Advanced Coatings by Thermal Spray Processes

Joshi Joshi,Nylen Nylen

doi:10.3390/technologies7040079

Abstract

Coatings are pivotal in combating problems of premature component degradation in aggressive industrial environments and constitute a strategic area for continued development. Thermal spray (TS) coatings offer distinct advantages by combining versatility, cost-effectiveness, and the ability to coat complex geometries without constraints of other in-chamber processes. Consequently, TS techniques like high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) are industrially well-accepted. However, they have reached limits of their capabilities while expectations from coatings progressively increase in pursuit of enhanced efficiency and productivity. Two emerging TS variants, namely high-velocity air-fuel (HVAF) and liquid feedstock thermal spraying, offer attractive pathways to realize high-performance surfaces superior to those hitherto achievable. Supersonic HVAF spraying provides highly adherent coatings with negligible porosity and its low processing temperature also ensures insignificant thermal ‘damage’ (oxidation, decarburization, etc.) to the starting material. On the other hand, liquid feedstock derived TS coatings, deposited using suspensions of fine particles (100 nm–5 µm) or solution precursors, permits the production of coatings with novel microstructures and diverse application-specific architectures. The possibility of hybrid processing, combining liquid and powder feedstock, provides further opportunities to fine tune the properties of functional surfaces. These new approaches are discussed along with some illustrative examples.

Highlights

Coatings have always played a pivotal role in enabling industries to combat problems of premature degradation of components that operate in harsh environments
While it is true that interest in suspension plasma spraying (SPS) thermal barrier coatings (TBCs) is motivated by the ability of the process to conveniently yield strain-tolerant columnar microstructures, it is to be emphasized that column formation in SPS
The high-velocity air-fuel (HVAF) and SPS coating techniques are the two most exciting thermal spray variants that have been the subject of growing research attention in recent years

Summary

Introduction

Coatings have always played a pivotal role in enabling industries to combat problems of premature degradation of components that operate in harsh environments. Its growing reputation within the global thermal spray community has been greatly aided by the availability of state-of-the-art facilities, comprising a high power Mettech Axial III plasma spray system, which is widely acknowledged to open new vistas in thermal spraying by enabling axial suspension plasma spraying (ASPS), and a UniqueCoat M3 HVAF facility, which permits the deposition of very dense and well-bonded coatings with minimal thermal damage to coating feedstock (Figure 1). These lend a rare uniqueness and versatility to the group’s processing capabilities that are available at few thermal spray groups worldwide.

HVAF for Wear

Corrosion

21 Cr9 Mo

SPS Coatings for TBC Applications

Cross-sectional

SPS Coatings for Non-TBC Applications

Prospects

Summary