高速气流对C/SiC复合材料激光烧蚀行为影响的实验研究

为了明确高速气流对C/SiC复合材料激光烧蚀行为的影响机制,开展了不同环境下强激光对C/SiC复合材料的烧蚀对比实验研究。利用激光器与高速风洞联合实验平台,完成了静态以及Ma 1.8,Ma 3.0,Ma 6.0气流环境下2D与3DN C/SiC复合材料激光烧蚀实验。结果表明,与静态环境相比,高速气流对C/SiC复合材料的激光烧蚀行为产生了显著的影响,气流的冲刷使得烧蚀坑呈现出更宽、更深、更光滑的变化趋势。随着气流速度的增长,线烧蚀速率与质量烧蚀速率逐渐增大,主要原因为当地静压降低引起的升华速率增大,以及动压增大引起的剥蚀速率增大。此外,通过实验对比了不同构型对C/SiC激光烧蚀行为的影响。结果表明：2D C/SiC复合材料由于厚度方向更低的导热能力、更低的孔隙率等原因,其在不同环境条件下抗烧蚀能力均强于3DN C/SiC复合材料。     

纤维增强复合材料激光烧蚀效应的数值模拟

考虑材料的热解、氧化、相变及辐射和内外对流换热等物理过程,给出了激光烧蚀纤维增强复合材料的物理模型及数学模型。以碳纤维/环氧树脂复合材料为例,编程计算了材料的激光烧蚀过程,计算结果与实验结果符合得较好。计算结果表明：考虑复合材料的内对流时得到的结果更准确;较强功率密度激光辐照时,氧化对烧蚀的贡献可以忽略;功率密度一定时,烧蚀质量随时间近似为线性变化,功率密度越高,烧蚀效率越高。以辐照结束时背表面温度及烧蚀质量为目标物理量,对烧蚀过程做了参数敏感性分析,结果表明：热容及热导率对背表面温度的影响较大;树脂含量对烧蚀质量的影响较大,但其相对敏感度随激光功率密度增加而下降;激光功率密度超过1 kW/cm2时,辐射系数对烧蚀质量影响较大,但其相对敏感度随激光功率密度增加而下降。     

两种纤维增强复合材料连续激光烧蚀阈值测量及吸收特性分析

 通过双积分球-光电管测试系统和摄像记录的方法,对芳纶纤维/环氧和碳纤维/环氧两种复合材料在1.319 μm连续激光作用下的烧蚀阈值和烧蚀过程中材料对激光能量的吸收特性进行了实验研究。结果表明：芳纶纤维/环氧复合材料的平均烧蚀阈值随材料厚度增加而降低,碳纤维/环氧复合材料的平均烧蚀阈值不受材料厚度影响,约为70 W/cm²;两种纤维增强复合材料烧蚀前的反射率随激光功率增加而缓慢增大,芳纶纤维/环氧材料从0.40变化到0.45,碳纤维/环氧材料从0.15变化到0.20;当发生烧蚀时,芳纶纤维/环氧材料的反射率急剧下降,吸收率增大,碳纤维/环氧材料的反射率无明显变化,吸收率约为0.80。     

掺杂相对ZrB2陶瓷涂层抗激光烧蚀性能的影响

耐高温陶瓷作为高熔点材料,具有优异的高温抗烧蚀性能,有可能满足未来抗激光防护的需求。为摸清ZrB₂陶瓷涂层抗激光防护性能,采用高功率固体激光器作为测试光源,搭建了激光烧蚀实验平台和激光耦合特性测量系统,重点对ZrB₂陶瓷涂层开展了激光烧蚀实验和涂层反射率测试。实验研究了不同激光参数条件下ZrB₂涂层抗激光烧蚀性能,以及掺杂相(SiC、MoSi2)的影响。结果表明,相比于未掺杂ZrB₂涂层,掺杂后ZrB₂涂层抗激光烧蚀能力明显下降。分析认为掺杂相可提高ZrB₂涂层抗氧化性能,但不利于发挥氧化生成物ZrO2的高反射和隔热作用,致使抗激光损伤阈值降低。激光损伤前后涂层反射率的测试结果,也证实了ZrO2的高反射率是增强ZrB₂涂层抗激光损伤阈值的关键。同时,利用有限元软件建立了连续激光烧蚀下ZrB₂陶瓷涂层温度计算模型,并以基底发生熔化为判据,仿真得到了陶瓷涂层典型的抗激光烧蚀阈值参数。     

氧化多孔硅/聚合物复合膜的折射率

用Bruggeman模型理论，分析了氧化多孔硅/聚合物复合膜的等效折射率与多孔硅孔隙率、氧化度和嵌入率的关系.实验研究了嵌入PMMA材料的氧化多孔硅/聚合物膜的等效折射率.证实了在多孔硅中嵌入聚合物可使薄膜的光学参量保持稳定.     

