zia valefi

The purpose of this research is to investigate the performance of vertical cracks along the thickness of YSZ-40%wtAl2O3 coating and to improve the resistance to oxidation and thermal shock of thermal barrier coatings by creating a layer of YSZ-Al2O3 applied on the YSZ by atmospheric plasma spraying. In this research, YSZ-40%wtAl2O3 powder was synthesized using co-precipitation process and then applied on the YSZ by atmospheric plasma spraying process. High temperature oxidation test at 1100 ˚C and thermal shock test at 1000 ˚C were performed. The structural and phase characteristics of the coatings were investigated using optical microscope, field emission scanning electron microscope (FE-SEM) and X-ray diffractometry (XRD). The structural comparison of the samples showed that the use of the YSZ-40%wtAl2O3 eutectic compound due to the decrease in the melting temperature of the compound and the increase in the melting which brings the proper contact between the splats. This coating showed 18.6% increase in oxidation resistance compared to conventional TBC. Also, the findings showed that the decomposition of the un-pyrolysed precursor in the coating structure of YSZ-40%wtAl2O3 leads to the creation of vertical cracks in the coating structure, which as the oxidation time increases, the number of vertical cracks with a certain distance in the coating structure will also increase. Also, the suitability of the vertical cracks created in the YSZ-40%wtAl2O3 coating, in terms of stress tolerance, increases the ability of the coating to release the stresses during thermal cycles and finally leads to increase in the durability of the coating.

In this research, silicon carbide (SiC) powder with a particle size of 30 to 45 micrometers and an irregular morphology was used for coating in an electroless nickel bath. The effect of particle surface roughening parameters in an acidic solution and electroless process time on uniformity and thickness of Ni-P coating on the SiC particles was investigated. SiC(Ni)-70vol.%Co blended powder was coated on simple carbon steel substrates by high-velocity oxy-fuel (HVOF) and atmospheric plasma spraying (APS) processes. The composite powder was obtained from the mechanical blending of cobalt powder with coated silicon carbide SiC(Ni) powder. The microstructure and morphology of powder particles and corresponding coatings were investigated by scanning electron microscopy (SEM). The results showed that the surface preparation of SiC powder by roughening the particles with HF/NaF solution and electroless time of 5 minutes led to the formation of a continuous and uniform Ni-P coating with a thickness of 1-3 micrometers on the SiC particles. Electroless nickel coating during thermal spraying by HVOF protected SiC particles against decomposition and oxidation and as a carrier led to the placement and distribution of SiC in SiC(Ni)-Co composite coating. In the APS process, due to the very high temperature of the plasma jet and thermal decomposition, SiC particles was insignificantly contributed to the coating microstructure. The micro-hardness of two mentioned coatings was measured as 570±40 and 460±30 HV0.3, respectively, and this difference was due to the effect of the significant distribution of SiC in the HVOF coating.

In this research, CoNiCrAlY powder was deposited by high-velocity oxy-fuel (HVOF) and low-pressure plasma spraying (LPPS) processes on IN738 nickel-based superalloy substrates. The high-temperature oxidation test was performed on the coatings at a temperature of 1050 ̊C and a time of 200 hours in a muffle furnace. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The porosity (volume percentage) and surface roughness (micrometer) were measured for HVOF coating as 0.6 and 4.4, and for LPPS coating as 2 and 6.62, respectively. The HVOF coating consisted of γ-CoNiCr and β-(Co,Ni)Al, while the LPPS coating included a single phase γ-CoNiCr. The disappearance of the β phase in the LPPS coating after spraying was due to dissolution in the plasma jet and its non-recovery in the conditions of rapid quenching and non-equilibrium solidification. This phase was recovered after heat treatment. The microstructure of the LPPS coating had much less oxide than the HVOF coating due to depositing at low oxygen pressure in the vacuum chamber. After 200 hours of oxidation test, the amount of β phase (as an oxidation resistance criterion) was completely consumed in the LPPS coating, while the HVOF coating contained the retained β deposits. The average thickness of TGO layer for HVOF and LPPS coatings was 5.2 and 7.1 μm, respectively. The dispersed oxides in the microstructure, lower roughness and denser structure of HVOF coating were reasons for the higher oxidation resistance of HVOF coating than LPPS.

