Modified ball bond shear test for determination of adhesion strength of cold spray splats

A modified ball bond shear test was developed to measure the adhesion of cold sprayed splats to both bulk substrates and cold sprayed coatings. The technique was applied to the deposition of Ti feedstock powder (spherical morphology, 29 μm average dia.) onto a commercially pure bulk Ti plate and onto Ti coatings prepared with the same feedstock powder. Both an adhesion strength and adhesion energy were measured, with trends in adhesion examined as a function of in-air particle velocities between 580 and 825 m/s. Comparisons between cold spray splat on bulk materials versus coatings were conducted for two deposition conditions (694 and 825 m/s). Generally, the adhesion for splats deposited at the higher deposition condition were insensitive to the substrate material, while splats deposited at the lower deposition condition adhered better to the bulk material than to the coatings.     

Mechanical Property Mapping of Cold Sprayed Ti Splats and Coatings

Profile nanoindentation and nanoindentation mapping were used to investigate the mechanical properties of commercially pure cold spray Ti splats and coatings deposited at increasing deposition velocities. Three regions in the cold spray Ti splats have been indentified: the impact region, the jetting region, and the upper splat region. The mechanical properties measured in these regions were tied to the cold spray deposition process with help of optical and scanning electron microscopes. The jetting region was observed to contribute to a metallurgical bonding of cold spray splats to cold spray splats and was measured to have low hardness in comparison to the splat impact site and similar to the hardness in the upper splat region. No increase in the profile coatings hardness with increase in the particle in-flight velocity and coating thickness was found. A correlation between the mechanical properties and the presently known deposition temperature, stress and dislocation density models was made.     

Influence of Powder Morphology and Microstructure on the Cold Spray and Mechanical Properties of Ti6Al4V Coatings

The powder microstructure and morphology has significant influence on the cold sprayability of Ti6Al4V coatings. Here, we compare the cold sprayability and properties of coatings obtained from Ti6Al4V powders of spherical morphology (SM) manufactured using plasma gas atomization and irregular morphology (IM) manufactured using the Armstrong process. Coatings deposited using IM powders had negligible porosity and better properties compared to coatings deposited using SM powders due to higher particle impact velocities, porous surface morphology and more deformable microstructure. To evaluate the cohesive strength, multi-scale indentation was performed and hardness loss parameter was calculated. Coatings deposited using SM powders exhibited poor cohesive strength compared to coatings deposited using IM powders. Images of the residual indents showed de-bonding and sliding of adjacent splats in the coatings deposited using SM powders irrespective of the load. Coatings deposited using IM powders showed no evidence of de-bonding at low loads. At high loads, splat de-bonding was observed resulting in hardness loss despite negligible porosity. Thus, while the powders from Armstrong process lead to a significant improvement in sprayability and coating properties, further optimization of powder and cold spray process will be required as well as consideration of post-annealing treatments to obtain acceptable cohesive strength.     

Influence of Substrate Characteristics on Single Ti Splat Bonding to Ceramic Substrates by Cold Spray

The cold spray technique may be used to fabricate metal matrix composites and to metallize ceramics. Both applications involve the creation of metal/ceramic interfaces, which are well researched for other processes but not nearly as much for cold spray. Here, the effect of ceramic substrate composition and surface roughness on adhesion strength of metallic splats is investigated. Splat adhesion testing was performed on Ti splats deposited on Al₂O₃ substrates with varying average reduced peak height roughness (R_pk) values. Ti splats sprayed onto Al₂O₃ with the lowest surface roughness had a higher bond strength (305?±?87 MPa) than splats deposited on the higher surface roughness Al₂O₃ (237?±?47 MPa). Failed interfaces revealed that the bonding mechanism for substrates with higher surface roughness is predominantly mechanical interlocking. Adhesion to the Al₂O₃ substrate with low surface roughness is predominantly along the periphery of the particle where jetting occurs. Splat adhesion testing was also performed on Ti splats deposited on SiC. Ti splats had a significantly higher bond strength to all Al₂O₃ substrates than to SiC. Posttest observations of SiC substrates showed little evidence of bonding. Several rebounded or detached splats left traces of Ti along the periphery of the impacted particle.     

