Abstract
Ceramics are priority applied for protective coating because of their excellent wear and corrosion resistance, heat resistance and high temperature oxidation resistance. However, ceramic coating show low adhesion prepared by surface coating technique due to huge difference of Ceramic coating material and metal matrix material in physical performance. Ceramic coatings are easy to failure as a result of huge thermal stress in the high temperature environment. In the paper, a fundamental research on the high bonding strength ceramic coating prepared by laser cladding. A series of research on the samples of MCrAlY/Al2O3-13％TiO2 ceramic coating and microporous structure adhesive layer/ Al2O3-13％TiO2 ceramic coating, which prepared by high frequency assisted laser cladding.
Prepared for the ceramic coating, firstly, the sintering mechanism of the mechanism of ceramic materials under the action of laser was researched. Meanwhile, the model of laser was analyzed. A research on the mechanism was carried out according to combination of experimental and theoretical methods. It can be divided into absorption, sintering, solidification three stages during ceramic materials sintering. According to the mechanism of sintering process, Al2O3-13％TiO2 ceramic were use to experimental analysis. The result showed that, ceramic particle, shape, size and density have a superior influence on the sintering properties of ceramic materials. Ceramic sintering was the most superior performance when the size reaches the nanometer ceramic and spherical shape. Meanwhile,sintering properties of ceramic were increased significantly with the density of green body increases.
While the temperature field of laser has a superior influence on sintering properties of ceramic materials. The thermal stress which induced microcrack will be reduced when the uniform temperature field, and the separation was precipitated between the grains, but the size increased.A whole design scheme of high bonding strength was proposed according to the existing mechanism of high temperature ceramic coating failure analysis. It is designed MCrAlY/Al2O3-13％TiO2 ceramic coating and microporous structure adhesive layer/Al2O3-13％TiO2 ceramic coating prepared based on idea of laser cladding. MCrAlY/Al2O3-13 ％ TiO2 ceramic coatings use conventional coating design. It is used MCrAlY bond layers reduce the differences of physical properties between the ceramic layer and matrix metal. Meanwhile, the feasibility verifying tests are carried under different cladding conditions. The results showed that, ceramic coating can be prepared by high frequency assisted laser cladding pre-high density ceramic powder. A creative design which proposed based on the needs of high-temperature work, it is microporous structure adhesive layer/Al2O3-13％TiO2 ceramic coating. In the design, the adhesive layer is designed to be porous structure, ceramic coating will has excellent high temperature properties result of microporous structure has a high thermal shock resistance, low thermal expansion coefficient, can be used to reduce thermal stress of ceramic layer and matrix metal. Similarly, the feasibility verifying tests of microporous structure sintering by laser are carried in order to prove scientific of the coating design.
Scanning electron microscopy (SEM) and X-ray diffraction (XRD) was applied to analyze the composition and microstructure of the MCrAlY/Al2O3-13％TiO2 ceramic coating and microporous structure adhesive layer/ Al2O3-13 ％ TiO2 ceramic coating. The result showed that, MCrAlY/Al2O3-13％TiO2 ceramic coating has a excellent chemical combination due to elements obviously diffused in the interface, will increase bonding strength of coating. In comparison with MCrAlY/Al2O3-13 ％ TiO2 ceramic coating, There is no obvious elements diffused in the interface of microporous structure adhesive layer/ Al2O3-13％TiO2 ceramic coating result of the existence of a large number of holes and changes in composition of adhesive layer. However, the interface is still tightly under the action of microporous riveting.
In order to estimate high temperature properties and failure mode of coating, bonding strength,high temperature oxidation and thermal shock of coating samples were tested respectively. The results showed that MCrAlY/Al2O3-13％TiO2 ceramic coating has good bonding strength and excellent high-temperature oxidation resistance, however，bonding strength of coating dramatically decreased with increasing thermal shock times. The microcracks are easily formed in the high temperature result of huge thermal stress by residual stress test, add to use at low temperatures. Comparatively,microporous structure adhesive layer/Al2O3-13％TiO2 ceramic coating had a slightly lower bonding strength and excellent high-temperature oxidation resistance, but displayed a good thermal shock resistance. Final failure mode is thermal fatigue failure.
Comprehensive result of work, a research on in-situ ceramic coating prepared by laser cladding was carried out. MCrAlY/Al were used to in-situ about 40 m ceramic coating prepared by laser cladding according to principle of Al in-situ oxidation. The result of study on microstructure showed that in-situ ceramic coating and reaction layer combined closely to each other, meanwhile, there are a lot of whiskers formed in the interface of the coating, the bonding strength will greatly increase，shows that the method is a very promising preparation of ceramic coatings.
Key words: ni-base superalloy, laser cladding, high bonding strength, ceramic coating, microporous structure
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