In this research, pure CoNiCrAlY and composite CoNiCrAlY-2, 4 wt% Al2O3 powders were deposited by the HVOF process on IN738 nickel-based superalloy substrates. Composite powders were produced using a water-based solution method and polyvinyl alcohol adhesive to distribute oxide nanoparticles on CoNiCrAlY microparticles. An oxidation test was performed on the coatings at 1050°C and 50, 100, 150, and 200 hours in the stationary air. The microstructure and phase composition of the coatings were investigated by SEM and XRD before and after the oxidation test. The results showed that nanoparticles are present with relatively uniform distribution around the powders and the microstructure of the corresponding coatings. Porosity and surface roughness of pure and composite coatings (2, 4 wt%) were measured as 0.6, 1, and 2.7 vol.% and 4.4, 4.8, and 7.1 µm, respectively. After 200 hours of oxidation test, the thickness of the oxide layer formed on pure and composite coatings (2, 4 wt%) was measured as 5.2, 4.3, and 5.9 micrometers, respectively. CoNiCrAlY coating containing 2 wt% of Al2O3 showed the best resistance to oxidation due to the creation of diffusion barriers by nanoparticles. In contrast, the composite coating containing 4 wt% of Al2O3 had the weakest resistance to oxidation, which was caused by the rough surface morphology and the occurrence of internal oxidation in it.

In this study, the effect of injection of argon and hydrogen shielding gases on the properties of ZrC coatings, applied by solid shield plasma spraying (SSPS) and comparison with APS method were investigated. The effect of type and rate of injected gases on the microstructure of the coating were investigated by studying the morphology of the particles in the Single-line scan-spray. Also, the microstructure and phases of the coatings along with their porosity and oxide content were evaluated. The results show that the coating applied by SSPS method and injection of internal hydrogen gas at a rate of 20 l/min leads to an increase in the amount of splats in the coating and a decrease of more than 50% in the amount of oxides and porosity in the microstructure compared to APS coating. The H2 gas is injected to solid shield burns with O2 that caused a reduction in the amount of oxides in SSPS coatings and increase the heat and temperature around the plasma jet and increase the amount of molten particles in the coatings.

Solution precursor thermal Spraying (SPTS) processes are suitable methods for producing nano-structured coatings. Due to the uncompleted reactions such as solvent evaporation and pyrolysis of the precursor, achieving coatings with controlled properties at a satisfactory precipitation rate remains an important challenge in these processes that needs to precise control of spray parameters. In this study, in order to investigate the effect of Solution precursor high velocity flame spraying parameters such as fuel and oxygen content, spraying distance and solution injection rate, single-scan spraying test was performed on glass substrates. The morphology of the formed splats and their structural characteristics were investigated using Scanning Electron Microscope (SEM). Structural comparison in the single-scan spraying test performed in two ratios of fuel to oxygen, showed that in the flame parameter with oxygen pressure of 6 bar and fuel 3 bar at the injection rate of Solution precursor 20 cm3/min and spray distance of 5 cm was selected as the optimal parameter. In this parameter, due to the low injection rate of the solution and higher heat transfer per drop of the solution precursor and completion of processes that resulting in melting and crystallization, the number of splats increased. Also, evaluation of single-scan spraying in the flame with oxygen pressure of 8 bar and fuel bar of 4 bar and spray distance of 5 cm showed that the injection rate of 40 cm3/min solution precursor would be more appropriate due to increasing the number of fine splats and improving coating efficiency.

