Thermal spray coatings produced from nanostructured ceramic agglomerated powders were tailored for different applications, some of which required almost completely opposite performance characteristics (e.g., anti-wear and abradable coatings). The influence of nanostructured materials on important areas, such as, thermal barrier coatings (TBCs) and biomedical coatings was also investigated. It was determined that by controlling the distribution and character of the semi-molten nanostructured agglomerated particles (i.e., nanozones) embedded in the coating microstructure, it was possible to engineer coatings that exhibited high toughness for anti-wear applications or highly friable for use as abradables, exhibiting abradability levels equivalent to those of metallic-based abradables. It is shown that nanozones, in addition to being very important for the mechanical behavior, may also play a key role in enhancing and controlling the bioactivity levels of biomedical coatings via biomimetism. This research demonstrates that these nanostructured coatings can be engineered to exhibit different properties and microstructures by spraying nanostructured ceramic agglomerated powders via air plasma spray (APS) or high velocity oxy-fuel (HVOF). Finally, in order to present readers with a broader view of the current achievements and future prospects in this area of research, a general overview is presented based on the main papers published on this subject in the scientific literature.

Thermal Spray Coatings Engineered from Nanostructured Ceramic Agglomerated Powders for Structural, Thermal Barrier and Biomedical Applications: A Review

http://me.eng.sunysb.edu/~compmech/downloads/N34.pdf

Effects of pores and interfaces on effective properties of plasma sprayed zirconia coatings

The microstructures of thermally sprayed coatings are very complex and incorporate process-dependent defects such as globular pores, interlamellar pores, cracks (in case of ceramics), etc. Porosity is a prevalent feature in the microstructure and affects a wide range of coating properties such as elastic modulus, thermal conductivity and dielectric behavior. Various methods are employed for quantitative measurement of porosity, which forms an important and integral part of microstructural characterization of thermal spray coatings. Studies have been conducted here to establish image analysis (IA) as a reliable method for characterization of porosity in thermally sprayed coatings. The versatility of IA methods for microstructural quantification has been investigated for diverse coatings deposited with distinct spray processes and material feedstock characteristics. The IA methods put forth have been successfully applied to a variety of thermally sprayed coatings; materials of interest in this study being produced from partially stabilized zirconia (PSZ, ZrO2+8% Y2O3) and alumina ceramics. The results have been complemented with the microstructural information obtained using advanced characterization techniques, such as small-angle neutron scattering (SANS) and computer micro-tomography (CMT) carried out on similar coatings. The trends shown by these different methods for variation in porosity with respect to feedstock characteristics were in agreement. This study confirms the applicability of image analysis as a straightforward, versatile, reliable and inexpensive method for the characterization of porosity.

Application of image analysis for characterization of porosity in thermal spray coatings and correlation with small angle neutron scattering

Thermal-barrier coatings (TBCs) are complex, defected, thick films made of zirconia-based refractory ceramic oxides. Their widespread applicability has necessitated development of high throughput, low cost materials manufacturing technologies. Thermal plasmas and electron beams have been the primary energy sources for processing of such systems. Electron-beam physical vapor deposition (EBPVD) is a sophisticated TBC fabrication technology for rotating parts of aero engine components, while atmospheric plasma sprays (APS) span the range from rotating blades of large power generation turbines to afterburners in supersonic propulsion engines. This article presents a scientific description of both contemporary manufacturing processes (EBPVD, APS) and emerging TBC deposition technologies based on novel extensions to plasma technology (suspension spray, plasma spray-PVD) to facilitate novel compliant and low thermal conductivity coating architectures. TBCs are of vital importance to both performance and energy efficiency of modern turbines with concomitant needs in process control for both advanced design and reliable manufacturing.

Processing science of advanced thermal-barrier systems

Wear behaviour of thermally sprayed ceramic oxide coatings

Thermal spray offers a variety of sub-sets of processing approaches to produce coatings. The various processes are classified based on the thermal spray source (from low velocity combustion spray to high temperature plasma jets) and method of material injection (in the form of powder, wire or rod). However, it is this intrinsic versatility which sets-up variations in characteristics of the applied coatings. Properties of thermally sprayed coatings, including process induced residual stress, are controlled by various parameters of the spraying process. This study examines three thermal spraying techniques with significantly different particle temperatures and velocities. They are air plasma spraying (APS), twin wire-arc spraying (TWA) and high velocity oxy-fuel (HVOF) spraying. For comparison purposes the recently developed cold spray processed materials were included in the study. For each method, in-flight particle diagnostics was performed; Ni–5 wt.%Al splats and deposits were fabricated and analyzed. Porosity, elastic modulus and thermal conductivity of the deposits were evaluated and correlated to the process variables. Using indentation at different loads and analysis of the indented region, stress–strain relationships for these coatings were obtained. Surprising differences in the properties were observed and were explained based on the fundamental variations in microstructure development. Through-thickness residual stress profiles in Ni–5 wt.%Al coatings on steel substrates were determined non-destructively by neutron diffraction. The stresses range from highly tensile in the APS coating to compressive in the HVOF coating. Various stress generation mechanisms—splat quenching, peening and thermal mismatch—are discussed with respect to process parameters and material properties.

