Microstructure Evolution Of Coatings Research Articles

Dilution induced variation of microstructure and mechanical properties on Co-Cr-Fe-Ni high entropy alloy coatings prepared by laser cladding

The remarkable properties of some series of high entropy alloys (HEAs) have garnered extensive interest in the development of HEA coatings. In this study, we prepared non-equiatomic Co-Cr-Fe-Ni HEA coatings using laser cladding method with Co-Cr-Fe-Ni cable-type welding wire (CTWW) as the filling material and investigated the effect of dilution rate on the microstructure evolution of coatings. By changing the wire feeding speed, a large dilution rate range was obtained which is from over 80 % to below 20 % and a variation of the coating's microstructure and mechanical properties was discovered within this large dilution rate range. All of the coatings are dense and uniform, with good metallurgical bonding. A pivotal discovery was made when the dilution rate decreased from approximately 35 %–40 % to below this threshold. The coating transitions from face centered cubic (FCC) and body centered cubic (BCC) dual-phase to FCC single-phase, accompanied by a precipitous drop in hardness from approximately 425 HV to below 175 HV. The interlaced distribution and dislocation pile-up of the layered BCC eutectic structure within the FCC phase are the main reasons for this dramatic change. Therefore, controlling the dilution rate is an effective way to regulate the performance of Co-Cr-Fe-Ni HEA coatings.

Failure behaviour of EB-PVD YSZ thermal barrier coatings under simulated aero-engine operating conditions

Thermal shock test simulating aero-engine operating conditions was carried out on electron beam physical vapor deposition (EB-PVD) yttria partially stabilized zirconia (YSZ) thermal barrier coatings (TBCs) using advanced multi factor coupling test equipment. Phase composition, thermally grown oxide (TGO) residual stress, surface morphology and cross-sectional microstructure evolution in coatings were systematically studied. Based on XRD results, coatings possessed tetragonal phase structure without any further transformation during testing. After 5000 cycles, the thickness of TGO gradually increased from 0.3 to 2 μm. TGO residual compressive stress also gradually increased and then drastically dropped when the coating spalled. Horizontal cracks first initiated from TGO/bond coat (BC) interface, then arose from top coat (TC)/TGO interface and finally coalesced through TGO to cause delamination. Repeated thermal shock and gas impact conditions promoted the emergence of vertical cracks in ceramic top coat. Once communicated with transverse delaminations, cracks eventually led to spalling failure of the coating system. The contribution to spallation failure was mainly from thermal stress as a result of thermal expansion mismatch between the coating and the substrate. In turn, elastic stress induced by high speed impact of gas and TGO growth stress promoted initiation and propagation of coating cracks.

Thickness-related failure behaviors of the thermal barrier coatings under thermal gradient cycling

The ZrO2–7 wt% Y2O3 (7YSZ) is the most commonly used ceramic layer in the thermal barrier coating (TBC) system due to its low thermal conductivity. However, the thermal stress-induced coating failure would damage its performance and limit its application. In the present study, a burner rig is employed to attain the operating temperature field and conduct the thermal gradient cycling tests of the coating. Moreover, the failure modes of the YSZ coating with different thicknesses (100, 200, and 400 μm) are investigated. The results show that the coating with 200 μm thickness has the most extended cyclic lifetime (>340 cycles), while 100 μm and 400 μm thick coatings have cyclic lifetimes of 240 and 80 cycles, respectively. The coating microstructure evolution, thermal growth oxides (TGO) thickness, and coating sintering behaviors are considered to explain the phenomenon. Furthermore, a TGO growth/sintering competitive failure model is proposed to describe the failure behaviors and elucidate the failure mechanisms of the coating with different thicknesses. The coatings with different thicknesses have different temperature distributions in the same external conditions. It results in different TGO growth rates and different degrees of coating sintering. The balance of TGO growth and coating sintering makes the 200 μm thick coating the most extended cyclic lifetime.

