Foreword
The partnership between Metro and the Materials and Engineering Research Institute (MERI) has continued to grow and prosper. MERI and Metro have continued the journey they began in 2003, a journey that has benefited both parties and one that has been recognised for its innovation.

In 2007 Metro and MERI were nominated for one of the prestigious Centres for Industrial Collaboration awards in recognition of their partnership which has developed new nozzle designs, introduced new materials for the manufacture of nozzles and explored the unknown phenomena taking place inside nozzles in operation.

Metro has become a major proponent in the use of innovative and state-of-the art analysis techniques such as electron microscopy and computational fluid dynamics to greater understand the applications in which their products are used. MERI itself has continued to develop and acquire new state of the art equipment in order to support its activities with companies like Metro. It has recently purchased the latest scanning electron microscope (FEI Nova Nano) which extends its ability to explore the microscopic and nano-scale worlds where the fundamental properties of materials are controlled. It has also secured funding for new surface analysis equipment (GDOES) which will allow it to analyse coating systems and surface treatments which are becoming increasingly important in the world of nozzles and other related equipment. MERI continues to expand the number of companies with whom it works and to whom its services and capabilities can be used to increase their commercial success.

Few other nozzle or equipment manufacturers appear to be taking the Metro approach of integrating fundamental science with business development yet this approach clearly works as Metro’s continued success and diversification are proof.

It continues to be both a challenging and a rewarding experience to work with such an enlightened and enthusiastic company who regularly bring new challenges to the attention of the scientists and engineers here in MERI.

Figure 1: An SEM image of a selection of nozzles. Top left, a steel single hole nozzle, top right a complex hole design with a diamond composite tip, bottom left a ceramic 3 hole design and bottom right an aluminium 5 hole design.

Roxton Nozzles
As SMT nozzles become smaller and the market becomes ever more competitive the challenges faced by manufacturers are continually growing. To compete on costs alone is not a viable option for most Western European manufacturers. For them success in the future lies in quality and innovation. One such company that has been at the forefront of both is Metro Technologies Ltd. By working with the Materials & Engineering Research Institute (MERI) at Sheffield Hallam University they have pushed the frontiers of nozzle materials and design in order to compete and grow.

One of the developments that shows how Metro are taking SMT design forward is Roxton™, a new and unique development for SMT nozzles. Roxton™ has been the fruit of a research project which began when Metroapproached MERI for help and guidance on how to develop better nozzles. Together MERI and Metro explored the issues surrounding SMT nozzles and their manufacture. Key issues that were identified were;

• Machining challenges for smaller and smaller nozzles
• Costs and properties of raw materials
• Quality of end product
• Colour and maintenance of colour
• Nozzle lifetime and blockages

MERI helped Metro identify and explore technologies that existed in other application areas such as antiwear coatings for racing car engines, and carried out feasibility studies to determine whether such technologies could also be used on SMT nozzles.

Many different approaches have been taken by those attempting to produce a better nozzle. This has primarily focussed on making the nozzle from a harder material in an attempt to increase wear resistance. Nozzles made from hard tool steel, nozzles with PVD coatings, nozzles made from bulk ceramics such as zirconia and nozzles with“diamond” tips are all available in various designs. All have increased hardness but all also have draw backs. Diamond tipped nozzles are not actually diamonds but a complex composite of diamond and metal and can be prohibitively expensive and prone to cracking or fracture. Zirconia nozzles are also expensive and are less tough than metallic materials and can be prone to fracture and chipping if abused. PVD coated parts can be exposed to significant heat during the coating process and require a rigorous cleaning procedure. It is also a line of sight process that cannot coat down holes, especially the small holes needed in nozzles.

Tool steel is hard to machine and relatively expensive. In order to create a better nozzle but also avoid the pit-falls of the material systems mentioned above Metro technologies have developed Roxton™. Roxton™ is a black, hard, wear resistant surface treatment that can be applied to aluminium substrates. This allows aluminium, a material which would otherwise be too soft and have too low a wear resistance, to be used to manufacturer quality nozzles. The use of aluminium can reduce both raw material and machining costs compared to hard tool steels or bulk ceramic nozzles.

