Thermal shock damage assessment of a BNW/SiO2 ceramic: Insight from temperature-dependent material properties

The high-temperature service performance of nearly fully dense 20 wt% BN_W/SiO₂ ceramic was systematically investigated. The oxidation damage and strength degradation of the whiskers combined with the surface microstructures of the samples predominantly influence the flexural strength from RT to 1000 °C. In previous work, the temperature dependence of the material properties is invariably ignored when evaluating thermal stress crack initiation and propagation behaviour. In this work, modified thermal shock models that include temperature-dependent material properties were established based on thermal-shock fracture (TSF) theory and thermal-shock damage (TSD) theory. Then, the thermal shock resistance (TSR) of the BN_W/SiO₂ ceramic was evaluated by preforming a water quenching test. The modified models could better explain the TSR behaviour of the ceramic, indicating that considering the temperature-dependent material properties will reveal the thermal shock damage mechanism more precisely.     

Thermal shock resistance of rare-earth doped in-situ SiAlON reinforced h-BN matrix ceramics under vacuum thermal cycling

In-situ SiAlON reinforced BN-matrix ceramics were prepared by hot pressing sintering, and the effects of different rare earth oxides on the thermal shock resistance of the materials were investigated. The effects of rare earth oxides on the phase composition, microstructure, bending strength, thermal properties and thermal shock resistance of the composites were studied. The results show that the phase composition and bending strength of ceramics with different rare earth oxides had no obvious change. However, the influence on the thermal expansion coefficient of the material was notable. The thermal expansion coefficient of the ceramics with CeO₂ increased by 24.6% compared with Sm₂O₃ in the test temperature range. After 50 cycles of thermal shock at Δt = 1150 °C, the residual strength of ceramics with CeO₂ was down to 157.1 MPa, decreased by 40.6% compared with the one tested in room temperature. And the Sm₂O₃-added ceramics reduced by 34.7%–167.1 MPa after thermal shock. The decrease of the residual strength of ceramics is mainly caused by the internal stress generated by the mismatch between the growth of quartz and SiAlON phase in the matrix and the thermal expansion coefficient of the matrix. However, no macro cracks were observed on the surface of the samples after thermal shock.     

纳米BN复合SiC材料的抗热震性研究

将分析纯H3BO3、CO(NH2)2按物质的量比为1∶2.5溶于无水乙醇中,搅拌过程中按质量分数为80%加入平均粒径0.2μm的β-SiC,在850℃氮气中(纯度99.99%,压力为0.92~0.93 MPa)反应15 h制得纳米BN复合SiC粉体,然后在0.92~0.93 MPa N2气氛中以30 MPa轴向压力于1 750~1 800℃保压0.5~1 h热压烧结上述粉体,制成纳米BN复合SiC试样,采用三点弯曲及透射电镜、扫描电镜等方法研究了纳米BN复合SiC材料的抗热震性。结果表明:在SiC材料中引入纳米BN,一方面可以降低材料的弹性模量,有利于抗热震性的提高。另一方面由于基体SiC与第二相六方氮化硼(h-BN)的热膨胀系数相差较大,热失配作用导致h-BN晶粒发生晶间脱层,在复相陶瓷材料内产生许多微孔,这些微孔的存在可以有效缓解由于高温引起的热膨胀作用,从而极大地改善材料的抗热震性。     

Thermal shock behavior of multilayer and functionally graded micro- and nano-structured topcoat APS TBCs

Performance of air plasma sprayed (APS) thermal barrier coatings (TBCs) with multilayer and functionally graded topcoat were investigated in thermal shock conditions. Ceria-yttria stabilized zirconia (CSZ) and micro- and nano-structured yttria stabilized zirconia (YSZ and YSZ-N) were used to produce coating samples. The samples were classified into four families, namely single-layer, double-layer, triple-layer and functionally graded (FG). To measure thermal shock resistance, the heating/water quenching cycles were repeated 70 times and 30% destruction of the coating was considered its functionality limit. Thus, cycles did not continue for those coatings that were destroyed more than 30%. At the end of each cycle, the surface and edge damage were determined from the photos of samples. Furthermore, scanning electron microscope (SEM) images and energy-dispersive spectrometer (EDS) analysis of samples’ cross-section were taken before and after the test. After collecting the experimental data, effects of various factors on outputs were investigated. The results showed that YSZ-N single-layer coating and triple-layer with CSZ as a top layer, has less thermally grown oxide (TGO) thickness and best performance in thermal shock conditions.     

