Microstructure and failure mechanism in As-deposited,vacuum plasma-sprayed Ti-6Al-4V alloy

The microstructure, phase composition, and chemical composition of vacuum plasma-sprayed Ti-6Al-4V alloys were examined in detail using a variety of techniques, including x-ray diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy. The observed chemistry and structure were related to the conditions under which the deposit was formed and the phase equilibria in the Ti alloy system. The porosity of the deposit was in the range of 3 to 5%. A slight decrease in the Al content and a slight increase in the amount of oxygen and hydrogen was found relative to the starting powder. Within individual splats, a columnar solidification structure can be seen. However, the as-deposited material is ≥90% α′ martensite that is present in the form of fine lathes on the order of 500 nm in width surrounded by residual β-phase. This herringbone structure obscures to some extent the preexisting columnar structure of the as-solidified β-phase. The material fails at low elongations (∼1%) when tested in tension, with a macroscopic stress-strain curve, which appears to be quite brittle. Examination of the fracture surface, however, reveals a ductile failure mode within individual splats, which is consistent with the structure described above. Sections perpendicular to the fracture surface show that failure occurs at the weak splat boundaries through the development and growth of voids between splats.     

Study on plasma sprayed boron carbide coating

The microstructure, phase composition, and mechanical properties of boron carbide coatings formed by atmospheric plasma spraying (APS) are studied in the present work. The boron carbide coating with high microhardness and low porosity could be produced by APS. The decomposition of boron carbide powder during the plasma spray process would result in the formation of the B_xC phase and an increase of the carbon phase, which is confirmed by transmission electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction results.     

Microstructure of a plasma-sprayed Mo-Si-B alloy

Powders of Mo₅₂Si₃₈B₁₀ were plasma sprayed under inert conditions onto stainless steel substrates to determine if high density free standing forms could be synthesized by this process. Thermal spray conditions were varied to minimize porosity and oxygen impurities while minimizing evaporative metal losses. The assprayed and sintered microstructures were characterized using scanning and transmission electron microscopy and quantitative x-ray diffraction (XRD). The as-sprayed microstructure consisted of elongated splats tens of microns in length and only one to three microns in thickness. The splats contained submicrometer grains of primarily MoB and Mo₅Si₃B _x (T1) and minor amounts of MoSi₂ and a glassy grain boundary phase. The interior of the splats typically consisted of a fine eutectic of MoB and T1. Small pieces were cut out of the cross section of the sample and pressureless sintered for 2, 6, and 10 h at 1800 °C in flowing Ar. After sintering for 2 h at 1800 °C, the samples exhibited a coarser but equiaxed microstructure (1 to 5 μm grain size) containing 78 vol.% T1, 16 vol.% MoB, and 6 vol.% MoSi₂ as determined by XRD. Approximately 8 at.% of the Si formed silica. The high-temperature anneal removed all vestiges of the layered structure observed in the as-sprayed samples.     

Deposition and properties of high-velocity-oxygen-fuel and plasma-sprayed Mo-Mo2C composite coatings

Molybdenum thermal-spray coatings, dispersion strengthened by molybdenum oxides and molybdenum carbides, play an important role in industrial tribological applications. Traditionally, they have been prepared by plasma and wire flame spraying. High porosity and lower cohesion strength limit their application in situations where both galling and abrasion wear is involved. In this study, high-velocity-oxygen-fuel (HVOF) deposition of molybdenum and molybdenum carbide coatings was attempted. Deposition was achieved for all powders used. Composition, microstructure, mechanical, and wear properties of the HVOF synthesized coatings were evaluated and compared with plasma-sprayed counterparts. The HVOF coatings possessed a very good abrasion resistance, whereas plasma deposits performed better in dry sliding tests. Measurements showed a close relationship between the coating surface hardness and its abrasion resistance. Results also suggested correlation between molybdenum carbide distribution in the molybdenum matrix and the sliding friction response of Mo-Mo₂C coatings.     

