How to select coatings to optimise performance

Paul Boughton

Considering the operating environment and its impact on the coatings selected. By Andrew Courtney

Specialised coatings are used to permanently enhance the surface of metals and other substrates on components and equipment used in a wide range of industries to solve wear, corrosion, lubrication and release problems. This article examines the challenges posed by different operating environments and the impact these challenges have on the choice of a suitable surface coating.

In the automotive industry the use of surface coatings has expanded exponentially. Fifty years ago the internal combustion engine was generally manufactured from cast iron, forged steels and die-cast aluminium alloys. Heat treatment, coupled with the use of white metal or bronze bearings, provided the wear surfaces. These were lubricated by the oil films pumped through generous clearances allowed between moving surfaces.

Although the same operating principles continue today, modern engines have to last for significantly longer distances as well as adhering to emissions regulations. The need for reduced fuel consumption means that the industry requires much lighter engines with lower engineering tolerances and lower friction between moving parts.

This change has necessitated the incorporation of a much wider range of materials, which has required the use of a wide variety of coatings to overcome the wear, corrosion and friction problems that the newer materials can sometimes introduce.

Piston rings, skirts and pins are often coated with hard-wearing, low friction coatings to aid initial running-in followed by long-life performance. This is due to the lower oil lubrication levels associated with cold engine starts and the reduced amount of lubricant permitted to reach the combustion chamber. The use of ceramic coatings to provide thermal insulation of piston crowns, particularly in diesel engines, is increasing and high temperature thermal spray or vacuum application methods can be used.

The mating cylinder linings can be coated with electrolytic or electroless plated composite finishes incorporating hard particles of silicon carbide, diamond or boron nitride. Vacuum deposited diamond-like coatings are also employed, which are designed to reduce wear and lower friction. The particular engine design and performance requirements influence the design engineer’s choice.

The increased use of bio fuels has greatly increased the corrosion problems of the fuel input systems and there is a wide range of thin coatings that can be applied to mitigate this problem on fuel injection systems. Even the fuel storage and delivery systems have corrosion issues and the use of electroless nickel plating, often enhanced with powder coating or e-coat top coats, are widely used.

Along with improved engine performance and tyre design, there has resulted in a need for better braking systems. The move to open spoke alloy wheels and consequently more visibility has also required the improvement of anti-corrosion coatings and appearance of the brake calipers. Originally brakes were usually made from ductile cast iron and zinc plating was sufficient corrosion protection. Weight reduction targets have increased the use of aluminium calipers. These require hard anodising to improve external corrosion protection and to improve the wear characteristics in the piston bores. Increasingly the anodising is being enhanced by applying paints and powder coating top coats, colour co-ordination is also very important, particularly on luxury vehicles.

Surface Technology’s specialist ductile zinc nickel plating, used extensively for brake systems, is now also commonly used on fluid delivery pipework and air suspension systems. The key benefit of this coating is its malleability, allowing for standard manufacture of the pipe system in straight form with post plating manipulation to the required design. Post forming to the required design offers component manufacturers both improved production efficiencies and reduced complexity and working capital through component standardisation. Surface Technology currently works with manufacturers and suppliers of components for high volume production vehicles and leading high-performance and motorsport companies.

The automotive industry also has an ever-increasing need to coat the components used in door latches, seatbelt mechanisms, under bonnet and seat adjustment assemblies. The coatings need to provide corrosion resistance, noise reduction and constant friction performance. Vehicles increasingly have multiple motors containing neodymium/iron/boron magnets that, while very powerful magnetically, are also very susceptible to corrosion. Surface Technology has a range of coatings that can overcome this ranging, from its proprietary TriCem coating to heat cured coatings designed to cure at temperatures that leave the magnetic performance of the various alloys unaffected.

Coatings for mining, construction and off-highway vehicles

Surface Technology coated pivot pins are extensively used in the heavy mechanical handling vehicle industry. It widely plates zinc and zinc alloy to high volumes of all sizes of pins, and also has a proprietary Molybdenum Disulphide coating. This provides both corrosion protection and permanent lubrication to pins that were originally sherardised, needing large clearances on diameter. The use of the molybdenum disulphide resin coating allows tighter dimensional fitting and movement of the hydraulically moved operating arms.

Hard chrome is widely used where components need to be able to withstand friction, extreme temperatures and corrosive environments while continuing to operate at optimum levels and has, therefore, been a mainstay of industrial coating in industries such as aerospace, heavy equipment and engineering applications for many years. However, the safeguarding EU and US regulations mean that hard chrome plating is incurring rising costs and extensive regulation and its use will be severely restricted under the REACH directive.

Successful alternatives are being sought that can offer enhanced protection against corrosion, wear, chemically aggressive media and enhanced fatigue life.  An emerging replacement solution to this problem, which is already being used on parts such as aviation landing gear, is high velocity oxy-fuel (HVOF) spray.  This most popular application of thermal spray is deposited with spray systems using gaseous fuels, generally to protect against wear. In turn this solution provides a wealth of other benefits around, but not restricted to, wear behaviour, no hydrogen embrittlement and higher ductility for lower microcracking tendancy.

Andrew Courtney is with Surface Technology

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