| A popular ways for surface engineering in our days are methods based on using ofconcentrated energy flows (electron and laser beams, low-temperature plasma, pulseddischarge etc.) Method of electro spark deposition is one of them.In short-term modern heatproof titanium alloys could be used at temperaturesabout600-650°C and higher. But in case of long-term using (hundred hours and more)in oxidizing mediums at temperature more than500°C is impossible because of strongsurface oxidation and corrosive destruction. Nowadays there are no reliabletechnologies for protection of surface layers that can improve characteristics of titaniumalloys and protect them from destruction.Purpose of our work is to investigate a possibility application of electro-sparkdeposition (ESD) method to improve a heat resistance, resistance to erosion and toincrease an operating temperature of the titanium alloy TC11(Ti-6.5Al-3.5Mo-1.5Zr-0.3Si). To achieve this purpose it is were chosen the operatingpractices of surface modification, anode materials that makes it possible to use ESDmethod effectively and to carry out research work to check properties of depositedcoatings.In present work were investigated heat-resistant properties of ESD coatings onTC11alloy. Oxidation was estimated by mass change of samples. Test revealed thataluminum ESD coating can provide the best oxidation resistance of used anodematerials. In this test (100h,700°C) uncoated TC11samples oxidized2.6times morethan samples with deposited aluminum. Were studied oxidation laws for all depositedlayers in a short-term oxidation test. Formation of two layer ESD coatings withaluminum and nickel was not effective. These protective layers are worse than ofaluminum.Combination coatings of ESD and PEB did not provide the desired protection.Analysis of microstructure showed that in a surface layer appeared multiple cracks thatcan be caused by formation of more complex compounds Al3Ti5O2and Al2Ti in aprocess of electron beam treatment of samples with ESD coatings. Results of oxidationtest of combined coating is similar to single ESDBecoming more common in the aerospace industry are α+β titanium alloys,including Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11). These alloys are successfully used in themanufacture of aircraft engine parts and other parts exposed to high temperatures forsmall values of strain.In our work we decided to create high-temperature protective coatings on the surfaces of titanium alloys by using of electric-spark deposition method (ESD). Anadvantage of this process is the ability to form ESD layers with a large range ofphysicochemical and mechanical properties on the working surfaces of any conductingparts and structural elements. Today this method is well studied by Russian specialistsand scientists from other countries and it is widely used to increase heat resistance, wearresistance and corrosion resistance of different steels and alloys.ESD is a micro-welding process by which coatings are applied in short pulses froman electrode to deposit nano-grained microstructure coatings to metal substratesFor simplification, process of ESD can be divided on few stepsa-the breakdown of the interelectrode gapb-the formation of erosion holes at the anode and cathode; mass transfer from anode tocathode;c-the moment of contact electrodes, which is accompanied by short-term joint andreturn mass transfer from cathode to anoded-the formation of secondary structure on the anode and the doped layer at the cathode.Electron beams are becoming an increased subject of interest for materialsprocessing. While continuous electron beams have already found wide applications indrilling, hardening, cutting and welding, the advantage of a pulsed electron beam hasjust emerged. It generates a high power density up to105—109W/cm2at the targetsurface. Such a high energy is deposited only in a very thin layer within a short tune,and causes superfast processes such as heating, melting and evaporation. A dynamicstress field induced in these processes leads to significant modification effects in thematerial. The combination of these processes provides the material with improvedphysicochemical and mechanical properties unattainable with ordinary surface treatmenttechniquesElectrode materials used in our work:Base material (cathode)-α+β titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si (TC11).Coating materials (anode)-1) Cr20Ni80(20%Cr,80%Ni);2) Cr;3) Ni;In order to determine the temperature at which rapid oxidation of the alloy, it wasdecided to hold short-term exposure. The samples were placed in an furnace heated to500°C, and with further soaking they were extracted one by one. The results are shownon this figure. Before oxidation test of ESD coatings was carried out test with uncoatedsamples of TC11. The samples were extracted from the furnace one by one as it shownon a FigureAlloying with Cr and Cr20Ni80is accompanied by a significant erosion of theanode material and substantial additional weights of cathode samples. As we can findout from result of weight increment of cathodes Δk in the study of the kinetics of mass transfer, the beginning of brittle fracture in the coatings formed by Cr and Cr20Ni80achieved only after10minutes. This is due to the high scale resistance of thesematerials, which prevents the