铝镁质钢包透气砖热震损毁研究

研究了w(Al2O3)为91.2%,w(MgO)为7.0%,高温抗折强度为33.4 MPa,弹性模量为146.1 GPa,热膨胀系数为7.97×10-6K-1的铝镁质钢包透气砖的热震损毁现象。采用脉冲激振法、三点弯曲法测量铝镁质钢包透气砖热震前后的热震参数、弹性模量、常温和高温抗折强度,并计算了弹性模量和抗折强度保持率。结果表明:铝镁质材料热震过程中产生大量微裂纹,裂纹扩展慢,并且铝镁质材料的细晶和网络穿插结构有助于提高其抗热震性能。     

Thermal shock behaviour of laminated multilayer refractories for steel casting applications reinforced by residual stresses

Ceramic multilayer structures based on tape cast alumina and zirconia substrates have been manufactured for use as carbon-free refractory materials. The laminates were reinforced via residual stresses due to shrinkage mismatch or differences in thermal expansion in order to achieve an improved thermal cycling capability. Thermal shock tests have been carried out using a plasma test stand. The impact of layer sequence and residual stresses has been demonstrated via measurement of Young's modulus and microstructure images of the layered structures. Hasselman parameters as well as the crack propagation behaviour at interfaces have been analysed via wedge splitting test.     

Microstructural,mechanical and thermal shock properties of triple-layer TBCs with different thicknesses of bond coat and ceramic top coat deposited onto polyimide matrix composite

In this study, a triple-layer thermal barrier coating (TBC) of Cu-6Sn/NiCrAlY/YSZ was deposited onto a carbon-fiber reinforced polyimide matrix composite. Effects of different thicknesses of YSZ ceramic top coat and NiCrAlY intermediate layer on microstructural, mechanical and thermal shock properties of the coated samples were examined. The results revealed that the TBC systems with up to 300 µm top coat thicknesses have clean and adhesive coating/substrate interfaces whereas cracks exist along coating/substrate interface of the TBC system with 400 µm thick YSZ. Tensile adhesion test (TAT) indicated that adhesion strength values of the coated samples are inversely proportional to the ceramic top coat thickness. Contrarily, thermal shock resistance of the coated samples enhanced with increase in thickness of the ceramic coating. Investigation of the TBCs with different thicknesses of NiCrAlY and 300 µm thick YSZ layers revealed that the TBC system with 100 µm thick NiCrAlY layer exhibited the best adhesion strength and thermal shock resistance. It was inferred that thermal mismatch stresses and oxidation of the bond coats were the main factors causing failure in the thermal shock test.     

Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy

The thermal shock behavior of a thermal barrier coating (TBC) prepared by plasma spraying at 1100 °C was investigated. The TBC consisted of a double layer structure of 8YSZ/CoCrAlYTaSi. The morphology, microstructure, phases and the elemental distribution of the TBCs were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), X-ray diffraction (XRD) and electron probe micro-analysis (EPMA). The characterization results showed that the film consisted primarily of metastable tetragonal phases (t′), and a large number of micro-cracks were present in the 8YSZ crystals. Following eighty-six thermal shock cycles of the specimens a large areal spallation was observed on the 8YSZ coating. The decreased concentration of yttrium at the coating interfaces weakened the inhibition of crystal growth and the phase transition of the Al₂O₃. The growth of TGO (Thermal growth oxide) and the diffusion into the 8YSZ coating produced deformation and stress in the ceramic coating. Tantalum appeared to absorb the oxygen that diffused into the coatings and delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, which was beneficial to prolonging the life of the TBC.     

Thermal shock behavior of a nanolaminated ternary boride: MoAlB

MoAlB has been the only known compound with attractive oxidation resistance in the MAB family so far. To widen the application of the MoAlB material in high temperature environments where temperature change is abrupt, the investigation of its thermal shock behavior is required. A dense MoAlB material was prepared by hot pressing of Mo, Al and B elemental powders at 1200?°C with 25?MPa for 1?h in Ar atmosphere. Thermal shock resistance of the synthesized MoAlB material was investigated by quenching in water from various temperatures. The residual strength of MoAlB showed a slow decrease trend after quenching from 600?°C to 1400?°C. The MoAlB material exhibits better thermal shock resistance than another MAB phase of Fe₂AlB₂. X-ray diffractometry was used to identify the phase composition, and scanning electron microscopy was adopted to characterize the microstructure.     

膨胀珍珠岩对镁钙质陶瓷热震稳定性的影响

以煅烧白云石、工业氧化镁为主要原料,通过添加膨胀珍珠岩制备镁钙质陶瓷试样;探讨膨胀珍珠岩含量对试样显气孔率、体积密度、抗热冲击性能及热冲击对试样抗折强度的影响。试验结果表明：添加适量的膨胀珍珠岩能有效地提高镁钙质陶瓷的抗热冲击性能。     

Thermal shock resistance behavior of auxetic ceramic honeycombs with a central crack or an edge crack

In this paper, the thermal shock resistance of an auxetic ceramic honeycomb plate is studied based on the fracture mechanics concept for the cases of a central crack or an edge crack. The transient temperature field and transient thermal stress field are obtained for both auxetic and non-auxetic structures. The relationship between the thermal stress intensity factor (TSIF) and the internal cell angle, crack length and time is determined and the critical temperature for the initiation of crack propagation is predicted. Results show that compared with the non-auxetic ceramic honeycombs which are at an internal cell angle of 30°, the critical temperature of the auxetic ceramic honeycombs whose cell are orientated at an angle of −30° increases by 78.5% and the TSIF at the crack tip decreases by 40%, respectively. Hence, the auxetic structures have better thermal shock resistance. This study indicates that auxetic ceramic honeycombs have significant potential applications in harsh temperature environments.     