It can achieve significantly longer lifetimes and also reduces overall weight putting less strain on machine heads.Roxton™ is formed by a proprietary Electrophoretic Ceramic Forming (ECF) Technology The Electrophoretic Forming utilizes the motion of electrically charged ceramic oxide powder particles in an electric field to produce highly dense layers. Due to close-packed structures of the layer this method enabled to produce low defect ceramic forming on complex surfaces. ECF of oxide powder is performed in highly loaded suspensions. The stability of the suspension is of critical importance for the resultant coating.

Optimal suspension properties are determined and achieved via electrokinetic and
electroacoustic measurements supplemented by phealogical investigations. ECF enables the deposit of uniform thickness layers of 10s of microns in several minutes.

The thickness of the surface layer is easily controlled by careful control of the processing parameters and is highly reproducible and as such tolerances can be easily maintained.

In essence the surface becomes alumina (Al2O3), which is well known for its hardness and wear resistant properties. However, unlike bulk ceramics, Roxton™ is not brittle as it is supported by the tough aluminium substrate. Alumina is also most commonly an offwhite colour but Roxton™ has been developed to have a gloss black appearance, a necessary requirement for many machines with automated vision systems. Roxton™ has excellent adhesion strengths (50MPa) and it can easily reproduce the shape of the substrate, even at sharp corners and down holes where PVD or anodising can be problematic.

Figure 2: SEM image and elemental analysis of a cross-section of an aluminium substrate treated with Roxton™. The element map shows that the Roxton™ surface consists of primarily Al and O in the form of Al2O3.

Scanning electron microscopy (SEM) was used as a key tool in this and other development projects. SEM provides a powerful and unique way of examining nozzles and its analysis capabilities aids the diagnosis of problems such as coating spalling, coating wear, nozzle blockage or nozzle fracture. It allows the examination of nozzles in unprecedented detail and can enhance quality control and provides stunningimages especially of the detail of the smaller nozzles. It has been especially useful while developing Roxton™ to assess the quality and properties of the surface layer. As can be seen in figure 1, Roxton™ has been examined using SEM to check its integrity (no pores or cracks), its thickness and consistency and its composition. Figure 2 show a typical Roxton™ layer on an aluminium substrate showing the high integrity of the coating and the excellent consistency of the surface finish. The elemental map also demonstrates the fact that the surface is an oxide of aluminium. The hardness of Roxton™ has been measured at MERI using a micro-hardness technique with a load of 50g (HV0.05). Micro-hardness is necessary because Roxton™ is a thin coating (100mm) and thus we need to avoid the influence of the (soft) substrate. For comparison the aluminium substrate was also measured. The result shows that Roxton™ is over 10 times harder than the aluminium substrate, with Roxton™ HV0.05 = 1570 compared with the aluminium substrate hardness of HV0.05 = 145. Tool steels nominally have hardness values of HV30=900 and in fact Roxton™ is more akin to ceramics such as silicon nitride (Si3N4) in hardness.

Figure 3: Scanning Electron Microscope in use at MERI

Extended testing of Roxton™ nozzles has taken place with up to 3 million simulated touch downs being carried out with no appreciable wear or damage being observed to the nozzle as observed using SEM.

Roxton™ nozzles have also been trialled in the field with over 18 months of operation resulting in a medium volume production environment with no failures and excellent reported performance.

Figure 4: The comparison of Vickers hardness of the aluminium substrate and the Roxton™ surface layer.

Roxton™ is just one of the ways in which Metro are working with MERI to push the envelope of nozzle materials and design. They are also engaged in several research and development programs with the university exploring everything from the way air flow can be effected by nozzle design to innovative new heat treatment processes to improve nozzle properties.