With seemingly minor factors such as the surface finish of compressor blades, for example, playing a key role in increasing fuel efficiency, OEMs and MROs (maintenance, repair and operations service providers) are turning to advanced coatings that modify the surface of turbine components to improve wear and corrosion resistance. Such coatings also provide a lower coefficient of friction for metal-on-metal contact of rotating parts such as shafts. One technology attracting increasing attention is physical vapour deposition (PVD) coating for both industrial gas turbine (IGT) and steam turbine components.
Gas turbine applications
IGT OEMs continue to seek new ways to incrementally increase turbine efficiency, for example by employing higher firing temperatures, further raising the temperatures inside turbines and leading to additional wear of components.
Erosion/corrosion & fouling of compressor blades, vanes and inlet guide vanes (IGVs) is a concern, given that the surface finish has a direct impact on optimum airflow and, therefore, engine efficiency. This can occur as a result of many factors, including particulates or moisture droplets that make it past air inlet filtration systems into the compressor section. An IGT located near a refinery, for example, can draw in polymers and sulphur from the air. Sand and salt are also common issues, depending on location.
Over time, the particulates stick to the blades, creating a rougher surface that degrades turbine performance. As the blades foul, the efficiency of the entire compressor system eventually drops. This can drive up operating costs until the parts are serviced. Even then, parts that are re-polished to a mirror finish become quickly fouled again.
“Ensuring that surfaces of compressor blades are maintained smooth during service will maximise air flow through the compressor and sustain compressor efficiency,” says Paul Brooks, lead segment manager of power generation, Oerlikon Balzers. “This is an area where PVD coatings excel because of their unique combination of extreme surface hardness and low friction coefficient. PVD coatings have only been used marginally in gas turbines to date, largely because of limited awareness in this sector.”
Although the base material used to manufacture compressor section blades and vanes varies, and continues to evolve, most IGT OEMs today use stainless steel, or apply a galvanic coating over a base steel. Unfortunately, galvanic coatings are relatively soft and erode over time. Due to the high cost of these components, solutions such as PVD coatings, which are much harder and last longer, are now considered a better solution.
PVD represents a variety of vacuum deposition methods that can be used to produce very thin coatings, typically 1-5 μm in thickness. The thin coatings, in conjunction with close tolerancing, means that the component retains its form, fit and dimensions after coating without the need for re-machining. Thicker coatings up to 25μm can also be applied, if needed, to increase erosion resistance.
PVD coatings also provide a viable alternative in replacing hard chrome plating. This comes at an opportune time, given the industry’s transition away from hard chromium plating to more environmentally friendly alternatives. For many years, hard chrome plating was the standard for achieving wear and corrosion protection, but in Europe, for example, due to REACH regulations, the application of hard chrome plating is now highly regulated.
One REACH compliant PVD coating, BALINIT® TURBINE PRO from Oerlikon Balzers, is specifically designed to protect engine compressor blades, vanes and integrated bladed rotors (blisks) from particle erosion by maintaining a highly polished surface finish to retain efficiency gains for the lifetime of the part.
The formulation delivers a metal aluminium nitride (MeAlN) structure that results in an optimal relation of high hardness to residual compressive stress even under demanding thermal conditions. The PVD coating can be applied to steels, super alloys and titanium components and has an extremely low surface roughness once applied.
The high hardness of BALINIT TURBINE PRO has already been proven in solid particle, liquid droplet, liquid cavitation, waterjet and other erosion tests with the coating on various substrates (steel, Inconel and titanium) in different coating thickness and high temperatures.
In the solid particle erosion test in which materials were evaluated based on mass loss, for example, BALINIT TURBINE PRO demonstrated more than five times higher erosion protection than other PVD coatings, including titanium nitride (TiN). That value increased to more than 40× when compared to uncoated titanium and even more for steel.
PVD coatings also show promise for the hot sections of IGTs. Oerlikon Balzers is currently working with leading IGT OEMs on development of PVD coatings that address erosion/corrosion in these parts of the turbine. Given the variety of metal substrates utilised for component parts and the variable demands in different sections of IGTs, the company regularly collaborates with OEMs to advance new coating formulations.
Steam turbine applications
Components of steam turbines also face high temperatures, erosion and fretting with the additional concern of steam-based corrosion.
The primary concern when coating steam turbine blades and vanes is shortened life due to solid particle or water droplet erosion. As hot steam comes through to the high-pressure stage of the steam turbine, blades and vanes are subject to solid particle erosion. As the steam cools and turns to liquid in the low pressure stages, water droplet erosion becomes the concern.
A companion solution to the gas turbine coatings, BALINIT D, was developed specifically for applications involving high temperature steam, to improve erosion and oxidation resistance.
Because of its extreme hardness and superior wear properties, the PVD coating allows steam turbine OEMs to substitute expensive alloys with less expensive stainless steel or chrome steel substrates.
