Development and Characterization of Cr3C2–NiCr Coated Superalloy by Novel Cold Spray Process

The coating deposition by cold spray process depends upon the mechanical properties of both the coating and substrate materials. This results in variation of the coating microstructure for different coating–substrate system materials. In this study cold sprayed 75%Cr₃C₂–25%(Ni–20%Cr) coating–superalloy system is developed. The microstructure of the coating obtained is suitable for restricting the penetration of corrosive species toward the substrate. The techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX), and elemental X-ray analysis were used for characterization of the newly developed cold spray coated superalloy.     

Measuring mechanical properties of plasma-sprayed alumina coatings by nanoindentation technique

AISI 1020 steel substrate is coated with alumina as feedstock material using plasma spraying process in order to correlate the microstructural features with mechanical properties of coating. The present work focuses on the effects of microstructural inhomogeneity on mechanical properties of alumina coating through nanoindentation technique. Young’s modulus and hardness of the alumina coating are analytically evaluated. Indentation stress–strain curves are generated from the experimentally obtained load–displacement curves to characterise the mechanical properties of the coating. The results have shown large variation in hardness and Young’s modulus of alumina due to microstructural inhomogeneity of the coating.     

冷喷涂CuO/ZnO/Al_2O_3催化涂层微结构

采用冷喷涂方法在铝基板上制备了CuO/ZnO/Al2O3催化涂层,并利用扫描电镜、能谱分析仪对涂层进行了表征。结果表明,冷喷涂前后催化剂颗粒和涂层的组成基本相同;涂层与基板和涂层之间以机械咬合结合为主;加热气体温度较低时,涂层呈现疏松扁平层叠和多孔结构;涂层形貌受加热气体温度与颗粒破碎的影响。在该涂层上进行甲醇水蒸气重整制氢试验,结果表明冷喷涂技术可以成功应用于制备催化功能涂层材料。     

Microstructural features of dynamic recrystallization in alloy 625 friction surfacing coatings

In friction surfacing (FS), material is deposited onto a substrate in the plasticized state, using frictional heat and shear stresses. The coating material remains in the solid state and undergoes severe plastic deformation (SPD) at high process temperatures (≈0.8 T_melt), followed by high cooling rates in the range of 30?K/s. Dynamic recrystallization and the thermal cycle determine the resulting microstructure. In this study, Ni-based alloy 625 was deposited onto 42CrMo4 substrate, suitable, for instance, for repair welding of corrosion protection layers. Alloy 625 is known to undergo discontinuous dynamic recrystallization under SPD, and the resulting grain size depends on the strain rate. The coating microstructure was studied by microscopy and electron backscatter diffraction (EBSD). The coatings exhibit a fully recrystallized microstructure with equiaxed grains (0.5–12?µm) and a low degree of grain average misorientation. Flow lines caused by a localized decrease in grain size and linear alignment of grain boundaries are visible. Grain nucleation and growth were found to be strongly affected by localized shear and nonuniform material flow, resulting in varying amounts of residual strain, twins and low-angle grain boundaries in different regions within a single coating layer’s cross section.

FS can be used to study dynamic recrystallization at high temperatures, strains and strain rates, while at the same time materials with a recrystallization grain size sensitive to the strain rate can be used to study the material flow during the process.     

Microstructural characterization of Co-based coating deposited by low power pulse laser cladding

A detailed microstructural study of Stellite 6 coating deposited on a low carbon ferritic steel substrate using preplaced powder method and low power Nd:YAG pulse laser is performed. The grain structure and solidification texture of the coating are investigated by orientation imaging microscopy (OIM) and scanning electron microscopy. In addition, the effect of consecutive pulses on the microstructure of the coating is examined. The orientation relationship (OR) at coating/substrate interface and the solid state phase transformation in heat-affected zone are studied as well as the Vickers microhardness profile measurement in order to support the microstructural observations. An important conclusion is reached that the shape of solidification front during pulsed laser cladding is similar to the shape of solidification front during continuous cladding with a doubled laser beam scanning speed. Further, OIM reveals the Greninger–Troiano OR between the face centered cubic coating and bcc substrate grains. It is concluded that at the moment of solidification epitaxial growth of the grains in the coating occur on the austenitic grains of the substrate and that an austenite–ferrite transformation occurs in the heat-affected zone upon subsequent cooling.     