In this research, the properties of the coating applied by conventional plasma spray and with inert gas shroud has been studied and compared, in the way that nozzle like part attached to plasma gun in order to protect the plasma jet by exiting nitrogen from the nozzle. The Microstructural characterization of the coatings was performed by optical microscope and scanning electron microscope equipped with energy dispersive spectroscope. Hardness of coatings is also measured by Vickers method under the applied load of 30 gram-force. Isothermal oxidation and thermal shock tests are done at 1000 and 950ºC respectively. Post-spray results show that the use of nitrogen gas shroud is useful and coating achieved by nitrogen shroud has less oxide and porosity and has more homogeneous structure. Results from isothermal oxidation show that TGO layer growth rate in the specimen sprayed by nitrogen shroud is less. Thermal shock test shows that the specimen sprayed by nitrogen shroud has more resistance against thermal shock due to layer by layer and regular growth of TGO and having less oxide and porosity in comparison with the same specimen sprayed without nitrogen shroud. Also, the microhardness of sprayed coating without nitrogen shroud was 35 Vikers more than the applied coating with nitrogen shroud.

In this paper ZrC coating was prepared on SiC coated graphite as a substrate by atmospheric plasma spray (APS) and solid shielding/ shrouded plasma spray (SSPS) methods. Microstructure observation and phase identification of the coatings were performed by scanning electron microscopy and X-ray diffraction. The ablation behavior of the coating was evaluated under supersonic flame for 60s. The results showed that the ZrC coating enhance the ablation resistance of SiC coated graphite remarkably. The results of ablation test revealed that the linear and mass ablation rates of the ZrC coating applied by APS method were 3.7×10-3 mm.s-1 and 22×10-3 g.s-1, while those for SSPS coating were 2.2×10-3 mm.s-1 and 14×10-3 g.s-1, respectively. The excellent ablation resistance is attributed to the formation of continuous zirconia (ZrO2) layer on the surface during the oxidation of the ZrC coating. Moreover, the SPS-ZrC coated sample with lowest pores and cracks have better ablation resistance during the ablation test and can protected the graphite substrate against ablation sufficiently.

In this research, firstly amorphous Alumina powder was produced by co-precipitation method. Then YSZ/Al2O3 coatings were applied by plasma spraying process in two types of pyrolyzed and crystalline nano-alumina. High temperature oxidation and thermal shock resistance test were done at 1100˚C. Microstructure and phase analysis of coatings were studied by optical and electron microscopes and XRD method. Comparison of the microstructure of coatings showed that the use of crystalline nano-alumina powder in the YSZ/Alumina layer composite upgrades the thermal properties. High temperature oxidation and thermal shock resistance of plasma sprayed YSZ/Al2O3 with un-pyrolysed nano-alumina and coatings with same composition with crystalline nano-alumina to created by plasma spraying were studied. Findings showed that the use of un-pyrolyzed nano-alumina powder in YSZ/Al2O3 layer composite resulted in increased porosity and shrinkage cavities in the coating, which increased the diffusion of O2 that causes the TGO growth rate. Also, high density and proper contact between the splats made of crystalline nano-alumina powder results in higher resistance of thermal cycles.

In this research, NiCrAlY powder was applied on steel, and Hastelloy X substrates with solid shielding shrouded plasma spray (SSPS) process and compared with atmospheric plasma spraying (APS). The high-temperature oxidation test was also performed on the coatings, and the microstructure of coatings was studied by optical microscopy (OM) and scanning electron microscopy (SEM). To investigate the influence of the SSPS process on the properties of metallic coatings, variable parameters; such as type of shroud gas (Ar, H2), the gas injection method (internal, external or simultaneous) and the flow rate of that, were examined. During the use of shroud gas, the temperature of the plasma jet has increased significantly. The oxidation test results showed the proper performance of NiCrAlY coating under the protection of argon internal shroud gas with a flow rate of 75SLPM, which was able to perform the best plasma flame protection during spraying. It can lead to a reduction in oxide and porosity of coating up to 8%. Also, the lowest thermally grown oxide (TGO) thickness was obtained for this sample after 200 hours of oxidation, indicating its excellent performance in maintaining the Al for the formation of the continuous α-Al2O3 layer during high-temperature oxidation.