Effect of substrate temperature on the splat formation of flame sprayed polypropylene

Polypropylene (PP) powder particles were thermally sprayed via a flame spray onto mild steel substrates at room temperature (RT), 70 °C , 120 °C, and 170 °C. Single solidified splats were collected from polished substrates. The splat morphology was characterized using optical microscopy and scanning electron microscopy (SEM). Micro-Raman spectroscopy examined the effect of substrate preheating on the molecular structure of the PP splats. The splat-substrate interface, porosity and a qualitative interpretation of the interface adhesion were obtained by cross-sectioning and imaging using the focused ion beam (FIB) technique.This study indicated that increasing the substrate temperature from room temperature to 170 °C produces PP splats of larger diameter that would be expected to exhibit improved adhesion. The influence of preheating dominates the splat formation and splat morphology. The Raman spectra indicate that the degree of crystallinity of the PP particles rises with increasing substrate temperature. Comparison of the spectra of the feedstock and the deposited PP particles confirms that there is no thermal degradation of the material under these processing conditions.This work demonstrates the influence of substrate chemistry on the PP splat morphology. Microstructural characteristics are related to the flame spray process variables for polymeric coatings.     

冷喷涂TC4涂层临界沉积速度计算及制备涂层性能研究

目的研究冷喷涂TC4涂层的临界沉积速度及粒子温度对临界沉积速度的影响规律,并研究气体压强对沉积涂层性能的影响规律。方法理论研究上,采用有限元LS-DYNA软件中的Johnson-Cook塑性模型,选取3D164计算单元建立模型,研究粒子在不同温度和不同速度下碰撞基体后的形貌特征,确定粒子沉积临界速度。试验研究上,采用N_2作为冷喷涂驱动气体,在TC4合金上制备TC4涂层,然后采用SEM、Image J图像分析软件、硬度计等分析已沉积涂层的孔隙率和硬度等性能。结果 25、400、500、600℃温度下,计算表明10μm的TC4合金粒子在TC4基板上的临界沉积速度分别为730、465、392、361 m/s,即随粒子温度升高,粒子临界沉积速度降低,粒子沉积成涂层更容易。采用冷喷涂工艺在TC4基板上沉积TC4涂层,在N_2温度600℃、气体压力3 MPa的条件下,制备的TC4涂层厚度约1000μm,与TC4钛合金基体结合紧密,涂层孔隙率约为6.46%。结论气体温度升高,粒子临界沉积速度降低;气体压强变大,制备的涂层厚度就大且更加致密。     

Effect of Droplet Characteristics and Substrate Surface Topography on the Final Morphology of Plasma-Sprayed Zirconia Single Splats

The morphology of atmospheric plasma-sprayed yttria-stabilized zirconia single splats has been studied. Single splats of plasma-sprayed ZrO₂-7%Y₂O₃ powder have been collected on polished stainless steel substrates kept at three different temperatures (room temperature, 300 °C, and 600 °C). The effect of heating on the substrates’ surface topography was evaluated. The effects of spray process parameters such as substrate temperature, particle temperature, and velocity on the morphology of single splats was studied. Variation of splat shape with location within the footprint of plasma spray was investigated. Pore and microcrack formation, splashing behavior, splat/substrate, and splat/splat interfaces were analyzed. Splat morphology and diameter, satellite particles, and splashing behavior were recorded using both scanning electron microscopy and image analysis. Splat/substrate and splat/splat interfaces were studied from cross sections prepared by focused ion beam milling. Results showed primarily disk-shape morphology and no evidence of delamination along the splat/substrate interface at 600 °C substrate temperature. Overlapped splats showed evidence of melting (microwelding) at splat boundaries. Splat thickness was measured to be less than 1 μm for all spray conditions.     