放电等离子烧结制备ZrB2-SiC超高温陶瓷的力学性能及氧化行为

在服役环境中,超高声速飞行器表面与空气剧烈摩擦导致温度极高。超高温陶瓷相较于一般陶瓷而言具有高熔点和良好的抗氧化烧蚀性能,是目前极具前景的热防护材料之一。采用放电等离子两步烧结工艺将ZrB2纳米粉末和SiC粉末在1700℃下制备超高温陶瓷材料ZrB2-20%SiC,通过纳米压痕微观实验、三点弯实验研究其力学性能及其在高温环境下的氧化行为,着重分析1000、1200、1400和1600℃4种不同氧化温度下ZrB2-20%SiC超高温陶瓷的氧化表面、氧化截面和氧化层厚度。结果表明:ZrB2-20%SiC超高温陶瓷的硬度为18 GPa,弹性模量为541 GPa,断裂韧性为5.7 MPa·m1/2;当氧化温度为1600℃时,超高温陶瓷内部的SiC由被动氧化转变为主动氧化,并且随着氧化温度升高,超高温陶瓷氧化层厚度与氧化温度呈正相关。     

SiC纤维增强Ti17合金复合材料轴向残余应力的拉曼光谱和X射线衍射法对比研究

准确测量和分析SiC纤维增强Ti合金复合材料(SiC_f/Ti)中残余应力状态对优化复合材料的成型工艺和理解其失效模式具有重要意义,但其残余应力的实验测量和分析仍是一个挑战.石墨C涂层作为SiC纤维与Ti17基体合金之间必需的扩散障涂层,承载了由纤维与基体之间热不匹配引入的残余应力.本文采用显微拉曼光谱法对比测量纤维表面C涂层在复合材料中和去掉基体无应力态下G峰的峰位,通过石墨C涂层应力态下峰位移动计算出SiCf/C/Ti17复合材料中SiC纤维受到～705.0 MPa的残余压应力.采用X射线衍射方法测量了不同方向上该复合材料中基体钛合金的晶面间距以获取其空间应变,根据三轴应力模型分析了复合材料中基体钛合金沿轴向方向的残余应力为～701.3 MPa的张应力,并通过线性弹性理论转化为SiC纤维的残余压应力为～759.4 MPa.两种测试方法都确定了SiC纤维在成型过程中受到残余压应力,且获得的应力值较为接近,都可以用于对SiC_f/Ti复合材料的残余应力测量.     

硅纳米线/氧化钒纳米棒复合材料的制备与气敏性能研究

采用纳米球光刻和金属辅助刻蚀法以p型单晶硅片制备了硅纳米线阵列, 并以此作为基底, 通过溅射不同时长的金属钒薄膜并进行热退火氧化处理, 制备出硅纳米线/氧化钒纳米棒复合材料. 采用扫描电子显微镜和X射线衍射仪表征了该复合材料的微观特性, 结果表明该结构增大了材料的比表面积, 有利于气体传感, 并且镀膜时间对后续生长的氧化钒纳米棒形貌有明显影响. 采用静态配气法在室温下测试了该复合材料对NO₂的气敏性能, 气敏测试结果表明沉积钒膜的时间对复合材料的气敏性能影响较大. 当选择合适的镀膜时间时, 适量氧化钒纳米棒增加了材料表面积并形成大量pn结结构, 相比纯硅纳米线对NO₂气体的灵敏度有明显提升, 且在室温下表现出优良的选择性. 同时, 对气敏机理做了定性解释, 认为硅纳米线与氧化钒纳米棒之间形成的pn结及能带结构在接触NO₂ 时的动态变化是其气敏响应提升的主要机制.     