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

For plasma sprayed thermal barrier coatings (TBCs), control of thermal conductivity is critical since low thermal conductivity depends not only on the intrinsic property of the yttria-stabilized zirconia (YSZ) TBC, but also on the morphology of pores and cracks introduced during spray process. They are closely linked to process methodology as well as to chemistry, structure and morphology of the ceramic feed materials. This paper addresses the influence of feedstock characteristics on particle state in the plasma and the resultant coating properties. In addition, substrate temperature, angle-of-impact and thermal cycling effects on porosity (quantity and morphology) and its resultant influence on thermal conductivity and elastic modulus of plasma sprayed YSZ TBCs. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted particles, leading to lower thermal conductivity and modulus. Furthermore, higher substrate temperatures and low particle velocity lead to lower porosity and improved inter-splat contact and, thus, enhanced coating properties. Sintering during thermal cycling reduces porosity and increases thermal conductivity and modulus.

Processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia coatings

Plasma-sprayed yttria-stabilized zirconia (YSZ) continues to play an important role in enhancing performance of both propulsion and land-based gas turbine engines. Tailoring the microstructure and properties of these thermal barrier coatings towards achieving both prime reliance and manufacturing reproducibility is a complex task due to the multitude of interrelated parameters that influence the plasma spray process and the deposit formation dynamics. In this article, we report on a study that connects thermal spray coatings through process science and materials science utilizing the concept of process maps. Process maps are representations of interrelationships among control parameters and measured responses. First-order process maps have been established for three YSZ powder morphologies, linking the plasma forming torch parameters to the particle state (responses) through a design of experiments approach and in-flight diagnostics. Refinements to representation of the raw particle characteristics are proposed through the use of group parameters (melting index and kinetic energy) from the experimental results. First-order process maps have been used for process parameterization and feedback control. Correlating the first-order responses with coating properties allows representation of coating properties in the form of second-order process maps and enables identification of process windows. As will be demonstrated in this paper, these advances provide a platform with which to construct comprehensive process-microstructure–property relationships with implications for coating design, process efficiency and full-field assessment of manufacturing reliability.

Process maps for plasma spraying of yttria-stabilized zirconia: An integrated approach to design, optimization and reliability

It is well known that the high velocity oxy-fuel based thermal spray process impart high density and reduced porosity in coatings compared to those produced by other ambient thermal spray processes. The benefits of HVOF have largely remained in the domain of metals and cermets and limited investigations have been carried out in ceramic coatings. The ability to produce high density ceramic coatings (e.g. alumina) offers potential in high performance applications in the field of wear, corrosion resistance and dielectric coatings. However, due to extreme operational limits of the HVOF process, the fundamentals of process–structure–property relationships are not fully understood. In this paper, we report an integrated approach to establish processing–microstructure–property correlations in order to optimize coatings for such applications. This approach involves diagnostic studies, microstructure development and its resultant influence on properties of high velocity oxy-fuel (HVOF) sprayed alumina coatings. The diagnostic studies were aimed to investigate the effects of fuel gas/oxygen ratio and amount of total gas flow on the particle temperature and velocity. Furthermore, splats and coatings were deposited to investigate the relationship between diagnostic data, melting behavior and droplet substrate interactions. Such a comprehensive study, coupled with property measurements of the coatings, demonstrates critical operational variables among deposition procedure, coating microstructure and the deposit properties.

On the role of particle state and deposition procedure on mechanical, tribological and dielectric response of high velocity oxy-fuel sprayed alumina coatings

A. Vaidya

Papers

Role of thermal spray processing method on the microstructure, residual stress and properties of coatings: an integrated study for Ni–5 wt.%Al bond coats

Processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia coatings

Process maps for plasma spraying of yttria-stabilized zirconia: An integrated approach to design, optimization and reliability

On the role of particle state and deposition procedure on mechanical, tribological and dielectric response of high velocity oxy-fuel sprayed alumina coatings