TiAlNb intermetallic compound coating prepared by high velocity oxy-fuel spraying

Ti_28.35Al_63.4Nb_8.25 (at.%) intermetallic compound coatings were sprayed onto 316 L stainless steel substrates by HVOF processes using various parameters. By varying the grit blasting pressure between 0.11 and 0.55 MPa, the effects of substrate roughness on the adhesion of TiAlNb thermal sprayed coatings were investigated. The microstructure, porosity and microhardness of the coatings were characterized by SEM, XRD, Image Analysis and Vickers hardness analysis. The tensile adhesion test (TAT) specified by ASTM C 633-79 was used to measure the tensile bonding strength of the coating. The results show that the coatings with substrate roughness of 8.33 μm displayed the best combined strength. TiAlNb coatings had a lamellar microstructure with different spraying parameters. The porosity, bonding strength, microhardness of coatings were assessed in relation to the spraying processes. The thickness of bond coat on the bond strength of coatings was also discussed.     

Effect of grit-blasting on substrate roughness and coating adhesion

Statistically designed experiments were performed to compare the surface roughness produced by grit blasting A36/1020 steel using different abrasives. Grit blast media, blast pressure, and working distance were varied using a Box-type statistical design of experiment (SDE) approach. The surface textures produced by four metal grits (HG16, HG18, HG25, and HG40) and three conventional grits (copper slag, coal slag, and chilled iron) were compared. Substrate roughness was measured using surface profilometry and correlated with operating parameters. The HG16 grit produced the highest surface roughness of all the grits tested. Aluminum and zinc-aluminum coatings were deposited on the grit-blasted substrates using the twin-wire electric are (TWEA) process. Bond strength of the coatings was measured with a portable adhesion tester in accordance with ASTM standard D 4541. The coatings on substrates roughened with steel grit exhibit superior bond strength to those prepared with conventional grit. For aluminum coatings sprayed onto surfaces prepared with the HG16 grit, the bond strength was most influenced by current, spray distance, and spray gun pressure (in that order). The highest bond strength for the zinc-aluminum coatings was attained on surfaces prepared using the metal grits.     

热喷涂基体表面前处理技术的研究进展

热喷涂涂层与基体机械咬合的结合机理决定了基体表面前处理是热喷涂涂层中非常重要的处理工艺。文中概述了当前广泛应用的喷砂处理的工艺特点,指出砂粒易在基体表面镶嵌和对基体造成损伤是喷砂工艺的主要缺点,讨论了喷砂对高温合金单晶材料和超高强钢疲劳性能的影响,研究了软质基体表面超音速火焰喷涂WC涂层的免喷砂工艺。同时介绍了近年来其他热喷涂基体表面前处理方面的研究热点,包括高压水射流处理技术、机械粗化技术以及激光表面前处理,并重点阐述了其基本原理、特点及应用情况。     

On the Interplay Between Adhesion Strength and Tensile Properties of Thermal Spray Coated Laminates—Part II: Low-Velocity Thermal Spray Coatings

In this two-part study, uniaxial tensile testing was used to evaluate coating/substrate bonding and compared with traditional ASTM C633 bond pull test results for thermal spray (TS) coated steel laminates. In Part I, the rationale, methodology, and applicability of the test to high-velocity TS coatings were demonstrated. In this Part II, the method was investigated for low-velocity TS processes (air plasma spray and arc spray) on equivalent materials. Ni and Ni-5wt.%Al coatings were deposited on steel substrates with three different roughness levels and tested using both uniaxial tensile and ASTM C633 methods. The results indicate the uniaxial tensile approach provides useful information about the nature of the coating/substrate bonding and goes beyond the traditional bond pull test in providing insightful information on the load sharing processes across the interface. Additionally, this proposed methodology alleviates some of the longstanding shortcomings and potentially reduces error associated with the traditional ASTM C633 test. The mechanisms governing the load transfer between the substrate and the coating were investigated, and the influence of Al in the coating material evaluated.     