30.4 nm波长SiC/Mg多层膜反射镜

波长30.4 nm的He-II谱线是极紫外天文观测中最重要的谱线之一,空间极紫外太阳观测光学系统需要采用多层膜作为反射元件。为此研究了SiC/Mg、B4C/Mg、C/Mg、C/Al、Mo/Si、B4C/Si、SiC/Si、C/Si、Sc/Si等材料组合的多层膜在该波长处的反射性能。基于反射率最大与多层膜带宽最小的设计优化原则,选取了SiC/Mg作为膜系材料。采用直流磁控溅射技术制备了SiC/Mg多层膜,用X射线衍射仪测量了多层膜的周期厚度,用国家同步辐射计量站的反射率计测量了多层膜的反射率,在入射角12°时,实测30.4 nm处的反射率为38.0%。     

航空复合涂层材料的激光烧蚀效应

针对航空激光防护技术研究需求,开展了复合涂层材料的激光烧蚀效应综合研究。采用圆棒固体激光器作为测试光源,搭建了具有在线温度测量功能的烧蚀实验平台。在此基础上,对聚碳硅烷(PCS)涂层样品开展了激光烧蚀实验。通过对烧蚀区域形貌和温度数据的对比分析,证明了PCS复合材料具备显著的激光防护作用。同时,从理论方面对涂层的激光防护机理进行了研究,基于材料热传导方程,建立了激光烧蚀过程的热力学模型,对温度场变化进行了模拟。研究结果表明,在复合涂层的保护下,kW级激光仅产生百℃的金属基底升温。     

Numerical simulation of the ablation of thin molybdenum films under laser irradiation

Laser irradiation of a molybdenum film on a quartz substrate is numerically studied. The simulated results prove the experimental effect lying in a threefold decrease in the size of the ablation region in comparison with the focal spot. The numerical experiment proves the hypothesis on the two-stage ablation of metal film with the primary formation of oxide phase. It is demonstrated that oxidation leads to a selective decrease in the thermal resistance of the film along the vertical direction, so that the anisotropic character of the ablation is enhanced.     

Surface modification and oxidation kinetics of hot-pressed AlN–SiC–MoSi2 electroconductive ceramic composite

The effects of oxidation on the microstructural modification and on the electrical resistivity and mechanical strength of a hot-pressed AlN–SiC–MoSi₂ electroconductive ceramic composite were studied. The kinetic of the oxidation was also evaluated. After the oxidation at temperatures below 1000 °C samples do not gain weight, due to simultaneous formation of SiO₂ and evaporation of MoO₃ formed by the oxidation of MoSi₂. However, the AlN/SiC matrix disables the “pesting” phenomena and strength degradation, despite the fact that at these temperatures MoSi₂ oxidizes rapidly. At temperatures above 1000 °C, the composite gains weight due to protective mullite layer formation on the surface, that provides a good oxidation resistance for use at higher temperatures. The kinetics of the oxidation follows the parabolic law. The possible rate-controlling mechanism is the diffusion of oxygen through the mullite-rich surface oxide scale.     

Features of film growth during plasma anodizing of Al 2024/SiC metal matrix composite

Plasma anodizing is a novel promising process to fabricate corrosion-resistant protective films on metal matrix composites. The corrosion-resistant films were prepared by plasma anodizing on SiC reinforced aluminum matrix composite. The morphology and microstructure of films were analyzed by scanning electron microscopy. Specifically, the morphology of residual SiC reinforcement particles in the film was observed. It is found that the most SiC reinforcement particles have been molten to become silicon oxide, but a few tiny SiC particles still remain in the film close to the composite/film interface. This interface is irregular due to the hindering effect of SiC particles on the film growth. Morphology and distribution of residual SiC particles in film provide direct evidence to identify the local melt occurs in the interior of plasma anodizing film even near the composite/film interface. A model of film growth by plasma anodizing on metal matrix composites was proposed.     