Microstructure and properties of WC-10%Co-4% Cr Spray powders and coatings: Part 1. Powder characterization

WC-10% Co-4% Cr¹ represents an important composition for thermally sprayed hardmetal-like coatings that are applied when simultaneous wear and corrosion resistance is required. In this paper, four commercially available spray powders obtained by various production techniques (sintering and crushing, agglomeration and plasma densification) were thoroughly characterized using a broad variety of physical and chemical methods, including scanning electron microscopy (SEM), energy-dispersive x-ray (EDX), x-ray diffraction, adsorption, mercury intrusion, and helium pycnometry. Special emphasis is given to the interdependence of the chemical and phase compositions. The cooling rate applied during preparation of the spray powders seems to be responsible for the appearance of equilibrium or nonequilibrium phases, as was established from the investigation of the spray powders after heat treatment at 1000 °C. The amount of Cr added to the composition, 4%, is higher than the solubility limit of chromium in the binder phase in the presence of WC; hence, a second carbide phase is formed. The carbon content determines which carbide phase is formed and how both cobalt and chromium are distributed between the hard and the binder phases. A substantial carbon deficiency leads to nearly complete bonding of both chromium and cobalt into carbide phases. As was shown by differential scanning calorimetry (DSC) experiments, such spray powders do not form a melt in the temperature range up to 1465 °C, while powders containing clearly detectable amounts of metallic cobalt form a melt above 1210 °C.     

催化剂对B4C-SiC-Si复合材料组织与性能的影响

采用间苯二酚甲醛水基凝胶体系,通过凝胶注模成形法制备B4C/C坯体,结合反应渗Si烧结工艺制备B4C-SiC-Si复合材料。重点考察Na2CO3、NaOH、KOH、Na2SiO3和NaHCO3 5种不同碱性催化剂对B4C-SiC-Si复合材料组织与力学性能的影响。结果表明:催化剂主要影响复合材料的组织均匀性及游离Si的尺寸。分别添加5种不同催化剂时复合材料均由BxC、B12(B,C,Si)3、SiC和Si组成。以NaOH、KOH或NaHCO3为催化剂时,复合材料中游离Si的分布较均匀、尺寸较小;以NaOH作为催化剂时,B4C-SiC-Si复合材料的综合力学性能最佳,其硬度、抗折强度和断裂韧性分别达到16.9 GPa、296 MPa和4.15 MPa·m1/2。     

Microstructure and adhesion of Cr3C2-NiCr vacuum plasma sprayed coatings

Vacuum plasma spraying (VPS) was used to spray a Cr₃C₂-NiCr coating of ∼ 150, 300 and 450 μm in thickness onto a plain carbon steel substrate, employing a commercially available Cr20Ni9.5C powder. The splat microstructures observed in the coating were found to consist of a NiCr matrix with a predominant Cr₃C₂ phase, besides Cr₇C₃ and Cr₂O₃. The adhesion of the coating to the substrate was evaluated by means of interfacial indentation techniques. It has been found that the interfacial toughness value changes from 7.6 to 10.1 MPa m^1/2 when the thickness increases from 150 to 450 μm. Also, it has been found that the parameter Kca_o, determined by linear regression from the Kca versus 1 / e² curve by means of the interfacial indentation model advanced by Chicot et al., has a value of ∼ 9.8 MPa m^1/2.     

Synthesis of B4C-Nano TiB2 Composite Powder by Sol-Gel Method

Production of (B₄C-nano TiB₂) composite powder by chemical method was evaluated in this study. Starting materials were boron carbide, carbon, and titanium (IV) iso propoxide (TTIP). Water was used as a hydrolyser agent. TTIP was hydrolyzed with water and, consequently, amorphous Ti(OH)₄ was formed. Heat-treatment of Ti(OH)₄ at 100 and 850 °C led to the production of TiO₂ and TiB₂ phases, respectively. The effect of heat-treatment time and temperature on the phase transformation and size of the produced nano powder were investigated. The produced nano powder was characterized by XRD, SEM, and DTA. It was found that heat-treatment time and temperature have significant effects on the amount and size of the produced TiB₂ powder. The data also reveal that the minimum temperature for TiB₂ formation is 650 °C.     

