Hub of information - the surface engineering microsite

Materials World magazine
,
1 Sep 2008

The new website for the Surface Engineering Division of IOM3 brings together all areas of the sector in a push for better design and manufacturing. The Division explains

Surface engineering is vital to the success of almost every commercial and industrial product, from aero engines to razor blades. It is a critical enabling technology that underpins every industrial and manufacturing sector. The new Surface Engineering Division Strategy of IOM3 encompasses the needs of British industry to enable predictive design of important components and products with minimum waste and the use of more sustainable materials and environmentally friendly processes.

The centre

To promote the drawing together of the surface engineering community and allow best practice to be communicated and adopted, the Divisional website has been designed around a hub which allows developments in a number of areas to be shared in a common format.

• Near the centre of the web page are articles that relate to the early stages of the product lifecycle, such as university-based research.
• Moving outwards, articles focus on the transition from academic research to industrial applications, and then move into proven applications and case studies.
• The outer most portion presents articles of general interest, or activities relevant to the whole community, such as standardisation of testing procedures, and education and training strategies.

As with other divisional sites, the surface engineering website will be managed through a number of divisional site administrators. Members are encouraged to contribute to the hub to create a sense of community. The schematic (left) lays out this structure and below we highlight some of these areas.

High value manufacturing

High performance coatings for glazing which allow the transmission of visible light but reflect infra-red radiation, have the correct colour, can be bent to shape and survive toughening are now commonplace. These coatings are based on doped tin oxide, applied via chemical vapour deposition (CVD) continuously on the hot glass ribbon as it is produced in the float glass process – this product is sold as Pilkington K-glass – or on thin (10nm) layers of silver deposited as part of a multi-layer stack via magnetron sputtering. In this way, the U-value (a measure of heat loss through the window) can be reduced from ~5.8W/m2/K, for a single sheet of four milimetre glass, to ~1W/m2/K, for a double-glazed system. The value of this market worldwide is growing rapidly, approaching £500m/year.

The knowledge of atmospheric pressure CVD gained during the development of Pilkington K-glass supported newer products such as Pilkington Activ and coated glass for thin film photovoltaics (PV). The resurgence of interest in solar power is due to high oil prices and concerns over carbon dioxide emissions from fossil fuel use. Mass production of thin-film PV offers a potential solution because it will be cheap, its power generation process is emission-free, and the sun provides an inexhaustible supply of energy that is thousands of times greater than current, and forecast, global demand.

The two main thin film systems are based on the semiconductor absorber layers cadmium telluride and amorphous silicon, but in both cases light must get to the absorber layer and charge carriers must leave the device. Finding materials that both transmit light and conduct electricity is not easy, though the doped oxides of tin, indium and zinc show this behaviour. Of these, the former is preferred – because of the high cost of indium metal and the poor environmental performance of zinc oxide. Also, the flat glass industry can produce large quantities of doped tin oxide reliably and cheaply.

Transport technology

Coatings were first used on steel compressor airfoils in the 1960s to limit unexpected levels of corrosion. These included aluminium-filled overlay coatings, such as SermeTel W. They are produced from water-based slurries which are sprayed and cured to form a tightly adherent coating where fine aluminium particles (<five microns in diameter) are cemented in place by a ceramic binder. This structure could be made electrically conductive to provide galvanically sacrificial protection.

In the mid 1970s, a polished version of SermeTel 5375 was introduced. Surface finishes of <0.38µm Ra could be achieved. Tested by airlines these coatings reduced exhaust gas temperatures and resulted in a one per cent reduction in thrust-specific fuel consumption. Later coatings such as SermeTel 5380 DP, combined improved galvanic protection with smooth finishes without the need for mechanical polishing.

SermaFlow represents a new generation of Sermatech coatings. This multi-coat system incorporates an ultra-smooth topcoat (<0.25µm Ra) which is inert, non-wetting and fouling resistant. The non-metallic topcoat resists the build up of charged particles, helping the surface remain clean in service. The smooth surface is an extremely effective alternative to superpolishing and resists environmental effects that would compromise a superpolished surface. Such products enhance and maintain the optimum airflow efficiencies needed for modern engines.

Medical importance

A porous metal coating, Gription, has been developed by DePuy, headquartered in the USA, for surgical joint replacements that combines the optimal porosity for bone ingrowth with a high initial locking friction with the patients bone. DePuy used titanium alloys with proven biocompatibility as part of its development of minimally invasive surgical tools.

The coating has an average pore size of 300um with 63% porosity through the structure. Traditionally, the best coatings for bone ingrowth have spherical beads, uniform sintering characteristics and control of porosity. Gription uses this with an additional surface of plate-like particles that give an immediate friction lock against the bone.

Although it is relatively easy to sinter titanium alloys, there are special problems with joint prostheses. Applying a uniform depth of particles to the shape is not easy, and the coating has to stay in place through thermal cycling until metallic bonds are formed. Also, hip joints are highly stressed long-life components and traditional sintering temperatures tend to form structures suited to low cycle fatigue. In the USA, Food and Drug Administration approval has been given for implantation of the Gription coating on some hip replacement components.

In joint replacement surgery stable fixation of the implant is a key factor for long-term success preventing damage to the muscles, tendons and ligaments. Biological fixation by the ingrowth of bone into the implant surface is a satisfactory way of achieving this. Porocoat, also devised by DePuy uses spherical beads to provide an optimum pore size fixation.

Raising the standard

When combined with the need for improved reliability and cost effective manufacture, the development of improved measurement methods and standards is essential.

Measurements are needed to control the quality of coating production so that the requisite reliability of coated components is achieved. Conventionally this takes place by measuring properties such as hardness, but increasingly, the focus is moving towards measurements related to the control of the coating process itself. Increased confidence in the reliability of this process will reduce, or even eliminate, the need for quality control measurements.

Measurements are also increasingly needed to enable effective design of components and engineering systems. Values for key properties of the coatings and materials systems, along with an understanding of how the variation and uncertainty in these parameters affects achievable performance, enable sound selection and design of the complete engineering system from an early stage. This allows the design process to be optimised, leading to more reliable, cost effective and higher performance products. It is normally not possible to evaluate the performance of new experimental coatings in their final applications, so some form of laboratory testing is used.

Once appropriate measurement methods have been developed, standards are needed so that organisations in the supply chain can agree on specification criteria that gives confidence for commercialisation. Many national and international groups such as ASTM International, the European Committee for Standardization – CEN and the International Organization for Standardization – ISO, are active in standardisation of measurement methods for surface engineering.

Facing the future

The six sectors that comprise the surface engineering hub face significant challenges in the development of material systems. In early applications, surface engineering was seen as a remedy to address design shortfalls, and the product was introduced into operation with a ‘build and test’ philosophy. To adopt robust design, the Institute recognises the importance of education and training, and at Congress 2008, in Grantham, UK, this May it held the first in a series of simulation workshops aimed at providing the tools necessary to support the change from reactive to proactive surface engineering. In this context, modelling techniques will become increasingly important to optimise materials and the associated manufacturing processes. These will be linked to design data for the coating system, which will collectively reduce development costs and product introduction timescales, maximising business benefits through the adoption of designed-in surface engineering applications.

Further information:

Surface Engineering Division of IOM3