Effect of boron on the microstructural evolution and wear resistance of high-hardness Fe-based alloy coatings prepared by laser cladding

One of the primary failure modes of 45 steel is severe surface wear, which significantly raises its application cost. In order to improve the wear resistance, high-hardness Fe-based alloy coatings with varying boron contents were prepared on 45 steel substrates by preplaced-powder laser cladding in this study. The effect of boron content on the phase composition and microstructural evolution of the coatings were systematically studied. The proportion, morphology, and distribution characteristics of boride phases in the coatings were discussed in detail. The results demonstrated that the Fe-based alloy coatings with different boron contents were composed of α-Fe, Cr7C3, and Fe2B phases. The boron content had a significant influence on the morphology and proportion of the boride phases. With the increase of boron content, the proportion of boride phases gradually increased and the morphology transition was reticular → cluster-shape → long strip-shape → plate-shape. The composition analysis revealed that the boride was Cr-rich Fe2B phase and the content of Cr in the borides increased with the boron content of the coating. The addition of boron could effectively improve the hardness and wear resistance of the Fe-based coatings, which could be attributed to the boride hard phases in the microstructures. The coating with 6.0 wt% B exhibited the highest microhardness and least amount of wear loss. The wear mechanism of the coating with different boron contents was also discussed.

Effect of spray angle and substrate material on formation mechanisms and properties of HVAF sprayed coatings

Thermally sprayed coatings are often used to enhance the surface properties (wear resistance, corrosion resistance, etc.) of engineering components in order to extend their performance and service lifetime. Typically, the industrial components to be coated possess complex geometries and are fabricated using different materials, which can influence the deposited coating's microstructure and performance. High-velocity air fuel (HVAF) process is a relatively new thermal spray processing technique that has shown tremendous potential to deposit high performance coatings for durable industrial components. However, no detailed studies have been reported on HVAF sprayed coating formation mechanisms so far in relation to the spray angle and substrate properties, and the influence of coating material on the above. The objective of this work was to study the influence of spray angles and substrate materials on splat characteristics, coating microstructure evolution, properties and performance for two distinct coating materials.In this study, one cermet (WC-CoCr) and one metallic (Inconel 625) feedstock were deposited onto three different substrates (aluminium alloy, carbon steel and Hastelloy-X) utilising different spray angles (40°, 60° and 90°). The coating evolution was analysed utilising SEM/EDS, image analysis, and micro-indentation. To determine the tribological performance, coatings were subjected to dry sliding wear test utilising alumina ball as counter surface and specific wear rates were obtained.The results showed that initial splat characteristics were substantially altered on changing the substrate and the spray angle. However, the final coating properties were not affected significantly even though the deposition rate was reduced significantly at lower spray angle, suggesting the versatility of the HVAF process.

Formation process and oxidation behavior of MCrAlY + AlSiY composite coatings on a Ni-based superalloy

An AlSiY coating and two MCrAlY + AlSiY composite coatings with different thickness of MCrAlY interlayer were prepared by arc ion plating (AIP) and vacuum annealing. The isothermal oxidation behavior of coatings at 1100 °C for 300 h was also investigated to characterize the microstructure evolution of coatings during annealing. The composite coatings exhibited a better high-temperature oxidation resistance at 1100 °C. The reason is that the addition of MCrAlY layer can greatly contribute to prevent the diffusion of refractory elements to the outer layer. The inhibition of Al inward diffusion can be much stronger, as the Si content increases in the outer layer during annealing.

Effect of Copper Addition on the Phase Composition and Microstructure Evolution of Fe-Based Amorphous Alloy Coatings Prepared by Laser Cladding

Laser clad coating’s susceptibility to cracking is extremely high due to the brittleness of the material itself and the enormous residual stress caused by rapid heating and cooling. In order to relieve the residual stress, a series of Fe-based amorphous-Cu composite coatings were fabricated on 20 steel using laser processing. The effects of copper addition on the phase composition and microstructure evolution of the coatings were then investigated. The as-prepared coatings, which are mainly composed of Fe-based amorphous, Cu, and (Fe, Cr)23 (C, B)6 have a network microstructure. A large amount of spherical-shaped copper metal is present in the coating and is evenly dispersed within the amorphous matrix. The copper in the coating has little effect on the crystallisation of Fe-based amorphous alloys due to the negligible solubility of solid copper and the Fe-based amorphous alloy. The coating average hardness reduces considerably and shows a significant difference, improving the coating stress distribution.