rapid formation of secondary structures on the surface ofthe electrodes during the erosive process of ESD. However, the use of Ni as awheat-resistant material does not provide the intense erosion of anode and mass transfertoe itghhe t siunrcfraecme eonft s∑am1=0p1l es. Moreover, within10minutes of ESD process, the rate ofhas a negative value. To explain this fact we take a look atthe selected alloys of electrode materials in order of increasing hardness, it’s obviousthat the total values of mass transfer can be arranged in a similar sequence: Cr,Cr20Ni80, TC11, Al and Ni (Table1).In the coating layer of Al time of brittle fracturing tх is6minutes, the mass transfercoefficient is88%. This fact can be explained by the low melting point of Al, whichallows to melt the anode electrode at the beginning of the ESD process and to performthe mass transfer in a drip-liquid phaseThese characteristic features of Ni and Al in the ESD are clearly reflected in thebehavior of the curves of mass transfer of two-layer coatings formed with these metals.Al anode forms electrospark coating on a substrate with a positive mass transfer, but theNi anode destroys this coating, thinned every minute of ESD processExistence of different laws of oxidation is conditional on processes in oxidationlayers, in scale and in substrate. Growth of the oxide scales as a function of time can bedescribed by growth laws.In practical applications a number of different growth laws are reported, some ofthe borderline cases are displayed on a scheme.For pure titanium oxidation kinetics varies with temperature and time. Fig.4.3summarizes the various rate laws followed by titanium at various temperatures.Oxidation kinetics follows logarithmic rate law below400°C during which a thin oxidelayer of TiO is formed. Between400to600°C kinetics are found lo follow cubic ratelaw. Oxygen dissolution becomes quite important in this temperature range. Between600to1000°C the kinetics are described by parabolic behavior, that involves bothoxygen dissolution and scale formation.. Above this temperature, the rate of oxidation isso fast that it can best be described by linear kinetics.All coated samples were subjected to high temperature oxidation in a mufflefurnace at700°C for four hours. Oxidizability was estimated by specific changes in themass of the samples measured every hour after complete cooling in air. During testingof the samples delamination of heat scale was not revealed, but increased the numberand size of microcracks on a surface.The oxidation kinetics curves of the Ti alloy TC11, gained from mass change are plotted on a fig. Plotting of regression with functions of oxidation laws allow to find outthe primary mechanism of oxidation and scale formation for each sample. To estimatequality of approximation was used software program OriginPro8. Coincidence degreebetween experimental results and plotted approximating curves are shown in TableEstimation of coincidence degree for all oxidation laws shows that eight of tensamples oxidizing according logarithmic law. According to Evans theory of logarithmicmetal oxidation, appearance of microcavities on a boundary between coating or oxidescale layer and substrate metal leads to increasing of self-stopping of oxidation. Similarmicrocavities were observed on cross-section of samples2,3,4,6. At700°C oxidationof pure titanium goes in correspondence with parabolic law according to Fig.4-3.Logarithmic type of oxidation curve of uncoated samples of TC11and samples withESD coatings can be explained by heat-resistant properties of this titanium alloy. Indescription of Evans theory was proved possibility of application of logarithmic law forthick films.[62]Similar results were obtained in works of Russian[64]and Chinesescientists[65]Therefore, for application of diagram to TC11alloy it must be shifted tohigh-temperature region.Next step was further investigation of high-temperature oxidation at700°C of thesame samples in long-term soaking in furnace for100hours. Process was continueduninterruptedly for all period of time. Figure3-12shows a comparison of mass changeafter these two stages of oxidation. The lowest mass gain of samples for4and for100hours showed ESD coating with aluminum1,225g/m2(100hours) that is about3timeslower than of uncoated samples.The samples with nickel (Ni/TC11) and chromium (Cr/TC11) coatings provideprotection only in a short-term test that is not enough for a standard use of TC11detailsat high temperatures. What about chromium-nickel alloy coating (Cr20Ni80), it doesnot demonstrate any protective properties. Long-term oxidation of these samples higherthan of TC11about2times.For two layer coatings gain of masses had insignificant difference after4and100hours. To compare with uncoated sample mass gain and oxidation rate were very similarafter short-term exposure, but for long-term experiment the two layer coatings improvedoxidation resistance of the samples about32%. So, from obtained results it also couldbe concluded that chemical composition of formed ESD coating is more important thansequence of formation of coating layers.By using scanning electron microscopy were obtained pictures of the samplessurface (Pic.7). On the photographs can be clearly seen that the coating exposed tosoaking in the furnace, is covered with cracks much stronger