A study on the microstructure and property development of d.c. magnetron cosputtered ternary titanium aluminium nitride coatings Part III effect of substrate bias voltage and temperature

Advanced ternary (Ti,Al)N coatings were produced by reactive magnetron co-sputtering technique with separate titanium and aluminium targets at a 30° magnetron configuration under various substrate bias voltages and temperatures. The effect of substrate bias and temperature on the microstructure and property development of the coatings was investigated. It was found that an increase in substrate bias and/or substrate temperature imposed no major effect on the composition and phase formation of the (Ti,Al)N coatings, but had significant influence on the development of their microstructure and surface morphology. As the substrate bias and/or temperature increased, the coating structure was densified with development of fine grain size and reduced surface roughness, resulting in a substantial increase of the coating hardness. However as the substrate bias increased over 200 volts, excessive residual stress was built up, causing a fracture of the coatings. It is believed that the microstructure and property enhancement is attributed to an increased translational kinetic energy of the depositing atoms and a greater thermal energy provided to the substrate and the coating material with increasing substrate bias and/or temperature. The adatom mobility and the surface diffusion of atoms are enhanced to reduce the detrimental effects induced by the statistical roughening and self-shadowing of asputter deposition process. A densified zone T structure with low porosity and improved properties is produced.     

Feasibility study on preparation of coatings on Ti–6Al–4V by combined ultrasonic impact treatment and electrospark deposition

A novel method combining ultrasonic impact treatment (UIT) with electrospark deposition was developed to prepare coatings on Ti–6Al–4V substrates. The microstructure, phase composition, residual stress, microhardness, and wear performance of the coating were studied, and new amorphous and nanocrystalline phases (titanium carbide nitride and iron titanium oxide) were found. In addition, the residual stress in the coating and in the substrate near the coating is compressive stress. The maximum compressive residual stress is about −717 MPa, and its depth is about 470 μm. Because of contributions from multiple factors, the wear volume loss of the sample subjected to combined UIT and electrospark processing was reduced by four orders of magnitude compared with that of the base material.     

用Sol-Gel法在PC上制备有机-无机复合耐磨涂层

用ｓｏｌ－ｇｅｌ法在聚碳酸酯ＰＣ上制备一层耐磨涂层。该涂层为有机－无机复合网络,由γ—缩水甘油醚氧丙基三甲氧基硅烷（γ—ＧＰＴＭＳ）和金属醇盐Ｔｉ（ＯＣ２Ｈ５）４合成。采用浸涂工艺,热固化后可得到几微米厚的透明涂层。用３—氨丙基三乙氧基硅烷（３—ＡＰＳ）对ＰＣ表面进行处理,可使涂层与基体的结合力大大提高。涂层的基本骨架由Ｓｉ－Ｏ－Ｔｉ组成,能显著改善ＰＣ材料的耐磨性能。     

Synthesis and Characterization of Cr-ZrO2 Composite Coating Formed by DC and Pulse Electrodeposition

Chromium–zirconia (Cr-ZrO₂) composite coatings were developed on low-carbon steel substrate by direct current (DC) and pulse electrodeposition (PED) technique with different pulse frequency and duty cycles to enhance mechanical properties of the coating. The phases and morphology of the coating were studied with scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. Surface mechanical properties were analyzed by micro-hardness and ball-on-plate wear study. It was found that pulsing and higher pulse frequencies refine the matrix and increase the ZrO₂ content in the coating. Apart from fine structure and dispersion, crystallographic orientation of Cr matrix also gives its effect on hardness and wear properties. Wear mechanism was found to be mainly abrasive in nature with little adhesive inclination in case of DC deposition.     

