Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

https://link.springer.com/content/pdf/10.1007%2Fs11666-016-0473-x.pdf

The 2016 Thermal Spray Roadmap

Yb2Si2O7 Environmental barrier coatings with reduced bond coat oxidation rates via chemical modifications for long life

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

A review on failure mechanism of thermal barrier coatings and strategies to extend their lifetime

This article is a detailed review of the measures to modify the high-temperature mechanical properties of silicon carbide ceramic matrix composites (SiC CMCs), namely toughness, high-temperature stability and wear resistance Additionally, it briefly describes the common processing methods of the SiC CMCs and their application in the high-temperature field of aerospace The advantages and disadvantages of various existing processing and molding methods for the SiC CMCs are also discussed The high-temperature mechanical properties of the SiC CMCs are mainly affected by the properties of the matrix, added phase and interface It is crucial to reduce the crystal defects of the matrix and select a suitable enhancement phase for an elevated performance Moreover, it is important to improve the bonding at the interface between the enhancement phase and the matrix This review is expected to provide useful information for the subsequent development of complex SiC CMCs for high-temperature applications

Advances in modifications and high-temperature applications of silicon carbide ceramic matrix composites in aerospace: A focused review

Variable microstructures of ZrO2-7 wt.% Y2O3 (7YSZ) coating can be achieved by plasma spray-physical vapor deposition (PS-PVD). Based on gas-phase prepared 7YSZ coating, the deposition mechanisms and their dependency on process conditions were investigated. When the spray distance was 950 mm, heterogeneous nucleation occurred on the surface of substrate. Porous oriented columnar coating could be obtained on the nickel-based superalloy at a substrate temperature of 850 °C. As the substrate temperature decreases to room temperature, porous columnar coating tends to be dense coating that is mainly made of randomly oriented fine grains. But when the substrate was replaced by graphite, columnar coating was observed again. As the spray distance increases from 950 to 2200 mm, homogenous nucleation occurs at the end of plasma plume leading to formation of cluster agglomerated by nano-sized grains. Finally, fine grain constituted coating was obtained on substrate at room temperature regardless of substrate materials.

Gas-deposition mechanisms of 7YSZ coating based on plasma spray-physical vapor deposition

Abstract Thermal barrier coatings (TBCs) can effectively protect the alloy substrate of hot components in aeroengines or land-based gas turbines by the thermal insulation and corrosion/erosion resistance of the ceramic top coat. However, the continuous pursuit of a higher operating temperature leads to degradation, delamination, and premature failure of the top coat. Both new ceramic materials and new coating structures must be developed to meet the demand for future advanced TBC systems. In this paper, the latest progress of some new ceramic materials is first reviewed. Then, a comprehensive spalling mechanism of the ceramic top coat is summarized to understand the dependence of lifetime on various factors such as oxidation scale growth, ceramic sintering, erosion, and calcium-magnesium-aluminium-silicate (CMAS) molten salt corrosion. Finally, new structural design methods for high-performance TBCs are discussed from the perspectives of lamellar, columnar, and nanostructure inclusions. The latest developments of ceramic top coat will be presented in terms of material selection, structural design, and failure mechanism, and the comprehensive guidance will be provided for the development of next-generation advanced TBCs with higher temperature resistance, better thermal insulation, and longer lifetime. 

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

In general, it is accepted that thermal barrier coating (TBC) with good CMAS (CaO–MgO–Al 2 O 3 –SiO 2 ) corrosion resistance has long lifetime. A columnar Al film was firstly deposited in this work at the top of 7 wt.% yttria-stabilized zirconia (7YSZ) TBC by magnetron sputtering. Then a vacuum treatment of Al-deposited TBC was carried out at 700 °C for 1 h and 900 °C for 5 h. An α-Al 2 O 3 layer was in situ synthesized at the top of 7YSZ TBC through the reaction of Al and ZrO 2 after vacuum heat treatment. The CMAS corrosion resistance of as-sprayed TBC and treated TBC was compared after isothermal test of 1200 °C for 24 h. The results indicated that the Al-deposited TBC after vacuum heat treatment has better CMAS corrosion resistance than the as-sprayed TBC. Two mechanisms are considered to contribute to the protective effect of in situ synthesized α-Al 2 O 3 layer: the first is α-Al 2 O 3 partially filling existed open pores and cracks in the top 7YSZ coating to hinder the penetration of molten CMAS; and the second is α-Al 2 O 3 reacted with CMAS forming CaAl 2 Si 2 O 8 and MgAl 2 O 4 that play the role in inhibiting further infiltration of CMAS.

Xiaofeng Zhang

Papers

Gas-deposition mechanisms of 7YSZ coating based on plasma spray-physical vapor deposition

Progress in ceramic materials and structure design toward advanced thermal barrier coatings

In situ synthesis of α-alumina layer on thermal barrier coating for protection against CMAS (CaO–MgO–Al2O3–SiO2) corrosion

Mechanisms governing the thermal shock and tensile fracture of PS-PVD 7YSZ TBC

Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion