Study On Microstructures And Properties Of Plasma Sprayed Nano-ceramic Thermal Barrier Coatings

The materials with low thermal coefficients were deposited on the surface machine parts that operate in high temperature environment to protect the metal substrate against heat erosion. The deposited surface coatings were usually referred to as thermal barrier coatings (TBCs). To improve the efficiency of turbine and aeronautical engine, the working gas temperature and pressure in combustion room had the tendency to be gradually increased. Ceramic thermal barrier coatings were developed in order to protect the vanes against thermal fatigue and heat erosion. Although conventional ceramic thermal barrier coatings played an important role in protecting superalloy substrate and improving the efficiency of engines, its properties were far beyond satisfaction as far as their properties and service life were concerned. With the rapid development of nanoscience and technology in 1980s, people were attracted to explore the theory and application of nanomaterials because the nanomaterials often demonstrated particular and promising properties as a result of their special microstructures. For instance, nanostructured materials usually had lower thermal coefficients because of boundary dispersion effect. This meant that nanostructured ceramic thermal barrier coatings had great potentials to further improve the properties of TBC. Among the industrial techniques for preparing thermal barrier coatings, plasma spray method was attached much more importance for the sake of its simple operation and high efficiency. Now, what changes will occur to the powders and whether nanostructured thermal barrier coatings can be obtained have become the hot topics of studies in the field of plasma sprayed thermal barrier coatings with nanostructured ceramic powders all over the world. The researchers of the USA, Japan and Canada had made some contributions in this respect. However, the research work has been carried out only for a short time in China. Up to now, little systematic investigations have been reported. Therefore, it has great theoretical and practical significance to carry out the study on the microstructures and properties of plasma sprayed thermal barrier coatings with nanostructured ceramic powders. In this paper, the microstructures and properties (bonding strength, thermal shock resistance, heat insulation capacity, oxidation resistance and thermal stability) of atmospheric plasma sprayed (APS) nanostructured thermal barrier coatings(NiCrAlY+ZrO2-8%Y2O3) were systematically studied compared with their conventional counterparts. The effects of plasma spray parameters (spray current, spray distance, feedstock rate and secondary gas flow) on the microstructures and properties of nanostructured thermal barrier coatings were also investigated. On this basis, a group of optimum plasma spray parameters for preparing nanostructured TBC were obtained under help of orthogonal experiments. Nanoceramic powders cannot be directly used for plasma spray as a result of their light mass and poor flowability. A new idea was put forward in this paper to solve this difficult problem. The nanostructured ceramic powders were agglomerated into micrometer-sized particles after being subjected to three physical processing steps: slurry preparation, spray drying and subsequent heattreatment. The agglomerated nanoceramic particles were spherical and still nanostructured with good flowability. Generally, during the plasma spray, once the conventional ceramic powders began to melt, they fully melted quickly. The study showed that agglomerated nanoceramic particles had different melting behavior. During the process of plasma spray, the agglomerated particles only melted on the surface part, but the inner of agglomerated particles did not melt and retained nanostructured. The melted part regrew into columnar structure and its grain size was more than 100nm. The study indicated that the melting degree of agglomerated nanoceramic particles had relationship with the particles size. The melted part covered bigger proportion for the small-sized agglomerated particles. On the other hand, the plasma spray parameters had strong effects on the melting degree of agglomerated nanoceramic particles. Compared with the conventional ceramic thermal barrier coatings, the nanostructured thermal barrier coatings had smoother and denser surface with fewer microcracks and smaller pores. Their microstructures consisted of melted zones and partially melted zones. The former were formed from the melt part of agglomerated nanoceramic particles and did not keep nanostructured anymore. The latter were formed from the combination of melt part and unmelt part of agglomerated nanoceramic particles and kept nanostructured. The XRD analysis showed that the phase composition of agglomerated nanoceramic particles mainly included t-ZrO2 and a little m-ZrO2, but the as-sprayed ceramic coatings mainly consisted of t-ZrO2 without m-ZrO2 phase. This was attributed to the rapid heating and cooling speeds during plasma spray. On the basis of study on the melting behaviour of agglomerated nanoceramic particles and microstructural features of nanostructured thermal barrier coatings, the forming process mode of nanostructured thermal barrier coatings prepared by plasma spray with agglomerated nanoceramic particles was established in the paper.Compared with conventional ceramic thermal barrier coatings, the nanostructured ceramic thermal barrier coatings had better comprehensive properties. For the ceramic thermal barrier coatings with different thickness(100μm ,300μm,500μm),the bonding strength of nanoceramic thermal barrier coatings and conventional thermal barrier coatings was 69MPa,60MPa,47MPa and 48MPa,42MPa,32MPa, respectively. The bonding strength of nanoceramic thermal barrier coatings and conventional thermal barrier coatings decreased with the increase of coating thickness. This had relationship with the rapid increase of residual stresses when coatings thickness grew. For the TBCs with same thickness, the bonding strength of nanostructured thermal barrier coatings was higher than that of their counterparts because the contact area of splats formed by nanostructured particles with the substrate was larger and resulted in high bonding strength. On the other hand, the residual stresses of nanostructured TBCs were lower than that of their counterparts because the nanostructured grains could slide or rotate to alleviate coating stresses. For the ceramic thermal barrier coatings with different thickness(100μm ,300μm,500μm),the thermal insulation capacity of nanoceramic thermal barrier coatings and conventional thermal barrier coatings was 87℃,112℃,118℃and 60℃,67℃,92℃, respectively. Obviously, it was understandable that the thermal insulation capacity increased with the increase of the coating thickness. For the TBCs with same thickness, the thermal insulation capacity of the nanostructured TBCs was higher than that of their counterparts because the nanostructured coatings had lower thermal coefficient resulting from the dispersion effect of nanosized grains boundary to the conducting electrons. For the ceramic thermal barrier coatings with different thickness(100μm ,300μm,500μm),the thermal shock resistance of nanoceramic thermal barrier coatings and conventional thermal barrier coatings was 52 times, 12 times, 2 times and 25 times, 6 times, 1 times, respectively. The study indicated that the decrease ofthermal shock resistance of TBCs with the increase of coating thickness was mainly associated with high residual stresses of thick coatings. For the TBCs with same thickness, the thermal shock resistance of nanostructured TBCs was higher than that of their counterparts not only because the nanostructured grains could slide or rotate to alleviate stresses of the coating, but also because nanostructured TBCs had better bonding strength and higher toughness, which would increase the energy of cracks initiation and propagation. The experiments also showed that although the nanosized ceramic grains grew to some degree, they did not coarsen seriously. In other words, the nanostructured thermal barrier coatings could retain nanostructured under the operation condition of high temperatures. It should be pointed out that the oxidation resistance of both nanostructured thermal barrier coatings and conventional thermal barrier coatings at high temperature showed little differences. In this paper, the effects of plasma spray parameters (spay current, spray distance, feedstock rate and secondary flow rate) on the microstructures and properties of nanostructured ceramic thermal barrier coatings were investigated and a group of optimum plasma spray parameters were obtained on the basis of orthogonal experiments and they were: spray current: 570A; spray distance: 80mm; feedstock rate: 33g/min.; secondary gas flow: 4.8L/min. The study well indicated that nanostructured thermal barrier coatings could improve the properties of TBCs and increase their service life compared with conventional thermal barrier coatings.