Moving from a 2-dimensional to a 3-dimensional approach to microstructure and properties has been expected eagerly for a long while to result in a dramatic increase in the knowledge of thermally-sprayed processes and coatings. To meet these expectations, in the present work, microtomography and electrochemical impedance spectroscopy (EIS) were carried out to simulate the microstructure of plasma-sprayed alumina. As-sprayed and excimer laser-processed deposits were studied. Some unexpected but relevant results, e.g. regarding pore orientation in the coatings, could be obtained. EIS simulation led to the establishing of an electrical circuit equivalent to the microstructure which simulated the insulating properties as a function of interfaces and pore interconnection. The latter was studied by microtomography. From this 3-dimensional simulation, a finite element analysis of mechanical properties was developed and compared to experimental measurements. Using this approach to microstructure and properties, excimer laser surface processing was shown to be an innovative process to modify insulating characteristics of plasma-sprayed alumina.

The application of thermal sprayed coatings has a broad industrial scope, not only applied where it concerns repairing but also as a preventive means. This article focuses on the applications of ceramics as wear and corrosion protection on hydraulic piston rods. It begins with a concise description of the different forms of wear and their mechanisms with some typical examples of worn surfaces. This is followed by an overview of the available coating techniques for enhancing metal surfaces. Next, the article discusses the principle and applications of thermal sprayed coatings. Then, it presents the scope and protection measures of thermal sprayed coatings to failure mechanism. Further, the article discusses the properties, quality control, and testing criteria of the coatings. It describes the chemical composition and chemical analysis of quality control spray powder. Finally, the article ends with a discussion on the examples of thermal sprayed coatings in OEM applications.

Zircon (ZrSiO 4 ) is a technologically important oxide ceramic material known for its high refractoriness and chemical stability. It shows excellent thermal shock resistance as a result of its very low thermal expansion coefficient and a low heat conductivity coefficient. Plasma spraying is a convenient method to produce large area coatings with high growth rates, necessary for many applications. ZrSiO 4 is among the least expensive spraying materials for refractory applications. In this study, a single-step process was used to prepare mullite/zirconia ceramic composites by plasma spraying zircon/alumina mixtures. Mixtures of ZrSiO 4 and Al 2 O 3 powders with Al 2 O 3 to SiO 2 molar ratios of 3:2 were milled for 2 h in a zirconia medium using a ball mill. The as–milled powders were dried in the furnace and sintered at 1300 and 1350 °C for 2h then crushed to a size less than 100 μm. The powders were sprayed by an atmospheric plasma spray gun (Metco 3MB) using C/C+SiC ceramic matrix composite substrates. Scanning electron microscopy (SEM) was used to analyze the microstructures of the powders and plasma coatings. The phase composition analysis of the powder showed the presence of alumina and zircon. After plasma coating, alumina, zircon, and zirconia phases were determined.

CaZrO 3 coatings were alternatively prepared by air plasma spray and flame spray processes. The microstructural characteristics and crystalline phases of the coatings were comparatively studied as a function of the spraying temperature achieved with each technique and the stand off distance. Image analyses were used to estimate their porosity. Thermal diffusivity was measured on free-standing thick coatings using the laser flash technique. Thermal conductivity was obtained from the experimental thermal diffusivity and density data. The hardness of the coatings was evaluated by Vickers indentation tests. Finally, different thermal treatments were carried out to evaluate the evolution of the crystalline phases and the properties of the coatings.

A copper electroplating method revealed that recently developed commercial Al 2 O 3 thermal spray coatings did not have the well-known lamellar structure but instead had a structure that was made up of tightly bonded particles, with many vertical microcracks in the coating. The coating in the study was prepared by the atmospheric plasma spray system. The copper electroplating method was applied to observation of the Al 2 O 3 thermal spray coating structure. Spray coating specimens were electroplated in an aqueous solution containing CuSO 4 , H 2 SO 4 , CuCl 2 , and additives so that copper was deposited in small pores and narrow cracks in the coatings. Copper in the Al 2 O 3 coatings was observed clearly by scanning electron microscopy, which also revealed the coating’s structure (boundaries of flattened particles and vertical cracks). The coating did not have a lamellar structure (horizontal apertures between flattened particles) and it was dense in spite of the many microcracks.