A cleaner finish — the evolution of surface engineering

Materials World magazine
,
1 Sep 2008

Keith Harrison, Chairman of the IOM3 Surface Engineering Division, reports on how surface engineering has become more environmentally friendly.

There is a need to change our ways for the sake of the environment, to conserve our natural resources, reduce carbon emissions and use less of our strategic materials such as nickel, tungsten, cobalt, molybdenum and indium. Surface engineering is helping by adapting its technologies to meet these challenges, enabling designers to specify a low-cost, lightweight, sustainable and easily workable substrate that can be modified with a functional coating exactly where needed.

The surface of a component not only reacts with the environment, but possibly with another surface or body. It can determine the chemical, physical, and mechanical behaviour of the entire system and the subsequent performance of the component. Functional coatings can alter properties such as wear, friction, corrosion, thermal conductivity and insulation, electrical conductivity and resistance, and even be decorative.

Whatever process is used, the technology is continually being refined. For example, the electroplating industry has drastically reduced the use of toxic chemicals such as chromates and cyanides. The treatment of waste solutions to eliminate the emission of heavy metals and other toxic compounds has also improved, while the heat treatment industry is continually adopting new methods to reduce energy consumption.

Something in the air

A good illustration of how technology has progressed is in the thermal spray sector where advances have been made in the last 30 years to reduce energy consumption, improve efficiency and cut emissions. The exhaust from thermal spray booths is now passed through pleated cartridge filters before being emitted, as environmental legislation limits the amount of particulate content that can be released.

These filtration systems have continuous indicative monitoring to prevent these limits being exceeded, and typically, emissions are under five milligrammes per cubic metre and can be lower in certain units. Dry filtration has therefore superceded the less efficient wet-back spray booth with wet collectors.

There has also been a step change in the design of plasma spray guns, which had seen only small improvements over many years. Plasma is a gas in the ionised state and is a fairly efficient way of converting electrical energy into thermal energy providing a suitable medium to melt and accelerate a fine powder to form a coating onto a prepared substrate. The overall layout remained largely the same as in the original Metco type MB gun until the development of the triple cathode plasma spray gun. This was driven by the need to reduce costs by increasing deposit efficiency and production rates, and extending hardware life.

The latest thermal spray guns have been developed using technologies not available 40 years ago, such as computational fluid dynamics. The output from these guns has been optimised using new techniques for analysing temperature and velocity of the individual feedstock particles in real time. These technologies have allowed effects to be observed that have never before been properly characterised and have resulted in a new generation of plasma spray guns with stable and well-defined arc behaviour. This results in uniform powder heating and significant improvements in gun performance.

Deposit efficiencies can be increased by up to 50% and, in some cases, throughput is increased by a factor of three compared to traditional guns. The arc is divided into three individual arcs of 100-200A compared with 500-1,000A in traditional guns for the same power levels, allowing industry to adopt fixed parameters for the first time as there is no process drift. The life expectancy of the gun consumables can be an order of magnitude better than traditional guns, allowing the unit to be used for a much longer operating time.

Machine driven

Automated spraying facilities not only produce surfaces of excellent and unvarying quality at higher output and enhanced process efficiency, they also reduce emissions and protect the operators. Automation in plasma spraying can cover all aspects of the process, from selection of the powder production method and testing, to quality assurance of the coatings. Also included is the pre-treatment of the workpiece surface, controlling the plasma generation and powder injection into the plasma, as well as monitoring the relative movement between the spray gun and workpiece.

Spray facilities installed in modern high-performance noise absorbing booths with an exhaust filter reduce operating noise level to below 75dB(A). Dust residues in the air are less than one milligram per cubic metre, and the working conditions of the operator are greatly improved by limiting the number of times he or she has to enter the spray booth.

The use of multi-axis robots and other advanced manipulation equipment maintains precise spray gun-to-workpiece distance whatever the component configuration. This results in optimum spray efficiency and coating microstructure, with controlled application temperature to minimise residual stress.

Spray development

Early thermal spray controllers had manual controls for the operator to set parameters such as gas flows, current and voltage values. The plasma had to be manually ramped up or down by the operator who also had to start the powder feed at the right time.

The latest generation of controllers offer unlimited thermal spray versatility and process control in one package. They combine the advantages of a computer, for process visualisation and data management, with the strength of a programmable logic controller that can log data and monitor the entire process.

With easy to use touch screens and mass flow gas control, some controllers can handle up to four separate thermal spray processes from a single operator station. Spray recipes can be programmed by a process engineer and accessed, but not modified, by the operator. The whole process is fully automatic.

Multilevel alarms have been built in to deliver warning messages to critical alarms with automatic shutdown. They can be interfaced with gas detection systems, exhaust flow monitoring, and spray gun and workpiece manipulation systems for rapid and controlled shutdown.

Online sensor and monitoring technology is used continuously to measure the position and geometry of the spray plume, overall intensity of the spray stream, mean particle temperature and velocity and substrate temperature. This maximises quality control and aids the development of optimum coating parameters to give specific characteristics.

Everyday applications

Surface engineering helps the environment by replacing older applications that use environmentally unfriendly processes or materials with coatings that use less energy during manufacture and do not pollute or use up stocks of strategically important materials.

The list of applications is almost endless, and the importance of the technology can be seen everywhere. Most new landing gear components such as hydraulic rams, shafts and trunnions, which were traditionally hard chrome plated, are now coated with tungsten carbide/cobalt/nickelusing the high velocity oxy-fuel process.

Surface engineering is used in solid oxide fuel cells to form the dense ceramic oxide electrolyte and porous anodes. The ion conducting membranes of lanthanum strontium manganite used for gas separation can also be manufactured. Alternative coatings to electroplated cadmium are being developed, such as thermal sprayed aluminium and thin film technology using physical and chemical vapour deposition.

Planning ahead

Recognised as important to the future of UK industry and manufacturing, surface engineering has a vital role in the future of power generation, especially fossil fuel combustion, to overcome problems with plant operation in the more arduous conditions associated with carbon capture and storage.

The use of synthetic gas and biomass fuels will also lead to difficult operating conditions. New coatings and coating systems to combat the corrosion and tribological problems associated with these conditions need to be developed.

Surface engineering will also have a large input to other generation techniques such as wind, tidal and wave power. Coatings will also have an important role to play in combating wear and corrosion in nuclear power generation. Furthermore, coatings have to be developed for enhanced corrosion protection of high performance rare earth magnets such as neodymium, iron and boron.

Limits of particulate content

Total particulate (per m3 of air): 50mg max
Chromium: 15mg max
Nickel: 15mg max
Copper: 7.5mg max
Cobalt: 3mg max

Further information:

IOM3 Surface Engineering Division