Electroless Ni–P Plating on Mullite Powders and Study of the Mechanical Properties of Its Plasma-Sprayed Coating

To effectively improve the properties of a mullite coating and its interfacial bonding with the substrate, a Ni–P layer is deposited on the surface of mullite powders by electroless plating. The original mullite powders and coated mullite powders are then deposited onto stainless-steel substrates by plasma spraying. The growth mechanism of the Ni–P layer during the plating, the microstructures of the coated powders and mullite coating and the properties of the mullite coatings are characterized and analyzed. The results indicate that the Ni–P layer on the surface of the mullite powder has cell structures with a dense uniform distribution and grows in layers on the surface of the mullite powder. The crystallization behavior of Ni-P amorphous layer is induced by heat treatment. Compared to the original mullite coating, the coating prepared by the coated mullite powders has better manufacturability, stronger adhesion to the substrate, lower porosity (7.40%, 65% of that of the original coating), higher hardness (500.1 HV, 1.2 times that of the original coating), and better thermal cycle resistance (two times that of the original coating). The method of preparation of high-temperature thermal barrier coatings with coated mullite powders has a high application value.

HVOF Sprayed Mullite Coatings for Use In Extreme Environments

Products used in industry and marine applications are exposed to extreme environments like high heat, humidity, acidic or alkaline or hyper saline environment, UV and IR radiation. Metals with good corrosion and oxidation resistance may be used but are restricted to Ni and Cr alloys, titanium and super alloys etc., which are costly and have their limitations. Hence ceramic coatings on low cost metals may be an answer to this problem. Ceramics are inherently chemically inert, high temperature resistant, corrosion and oxidation resistant. Flame spraying of ceramics is a good and reliable method for applying ceramic coatings on metallic substrates with good bond strength (> 80 MPa) and 1% porosity. In this work, HVOF technique is applied to obtain 100 microns thick mullite coatings on MS substrates with a NiCr bond coat. Mullite has a high oxidation and corrosion resistance. It is chemically inert. It has high temperature resistance even at 2000 C. These properties are ideal for industrial components exposed to salty environments. Characterization studies like XRD, SEM/EDS, Corrosion tests using polarization technique, coating thickness and surface roughness have been studied and reported. A corrosion rate of 1.55 mm/ year has been achieved in a sea water environment.

Slurry Sprayed Mullite Coatings and Their Corrosion Performances

Slurry spray technique (SST) is a distinctive variant among the numerous and already established coating techniques. Functionally graded thermal barrier and environmental barrier coating have been the functionalities developed so far for the process. Among the choice of the various ceramic feedstocks available mullite and partially stabilized zircona have been found suitable and investigated for the coating deposition via SST. This chapter reports the findings of the corrosion studies in simulated industrial corrosive environments and characterization results of the six sets of slurry sprayed mulite-nickel based coatings. Decent protection against coating has been found during the immersion test performed on these coatings for evaluating their corrosion performance. The developed coatings are recommended for use in applications to endure the elevated temperature and inflict corrosion. Thermal cycling test was performed to support the acceptable thermal shock resistance and coating compliance of the developed coatings.

Effect of particle size on anti-oxidation property of mullite coating prepared by pulse arc discharge deposition

Mullite coatings with different particle sizes were prepared on carbon/carbon (C/C)–SiC composites surface by pulse arc discharge deposition (PADD).