Splat microstructure of plasma sprayed cast iron with different chamber pressures

Achieving a plasma sprayed cast iron coating containing graphite requires stringent control on spray parameters that synergistically influence the coating properties and thus the performance. The microstructure of cast iron splats greatly depends on spray parameters such as substrate temperature, chamber pressure, and spray distance. This paper presents the effect of chamber pressure on the splat microstructure, including oxides and graphite. At low chamber pressures, most splats exhibit a disk shape with high flattening ratios, whereas star-shaped splats extensively appear at high chamber pressures. Spraying at high chamber pressures causes the formation of pores and thick oxide zones at the splat/substrate interface, mainly due to the atmospheric gases, which are responsible for a decrease in splat adhesion. Spraying in Ar atmosphere reduces the splat oxidation due to a decrease in the oxygen partial pressure. Small deformed substrate ridges are observed adjacent to the periphery of splats sprayed at low chamber pressures whereas no ridges are detected at high chamber pressures. Ridge formation generates a kind of mechanical bond, which increases the adhesive strength. Since the molten droplets impinge with high velocity and thus high flattening ratio at low chamber pressures, the solidification rate becomes faster, and graphite formation is resultantly hindered.     

Deposition Features of Ti Coating Using Irregular Powders in Cold Spray

The investigation on the deposition features of irregular Ti powders coated on Al and stainless steel (SS) substrates was conducted in this study. Ti coating was experimentally produced at different gas inlet temperatures, and then detailed analysis on the critical velocity, coating microstructure, porosity, and microhardness was performed. As an assistant method, numerical simulation was also adopted to study the deformation features of single-irregular Ti particle during the cold spray process. The results reveal that the critical velocity of Ti particle on Al substrate is higher than that on SS substrate due to the different substrate hardness. The coating porosity is shown to be decreasing as the inlet temperature increases and independent to the substrate material. Similar phenomenon is also noticed for coating microstructure, but the changing trend is reverse in this case. Finally, the most important finding is that an additional torque imposed on the irregular particle is generated during the deposition process, which tends to detach the bonded particle from the substrate surface.     

Effects of Temperature of In-flight Particles on Bonding and Microstructure in Warm-Sprayed Titanium Deposits

Micron-sized titanium particles were deposited on steel substrates by the warm spraying, which is a modified high velocity oxy-fuel (HVOF) spraying technique. In the process, nitrogen gas is mixed with the HVOF flame jet to lower the temperature of injected powder particles. Detailed observations of splats formed on polished substrates by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were conducted to investigate the effects of particle temperature on the bonding of splats with the substrate and the microstructure within the splats. At lower nitrogen flow rates, the particles observed were heavily deformed and exhibited diverse splat morphologies and microstructures. At higher nitrogen flow rates, most of the particles were impacted in the solid state and the oxidation of particles was remarkably less. The TEM observation revealed distinctively different microstructures within the splats as well as the splat/substrate interfaces depending on whether the particle was molten or solid before the impact.     

Effects of Traverse Scanning Speed of Spray Nozzle on the Microstructure and Mechanical Properties of Cold-Sprayed Ti6Al4V Coatings

Cold spray has the potential to restore damaged aerospace components made from titanium alloy, Ti6Al4V at low temperature (200-400 °C). Traverse scanning speed during deposition is one of the key factors that affect the quality of the Ti6Al4V coatings as it influences the thermal build-up and coating thickness per pass. As there are fewer reported studies on this, this work investigated the effects of different traverse scanning speeds (100, 300 and 500 mm/s) of cold spray nozzle on the microstructure and mechanical properties of cold-sprayed Ti6Al4V coatings. The cross-sectional analysis showed coating porosities reduces with slower traverse speed, from 3.2 to 0.5%. In addition, the microhardness of the coatings increased from about 361-385 HV due to strain hardening. However, the adhesion strength of the coatings to the substrates significantly decreased with reduced traverse speed from about 60 MPa (glue failure) at 500 mm/s to 2.5 MPa (interface failure) at 100 mm/s. Therefore, this study revealed that the control of heat build-up and thickness per pass during the cold spray deposition of the Ti6Al4V coatings is crucial to attain the desirable properties of the coatings.     