Electron beam-physical vapor deposition of SiC/SiO2 high emissivity thin film

When heated by high-energy electron beam (EB), SiC can decompose into C and Si vapor. Subsequently, Si vapor reacts with metal oxide thin film on substrate surface and formats dense SiO₂ thin film at high substrate temperature. By means of the two reactions, SiC/SiO₂ composite thin film was prepared on the pre-oxidized 316 stainless steel (SS) substrate by electron beam-physical vapor deposition (EB-PVD) only using β-SiC target at 1000 °C. The thin film was examined by energy dispersive spectroscopy (EDS), grazing incidence X-ray asymmetry diffraction (GIAXD), scanning electron microscopy (SEM), atomic force microscopy (AFM), backscattered electron image (BSE), electron probe microanalysis (EPMA), X-ray photoelectron spectroscopy (XPS) and Fourier transformed infra-red (FT-IR) spectroscopy. The analysis results show that the thin film is mainly composed of imperfect nano-crystalline phases of 3C-SiC and SiO₂, especially, SiO₂ phase is nearly amorphous. Moreover, the smooth and dense thin film surface consists of nano-sized particles, and the interface between SiC/SiO₂ composite thin film and SS substrate is perfect. At last, the emissivity of SS substrate is improved by the SiC/SiO₂ composite thin film.     

Cycle oxidation behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

Cycle oxidation resistance at 800 °C in static air was investigated for a nanostructured Ni60-TiB₂ composite coating sprayed by high velocity oxy-fuel (HVOF). For comparison, a Ni60-TiB₂ conventional composite coating was also studied. The results indicate that, the oxidation processes of both composite coatings are controlled by diffusion mechanism, and the nanostructured composite coating has better cycle oxidation resistance than that of the conventional composite coating. The reasons for this improvement can be attributed to the formation of the intact SiO₂ and Cr₂O₃ protective layer, and the enhanced adhesion between oxide film and nanostructure coating.     

Investigation of isothermal and no-isothermal oxidation of SiC powder using an analytic kinetic model

The oxidation of SiC powder has been investigated under non-isothermal and isothermal conditions. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) were employed to investigate the morphological development during the oxidation. The results show that the major oxidation product was amorphous silica and the oxidation reaction was mainly diffusion-controlled. Based on limited experimental data, an analytic kinetic model, which expresses the oxidation weight gain as a function of time and temperature explicitly, has been used to predict the oxidation behavior of SiC powder. The comparison between experimental results and theoretical calculation shows that this new model works very well. The activation energy of non-isothermal and isothermal oxidation of SiC powder has been derived to be 226.5 kJ/mol and 187.5 kJ/mol, respectively.     

Electroless plating of silver on cenosphere particles and the investigation of its corrosion behavior in composite silicon rubber

In this paper, silver coating on the surface of cenosphere particles was prepared by electroless plating method. The adhesion, oxidation resistance and corrosion resistance properties of silver coating mixed in silicone rubber were investigated. The corrosion characteristic of silver coating was evaluated by anodic polarization curves of the silicone rubber composite in sulfuric acid solution. The results showed that the silver coating on the surface of cenosphere particles was smooth and uniform. The silver film was not oxidized and peeling off during preparation of composite silicone rubber. The adhesion between the cenosphere particle and silver film was good enough. The anodic polarization curves of the silicone rubber composite showed typical activation and passivation transformation. The values of corrosion potential, the initiating passive potential and maintaining passivity potential of composites filled with different contents of Ag-coated cenosphere particles were the same and related to the nature of silver coating. The passive current density of composite increased with increase of the amount of Ag-coated cenosphere particles and was inversely proportional to the resistance of silicone rubber composite. The better the conductivity of silicone rubber composite is, the higher corrosion rate will be.     

The influence of ablation products on the ablation resistance of C/C-SiC composites and the growth mechanism of SiO2 nanowires

Ablation under oxyacetylene torch with heat flux of 4186.8(10%kW/m2 for 20 s was performed to evaluate the ablation resistance of C/C-SiC composites fabricated by chemical vapor infiltration（CVI） combined with liquid silicon infiltration（LSI） process.The results indicated that C/C-SiC composites present a better ablation resistance than C/C composites without doped SiC.The doped SiC and the ablation products SiO₂ derived from it play key roles in ablation process.Bulk quantities of SiO₂ nanowires with diameter of 80 nm-150 nm and length of tens microns were observed on the surface of specimens after ablation.The growth mechanism of the SiO₂ nanowires was interpreted with a developed vapor-liquid-solid（VLS） driven by the temperature gradient.