稀释气体对化学气相沉积SiC涂层生长行为的影响

以CH3SiCl3-H2为反应气体,采用Ar和H2作为稀释气体。在1100℃、负压条件下,由化学气相沉积制备了SiC涂层,研究了稀释气体对涂层沉积速率、形貌以及晶体结构的影响。以Ar为稀释气体时,随着稀释气体流量的增加沉积速率迅速减小;用Ar作稀释气体制备的SiC涂层相对粗糙,随着Ar流量的增加,晶粒簇之间的空隙较大,涂层变得疏松。XRD分析表明：当稀释气体Ar流量超过200ml／min时,涂层中除了β-SiC外,还逐渐出现了少量的α-SiC。以H2为稀释气体时,当H2流量增加到400ml／min时,涂层的沉积速率迅速增大;以H2为稀释气体制备的SiC涂层致密、光滑,沉积的SiC涂层全部是β-SiC,且具有非常强的(111)晶面取向,涂层中无α-SiC出现。     

低温化学气相沉积SiC涂层显微结构及晶体结构研究

在CH_3SiCl_3-H_2体系中,采用化学气相沉积法(CVD)在1000～1300℃制备了SiC涂层。研究了SiC涂层的沉积速率和温度之间的关系,发现低温化学气相沉积SiC为动力学控制过程,反应的表观活化能为85～156 kJ/mol。SiC涂层的外观颜色及涂层表面的显微结构随沉积温度变化而呈现规律的变化:当沉积温度<1150℃时,SiC涂层的外观颜色为银白色,涂层表面致密、光滑;当温度≥1150℃时,SiC涂层外观颜色逐渐变暗,涂层表面变得疏松、粗糙。利用XRD分析了不同沉积温度下SiC涂层的晶体结构,随着温度的升高,SiC涂层的结晶由不完整趋向于完整;当沉积温度≥1150℃,SiC涂层的XRD谱图中除了β-SiC外还出现了少量α-SiC。     

CVD沉积工艺对SiC涂层结晶度与耐腐蚀性能的影响

通过化学气相沉积(CVD)SiC涂层来提高SiC_f/SiC复合材料的耐腐蚀性能,本文以CH_3SiCl_3(MTS)为源气体,在反应烧结SiC基体上制备SiC涂层,控制沉积温度、炉压及H_2/MTS摩尔比等工艺参数,通过X射线衍射实验(XRD)得到不同工艺条件下生成的碳化硅涂层的物相组成和结晶度,通过高温水腐蚀实验检测涂层的耐腐蚀性,并利用扫描电子显微镜(SEM)观察腐蚀前后的表面形貌。结果表明:当沉积时间为8 h,沉积温度从1050℃到1250℃,β-SiC涂层表面平整性提高,沉积厚度由12.97μm急剧增加至71.10μm, SiC晶粒尺寸逐渐增大,最终呈金字塔状;碳化硅涂层腐蚀60 d后,表面呈现针状结构,1250℃下沉积的SiC涂层耐腐蚀性能较好;β-SiC涂层的晶粒尺寸随沉积炉压的增大而增大,结晶度随沉积炉压增大而减小,在200 Pa以下,获得的β-SiC晶粒的结晶度最高(81.08%)、晶粒尺寸最小(13.7 nm);随着H_2/MTS摩尔比增加,β-SiC晶粒结晶度迅速下降,当H_2/MTS=6.5时,结晶度最高(95.91%)。     

SiC沉积的机理研究

在石墨基底上采用低压化学气相沉积法沉积SiC,沉积室是热壁.H2载气通过鼓泡法将三氯甲基硅烷(methyltrichlorosilane,MTS)送入沉积室,沉积基底的面积和沉积室的体积比为1×10-2mm-1.沉积参数为:压力5kPa,温度1 100℃,H2的流量为150mL/min,气相成分中H2和MTS的流量比为10.结果表明:较大的液态聚基体表面能低,小液滴在大液滴表面沉积.H2流量小时,小液滴沿大液滴的生长锥顶部环状沉积,各生长锥问有间隙.H2流量增加时,小液滴无序的分散在大液滴的表面上.据此提出了一种可能的SiC的沉积和生长机理.     