Composite ceramic-metal coatings by means of combined electrophoretic deposition and galvanic methods

A method based on the combination of electrophoretic and galvanic deposition techniques has been developed to fabricate metal-ceramic composite coatings on metallic substrates. A ZrO₂-Ni composite coating with interpenetrating microstructure was produced on stainless steel plates. For electrophoretic deposition of the ceramic component, a non-aqueous suspension consisting of zirconia nanoparticles, ethanol and addition of 4-hydroxybenzoic acid was optimised by electrokinetic sonic amplitude (ESA) measurements. The zirconia deposits were partially sintered to create an open porous structure (porosity = 40–50%), which was subsequently filled with Ni by galvanic deposition. The bonding strength between the composite coating and the stainless steel substrate was improved by a final heat-treatement at 950°C for 3 h which promoted the diffusion of Ni into the steel substrate and the formation of a diffusion interlayer. The high adhesion strength of the composite coating to the stainless steel substrate after the diffusion bonding heat-treatment was confirmed by 3-point flexural strength tests. The coating exhibited a homogeneous interpenetrating microstructure with hardness values >6 GPa.     

Characterizing microstructure of refractory porous materials

Liquid Expulsion technique was used in this study to quantify the transport properties and microstructure of a refractory coating used in the Lost Foam Casting process. The pore size information obtained from the Liquid Expulsion Method is found to be well correlated with the transport properties of the porous coating material. For manufacturing process control, the viscosity of these coating slurries prior to its application on expanded polystyrene foam is often reduced by dilution with water and/or using a dispersant. In this paper, the effects of diluting or dispersing the slurry on the microstructure and transport properties of the dried refractory coatings are evaluated. Results show that the dilution and dispersion have opposing influences on the pore size and transport properties. Adding dispersant was found to reduce the transport properties of the refractory coatings significantly, potentially leading to defects in metal castings. The pore characterization technique developed in this paper is also used to determine the effects of drying methods (oven versus air dry) on the pore size and transport properties.     

激光熔覆制备原位自生TiC增强Ni3（Si,Ti）复合涂层研究

运用激光熔覆技术修复受损的烟汽轮机叶片,在GH864镍基合金表面制备原位自生TiC颗粒增强Ni3(Si,Ti)金属间化合物复合涂层.利用扫描电镜、能谱分析仪、X射线衍射仪及显微硬度计研究了(Ti+C)的加入量对熔覆层组织及硬度的影响.研究表明:在优化的工艺参数下可获得宏观质量完好,无裂纹、气孔等缺陷,且与基体呈冶金结合的激光熔覆层,熔覆层由Ni(Si)、Ni3(Si,Ti)和TiC组成;当合金粉末中(Ti+C)的加入量为20%时熔覆层的硬度最高可达780Hv,是基体材料的2.4倍.     

超音速冷喷涂Cu-Al_2O_3复合涂层特性

采用冷喷涂技术在铝基板上制备了Cu-Al2O3复合涂层,以复合涂层为催化荆进行了甲醇水蒸气重整制氢实验,并利用扫描电镜、能谱分析仪和X射线衍射仪对实验前后的涂层进行了表征.结果表明,涂层之间和涂层与基板问的结合主要为机械咬合,孔隙率较高,与喷涂颗粒相比,涂层没有相的变化;喷涂后Al2O3颗粒发生了破碎,而Cu颗粒变形不充分,Cu颗粒特性导致涂层中铜含量比粉末中的低;甲醇水蒸气重整制氢实验表明,Cu-Al2O3复合涂层由于含氧高而具有比纯铜涂层更好的稳定性.     

Nanostructured Al2O3–TiO2 coatings for high-temperature protection of titanium alloy during ablation

Plasma-sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings were successfully fabricated on titanium alloys (Ti–6Al–4V) using as-prepared feedstock. Ablation experiments for the titanium alloy samples with or without a coating were carried out using a Metco 9MB plasma gun. The microstructure, phase constituents and mechanical properties of the titanium alloys before and after ablation were investigated by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Vickers hardness tester. The surface morphologies, cross-sectional microstructure and hardness of titanium alloys with coatings are similar before and after ablation. In contrast, the microstructure and mechanical properties of the titanium alloy without coating are significantly changed after ablation. The surface coating is found to serve as a protective coating during ablation.     

Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying

Hypereutectic Al-Si-Cu coatings were prepared by supersonic atmospheric plasma spraying to enhance the surface performance of lightweight alloys.To find out optimum process conditions and achieve desirable coatings,this work focuses on the influence of three important parameters (in-flight par-ticle temperature,impact velocity,and substrate temperature) on the collected splats morphology,coatings microstructure and microhardness.Results show that appropriate combinations of temper-ature and velocity of in-flight particles cannot only completely melt hypereutectic Al-Si-Cu particles,especially the primary Si phase,but also provide the particles with sufficient kinetic energy.Thus,the optimized coating consists of 98.6 ％ of fully-melted region with nanosized coupled eutectic and 0.9 ％of porosity.Increasing the substrate deposition temperature promotes the transition from inhomoge-neous banded microstructure to homogeneous equiaxed microstructure with a lower porosity level.The observations are further interpreted by a newly developed phase-change heat transfer model on quan-titatively revealing the solidification and remelting behaviors of several splats deposited on substrate.Besides,phase evolutions including the formation of supersaturated α-Al matrix solid solution,growth of Si and Al2Cu phases at different process conditions are elaborated.An ideal microstructure (low frac-tions of unmelted/partially-melted regions and defects) together with solid solution,grain refinement,and second phase strengthening effects contributes to the enhanced microhardness of coating.This inte-grated study not only provides a framework for optimizing Al-Si based coatings via thermal spraying but also gives valuable insights into the formation mechanisms of this class of coating materials.     

Comparison of Tantalum and Hydroxyapatite Coatings on Titanium for Applications in Load Bearing Implants

Over the years hydroxyapatite (HA) coatings have been used to improve biologic properties of Ti‐based load bearing metal implants. However, applicability of HA coated implants is subjected to physical stability of the HA phase and mechanical integrity of the coating‐substrate interface. In this study, we have compared the microstructure and in vitro cell–materials interactions of newly developed laser deposited Ta coatings and radio frequency (RF) induction plasma sprayed HA coatings on Ti substrate. In vitro biocompatibility study, using human osteoblast cell line hFOB, showed equally excellent cellular adherence and growth on Ta and HA coatings. Quantitative assay of cell survivability on these coatings showed that the Ta coatings provide comparable initial cell attachment to that of HA coatings. Microstructural analysis of the coatings showed strong metallurgical bonding without sharp interface between the Ta coating and the Ti substrate, while the interface between HA coating and the Ti substrate was sharp. The interface microstructural features and in vitro cell–materials interactions of Ta coatings on Ti clearly demonstrate their potential to replace HA based coatings for enhanced/early biologic fixation. Other significant benefits of these dense Ta coatings include high toughness, strong bonding with the substrate, and long‐term stability of the interface.     

Development of dense corrosion resistant coatings by an improved HVOF spraying process

For 6 years, we have developed corrosion resistant coatings to protect steel structures in the marine environment by using a thermal spray technique. This paper summarizes the major developments and results obtained. Such a coating requires primarily impermeability and secondarily homogenous and clean microstructure. In order to make denser and highly corrosion resistant coatings, we selected spray materials and improved fabrication processes. HastelloyC was a suitable material for High Velocity Oxy-Fuel (HVOF) spraying to form corrosion resistant coatings because of its high resistance against thermal oxidation as well as seawater corrosion, especially crevice and pitting corrosion. An inert gas shroud system was attached with a commercial HVOF apparatus and this attachment increased the in-flight velocity of spray particles over 750 m s^?1 and simultaneously suppressed oxidation significantly. In addition, some new methods were designed to evaluate the sprayed particle’s state and the coating properties with high accuracy and sensitivity. Thermal energy of in-flight spray particles was revealed by molten fraction of spray particles, determined by quantitative analysis of melted and unmelted particles captured in an agar gel. Through-porosity of the coatings with open porosity below 0.1% was determined by using Inductively Coupled Plasma analysis of dissolved substance from substrate through the penetrating path of the coatings. The coating of HastelloyC nickel base alloy by the HVOF spraying with the gas shroud attachment had zero through-porosity and 0.2 mass% of oxygen content. The laboratory corrosion tests showed that the on-shroud HastelloyC coating was comparable to the bulk material of HastelloyC in terms of corrosion resistance. This coating, formed on steel, demonstrated an excellent protective performance over 10 months in the marine exposure test.     