Microstructure evolution during reactive plasma spraying of MoSi2 with methane

The mechanisms that govern microstructure evolution during reactive plasma spraying of MoSiz using 100% methane were investigated, with particular emphasis on the thermodynamics and kinetics of the relevant phase transformations and chemical reactions. The reactive plasma-sprayed M0Si₂ exhibited a dense, multilayered microstructure. In addition to the M0Si₂ matrix, significant amounts of M0₅Si₃ and elemental carbon were observed, along with a small amount of SiC. Thermodynamic and kinetic analysis predicted a large volume fraction of M0₅Si₃ and a small amount of SiC in the as-deposited reactive plasma-sprayed MoSi₂, in agreement with the experimental observations.     

Spark plasma sintering of B4C ceramics: The effects of milling medium and TiB2 addition

Boron carbide (B₄C) ceramics, with a relative density up to 98.4% and limited grain growth, were prepared at 1600-1800 °C by spark plasma sintering (SPS) technique. The effects of powder milling medium (water and 2-propanol) on the powders' surface characteristics and TiB₂ addition on the sintering densification were investigated. The ball milling processing of B₄C powders in water can promote the sintering of B₄C ceramics. A B₂O₃ layer on B₄C particle surface is concluded to promote the densification of the B₄C ceramics at an early sintering stage. This B₂O₃ layer, which normally inhibits the densification process at the final stage of the sintering, can be reduced through reaction with TiB₂ particles, resulting in further densification of the B₄C ceramics.     

等离子熔覆-注射B4C铁基熔覆层组织耐磨性研究

目的采用等离子熔覆-注射工艺在Q235基体上制备B4C铁基熔覆层并研究其耐磨性。方法通过OM,SEM,EDS等分析熔覆层及界面的组织特征,并进行耐磨性测试。结果当B4C质量占主体熔覆材料质量的18%时,注射熔覆层表面比较平整,无裂纹。注射熔覆层组织致密,界面呈现平直的亮白色过渡层,稀释率小,与基体形成了良好的冶金结合。B4C陶瓷颗粒表面溶解会形成Fe,Cr等元素的硼化物。等离子熔覆-注射B4C熔覆层的耐磨性是42CrMo的22倍,是16Mn钢的41倍。结论等离子熔覆-注射B4C工艺能够增强B4C与熔覆层之间的结合力,提高熔覆层的硬度和耐磨性。     

Photoresponse and donor concentration of plasma-sprayed TiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript>-ZnO electrodes

The photoelectrochemical characteristics of plasma-sprayed porous TiO₂, TiO₂-5%ZnO, and TiO₂-10%ZnO electrodes in 0.1 N NaOH solution were studied through a three-electrode cell system. The microstructure, morphology, and composition of the electrodes were analyzed using an electron probe surface roughness analyzer (ERA-8800FE), scanning electron microscopy, and x-ray diffraction. The results indicate that the sprayed electrodes have a porous microstructure, which is affected by the plasma spray parameters and composition of the powders. The TiO₂-ZnO electrodes consist of anatase TiO₂, rutile TiO₂, and Zn₂Ti₃O₈ phase. The photoresponse characteristics of the plasma-sprayed electrodes are comparable to those of single-crystal TiO₂, but the breakdown voltage is close to 0.5 V (versus that of a saturated calomel electrode). The short-circuit photocurrent density (J _SC) increases with a decrease of donor concentration, which was calculated according to the Gartner-Butler model. For the lowest donor concentration of a TiO₂-5%ZnO electrode sprayed under an arc current of 600 A, the short-circuit J _SC is approximately 0.4 mA/cm² higher than that of the TiO₂ electrodes under 30 mW/cm² xenon light irradiation. The J _SC increases linearly with light intensity. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

