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Core research pushes the boundaries

Connect, no. 135, March/April 2005, p.6

Fig.1. BSE image of nanosteel coating produced by HVOF spraying showing mixed crystalline and amorphous structure
Fig.1. BSE image of nanosteel coating produced by HVOF spraying showing mixed crystalline and amorphous structure

It is forecast that nano-materials will revolutionise surface engineering because remarkable combinations of properties, such as strength, hardness, toughness, impact and corrosion resistance, will be achievable simultaneously due to nano-scale grain refinement.

At present, nano-structured coatings are mostly produced by vapour phase deposition, ion or sol-gel processes and are limited to high value components.

To introduce nano-structured coating benefits to a wider market more processes such as HVOF spraying and low cost materials such as steel will be needed. To date, nano-structured thermal spray powders have been produced by expensive processes such as the agglomeration and sintering of mechanically milled nano-sized particles to form macro-powders.

Fig.2. Potential application - gas turbine journals
Fig.2. Potential application - gas turbine journals

TWI, however, has recently completed the preliminary evaluation of HVOF coatings prepared from powder containing a mixture of crystalline and amorphous structures. Initial trials show these coatings are characterised by very low porosity (0.2%), high hardness (960HV0.2) and low oxygen content (0.2%).

Work continues in the Core Research Programme comparing the wear and corrosion behaviour of coatings produced from amorphous and nano-structured powders with HVOF carbide coatings and hard chrome plating.

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