In this research, a double layer thermal barrier coating was applied and then an alumina diffusion barrier layer was deposited on the YSZ by two solution precersore plasma and solution precersore flame spraying. High temperature oxidation and thermal shock resistance tests were done at 1100˚C. Microstructure of coatings were studied by optical Microscopy and Field Emission Scanning Electron Microscopy. Comparison of the microstructures of coatings showed that applying of Alumina with the solution precursor flame spray process upgrades the thermal properties. High temperature oxidation and thermal shock resistance of YSZ/Al2O3 coatings with Alumina applied by the solution precersoure thermal spray with the same compound were studied. Findings showed that applying alumina with the solution precursor flame spray process leads to increase the amount of the deposited splats and proper contact between them, causes to decrease the diffusion of O2 and as a result TGO thickness decreases and also thermal shock resistance increases.

In the present research, wear behavior of Cr2O3-20YSZ (CZ) and Cr2O3 (C) coatings created using atmospheric plasma spray (APS) process was evaluated. For this purpose, Cr2O3 and YSZ nanopowders were produced after milling for 5h in a high-energy ball mill and subsequently plasma spraying of the agglomerated powders was carried out on 304L substrates. Microstructural characterization of these ceramic coatings was performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectroscopy (EDS) and optical microscope. Mechanical tests including adhesive strength, fracture toughness, and micro-hardness were used so as to explain the coatings wear behavior. Sliding wear test was conducted using a ball-on-disk configuration against an alumina counterpart at room temperature. Addition of YSZ particles to the Cr2O3 matrix due to the phase transformation toughening mechanism associated with tetragonal zirconia resulted in an increase in the fracture toughness while decreased the micro-hardness. It was found that the composite coatings had the friction coefficients of the proper order of 0.11-0.15. The CZ composite coating compared to the C coating showed the higher wear resistance so that the weight loss was obtained as 11 and 31 mg, respectively. Observation of the wear tracks of the coatings indicated that the lower wear rate of the CZ coating was attributed to its higher toughness and therefore filling the pores due to the higher plastic deformation of its wear debris.

To improve the efficiency and power output of turbine engines, the turbine inlet temperature should be increased which requires higher oxidation resistance of MCrAlY coatings. Moreover, the oxidation resistance of the MCrAlY coating depends on the chemical composition and coating’s microstructure, which can be improved by optimizing the parameters of coating process. In this study the parameters of LPPS process on the microstructure of NiCoCrAlY coating is evaluated and compared with the microstructure of NiCoCrAlY coating applied by the HVOF method. Thus, microstructure, phase composition and elemental analysis of the coatings are characterized by optical and field emission scanning electron microscopy (FESEM), X-ray diffractometer (XRD) and energy dispersive spectroscopy (EDS), respectively. The results reveal that the optimum microstructure of the coating with minimum porosity, non-melted particles and oxide phases was obtained at the highest temperature and particle’s velocity impact to the substrate’s surface. Hence, the optimized values for Ar and H2 flow rate (75 and 16 SLPM, respectively), and the current of 650 A at a spray distance of 12 cm has been optimized for for maximum amount of remaining particles at maximum speed in plasma jet. In addition, EDS and XRD analysis show that the NiCoCrAlY coating contains continuous γ nickel rich solid solution with dispersed β- (Ni, Co) Al phases and negligible ratio of oxide phase.

In this work, self-fluxing NiCrBSi coatings were deposited by plasma spraying. Simultaneous effect of temperature and time of the fusing heat treatment on microstructure, surface roughness and microhardness as well as wear performance of these coatings was evaluated. Fusing process was carried out at 1000, 1050 and 1100˚C for 5, 15 and 25 min. The morphologies and microstructures of the coatings as well as the wear tracks were characterized using optical microscope and scanning electron microscope. X-Ray Diffraction was applied to determine the phase composition of coatings. Wear performance of the fused coatings was investigated by Pin-On-Disk test. In consequence of the fusing process, the thickness, porosity and surface roughness decreased, the splat boundaries were eliminated, the microhardness increased, a metallurgical bond was created between the coating and the substrate, and hard carbide and boride precipitates (CrB and Cr7C3) were formed. Exceeding the optimum parameters of the fusing caused over-fusing phenomenon and thereby, degradation of coating properties. It was found that the temperature of 1000˚C and the time of 5 min are the optimum conditions of fusing process in this study, as the lowest porosity, the highest microhardness as well as the best wear performance were obtained in coating fused at these parameters. Dominant wear mechanism in this sample was abrasive wear.

