Structure And Properties Of Zr And Y Containing Silicide Coatings Prepared On Nb-Ti-Si-Cr Based Alloys

Nb-Si based ultra-high temperature alloys have great potential for high temperature structural applications because of their high melting points,low densities,good high-temperature strength and room temperature fracture toughness.Unfortunately,their rather poor oxidation resistance remains a key challenge to the practical application of this kind of alloys at high temperatures.Although alloying can improve the high-temperature oxidation resistance of these alloys to some extent,the improvement via only alloy modification is still not enough to meet the requirements of practical applications.For example,the excessive addition of some alloying elements that are beneficial to alloy's oxidation resistance would impose a deleterious influence on their high-temperature strength and melting points.Different from alloying,surface protective coatings can effectively improve the high-temperature oxidation resistance of Nb-Si based alloys and have less influence on their mechanical properties.Thus,a combination use of alloying and protective coatings should be a feasible way for the practical use of Nb-Si based alloys.In the present work,two kinds of Zr-Y modified silicide coatings were prepared on an Nb-Ti-Si-Cr based ultra-high temperature alloy,one is a Si-Zr-Y co-deposition coating prepared by one step pack cementation process,and another is a ZrSi₂-NbSi₂ compound coating prepared by firstly magnetron sputtering Zr alloy films,and then Si-Y co-deposition using pack cementation process.The phase constituents,compositional distribution and microstructure of the coatings were characterized by XRD,SEM and EDS analyses.The oxidation resistance,wear and solid particle erosion properties of the Si-Zr-Y co-deposition coating,and the high-temperature oxidation behavior of the ZrSi₂-NbSi₂ compound coating were investigated,respectively.Moreover,the corresponding anti-oxidation,wear and solid particle erosion mechanisms of the coatings were also discussed.The main contents and conclusions of this work are as follows:?1?Si-Zr-Y co-deposition coatings have been prepared on an Nb-Ti-Si-Cr based ultra-high temperature alloy using one step pack cementation process.The influences of different kinds of activators?NaF,NH4 F,NH4Cl,NaBr and NaCl?and their contents in the pack mixtures?1⁸wt.%?,co-deposition temperatures?1100¹350??,amount of Zr powders in the pack mixtures?5¹5wt.%?and co-deposition time?0¹2h?on the coating structures have been revealed.The coatings prepared with different kinds of activators?NaF,NH4 F,NH4Cl,NaBr and NaCl?are mainly composed of an?Nb,X?Si2?X represents Ti,Cr,Zr and Hf?outer layer,a?Ti,Nb?5Si4 middle layer,and an Al and Y-rich?Nb,X?5Si3 inner layer.The coating prepared with NaF as the activator is relatively thicker and denser,and furthermore,the concentrations of both Zr and Y in the coating prepared with NaF are higher than those prepared with NH4 F,NH4Cl,NaBr and NaCl.The content of Na F in the pack mixtures would not change the coating structures.However,the coating thickness increases with increasing the NaF content from 1wt.% to 5wt.%,and then decreases with further increasing the NaF content to 8wt.% in the pack mixtures.Co-deposition temperature imposes an obvious influence on both structure and thickness of the coatings.The coatings prepared at 1150 and 1200? for 8h have similar structures to those prepared at 1250?,however,a?Ti,Nb?5Si4 outermost layer with numerous holes forms in the coatings prepared at temperature above 1300?.Moreover,both coating thickness and Zr,Y concentrations in the?Nb,X?Si2 outer layer increase with increase in co-deposition temperature.An increase in the amount of Zr powders in the pack mixtures leads to a lower coating growth rate but higher Zr concentration in the outer layers of the coatings,and also,a higher content of Zr powders in the pack mixtures?15wt.%?would cause the formation of a Zr-rich?Ti,Nb?5?Si,Al?4 outermost layer in the coating.Prolonging the co-deposition time would not change the coating structure,but lead to an increase in coating thickness.The coating formation process is controlled by diffusion,as evidenced by the linear relationship between the coating thickness?h?and the square root of co-deposition time?t1/2?.?2?The coating specimens prepared at 1250? for 8h with the pack mixtures composed of 10Si-10Zr-3Y2O3-5NaF-72Al2O3?wt.%?have been chosen for static oxidation,wear and solid particle erosion tests.The Si-Zr-Y co-deposition coating exhibits much better oxidation resistance than the base alloy,as the parabolic oxidation rate constant of the Si-Zr-Y co-deposition coating is lower than that of the base alloy by about 3 orders of magnitude.The coating can protect the base alloy from oxidation at least for 200 h at 1250? in air.However,slight spallation and cracks can be observed on the surface of the scale formed on the coating after oxidation for 200 h.The good oxidation resistance of the Si-Zr-Y co-deposition coatings is attributed to the formation of a dense scale consisting of SiO2 and TiO2.The Si-Zr-Y co-deposition coating possesses much higher micro-hardness than the base alloy.The coating has better anti-friction property than the Nb-Ti-Si-Cr based alloy under the same sliding loads,irrespective of the sliding temperatures.During sliding,oxidation products such as SiO2 that can offer effective protection for the substrates at both room temperature and 800? form on the worn surfaces of the coating.The coating can provide a greater SPE resistance at a lower impingement angle such as 30°,while its SPE resistance at a higher impingement angle such as 90° is lower than that of the base alloy.?3?ZrSi₂-NbSi₂ compound coatings have been prepared through firstly magnetron sputtering 2,5 and 10?m thick Zr films,and then co-depositing Si-Y at 1150,1250,and 1350? respectively.The influences of Zr film thickness and Si-Y co-deposition temperature on the coating structure have been revealed.The coatings prepared with different Zr film thicknesses and Si-Y co-deposition temperatures possess a similar structure,consisting of a ZrSi₂ outer layer,a?Nb,X?Si2?X represents Ti,Cr,Zr and Hf?)middle layer and a?Ti,Nb?5Si4 inner layer.A higher Si-Y co-deposition temperature?1350??would result in a thicker coating,but cause cracks at the interfaces between the constituent layers of the coatings.The coatings prepared by Si-Y co-deposition at 1250? for 4h,with Zr film thickness of 10?m were chosen for the oxidation test.The coating shows much better oxidation resistance than the base alloy,as the mass gains per unit area of the coating specimens are much lower than those of the base alloy after exposure at 1250? for the same time in air.After oxidation,a dense scale consisting of a mixture of SiO2,ZrSiO4,ZrO2,Al2O3,TiO2 and Cr2O3 oxides forms on the coating,which can protect the substrate from oxidation for at least 100 h at 1250? in air.The oxidation process of the ZrSi₂-NbSi₂ compound coating can be divided into two stages.The first stage mainly involves the rapid oxidation of the ZrSi₂ outer layer of the coating,and this stage is dominated by the inward diffusion of oxygen into the ZrSi₂ matrix.The second stage includes the growth of the inner scale part,which is dominated by the outward diffusion of Si,Ti,Cr and Al into the scale.