African Fusion March 2015

AFRICAN FUSION

March 2015

SAIW: Thermal spray coatings seminar

“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-

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.

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

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

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

The thermal spray process map shows the combination of particle velocity

and particle temperature inherent in the different thermal spray processes.

Heidi Lovelock, surface engineering specialist with

TWI in the UK, at the TSASA-hosted thermal spray

coatings seminar at Emperor’s Palace

Courtesy of TWI Limited.