Effect of Al2O3 scales from pre-oxidation on the microstructural evolution and phase transition of NiAlHf coatings at 1200 °C

The pre-oxidation treatment was conducted for NiAlHf coatings in a sealed glass tube with low oxygen atmosphere at 900 °C. The microstructure evolution and oxidation resistance of the NiAlHf coatings with and without pre-oxidation treatment were systematically investigated via isothermal oxidation at 1200 °C for up to 200 h in laboratory air. The results revealed that a dense Al2O3 scale was formed on the NiAlHf coating surface after pre-oxidation treatment, which could effectively reduce oxygen pressure and Al consumption on the NiAlHf coating surface. Furthermore, the NiAlHf coating microstructure evolution was effectively delayed from β-NiAl to the γ′-Ni3Al phase due to Al2O3 scale formation by pre-oxidation treatment. Oxidation resistance of the NiAlHf coating with pre-oxidation treatment was improved after 200 h of oxidation, of which the oxidation rate constant was enhanced by 40% compared to the NiAlHf coating without pre-oxidation treatment. The mechanism of oxygen diffusion inhibition by pre-oxidation was also discussed. This research was beneficial to understand the effect of pre-treatment on the NiAHf coating microstructure evolution and could further guide the improvement of the oxidation resistance of NiAlHf coatings for long-term oxidation.

Effects of sample bias on adhesion of magnetron sputtered Cr coatings on SiC

Swelling of SiC at 300 ∘C due to in-service neutron irradiation causes tensile residual stresses in coatings which are expected to adversely affect the performance of coated SiC composite fuel cladding for light water reactors. Matching the coating swelling with the substrate, a solution common for thermal expansion, is not practical in the case of neutron irradiation. Biasing samples during magnetron sputtering deposition induces compressive residual stress which may counteract this. In this study, chromium coatings were deposited on SiC by DC magnetron sputtering with no external heating at bias voltages of –50V, –75V, and –100V. The effects of the bias voltage on morphology, residual stress, microstrain, texture, and adhesion are shown. The low deposition temperature resulted in the coating microstructure evolution following an energetic particle bombardment dominated trend. At the two lower bias voltages knock-on implantation dominated increasing the residual stress and microstrain while at the highest bias voltage, thermal spike migration allowed for defect relaxation. When the knock-on induced compressive residual stress exceeded 0.8 GPa microcrack formation in the SiC substrate decreased coating adhesion. While no microcracks formed at the lowest bias voltage, insufficient atomic mobility during coating growth lead to voids forming in the coating. A balance is needed to form void-free coatings that have high compressive residual stress.

The influence of nano-CeO2 on tribological properties and microstructure evolution of Cr3C2-NiCrCoMo composite coatings at high temperature

With the aim of further improving the high-temperature tribological properties and extending the application of typical chromium carbide cermet coating, a novel Cr3C2-NiCrCoMo/nano-CeO2 (NCE) composite coating deposited by high-velocity oxygen fuel spray (HVOF) was developed based on the collaborative modification of the multielement alloy adhesive phase. The influence of nano-CeO2 content on high-temperature tribological properties and microstructure evolution of the coating during high-temperature friction was studied. Tribological properties, microstructure and phase composition of the coating after friction at 400 °C, 600 °C and 800 °C were carefully investigated. The results indicate that the novel NCE coating exhibits improved wear resistance at high temperature with lower friction coefficient and wear depth due to the lubrication of oxides and improvement of oxide film adhesion. The wear mechanism of the NCE coating mainly consists of abrasive wear and surface fatigue wear. With the increase of friction temperature, the oxide film replaces the coating surface as the main object of wear. The addition of nano-CeO2 decreases the threshold temperature of Ostwald ripening triggered by the secondary precipitated nano-Cr3C2 phase and promotes the growth and aggregation of the nano-Cr3C2 precipitated from the adhesive phase of the NCE coating, which contributes to wear resistance of the coating in the subsequent friction.

Effect of galvannealing temperature on coating microstructure evolution correlated to flaking degradation on galvannealed interstitial-free steel

Galvannealed (GA) steel sheets have been widely used in the automotive industry because of their good weldability, paintability, and corrosion resistance. However, coating failure and exfoliation originating from the brittle and hard FeZn phases during press forming is a serious problem. This study investigates the mechanism of flaking deterioration in GA coatings with high Fe content. For this, specimens with various Fe contents were fabricated with different galvannealing temperatures. As the temperature increased, the η and ζ phases became extinct and both δ and Γ phases grew simultaneously. Accordingly, surface became homogeneously flattened with crater degeneration. Simultaneously, the interface was flattened by lateral Γ phase growth. The flat region where the Γ existed was widened by broadening the Γ phase area, by which the etch-pit shape rough knoll found under craters was gradually eliminated. As a result, the Γ phase growth resulted in interface flattening, which deteriorated the flaking resistance.