Microstructural characterisation of high velocity oxyfuel thermally sprayed Stellite 6

Abstract

High velocity oxyfuel (HVOF) thermal spraying with an oxygen-propylene combustion flame was used to deposit coatings of a Co–Cr–W–C alloy (similar in composition to Stellite 6) from gas atomised feedstock powder. The microstructural characteristics of the deposits were investigated using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In particular, the tripod polishing technique was used to prepare large areas of electron transparent membrane for TEM analysis; these areas were typically of the order of several hundred square micrometres. Accordingly, SEM and TEM analysis could be directly correlated, providing a clear understanding of the overall nature of the coatings. The coatings exhibited characteristic splatlike, layered morphologies due to the deposition and resolidification ofmolten or semimolten powder particles. The as sprayed microstructure consisted of a face centred cubic Co rich metallic matrix with a small fraction of CoCr₂O₄ spinel oxide. The latter formed either as intersplat lamellae or intrasplat globules. Under the HVOF spray conditions employed, a high proportion of the feedstock powder appeared to have been fully melted prior to impact on the substrate, with partially or unmelted powder accounting for < 20% of the microstructure. Formation of the M₇C₃ phase, normally present in Stellite 6, appeared to have been suppressed by the high cooling rate during spray deposition.     

超音速火焰喷涂制备纳米和微米结构WC-η涂层的耐熔锌腐蚀性能

以钨氧化物、钴氧化物和炭黑为原料, 通过原位还原碳化反应制备纳米WC-η(η为Co₃W₃C、Co₆W₆C等缺碳相)复合粉, 粉末平均粒径为155 nm。该复合粉经团聚造粒制备得到具有高致密性和良好流动性的热喷涂粉末。以此纳米结构和商业化的微米结构低碳WC-12Co粉末作为喂料, 通过超音速火焰喷涂制备硬质合金涂层。结果表明, 纳米结构涂层中生成了一定量等轴状的W₂C相, 裂纹主要沿晶界或相界面扩展, 而微米结构涂层中除W₂C外还含有较多的W相, 主要包裹在WC颗粒表面, 穿晶断裂比例较高, 裂纹扩展路径较平滑。由于纳米结构涂层组织致密、晶粒细小、界面积大, 因此比微米结构涂层具有更高的硬度和断裂韧性。两种涂层在熔融锌液中浸泡200 h后, 微米结构涂层中产生了较多的横向和纵向裂纹, 导致材料的大面积剥落和基材腐蚀; 纳米结构涂层中没有发生锌的浸蚀, 在局部产生了少量纵向裂纹, 裂纹间隙被钨钴氧化物所填充, 反而抑制了熔锌对涂层的腐蚀, 因此纳米结构涂层表现出更高的耐熔锌腐蚀性能。     

Effect of heat treatment on the microstructure and property of cold-sprayed nanostructured FeAl/Al2O3 intermetallic composite coating

It is difficult to deposit dense intermetallic compound coatings by cold spraying directly using compound feedstock powders due to their intrinsic low temperature brittleness. A method to prepare intermetallic compound coatings in-situ employing cold spraying was developed using a metastable alloy powder assisted with post heat treatment. In this study, a nanostructured Fe(Al)/Al₂O₃ composite alloy coating was prepared by cold spraying of ball-milled powder. The cold-sprayed Fe(Al)/Al₂O₃ composite alloy coating was evolved in-situ to FeAl/Al₂O₃ intermetallic composite coating through a post heat treatment. The effect of heat treatment on the phase formation, microstructure and microhardness of cold-sprayed Fe(Al)/Al₂O₃ composite coating was investigated. The results showed that annealing at a temperature of 600 °C results in the complete transformation of the Fe(Al) solid solution to a FeAl intermetallic compound. Annealing temperature significantly influenced the microstructure and microhardness of the cold-sprayed FeAl/Al₂O₃ coating. On raising the temperature to over 950 °C, diffusion occurred not only in the coating but also at the interface between the coating and substrate. The microhardness of the FeAl/Al₂O₃ coating was maintained at about 600HV_0.1 at an annealing temperature below 500 °C, and gradually decreased to 400HV_0.1 at 1100 °C.