| Phosphate binder was widely used in high temperature coatings because of its thermal stability. But the curing mechanism of phosphate binder was seldom studied. Normaly phosphate coatings have some disadvantages such as low strength, high brittleness, and low glossiness. In this paper, the way of improving the coating quality and curing mechanism of phosphate binder were studied.Phosphate binder was synthesized by H3PO4, Al(OH)3 and CrO3. Based on IR measurement, curing mechanism of phosphate binder was studied. The changes of the bonds P-O, P=O, and P-OH were analyzed while phosphate binder was curing. It found that the presence of Cr enhanced the dehydrolytic condensation. As filler, Cr was beneficial to form amorphous structure and restrained crystallization behavior of aluminum phosphate binder. And the Cr compound enhanced stability of the system by decreasing stress and strain during phase transformation and increasing disorder of the system. Reaction mechanism and kinetic behavior of Al(H2PO4)3, AlCr(H2PO4)5 and Al3Cr(H2PO4)12 binders, especially Al(H2PO4)3 binder was studied by means of the TG-DSC technique. The results showed that there were four steps in the thermal decomposition of Al(H2PO4)3 binder. The decomposition action of four steps was controlled by nucleation and nuclei growth. Based on the results of apparent active energy (E) and frequency factor (A) in each stage calculated by Doyle equation, the kinetic equations were obtained. The thermal decomposition kinetic behavior of AlCr(H2PO4)5 and Al3Cr(H2PO4)12 binders were studied in the range of 80～208℃. Compared with first stage decomposition of Al(H2PO4)3 binder, apparent active energy decreased with increasing Cr content and dehydrolytic condensation accelerated gradually.By using phosphates binder as filmogen, electroquartz powder as aggregate, phosphate coatings were fabricated. To stiffen the coating, thermal curing process and chemical curing process were used. The transitional layer formed by mutual diffusion on the interface and mechanical interlinkage combined coating with metal matrix firmly. At low temperature, phosphate coatings were combined with ceramic material by physical bonding. When treated at high temperature, interface layer of aluminum orthophosphate was formed by solid phase reaction between coating and matrix. So after heat-treatment phosphate coatings were combined with ceramic material mainly by chemical bonding. With mixed phosphate binder and acid silica sol as composite filmogen and calcined talc and magnesia as active fillers, the modified coatings were fabricated. Compared with the original coatings, toughness of the modified coatings was improved. The coatings with aluminum-chromium-phosphates binder as filmogen showed higher hardness, adhesion, and glossiness than the coatings with aluminum phosphates binder as filmogen after drying at room temperature. Phosphate coatings perform excellent heat-durability. They will not break off at 1200℃. Phosphate coating material shows good microwave dielectric properties. Their dielectric constant ranges from 3 to 4 and dielectric loss is 61×10-4 under frequency of 10GHz.
|
PDF Full Text Request