TWI has been working with coatings and surface engineering for decades, providing impartial and independent expert advice and support to industry. Solving challenges across all industry sectors, our work includes core research programme (CRP) projects for the wider benefit of industry, joint industry programme (JIP) projects that allow interested parties to pool their resources to gain exclusive access to project results related to their field of interest, collaborative projects working with industry and academia, and projects designed for specific Industrial Members. This body of work has created a deep level of understanding and expertise at TWI that allows us to take solutions from one industry area and adapt and apply them to another, providing bespoke solutions for the benefit of our Members.
Core Research Programme (CRP)
Our CRP projects are created to solve set challenges that often provide benefits for a range of industries and applications. With regards to coatings and surface testing, these projects have investigated different coatings, environments, deposition methods and more.
Some of our earlier work in this area was concerned with porosity produced in protective coatings deposited using arc spraying. This porosity was a problem as it allowed corrosive liquids and gases to permeate to the substrate. To minimise this, TWI’s experts studied the effects of parameters including arc current, arc voltage, air pressure and standoff distance. Porosity was also part of a 1992 project that looked into active / inert gas atomisation and arc spray coating quality. The aim was to solve the issue of excessive oxide content in coatings, which can reduce bond strength, create porosity and cause coatings to flake off prematurely. Another disadvantage is an excessive burnoff of the alloying elements, which are the essential ingredients contained in the parent wires, so that the coatings cannot be reliably produced with a specified composition. All these issues can be minimised with nitrogen or argon gas atomisation, leading us to review the effects of atomising gases on coating characteristics.
Arc spraying has the highest deposition rates among thermal spraying processes and has been widely used to deposit corrosion resistant coatings onto large outdoor steel structures and for reclamation of over-machined or worn components. We undertook a study in 1992 to review the factors determining spray particle characteristics with regard to particle size, temperature and velocity, and the effects of the spray particle characteristics on coating quality in terms of bond strength, porosity, surface roughness, oxidation and residual stresses.
By 1996 our attention had turned to high velocity oxyfuel (HVOF) sprayed coatings with a project to test the adhesion of coatings that defy testing using the ASTM C633-79 method. A review was carried out to assess the principal methods of adhesion testing at the time and their potential for ranking high adhesion coatings. The same year, HVOF spraying was investigated as a technique for co-spraying a metal and a ceramic powder simultaneously, depositing metal/ceramic coatings designed to improve ceramic coating stability on metal substrates.
1996 saw us present studies into the surface preparation and adhesion of sprayed alumina coatings, which are resistant to many corrosive liquids, provide good electrical insulation, and has good resistance to abrasive wear. This project sought to identify the best method for preparing steel surfaces for alumina spraying and determine whether the particle velocity, fuel gas or interpass time had a significant effect on adhesion. In addition, we determined whether the P.A.T. test method for coating adhesion gave comparable results to the ASTM C633 pull-off test. Research into alumina based coatings continued with a review of the ability of HVOF to deposit alumina-based coatings, as well as measuring abrasive wear, erosive wear and protection against aqueous corrosion given by the prepared coatings. This work was extended in 1998 with a study into the deposition of nickel aluminide and iron aluminide coatings using the HVOF spraying process. The research investigated the microstructure and quality of the prepared coatings and also identified industrial applications that could benefit from using aluminide coatings deposited by the HVOF process. There was yet more research into HVOF and coatings as TWI researched multi-layer HVOF coatings of alumina and nickel alloy in 2002, with the aim of producing engineered microstructures for multi-functional surfaces or devices. This project aimed to demonstrate the feasibility of preparing a multi-layer coating consisting of alternating 40mm layers of alumina and nickel alloy using the HVOF spraying process before testing the coating for wear and corrosion resistance.
Returning to arc spraying in 1997, TWI studied the use of cored wire consumables during arc spraying to produce hard coatings. The project demonstrated the feasibility of using cored wire for arc spraying, assessed the coating efficiency, and determined the hardness and composition of the deposits.
