Microstructural and Mechanical properties | My Assignment Tutor

0PROJECT REPORTSURFACE ENGINEERINGTopic: – Microstructural and Mechanical propertiesanalysis of Two different High Entropy Alloys (HEA)coated using Thermal Spray ProcessName – AKSHAY PRAKASHBHAI JOTANIEnrolment – 1028724921INDEX: –1) Abstract and Summary 22) Introduction to thermal spray process 3• 2.1) Working in detail 4• 2.2) Classification 4• 2.3) D-Gun Process 5• 2.4) Plasma Transferred Arc Process 6• 2.5) Surface Treatments 63) High Entropy Alloys 74) Microstructural and mechanical properties of 2HEA’s coated using Thermal spray process 8• 4.1 Alloy 1 FeCoCrNiMo0.2 9• 4.2 Setup parameters 9• 4.3 Results 10• 4.4 Mechanical Properties 12• 4.5 Alloy 2 AlCoCrFeNiTi 13• 4.6 Setup parameters 14• 4.7 Results 145) Conclusion and Discussion 16• Alloy 1 FeCoCrNiMo0.2 16• Alloy 2 AlCoCrFeNiTi 166) References 172Abstract and SummaryAs we know that in engineering we try to design something which is not only given100% thought process but also considered it’s all aspects and different possibility. Inengineering when designing something right from its atomic structure to its simulationin real life is considered as equally important. By doing these we are not only improvingoverall quality but also its ability to outperform. There are many aspects of designingthe part in engineering for example geometrical (aesthetic and ergonomics), materialselection, durability etc. now if we see one way there’s lot riding on which material weselect for any application, because it will determine its cost and overall performance.In this study we will focus on two major aspects of materials. First are new types ofmaterials which are called High Entropy Alloys and Second is material coating usingthe thermal spray process.Now question arises why not use just a simple material and go ahead with ourmanufacturing part. Due to advanced development in materials we are now able tocombine more than 2 materials to customize the material outcome as per ourrequirement. Earlier in iron age we developed a technique which taught us to preventcorrosion and now we can improve specific properties of material as per ourrequirement for example if in a material we want higher thermal conductivity andhardness we can easily achieve such properties. For example, the carbon ceramicstiles which were used in Space Shuttle to prevent it from burning upon re-entry in tothe earth atmosphere can withstand extreme heat and force, but it was brittle at thesame time.Changes to the material can be made using 2 major approaches. Internally andexternally. As discussed earlier changes internally altering its chemical compositionand externally it can be made by a technique called thermal spray process. Thermalspray process is an external process which allows us to develop new layer of materialon top of existing material to improve certain aspects in terms of properties. Thermalprocess allows us to externally customise material such as a coating of certain materialcan provide ability to withstand higher thermal temperature, protection againstcorrosion, increase surface hardness and many more. Basically, it is a clevertechnique to customise certain material. For, example if the material is not able to withstand certain hardness due to its internal chemical composition we can just simplyapply a layer of coating which will be able to withstand certain level of hardness.Brief explanation is provided below regarding different types of thermal sprayprocesses and different approaches as per requirement. Also, various techniques areexplained to give better understanding of the spray process.Further in this study/paper literature review or in other words analysis is done basedon Mechanical and Microstructural Properties of 2 different high entropy alloys coatedusing different thermal spray process.32. Introduction to Thermal SprayThermal spray is defined as the coating or spraying metallic and non-metallic materialon to the prepared substrate in molten or powder form to form a separate layer. Thematerial which are sprayed can be in any form such as ceramics, powder, rod or wire.Thermal spray process is divided into many sub-processes but first we need tounderstand the basic working.Fig 1: – basic concept of thermal spray process {4}In the above figure we can observe and understand the basic working. Firstly, thesubstrate is prepared (cleaned). The equipment’s required are Gas, Powder or feedmaterial and spray torch. The (feed)material is deposited or loaded and then passedthrough the spray torch in which due to combustion or electrical discharge the materialis melted then streamed towards the substrate with the help of high velocity gas streamgets deposited. The torch is moved at certain angel to achieve a quality layer. Moltenparticles stick onto the surface due to the temperature