Study of plasma- and detonation gun-sprayed alumina coatings using taguchi experimental design

Atmospheric plasma spraying (APS) is a most versatile thermal spray method for depositing alumina (Al₂O₃) coatings, and detonation gun (D-gun) spraying is an alternative thermal spray technology for depositing such coatings with extremely good wear characteristics. The present study is aimed at comparing the characteristics of Al₂O₃ coatings deposited using the above techniques by using Taguchi experimental design. Alumina coating experiments were conducted using a Taguchi fractional-factorial (L₈) design parametric study to optimize the spray process parameters for both APS and D-gun. The Taguchi design evaluated the effect of four APS and D-gun spray variables on the measured coating attributes. The coating qualities evaluated were surface roughness, porosity, microhardness, abrasion, and sliding wear. The results show that the coating quality is directly related to the corresponding coating microstructure, which is significantly influenced by the spray parameters employed. Though it is evident that the D-gun-sprayed coatings consistently exhibit dense and uniform microstructure, higher hardness, and superior tribological performance, the attainment of suitable plasma-sprayed coatings can be improved by employing the Taguchi analysis.     

Laser texturing as an alternative to grit blasting for improved coating adhesion on AZ91D magnesium alloy

Substrate preparation plays an important role in the performance of thermal spray coating, especially on softer materials like magnesium and aluminium alloys. Conventional substrate preparation methods such as grit blasting may not be the most suitable choice due to grit embedding, lower coating adhesion strength and environmental concerns. Laser texturing can be an attractive alternative to the grit blasting method for such materials. AZ91D substrate was prepared for thermal spray coating using grit blasting and laser texturing techniques. WC-12Co powder was thermally sprayed on AZ91D magnesium alloy using the high-velocity oxygen fuel technique. The adhesion strength of the coating, thus produced, was determined using the ASTM 633C adhesion strength test. Scanning electron microscopy was used to investigate substrate morphology and to qualitatively analyse substrate and coating interface. X-ray diffraction was used to identify phase compositions. The coating was characterised for roughness, porosity, micro-hardness and fracture toughness. Laser texturing as a substrate preparation technique has been able to produce well-adhered coatings, with adhesion strength of 45.6?MPa, and comparable coating characteristics with those of the grit blasting technique.     

Residual stress development in cold sprayed Al,Cu and Ti coatings

Residual stresses play an important role in the formation and performance of thermal spray coatings. A curvature-based approach where the substrate–coating system deflection and temperature are monitored throughout the coating deposition process was used to determine residual stress formation during cold spray deposition of Al, Cu and Ti coatings. The effect of substrate material (carbon steel, stainless steel and aluminium) and substrate pre-treatment (normal grit blasting, grit blasting with the cold spray system and grinding for carbon steel substrate) were studied for all coating materials with optimized deposition parameters. Mainly compressive stresses were expected because of the nature of cold spraying, but also neutral as well as tensile stresses were formed for studied coatings. The magnitudes of the residual stresses were mainly dependent on the substrate/coating material combination, but the surface preparation was also found to have an effect on the final stress stage of the coating.     

Influence of the Spray Angle on the Characteristics of Atmospheric Plasma Sprayed Hard Material Based Coatings

This paper presents an investigation of the influence of the spray angle on thermally sprayed coatings. Spray beads were manufactured with different spray angles between 90 and 20° by means of atmospheric plasma spraying (APS) on heat-treated mild steel (1.0503). WC-12Co and Cr₃C₂-10(Ni20Cr) powders were employed as feedstock materials. Every spray bead was characterized by a Gaussian fit. This opens the opportunity to analyze the influence of the spray angle on coating properties. Furthermore, metallographic studies of the surface roughness, porosity, hardness, and morphology were carried out and the deposition efficiency as well as the tensile strength was measured. The thermally sprayed coatings show a clear dependence on the spray angle. A decrease in spray angle changes the thickness, width, and form of the spray beads. The coatings become rougher and their quality decreases.     

喷砂预处理对HVOF喷涂TiAl-Nb/NiCrAl涂层结合强度的影响

采用不同的喷砂压力对基体表面进行喷砂预处理,研究了基体表面状态的变化对HVOF喷涂TiAl-Nb/NiCrAl涂层结合强度的影响。结果表明：随着喷砂压力的增大,基体粗糙度及表面凹坑的深度和宽度增大,NiCrAl层与基体结合界面处孔洞等缺陷增多,同时基体表面残余砂粒的面积分数增加;涂层结合强度随基体粗糙度的增大,先增大后减小,当基体粗糙度为8.33μm时,结合强度达到最大值44.5 MPa。     

Examination of a grit-blasting process for thermal spraying using statistical methods

An experimental study was conducted to develop an understanding of how the grit blasting process, prior to plasma spray coating, affects various properties of the substrate and coatings. A statistical design of experiment approach was used and the results were analyzed using both the linear regression method and average response of factors calculations. The following process variables were studied: grit size (20, 36, 54), blasting pressure (20, 35, 50 psi), blasting duration (4, 6, 8 passes), blasting distance (4, 6 in.), and blasting angle (45°, 90°). Properties such as bond strength, grit contamination, surface roughness, and substrate distortion were evaluated and correlated to the process variables. Based on multiple linear regression results, it was shown that the bond strength can be improved by increasing all of the parameters within the range studied here. No relationship between the surface roughness and bond strength was observed. Grit contamination is mostly influenced by grit size, blasting pressure, and number of blasting passes. The average response method provided indications to the direction of modifying the required properties as a function of process variables. While the average response method agreed mostly with the linear regression predication, some differences are further discussed in the study.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.     