C/SiC复合材料表面化学气相沉积涂覆SiC涂层及其抗氧化性能

采用化学气相沉积法,在1 100 ℃,在碳纤维增强碳化硅复合材料表面制备SiC涂层,研究了涂层连续沉积和分4次沉积(每次沉积时间为6 h)所制备的SiC涂层的微观结构和涂层样品的氧化性能.结果表明:两种SiC涂层的厚度均约为40 μm,且4次沉积制备的SiC涂层为一个连续的整体.涂层连续沉积时,表面只出现裸露裂纹;分4次沉积制备时,表面出现大量边缘有SiC生长锥的附着裂纹,附着裂纹在高温氧化时易发生自愈合.与连续涂层样品相比,4次涂层能显著提高C/SiC样品的抗氧化性能.4次涂层样品经1 400 ℃,50 h氧化后,质量损失为0.88%,质量损失速率稳定在6.30 × 10-5 g/(cm2?h),且4次涂层样品具有优异的抗热震性能.     

SiC复合涂层的制备及对机电传动轴耐磨性能的影响

采用双辉等离子技术在机电传动轴表面制备了SiC/Ta涂层,研究了反应气体四甲基硅烷对涂层显微组织和耐磨性能的影响。结果表明,不同H_2/TMS流量比例的复合涂层截面都为表面SiC层和Ta过渡层的双层结构特征,涂层截面厚度介于4.8～6μm,当TMS流量至1.5 sccm时,SiC涂层和Ta过渡层较为致密,层间结合良好;4Cr13基体物相主要为Fe-Cr,而不同H_2/TMS流量比例的复合涂层处Fe-Cr衍射峰外,还出现了TaC/SiC、Ta_2C和Ta_2C/SiC衍射峰;基体的磨痕宽度、深度和比磨损率分别为860μm、15.91μm和10~2×10~(-5) mm~3/N·m,不同H_2/TMS流量比例复合涂层的磨痕宽度、深度和比磨损率都要低于基体,且TMS流量为1.5 sccm时,复合涂层磨痕宽度、深度和比磨损率最小。通过双辉等离子技术在机电传动轴表面制备SiC复合涂层有助于改善基体的耐磨性能,且当TMS流量为1.5 sccm时复合涂层的结构较为致密、摩擦系数较低,具有相对更好的耐磨性能。     

CVI法快速制备C/SiC复合材料

为缩短CVI法制备C／SiC复合材料的工艺周期并降低成本，研究了CVI工艺过程中沉积温度、MTS（CH3SiC3)摩尔分数和气体流量对SiC沉积速率和MTS有效利用率的影响，实验结果表明：提高沉积温度，常压下1100℃时增大MTS摩尔分数（11％→19％），都有利于提高SiC沉积速率；提高沉积温度和降低反应物气体流量，能提高MTS有效利用率，在优化的工艺条件下，预制体的微观孔隙内沉积了致密的SiC基体，沉积速率达到142μm/h左右，并有效消除了基体中裂纹的形成，MTS的有效利用率为11％－27％。     

炭纤维表面用化学气相沉积法涂覆碳化硼的研究

以CH4，BCl3，H2为原料气，采用化学气相沉积法(常压CVD)在炭纤维表面连续涂覆B4C，通过正交实验得到最佳涂覆条件；采用IR、TG—DTA、XRD等技术考察了涂层的组成、结构和形貌，并对最佳涂覆条件下炭纤维的拉伸强度进行测试。实验结果表明：当υH2／υBCl3=3．5、υBCl1／υCH4=1．7、气体总流速=160mL／min，沉积温度1100℃，走丝速度5转／min时，涂层表面有明显的一层致密物质，表面较平整，涂层纤维的氧化温度由未涂层时的350℃提高到630℃，纤维的单丝强度由未涂层时的1．93GPa提高到3．15GPa。在炭纤维表面采用化学气相沉积法涂覆B4C不仅装置简单、操作方便，而且可以明显地提高炭纤维的抗热氧化性和单丝强度。     

