Preparation and Properties of Plasma Spraying Cu-Al_2O_3 Gradient Coatings

In order to overcome the limitations of low adhesion strength and poor thermal-shock resistance of pure ceramic coatings, Cu-Al2O3 gradient coatings were fabricated by plasma spraying. The microstructure and distribution of Cu-Al2O3 gradient coatings were analyzed. The adhesion strength, thermal-shock resistance and porosity of the coatings were tested. The results show that the composition of the gradient coatings has a gradient distribution along the thickness of coatings. As copper has a relatively low melting point and the molten copper has good wettability on the surface of Al2O3, it can be melted sufficiently and could fill the interstices and pores among the spraying particles effectively, thus improves the adhesion strength, thermal shock resistance and reduces the porosity. The adhesion strength of the gradient coating is 15.2 MPa which is two times of that of the double-layer structure coating.

In-situ preparation, modulation and mechanism of refractory metal carbide gradient coating

TiC, ZrC and TaC modified layers were in-situ prepared on graphite matrix by chemical vapor infiltration method with metal salts as the activator. Taking the TiC modified layer as an example, through thermodynamic calculation and experiment, the thermal decomposition process of raw materials(Ti/K_(2)TiF_(6)) was analyzed, the formation mechanism of TiC was determined, and the distribution of TiC modified layer was modulated. The results show that activator K_(2)TiF_(6)has higher decomposition temperature than NH4Cl, which is conducive to improving the utilization rate of raw materials in the gas infiltration process. Increasing the content of Ti powder can increase the concentration of reaction gas and contribute to the formation of TiC modified layer. When the molar ratio of Ti to K_(2)TiF_(6)is 3:1, the surface thickness and infiltration depth of Ti C are 5.42 and 136.24 μm, respectively. Increasing the reaction temperature can improve the rate of in-situ reaction and the thickness of TiC surface layer. When the experimental temperature rises to 1600 °C, the TiC surface layer thickness increases to 20.27 μm.

Structure design of gradient hard coatings on YG8 and their residual stress analysis by ANSYS

A structure of gradient hard coatings( Ti,TiN,TiCN and TiAlN) is designed,and residual stress is simulated by a finite element method with ANSYS. The influence of the realistic situation including load and temperature on the residual stress of the coatings is investigated. Simulated results show that the realistic situation strongly affects the residual stress. To be specific,i) The main residual stress concentrates on the coatings prepared on YG8 substrate,and the residual stress and its gradient of the coatings are bigger than that of the substrate; ii) TiAlN and TiCN coatings have better resistance compression than that of TiN coatings in the same condition; iii) The improved multilayer structure of the gradient hard coatings produces weaker residual stress but higher anti-pressure of the substrate.

Effect of Y_2O_3 Content on Microstructure of Gradient Bioceramic Composite Coating Produced by Wide-Band Laser Cladding

To eliminate thermal stress and cracks in the process of laser cladding, a kind of bioceramic coating with gradient compositional design was prepared on the surface of Ti alloy by using wide-band laser cladding. And effect of Y2O3 content on gradient bioceramic composite coating was studied. The experimental results indicate that adding rare earth can refine grain. Different rare earth contents affect formation of HA and β-TCP in bioceramic coating. When the content of rare earth ranges from 0.4% to 0.6%, the active extent of rare earth in synthesizing HA and β-TCP is the best, which indicates that “monosodium glutamate” effect of rare earth plays a dominant role. However, when rare earth content is up to 0.8%, the amount of synthesizing HA and β-TCP in coating conversely goes down, which demonstrates that rare earth gradually losts its catalysis in manufacturing HA and β-TCP.

Oxidation and Hot Corrosion of Gradient Thermal Barrier Coatings Prepared by EB-PVD

The performances of gradient thermal barrier coatings (GTBCs) produced by EB-PVD were evaluated by isothermal oxidation and cyclic hot corrosion (HTHC) tests. Compared with conventional two-layered TBCs, the GTBCs exhibite better resistance to not only oxidation but also hot-corrosion. A dense Al2O3 layer in the GTBCs effectively prohibites inward diffusion of O and S and outward diffusion of Al and Cr during the tests. On the other hand, an "inlaid" interface, resulting from oxidation of the Al along the columnar grains of the bond coat, enhances the adherence of AI2O3 layer. Failure of the GTBC finally occurred by cracking at the interface between the bond coat and AI2O3 layer, due to the combined effect of sulfidation of the bond coat and thermal cvcling.

Effect of pH Value on Nano-Al_2O_3/Ni+Co Gradient Bionic Composite Coating by Electrodeposition

The metal and nano-ceramic composite coatings were formed on gray cast iron surface by electrodepositon method.The Ni-Co was used as metal matrix,and the nano-Al2O3 was chosen as second-phase particulates.The gradient structure of biology material was the model to avoid bad interface bonding and stress distribution,therefore the gradient composite coating was prepared.The morphology of composite coatings was flatter and the microstructure was more com-pact than the pure Ni-Co coatings.Effect of pH value on surface morphology was analyzed,which some microcracks appeared when pH value was greater than 5.The content of codeposition na-no-Al2O3 reached a maximum value at pH value of 3-4,at the same time the properties including mi-crohardness and wear-resistance were analyzed.The result indicated that the mirohardness reached a maximum value and the wear loss volume was less at pH value 3-4.

FEM ANALYSIS OF THERMAL STRESSES IN GRADIENT THERMAL BARRIER COATINGS PRODUCED BY EB-PVD

Gradient thermal barrier coatings (GTBCs) produced by co-deposition of mixtures ofAl-Al2 O3-YSZ onto metallic bond coat exhibited longer lifetimes than the two-layeredTBCs. The finite element method (FEM) numerical models were used to investigatestress and strain states in the GTBCs and traditional two-layered TBCs as they cooledto 750℃ from a stress-free state at 850℃.

Coherent gradient sensing method for measuring thermal stress field of thermal barrier coating structures

Coherent gradient sensing （CGS） method can be used to measure the slope of a reflective surface, and has the merits of full-field, non-contact, and real-time measurement. In this study, the thermal stress field of thermal barrier coating （TBC） structures is measured by CGS method. Two kinds of powders were sprayed onto Ni-based alloy using a plasma spraying method to obtain two groups of film-substrate specimens. The specimens were then heated with an oxy-acetylene flame. The resulting thermal mismatch between the film and substrate led to out-of-plane deformation of the specimen. The deformation was measured by the reflective CGS method and the thermal stress field of the structure was obtained through calibration with the help of finite element analysis. Both the experiment and numerical results showed that the thermal stress field of TBC structures can be successfully measured by CGS method.

Design of DLC/Titanium Alloy Bio-Functionally Gradient Coat

The material design is used to direct the magnetron cosputtering process. At first, according to the particularity of functionally gradient coat (FGC) the thermal elastic stress analysis for FGC was carried out based on the plane stress hypothesis. It is obtained that the peak value of plane thermal stress within FGC is only determined by the physical properties of materials of FGC and substrate, the composition distribution coefficient only influences the distribution and trend of plane thermal stress. And the plane thermal stress criterion for design of FGC was presented. Then the plane thermal stress of diamond like carbon/titanium alloy FGC was calculated.