than a before a heattreatment. During ESD process, the formation of micro cracks is inevitable, with sufficient magnification shows that the coating before heat treatment has it too (Pic.7).However, we see that exposure to high temperatures provoke the increase of a size and anumber of cracks. This phenomenon has a negative effect, reducing the effectiveness ofESD coatings, as is known, penetration of oxygen to the substrate go through the poresand cracks and it is the result of further oxidation of the base metal.As we can see from this tables, concentration of oxygen in the surface layersbefore and after heat treatment differ about10%, that can be caused only by oxidationduring a soaking in furnaceQuantitative analysis of the base material showed different results for differentcoverages. For a point analysis of samples before and after heat treatment is almostidentical we can see that content of oxygen did’t change, so this fact let us to supposethat oxidation of the alloy surface happened at room temperature after production of thesection. In the areas of pearlite structure, the oxygen content is much lower than in areasrelated to cementite (an average of1.8%and5.7%). These data apply to samples withthe electric spark deposition formed with pure chromium, which showed the greatestweight gain after heat treatment in a furnace.Metallographic examination of modified TC11samples revealed microstructure ofthe substrate after the ESD. Fig.2shows that the deposited layer has a clear boundaryand the diffusion layer extending in the substrate material along the borders of DL.However, on all samples coated with selected electrode materials there is no pronouncedheat-affected zone in the substrate, characterized by a decrease in grain size of thematrix material, that usually appear under the surface layers due to exposure to hightemperatures during electro-spark treatment.By mean of EDX method of electron microscopy, there was analyzed the linearconcentration of chemical elements in the microsection of the samples. It is ascertainedthat after ESD of TC11alloy with anodic materials of Cr and Cr20Ni80, the percentageof oxygen measured in all depth of deposited layer does not exceed5%. Diagram oflinear concentration of an elements in the sample Cr20Ni80/TC11(Fig.3) shows somejumps of oxygen concentration in the substrate material that can explained by differentsolubility of oxygen in the α-and β-titanium and by inhomogeneities of the phasecomposition of coating layer.Such a small concentration of oxygen in the deposited layers is due to highoxidation resistance of Cr and Cr20Ni80. It reduces the rate of formation of oxides inthe secondary structure on the surface of the anodes. This fact is matched with theresults of researching of the kinetics of mass transfer (Fig.1). Low concentration of Niin DL (6%and less) was set because of substrate destruction, started in a very firstminute of the ESD process by nickel electrode (Fig.3b) и. It is accompanied by increasing concentration of oxygen. During the ESD process, there is going substratedestruction by Ni anode. In spite of this process, the thickness of the DL is increasing to36.1μm. As a result, there was assumed, that the other recrystallization andreorientation processes have occurred with (α-β) titanium alloy crystals. All theseprocesses are accompanied with thermal treatment of electrical discharges andmechanical stress.Creation of complex coatings on surface of metallic parts by using of electro-sparkdeposition method with other methods is one of possible ways to improve coatingproperties. In this case result coating can possibly get advantages of both methods. Toprove this effect should be done special tests and measurements. In our work to improveelectro-spark coating was used high current pulsed electron beam.8groups of samples were prepared to carry out further tests of electrosparkdeposition coatings with additional PEB treatment.The oxidation resistance of the samples with aluminum ESD coatings is muchhigher than of uncoated TC11as in previous tests for both Al/TC11and Al/TC11+PEBgroups of samples. Resistances for these coatings differ about2%that can be caused byinaccuracy of measurements. So we will consider this parameter does not change afterelectron beam treatment for aluminum ESD coatings.It was studied morphology of coating after PEB. Surface layer of I and I*groupssamples have fine-grain microstructure that appeared in a process of electron beamtreatment. The grain size is about50-150nm. According to this data we can assume thatin a process of electron beam treatment surface layer has been melted and crystallizedvery rapidly, so appeared great number of crystallization centers.The new structures appear in a surface layer in the depth of5μm. In oxidation testit has been shown that this new layer has slight protective properties. Oxidation ofsamples from IV*group is stronger than of I*group at26%Complex coatings of II and II*groups have great number of cracks of the surface,but according to the oxidation test these cracks do not effect on protective properties ofESD coating. Nevertheless after oxidation of these samples there are some differencebetween II and III group. It was revealed that after oxidation on the cracks edges startsgrowing of needle-shaped crystals. The crystals were