Microstructures And Oxidation Resistance Of Si-Al-Y Diffusion Coatings On Nb-Si Based Alloys

Nb-Si based alloys have great potential for high temperature structural applicationsdue to their high melting points, low densities, excellent creep strength and acceptableroom temperature fracture toughness. However, the poor oxidation resistance of thesealloys has been a major barrier to their high temperature applications. Although theoxidation resistance of the Nb-Si based alloys can be improved obviously by addingalloying elements such as Ti, Al, Cr and Hf, but unfortunately, this improvement wasnot enough for the need of actual high-temperature applications in oxidizingenvironments, thus, a protective coating is still necessary to the applications of Nb-Sibased alloys. In the present work, Si-Al-Y multi-component coatings were preparedon an advanced Nb-Si based ultrahigh temperature alloy by halide-activated packcementation processes (HAPC), and several kinds of coatings, such as Y, Al modifiedsilicide coatings, Y modified Si-Al composite coatings and two-step Si-Al-Y diffusioncoatings were obtained by using one-step or two-step processes respectively. Theconstituent phases, microstructure and compositional distribution of the coatings werecharacterized by XRD, SEM and EDS analyses. The oxidation behavior of thecoatings at1250℃was investigated, and both anti-oxidation mechanisms and thefailure mechanisms of the coatings were discussed. The main contents andconclusions of this work are as follows:Y, Al modified silicide coating was prepared on an Nb-Si based alloy at1150℃for10h with a pack mixture composed of Si-Al-Y2O3. The coating was about70μmthick and had a multi-layer structure, including a58μm thick (Nb,X)Si2outer layer (Xrepresents Ti, Cr and Hf), a5μm thick (Ti,Nb)5Si4middle layer and a7μm thick Al,Cr-rich inner layer composed of Al3(Nb,X) and (Cr,Al)2(Nb,Ti) phases. The coatingexhibited excellent oxidation resistance at1250℃; the mass gain of the coatingspecimen after oxidation at1250℃for100h was only2.3mg/cm2. The existence of asuitable content of Al in the coating could promote the preferential formation of SiO2on its surface upon oxidation, and thus forming a dense and compact scale mainlyconsisting of SiO2·Al2O3.The deposition of Al and Si occurred in a sequential manner during the packcementation process. The coating growth was proceeded by inward diffusion of Aland Si, and the growth kinetics at1150℃followed a parabolic law. A higherco-deposition temperature would be favorable for the deposition of Si, thus the (Nb,X)Si2outer layer of the coatings thickened and its porosity was ameliorated,while some pores and cracks could be observed in the coatings prepared by aexorbitant temperature (such as higher than1250℃) due to a higher growth rate of the(Nb,X)Si2outer layer in the coatings. When the Al content in the pack mixtures waslower than15wt%, the structure of the coatings was slightly effected, while both thethickness of the Al, Cr-rich inner layer and the Al content of the (Nb,X)Si2outer layerin the coatings increased with increase in the Al content in the pack mixtures.Y modified Si-Al composite coating was prepared on an Nb-Si based alloy at1050℃for10h with a pack mixture composed of Si-Al-Y2O3. The coating was about50μm thick and had a multi-layer structure, including a10μm thick outer layerconsisting of (Nb,X)3Si5Al2matrix and dispersed (Nb,X)Si2particles, a32μm thickmiddle layer consisting of Al3(Nb,X) matrix and dispersed (Nb,X)5Si3particles, and an8μm thick Al, Cr-rich inner layer consisting of Al3(Nb,X) and (Cr,Al)2(Nb,Ti) phases.The coating could only endure for less than20h at1250℃in air, and a dense Al2O3scale formed on its surfaces. When the oxidation was proceeded for more than20h,due to the insufficient supply of Al and formation of (Nb,X)5Si3in the outer layer ofthe residual coating, a layer containing TiO2, Nb2O5, SiO2, TiNb2O7and Ti2Nb10O29formed underneath the Al2O3scale upon oxidation, and the existence of some poresand micro-cracks in this mixed oxide layer resulted in the scale shelled off during thecooling process from1250℃to room temperature.The formation process of the Y modified Si-Al composite coating was in asequence manner of first Al depositing and then Al-Si co-depositing, and the latter onewas a prerequisite condition for the formation of (Nb,X)3Si5Al2phase in the outerlayer of the coating. Both a higher co-deposition temperature and a lower Al contentin the pack mixtures would be favorable for Al-Si co-depositing or Si depositingdominantly, while the latter would result in the transformation of (Nb,X)3Si5Al2to(Nb,X)Si2in the outer layer of the coatings. The deposition of Al into the coating wasaffected slightly by decreasing the activator content in the pack mixtures, but itslasting time would be prolonged by increasing the activator content in the packmixtures.A two-step pack cementation process, including first Al-Y2O3co-deposition at950℃for5h and then Si-Y2O3co-deposition at1150℃for0-7h, was utilized toprepare Si-Al-Y diffusion coatings on an Nb-Si based alloy. The first aluminizedcoating consisted of a single Al3(Nb,X) layer with the thickness about120μm. During the second siliconizing process, an interesting and gradual replacement of the originalAl3(Nb,X) layer by a porous (Nb,X)Si2layer occurred, certainly accompanied with theoutflow of Al from the original aluminized coating into the pack. In addition, beneaththe porous (Nb,X)Si2layer, a relatively compact intermediate zone composed of a(Nb,X)Si2middle layer, a (Ti,Nb)5Si4sub-inner layer and an Al, Cr-rich inner layerformed in the coatings through the inward diffusion of Al and Si.Another two-step pack cementation process, including first Si-Y2O3co-depositionat1150℃for5h and then Al-Y2O3co-deposition at950℃for0-7h, was also utilizedto prepare Si-Al-Y diffusion coatings on an Nb-Si based alloy. The first siliconizedcoating was composed of a47μm thick (Nb,X)Si2outer layer and a6μm thick(Ti,Nb)5Si4inner layer. During the second aluminizing process, the original (Nb,X)Si2outer layer was retained and the Al content in it increased, while the original(Ti,Nb)5Si4inner layer disappeared and was replaced by a thick Al3(Nb,X) inner layer.Besides, a thin Al3(Nb,X) outmost layer was also observed on the surface of the(Nb,X)Si2outer layer in the coating. The formation of the Al3(Nb,X) inner layerresulted in the (Nb,X)Si2outer layer being separated from the substrate and pushedoutward, and thus some cracks generated in it.The oxidation behavior of the two-step Si-Al-Y diffusion coating prepared by aselected process consisting of first Si-Y2O3co-deposition at1250℃for4h and thenAl-Y2O3co-deposition at900℃for1.5h was studied. A dense and compact scaleformed on the coating specimen after oxidation at1250℃for100h, and its mass gainwas3.73mg/cm2, lower than that of the coating specimen prepared by the firstSi-Y2O3co-deposition (5.95mg/cm2). The scale possessed a double-layered structure,including an Al2O3outer layer and a SiO2·Al2O3, TiO2, Al2TiO5and CrNbO4mixedoxide inner layer.