Down-selection and optimization of thermal-sprayed coatings for aluminum mould tool protection and upgrade

This article details the down-selection procedure for thermally sprayed coatings for aluminum injection mould tooling. A down-selection metric was used to rank a wide range of coatings. A range of high-velocity oxyfuel (HVOF) and atmospheric plasma spray (APS) systems was used to identify the optimal coating-process-system combinations. Three coatings were identified as suitable for further study; two CrC NiCr materials and one Fe Ni Cr alloy. No APS-deposited coatings were suitable for the intended application due to poor substrate adhesion (SA) and very high surface roughness (SR). The DJ2700 deposited coating properties were inferior to the coatings deposited using other HVOF systems and thus a Taguchi L18 five parameter, three-level optimization was used to optimize SA of CRC-1 and FE-1. Significant mean increases in bond strength were achieved (147±30% for FE-1 [58±4 MPa] and 12±1% for CRC-1 [67±5 MPa]). An analysis of variance (ANOVA) indicated that the coating bond strengths were primarily dependent on powder flow rate and propane gas flow rate, and also secondarily dependent on spray distance. The optimal deposition parameters identified were: (CRC-1/FE-1) O₂ 264/264 standard liters per minute (SLPM); C₃H₈ 62/73 SLPM; air 332/311 SLPM; feed rate 30/28 g/min; and spray distance 150/206 mm.     

微束等离子喷涂NiCrBSi涂层

采用微束等离子喷涂方法在St37低碳钢上制备了NiCrBSi涂层.研究了基材温度、等离子气体流量、喷涂距离和电流强度等工艺参数对涂层组织结构和性能的影响.采用光学显微镜观察涂层的组织结构,用Perthometers M4P 150测定涂层的表面粗糙度,以及用LECO TC316气体抽提仪检测涂层的氧含量.结果表明,在试验条件下,喷涂参数对涂层组织和性能产生较大的影响.随着电流和基材温度的增加涂层的粗糙度降低,涂层的氧含量随着基材温度和喷涂距离的增加而增加.大多数涂层的显微硬度大于600 HV0.2.尽管粒子速度较低,涂层的平均结合强度仍然大于50 MPa.     

超音速火焰喷涂316L不锈钢涂层性能研究

采用超音速火焰喷涂技术(HVOF)在高强钢表面制备了316L不锈钢涂层,利用扫描电镜、显微硬度仪、电化学测试系统等设备对涂层金相组织、硬度、结合性能和抗腐蚀性能等进行了测试,并分析了WC-CoCr中间层对316L不锈钢粉末涂层结合强度及涂层界面的影响。结果表明:超音速火焰喷涂316L不锈钢粉末颗粒在喷涂中变形充分,形成较致密的涂层,并具有超过400 HV0.1的显微硬度;涂层具有较高自腐蚀电位,耐蚀性优于高强钢;涂层结合强度随着涂层厚度的减小、基体硬度的增加而提高;WC-CoCr底层可改善涂层界面结合,从而改善316L不锈钢涂层的结合性能。     

Coatability and characterization of fly ash deposited on mild steel by detonation spraying

Recently, considerable emphasis has been placed on the processing of low-grade ore minerals through thermal spray techniques. In the present investigation, the suitability of detonation spray system for coating fly ash onto a mild steel substrate has been demonstrated. Resultant coatings are 2–3 times harder than the substrate material and also exhibit a 3-fold reduction in coefficient of friction under sliding wear conditions. However, these coatings exhibit poor sliding wear resistance.     