Another method investigated in 1997 was the use of resistance surfacing to deposit wear and corrosion resistant coatings. This novel process had showed the desirable benefits of a full metallurgical bond with minimal dilution, using conventional powder consumables, so our experts researched its use to produce a range of claddings on low carbon steel using standard metallic thermal spraying powders and to evaluate the basic overlay characteristics.
Alongside methods for the deposition of coatings, it is also important to be able to assess their quality and detect any flaws. A 1997 CRP project was created for this purpose, evaluating the suitability of infrared non-destructive testing (NDT) techniques to detect and characterise flaws associated with thermally sprayed coatings. This research in NDT continued through a CRP project in 2000 to provide a critical review of all NDT methods relevant to the inspection of thermally sprayed coatings, thus defining the capabilities of existing techniques and highlighting lines of future development.
Around the same time, our technical experts were investigating Sol-gel, an emergent technology for the fabrication of glasses, ceramics and a new class of polymer/ceramic composite materials called ormocers. Sol-gel is a liquid phase route that involves chemical processing yet has the simplicity in the processing of product forms. Plus, because it starts with pure materials the final product can be designed with a high purity or tailored for specific applications ranging from powders and highly porous materials to thin inorganic films.
HVOF continued to be an area of interest in our core research endeavours, with a 1998 investigation of HVOF using hydrogen fuel that determined the generic characteristics of NiCr-Cr 3C 2 coatings produced from a range of different commercial powders before optimising the spraying parameters for the most promising powder, using hydrogen fuel. As we reached 1999, new legislation was introduced that aimed to minimise the toxic waste generated by the chromium plating process. This led to interest in thermal sprayed coatings as an alternative and a subsequent project at TWI to investigate the efficacy of the HVOF and other thermal processes to deposit amorphous and nanocrystalline metallic coatings as an alternative to hard chromium plating.
The quality of coatings can be measured by the level of adhesion to a substrate, with a number of destructive testing methods being undertaken with specially prepared test specimens. However, by 2002, TWI’s experts examined a range of NDT techniques to determine their the suitability for the characterisation of adhesion strength in high velocity oxyfuel (HVOF) and arc sprayed coatings. Around the same time TWI also evaluated a high velocity wire flame spraying process, checking the coating characteristics and corrosion performance as well as comparing these to coatings made with more conventional electrical wire arc and HVOF processes.
In 2003, we completed a project to investigate sol-gel coatings to deliver improved corrosion protection compared to the parent metal substrate. This project looked into sol-gel coatings for their capability to provide protection while maintaining a clear appearance, making them better suited to jewellery, ironmongery and cutlery than alternatives that are coloured and thereby change the appearance of the underlying substrate. Our work into transparent sol-gel coatings continued into 2005 with a project aimed at depositing sol-gel derived indium tin oxide coatings on glass and characterising them in terms of microstructure and electrical resistance and another project to investigate the curing and stability of silica-based sol-gel coatings, including for high temperature uses and for potential use in anti-fouling applications.
By 2005, TWI had returned to evaluating HVOF coatings with a CRP project to deposit and characterise HVOF coatings prepared from amorphous and nanocrystalline powders, as well as assessing the corrosion and wear performance of the coatings, including a comparison against a standard WC-Co-Cr HVOF coating. This work continued into 2006, with a project to assess HVOF sprayed alternatives to hard chromium plating. HVOF was investigated again in 2009 with a project aimed at corrosion testing HVOF coatings for biomass use as well as for waste incineration and co-fired combustion power plant environments. This included the design and build of a test facility to allow for a comprehensive set of corrosion tests representative of elevated temperature biomass combustion at which the coatings could be assessed, with the work continuing into 2010 with a follow-up CRP project.
Although thermal coatings had been used for a wide range of corrosion, wear and thermal barrier applications, coating processes powered by oxy-fuel combustion and electric arc were characterised by voids, oxidation, weak mechanical bonding and highly stressed, non-equilibrium microstructures. Cold spray was seen as a potential solution because the spraying consumable is not oxidised during spraying and is not subject to rapid changes of state, volume and phase. Bearing this in mind, TWI completed a project in 2011 to assess spray-formed Titanium (Ti) and Ti coatings prepared by cold spray. Research into cold spray continued into 2012 with another project to assess the corrosion of cold sprayed tantalum coatings. The aim was to assess the ability of these coatings to protect less noble substrates, such as steel, from corrosion.