determined to be rich in titanium(32%Wt.), aluminum (13%Wt.) and oxygen (39%Wt.) by EDX tests. The nearby zoneof coating contain same materials, but in other percentage–43%of titanium,37%ofaluminum and14%of oxygen.To explain formation of cracks in the coating it should be considered that usuallyelectron beam treatment is used for process with homogenous metal materials. As resultof heat treatment the surface layer melts in the depth of few micrometers and it goes to recrystallization. In this process it goes on formation of new layer with new structurebut still homogeneous generally. As for ESD coatings, the surface layer is a mixture ofdifferent phases (metal and nonmetal). So the coating does not have same meltingtemperature for all these phasesCoating layer accumulates energy and microstresses. Asresult of microstresses in this non-homogeneous structure appear numerous cracks.After comparison of phase analysis data for different groups of samples in pairs it willbe possible to conclude which phases improve oxidation resistance of our titanium alloyand analyze conditions that lead to appearing of these phases. Results of XRD analysisare shown on Fig.5-8,5-9,5-11,5-12,Samples of I, I*, IV and IV*groups do not have any additional elements, so theyshould be considered altogether. The initial alloy TC11consist of α-Ti, β-Ti, andchemical compounds of titanium and aluminum–AlTi3and Al3Ti (Fig.5-11). Forphase analysis elements with low percentage (Zr, Mo, Si, Fe) were not considered.Electron beam treatment destruct the Al3Ti phase in structure of titanium alloy (Fig.5-12). This treatment also decreases an amount of β-Ti in coating layer. As result it wasobtained new coating with redistributed elements. As XRD analyses does not revealpure aluminum in the structure of surface material, we can conclude, that Al3Ti wastransformed into AlTi3.After long-term oxidation of samples of at700°C (I*and IV*groups) in thestructure appears titanium dioxide–rutile TiO2. Rutile is one of common titaniumoxides (Table1). As the free titanium in the coating material mostly presented byα-Ti, so we can assume that rutile in the coating is the product of α-Ti oxidation.To analyze XRD results for samples of II, II*, III and III*groups is morecomplicated task as ESD coating layer is complex material even without additionaltreatment and oxidation. After formation of ESD coating, surface layer consist of purealuminum, aluminum oxide Al2O3, aluminum titanium compounds Al2Ti and Al3Ti,and titanium nitride TiN (osbornite)(Fig.4-14). Possibility of formation of intermetalliccompounds Al3Ti, Al2Ti and AlTi3from solid phase at temperatures over600°C wasapproved by detailed investigation.[69]Formation of oxide and nitride phases becamepossible in connection with high temperature of electric spark during ESD process.Energy of electric spark made it possible to transform highly stable molecule ofnitrogen N2into ion form for further reaction with titanium.Electron treatment of samples with ESD coatings caused formation of morecomplex compounds Al3Ti5O2and Al2Ti, that were not detected in samples of III andIII*groups. Al3Ti5O2compound can be formed in reaction between TiO2and Al2O3at1100K.[72]In long-term oxidation at700°C pure aluminum in coating layer of samples of II* and III*groups was transformed to Al2O3. Alpha phase alumina that was detectedcoating layer in samples of II and III groups is the strongest and stiffest of the oxideceramics. Its high hardness, excellent dielectric properties, refractoriness and goodthermal properties make it the material of choice for a wide range of applications.[70]Asaluminum oxide is well known of its corrosion protective properties, aluminum ESDcoating after oxidation can have corrosion resistance properties.Long term oxidation also cause particular transformation of aluminum reach Al3Tiphase-second peak on phase diagram (Fig.5-12) that mostly formed by this phase intoanother intermetallic compounds Al2Ti and AlTi3(Fig.5-13).By mean of microhardness test on the samples cross-section there were studiedchanges of TC11alloy microhardness after different treatments and processes. It wasmeasured that process of electron beam treatment slightly increases microhardness ofsurface layer. For samples without ESD coatings it changes from393.5HV0.1to399.3HV0.1and it caused by formation of new fine structure on the surface and bydestruction of TiAl3compound. For samples with aluminum ESD coatings it changesfrom718HV0.1to730HV0.1and it caused by formation of Al3Ti5O2phase and bydestruction of Al2Ti phase.For samples of I and IV groups after oxidation test surface microhardness increases2.7and2.6times respectively. This change of microhardness can be caused byformation of titanium oxide rutile (TiO2) in the surface layer, as it is the only differencebetween structures of these groups material.Oxidation also improves microhardness of samples with ESD coatings (II and IIIgroups), but in structure of these samples were not revealed rutile as for I and IV groups.Hereby hardening of surface material can be caused by increasing of Al2O3phasequantity after oxidation of pure aluminum. As for uncoated samples of TC11formationof electrospark aluminum coating on the surface improve microhardness of samples at95%. |
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