Thermal spray can be used to create coatings from molten or semi-molten materials that are deposited on a surface to improve or modify its properties, including to make it more corrosion resistant, wear resistant, or durable.
The process can deposit metals, cermets, ceramics or polymers in layers typically 60 μm to 10 mm thick. Almost any material that melts or becomes plastic during the spraying process can be used for the coating, including blends of different materials to produce composite or graded thermal spray coatings.
TWI has decades of experience with thermal spray coatings, delivering support and advice to industry on all aspects of thermal spraying. This includes numerous research projects related to thermal spraying for the benefit of a range of different industry sectors.
This insight provides details of some of these projects, including those undertaken as part of our Industrial Member-funded core research programme (CRP), joint industry programme (JIP) projects, work as part of wider collaborations between several organisations, and projects to solve challenges for individual Industrial Member companies.
Core Research Programme (CRP)
Through the core research programme (CRP), TWI conducts projects to address challenges faced by our Industrial Members, advancing technologies and processes, and finding new solutions for industry problems. These projects are jointly funded by our Industrial Members, with the outcomes made available to all our Members.
Microstructural Characterisation of HVOF Sprayed Tungsten Carbide-Cobalt Coatings
At the time of this project, in 1997, high velocity oxyfuel (HVOF) spraying was increasingly being considered as a viable alternative thermal spraying process to air plasma and detonation flame spraying for the preparation of high-quality coatings of tungsten carbide-cobalt (WC-Co). These coatings were being used by industry to provide protection against wear such as abrasion, adhesion, fretting and particle erosion. Despite the effectiveness of this protection being dependent on the microstructure of the deposited coating, there was very little published information on the microstructure of HVOF sprayed WC-Co coatings and how this influences wear performance. This project addressed this gap by characterising the microstructure of two HVOF sprayed WC-Co coatings using scanning electron microscopy, X-ray diffraction and chemical etching techniques in order to determine the microstructure necessary to give best resistance to abrasive wear.
- Thermal Spraying of Amorphous and Nanocrystalline Metallic Coatings - A Review
Legislation designed to reduce the toxic waste generated by the chromium plating process led to industry interest in the use of thermal sprayed coatings as an alternative to electrolytic hard chromium plating (EHC) at the time of this 1999 CRP project. Developments in the high velocity oxyfuel (HVOF) process had enabled the deposition of amorphous and nanocrystalline metallic coatings as a potential alternative to hard chromium plating. TWI’s experts undertook a review of recent literature on these coatings deposited using advanced thermal spray processes, in particular the HVOF process.
- Evaluation of Adhesives and Test Methods for Measurement of Adhesion and Cohesion Strength of Thermal Spray Coatings
This project addressed testing of the bond between a thermal spray coating and substrate as covered by standards such as:
- ASTM C633-01 - Standard test method for adhesion or cohesion strength of thermal spray coatings
- BS EN 582:1994 - Thermal spraying. Determination of tensile adhesive strength
- ASTM D4541-09 - Standard test method for pull-off strength of coatings using portable adhesion testers
- ASTM F1044-05 - Standard test method for shear testing of calcium phosphate coatings and metallic coatings
This 2013 CRP project began by identifying alternative high strength glues for use in thermal spray coating adhesion tests according to ASTM C633 (tensile test); ASTM D4541-09 (portable adhesion test), and ASTM F1044 (shear test). This alternative glue was then used while measuring the adhesion and cohesion of a range of thermal spray coatings in tension in accordance with ASTM C633.
The second phase of the project involved assessing the suitability of the ASTM F1044 shear strength test (originally developed for medical coatings) for measuring the shear strength of a range of thermal spray coatings, as well as assessing the suitability of the 'tubular coating tensile' (TCT) test (developed for cold spray coatings) for measuring the cohesive strength of thermal sprayed coatings, generating TCT test data for of a selection of industrially representative high velocity oxy-fuel, twin wire arc and flame spray coatings.
- Surface Characterisation and Preparation for Thermal Spray Coatings - Technical Review
The bond strength between a substrate and a coating is dependent on surface preparation of the substrate and using optimised spray parameters. Although the parameters are usually carefully monitored and controlled, most surface preparation is performed via manual grit blasting with little to no process monitoring or calibration. In 2015, our experts undertook a review of surface preparation techniques for the application of thermal spray coatings in order to improve surface preparation using mechanised processes to ensure a tighter control of grit blasting and repeatability. We also investigated different methods of surface characterisation and other surface preparation techniques.
