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Superplastic deposition of ceramic coatings


Experimental trials carried out in this project showed that the addition of oxide systems to silicon nitride markedly increases the flow of feedstock particles at impact with the substrate during thermal spray deposition. Extensive particle flow is essential for the formation of high-performance coatings. A quantitative model for the flow of particles at impact has been developed in terms of strain rate, impact velocity, particle size and second-phase content. A number of matrix systems including yttria-alumina-silica, yttria-alumina, alpha-alumina, gamma-alumina and mullite have been selected on the basis of the model and investigated. Process routes for composite feedstock powders have been developed using sol-gel techniques, spray-drying and nitrogen-sintering. An individual particle in the feedstock powders consists of a dispersion of fine silicon nitride particles in a matrix of each of the latter oxide systems. A series of powder compositions with a range of silicon nitride contents and sizes for each matrix type was prepared. Subsequent plasma spraying trials showed that the experimental results were consistent with the theoretical model. It was shown, for example, that the presence of fine silicon nitride particles and silica in glassy phases drastically reduces particle flow. The model has enabled the powders for each type of matrix system to be optimized.

Evaluation of the coatings showed that major improvements in coating quality were achieved throughout the project by following the model. The performance of the coatings as assessed by the industrial partners was comparable with that of existing industrial state-of-the-art coatings. The problems of agglomeration of the fine silicon nitride particles and poor splat flow have been overcome such that well separated dispersions can be obtained. This shows that the model is successful and has considerable potential for the development of ceramic-matrix coatings in general.
Objectives and content
Advanced non-oxide ceramics, such as silicon nitride and
silicon carbide, have a unique combination of high
strength, high toughness, wear resistance, thermal and
chemical stability. There has been a considerable
increase in the use of these materials as monolithic
components but their application is limited by their
brittleness and poor formability. Surface treatment has
major potential in overcoming these weaknesses but there
is no suitable industrial process for producing thick
coatings of non-oxide ceramics. The proposed project
will design a new thermal spray process based on
superplasticity and develop tailor-made feedstock
Non-oxide ceramics cannot be thermally sprayed to produce
coatings because they decompose rather than melt in the
flame. Although deposition in the solid state has been
shown to be practicable with metals, it is not applicable
to ceramics owing to their brittleness. However, recent
research has shown that ceramics become superplastic and
exhibit exceptionally large ductility at elevated
temperatures and stresses, providing they have
sufficiently fine grain structures and adequate grainboundary cohesion. The principle behind this proposal is
to use superplasticity as a basis for the thermal spray
deposition of non-oxide ceramics in the solid state so as
to avoid decomposition. An extensive review of the
literature has shown that no research has been undertaken
in this field. Theoretical calculations based on heat
transfer and fluid mechanics indicate that plasma
spraying is the most suitable thermal spray technique but
that it requires substantial modification before ceramic
superplasticity can be achieved during deposition. The
proposed project is directed at understanding the
phenomenon of superplastic deposition, developing a new
plasma spray process and new feedstock materials with the
aim of exploiting the unique advantages of non-oxide
ceramics for coatings and composites.
The successful development of superplastic ceramic
coatings will have a major impact on the world market for
high-quality coatings for heavy-duty applications in
industrial sectors such as chemical processing, power
generation, automotive/aero-engine manufacture and
machinery. Potential usage includes layers on metals,
ceramics and composites to protect against wear, erosion,
gaseous or liquid corrosion in applications such as gatevalves, pumps, combustion chambers and machine parts.
The proposed process has further advantages in that it
enables scrap from expensive monolithic ceramics to be
granulated to feedstock powder and recycled as coatings,
the reclamation of components by the repair of localised
surface damage and as a novel technique for the
manufacture of ceramic matrix composite monolithic
components and near net shapes. Environmental advantages
include no emission nor sludges and the ability to
recycle the overspread powder.
An important characteristic of the proposed technique is
that it is amenable for use by SMEs in component
manufacture across the EU. The equipment can be readily
installed in small premises and easily integrated into
existing production lines in order to upgrade the quality
of a wide range of components.
The proposal complies with the Workprogramme by
developing the promising advances in superplasticity made
recently in ceramic science, undertaking modelling
studies for high-performance materials and providing an
environmentally acceptable process (2.1.3.L, 2.3.1.L,
2.3.2L and 2.4.1.L).

Funding Scheme

CSC - Cost-sharing contracts


South Bank University
103,Borough Road 103
SE1 0AA London
United Kingdom

Participants (4)

Bayer AG
47829 Krefeld
Taylor Valves Limited
United Kingdom
Dowker Works
HD3 4JX Huddersfield
Laederhyddsvaegen 1
751 21 Uppsala
Universidad de Sevilla
41080 Sevilla