The coatings can not be compared with the bulk ceramic and metallic materials, so they do not function by themselves but only together with the substrate. It is the substrate responsible for the mechanical properties of the system substrate-coating. Coating properties such as strength and toughness are of secondary importance, more important to obtain the following properties:

By the way, the compressive stress in the coating causes a significant increase in the apparent toughness of the coating. Conversely tensile stresses, which are almost at all standard thermally spraying coatings, reduce their toughness to almost zero.

Thanks for reply,

If we talk about HVOF sprayed wear resistant coatings especially for abrsive wear conditions hardness and fracture toughness are important properties. I am talking about this after acheiving a adhesive bond strength of 85-95 MPa in case of HVOF coatings which is possible. Particularly the intersplat cohesive strength governs the fracture toughness of the coating. I would like to know two things, firstly, is there any technique by which we can increase the cohesive strength between the splats. Secondly how to develop compressive residual stresses in the coating ? One of technique to impart compressive residual stresses is grinding. Is there any other technique or reference paper on development of compressive residual stresses in thermal spray coatings. Some researchers have claimed to acheive coating properties very close to sintered materials by  Warm Spraying and post-treatment and one of the researcher I know is Prof. Seiji Kuroda, National Institute of Material Science, Japan.

Hi Lalit,

To dramatically increase the cohesion and adhesion of coatings (> 200 MPa) would have to abandon the method of HVOF and go to the high-speed plasma spraying. Also needed are quite different powders for spraying. Why do I think HVOF hopeless? The fact that the particles and the substrate during spraying or HVOF be oxidized and not can to form strong metallurgical bond. Only plasma spraying (oxygen-free flame) can restore the oxide film on the metal particles and substrate.

To obtain the best adhesion and cohesion of coating, plasma spraying powder should consist of small particles of metal, which are completely melted in the flame and large, hard, nonmetallic refractory particles which do not melt. Such particles act as an abrasive (destroy oxide films) and allow metal droplets flow over the substrate and over solid metal interlayers. The third component is useful to introduce a bit of a fine powder of silica glass. Molten glass is also destroys the oxide films and promotes the formation of a metallurgical bond (analogy - brazing with flux).

To lower the elastic modulus and reduction of tensile residual stress  in the coating, coating must be a multiphase (comprise various metallic and ceramic components). For such coatings can be based on known commercial powders, for example, Cr3C2-NiCr and admixed other materials thereto. Furthermore, by introducing additional components can optimize the coefficient of linear thermal expansion of the coating; it should be slightly less than the coefficient of linear thermal expansion of the substrate.

If the coating comprises a glass, it becomes possible to obtain a compressive stress in the coating upon heating the coated part > 550 ° C. At this temperature, glass can plastic deform and relaxes all residual tensile stress. After cooling, the coating compressive stresses arise automatically, if its coefficient of thermal expansion less than that of the substrate.

Some of these new coatings obtained using plasma torch Axial III you can see on my page.

Regards

Vadim

Dear A. Reddy, we are not discussing here about the adhesive bond strength of coating/substrate interface. We are interested in the improvement in inter-splat bond strength which affects the fracture toughness of the coating. In my case I am exploring about various methods and technique to improve the fracture toughness of High velocity oxy-fuel spray coatings for wear resistance applications. I think burnishing will improve the properties up to few microns (50-100) depth. I am interested in developing compressive stresses in all the layers of coating. 

http://www.gtv-mbh.com/_old/gtv-mbh-englisch/www.gtv-mbh.de/cms/upload/publikat/Siegmann/english/2005_03_eng_Siegmann.pdf

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