The Effect of Molybdenum Substrate Oxidation on Molybdenum Splat Formation

Disk splats are usually observed when the deposition temperature exceeds the transition temperature, whereas thick oxide layer will reduce the adhesion resulting from high deposition temperature. In present study, single molybdenum splats were deposited onto polished molybdenum substrates with different preheating processes to clarify the effect of surface oxidation on the splat formation. Three substrate samples experienced three different preheating processes in an argon atmosphere. Two samples were preheated to 350 and 550 °C, and another sample was cooled to 350 °C after it was preheated to 550 °C. The chemistry and compositions of substrate surface were examined by XPS. The cross sections of splats were prepared by focus ion beam (FIB) and then characterized by SEM. Nearly disk-shaped splat with small fingers in the periphery was observed on the sample preheated to 350 °C. A perfect disk-shape splat was deposited at 550 °C. With the sample on the substrate preheated to 350 °C (cooling down from 550 °C), flower-shaped splat exhibited a central core and discrete periphery detached by some voids. The results of peeling off splats by carbon tape and the morphology of FIB sampled cross sections indicated that no effective bonding formed at the splat–substrate interface for the substrate ever heated to 550 °C, due to the increasing content of MoO₃ on the preheated molybdenum surface.     

Study of solidification behavior and splat morphology of vacuum plasma sprayed Ti alloy by computational modeling and experimental results

The microstructure of Ti-6Al-4V alloy manufactured by vacuum plasma spraying consists of individual lamellae, inter-lamellae boundaries, and porosity. Mechanical properties of the as-sprayed structure depend mainly upon the solidification behavior and resultant microstructure and morphology of the individual splats and cohesion between splats. Using a three-dimensional numerical model, the cooling rate and solidification behavior of a single Ti-6Al-4V droplet (50 μm) impacting on a titanium substrate under vacuum plasma spray conditions were investigated. Results were verified with experimental observations in single splats and as-sprayed microstructures obtained by vacuum plasma sprayed form of Ti-6Al-4V alloy. The average cooling rate of a single splat obtained from the numerical simulation was on the order of 10⁸ °C/s and the solidification front velocity was approximately 63 cm/s which is in the range of the rapid solidification. The thickness of the splat was calculated to be around 3 μm and the deposition efficiency was approximately 70%. These results illustrated good agreements with those obtained from experiments.     

梯度基体温度法和反应溅射TiC过渡层对钛合金基体沉积金刚石薄膜的影响

以H2和CH4作为反应气体,采用热丝化学气相沉积法(HFCVD)在钛合金(Ti6Al4V)平板基体上制备金刚石薄膜,利用扫描电镜(SEM)、X射线衍射仪(XRD)、激光拉曼光谱(Raman)和洛氏硬度仪分析薄膜的表面形貌、结构、成分和附着性能,研究了高温形核-低温生长的梯度降温法对原始钛合金和反应磁控溅射TiC过渡层的钛合金表面沉积金刚石薄膜的影响。结果表明:原始基体区和TiC过渡层区沉积的金刚石薄膜平均尺寸分别为0.77μm和0.75μm,薄膜内应力分别为-5.85GPa和-4.14GPa,TiC层的引入可以有效提高金刚石的形核密度和晶粒尺寸的均匀性,并减少薄膜残余应力;高温形核-低温生长的梯度降温法可以有效提高金刚石的形核密度和质量,并提高原始基体上沉积金刚石薄膜的附着性能。     

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.     

热喷涂NiCoCrAlYTa+7YSZ热障涂层颗粒沉积行为

分别采用低压等离子喷涂和大气等离子喷涂在K4169基体上收集了NiCoCrAlYTa颗粒沉积物及涂层,并对颗粒沉积物的形貌及涂层性能进行了观察分析。结果表明:低压等离子喷涂收集到的单个NiCoCrAlYTa扁平颗粒主要呈圆盘状,涂层致密且氧含量低。而大气等离子喷涂收集到的扁平颗粒主要呈溅射状,涂层孔隙率和氧含量均较高。又在经镜面抛光的NiCoCrAlYTa涂层和K4169基体上分别收集了7YSZ颗粒沉积物,并对其沉积形貌进行了观察分析,结果表明:在K4169基体上收集到的7YSZ颗粒沉积物主要呈圆盘状,表面存在大量的网状微裂纹及宏观环状贯通裂纹。在镜面抛光的NiCoCrAlYTa涂层表面收集的7YSZ颗粒沉积物,周围有少量的指状溅射物,中心部存在一定数量的网状微裂纹,但宏观环状裂纹消失。     

