Substrates key to harder coatings

By gaining NADCAP approval for its hard coatings, a UK firm has now become the first company in Europe to secure the stringent aerospace standard for PVD coatings. Dean Palmer reports Although PVD (physical vapour deposition) coatings are not new, the methods being used to apply these hard coatings to components, and to heat treat them, differ considerably from company to company. Cambridge-based coatings specialist Tecvac, for example (part of the Wallwork Heat Treatment group since 1997) has recently gained NADCAP approval (the dominant global aerospace standard that originates from the US Aerospace industry) for its PVD coating services, the first company in Europe to gain this approval for PVD coating of parts. PVD allows very thin, high integrity, high adhesion coatings of very hard or lubricious materials to be applied to metal surfaces. These coatings, between 1 and 30 microns, extend from super-hard titanium nitride and chromium nitride - three times as hard as high quality chrome plate - to highly lubricious diamond-like coatings with very low coefficients of friction. Peter Carpenter, director at Wallwork Heat Treatment explained to Eureka: "The company has recently completed the first phase of new investment [around £1 million] in PVD and PACVD processes, to expand facilities to meet the needs of the aerospace, biomedical and high-end automotive engineering businesses." The accreditation process took more than 18 months to complete. As John Rushforth, operations director at Tecvac in Cambridge, explained: "While the process of accreditation to this new standard is, as it should be, very rigorous, our electron beam evaporation technique used for our PVD coatings produces a highly consistent, defect-free and very smooth coating. This facilitated much of our validation work and covers all the flying parts currently coated, including wear-resistant coatings for titanium and steel aerospace bearings and erosion-resistant coatings to increase the life of gas turbine blades." The company is currently working with aerospace customers. It provides hard coatings for the turbine blades on Rolls-Royce engines and is also case hardening and heat treating steel bearings with titanium nitride, for a major bearings manufacturer to the aerospace industry. But the performance of a hard coating is only as good as the substrate used. The key to Tecvac's success therefore lies in the combined expertise of its own PVD hard coatings, and parent company Wallwork's heat treatment and surface cleaning know-how. There are other benefits too. As Carpenter explained: "If a customer chooses to use two separate suppliers for heat treatment and hard coatings, you invariably get the 'blame each other' syndrome when things go wrong, whereas with us, you get the complete service from one supplier." Rod Hunter, technical director at Tecvac also pointed out: "There's a bit of an education issue on hard coatings. We need to talk to design engineers at the drawing board stage, when they are considering different materials for their components. Choosing a coating and heat treatment is often an afterthought with designers, but it shouldn't be. They often tend to stick with what they know and are comfortable with and we only become involved after they've selected the base material." Tecvac has also just launched a new vacuum brazing service for aerospace customers, using its dedicated high capacity vacuum furnaces at Cambridge. Components can be vacuum brazed using alloys of copper, nickel, gold and silver, to join similar or dissimilar metals and/or ceramics and cermets. The service covers components using alloy steels, tool steels, titanium alloys, nickel and superalloys, copper, tungsten and molybdenum alloys. As Carpenter explained: "The vacuum furnaces guarantee fully temperature controlled performance down to a vacuum of 10-5 atmospheres at temperatures above 750°C. "This enables us to offer exceptional brazing integrity and high quality surface finish using high melting point brazes with the most advanced component materials." Parts can be brazed with envelopes up to 1,200mm x 600mm x 600mm. Head of brazing at Tecvac, Glen Davis told Eureka that he had brazed complex parts "with up to 25,000 joints per unit". Tecvac has also recently completed a three-year EC-funded project, 'Coming Dry', which was aimed at developing a cost-effective method of dry or near-dry machining of exotic materials such as Inconel and organic matrix composites. Working with the Fiat Research Centre and Airbus Spain, the resulting Nitron-CA coating actually reduced the cost per hole of an Airbus drilling operation (12mm diameter through-holes reamed in situ on Airbus tailplanes) by six times and cut production time for this operation by 43%, for just 15% on the tooling costs. Using Nitron-CA, drill life was extended from 2.7 to 22 holes on average, well above Airbus' expectations. Tecvac pioneered the commercial development of ion implantation for engineering components in the UK. This surface transformation process operates at around 200°C in a vacuum chamber and involves the bombardment of a steel surface with a high energy beam of nitrogen ions. As the nitrogen ions reach the surface of the workpiece, they have sufficient kinetic energy to penetrate the surface and combine with the constituents of the steel alloy, transforming chrome to chromium nitride, and so on. This means a vast range of metals not suitable for hard coating can be provided with a very thin, but very hard, surface layer. The low application temperature, often below 150°C, means that surface finish, appearance and dimensional stability are not affected by the process, and wear lifetime of treated components can be extended by a factor of three to five, or more.