低铯含量KF-CsF-AlF3铝钎剂的物相结构与熔化特性的关系

采用差示扫描量热法(DSC)对湿法合成的低铯含量KF-CsF-AlF3铝钎剂样品进行了熔化特性测定,归纳总结了该钎剂的熔化特性随成分的变化规律.利用XRD,SEM和EDS分析技术,研究了典型样品的物相结构和微观形貌.结果表明,低铯含量钎剂主要由棒状K2AlF5和絮状Cs2KAl3F12机械混合物组成,致使微观上局部成分偏离三元共晶成分点E4或E5.因此在加热过程中难以发生三元共晶反应,而是在温度505℃附近发生K2AlF5固态相转变反应.在温度525℃附近发生Cs2 KAl3 F12相的熔化反应,这是造成实际湿法合成的钎剂样品熔化温度与理论相图差异较大、特别是液相线升高的根本原因.     

热喷涂TiB2-Ni复合涂层组织结构和力学性能

采用团聚烧结方法制备TiB₂-Ni复合粉末喂料,并采用大气等离子喷涂和高速火焰喷涂两种喷涂方法制备了TiB₂-Ni涂层,比较分析了两种涂层的显微组织、物相组成、孔隙率、硬度和断裂韧性.结果表明,与等离子喷涂相比,高速火焰喷涂制备的TiB₂-Ni涂层具有更高的致密度,TiB₂含量,硬度和断裂韧性.两种涂层中TiB₂都没有发生明显的脱硼,氧化,但等离子喷涂过程中TiB₂向金属相中发生了溶解生成了大量脆性Ni₂₀Ti₃B₆相,并降低了涂层中TiB₂的含量,这是涂层硬度和断裂韧性相对较低的主要原因.     

Cu元素对Al-Zn-Mg-Er-Sc-Zr合金组织演变和腐蚀性能的影响

利用Cu元素的含量变化研究了Al₈Cu₄Er相的形成与演变规律及其对Al-Zn-Mg-Er-Sc-Zr合金腐蚀性能的影响。结果表明：随着Cu含量的增加,合金晶粒得到显著细化,但同时固溶态合金不同类型的残余相增多;Al₈Cu₄Er相与Al-Fe相存在伴生关系,二者通过Cu与Fe交互作用实现相的转化,且可表述为如下关系式：;不同成分合金的晶间腐蚀均表现出与残余相密切相关的点蚀特征,含Cu、Er的Al-Fe相虽然具有更小的腐蚀坑尺寸,但网状分布特征使腐蚀深度有所增加;而具有更好耐蚀性能的Al₈Cu₄Er则因相的粗化和它与Al-Fe相的伴生关系导致合金耐蚀性能严重下降。     

Behavior of plasma-sprayed thermal barrier coatings during thermal cycling and the effect of a preoxidized NiCrAlY bond coat

The failure mechanisms of thermal barrier coatings (TBCs) subjected to a thermal load are still not entirely understood. Thermal stresses and/or oxidation cause the coating to fail and hence must be minimized. During the present investigation, TBCs up to 1.0 mm were sprayed and withstood high thermal stresses during thermal testing. Owing to the substantial thickness, the temperature at the top coat/bond coat interface was relatively low, resulting in a low oxidation rate. Furthermore, bond coats were preoxidized before applying a top coat. The bond strength and the behavior during three different thermal loads of the preoxidized TBCs were compared with a standard duplex TBC. Finite-element model (FEM) calculations that took account of bond coat preoxidation and interface roughness were made to calculate the stresses occurring during thermal shock. It is concluded that the thick TBCs applied during this research exhibit excellent thermal shock resistance and that a preoxidizing treatment of the bond coat increases the lifetime during thermal loading, where oxidation is the main cause of failure. The FEM analysis gives a first impression of the stress conditions on the interface undulations during thermal loading, but further development is required.     