- Damage Tolerance of Thermal Spray Aluminium (TSA) Coatings
Carbon steel is the material of choice for offshore wind structures although it is prone to corrosion in marine environments, which means that corrosion mitigation systems such as coatings or cathodic protection need to be used. Thermally sprayed aluminium (TSA) is often used as a barrier system in all marine environments, offering cathodic protection to immersed steel structures. However, despite the frequent use at the time of this project in 2021, it was not clear how these coatings perform over long periods in the presence of damage. Previous studies had focused on single, small-scale damage exposing up to 5% of the area. However, the effect of multiple or large-scale damage exposing around 20% of the area had not been studied in detail. In addition to the dissolution of the aluminium coating, the deposits (if any) formed on the damaged regions also play a role in the corrosion kinetics. The evaluation of the deposits on the defect regions was carried out to understand their role in the corrosion process and thus ascertain if the deposits offer any protection (Figures 1-3). Twin wire arc spray was used to coat S355 steel with Al (1050) before the coated steel specimens were exposed to synthetic seawater in a bath kept at 25°C with no applied potential. The open circuit potentials were monitored against a standard Ag/AgCl reference electrode and the coated steel specimens had various levels of holidays (~5, 10, 15 and 18%) drilled on the front surface while the back surface was coated in lacquer. Microstructural characterisation and phase determination was carried out after testing and the data was analysed and used to develop an understanding of the corrosion mechanisms.
Joint Industry Projects
Our joint industry projects (JIPs) allow interested parties to come together as project sponsors, pooling resources for greater research outcomes, while also being given exclusive access to the results and the opportunity to guide the direction of the projects themselves. These projects typically focus on challenges faced by specific industry sectors, providing industry-relevant information, guidance and solutions:
- Thermally Sprayed Aluminium Alloys for the Prevention of Corrosion and Environmentally Assisted Cracking of Welded Corrosion Resistant Alloys
Started in 2006 and completed in 2008, this JIP investigated thermally sprayed alloy compositions for the control of polarisation to mitigate against the risk of hydrogen-induced stress cracking (HISC).
- Development of Coating Technologies for High Temperature, Chlorine Induced, Corrosion Mitigation in Biomass, Waste to Energy and Other Process Plants
This 2009 JIP was created to address the high temperature, chlorine-induced corrosion encountered with biomass and municipal solid waste combustion. Through the development of thermal spray coatings with improved properties for aggressive power generation environments, the project team sought to deliver increased combustion temperatures, recoverable energy efficiency and plant maintenance intervals, as well as reduced life cycle costs, pollutants, unscheduled breakdowns and lost days per annum. The coating performance was ranked using TWI’s high temperature test facility and field trials were undertaken to generate real life performance data.
- CompoSurf Coating Technology for Increased Functionality of Composite Materials
Although composites had found an increasing number of applications as structural components due to their many favourable properties, the limited surface properties prevented their use in applications where thermal management is required. This 2011 project evaluated an advanced coating technology developed at TWI called ‘CompoSurf™’ for selected potential industrial applications. Work was undertaken to improve the understanding of materials selection, spraying process and parameter selection to produce coatings for more extreme environments.
- Measuring the Effect of Cathodic Protection on the Performance of Thermally Sprayed Aluminium Coatings at Elevated Temperature
This project was created in 2013 to address the lack of data regarding the interaction of thermally sprayed aluminium (TSA) coatings and cathodic protection (CP) when applied to hot and thermally cycled risers. The project also investigated hydrocarbon transportation. Quantitative methods were used to measure current coating demands under selected CP values along with long-term testing at elevated temperatures and throughout thermal cycling.
- Sustainable Approaches to Surface Pretreatment of Additively Manufactured Aluminium Alloys
This 2024 JIP project aimed to address the crucial need for environmentally responsible, corrosion-resistant treatments for additively manufactured (AM) aluminium alloys used in aerospace, defence, automotive, and other industrial applications. The primary focus of the project was the identification of REACH-compliant, Cr(VI)-free replacements, Cr(VI)-free paint systems, as well as an understanding of how pre-treatments may differ when applied to AM components, compared with cast or wrought versions. The aim was to provide industrial guidance using service-relevant data on the corrosion performance of various pre-treatments in different environments, along with an investigation into how microstructural variations in AM aluminium alloys and traditional cast and wrought alloys influence pretreatment performance, durability, and reliability.