Microstructure, surface stress and surface temperature response of ZrB2–SiC based coatings

SiC/ZrB2–SiC, SiC/ZrB2–SiC/SiC, SiC/ZrB2–SiC/ZrB2 and SiC/ZrB2–SiC/ZrB2/SiC coatings were prepared on graphite by pack cementation and chemical vapor deposition (CVD). The microstructure, surface stress, ablation resistance and surface temperature response during ablation of these coatings were systematically studied. The results indicated that pack cemented SiC/ZrB2–SiC coating can effectively relax mismatch between graphite substrate and outer dense coatings. The CVD ZrB2 layer with loose structure can further increase the compressive stress of outer CVD SiC layer, and avoid the formation of cracks in SiC layer. The surface temperature response during ablation is mainly affected by coating microstructure evolution and emissivity of different components.

Plasma electrolytic oxidation ceramic coatings proceed by porous anodic film

Ceramic coatings were performed on aluminum through plasma electrolytic oxidation (PEO) in a Na5P3O10 solution with various concentrations of NaOH. It was revealed that the concentration of NaOH affected the types of anodic films and the activity of plasma discharges. Emphatic investigation was performed on the coatings fabricated in the electrolyte with addition of 1 g L−1 NaOH, focusing on the coating microstructure evolution and coating corrosion resistance. Some coatings were detached from the substrate, showing that the oxide particles on the anodic film C/S interface underwent profile variations from hemisphere to polygon, and its interpore distance decreased over time. The formations of pores of anodic films and plasma discharge of the PEO coatings were both easy to occur around the existing pores and the existing discharge channels. Fractured cross-section showed that a pancake-like structure corresponded to a discharge channel, a cavity, and a barrier layer protrusion whose area was similar to that of the corresponding pancake-like structure. The data of potentiodynamic polarization measurements showed that the sample treated with PEO for 10 min possessed the lowest icorr value of 4.97E-8 A cm−2, the most positive Epit value of −0.63 V, and the highest Rp value of 2.66E5 Ω cm2. EIS data of 10 min sample exhibited the highest R2 value of 1.61E6 Ω cm2, demonstrating the best corrosion resistance among these samples.

Chromium carbide coatings produced on ductile cast iron QT600-3 by thermal reactive diffusion in fluoride salt bath: Growth behavior, microstructure evolution and kinetics

A series of chromium carbide coatings were formed on ductile cast iron QT600-3 by thermal reactive diffusion (TRD) processes in fluoride salt bath at 850–980 °C for 10 min ~ 40 h under an argon atmosphere. Coatings were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). The results demonstrate that the microstructure of chromium carbide coatings formed on ductile cast iron QT600-3 at 850 °C is related to the treatment time. At first 10 min, there is only a thin layer of Cr7C3 in the coating. At 1 h, the outermost layer of the coating is a thin Cr23C6 layer, and there is a thin M3C layer beneath the M7C3 layer. With increasing treatment time, the thickness of all layers in the coatings increases. For the sample treated for longer than 1 h, the microstructure of coating is Cr23C6, M7C3 and M3C from the outermost layer to the substrate. The microstructure of chromium carbide coatings formed on high C content steel substrates via TRD processes is independent with the composition of the base salt. The growth kinetics of chromium carbide coating follows a parabolic law with an activation energy of 129.906 kJ/mol. The evolution of hardness of coatings is consistent with the microstructure evolution of coatings.

Rumpling of nickel aluminide coatings: a reassessment of respective influence of thermal grown oxide and phase transformations

The respective influence of the oxidation and phase transformation on the rumpling of nickel aluminide coatings is reviewed using the thermal treatments in air and Ar atmosphere: β-(Ni,Pt)Al coating deposited on AM1 Ni-base superalloy was subjected to the isothermal and cyclic oxidation at 1100 °C as well as to the thermal cycling in Ar atmosphere. A comparative study of the coating microstructure evolution and surface roughness increase in different conditions suggests that the presence of adherent oxide layer is essential for the rumpling of nickel aluminide coatings on Ni-base superalloys.

Microstructural Evolution of SAPS/HVOF CoNiCrAlY Alloy Coating During Thermal Cycling Test

In this paper, CoNiCrAlY alloy coatings were deposited by high-efficiency supersonic atmospheric plasma spraying (SAPS) and high-velocity oxygen fuel (HVOF) spraying. The microstructural evolution of coatings during thermal cycling test was investigated. The results suggested that the as-sprayed SAPS coating consisted of lamellar structures and unmelted particles. However, the as-sprayed HVOF coating primarily consisted of the unmelted particles. The β-NiAl phase mainly existed in the unmelted particles, and its content increased with the increase of unmelted particles. The thermal cycling life of SAPS coating was 258 cycles, about 117 % higher than that of HVOF coating. During thermal cycling, significant internal oxidation and large cracks formed in the HVOF coating, which was one of the reasons that led to the spallation of HVOF coating.

