2000 IRISH SCIENTIST YEAR BOOK

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Platinum plating of gas turbine components

SIFCO Ireland
Liam O'Neill, Mark Muckian & Aidan Kennedy

Figure 1. Platinum plating line recently installed by SIFCO.

Modern aero engines operate under highly corrosive conditions. The continuing demand for increased engine power and improved fuel efficiency has pushed the operating temperature of modern engines beyond 1000°C. This exposes the engine components to high pressures, temperatures and oxidising gases, resulting in increased wear rates. Metallic coatings have been widely adopted to protect turbine parts from these extreme conditions.

In recent years, SIFCO Ireland has become involved in the development of various coating technologies for such applications. SIFCO specialises in the remanufacture and repair of turbine blades and nozzle guide vanes from the 'hot section' of gas turbine engines, with three plants located in the Cork area.

Platinum based coatings

The most common protective coatings are those based on aluminium. The durability of these coatings is significantly lengthened by the incorporation of other metals. In particular, the addition of platinum leads to the development of coatings that can significantly extend the lifetime of engine components. These platinum-aluminide (PtAl) coatings are applied through a stepwise process. Platinum is first electrolytically plated onto the substrate. The platinum then diffuses into the material during a heat treatment. Subsequent aluminium deposition can be carried out using either a chemical vapour deposition (CVD) route or older, more conventional, processes. A further heat treatment results in the formation of the required PtAl layer.

Platinum plating is a complex process involving the electrochemical deposition of platinum metal onto superalloy components with complex shapes. SIFCO has recently expanded its plating capabilities through the installation of a new plating line at the Carrigtwohill plant (Figure 1) .


Figure 2. Schematic outline of rack with blades attached.

Before plating, the turbine blades undergo thorough cleaning and polishing cycles. This is essential to ensure binding of the platinum layer to the superalloy substrate. The blades are then mounted on a rack and checked to ensure high conductivity (Figure 2) . As only certain parts of the turbine blades are to be plated, layers of resin are then applied to the blade root to mask it during the plating process. The parts are then rinsed in de-ionised water and further cleaned in an ultrasonic bath.

Platinum is then deposited in the plating tank. De-ionised water is first added to the tank, and the pH adjusted by the addition of ammonia. A platinum salt, Pt(NH 3 ) 2 (NO 2 ) 2 , is then dissolved in the solution. As de-ionised water is a poor conductor of electricity, several phosphate salts are added to improve electrical conductivity. These also aid pH control. Finally, the entire bath is heated to between 80 and 90°C. As a number of variables can adversely affect the plating thickness and quality, the system is stringently monitored. Temperature and pH are determined using appropriate sensors. The composition of the bath is routinely monitored using x-ray fluorescence (XRF) and atomic adsorption spectroscopy (AAS) and the required compounds replenished by addition of chemicals.


Figure 3. Scanning electron microscope image of PtAl coating on superalloy substrate.

Once the tank is ready and the temperature is between 80 and 90°C, the rack of blades is placed between two insoluble platinised titanium anodes. The blades act as the cathode of the electrical circuit. The platinum-based solution completes the circuit. Passing a current through the circuit leads to the decomposition of the platinum salt into platinum cations and residual anions. The positively charged platinum ions are drawn towards the negative cathode, which in this case is the turbine blade, and a thin platinum layer deposits here. Figure 3 depicts an electron microscope image of a typical PtAl layer deposited onto a superalloy substrate. Adjusting the current density, platinum concentration and the plating time allows precise control of the coating thickness. This process deposits thin, adherent, crack-free, ductile layers of platinum metal onto the superalloy substrate. A typical platinum coated blade is shown in Figure 4 .


Figure 4. Platinum plated turbine blade

Future developments

Research is continuing into the development of these coatings. In particular, there is interest in producing PtAl coatings with improved mechanical and oxidative resistance properties. This may be achieved through the incorporation of other elements, or through modification of the coating procedures. Both of these options are currently being investigated.

Contact: Dr Liam O'Neill,
SIFCO Turbine Components,
Carrigtwohill, Co. Cork;
Tel: 021-4287300;
Fax: 021-4287301;
E-mail: liamoneill@sifco.ie ;
Web: www.SIFCO.ie