and in cold spraying due to itsvelocity.Fig 2: – Shows the Plasma Spray Torch from thermal spray family {4}42.1 Working in Detail: –It can it easily understood with the help of sub-steps listed below: –1) Torch which can be considered as the most important part of any process whichhelps to generate high velocity stream of jet which includes energy and powersupply2) Second major player is powder. Some cases powder is to be prepared andaccording to the specific grain size and injection into high velocity gas stream,so it can melt before being deposited.3) Surroundings also matter in some of the process which means they might needcontrolled environment in terms of humidity, specific gases, dust proof etc.4) Substrate preparation which includes cleaning the surface, holding it positionat certain angle and safe surroundings5) Lastly controlling the equipment which controls the motion of the torch, it canbe hand held in some cases.The steps listed above can vary according to the requirement and selected criteria.Fig 3: – Simple Classification tree of Thermal Spray Processes {4}2.2 Classification: –Thermal spray processes are classified into several sub-processes which furtherdivided according to their way of working. First and foremost, they are dividedaccording to their discharge i.e. combustion and electric discharge. In (1)combustion process, combustion takes place inside the torch and feed material interms of powder or a wire gets melted then it is sprayed with the help of highvelocity stream. (2) in electric discharge process the feed material in terms ofpowder or a wire is melted due to high voltage current. An arc is generated due towhich the material melts and then with the help of high velocity stream it getsdeposited.5Under combustion processes in flame thermal spray process comes and it is oneof the oldest techniques and simplest to understand. The powder is fed through asimple hopper then with the help of oxy-fuel combustion it generates hightemperature and powder, or wire gets melted and due to its own pressure andvelocity it gets deposited on to the substrate. Secondly HVOF or High Velocity OxyFuel comes in which by using a specially designed nozzle because as the namesuggests high velocity which means the particles speed coming out of the nozzleis subsonic to supersonic {4}. Thirdly D-Gun or detonation gun comes which worksaccording to its name. A mixture of explosive materials is exploded inside the Dgun and then sprayed through the nozzle and during that powder is heated andsprayed onto the substrate {4}.In electric discharge method as described earlier electric arc is generated due tohigh voltage current. In wire arc the feed material is wire and it is melted with thehelp of arc and then deposited to the part. The arc is generated between twoconsumable electrodes and gas is included which helps in atomising anddepositing the substrate. In plasma transferred arc the feed material is fed in to theplasma device and then heated and sprayed into the substrate. In PTA the sprayingoperation is performed in horizontal calibration due to its equipment position {4}.In flame spray process it may affect the uppermost layer of the surface and mightchange its granular structure which may affect later duration its application. As faras plasma process go it falls under the same category but it does a slightly betterjob because it melts the layer like in welding process by creating a pool fromplasma and then depositing the coating material.2.3 G-Gun process in Detail: –Fig 4: – Structural diagram of D-Gun {4}As we can see in the figure above and understand the working of the D-gun issimple to understand. It was developed in Russia in early 1950’s. In working it is6like the HVOF, the variation in this process is that a controlled detonation takesplace which includes oxygen combined with fuel and powder coating material. Aswe can see that gun is open from one side and closed from the other it’s for areason for how the overall process works. Oxygen along with the fuel gas which ismost likely to be acetylene is induced to the barrel as we can see in the figure andspark plug generates the spark that ignites the mixture which then creates anexplosion. Pressure which is created by that explosion is about 2Mpa and it isutilized, or it automatically creates a flow towards the open side of the barrel. Alongthat powder is fed which heats up and in some cases melts and then gets depositedto the substrate. This is a cyclic process which is carried out again and again insidethe D-gun at the rate of 4 to 8 times per seconds. Now due to detonation and thegas velocity the spraying speed is around or can reach 1050 to 3050 m/s. It canproduce very high-quality coatings with smooth surfaces. If one has therequirement to produce a high-quality coating with smooth surface and propertiessuch as high hardness, high wear resistance, high corrosion resistance, highbonding strength along with cohesive strength the D-gun is capable of spraying.2.4 Plasma-Transferred Arc deposition (Electric discharge)Fig 5: – Working of Plasma Transferred Arc {4}This process is multi-functional process as far as its working goes. Feed materialis powder and fed into plasma arc which gets heated and then defused to thesubstrate. It basically works like welding process as a result it only employs metallicmaterial as a feeding material. It uses argon gas as plasma forming gas with gasflow rate of 0.5 g/s and the distance is relatively shorter between the nozzle andthe workpiece. As it works like welding process it creates a pool of molten metalwhich is protected by shielding gas and then powder is heated and then meltedwith the help of transferred arc. The coating deposition rates depends on the flowrate of the gas and plasma. PTA can also coat 2 different types of powder such as1 which melts in the process and other ceramic based which is sprayed as a toplayer which drastically increases its area of application, but the material range islimited to some specific material. It produces a smooth and uniform layer of coat.2.5 Surface Treatments: –7Before starting thermal spray process, it is equally important to do surfacetreatment. It includes preparing the substrate cleaning the surface and making sureit is properly done because if not prepared properly then unwanted material mightget stuck between the coat layer and the layer of the substrate creating a separatelayer which later can be a medium to let the coated surfaces peel away. Manymethods such as strain hardening, surface hardening, thermo chemical etc. areused. In surface hardening the part is heated to certain austenitic temperature andrapidly cooled down which helps in hardening the surface of that part hence thename surface hardening. Other way is to treat the substrate chemically in whichincludes methods such as carburizing, nitriding, carbonitriding etc. all thesemethods involves forming a layer of carbon which is done in very controlledenvironment and surroundings. Chemically treating a material takes a bit longerthen surface hardening.3. High Entropy alloysHigh entropy alloys are defined as the materials with mixture of 5 or more principlematerials. When we look at the conventional materials we know that they have 1or 2 primary materials present them. If there’s one major element it is called as amaterial for example iron. If more than 1 material is present such as in stainlesssteel which is chromium and carbon it is called as an alloy of that mother material.Similarly, high entropy alloy consists of the more than 5 materials with eachcontaining their own concentration between 5 % to 35 % along with the basematerial.As we know that in conventional materials the microstructure and the chemicalcomposition is simple and easy to understands because of less entropy but due tohigh concentration of the elements present in the HEA’s the entropy increasesbecause of complex microstructure and chemical composition. The microstructureand mechanical properties are highly customizable in HEA’s as there are manyelements present. Due to high entropy it generates complex BCC and FCCstructure. Some elements are FCC based yet others are BCC based. Withpresence several elements it produces alloys with unique and special mechanicalproperties with enrich in high hardness, high strength, extreme wear resistance,corrosion resistance and many more. While it is easy to produce HEA’s itautomatically widens the area of application due to its properties.HEA’s are different than conventional materials as a result it has its own uniqueeffects which are exclusive to HEA’s they are often called “core effects”.Core Effects: –• High Entropy Effect• Sluggish diffusion Effect• Severe Lattice-distortion Effect• Cocktail Effect2 effects are explained below1) High Entropy effect: –8It is defined as the effect which helps in the formation of phase during the solidformation state. HEA’s have greater than normal entropy rate as we know it andthis effect tends to stabilize that phase and helps in overall process.Fig 6: – XRD diagram of Alloys added one after another to record Structure {5}The concrete evidence of this effect can be proved from the XRD pattern shownin the figure above. As we can observe that almost all the materials consist of2 major phase structure which are clearly FCC and BCC with minor phaseswhich did not appeared during the XRD scan {3}.2) Severe Lattice distortion effect: –As we know in HEA’s many elements are present which creates a different typeof lattice structure as compared to conventional materials where it contains onlyone major or principle elements and it forms uniform and continuous grain orlattice structure {3}, {5}. However, in HEA’s some elements atom may be largerthan the other elements alloy which pushes one another upon settlement orsolidification and hence create a distortion in a lattice structure and called asan effect.Fig 7: – Lattice structures (a)same size atoms in pattern & (b) Irregular atomwith distorted pattern {3}9In the figure above (a) shows near perfectly aligned atoms which creates a gooduniform pattern and (b) shows distortion due to imperfect atom size whichcreates irregular pattern upon solidification and as a result we get distortedlattice structure.4 Microstructural and mechanicalproperties of 2 HEA’s coated usingThermal spray processAbout: –Below described are reviewed experiments in which High Entropy alloys werecoated using thermal spray process. The process is described in detail rightfrom start to finish which includes material preparation to the results discussedas achieved during the experiment. Both the materials were coated usingdifferent methods such as HVOF (High Velocity Oxy-Fuel) and Air PlasmaSpray.High Entropy alloy used are: –• FeCoCrNiMo0.2• AlCoCrFeNiTi4.1 Alloy 1 FeCoCrNiMo0.24.1 Introduction: –The coating material of FeCoCrNiMo0.2 was prepared by gas atomizationprocess and further it was coated using the High Velocity Oxy-fuel (HVOF) andAir Plasma Spray (APS). All the microstructural and mechanical properties wereinvestigated in detail such as phase formation, wear resistance (mechanical)and many other. Also, the comparison of both the spray process is describedin detail. In shorter version the wear resistance in APS technique was noted anotch higher than the HVOF process. But it was noted that both processes canaffect the microstructure due to their extreme temperature thus affecting all theother properties such as oxidation, mechanical and chemical.4.2 Setup and Parameters: –Powder preparation was done using high quality materials with purity of 99.9%FeCoCrNiMo0.2. The material was melted using nearly pure argon and theparameters were Gas flow rate was 0.25 m^3/s, atomization pressure 4 MPaand metal flow rate was at 50 g/s {2}. The powder was formed using a regulartechnique with melted droplets dropping directly into atomization chamber and10getting solidified on cooling. The spherical shaped powder particles were ofcertain range between 15 to 45 μm {2}. The oxygen content was calculatedusing the oxygen/nitrogen determinator.The substrate of 100mm X 30mm X 20mm was pre-cut and polished using thesand paper to 140 grit level {2}. Steel was used as a primary material for thisexperiment. Layer of coating was coated using the APS and HVOF as statedearlier. The exact parameters used are shown in image below.Fig 8: – Setup parameters for HVOF and APS {2}4.3 Results: –Feedstock powder was noted to be spherical shaped as it was manufacturedduring the gas atomization process. Characteristics such as satellite formationon that spherical shaped elements were due to rapid cooling during thesolidifying process. Other properties were nominal to the previous results.Fig 9: – XRD pattern of Gas atomization process and coating along withoxides Fe3O4, Fe2O3 and AB2O4 {5}11As we can observe from the XRD graph above we can clearly that both thecoatings were primarily formed based on FCC. In both the processes slightoxidation might have occurred in the feedstock powder. As we know fromprevious studies that HEA’s displays excellent properties in terms of thermalconductivity and stability which is the result of the high entropy effect andsluggish effect which are 2 of the core effects of the HEA’s. As stated earlierthe feedstock powder was subjected to oxidation during gas atomizationprocess it was reported that it was even more affected by that and due to rapidlycooling droplets of molten metals. Later it was also observed that it was differentreaction in both the methods. For example, in APS oxidation was more ascompared to HVOF and it affected the grain structure while the graph showssudden rise at certain level which clearly states generating a good granularstructure.Fig 10: – image from SEM containing low and high magnification of (a)&(b)APS and (c)&(d) HVOF coating, along with pore, (A)HEA & (B)Oxide phase{2}AS we can observe in the figure above where both the materials are displayinga fine and uniform lamellar structure. In APS thermal spraying process, it wasobserved that the structure had more defects because of previous oxidationeffect which was observed during the gas atomization process. This resultedinto the structural defects such as Splats, pores and cracks. During theoperation due to extreme (>10,000C) in APS it is possible that the feed powderwas completely melted and resulted in high oxidation rate before gettingdeposited onto the substrate. While in HVOF temperature is relatively low atabout 3000 C and the powder is accelerated at high