等离子喷涂ZrO_2NiCoCrAlY梯度涂层的界面强化热处理工艺

在大气条件下经高温长时间保温后，ＺｒＯ２ＮｉＣｏＣｒＡｌＹ 梯度涂层中的ＮｉＣｏＣｒＡｌＹ 组元发生严重氧化， 形成大量的ＮｉＯ， Ａｌ２Ｏ３ 和Ｃｒ２Ｏ３ 等氧化物， 导致ＺｒＯ２ 表面层中产生粗大网状裂纹， 并最终导致ＺｒＯ２ 表面层碎裂而失效。对梯度涂层进行（９００ ℃） 保温（８ ｈ） 的真空热处理后， 涂层完好无损， ＮｉＣｏＣｒＡｌＹ 底层与ＴＣ４ 基体发生了冶金化学反应， 在涂层与基体界面形成了含ＮｉＴｉ， ＮｉＴｉ２ 和ＴｉＡｌ３ 等化合物的反应层，

Mechanical Properties and Thermal Shock Resistance of SrAl_(2)Si_(2)O_(8) Reinforced BN Ceramic Composites

Hexagonal boron nitride(h-BN)ceramics have become exceptional materials for heat-resistant components in hypersonic vehicles,owing to their superior thermal stability and excellent dielectric properties.However,their densification during sintering still poses challenges for researchers,and their mechanical properties are rather unsatisfactory.In this study,SrAl_(2)Si_(2)O_(8)(SAS),with low melting point and high strength,was introduced into the h-BN ceramics to facilitate the sintering and reinforce the strength and toughness.Then,BN-SAS ceramic composites were fabricated via hot press sintering using h-BN,SrCO_(3),Al_(2)O_(3),and SiO_(2) as raw materials,and effects of sintering pressure on their microstructure,mechanical property,and thermal property were investigated.The thermal shock resistance of BN-SAS ceramic composites was evaluated.Results show that phases of as-preparedBN-SAS ceramic composites are h-BN and h-SrAl_(2)Si_(2)O_(8).With the increase of sintering pressure,the composites’densities increase,and the mechanical properties shew a rising trend followed by a slight decline.At a sintering pressure of 20 MPa,their bending strength and fracture toughness are(138±4)MPa and(1.84±0.05)MPa·m^(1/2),respectively.Composites sintered at 10 MPa exhibit a low coefficient of thermal expansion,with an average of 2.96×10^(-6) K^(-1) in the temperature range from 200 to 1200℃.The BN-SAS ceramic composites prepared at 20 MPa display higher thermal conductivity from 12.42 to 28.42 W·m^(-1)·K^(-1) within the temperature range from room temperature to 1000℃.Notably,BN-SAS composites exhibit remarkable thermal shock resistance,with residual bending strength peaking and subsequently declining sharply under a thermal shock temperature difference ranging from 600 to 1400℃.The maximum residual bending strength is recorded at a temperature difference of 800℃,with a residual strength retention rate of 101%.As the thermal shock temperature difference increase,the degree of oxidation on the ceramic surface and cracks due to thermal stress are also increased gradually.

Research on application of different particle sizes of cordierite on castable refractory

Effect of different particle sizes of cordierite on properties of castable refractory by different heat treatment temperatures were investigated respectively with mullite and bauxite as raw materials, calcium aluminate cement as binders. After 24 h curing in mould and another 24 h curing at 110 ℃ after demoulding, the specimens were heat treated at 1 000 ℃, 1 300℃ and 1 500℃ for 3 h, respectively. The bulk density （ BD）, permanent linear change （ PLC）, modulus of rupture（MOR） and clod crushing strength（CCS）, thermal expansion coefficient and thermal shock resistance were examined. The results show that there is no obvious effect on adjusting permanent linear change and bulk density of castables by adding different particle sizes of cordierite at low temperature and intermediate temperature. Modulus of rupture of castable increase with the decreasing of the particle sizes of cordierite after heat treated by 1 000 ℃ and 1 300℃. In this experiment, thermal shock resistance of the castable with cordierite whose particle size is 0 - 1 mm is the best.

Fabrication and plasma arc thermal shock resistance of HfB_2-based ultra high temperature ceramics

Two hafnium diboride based ceramic matrix composites containing 20% （volume fraction） SiC particle and with or without AIN as sintering additives were fabri,aated by hot-pressed sintering. The mechanical properties and microstructures of these two composites were tested and the thermal shock resistances were evaluated by plasma arc heater. The results indicate that the composite with A1N as sintering additive has a denser and finer microstructure than composite without sintering additive, and the mechanical properties, thermal shock resistance of the composite with A1N as sintering additive are also higher than those of the composite without A1N. Microstructure analysis on the cross-section of two composites after thermal shock tests indicates that a compact oxidation scale contains HfO2 and Al2O3 liquid phase is found on the surface of composite with A1N, which could fill the voids and cracks of surface and improve the thermal shock resistance of composite.

Improving the thermal shock resistance of ceramics by crack arrest blocks

Ceramics used in the high temperature environment are inevitably subjected to sudden temperature change, which may lead to catastrophic thermal shock failure due to the intrinsic brittleness of ceramics. In this paper, an experimental platform is designed to realize the in-situ observation during the thermal shock experiments. Experimental results show that all the cracks initiate from one of the edge midpoints and propagate to another one for square specimens. Such experimental observation is consistent with the maximum tensile stress zone with the maximum temperature gradient given by the finite element method(FEM). The different crack modes resulting from different heating rates after thermal shock experiments are observed and analyzed. Comparison between different clamping methods is conducted to study the effects of boundary conditions on the thermal shock experiments. Furthermore, in order to improve the thermal shock performance of alumina ceramics, crack arrest blocks are added near the edge midpoint. The thickness, shape and arrangement of the blocks are systematically investigated to understand the mechanism of improvement of thermal shock resistance.