This paper describes how effective medium theory and fractal analysis are used to investigate nonlinear microstructure-property relationships in HVOF-sprayed composite coatings produced from nano Fe-(β-SiC/SiO 2 ) agglomerate powders in order to optimize microwave absorption performance. The powder used in the study was prepared by spray granulation and deposited on Fe substrates. The microstructure of the powder and coatings was examined by SEM, the phase structure was determined by XRD analysis, and electrical permittivity and permeability were measured. To simplify calculations, electromagnetic absorption phases in the coating were assumed to be periodically distributed cubes. The results of the study indicate that multi-fractal diffraction in the coating microstructure facilitates the absorption of microwaves and is optimized when the mass fraction of nano βSiC in the composite is 28 wt%.

Flame and particle velocity mathematical model were constructed according to jet dynamics. Velocity of flame, and WC-17Co and NiCrBSi particles were simulated. Results show that velocity of the flame decreases continuously from supersonic to subsonic with the increasing of spray distance. Velocity of particles distributes in 300~800m/s, which relates to particle diameter, density and flame velocity.

The fracture behavior of polymer coatings fabricated by low temperature high velocity air fuel (LTHVAF) spray was investigated. It was shown that the coatings were dense and had a mean bonding strength of 13.5 MPa. During the tensile stress process, the fracture occurred in the interior of the coatings, which indicates that the cohesive strength within coatings is less than the bonding strength between the coatings and substrate. A significant amount of crazing and honeycombed holes existed in the fracture surface. Under a tensile stress, cracks initiated at porous defects in the coatings. Dispersed particles interrupted the crack expansion, and caused a change in direction of crack propagation.

A supersonic flame was produced in HVO/AF through the combustion of kerosene and the mixture of oxygen and air. The flame Temperature can be adjusted between 1400 and 2800 °C through changing the flux ratio of oxygen and air in the mixture. Three kinds of nanostructure WC-12Co coatings were sprayed by HVO/AF under three different spray conditions. Experiments results show that the coatings bond well with the substrates with very dense structure. The decarburization reduces with the decrease of flame temperature. Nanostructure coatings were much denser than microstructure coatings with about 0.4 to 0.5 times increase in microhardness.

According to aerodynamics and thermodynamics, High Velocity Oxygen/Air Fuel Spray system was successfully developed. The system introduced stream atomization, high-pressure combustion chamber, converging/diverging nozzle, spark plug ignition, radial powder injection with reliable operation. The system has the function of both of HVOF and HVAF. It can not only use air and oxygen to spray respectively, but also the mixture of air and oxygen, so the velocity and temperature of the flame can be changed by adjusting the flux of air and oxygen. The system can produce high quality cermets, metal and alloy coatings for the adjustable of flame velocity and temperature.

The paper analyzed microstructure and property of WC-17Co coatings sprayed by High Velocity Oxygen/Air Fuel Spray under three kinds of spray conditions, which are HVOF, HVO-AF and HVAF. Coatings bond well with the substrate. The average bonding strength exceeds 70Mpa. Coatings are dense and hard, and the average porosity is about 1%. Microhardness of coatings is between HV1000 0.2 and HV1200 0.2 . Coatings are mainly composed of WC with little W 2 C and Co 3 W 3 C. With the increasing of Nitrogen, decarburization of WC was reduced.