Thermal shock resistance of Al2O3/SiO2 composites by sol-gel

In this paper, the SiO₂ ceramic matrix composites were reinforced by the two-dimensional (2D) braided Al₂O₃ fibers by sol-gel. To develop the high performance aeroengine with excellent resistance to thermal shock for advanced aerospace application, two different thermal shock temperatures (1100?°C and 1300?°C) and three different thermal shock cycles (10, 20 and 30 cycles) were tested and compared in this paper; besides, the thermal shock resistance of Al₂O₃/SiO₂ composites was investigated in air. Our results suggested that, the flexural strength of the untreated composites was 78.157?MPa, while the residual strength of Al₂O₃/SiO₂ composites under diverse thermal shock cycles and temperatures had accounted for about 95% and 50% of the untreated composites, respectively. Meanwhile, the density and porosity of the composites were gradually increased with the increase in test temperature. Moreover, the changes in fracture morphology and micro-structural evolution of the composites were also observed. Our observations indicated that, the fracture morphology of the composites mainly exhibited ductile fracture at the thermal shock temperature of 1100?°C, whereas brittle fracture at the thermal shock temperature of 1300?°C. Additionally, Al₂O₃/SiO₂ composites belonged to the Oxide/Oxide CMCs, so no new phase was formed after thermal shock tests. Above all, findings of this paper showed that Al₂O₃/SiO₂ composites had displayed outstanding thermal shock resistance.     

反应烧结碳化硅材料的抗热震性能研究

通过对比不同温差热震后材料的残余强度 ,对反应烧结碳化硅材料的抗热震性能进行了研究。结果表明 :反应烧结碳化硅材料的抗热震性能与显微组织密切相关 ,低游离硅含量与小粒径的反应烧结碳化硅材料具有较好的抗热震断裂性能 ,而高游离硅含量或大碳化硅粒径的材料具有相对优异的抗热震损伤性。对反应烧结碳化硅材料的抗热震性与显微组织的关系进行了探讨。     

Improvement of thermal shock performance by residual stress field toughening in periclase-hercynite refractories

In this paper, the periclase-hercynite bricks suffered ten cycles of thermal shocks were characterized, and the micro-mechanism of enhancement thermal shock performance for periclase-hercynite refractories was analyzed by using residual stress field theory. Due to the diffusion of Fe²⁺ (or Fe³⁺) and Al³⁺, the composite spinel particles formed inside of magnesia grains in contract with hercynite. When temperature fluctuates, the caused residual stress field and composite spinel particles prevent the extension and propagation of microcracks, enhance fracture toughness, improving of thermal shock stability of magnesia grains. The estimated value of residual stress field and increment of fracture toughness for magnesia grains with composite spinel particles inside by residual stress field toughening model are ?1339.19 MPa and 0.49 MPa m^1/2, respectively. The excellent thermal shock resistance of periclase-hercynite refractories is derived from the improvement of the internal microstructure of large quantities of magnesia grains around the hercynite.     

Effect of CaF2 on sintering behavior and thermal shock resistance of Y2O3 materials

Y₂O₃ materials have become a popular candidate for preparing refractory crucibles for ultra-pure high-temperature alloy melting in recent years. However, its difficulty in sintering and poor thermal shock resistance limited its industrial application. The effect of CaF₂ on the densification microstructure, mechanical properties, and thermal shock resistance of Y₂O₃ materials was investigated in this paper. The main purpose of this study was to optimize the amount of CaF₂ added in the preparation of Y₂O₃ materials to improve its thermal shock resistance and get better mechanical properties. The mechanism of the densification process of CaF₂-doped Y₂O₃ materials was analyzed by phase analysis and microstructure. The results showed that successive doping of large Ca²⁺ ions caused more lattice distortion in the Y₂O₃ materials, and the diffusion rate of Y³⁺ was increased, thus enhanced grain boundary diffusion and promoted sintering densification in the Y₂O₃ materials. Meanwhile, the addition of CaF₂ also significantly reduced the apparent porosity and enhanced the mechanical properties of the materials. The improvement of these properties was attributed to the increased relative density of CaF₂-doped Y₂O₃ materials and the high sintering activity of CaF₂. In addition, crack deflections effectively improved the thermal shock resistance of the materials. The residual flexural strength ratio of Y₂O₃ materials doped with 1 wt % CaF₂ was increased by 21.2% after thermal shock test compared with undoped specimens.     