Adhesives and test methods for thermal spray coatings were studied for a 2013 CRP project, with tests conducted according to several standards to identify and assess high strength glues for use in thermal spray coating adhesion. This work generated shear strength data for a selection of industrially representative thermal spray coatings and metallic substrates, using an adhesive selected during the study and tested in accordance with ASTM F1044.
Cohesive strength of a coating is also important, especially in applications where coatings are applied for dimensional restoration or to spray form a component. It is also relevant where failure might occur at particle boundaries or within the spray particles which make up the coating. This study evaluated the tubular coating tensile (TCT) test method to measure the cohesion strength of a range of industrially relevant thermal spray coatings.
Where the previous project researched cohesion strength, our next CRP project related to coatings evaluated the durability of low surface energy coatings that are able to maintain a surface finish by not allowing foreign materials to adhere to the surface. This mitigates against surface contamination that could cause an increase in weight, reduced aerodynamic or hydrodynamic efficiency, or act as a source of corrosion. To find the best solutions to these challenges, TWI conducted a series of tests on current products to determine if they possessed the required properties. This 2014 project had potential advantages for a range of industry sectors and was followed by a 2015 review of the state of the art and methodologies for coating composites, including technical challenges and potential applications. 2015 also saw the publication of three more coating-specific project reports; ‘Thermal Spray Coating Surface Characterisation/Preparation,’ a technical review of surface preparation techniques for the application of thermal spray coatings, an assessment of ‘Titanium Dioxide Coatings for Photocatalytic Applications,’ and a benchmarking of ‘Cold Spray Systems for Nickel Alloy 718 Coating Deposition,’ Which also describes the effects of process gas type, temperature and pressure on the properties of cold spray deposited nickel alloy 718.
Our experts returned to the subject of low surface energy (or ‘non-stick’) coatings to produce a 2018 comparative review of their use in mechanically aggressive environments, following a series of tests to assess the functional properties and assess typical examples of commercial low surface energy coatings.
Coating adhesion tests typically used a stud attached to the coating surface with an adhesive before the bonded assembly is subjected to a controlled strain rate tensile test until the point of failure. However, advances in thermal and cold spray technology had resulted in coating bond strengths exceeding the maximum strength of available adhesives (70MPa). In order to measure the adhesion values of coatings with bond strengths in excess of 70MPa and order to validate their performance for load bearing applications it was necessary to develop and validate a bond strength test that didn’t require the use of an adhesive. To allow these higher bond strength coatings to be confidently used by industry, TWI delivered a 2020 project to develop and validate an adhesive-free bond strength test method for cold spray coatings (Figures 1-2).
Thermally sprayed aluminium (TSA) coatings had been used to act as a barrier in all marine environments and offer cathodic protection to immersed steel structures in industries such as offshore wind. However, tests on the damage tolerance of these coatings had only been tested with single, small-scale damage such as scribes or holidays exposing up to 5% area of the specimen on the corrosion performance, and only for a limited time period. TWI recognised a need for information on the damage tolerance of thermal spray aluminium (TSA) coatings where a larger damaged area was used, over a longer term, and also taking account of any deposits on the defect regions to understand their role in the corrosion process and if they offer any protection Figures 3a, 3b and 3c show photographs of specimens before testing showing holiday (defect) size and distribution. Specimens were approximately 40mm×40mm with percentage defect areas of approximately 5% (a), and ~18% (b and c).
Corrosion was also the focus of a 2023 CRP project that addressed the corrosion behaviour of PVD and CVD wear resistant coatings in synthetic seawater. Although there had been extensive research into the tribological properties of wear resistant coatings used in corrosive environments, not as much was known about their corrosion properties, particularly in conjunction with industrially relevant substrates and their use in final service environments. This project sought to address the need for more corrosion performance information for wear resistant coatings and to develop a more comprehensive, application focussed, testing approach to assist in future evaluation.
