Plasma sprayed (PS) ceramic coatings have found widespread commercial use, particularly for wear resistant surfaces. Understanding of the micromechanical basis of macroscopic weight loss through wear is very important for the development of improved coatings and better uses for existing coatings. This paper aims to elucidate the wear mechanisms that occur when a PS ceramic coating is in sliding contact with a harder asperity, using controlled scratch testing as a model abrasion test. It attempts to relate the scratch test results to the microstructural integrity, or cohesion, of the various coatings. Paper includes a German-language abstract.

In an effort to obtain a series of plasma-sprayed coatings of controlled microstructure, three monosized sapphire powders were deposited using an axial injection torch in which the plasma gas composition and nozzle diameter were the only parameters varied. The effect of these parameters on splat morphology, porosity, angular crack distribution, and hardness is reported. Uniform, dense microstructure with a hardness rivaling chromia coatings was achieved through a combination of tightly controlled processing conditions and the use of mono-disperse precursor powders.

This paper explains how laser-beam remelting influences the quality of ceramic coatings on superalloy substrates. The coatings studied consist of Al2O3-Ni ceramic layers. Test results showed that a laser power density of 0.103 x 109 W/m2 is ideal for a beam rate of 1 to 2 m/min. It was also found that coating quality could be further improved by adding a diffusive Cr-Al layer prior to laser remelting. This added step reduces pore volume and layer thickness and increases adhesion without cracks, chips, or spalls. The examination also showed that, depending on cooling rate, it is possible to obtain layers that are similar in composition but different in structure or even amorphic.

The use of aluminum in the automobile engines and other critical parts require a superior surface property of the same. This has led to the development of plasma sprayable surface coatings in the automotive components. To impart the maximum bonding strength, along with hardness to the coatings, an aluminum based composite (Al-SiC) was chosen to be the most suitable. The presence of a hard second phase within a soft matrix improves the wear resistance of the material. The metal matrix composite powders were made by mechanical alloying of 6061 aluminum alloy (particle size 40-60 μm) along with fine SiC particles (≈ 8μm). Content of SiC was varied from 20-75vol% the balance being aluminum alloy. An organic material was used as Process Control Agent to optimize distribution of ceramic within metal matrix. The coatings obtained by plasma spraying the powders were characterized for their microstructure, adherence, wear and other physical properties.

Thermal sprayed coatings are frequently used in corrosive environments, even when their major purpose is to provide wear or thermal resistance, rather than corrosion resistance. This includes Thermal Barrier Coatings (TBC), where high porosity is a desired feature to give good thermal protection. However, as this proves to be a limiting factor in the corrosion protection, a trade off is involved. This is because the interconnected porosity in TBCs allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. This work addresses the problem of permeability of TBCs which can lead to premature delamination due to interfacial corrosion. The coatings studied were yttria-stabilized zirconia TBCs. A simple infiltration technique has been proposed using sol-gel ceramic precursors. The precursors studied include aluminum isopropoxide or pre-hydrolyzed ethyl silicate, which decomposed to alumina and silica respectively, at surface heat treatment temperatures as low as 550°C. In addition to sealing the surface, it is believed that some level of compressive stress is generated on the surface of TBCs on cooling from the processing temperature. Electrochemical tests in 3.0% NaCl have been carried out to study the effectiveness of the sealant. These potentiodynamic tests as well as permeability tests show a considerable decrease in interconnected porosity with sol-gel modifications of the coatings.