半固态热等静压Al/B4C复合材料液相流动行为与致密化机理

铝基碳化硼复合材料是一种重要的中子吸收材料。为了制备具有更高密度的铝基碳化硼材料,研究采用粉末冶金半固态热等静压方法制备了含量为30%碳化硼的铝基碳化硼材料,采用WANCE100型材料力学性能试验机和SIRION200型扫描电镜研究了复合材料的力学性能及显微形貌。结果表明：半固态热等静压工艺可制备获得接近理论密度的Al/B4C复合材料;虽然Al/B4C材料抗拉强度可提升至约300Mpa,但过高碳化硼含量也使得该材料脆性特征十分明显;研究同时采用间接的方法观察到了半固态工艺过程中生成的液相,该液相不仅可改善碳化硼颗粒与铝基体的结合性,在高温高压下液相的流动还起到填充复合材料内部空隙的作用。半固态热等静压工艺过程中产生的液相是复合材料密度和机械性能提升的主要原因。     

YbB6 对Ti-6Al-4V钛合金组织和性能的影响

采用放电等离子烧结（SPS）制备了含YbB₆的Ti-6Al-4V钛合金,并研究了YbB₆对Ti-6Al-4V钛合金显微组织和力学性能的影响。结果表明,随着YbB₆含量的增加,复合材料的显微组织发生转变,晶粒明显细化,原位反应生成的TiB晶须和Yb₂O₃颗粒有利于复合材料力学性能的提高。此外,当添加0.6%（质量分数）YbB₆后,烧结样品的相对密度、显微硬度、屈服强度、极限拉伸强度和延伸率分别为99.43%、4030 MPa、903 MPa、1148 MPa和3.3%。与Ti-6Al-4V试样相比,其数值分别提高了0.37%、13.8%、38.07%和17.14%。强化机制主要是组织转变、晶粒细化和弥散强化。随着YbB₆含量的增加,断裂方式主要为韧性断裂和脆性断裂。     

焊接热循环及峰值温度对C-HRA-2镍基合金组织演变的影响

通过焊接热模拟方法得到了C-HRA-2镍基合金在不同一次峰值温度(T_p1=1150,1250,1350℃)和二次峰值温度(T_p2=850,950,1050,1150,1250,1350,1450℃)下的焊接热影响区(HAZ),研究了峰值温度和热循环次数对C-HRA-2合金HAZ微观组织演变的影响,并测试了其显微硬度。采用光学显微镜、扫描电镜和透射电镜对HAZ的微观结构和碳化物进行表征。结果表明,在T_p1=1150℃的HAZ中,可观察到沿晶界析出的细小的M₂₃C₆碳化物。在T_p1>1250℃的HAZ中,在晶界附近发现由于成分液化而导致的γ基体与M₂₃C₆碳化物组成的微观结构。当T_p2在1050～1250℃,可在HAZ中的晶界附近观察到与在T_p1     

等离子喷涂及真空热处理过程中MoSi2涂层的相演变

分别采用MoSi2Mo-66.7%Si和Mo-70%Si(摩尔分数)粉末为原料,在45钢上大气等离子喷涂MoSi2涂层,对喷涂前后相以及真空热处理对涂层相和组织的影响进行了研究.结果表明,以MoSi2为原料喷涂后,涂层主要是由MoSi2和Mo5Si3相组成,MoSi2由亚稳定的MoSi2(h)六方结构和稳定的MoSi2(t)四方晶体结构组成.而Mo-66.7%Si和Mo-70%Si两种粉末喷涂后,涂层均由Mo,Si单质粉组成,喷涂过程没有形成硅化物.Mo-66.7%Si涂层在1 050,1 200℃热处理1 h后,涂层中主相是Mo5Si3,次相为MoSi2.Mo-70%Si涂层在800,1 000 ℃热处理30 min后,生成了少量的MoSi2,Mo5Si3和Mo3Si.在1 050 ℃热处理时,随着时间的延长,MoSi2相含量相对增多,当热处理时间为30,45 min时,由于Mo,Si和Fe之间的反应扩散,还生成了FeMoSi和Fe3Si相.