Effects of Dissimilar Alumina Particulates on Microstructure and Properties of Cold-Sprayed Alumina/A380 Composite Coatings

In this study,alumina/A380 composite coatings were fabricated by cold spray.The influence of alumina particulates' morphology(spherical and irregular) and content on the deposition behavior of the coatings(including surface roughness,surface residual stress,cross-sectional microstructure and microhardness) was investigated.Results revealed that the spherical alumina mainly shows micro-tamping effect during deposition,which result in remarkable low surface roughness and porosity of the coatings.In addition,very low deposition efficiency and good interfacial bonding between the coating and the substrate were achieved.For irregular alumina particles,the embedding of ceramic particulates in the coating was dominant during deposition process,resulting in high retention in the final deposit.However,it showed limited influence on porosity,surface roughness and interfacial bonding of the deposit.The coatings containing irregular alumina particulates exhibited much higher microhardness than those containing spherical alumina due to the higher load-bearing capacity of deposited alumina.     

铝合金基体电热爆炸喷涂钼涂层的制备工艺与性能

采用电热爆炸喷涂技术在铝合金基体上制备钼涂层,用SEM、划痕仪、显微硬度计、表面粗糙度仪、微磨损试验机等表征了涂层性能。借助正交试验方法分析了钼涂层制备中各工艺参数对涂层质量的影响,确定了最佳工艺参数。结果表明:用电热爆炸喷涂方法制备的钼涂层均匀致密,孔隙率低;涂层与基体形成冶金结合,结合强度高;涂层硬度为基体的4.9倍,体积磨损量仅为基体的11.2%,耐磨性大幅增加。电爆炸喷涂的工艺参数中,对涂层质量和结合力影响最大的是喷涂距离,其次是喷涂次数,影响最小的是喷涂电压。     

Composite zinc-fly ash coating on mild steel

Zinc-fly ash composite coatings were deposited on mild steel substrates with the help of electrodeposition technique. Metallographic and chemical characterization of the produced composite coatings was performed with the aid of Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray Analysis (EDX). The corrosion behaviour of the composite coated mild steel substrate was studied in a 0.3 M NaCl solution (pH = 5.5) by means of anodic polarization curves. The wear of the zinc-fly ash coating was also investigated by using a pin-on-disk apparatus. The composite coating exhibited increased wear resistance, compared to the pure zinc coating and the mild steel substrate. The adhesion strength between the zinc-fly ash coating and the mild steel substrate was examined with a scratch testing apparatus. The adhesion strength between zinc-fly ash composite coating and the mild steel substrate was found to be higher than that of the pure zinc coating to mild steel.     

Adhesion of Ceramic Coating on Thin and Smooth Metal Substrate: A Novel Approach with a Nanostructured Ceramic Interlayer

The adhesion of plasma-sprayed coating is, to a large extent, controlled by the cleanness and roughness of the surface on which the coating is deposited. So, most of the plasma spray procedures involve surface pretreatment by grit-blasting to adapt the roughness of the surface to the size of the impacting particles. This preparation process brings about compressive stresses that make it inappropriate for thin substrates. The present works aim to elaborate a thick ceramic coating (about 0.5?mm thick) on a thin metal substrate (1?mm thick) with a smooth surface (Ra of about 0.4???m). The coating system is intended for use in a Generation-IV nuclear energy system. It must exhibit a good adhesion between the ceramic topcoat and the smooth metal substrate to meet the specifications of the application. Our approach consisted of depositing the ceramic topcoat by air plasma spraying on a few micrometers thick ceramic layer made by suspension plasma spraying. This nanostructured layer played the role of a bond coat for the topcoat and made it possible to deposit it on the as-received substrate. The adhesion of the nanostructured layer was measured by the Vickers indentation cracking technique and that of the ceramic duplex coating system by tensile test.     

Tensile strength of plasma-sprayed alumina and/or zirconia coatings on titanium

The heat treatment effect on the characteristics and tensile strength of plasma-sprayed alumina, yttria-stabilized zirconia (YSZ), and mixtures of alumina and YSZ coatings on titanium was investigated. The as-sprayed structures of alumina and YSZ coatings consists of a and y alumina phases, and cubic and tetragonal zirconia phases, respectively. The tensile strength of the coatings containing a large amount of YSZ is increased from 25 to 50 MPa by heat treatment at 800 °C. The 60% YSZ-AI₂O₃ coating showed the highest tensile strength. The tensile strength increase of the YSZ-containing coating by heat treatment is caused by formation of 10 to 100 nm wide microcracks. The interface adhesion strength between the heat-treated titanium substrate and the alumina-containing coating is increased by chemical reaction at the in-terface. Thus, a heat-treated alumina and zirconia mixture coating may be favorable in obtaining high tensile strength due to microcrack formation in the coating and the chemical reaction at the interface. During this work, S. Baba was a graduate student at Kyushu Institute of Technology, Sumitomo Metal Ind. Ltd., Osaka, Japan.