喷距对低温超音速火焰喷涂钛涂层显微结构与性能的影响

目的分析喷距对钛涂层显微结构和性能的影响。方法采用低温超音速火焰喷涂工艺,在不同的喷距下制备钛涂层,分析钛涂层的显微组织、相结构及显微硬度,探讨涂层受喷距影响的机制。结果低温超音速火焰喷涂钛涂层的表面粗糙度较高,涂层的孔隙率随喷距的增加呈先减小、后增大的趋势,当喷距为150 mm时,涂层的孔隙率最小。钛涂层没有发生明显的氧化,粒子主要以固态形式沉积。结论当喷距为150 mm时,粒子在焰流中加热较为充分,同时具有较高的飞行速度,粒子的扁平化程度高,因此涂层具有较高的致密度。     

Influence of surface laser cleaning combined with substrate preheating on the splat morphology

The morphology of sprayed splats influences the coating adhesion and properties, which are determined by the spraying parameters. Many studies in this field show that the substrate surface temperature is a very relevant factor for the splat shape: the hypotheses of substrate surface wettability and contamination or adsorption layer on the surfaces are supported by the fact that the near-disk-shaped splat can be obtained by increasing the substrate temperature. In this work, a short-duration pulsed laser was used to ablate the substrate just before powder spraying. This ablation was powerful enough to eliminate the contaminants on the substrate surface and to improve the adhesion. In this study the analyses of NiAl splat morphology on the polished TA6V (Ti-6Al-4V) substrate were carried out using laser ablation with different substrate temperatures and different heating modes: the flame and another laser. Results show that the temperature at which the disk-shaped splat can be obtained decreased dramatically by laser ablation. Moreover, laser ablation combined with another laser increased the adhesion strength of the coatings. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

The Effect of CFRP Surface Treatment on the Splat Morphology and Coating Adhesion Strength

Metallization of Carbon Fiber-Reinforced Polymer (CFRP) composites aggrandized their application to aircraft, automobile, and wind power industries. Recently, the metallization of CFRP surface using thermal spray technique, especially the cold spray, a solid state deposition technique, is a topic of research. However, a direct cold spray deposition on the CFRP substrate often imposes severe erosion on the surface owing to the high-impact energy of the sprayed particles. This urges the requirement of an interlayer on the CFRP surface. In the present study, the effect of surface treatment on the interlayer adhesion strength is evaluated. The CFRP samples were initially treated mechanically, chemically, and thermally and then an interlayer was developed by atmospheric plasma spray system. The quality of the coating is highly dependent on the splat taxonomy; therefore the present work also devoted to study the splat formation behavior using the splat-collection experiments, where the molten Cu particles impinged on the treated CFRP substrates. These results were correlated with the coating adhesion strength. The coating adhesion strength was measured by pull-out test. The results showed that the surface treatment, particularly the chemical treatment, was fairly successful in improving the adhesion strength.     

Investigating the microstructure and composition of cold gas-dynamic spray (CGDS) Ti powder deposited on Al 6063 substrate

The compositional variation, morphology and microstructure of cold gas-dynamic spray are of great importance for its proper application. This study investigates titanium powder deposition on an Al 6063 substrate using light optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition was examined using energy dispersive X-ray spectroscopy (EDX), X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF SIMS). Optical and electron microscopy revealed heavily deformed Ti powder particles penetrating 10 to 30 μm into the Al substrate. Examination using TEM did not reveal any evidence of second phases at the interface suggesting a sharp transition between the two metals. The presence of nanocrystals and grain refinement of both the coating and the substrate suggest the formation of a partial hetero-epitaxy condition near the interface. EDX results from a dedicated high-resolution scanning transmission electron microscope showed a sharp compositional change with a maximum inter-diffusion region of about 5 nm. Bonding of the coating to the substrate is therefore thought to be achieved by the particle/substrate interlocking and direct metal to metal bonding. However, it is most likely that the refine crystalline structure near the interface will be beneficial to the adhesion of the coating. XPS and ToF SIMS provided evidence of nitrogen pick-up during the spray process in the form of N and TiN even when utilizing Helium as the gas carrier. The presence of TiN suggests reaction of the Ti with the entrained air during spraying which explains the occurrence of flashing jet outside the nozzle. Investigation of the material properties using nanoindentation showed reasonably consistent hardness and elastic modulus values throughout the titanium coating and at the transition region. The hardness was slightly higher than typical commercially available bulk Ti.