多尺度协同强韧化超高温陶瓷构件形/性一体化制造基础研究

Lots of key components of hypersonic flight, reusable oribital vehicles for use in the extreme conditions usually undergo thermal shock, high-temperature oxidation, sharp particles impacting on their exposed surfaces and so on, which strongly require the development of both new advanced ultra-high-temperature ceramic (UHTC) materials wth the combination of lighweight, strong and high toughness, excellent resistance to thermal shock and high-temperature oxidation, and novel manufacturing technology making these ceramic materials readily formed to complex shapes. Based on the facts of excellent mechanical properties, good resistance to high-temperature oxidation and relatively high thermal conductivity of BN nanotubes (BNNTs),the primary emphasis of this proposal is to develop key techniques of integrated manufacturing for plasma spray formed near-net-shapes bulk BNNT/(ZrB2-SiC)ceramic composites with bimodal microstructure under the guidance of material design concept of "Multi-scale synergistic reinforcing and toughening splat boundaries and splat". The objective will be achieved by dividing reseach plan into three broad categories:.1. Optimization of plasma spray processing maps: In this section, the degree of melting and re-solidification of the in-flight agglomerated particels and splat morpholgy should be well controlled, regulation mechanisms of both multiple interface structures and bimodal microstructure will be also highlihted, and then relationship among processing parameters,deposit thickness and residual stress should be investigated..2. Plasma spray formed UHTC's components with complex shapes: Fundamental understanding of plasma spray formed near-net-shape UHTC's thick-walled componens with lower porosity and free of cracking, revealing the defect characteristics inside the components and their forming mechanisms, and achieveing quality-control methodologies of near-net-shaped BNNT/(ZrB2-SiC) ceramic composite components. Then, densification technology such as hot isosatic pressing (HIP) will be employed to achieve more denser near-net-shaped components, densification mechanisms of plasma spray formed components and correlation between HIP processing parameters and microstructure evolution will be also revealed..3. Evaluation of mechanical properties of UHTC's componens: Mechanical properties such as bend strength, fracture toughness, elastic modulus, wear resistance of near-net-shaped components before and after densification will be evaluated at ambient temperature and different thermal shock conditions, respectively. Fracture surfaces, surface morphologies of indent marks and scratch tracks of these samples cut from near-net-shaped components will also be carefully characterized to reveal reinforcing and toughening mechanisms of BNNT/(ZrB2-SiC) composites and to achieve mechanisms of thermal shock failure.

高超声速飞行器、可重复使用跨大气层飞行器中大量关键构件在其服役环境中要承受热冲击、高温氧化、微粒及碎片高速撞击等综合作用，急需同时具有轻质、高强韧性、优异抗热冲击和抗高温氧化性能的先进超高温陶瓷新材料及其异形构件制造新技术。本项目依据BN纳米管优异的力学性能、良好的抗高温氧化性能和较高的传热系数，以"扁平粒子界面及扁平粒子内部协同强韧化"的多尺度强韧耦合为材料强韧化设计思路，采用等离子喷涂近净成形技术，重点研究集材料设计和构件成形于一体的BNNT/（ZrB2-SiC）超高温陶瓷异形构件制造成形成性规律，揭示构件内部缺陷特征及其形成机理，提出等离子喷涂近净成形构件内部质量控制方法，完善等离子喷涂近净成形构件热等静压致密化工艺，阐明复合材料构件强韧化的物理、力学机制及其在不同热冲击条件下的失效机制，为超高温陶瓷关键异形构件一体化制造技术提供理论方法和技术支撑。

本项目依据BN纳米片(BNNP) 优异的力学性能、良好的高温稳定性能及较高的传热系数，利用等离子喷涂近净成形制备出了BNNP/(ZrB2-SiC) 超高温陶瓷复合材料薄壁件，并对其室温力学性能及热冲击性能开展了研究。为确保BNNPs在超高温复合材料中均匀分布，采用喷雾干燥制得了40-80 μm的复合材料微米团聚颗粒作为等离子喷涂喂料，而后采用等离子喷涂近净成形技术及热等静压后续致密化制备出厚度为1.5 mm的圆柱形薄壁件。通过在线监测等离子焰流中等离子喷涂喂料的平均飞行速度和平均表面温度，对等离子喷涂工艺参数进行全面优化。显微组织分析表明，BNNPs优异的高温稳定性，使其原始纳米片状结构经等离子喷涂高温加工后保留于等离子喷涂近净成形超高温陶瓷复合薄壁件显微组织中。此外，经热等静压后，等离子喷涂典型的层状组织结构特点消失，这有利于消除等离子喷涂扁平粒子界面引起的孔隙。相比于ZrB2-SiC薄壁件，2wt.%BNNP/(ZrB2-SiC)的室温断裂韧性提高了~29.3%，弯曲强度提高了~32.8%。BNNP/(ZrB2-SiC)复合材料薄壁件室温断裂韧性及弯曲强度的同步提升，主要源于BNNP拔出、裂纹桥接、裂纹偏转等外部增韧机制，而奥罗万机制及位错密度提高（Enhanced dislocation density）机制则是复合材料的主要强化机制。2wt.%BNNP/(ZrB2-SiC)在400℃、600℃、800℃的热冲击抗力较ZrB2-SiC分别提高了~75%，~39%和~13%。研究发现，BNNP/(ZrB2-SiC) 超高温陶瓷复合材料抗热冲击性能的提升，一方面得益于高热传导系数BNNP的添加，降低了高温环境下材料内部的温度梯度以减小由此产生的热应力；另一方面，BNNP的加入，有效地提高了复合材料的室温综合强韧性。上述研究，为超高温陶瓷关键异形构件一体化制造技术提供理论方法和技术支撑。