrix, but still lead to a declining trend in the strength following ablative thermal cycles.

Effect of Fe Composition and Heat Treatment Temperature on the Transformation Temperature of R-Phase of Ti-Ni-Fe Alloy

R-phase transformations in TiNi shape memory alloys (Nitinol) have various applications because of their small thermal hysteresis and low fatigue resistance. Adding a third element such as Fe or Al is one of the useful ways to induce R-phase transformation. Controlling the R-phase transformation temperature is crucial for industrial and bio-field applications. However, the effects of adding Fe have rarely been reported. In this study, the effects of Fe addition and heat treatment temperature on the R-phase transformation temperature of TiNiFe shape memory alloys were systematically investigated. Results showed that increasing Fe composition decreases the R-phase and martensitic transformation temperatures of 19K/1at%Fe and 51 K/1at%Fe, respectively. Additionally, in the Ti–49Ni–1Fe alloy, rather than a martensite transformation, the R phase transformation temperature was constant irrespective of heat treatment temperature and increasing number of thermal cycles. This means that the R-phase transformation temperature is not affected by dislocation density resulting from the heat cycling or cold working processes. This allows practical applications of R-phase transformation to be easily realized. Furthermore, this means that the R-phase transformation temperature is only affected by the Ni content of the matrix.

Effect of construction angles on the microstructure and mechanical properties of LPBF-fabricated 15-5 PH stainless steel

In this study, 15-5 PH stainless steel were fabricated by laser powder bed fusion (LPBF) at different construction angles (0°, 30°, 45°, 60°, and 90°). Effects of construction angles on the microstructure, mechanical properties, electrochemical properties and surface characterization of 15-5 PH stainless steel were investigated. The LPBF-fabricated 15-5 PH stainless steel exhibited a “track-track” overlap trajectory on the horizontal plane, and the vertical plane displayed the “layer-layer” overlap of the melt pool. All samples were composed of martensitic and austenite phases. Irregular grains and random grain orientation were found in each sample. In fact, the construction angle has an important role in the microstructure. The 0° sample has the lowest martensite content (75.8%), since fewer deposited layers correspond to less number of thermal cycles. Due to the largest amount of martensite (90.1%), the 45° sample exhibits the highest hardness of 308.9HV0.2. The similar tensile strength and yield strength are achieved for samples fabricated at different forming angles. Layer by layer melt pool boundary (MPBs) contributes to the ductile deformation, whereas track by track MPBs is favorable for brittle deformation. Samples with a larger construction angle have more layer by layer MPBs, resulting in the increased elongation. Therefore, the 0° sample shows a minimum elongation of 20.1%, but the 90° sample reaches a maximum elongation of 23.9%. The construction angle affects the corrosion resistance of the 15-5 PH SS by influencing the martensite content. As a result, the 0° sample, with the highest austenite content (24.2%), shows the best corrosion resistance and lowest roughness.

Effect of thermal cycling on the mechanics and microstructure of ultra-high performance concrete

This study investigates the mechanical and microstructural responses of Ultra-High Performance Concrete (UHPC) subjected to a substantial number of thermal cycles upto 300 times from ambient temperature to 60°C. The experimental findings reveal a distinctive pattern of behavior in the static compressive strength, characterized by an initial increase, followed by a subsequent decline, and a subsequent modest resurgence as the thermal cycling progressed. Notably, the peak compressive and flexural strengths were attained after 120 thermal cycles, whereas the maximum dynamic compressive strength was observed after 60 thermal cycles. It is worth highlighting that the dynamic increase factor (DIF) exhibited an inverse trajectory in comparison to the static compressive strength. After 120 thermal cycles, the stress-strain curves under impact loads exhibited pronounced strain softening characteristics, with an oscillation state duration of 250μs, which surpassed that of the ascending and descending phases. The observed variances in macroscopic mechanical properties can be attributed to several pivotal factors, including the quantity of hydration product, the mean chain length (MCL) of the calcium-silicate-hydrate (C-S-H) gel, the distribution of pore volume, and the presence of micro-cracks. This comprehensive examination serves as a valuable theoretical and empirical foundation for advancing the utilization of UHPC in applications subjected to prolonged and intricate thermal conditions.

