Mechanisms controlling the durability of thermal barrier coatings

Preceramic polymers were proposed over 30 years ago as precursors for the fabrication of mainly Si-based advanced ceramics, generally denoted as polymer-derived ceramics (PDCs). The polymer to ceramic transformation process enabled significant technological breakthroughs in ceramic science and technology, such as the development of ceramic fibers, coatings, or ceramics stable at ultrahigh temperatures (up to 2000°C) with respect to decomposition, crystallization, phase separation, and creep. In recent years, several important advances have been achieved such as the discovery of a variety of functional properties associated with PDCs. Moreover, novel insights into their structure at the nanoscale level have contributed to the fundamental understanding of the various useful and unique features of PDCs related to their high chemical durability or high creep resistance or semiconducting behavior. From the processing point of view, preceramic polymers have been used as reactive binders to produce technical ceramics, they have been manipulated to allow for the formation of ordered pores in the meso-range, they have been tested for joining advanced ceramic components, and have been processed into bulk or macroporous components. Consequently, possible fields of applications of PDCs have been extended significantly by the recent research and development activities. Several key engineering fields suitable for application of PDCs include high-temperature-resistant materials (energy materials, automotive, aerospace, etc.), hard materials, chemical engineering (catalyst support, food- and biotechnology, etc.), or functional materials in electrical engineering as well as in micro/nanoelectronics. The science and technological development of PDCs are highly interdisciplinary, at the forefront of micro- and nanoscience and technology, with expertise provided by chemists, physicists, mineralogists, and materials scientists, and engineers. Moreover, several specialized industries have already commercialized components based on PDCs, and the production and availability of the precursors used has dramatically increased over the past few years. In this feature article, we highlight the following scientific issues related to advanced PDCs research: (1) General synthesis procedures to produce silicon-based preceramic polymers.
(2) Special microstructural features of PDCs.
(3) Unusual materials properties of PDCs, that are related to their unique nanosized microstructure that makes preceramic polymers of great and topical interest to researchers across a wide spectrum of disciplines.
(4) Processing strategies to fabricate ceramic components from preceramic polymers.
(5) Discussion and presentation of several examples of possible real-life applications that take advantage of the special characteristics of preceramic polymers. Note: In the past, a wide range of specialized international symposia have been devoted to PDCs, in particular organized by the American Ceramic Society, the European Materials Society, and the Materials Research Society. Most of the reviews available on PDCs are either not up to date or deal with only a subset of preceramic polymers and ceramics (e.g., silazanes to produce SiCN-based ceramics). Thus, this review is focused on a large number of novel data and developments, and contains materials from the literature but also from sources that are not widely available.

Polymer-Derived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics

Failure mechanisms associated with the thermally grown oxide in plasma-sprayed thermal barrier coatings

The high output of sewage sludge, which is increasing during recent years, and the limitations of the existing means of disposing sewage sludge highlight the need to find alternative routes to manage this waste. Biomass and residues like sewage sludge are the only renewable energy sources that can provide C and H, thus it is interesting to process them by means of treatments that enable to obtain chemically valuable products like fuels and not only heat and power; pyrolysis can be one of these treatments. The main objective of this review is to provide an account of the state of the art of sewage sludge pyrolysis for liquid production, which is under study during recent years. This process yields around 50 wt% (daf) of liquid. Typically, this liquid is heterogeneous and it usually separates into two or three phases. Some of these organic phases have very high gross heating values, even similar to those of petroleum-based fuels. The only industrial sewage sludge pyrolysis plant operated to date is currently closed due to some technical challenges and problems of economic viability.

Sewage sludge pyrolysis for liquid production: A review

An overview is presented of the development of residual stresses in thermal spray coatings and their ef-fects on interfacial debonding. The main experimental techniques for measurement of residual stresse are briefly described, with particular attention given to the method of continuous curvature monitoring. Boundary conditions satisfied by all residual stress distributions are identified and expressions derived for the curvatures and stress distributions arising from a uniform misfit strain between coating and sub-strate.It is noted that stress distributions in thick coatings rarely correspond to the imposition of such a uniform misfit strain, so that recourse to numerical methods becomes essential for quantitative predic-tion of stress distributions. Relationships are presented between residual stresses and corresponding strain energy release rates during interfacial debonding. The effect on this of superimposing stresses from an externally applied load is outlined. The initiation of debonding is then considered, covering edge effects and other geometrical considerations. Finally, some specific case histories are briefly outlined to illustrate how the various theoretical concepts involved relate to industrial practice.

Residual Stresses in Thermal Spray Coatings and Their Effect on Interfacial Adhesion : A Review of Recent Work

Isothermal and cyclic heat treatments of a plasma-sprayed zirconia-8 wt% yttria thermal barrier coating on a nickel superalloy substrate highlighted coating failure mechanisms. A reaction layer formed at the bond coat/ceramic interface, and failure of the coating initiated in the reaction layer due to opening thermal expansion mismatch stresses associated with a nonplanar interface. Crack propagation occurred in the ceramic in all cases, due to the high fracture energy of the interface. The relatively low fracture energy of the coating, in planes parallel to the interface, is the ultimate factor limiting coating lifetime.

Failure Mechanisms of a Zirconia-8 wt% Yttria Thermal Barrier Coating

Clay–sewage sludge co-pyrolysis. A TG–MS and Py–GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Polymeric fibers were produced from a polyaluminocarbosilane obtained by reacting polycarbosilane and an aluminum alkoxide modified with a [beta]-ceto ester. The pre-ceramic fibers were converted into amorphous Si-Al-O-N ceramic fibers after a pyrolysis process under flowing ammonia at 1000 C. [sup 29]Si and [sup 27]Al magic-angle spinning nuclear magnetic resonance investigations were performed to characterize the conversion process of the polymeric precursor fibers into the ceramic product. However, because of the amount of matter required for the MAS-NMR experiments, the heat treatment applied for fibers was done on powders assuming identical evolution for both materials. Tensile strength was measured at various stages of the pyrolysis process and related to the corresponding structural evolution.

Si-Al-O-N Fibers from Polymeric Precursor: Synthesis, Structural, and Mechanical Characterization

Production of sharp cracks in ceramic materials by three-point bending of sandwiched specimens

A Zr-modified polycarbosilane has been obtained reacting a zirconium alkoxide with a polycarbosilane. This new preceramic polymer has been nitridated in flowing ammonia up to 1000 °C. The conversion process of the polymer precursor to the Si–N–Zr–O ceramic has been followed mainly by FT-IR, XRD, and TEM investigations. The formation of the Si–N–Si network starts at 600 °C. At 1000 °C the system can be described as an amorphous silicon nitride ceramic in which very fine zirconia-based particles are dispersed. Increasing the temperature to 1300 °C results in the crystallization of t-ZrO2 microcrystals (35 A in size). At higher temperatures the crystallization of the silicon nitride matrix into β–Si3N4 has been observed.

Roberto Dal Maschio

Papers

Failure Mechanisms of a Zirconia-8 wt% Yttria Thermal Barrier Coating

Clay–sewage sludge co-pyrolysis. A TG–MS and Py–GC study on potential advantages afforded by the presence of clay in the pyrolysis of wastewater sewage sludge

Si-Al-O-N Fibers from Polymeric Precursor: Synthesis, Structural, and Mechanical Characterization

Production of sharp cracks in ceramic materials by three-point bending of sandwiched specimens

Polymer-derived Si3N4−ZrO2 nanocomposite powders