采用在SiC毡上CVD—SiC涂层提高C／C复合材料抗氧化性能

最近发展起来的SiC纤维复合涂层，也就是SiC/SiC层与化学气相沉积（CVD）SiC结合形成复合涂层，已能够在高温下提高C／C复合材料的抗氧化性。形成的SiC纤维复合涂层约300μm厚，生产时先将SiC毡覆盖在3D－C／C基体材料上，然后浸渍一种碳粉与硅粉均匀分散的料浆进行化学气要沉积。通过化学气相沉积（CVD）过程，在复合材料上形成致密的涂层。在CO2－H2O－N2组成的混合气体（CO2 9％、N273％、H2O18％），1700℃下进行5h氧化实验，结果发现有SiC毡增强复合涂层比没有SiC毡增强复合材料失重率低。SiC纤维毡复合涂层由双层结构组成，里层是多气孔的SiC/SiC纤维层，外层为致密的SiC涂层。由于SiC/SiC纤维层热膨胀系数介于C／C复合基体材料与CVD－SiC涂层之间，因此，SiC/SiC中间层在复合材料中起了重要作用，从而由于热膨胀系数不同产生的热应力致使涂层开裂降低到最低程度。涂层试样氧化后，采用缓冲冲床（MSP）测试其残余强度。MSP测试结果表明氧化后C／C复合材料强度值呈发散性，从纤维折断面看有z轴方向分布纤维存在。然而，这种方法仅适用于测试小尺寸试样。从这篇论文中，可看出涂层后的C／C复合材料有高的抗氧化性，其氧化后仍能保持高的残余强度。     

化学气相反应法制备SiC涂层

采用化学气相反应法,以3种不同工艺在C／C复合材料表面制备了SiC涂层,并检测了其抗氧化性能．以工业用Si和辅助剂SiO2为原料,在高温、惰性环境中反应产生SiO蒸气,将其引入反应室与C／C复合材料在不同温度下进行气相反应,在试样表面生成一层致密的SiC涂层。X射线衍射分析表明：涂层是由β-SiC组成。从试样截面的扫描电镜可知：不同工艺制得的SiC涂层界面过渡带颗粒的微观形貌各异。经最优工艺制备的涂层过渡带很窄,有β-SiC纳米晶须生成,且其抗氧化性能最佳。     

A study of surface-coated SiC whiskers on carbon fiber substrates and their properties for diesel particulate filter applications

β-Silicon carbide (β-SiC) whiskers were synthesized on carbon fiber substrates using a chemical vapor infiltration (CVI) vapor-solid (VS) growth mechanism. An additional SiC surface coating process was utilized after whisker deposition by controlling the input gas ratio of the source gas flow and changing the H₂ (hydrogen) diluent gas to N₂ (nitrogen) under the same deposition temperature of 1,300 °C. As the surface coating deposition time increased, whiskers thickness and spherical blunt tips which were seen at the top edge of the whiskers went thicker. Observing the microstructure of the resulting tips by transmission electron microscopy (TEM) revealed that uncoated whiskers showed few stacking faults, whereas surface-coated whiskers were completely filled with stacking faults. The effect of surface coating deposition time was also evaluated by measuring the properties of a filtration system. Specifically, as the surface coating deposition time increased, gas permeability decreased; however, even at 30 min, the gas permeability of the thickest surface coated whisker filters was five times higher than that of cordierite honeycomb, which is currently used in commercial diesel particulate filter (DPF) devices. A specimen that had been surface coated for more than 20 min almost completely maintained its prime line density under high-pressure (5 MPa) gas. Moreover, we confirmed that SiC surface coating on whiskers and carbon fiber substrates enhanced oxidation resistance and filtration efficiency.     

Deposition Rate,Texture, and Mechanical Properties of SiC Coatings Produced by Chemical Vapor Deposition at Different Temperatures

Silicon carbide (SiC) coatings were produced on carbon/carbon (C/C) composites substrates using chemical vapor deposition (CVD) at different temperatures (1100°C, 1200°C, and 1300°C). The deposition rate was found to increase with deposition temperature from 1100°C to 1200°C. From 1200°C to 1300°C, the deposition rate decreased. SiC coating produced at 1200°C exhibited a strong (111) texture compared with the coatings produced at other temperatures. Both hardness and Young's modulus were also found to be higher in the coating produced at 1200°C. The variation in mechanical properties with the increase in temperature from 1100°C to 1300°C showed a direct correlation with the change in deposition rate and (111) texture. Microstructure analysis shows that the change in CVD temperature leads to the change in grain size, crystallinity, and density of stacking faults of SiC coatings, which appears to have no significant effect on mechanical properties of SiC compared with the texture observed in SiC coating. For the coating deposited at 1200°C, both the hardness and Young's modulus increased gradually from the substrate/coating interface to the top surface. The nonuniformity of mechanical properties along the cross‐section of the coating is attributed to the nonuniform microstructure.     

Deposition of a PMMA coating with an atmospheric pressure plasma jet

Atmospheric pressure plasma jet polymerization of methyl methacrylate (MMA) was performed in order to deposit a PMMA-like coating on ultrahigh molecular weight polyethylene (UHMWPE). This study is a first step in the transfer from MMA plasma polymerization experiments previously performed in a dielectric barrier discharge (DBD) reactor to a newly designed atmospheric pressure plasma jet. In this novel plasma setup, the substrate is not directly exposed to the plasma region, but placed in the plasma jet afterglow. The effect of several plasma jet process parameters on the coating properties was investigated using different surface characterization techniques such as XPS, FTIR, AFM, and OPS. Results show that the stationary deposition of PMMA-like thin films results in a radial gradient in surface chemistry, surface morphology, and coating thickness. Additionally, the coating properties were found to significantly depend on the monomer-containing gas flow rate. This observation is also confirmed by CFD modeling, which shows that the monomer-containing gas flow rate strongly influences the gas flow pattern of the plasma afterglow and therefore the final properties of the deposited PMMA-like film.     

裂解碳包覆对SiC纳米纤维增强SiC陶瓷基复合材料性能的影响

以SiC纳米纤维(SiC_nf)为增强体,通过化学气相沉积在SiC纳米纤维表面沉积裂解碳(PyC)包覆层,并与SiC粉体、Al₂O₃-Y₂O₃烧结助剂共混制备陶瓷素坯,采用热压烧结工艺制备质量分数为10%的SiC纳米纤维增强SiC陶瓷基(SiC_nf/SiC)复合材料。研究了PyC包覆层沉积时间对SiC_nf/SiC陶瓷基复合材料的致密度、断裂面微观形貌和力学性能的影响。结果表明:在1 100 ℃下沉积60 min制备的PyC包覆层厚度为10 nm,且为结晶度较好的层状石墨结构;相比于纤维表面无包覆层的复合材料,复合材料的断裂韧性提高了35%,达到最大值(19.35±1.17) MPa·m^1/2,抗弯强度为(375.5±8.5) MPa,致密度为96.68%。复合材料的断裂截面可见部分纳米纤维拔出现象,但SiC_nf/SiC陶瓷基复合材料界面结合仍较强,纳米纤维拔出短,表现为脆性断裂。     

Fabrication of an ultra-thick-oriented 3C-SiC coating on the inner surface of a graphite tube by high-frequency induction-heated halide chemical vapor deposition

Cubic SiC (3C-SiC) is a promising material for nuclear industry applications due to its excellent properties. In this report, a highly oriented thick 3C-SiC coating with good crystallinity was prepared on the inner surface of a monolithic graphite tube via high-frequency induction-heated halide chemical vapor deposition using SiCl₄, CH₄, and H₂ as precursors. The texture coefficient (T_C(hkl)), microstructure, and deposition rate along the tube axis was studied. 3C-SiC coating with a high (111) orientation and crystallinity was obtained. Along the tube axis, T_C(111) was consistent with the temperature distribution. The surficial morphology of the 3C-SiC coating changed from pebble-like to hexangular facet and then to hemispherical. The deposition rate and coating thickness were 300 μm/h and 615 μm, respectively, which is sufficiently rapid and thick enough to obtain free-standing SiC tubes for nuclear reactors.     