Joint Industry Programme (JIP) Projects
While the core research programme is funded from Industrial Memberships for the wider benefit of our Members, our joint industry programme (JIP) projects allow us to focus in on specific challenges or innovations for the benefit of a group of sponsors. This allows our Industrial Members the opportunity to invest in work that is of interest to them, gaining exclusive access to the results and the chance to have input into the direction of the project in return. This programme of work has delivered solutions across industry, including for coatings and surface testing.
These projects include thermally sprayed aluminium alloys (TSA) for the prevention of corrosion and environmentally assisted cracking of welded corrosion resistant alloys and the friction stir spot welding of high strength steels for transport industries in 2006 and the 2009 ‘Cold Spray - Improved Corrosion and Wear Resistant Coatings by Cold Spray,’ project that addressed technology gaps so that cold spray could be validated for applications such as corrosion resistant vessels, hard-facing, spray-forming and additive manufacturing. There was also little understanding of the relationship between cold spray particle flight characteristics and coating properties, and no published process economic data at the time of this project. Corrosion was also the subject of a 2009 project to improve splash and tidal zone coatings through the creation of a specification for a coating with a 40-year design life, delivering long-term corrosion mitigation for splash and tidal zones on offshore structures.
Increasing efficiencies and reducing costs were two of the benefits for the waste industry from TWI’s JIP project to develop coating technologies for high temperature, chlorine induced, corrosion mitigation in biomass, waste to energy and other process plants. This project addressed high temperature, chlorine-induced corrosion from biomass and municipal solid waste combustion by the development of thermal spray coatings with improved properties in aggressive power generation environments, removing the need for expensive superalloys substrates, increasing combustion temperatures and recoverable energy efficiency while reducing the need for maintenance, life cycle costs, pollutants, unscheduled breakdowns and lost days per annum. This technology is applicable to a range of different types of process plant, expanding the industrial reach of the project outcomes. Also providing benefits for several industries was TWI’s 2011 JIP-based development of the CompoSurfTM coating technology, which used thermal spraying processes to deliver increased functionality of composite materials.
A 2013 project created data for the oil and gas industry in light of the new fields that were emerging involving the installation of increasingly difficult-to-maintain and remote deep-water facilities, paired with the extraction of hotter hydrocarbons. This data measured the effect of cathodic protection on the performance of thermally sprayed aluminium (TSA) coatings at elevated temperatures, increasing the confidence in the long-term reliability of TSA coatings in subsea service.
In more recent years, industry required REACH-compliant solutions to meet the changing legislative landscape amid environmental concerns for different materials and products. A 2021 project involved the extensive testing of coatings deposited using EHLA (extreme high-speed laser application) and HVAF (high-velocity air fuel) spraying, allowing them to be benchmarked against hard chrome plating and HCP alternatives such as high velocity oxy-fuel spraying.
We also created a JIP project to address the wider industry concern over the continued use of PFAS (Perfluoroalkyl and polyfluoroalkyl substances), an umbrella term for a class of thousands of chemicals and polymers that are widely used as surfactants, lubricants, surface treatments, coatings, seals and liners. Primarily used for their chemical and thermal stability, this very durability means that PFAS are persistent when they are present in the environment and some show signs of bioaccumulation potential as they are detected as pollutants, contaminating groundwater, surface water and soil. With an estimated 10,000+ different PFAS types used by a £25 billion PFAS market, concerns over human health have already led to the voluntary withdrawal of two historically significant types of PFAS in the early 2000s, PFOA (perfluorooctanoic acid) and PFOS (perfluorooctane sulfonic acid). With alternatives and replacements subject to scrutiny with regards to their health and environmental effects and just a small number of chemicals having been screened from the number available, TWI created a project to assess and address the PFAS business risk within a changing regulatory and ESG landscape.
Another recent environmentally-focused JIP project launched by TWI 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. This project sought REACH-compliant alternatives to other widely used, but now restricted, coatings.
Public-Funded Projects
TWI also brings its expertise, knowledge and experience to collaborative, publicly-funded projects. Working alongside partners from industry and academia, our experts provide vital input to solve problems from across a range of industry sectors and applications.