Effects of Thermal Cycling on the Mechanical Strength of TPU 3D-Printed Material

Fused Deposition Modelling (FDM) is a three-dimensional (3D) printing technology known for its low-cost rapid manufacturing of parts. Nowadays, various industries such as automotive, aerospace, and maritime are using this technology to manufacture 3D-printed parts that have undergone high temperatures. The material used in this study is the Thermoplastic Polyurethane (TPU), which is the most commonly-used type of Thermoplastic Elastomer (TPE) in 3D printing. This material is a combination of substances from the qualities and characteristics of both thermoplastic and vulcanized thermoset rubber. TPU has excellent abrasion resistance, hardness, chemical, and thermal resistance properties. In addition, TPU is a great fit for making hoses, gaskets, and seals due to its oil and grease resistance properties. Due to the growing application of 3D-printed materials at elevated temperatures, this study aims to characterize the tensile strength of TPU 3D-printed materials when thermal cycled. The test results concluded that the tensile properties of TPU 3D-printed specimens were significantly influenced by the number of thermal cycles it was subjected to. The samples that underwent four thermal cycles exhibited the highest modulus of elasticity and stress at 200% strain. While samples which underwent 2, 8, and 16 thermal cycles resulted to a higher modulus of elasticity and tensile stress at 200% strain than the untreated specimen.

Study on thermal cycling ageing resistance of performances of polypropylene/elastomer/boron nitride nanocomposites for high voltage direct current cable insulation

AbstractThe influences of thermal cycling ageing on structures and insulation performances of polypropylene (PP)/elastomer/boron nitride (BN) nanocomposites are investigated. The Melt blending method was used to prepare the nanocomposites, in which propylene‐based elastomer (PBE) or ethylene‐octene copolymer elastomer (EOC) was contained for comparison. Then, the samples were treated using a thermal cycling process with a temperature range from −30 to 150°C, and the number of thermal cycles was set from 0 to 15. Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry and X‐ray diffraction measurements were taken to facilitate the comprehension of structural changes. Additionally, measurements were taken to assess the trap distribution and direct current (DC) breakdown strength of the samples. The obtained results revealed that following thermal cycling ageing, the morphology and crystallinity of PP/PBE/BN remained almost unchanged. In contrast, PP/EOC/BN exhibited substantial microstructural damage accompanied by a significant reduction in crystallinity. As the number of thermal cycles increased, the trap level and DC breakdown strength of PP/PBE/BN were maintained at high levels, while those of PP/EOC/BN initially remained stable but then experienced a sharp decline. It is suggested that the addition of BN nanoparticles enhances the thermal cycling ageing resistance of the PP/PBE blend, whereas it weakens this resistance of the PP/EOC blend.

Effect of Thermal Cycles and Curing Age on Bonding Strength of Cement Mortar Using Manufactured Sand

The bonding of cement mortar to the concrete substrate is crucial in buildings. In this study, cement mortar was prepared using manufactured sand (MS) instead of river sand (RS). The bonding strength between manufactured sand mortar (MSM) and concrete substrate was evaluated and the effects of curing age, water-to-cement ratio (w/c) and thermal cycling on the bonding strength were discussed and compared with those of the river sand mortar (RSM). The compressive strength of the MSM was consistently higher than that of the RSM, while the bonding strength of RSM was consistently higher than that of MSM, indicating that the bonding strength does not depend on the compressive strength of the mortar. As the number of thermal cycles increased, the pull-off strengths at the interface between the concrete and MSM or RSM at different w/c ratios all decreased, and the RSM experienced a larger reduction. After 400 cycles, the percentage decrease in bonding strength of MSM sample ranged from 18.62% to 30.86%.

Development of a duplex droplet digital PCR assay for the detection of Burkholderia cepacia complex and Stenotrophomonas maltophilia in bloodstream infections.