speeds 190 to 1100 m/s{2}. Feedstock powder which has been through such temperature and velocitymay not be melted completely or just partially melted which again results intolow oxidation when compared to one another. In images c & d it can be12observed and understood based on the oxidation process which might haveoccurred during the travel process from spray gun to substrate it producesmicrostructural defects such as interlamellar gaps, pores, splats and notcreating enough lamellar structures.4.4 Mechanical Properties: –The microhardness was tested to determine the hardness of the coating. Thecoating coated using the APS thermal spray process exhibited the average of356 HV (0.2) and the coating coated using the HVOF process shown 390 HV(0.2) {2}Fig 11: – Microhardness of both the coatings {2}As we can observe and understand from the graph above which is ofmicrohardness to the distance of interface we can clearly see that the hardnessof the APS process varies more than the HVOF process in comparison. Due tothe oxidation phase that occurred during the preparation of powder through gasatomization process it resulted into the varying result of the microhardness test.Because of that phase it formed the non-uniformed structure with all the defectsthat stated earlier such as pores, gaps etc. while other study shows that coatingcoated from APS should have much higher HV(hardness) value. However,many studies also state that it degrades overtime and decreases its overallvalue. In HVOF due to high velocity spray and a hospitable temperature itproduces the coating with lesser defects on oxidation and forms a uniformlamellar structure which holds up uniformly in the hardness test.13Fig 12: – Result after wear in friction test with different duration (a&b) Steel,(c-e) APS and (f-h) HVOF {2}Scratch test was performed during this experiment and it resulted into a finedata analysis. The image/figure above clearly shows worn out surfaces in boththe spraying methods in 3 different stages in terms of time interval of 3, 5 and15 min. In APS scratch test the layer of oxide formed and acted as an adhesiveparticle which only grew on the 5 and 15 min test. Specifically, it was observedthat in 15min scratch test the layer of oxide formed a crack which later becamechip and flew of the original position which resulted into relatively large amountof pit like structure and produces a huge derby. While in HVOF only smallamount of oxides were formed due to friction and several cracks were observedwithin but it showed noticeable amount of fight against the scratch.4.5 Alloy 2 AlCoCrFeNiTi4.6 Introduction: –It is inevitable to avoid the fact that aluminium and titanium are used inapplication where weight matters along with their superior mechanical andphysical properties. Thus, the study of special alloy AlCoCrFeNiTi wasundertaken to create an experiment which later resulted in producing noticeableresults such as high hardness, high wear resistance, high ductility, high strengthand many other qualities. The coating was coated using the HVOF (highvelocity oxy-fuel) thermal spraying process due to its capability of producingdenser coats at relatively lower temperature. Hardness test along with the14scratch was performed in which the material exceeded the expected results andperformed well.4.7 Setup and parameters: –Proposed material was prepared by melting the entire batch so that it can createa homogeneous mixture and produced to be the feed stock material from gasatomization with using 99.9 % pure argon gas to avoid any foreign particle. Theparticle size was kept at -60+20 μm. Before coating the substrate was cleanedwith Ethanol. The substrate with having the diameter of 40 mm was used andthe particles were blasted sizing -600+425 μm at 3.5 bar pressure from an angleof 60 degrees and 100 mm distance from actual substrate. {1}HVOF parameters was such as Oxygen was 810 L/min, nitrogen 11L/min,powder feed rate was 80 g/min, no. of layers 9 and spray distance was 360mmfrom the substrate. 3 wear tests were done 1) ball on disk test (20N), 2)oscillating wear test (26N) and 3) scratch test (1-200N). {1}4.8 Results: –Feedstock powder particles were noted to be spherical in shape and withoutsatellite structure. Particles sizing 20 μm were observed during this withhomogeneous mixture of elements.Fig 13: – XRD pattern of powder (a) full & (b) zoomed view of peak of BCC {1}Surprisingly in this thermal spray process a BCC based structured was formedand with furthermore investigation of the peak it was known that it producessome fine grained and coarse particles. This resulted into significantly changingof the BCC due to low intensity radiation which reacted during the formation.15Fig 14: – SEM images of coating (a) Overview (b) detailed view with (I)spray,(II)oxides and (III)pores {1}As we can observe from the image above which clearly shows that structure orlayer of coat formed during the spraying process was homogeneous. Also, weknow from the previous experiment with ALLOY 1 that oxidation during theHVOF process is significantly less as compared to APS. In this experiment theoxidation occurred less, and the coating was lamellar lattice structure and lessdefects. However, the pores and lamellar oxides are still visible in the imageabove as (II) & (III). {1}Fig 15: – (a)Ball-a-disk, (b)oscillating and (c) scratch test with (I)chrome plated& (II) HVOF Coating {1}In all 3 tests the material revealed significant results as compared to APSthermal spray process. In ball disk test it revealed that coating was productiveand showed very little wear depth in HVOF as compared to hard chromedmaterial. In oscillating wear test the results are very similarly as we can seefrom the bar graph above. Lastly in scratch test the depth of wear wasmeasured little bit lower.Fig 16: – (a)progressive load scratch (b) wear depth profile {1}16The coating coated using the HVOF process produced fine layer of coat as aresult it only had minor cracks as expected due to no oxidation and lowertemperature upon spraying. Above in the wear depth VS scratch length diagramwe can observe that the wear is continuous along with the scratch length.5 Conclusion and Discussion: – (Both the Alloys)The experiments which were conducted were exceptional in terms of datawhich we got after conducting the experiment. Both the alloys experimentprovided tons of data to study and discuss such as their microstructural andmechanical properties.5.1 Alloy 1 FeCoCrNiMo0.2For alloy 1 FeCoCrNiMo0.2 coating was done with 2 different methods APSand HVOF. On both the methods APS and HVOF the hardness of 356.4 and390.09 HV0.2 was achieved with microstructure being mostly homogeneous inHVOF and lamellar in both the processes.In both the FCC structure was dominant. Though the oxide formation washeavily noted in the APS method which later created cracks during the scratchtest and formed derbies. While in HVOF due to comparatively lowertemperature the oxidation was far less and exhibited good hardness againstscratch test. As a result, the coating from APS got value of 3.9 × 10-5 mm3/ N·mand HVOF of 4.8 × 10-4 mm3/N·m.5.2 Alloy 2 AlCoCrFeNiTiFor alloy 2 the coating was only done with the HVOF method. Themicrohardness of 730 ± 82 HV 0.1 was determined. The structure was formedhomogeneous and lamellar {1}.Surprisingly in this experiment BCC structure was formed. In terms ofmicrostructure a fine grained and less defective structure was formed withdefects being pores, gaps and splats.Lastly 3 tests were performed to determine harness 1) ball on disk, 2)Oscillating wear test and 3) scratch test. In all 3 tests it out performed andexhibited good mechanical properties.Both the alloys FeCoCrNiMo0.2 & AlCoCrFeNiTi displayed significantmicrostructural and mechanical properties with High strength, high wearresistance, high corrosion resistance and many more. Which proves that HighEntropy alloys exhibits far better properties which are also customizable.17REFERANCES: –1.1.1 {1} Microstructure and Wear Resistance of AlCoCrFeNiTi HighEntropy Alloy Coatings Produced by HVOFM. LöbelISSN: 2079-6412 , 2079-6412; DOI: 10.3390/coatings7090144Coatings , 2017, Vol.7(9), p.144 1.1.2{2}Microstructure and Wear Behavior of FeCoCrNiMo0.2 High Entropy CoatingsPrepared by Air Plasma Spray and the High Velocity Oxy-Fuel Spray Processes Li, Tianchen; Liu, Yong; Liu, Bin; Guo, Wenmin; Xu, LiyouISSN: 2079-6412 , 2079-6412; DOI: 10.3390/coatings7090151Coatings , 2017, Vol.7(9), p.1511.1.3 {3} High-Entropy Alloys: A Critical ReviewTsai, Ming-Hung; Yeh, Jien-WeiISSN: 2166-3831 , 2166-3831; DOI: 10.1080/21663831.2014.912690{4} Materials research letters. , 2014, Vol.2(3), p.107-1231.1.4 Thermal spray fundamentals: From powder to partMaher I BoulosISBN: 9780387689913 , 9780387689913; DOI: 10.1007/978-0-387-68991-3Thermal Spray Fundamentals From Powder to Part / , 2014, p.1-1566{5}High-Entropy Alloys: A Critical ReviewMing-Hung Tsai & Jien-Wei YehTo cite this article: Ming-Hung Tsai & Jien-Wei Yeh (2014) High-Entropy Alloys: A Critical Review,Materials Research Letters, 2:3, 107-123, DOI: 10.1080/21663831.2014.912690Other minor references: –1 Recent progress in antireflection and self-cleaningtechnology – From surface engineering tofunctional surfacesLin Yao, Junhui He Article history:Received 6 March 2013, Received in revised form 30 August 2013,Accepted 17 September2013, Available online 14 December 2013 1.1.52 The microstructure and strengthening mechanism of thermal spray coatingNixCo0.6Fe0.2CrySizAlTi0.2 high-entropy alloysWang,LM;Chen,CC;Yeh,JW;Ke,STISSN: 0254-0584,1879- 3312; DOI: 10.1016/j.matchemphys.2010.12.022


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