Cr2O3, YSZ and SiC nano-powders were prepared by ball milling and subsequently mixed and agglomerated to reach the proper composition and size for spraying. Morphological investigations and particle size analysis showed that powder particles after 5h of milling time reached to an ultrafine/nano size. The powder mixtures were then deposited onto 304L steel substrates using Atmospheric Plasma Spray (APS) to deposit Cr2O3-20vol%YSZ-10vol%SiC composite coating. Microstructure and morphology of the elemental/milled powders and plasma sprayed coatings were characterized using Field Emission scanning electron microscopy (FESEM) equipped by EDS. X-ray diffraction (XRD) patterns of powder particles and coatings included only the elemental peaks without any traces of impurities and new appearance peaks. Using image analysis method, the coatings porosity content increased from 8.7 to 12.8 through formation of composite coating. Also mechanical properties of the coatings including bonding strength, micro hardness and fracture toughness were evaluated. The results showed that coatings have high bonding strength of 40-49 MPa. Regarding hardness and toughness results, adding reinforcements to Cr2O3 coating increased hardness and toughness to 910 HV and 8.1 MPam1/2, respectively.

In this research, nanostructured Cr2O3 (C) and Cr2O3-20YSZ (CZ) coatings were deposited using Atmospheric Plasma Spray (APS) process and subsequently their mechanical properties including fracture toughness, hardness and bonding strength were evaluated and compared with each other. For this purpose, Cr2O3 and YSZ nano-powders were firstly prepared by high-energy planetary ball milling and then mixed in the given volume. To use the powder mixture in the plasma spraying, the powders were agglomerated and sprayed onto a 304L steel substrate. Microstructural studies of the powders and coatings were carried out through Filed Emission Scanning Electron Microscope (FE-SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD) analysis. XRD patterns of the milled powders and the coatings indicated there was no newly-appeared phase during the milling and plasma spraying process. Porosity of the coatings was measured using an optical microscope by image analysis of their cross-sectional images. Mechanical properties evaluation showed that both coatings have high adhesive strength in the range of 43-49 Mpa. Although the addition of YSZ nanoparticles to the chromia coating slightly decreased the coating hardness, it caused a considerable increase in the coating toughness due to the zirconia transformation toughening mechanism.

In this research to rich nano structure bond coat uses in TBC, firstly with ball milling process the micron size NiCrAlY convert to nano particle size NiCrAlY, and after agglomeration nano particle NiCrAlY, powder were deposited upon stainless steel 420 substrate with atmospheric plasma spray (APS) process. Injection distance of powder, hydrogen rate, and utilizationof argon shroud parameters were investigated for suitable deposition agglomeration nano NiCrAlY particles. The result shown, utilization of argon shroud in nano NiCrAlY bond coat was decreased oxidation salient. Also increase of injection distance of powder and hydrogen rate cause to oxidation enhancement. Moreover the nano particle NiCrAlY in comparison micron NiCrAlY toward oxidation was very tender.

Flame spray is a widely used coating process due to its low cost and wide variety of usable materials. But weak adhesion between splats as well as between coating and substrate and also high amounts of coating porosity are main drawbacks of this technique. To overcome these problems, usually heat treatment process is applied on these coatings. The NiCrBSi coatings which have high wear and corrosion resistance are used at severe tribological conditions. In the present research, Ni-Based NiCrBSicoatings were applied on CK45 steel substrates by flame spray and the effect of spray and heat treatment parameters on their microstructure were studied. The results showed that applied coatings on the sand blasted substrates under the condition of 150 g/min powder feed rate and 15 cm spray distance, have laminar splat microstructure with the lowest amounts of porosity. Also the effect of heat treatment’s temperature and time on the microstructure and generated phases in the coatings was investigated.Phase analysis showed that Ni, CrB, (Cr,Fe)7C3, Ni3B and Ni5Si2 phases are generated in the coatings microstructure which were heat treated at 1075°C. In addition to theses phases, Cr2Ni3B6 phase was also detected in the coatings which were heat treated at 1100°C.