African Fusion March 2015

SAIW: Thermal spray coatings seminar

Introduction to thermal spray coatings Heidi Lovelock

On January 21, 2015, SAIW and the Thermal Spray Association of Southern Africa (TSASA) hosted South African-born thermal spray specialist, Heidi Lovelock, who is now with TWI in the UK, to present aone-day seminar at Emperor’s Palace. African Fusion summarises the opening session.

Heidi Lovelock, surface engineering specialist with TWI in the UK, at the TSASA-hosted thermal spray coatings seminar at Emperor’s Palace .

“People often don’t realise just how important surface engineering is to modern life. While the technology is largely invisible, it is also indispensable,” Lovelock begins. The general role of surface engineering is to optimise the surface of a component to better suit its environment. While one can optimise mechanical properties such as strength and toughness of a product by selecting an appropriate base material, surface engineering allows one to separately optimise the surface for better wear, corrosion, high temperature oxidation or thermal degradation resistance. And the material property requirements for the bulk materials of a product often contradict the requirements at the sur- face,” she points out. Also, from a cost, design flexibility and fabrication point of view, corrosion and wear resistant materials are often very difficult to fabricate or they cost too much. Citing some examples, she says that in the transport industry, coating technologies are used in catalytic con-

verter cartridges, jet engines and aircraft landing gear. In the energy field, the technology is indispensable for coating solar cells, for the cathodic protection of offshore wind turbines and, for conven- tional fossil fuel power plants, coatings are usedon the blades of steamturbines and for erosion protection of critical boiler tubes. “In the healthcare industry, people are living longer, so knee and hip im- plants are needed. The success of these depends on thermally sprayed ceramic and titanium alloy coatings that ensure the implant is biocompatible andable to bind to bone,” Lovelock reveals. Surface modification treatments and coatings include a multitude of processes such as carburising, nitriding and anodising surface layers, for im- proving the hardness of gear teeth, for example. Hard chrome andother plating processes, such as copper, nickel, zinc, cadmiumand tin coatings, are inexpen- sive but, in the case of hard chrome, in particular, are under pressure due to environmental considerations. Other

coatings, many of which can be very sophisticated. Thermal spray coating characteristics “Thermal spray coatings adhere via a mechanical bond as opposed to a met- allurgical one. Sometimes people react to this fact with nervousness, think- ing the surface layer can’t have much bond strength. But it most certainly can. Tensile bond strengths of greater than 80 MPa are achievable but the true strength is difficult to determine accurately due to the limitations of the glues used in testing,” Lovelock reveals. “The bonding mechanisms at the coating/substrate interface and be- tween the coating ‘splats’ is still subject to some speculation, though, but while both mechanical interlocking and dif- fusion bonding may occur, mechanical bonding predominates.” In principle, all thermal spray pro- cesses involve the use of a coating con- sumable, usually in wire or powdered form. An energy source is used to heat the consumable tomelt or soften it, after which the particles are propelled toward the surface being coated. Representing thermal spray coating processes in two dimensions, Lovelock displays a pro- cess map with particle temperature on the y-axis and particle velocity on the x-axis. There are four broad processes: the flame spray processes on the left at medium temperatures and low par- ticle velocities; the plasma processes that give very high temperatures but relatively low particle velocities; the high velocity oxy-fuel (HVOF) processes, which can produce particle velocities of

modern process- es include thin film processes such as chemical and physical va- pour deposition (CVD and PVD), which offer layer thicknesses from a fewnanometres to 50 µm; as well as a wide range of paints, epox- ies, synthetic lay- ers and powder

Courtesy of TWI Limited.

The thermal spray process map shows the combination of particle velocity and particle temperature inherent in the different thermal spray processes.

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March 2015

AFRICAN FUSION