Burkholderia cepacia complex (BCC) and Stenotrophomonas maltophilia are nosocomial pathogens that cause various infections and exhibit high resistance to multiple antimicrobial agents. In this study, we aimed to develop a duplex droplet digital PCR (ddPCR) assay for detecting BCC and S. maltophilia in bloodstream infections. We optimized the experimental conditions by setting the annealing temperature to 51°C and determining the optimal concentrations of primers and probes, as well as the thermal cycle numbers. The feasibility of the duplex ddPCR reaction system with the optimal conditions was established and verified through parallel reactions with reference strains of BCC and S. maltophilia. The specificity of the assay, tested with 33 reference strains, was found to be 100%. The duplex ddPCR assay demonstrated good repeatability and could detect as low as 5.35 copies/reaction of BCC and 7.67 copies/reaction of S. maltophilia. This level of sensitivity was consistent in the simulated blood and blood bottle samples. We compared nucleic acid extraction methods and found that the Chelex-100 boiling method and kit extraction method exhibited similar detection sensitivity, suggesting the potential application of the Chelex-100 boiling method in the ddPCR assay. In the clinical samples, the duplex ddPCR assay accurately detected BCC and S. maltophilia in 58 cases. In conclusion, our study successfully developed a duplex ddPCR assay that provides accurate and convenient detection of BCC and S. maltophilia in bloodstream infections.IMPORTANCEBurkholderia cepacia complex (BCC) and Stenotrophomonas maltophilia are implicated in a wide range of infections, including bloodstream infections (BSIs), pneumonia, and meningitis, and often exhibit high intrinsic resistance to multiple antimicrobial agents, limiting therapeutic options. The gold standard for diagnosing bloodstream infections remains blood culture. However, current blood culture detection and positivity rates do not meet the "rapid diagnosis" required for the diagnosis and treatment of critically ill patients with BSIs. The digital droplet PCR (ddPCR) method is a potentially more powerful tool in the diagnosis of BSIs compared to other molecular methods due to its greater sensitivity, specificity, accuracy, and reproducibility. In this study, a duplex ddPCR assay for the detection of BCC and S. maltophilia in BSIs was developed.

Thermal shock effects on the microstructure and mechanical properties of iron-based composites reinforced by in-situ Fe2B/FeB ceramic phases

Recently, high-temperature composites reinforced with in-situ phases have been developed as a new class of composites for harsh environments. The behaviour of these composites under sudden heating and cooling conditions is greatly influenced by the type and fraction of components. The aim of this study is to investigate the microstructural properties and mechanical response of iron based composites after thermal shock cycling. The fabrication process involved in-situ powder metallurgy (IPM) sintering of Fe–B4C starting powders. The samples were subjected to thermal shock with a constant temperature differences (ΔT: 600 °C) and varied cycles (N: 1–50). Comparison of microstructural changes and mechanical behaviour before and after thermal shock has been carried out. The boron atoms released from the initial B4C generated diffusion zones in the matrix dominated by iron boride phases (Fe2B/FeB). An effective matrix-reinforcement interface has been created by in-situ powder metallurgy, where boride phases are almost homogeneously distributed in the iron matrix. While the hardness of the composites was significantly developed by the Fe2B/FeB phases, the addition of B4C resulted in a reduction in the coefficient of thermal expansion (CTE). The in-situ phases contributed in the minimisation of the coefficient mismatch by balancing the thermal expansion coefficients between iron and the initial B4C. Under the influence of thermal shock, microcracks and then macrocracks formed on the sample surfaces, increasing in width and depth with thermal cycles. This resulted in accelerated fracture damage under post-thermal loading. Both the impact and flexural strengths of the samples were significantly reduced as the number of thermal cycles increased. Although the 20% B4C reinforced composite provided maximum hardness, it showed poor resistance to thermal loading. The best results for mechanical loads were achieved by the 5% B4C reinforced composite, which showed no cracks even after 50 thermal cycling.

Thermal fatigue failure mechanism in the joint of nano Cu/Ti–Si3N4 ceramic substrates after thermal fatigue test

Thermal stress fatigue failure caused by mismatch in the coefficients of thermal expansion (CTE) between the metal layer and ceramic sheet during a high and low temperature cycle is considered as the big issue of high-power ceramic substrates. The aim of this study is to explore the thermal fatigue mechanism of nano Cu/Ti–Si3N4 ceramic substrates after thermal fatigue test. Thermal fatigue test has been performed by thermal cycles experiment from −30 °C to 150 °C. After thermal cycles, the microstructure of the interface is observed by SEM and TEM. The results show after 400 thermal cycles, the nano-Cu metal layer oxidizes severely. Various oxides are found in the corrosion zone, such as CuO, CuAlO2 and CuAl2O4. Higher dislocation density and nanoscale-twins are observed in Cu or Cu2O, which indicate that Cu and Cu2O alleviates thermal stress. As the number of thermal cycles increases, CuO CuAlO2, CuAl2O4 as a corrosion layer in the corrosion zone were formed. The corrosion layer is difficult to alleviate interfacial thermal stress, and easy to form cracks in the joint zone.