Flexural fatigue behavior and residual strength evolution of SiCnws-C/C composites

The flexural fatigue behavior and residual flexural strength evolution of SiC nanowires reinforced carbon/carbon (SiCnws-C/C) composites were investigated. Specimens were loaded at a stress level of 65% of their static flexural strength for 10⁵, 5?×?10⁵ and 10?×?10⁵ cycles, and their residual flexural strength was increased by 4.87%, 13.73% and 62.45% respectively after cyclic loading. Results indicate that the residual strength after cyclic load is affected by the formation and propagation of cracks, interfacial degradation, as well as the relief of residual thermal stress. An appropriate interfacial debonding and releasing of residual thermal stress are responsible for the large improvement of residual strength of SiCnws-C/C composites after 10?×?10⁵ fatigue cycles. Compared with carbon/carbon composites, SiCnws-C/C composites demonstrate higher mechanical strength and stronger resistance to crack propagation, which are ascribed to the strengthening effect brought by the SiC nanowires, including their pull-out, breaking and bridging.     

低碳MgO-C材料的抗热震性研究

以电熔大结晶镁砂、天然鳞片石墨、纳米炭黑、酚醛树脂、铝粉等为主要原料制备w(C)=3%的低碳MgO-C材料,以其抗热震性为考核指标,选取颗粒级配、复合抗氧化剂、石墨粒度和复合结合剂4个因素,进行了四因素三水平正交试验。结果表明:在本试验范围内,颗粒级配是影响低碳MgO-C材料抗热震性的主要因素,复合抗氧化剂次之,石墨粒度和复合结合剂的影响基本相当;通过极差分析确定,镁砂颗粒级配(3～1、1～0.088和≤0.088mm的镁砂的质量比)采用50:23:27,复合抗氧化剂采用Al2.5+Mg-Al0.5+B4C0.5,石墨粒度采用10μm的,复合结合剂采用炭黑N220+沥青+酚醛树脂,可制备出抗热震性最佳的低碳MgO-C材料。     

Thermal shock resistance and fatigue of Zircon-Mullite composite materials

Zircon-Mullite (ZrSiO₄-3Al₂O₃·2SiO₂) composites are refractory materials widely employed in many industries. Thermal shock behaviour of these materials must be considered because it is sometimes its failure mechanism.In this work both thermal shock resistance (TSR) and fatigue (TFR) of Zircon-Mullite composites with different compositions and microstructure configurations were experimentally evaluated by a non destructive measurement of the elastic modulus (E) and compared with the prediction made from the theoretical parameters (R, R? and R_ST).A typical solid brittle material behaviour was found; a simple mathematical expression facilitated the TFR analysis. Although the microstructural configurations studied differed, the experimental behaviour of this group of materials was almost equal. This fact was satisfactorily predicted by the theoretical parameters (R and R?) showing the importance and potential of the evaluation mechanical evaluation of the ceramic material that define these two parameters. On the other hand the slight difference evaluated in the TFR was correlated with the only parameter that takes into account the fracture toughness (R_ST) showing that the significance of this property in a more deep characterization of the ceramic materials.     

Thermal shock resistance study of stereolithographic additive-manufactured Al2O3 ceramics by in situ digital radiography

Thermal shock resistance is critical to ensure the service safety of ceramic hot-end components. The thermal shock performance of stereolithographic additive-manufactured ceramics has not yet been studied. In this study, a series of thermal shock experiments with various temperature differences was conducted on stereolithographic additive-manufactured Al₂O₃ ceramics. The surface cracks were analysed based on photographs captured before and after the thermal shock experiments. Three-point bending tests with in situ X-ray digital radiography were conducted to determine the thermal shock resistance. Crack initiation, propagation, and coalescence were observed under flexural loads. The critical temperature difference of the stereolithographic additive-manufactured Al₂O₃ ceramics was determined to be 267.22 °C. The crack length increased and residual strength decreased with increasing temperature differences. The layered structure of the stereolithographic additive-manufactured ceramics slowed crack propagation. We expect that this study will serve as a reference for the performance of stereolithographic additive-manufactured Al₂O₃ ceramics in extreme environments.     

差热分析若干影响因素探讨

以氟铝酸盐玻璃、氟磷酸盐玻璃和BGGFO玻璃为研究对象,探讨了在进行DTA测量时的一些影响因素.根据研究结果,选用退火玻璃进行DTA测量有助于准确判定Tg温度;DTA测量玻璃样品粒度应在1～2.5mm之间,可以获得较好的测量重复性;同时,文中对几种判定玻璃热稳定性的指标进行了比较.