Multi-layered thermal barrier coatings fabricated by plasma-spraying and dry-ice blasting: Microstructure characterization and prolonged lifetime

The quality of bond coat and top coat of thermal barrier coatings (TBCs) has strong influences on their lifetime at high temperature. Especially, the oxygen content level and the surface roughness of bond coat as well as the porosity of top coat have been demonstrated to play an important role in the thermomechanical behavior of TBCs. In this study, dry-ice blasting was used during the preparation of TBCs which were composed of atmospheric plasma sprayed CoNiCrAlY bond coat and YSZ top coat. Three sets of TBCs deposited with different dry-ice (CO2) blasting treatments were compared with respect to the coating microstructure evolution, the growth behavior of thermally grown oxide (TGO), crack propagation and thermal shock resistance during thermal cycling exposure. It was interesting to find that different microstructures of bond coat and top coat were obtained under different spraying conditions. Moreover, the results showed that CoNiCrAlY bond coats continue to be oxidized during the deposition process of YSZ top coats when they are plasma-sprayed without dry-ice blasting. The three sets of prepared TBCs have different thermal shock lifetime. TBC with APS dry-ice blasted bond coat and APS dry-ice blasted top coat was the most durable and exhibited significant improvement in lifetime. The remarkable decrease in the oxide content and the porosity of bond coat, the increase in the “vertical” porosity of top coat and the improvement in the bonding strength at the bond coat/top coat interface appeared to contribute to the prolonged lifetime.

Engineering HVOF-Sprayed Cr3C2-NiCr Coatings: The Effect of Particle Morphology and Spraying Parameters on the Microstructure, Properties, and High Temperature Wear Performance

Chromium carbide-based thermally sprayed coatings are widely used for high temperature wear applications (typical temperature range from 540 to 900 °C). In these extreme environments at those temperatures, several phenomena will degrade, oxidize, and change the microstructure of the coatings, thereby affecting their wear behavior. Although it can be easily conceived that the Cr3C2-NiCr coating microstructure evolution after high temperature exposure will depend on the as-sprayed microstructure and spraying parameters, very little has been done in this regard. This study intends to develop a better understanding of the effect of spraying parameters on the resulting chromium carbide coating microstructure after high temperature operation and high temperature sliding wear properties. The microstructures of different coatings produced from two morphologies of Cr3C2-NiCr powders and under a window of in-flight particle temperature and velocity values were characterized through x-ray diffraction and scanning electron microscopy. Sliding wear at 800 °C was performed and the wear behavior correlated with the spraying parameters and coating microstructure. Vickers microhardness (300 gf) of the coatings before and after sliding wear was also measured.

Modeling and Control of High-Velocity Oxygen-Fuel (HVOF) Thermal Spray: A Tutorial Review

This work provides a tutorial overview of recent research efforts in modeling and control of the high-velocity oxygen-fuel (HVOF) thermal spray process. Initially, the modeling of the HVOF thermal spray, including combustion, gas dynamics, particle in-flight behavior, and coating microstructure evolution is reviewed. The influence of the process operating conditions as predicted by the fundamental models on particle characteristics and coating microstructure is then discussed and compared with experimental observations. Finally, the issues of measurement and automatic control are discussed and comments on potential future research efforts are made.

Thermal shock cycling behavior of NiCoCrAlYSiB coatings on Ni-base superalloys: II. Microstructure evolution

NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated superalloys were investigated by cycling between high temperature (900, 1000 and 1050 °C) and room temperature (forced water quenching). The microstructure evolution in coatings and the underneath substrates was studied. The results showed that phase transformation from γ′ to γ occurred at the surface of the coated and bare specimens due to the consumption of Al and Ti by oxidation. A directional coarsening of the γ′ precipitates (γ′ rafting) just beneath the γ′-impoverishing zone (or interdiffusion zone) is observed. This kind of rafting is believed to come into being from the compressive stresses by temperature gradients due to high cooling rate during cooldown. The relationship between microstructure evolution and cracks formation and propagation is also discussed.