Wear behavior of SiC nanowire-reinforced SiC coating for C/C composites at elevated temperatures

To improve the wear resistance of SiC coating on carbon/carbon (C/C) composites, SiC nanowires (SiCNWs) were introduced into the SiC wear resistant coating. The dense SiC nanowire-reinforced SiC coating (SiCNW-SiC coating) was prepared on C/C composites using a two-step method consisting of chemical vapor deposition and pack cementation. The incorporation of SiCNWs improved the fracture toughness of SiC coating, which is an advantage in wear resistance. Wear behavior of the as-prepared coatings was investigated at elevated temperatures. The results show that the wear resistance of SiCNW-SiC coating was improved significantly by introducing SiC nanowires. It is worth noting that the wear rate of SiCNW-SiC coating was an order of magnitude lower than that of the SiC coating without SiCNWs at 800 °C. The wear mechanisms of SiCNW-SiC coating at 800 °C were abrasive wear and delamination. Pullout and breakage of SiC grains resulted in failure of SiC coating without SiCNWs at 800 °C.     

Fabrication of AMCs by spray forming: Setting of cognition and social parameters to accelerate the convergence in optimization of spray forming process

Spray forming (also known as spray atomization and deposition) is of interest as a manufacturing technique that combines near-net-shape capabilities with the structural control available through rapid solidification. The present article focuses on development of a comprehensive method to optimize a number of parameters in the spray deposition processing of Al matrix composites including melt superheat, nozzle to substrate distance, metal to gas flow rate and high heat removal rate at the droplet–gas interface. The SiC particles reinforced Al matrix composites were produced by the spray atomization and co-deposition process using optimized parameters. Microscopic investigations of the composite specimens indicate that the distribution of SiC in the matrix alloy is fairly uniform. The wear rate and volume loss showed the two stages (run-in and steady state) of wear for all the applied loads. Microstructural analyses of the wear surfaces and debris were carried out in order to understand the wear mechanism.     

Effect of the surface microstructure of SiC inner coating on the bonding strength and ablation resistance of ZrB2-SiC coating for C/C composites

The present study has been conducted in order to investigate the effect of the surface morphology of SiC inner coating on the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating for C/C composites. The microstructure of SiC inner coatings prepared by chemical vapor deposition and pack cementation at different temperatures were analyzed by X-ray diffraction, scanning electron microscopy, and 3D Confocal Laser Scanning Microscope. Tensile bonding strength and oxyacetylene ablation testing were used to characterize the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating, respectively. Results show that SiC inner coating prepared by chemical vapor deposition has a smooth surface, which is not beneficial to improve the bonding strength and ablation resistance of the sprayed ZrB₂-SiC coating. SiC inner coating prepared by pack cementation at 2000 °C has a rugged surface with the roughness of 72.15 µm, and the sprayed ZrB₂-SiC coating with it as inner layer exhibits good bonding strength and ablation resistance.     

Tribological performance of SiC coating for carbon/carbon composites at elevated temperatures

SiC coating was deposited on carbon/carbon (C/C) composites by chemical vapor deposition (CVD). The effects of elevated temperatures on tribological performance of SiC coating were investigated. The related microstructure and wear mechanism were analyzed. The results show that the as-deposited SiC coating consists of uniformity of β-SiC phase. The mild abrasive and slight adhesive wear were the main wear mechanisms at room temperature, and the SiC coating presented the maximum friction coefficient and the minimum wear rate. Slight oxidation of debris was occurred when the temperature rose to 300?°C. As the temperature was above 600?°C, dense oxide film formed on the worn surface. The silica tribo-film replaced the mechanical fracture and dominated the frication process. However, the aggravation of oxidation at elevated temperatures was responsible for the decrease of friction coefficient and the deterioration of wear rate. The SiC coating presented the minimum friction coefficient and the maximum wear rate when the temperature was 800?°C.     

Deposition of ZnO Films by Combustion Flame Pyrolysis of Solution Precursors

The structural features, including preferred orientation and surface morphology of zinc oxide (ZnO) films deposited by combustion flame pyrolysis were investigated as a function of process parameters, which include precursor solution concentration, substrate–nozzle (S–N) distance, gas flow rate, and duration of deposition. In this technique, the precursor droplets react within the flame and form a coating on an amorphous silica substrate held in or near the flame. Depending on the process parameters, the state of decomposition at which the precursor arrives on the substrate varies substantially and this in turn dictates the orientation and microstructure of the films.